materials Article
Magnetic Force Microscopy of Nanostructured Co/Pt Multilayer Films with Perpendicular Magnetization O. L. Ermolaeva 1, * and V. L. Mironov 1 1
2
*
ID
, N. S. Gusev 1 , E. V. Skorohodov 1 , Yu. V. Petrov 2 , M. V. Sapozhnikov 1
ID
Institute for Physics of Microstructures RAS, GSP-105, 603950 Nizhny Novgorod, Russia; [email protected] (N.S.G.); [email protected] (E.V.S.); [email protected] (M.V.S.); [email protected] (V.L.M.) Saint Petersburg State University, University Embankment, 7/9, 199034 St. Petersburg, Russia; [email protected] Correspondence: [email protected]; Tel.: +10-910-3963589
Received: 29 June 2017; Accepted: 31 August 2017; Published: 5 September 2017
Abstract: We present the results of magnetic force microscopy investigations of domain structures in multilayer [Co (0.5 nm)/Pt (1 nm)]5 thin film structures (denoted hereafter as Co/Pt) modified by additional Co capping layers and by ion irradiation. It is demonstrated that a Co capping layer essentially changes the domain structure and decreases the threshold of magnetization reversal, due to the formation of noncollinear magnetization in Co/Pt. It is shown that local irradiation with a focused He+ ion beam enables the formation of regions with decreased easy-axis anisotropy (magnetic bubbles) that have the inverse magnetization direction in the demagnetized state of Co/Pt. The experimental results demonstrate that the magnetic bubbles can be switched using a probe of a magnetic force microscope. The possible application of these effects for the development of magnetic logic and data storage systems is discussed. Keywords: perpendicular anisotropy; magnetic domains; magnetic nanostructures; magnetic nanowires; magnetic force microscopy
1. Introduction The engineering of magnetic states in ferromagnetic nanosystems is one of the topical problems closely connected with the development of modern magnetic data processing systems and element base of spin electronics [1]. One effective solution to this problem is the creation of planar patterned ferromagnetic nanostructures using lithographic methods. The control of shape and size of the patterned elements allows ferromagnetic nanostructures with various distributions of magnetization to be obtained [2–7]. Additional possibilities are opened with the use of magnetic films with perpendicular (easy-axis) anisotropy. The fabrication of nanostructures based on multilayer systems consisting of films with easy-axis and easy-plane anisotropy enables the realization of structures with essentially noncollinear magnetization distribution [8–10] for magnetic devices with vertical architectures. Additionally, in recent years, new approaches to modifying the properties of magnetic films by focused ion beams have been developed [11–15]. These methods enable the local modification of the magnetic properties of thin-film structures without changing the surface relief. The combination of all these methods is a powerful tool for micromagnetic engineering. In the present study, we investigate the possibilities of local modification of the domain structure in Co/Pt multilayer films with perpendicular anisotropy through the growth of additional patterned Co layers, and by local modification of the Co/Pt anisotropy parameter using a focused He+ ion beam.
Materials 2017, 10, 1034; doi:10.3390/ma10091034
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2. Experiment Materials 2017, 10, 1034
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Multilayer thin film structures [Co (0.5 nm)/Pt (1 nm)]5 were grown by DC magnetron sputtering on Si(100) substrate with Ta(10 nm)/Pt(10 nm) buffer sublayer. Additionally, we deposited Co capping 2. Experiment layers with variable thickness. The Co capping layers were patterned using lithographic methods Multilayer thin film structures [Co (0.5 nm)/Pt (1 nm)]5 were grown by DC magnetron sputtering to locally changesubstrate the domain The Carl 50VP” scanning microscope on Si(100) with structure. Ta(10 nm)/Pt(10 nm) Zeiss buffer “Supra sublayer. Additionally, weelectron deposited Co with “ELPHY Plus” exposure system (Carl Zeiss company, Jena, Germany) was used for the e-beam capping layers with variable thickness. The Co capping layers were patterned using lithographic lithography [4,16]. Additionally, wedomain studied the Co/Pt modified by a focused methods to locally change the structure. Thestructures Carl Zeiss when “Supraspatially 50VP” scanning electron + ion microscope, microscope with “ELPHY Plus”He exposure system (Carl equipped Zeiss company, Germany) was used generator for ion beam. The Carl Zeiss “Orion” withJena, “Nanomaker” pattern the e-beam lithography Additionally, we studied the Co/Pt structures when spatially (Carl Zeiss company, Jena, [4,16]. Germany), was used to fabricate arrays of modified areas inmodified Co/Pt [17]. by a focused ion beam. The Carl Zeiss “Orion” He+ ion microscope, equipped with “Nanomaker” The magnetic properties of the samples were investigated by home-made magneto-optical Kerr pattern generator (Carl Zeiss company, Jena, Germany), was used to fabricate arrays of modified effect (MOKE) technique. Domain structure was studied by magnetic force microscope (MFM) areas in Co/Pt [17]. “Solver HV” company, Moscow, Russia) equippedmagneto-optical by DC magnet with 500 The(NT-MDT magnetic properties of the samplesZelenograd, were investigated by home-made Kerr Oe perpendicular the sample) magnetic field. by The MFM probes were standard effect (MOKE)(relatively technique.toDomain structure was studied magnetic force microscope (MFM)NSG-11 “Solvercoated HV” (NT-MDT Moscow, Zelenograd, by DC magnet with 500 were cantilevers with Cocompany, with a thickness of 30 nm. Russia) Before equipped the measurements, the probes Oe perpendicular (relatively to the sample) magnetic field. The MFM probes were standard NSG-11 magnetized along the tip axes in a 1T external magnetic field. The MFM measurements were performed with Co with of 30constant-height nm. Before the measurements, the probes both incantilevers two-passcoated tapping mode and ainthickness noncontact mode. The phase shift ofwere cantilever magnetized along the tip axes in a 1T external magnetic field. The MFM measurements were oscillations under the gradient of the sample magnetic force was registered as the MFM contrast. performed both in two-pass tapping mode and in noncontact constant-height mode. The phase shift The MFM measurements were performed in a vacuum chamber with the rest gas pressure of 10−4 of cantilever oscillations under the gradient of the sample magnetic force was registered as the MFM Torr that improved the MFM signal due an increase cantilever quality contrast. The MFM measurements weretoperformed in aof vacuum chamber with factor. the rest gas pressure of 10−4 Torr that improved the MFM signal due to an increase of cantilever quality factor.
3. Results and Discussion
3. Results and Discussion
The typical hysteresis loop (the dependence of perpendicular component of magnetization on The typical hysteresis loop field) (the dependence of perpendicular on curve an out-of-plane external magnetic of the initial Co/Pt film iscomponent presentedofinmagnetization Figure 1a. This an out-of-plane external magnetic field) of the initial Co/Pt film is presented in Figure 1a. This curve has a practically rectangular shape with the remanent magnetization equal to the magnetization in has a practically rectangular shape with the remanent magnetization equal to the magnetization in saturation M0 . The coercive field is about 200 Oe. The MFM image of Co/Pt labyrinth domain structure saturation M0. The coercive field is about 200 Oe. The MFM image of Co/Pt labyrinth domain structure in a demagnetized statestate (demagnetizing 200Oe) Oe)isisshown shown in Figure in a demagnetized (demagnetizingfield field was was 200 in Figure 1b. 1b.
Figure 1. The normed MOKE hysteresis curves and MFM images of the samples’ domain structure in
Figure 1. The normed MOKE hysteresis curves and MFM images of the samples’ domain structure demagnetized state. (a,b) are for Co/Pt structure without capping layer; (c,d) are for the Co/Pt in demagnetized (a,b) are for Co/Pt without capping are forlayer. the Co/Pt structure withstate. 1 nm Co capping layer; (e,f) structure are for the Co/Pt structure withlayer; 1.3 nm(c,d) Co capping structure nm Co (e,f)MFM are for the Co/Pt structure with 1.3 nm Co capping layer. The with MFM1frame sizecapping is 5 μm ×layer; 5 μm. The contrast is normalized to the maximum. The MFM frame size is 5 µm × 5 µm. The MFM contrast is normalized to the maximum. As can be seen from Figure 1b, the typical lateral size of magnetic domains is about 1 μm. The additional Co capping layer changes the domain structure considerably. The hysteresis curve As can be seen from Figure 1b, the typical lateral size of magnetic domains is about 1 µm. and MFM image for Co/Pt structure with 1 nm Co capping layer are represented in Figure 1c,d. The additional Cowe capping changes of thethe domain structure considerably. The hysteresis and In this case, observelayer the narrowing hysteresis loop (the remanent magnetization is 0.8curve M0
MFM image for Co/Pt structure with 1 nm Co capping layer are represented in Figure 1c,d. In this case, we observe the narrowing of the hysteresis loop (the remanent magnetization is 0.8 M0 and the
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coercive field is 50 Oe) and a reduction in the size of the domains (typical width is about 350 nm). In the field is 50layer, Oe) and reduction in the size of the is domains width isfield about case ofand the the 1.3 coercive nm Co capping thearemanent magnetization 0.2 M0(typical , the coercive is 10 Oe 350 nm).2017, In the case of the 1.3 nm Co capping layer, the remanent magnetization is 0.2 M0, the coercive Materials 10, 1034 3 of 7 and the typical domain width is 150 nm. The decrease of the domain structure scales with increasing field is 10 Oe and the typical domain width is 150 nm. The decrease of the domain structure scales thickness ofincreasing the Co capping layer cancapping be explained bybeeffects of exchange interaction. The Co-Co/Pt with thickness of the layer can explained effects of(typical exchange interaction. and the coercive field is 50 Oe)Co and a reduction in the size of thebydomains width is about exchange coupling leads to a decrease in the effective perpendicular anisotropy, and to an increase The Co-Co/Pt to alayer, decrease in the effective perpendicular anisotropy, 350 nm). In the exchange case of thecoupling 1.3 nm Coleads capping the remanent magnetization is 0.2 M0, the coercive in the density ofOe theand domain in of thethe Co/Pt subsystem. Similar effects of changes in the domain and increase in typical thewalls density domain walls in the Co/Pt of subsystem. Similar effects of fieldtois an 10 the domain width is 150 nm. The decrease the domain structure scales changes inmentioned the domain structure were mentioned in [18], Co/Pd structures withlayer a NiFe capping structure were in [18], Co/Pd structures with aby NiFe capping were studied. with increasing thickness of thewhere Co capping layer can bewhere explained effects of exchange interaction. were studied. The Co-Co/Pt exchange coupling leads to astructures decrease ininthe perpendicular anisotropy, Tolayer create laterally inhomogeneous domain theeffective Co/Pt sample, we fabricated spatially To create laterally inhomogeneous domain structures in the Co/Pt sample, we fabricated and to an increase in the density of the domain walls in the Co/Pt subsystem. Similar effectsstructure of inhomogeneous Co coatings. As an example, Figure 2 shows the MFM image of the Co/Pt-Co spatially Co coatings. an example, Figure 2 shows MFM image the Co/Pt-Co changes inhomogeneous in the domain structure were As mentioned in [18], where Co/Pdthe structures withof a NiFe capping in the region near the edge of the 1 nm Co capping layer. structure instudied. the region near the edge of the 1 nm Co capping layer. layer were To create laterally inhomogeneous domain structures in the Co/Pt sample, we fabricated spatially inhomogeneous Co coatings. As an example, Figure 2 shows the MFM image of the Co/Pt-Co structure in the region near the edge of the 1 nm Co capping layer.
Figure 2. The MFM image of domain structure in demagnetized state of Co/Pt-Со (1 nm) sample near
Figurethe 2. The image of domain in layer demagnetized state of The Co/Pt-Co (1 nm) sample edgeMFM of Со (1 nm) capping layerstructure (Co capping is on the right side). border between Co/Pt near the edge Co (1 nm) capping layerThe is on theframe rightsize side). between andofCo/Pt-Co structures is layer shown(Co by acapping dashed line. MFM is 4The μm ×border 4 μm. The MFM Co/Pt and Co/Pt-Co structures is shown by a dashed line. The MFM frame size is 4 µm × 4 µm. The MFM contrast is normalized to the maximum. Figure 2. The MFM image of domain structure in demagnetized state of Co/Pt-Со (1 nm) sample near contrastthe isedge normalized to the maximum. of Со (1 nm) capping layer (Co capping layer is on the right side). The border between Co/Pt Itand can clearlystructures be seen is that the by deposition of a The thinMFM Co film lateral Co/Pt-Co shown a dashed line. framesubstantially size is 4 μm × changes 4 μm. Thethe MFM domain structure of the Co/Pt. From this point of view, the patterning of the upper Co layer enables contrast is normalized to the maximum. It the cancontrol clearlyofbe thatstructure the deposition a thin film substantially changes the lateral domain theseen domain of Co/Ptof layers onCo small spatial scales. structure ofWe Co/Pt. this point view, the thesubstantially upperCo Coislands layer enables the investigated change of of magnetic domains caused by circular covering thecontrol Itthe can clearlyFrom be the seen that the deposition ofpatterning a thin Co of film changes the lateral Co/Pt. Instructure Figures 3of and 4 Co/Pt. we show thethis sequence MFM during of magnetization of the of the domain structure of layers on point smallofspatial scales. domain theCo/Pt From view, images the patterning the upper Coreversal layer enables Co/Pt film with Cothe circular discs (1.5magnetic nm of thickness and 2 μm in diameter) in an external perpendicular theinvestigated control of the domain structure of Co/Pt layers on small spatial scales. We change of domains caused by circular Co islands covering the magnetic field. We investigated the change of magnetic domains caused by circular Co islands covering the Co/Pt. In Figures 3 and 4 we show the sequence of MFM images during magnetization reversal Co/Pt. In Figures 3 and 4 we show the sequence of MFM images during magnetization reversal of the of the Co/Pt film with Co circular discs (1.5 nm of thickness and 2 µm in diameter) in an external Co/Pt film with Co circular discs (1.5 nm of thickness and 2 μm in diameter) in an external perpendicular perpendicular magnetic field. magnetic field.
Figure 3. The sequential MFM images of first stage of magnetization reversal for Co/Pt sample with Co (1.5 nm) discs. (a) Initial state; (b) The remanent state after applying a 100 Oe reversed magnetic field; (c) The state after applying a 150 Oe magnetic field. The MFM frame size is 7 μm × 7 μm. The MFM contrast is normalized to the of maximum. Figure 3. The sequential MFM images first stage of magnetization reversal for Co/Pt sample with
Figure Co 3. The sequential MFM images of The firstremanent stage of magnetization reversal Co/Pt sample with Co (1.5 nm) discs. (a) Initial state; (b) state after applying a 100 for Oe reversed magnetic (1.5 nm) discs. state;applying (b) Thearemanent state after a 100 Oesize reversed field; (c) (a) TheInitial state after 150 Oe magnetic field.applying The MFM frame is 7 μmmagnetic × 7 μm. field; (c) The The state after applying a 150 Oetomagnetic field. The MFM frame size is 7 µm × 7 µm. The MFM MFM contrast is normalized the maximum. contrast is normalized to the maximum.
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(a)
(b)
Figure 4. The MFM images of the second stage of magnetization reversal for Co/Pt sample with Co
Figure (1.5 4. The MFM images of the second stage of magnetization reversal for Co/Pt sample with Co nm) discs. (a) The remanent state after applying a 200 Oe reversed magnetic field; (b) The state (1.5 nm) discs. (a) The remanent state after applying a 200 magnetic field; (b) The after applying a 250 Oe magnetic field. The MFM frame sizeOe is 7reversed μm × (b) 7 μm. The MFM contrast is state (a) after applying a 250 Oemaximum. magnetic field. The MFM frame size is 7 µm × 7 µm. The MFM contrast is normalized to the Figureto4.the Themaximum. MFM images of the second stage of magnetization reversal for Co/Pt sample with Co normalized (1.5 nm) discs. (a) The remanent state after applying ain200 reversed magnetic (b) The state Initially, the sample was uniformly magnetized anOe external field of 500field; Oe. The MFM image applying a 250 Oe magnetic The contours MFM frame 7 μm μm. The contrast is of of theafter initial state is shown in Figurefield. 3a. The of size the is discs are× 7visible dueMFM to the influence Initially, the sample uniformly inreversal an external of occurred 500 Oe. The MFM image of normalized to the thewas maximum. the edge relief on MFM image. Themagnetized magnetization of thisfield sample in two stages. the initial state is shown in Figure 3a. The contours of the discs are visible due to the influence In the first stage (in the fields less than the CoPt coercive field of 200 Oe), we observed the of the Initially, theof sample was uniformly magnetized an of external field 500 The image edge relief on the MFM image. The magnetization this sample inMFM two stages. remagnetization the regions of Co/Pt under thereversal Coindisks (Figure 3b,c).ofAtoccurred theOe. second stage, in the In the of the exceeding initial state200 is shown in Figure 3a. The contours of outside the discsthe arediscs visible due to the influence of fields Oe, the magnetization was reversed (Figure 4a,b). first stage (in the fields less than the CoPt coercive field of 200 Oe), we observed the remagnetization the edge relief on these the MFM image. The magnetization reversal of this sample occurred in two stages. We of applied effects to (Figure control the domain structure instage, planar Co/Pt nanowires of the regions Co/Pt under theinCoorder disks 3b,c). At the second in the fields exceeding In the first stage (in the fields less than the CoPt coercive field of 200 Oe), we observed (NWs). The NWs were fabricated by e-beam lithography and Ar+ ion etching [16]. The lateral sizesthe of 200 Oe,remagnetization the magnetization was reversed outsidethe theCodiscs 4a,b). of the of Co/Pt disks(Figure (Figure(AFM) 3b,c). At the second stage, in the the NWs were 100 nm regions × 3000 nm. The under atomic force microscopy image of the NW array is Wefields applied effects order to control the domain in planar exceeding 200 Oe, thein magnetization was reversed outsidestructure the discs (Figure 4a,b). Co/Pt nanowires presented inthese Figure 5a. applied effects inby order to control the domain planar Co/Pt nanowires (NWs). TheWe NWs werethese fabricated e-beam lithography andstructure Ar+ ioninetching [16]. The lateral sizes + (NWs). The NWs were fabricated by e-beam lithography and Ar ion etching [16]. The lateral sizes of the NWs were 100 nm × 3000 nm. The atomic force microscopy (AFM) image of the NWofarray is the NWs were 100 nm × 3000 nm. The atomic force microscopy (AFM) image of the NW array is presented in Figure 5a. presented in Figure 5a.
Figure 5. The sequential stages of remagnetization for Co/Pt NWs partly covered by Co capping layer. (a) The AFM image of the sample with Co/Pt NWs. The area below line A-B is covered by Co (1.3 nm) capping layer. The border of Co capping layer is shown by arrows and dashed line; (b) The MFM image of up-magnetized sample; (c) The MFM image of partly remagnetized NWs after applying of Figure The sequential stages remagnetization forofCo/Pt NWs partly covered bythe Cofield capping layer. 150 Oe 5. reversed magnetic field;of(d) The MFM image down-magnetized NWs in of 200 Oe. Figure (a) 5. The sequential stages of remagnetization for Co/Pt NWsline partly covered by Co(1.3 capping The AFM image of the sample with Co/Pt NWs. The area below A-B is covered by Co nm) layer. The frame sizes are 1.5 μm × 1.5 μm. The MFM contrast is normalized to the maximum. (a) The capping AFM image the sample with Co/Pt NWs. The area belowand linedashed A-B isline; covered byMFM Co (1.3 nm) layer. of The border of Co capping layer is shown by arrows (b) The capping layer. The border ofsample; Co capping layer image is shown by arrows and dashed line; (b) The image of up-magnetized (c) The MFM of partly remagnetized NWs after applying of MFM 150up-magnetized Oe reversed magnetic field;(c) (d)The The MFM of down-magnetized NWs in the fieldafter of 200 Oe. image of sample; MFMimage image of partly remagnetized NWs applying of frame sizes are 1.5field; μm × 1.5 MFM contrast is normalized to the maximum. 150 Oe The reversed magnetic (d)μm. TheThe MFM image of down-magnetized NWs in the field of 200 Oe.
The frame sizes are 1.5 µm × 1.5 µm. The MFM contrast is normalized to the maximum.
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The NW array was partly covered by a Co (1.3 nm) layer (the border of Co layer is indicated Materialsand 2017, a 10, dashed 1034 5 of 7of this by arrows line in Figure 5a). We investigated the magnetization reversal sample in an external perpendicular magnetic field. The sequential stages of remagnetization are The NW array was partly covered by a Co (1.3 nm) layer (the border of Co layer is indicated by presented in Figure 5b–d. In the initial state, all NWs were magnetized uniformly (Figure 5b) in arrows and a dashed line in Figure 5a). We investigated the magnetization reversal of this sample in the upward directed 500 Oe magnetic magnetic field. we applied a 150 Oeare reversed field an external perpendicular field. The Afterward sequential stages of remagnetization presented in (H1 ), and observed a partial remagnetization of thewere NWsmagnetized in remanent state (Figure 5c).5b) This state is stabilized Figure 5b–d. In the initial state, all NWs uniformly (Figure in the upward by magnetostatic interaction between partswe of applied the NWs having opposite When we directed 500 Oe magnetic field. Afterward a 150 Oe reversed fieldmagnetization. (H1), and observed a partial remagnetization of the NWs in remanent state (Figure 5c). This state is stabilized by applied a 200 Oe magnetic field (H2 ) all NWs were remagnetized completely. The magnitude of the magnetostatic of is thequite NWsenough having for opposite magnetization. Whenofwe gap ∆H = H2 − H1 interaction was equal between to 50 Oe,parts which applications in a variety devices applied a 200 Oe magnetic field (H2) all NWs were remagnetized completely. The magnitude of the based on NW remagnetization. gap ΔH = H2 − H1 was equal to 50 Oe, which is quite enough for applications in a variety of devices Another method for local modification of the magnetic properties of Co/Pt films is their irradiation based on NW remagnetization. by focusedAnother ion beams [17,19,20]. Depending of onthe themagnetic dose ofproperties the irradiation, perpendicular easy-axis method for local modification of Co/Ptthe films is their irradiation magnetic anisotropy can be reduced or even transformed into easy-plane anisotropy. This method by focused ion beams [17,19,20]. Depending on the dose of the irradiation, the perpendicular easyallowsaxis themagnetic formation of submicron with different topologies [19,20]. anisotropy. This anisotropy can bestructures reduced or even transformed into easy-plane method allows theCo/Pt formation of submicron structures with different topologies We investigated film with an array of locally irradiated spots[19,20]. of 100 nm in diameter, We investigated Co/Pt film with an array of locally irradiated spots of 100 in Co/Pt diameter, ordered in a square lattice with a 200 nm period. The domain structure ofnmthe sample ordered in a square lattice with a 200 nm period. The domain structure of the Co/Pt sample (in demagnetized state after applying a 200 Oe reversed magnetic field) near the boundary between (in demagnetized state after applying a 200 Oe reversed magnetic field) near the boundary between the irradiated and unirradiated regions is shown in Figure 6. the irradiated and unirradiated regions is shown in Figure 6.
Figure 6. The MFM image of the Co/Pt film locally irradiated by focused He+ ion beam. The irradiated
+ ion beam. The irradiated Figurearea 6. The image the Co/Pt area film islocally bythe focused HeThe is onMFM the left side,of unirradiated on theirradiated right side of picture. MFM frame size is area is5 on left The side,MFM unirradiated area is on to the side of the picture. The MFM frame size is μmthe × 5 μm. contrast is normalized theright maximum. 5 µm × 5 µm. The MFM contrast is normalized to the maximum. It can clearly be seen that periodic ion irradiation essentially changes the spatial structure of magnetization, and allows the realization of high-density lattices of inverted domains (bubbles). It can clearly be seen that periodic ion irradiation essentially changes the spatial structure of These objects can be used for application in data storage systems. magnetization, and ofallows the realization high-density lattices of bubbles inverted domains (bubbles). The effects magnetization reversal inofthe arrays of Co/Pt magnetic under an external These magnetic objects can used for application dataHere, storage systems. fieldbe were investigated earlier in in [17]. we studied the switching of bubbles under the The effectsofofanmagnetization in the arrays Co/Pt magnetic bubbles under of anthe external influence inhomogeneousreversal stray field of the MFMofprobe. The successive MFM images same region of bubble array are presented in Figure 7. Initially, the sample was magnetized magnetic field were investigated earlier in [17]. Here, we studied the switching of bubbles under the downward in the 500 Oe external we applied reversed magnetic field tooflead influence of an inhomogeneous strayfield. fieldAfterward of the MFM probe. 200 TheOe successive MFM images the same the sample in demagnetized state. All bubbles had upward-directed magnetic moments (Figure 7a), region of bubble array are presented in Figure 7. Initially, the sample was magnetized downward in while the MFM probe had magnetic moments directed in the opposite direction. Nonperturbing the 500 Oe external field. Afterward we applied 200 Oe reversed magnetic field to lead the sample MFM imaging was carried out in constant-height mode with a scanning height of about 30 nm (the in demagnetized All was bubbles had magnetic moments (Figure 7a), amplitude of state. oscillation 10 nm). Atupward-directed the second stage, we reduced the scanning height to 15 while nm the MFM to probe had magnetic moments directed in the opposite direction. Nonperturbing MFM imaging observe the switching of bubble magnetization under the MFM probe field. The corresponding was carried out inisconstant-height with a scanning ofofabout nm (the amplitude of MFM image shown in Figure mode 7b. Some cases of sharp height changing MFM30 contrast during the scanning Asstage, a result, observed thescanning switchingheight of contrast oscillation wasare 10indicated nm). At by thearrows. second wewe reduced the to 15(disappearance nm to observe the of white dots) in magnetization some areas of theunder MFM the image (Figure 7c). field. The mechanism of bubble switching is switching of bubble MFM probe The corresponding MFM image is similar to the mechanism for changing the magnetization of Co/Pt discs described in Ref. [21]. shown in Figure 7b. Some cases of sharp changing of MFM contrast during the scanning are indicated
by arrows. As a result, we observed the switching of contrast (disappearance of white dots) in some
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areas of the MFM image (Figure 7c). The mechanism of bubble switching is similar to the mechanism Materials 2017, 10, 1034 6 of 7 for changing the magnetization of Co/Pt discs described in Ref. [21]. Through manipulation of the probe Through positionmanipulation over the sample [22], it is possible tosample realize[22], theitselective reversal of of the probe position over the is possiblemagnetization to realize the selective individual bubbles.reversal of individual bubbles. magnetization
Figure 7. The successive MFM images of magnetic bubble array in Co/Pt. (a) The initial state of
Figureuniformly 7. The successive MFM images of magnetic bubble array in Co/Pt. (a) The initial state of magnetized array; (b) The MFM imaging with low scanning height. The places of sharp uniformly magnetized array; The MFM with(c)low height. The places of sharp switching of MFM contrast (b) are indicated byimaging white arrows; Thescanning final state of magnetic bubble array switching of MFM contrast are indicated by white arrows; (c) The final state of magnetic bubble after the MFM probe switching. The switched bubbles are indicated by white arrows. The MFM frame array after the probe switching. switched bubbles to are by white arrows. The MFM frame sizeMFM is 2 μm × 2 μm. The MFMThe contrast is normalized theindicated maximum. size is 2 µm × 2 µm. The MFM contrast is normalized to the maximum. 4. Conclusions
4. Conclusions We have studied the peculiarities of domain structure in multilayer Co/Pt thin-films modified by patterned Co capping layers and by a focused He+ ion beam. It was shown that the application of
We have studied the peculiarities of domain structure in multilayer Co/Pt thin-films modified an additional Co layer allows a reduction in the characteristic scale of the domain structure and the by patterned Co capping layers and by a focused He+ ion the beam. It was shownofthat thefilms application of magnitude of magnetization reversal fields. Therefore, nanostructuring Co/Pt and an additional layer allows a reduction characteristic of theand domain structure and the covering Co Co layers enables the possibilityin tothe control domain wallscale nucleation pinning/depinning magnitude of magnetization reversaloffields. Therefore, films and covering processes for the development magnetic devices.the Onnanostructuring the other hand, of theCo/Pt application of local irradiation ofthe Co/Pt films by ion beams opens up significant opportunities for creating highly dense Co layers enables possibility to control domain wall nucleation and pinning/depinning processes of controllable magnetic devices. domains that can be usedhand, to develop novel and promising data for thearrays development of magnetic On the other the application of local irradiation storage systems. of Co/Pt films by ion beams opens up significant opportunities for creating highly dense arrays of controllable magnetic domains that used to develop promising storage systems. Acknowledgments: The authors arecan verybe thankful to S. A. Gusevnovel and S.and N. Vdovichev for data their assistance in sample preparation. This research is supported by the Russian Science Foundation (project No. 16-12-10254).
Acknowledgments: The authors arebeam verywas thankful to S. and S. N. Vdovichev their assistance in performed at A. the Gusev Saint Petersburg State University for in the framework The sample irradiation by He+ ion sampleofpreparation. This research is supported by the Russian Science Foundation (project No. 16-12-10254). project No. 15-02-10254 of the Russian Foundation for Basic Researches The sample irradiation by He+ ion beam was performed at the Saint Petersburg State University in the framework Author and O.L.E. conceived designed the experiments; O.L.E., M.V.S., N.S.G., of project No. Contributions: 15-02-10254 ofV.L.M. the Russian Foundation forand Basic Researches. Y.V.P. performed the experiments; V.L.M. and O.L.E. analyzed the data; N.S.G. and E.V.S. contributed in
Authorsamples Contributions: V.L.M. and O.L.E. conceived and designed the experiments; O.L.E., M.V.S., N.S.G. and preparations; V.L.M. and O.L.E. wrote the paper. Authorship must be limited to those who have Y.V.P. performed the experiments; V.L.M. and O.L.E. analyzed the data; N.S.G. and E.V.S. contributed in samples contributed substantially to the work reported. preparations; V.L.M. and O.L.E. wrote the paper. Authorship must be limited to those who have contributed substantially to of theInterest: work reported. Conflicts The authors declare no conflict of interest. Conflicts of Interest: The authors declare no conflict of interest. References 1. Stamps, R.L.; Breitkreutz, S.; Akerman, J.; Chumak, A.V.; Otani, Y.; Bauer, G.E.W.; Thiele, J.-U.; Bowen, M.; References Majetich, S.A.; Klaui, M.; et al. The 2014 magnetism roadmap. J. Phys. D Appl. Phys. 2014, 47, 333001.
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Stamps, R.L.;W.; Breitkreutz, Akerman, Chumak, Otani, Y.; Bauer, G.E.W.; Thiele, J.-U.; Bowen, M.; 2. Scholz, Guslienko,S.; K.Y.; Novosad,J.;V.; Suess, D.;A.V.; Schrefl, T.; Chantrell, R.W.; Fidler, J. Transition from single-domain to vortex state soft magnetic cylindrical roadmap. nanodots. J. J.Magn. Mater. 2003, 155–163. Majetich, S.A.; Klaui, M.; et al.inThe 2014 magnetism Phys.Magn. D Appl. Phys.266, 2014, 47, 333001. 3. Buchanan, K.S.; Roy, P.E.; Grimsditch, M.; Fradin, F.Y.; Guslienko, K.Y.; Bader, S.D.; Novosad, V. [CrossRef] dynamics in patterned ferromagnetic ellipses. Nat.T.; Phys. 2005, 1, 172–176. Scholz,Soliton-pair W.; Guslienko, K.Y.; Novosad, V.; Suess, D.; Schrefl, Chantrell, R.W.; Fidler, J. Transition from 4. Mironov, V.L.; Ermolaeva, O.L.; Gusev, S.A.; Klimov, A.Y.; Rogov, V.V.; Gribkov, B.A.; Udalov, O.G.; single-domain to vortex state in soft magnetic cylindrical nanodots. J. Magn. Magn. Mater. 2003, 266, 155–163. Fraerman, A.A.; Marsh, R.; Checkley, C.; et al. Antivortex state in crosslikenanomagnets. Phys. Rev. B 2010, [CrossRef] 81, 094436. Buchanan, K.S.; Roy, P.E.; Grimsditch, M.; Fradin, F.Y.; Guslienko, K.Y.; Bader, S.D.; Novosad, V. Soliton-pair 5. Lendínez, S.; Jain, S.; Novosad, V.; Fradin, F.Y.; Pearson, J.E.; Tejada, J.; Bader, S.D. Dynamic decay of a dynamics in patterned ferromagnetic ellipses. Nat.Phys. Phys. 2005, 172–176. [CrossRef] single vortex into vortex-antivortex pairs. J. Appl. 2014, 115,1,doi:10.1063/1.4862219. Mironov, V.L.; Ermolaeva, O.L.; Gusev, S.A.; Klimov, A.Y.; Rogov, V.V.; Gribkov, B.A.; Udalov, O.G.; Fraerman, A.A.; Marsh, R.; Checkley, C.; et al. Antivortex state in crosslikenanomagnets. Phys. Rev. B 2010, 81, 094436. [CrossRef]
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Lendínez, S.; Jain, S.; Novosad, V.; Fradin, F.Y.; Pearson, J.E.; Tejada, J.; Bader, S.D. Dynamic decay of a single vortex into vortex-antivortex pairs. J. Appl. Phys. 2014, 115. [CrossRef] Zhu, X.; Grütter, P.; Hao, Y.J.; Castaño, F.; Haratani, S.; Ross, C.A.; Vögeli, B.; Smith, H.I. Magnetization switching in 70-nm-wide pseudo-spin-valve nanoelements. J. Appl. Phys. 2003, 93, 1132–1136. [CrossRef] Fraerman, A.A.; Gribkov, B.A.; Gusev, S.A.; Klimov, A.Y.; Mironov, V.L.; Nikitushkin, D.S.; Rogov, V.V.; Vdovichev, S.N.; Hjorvarsson, B.; Zabel, H. Magnetic force microscopy of helical states in multilayer nanomagnets. J. Appl. Phys. 2008, 103. [CrossRef] Bersweiler, M.; Lacour, D.; Dumesnil, K.; Montaigne, F.; Hehn, M. Phase diagram in exchange-coupled CoTb/[Co/Pt] multilayer-based magnetic tunnel junctions. Phys. Rev. B 2015, 92, 224431. [CrossRef] Demidov, E.S.; Gusev, N.S.; Budarin, L.I.; Karashtin, E.A.; Mironov, V.L.; Fraerman, A.A. Interlayer interaction in multilayer [Co/Pt]n /Pt/Co structures. J. Appl. Phys. 2016, 120. [CrossRef] Fraerman, A.A.; Ermolaeva, O.L.; Skorohodov, E.V.; Gusev, N.S.; Mironov, V.L.; Vdovichev, S.N.; Demidov, E.S. Skyrmion states in multilayer exchange coupled ferromagnetic nanostructures with distinct anisotropy directions. J. Magn. Magn. Mater. 2015, 393, 452–456. [CrossRef] Chappert, C.; Bernas, H.; Ferre, J.; Kottler, V.; Jamet, J.-P.; Chen, Y.; Cambril, E.; Devolder, T.; Rousseaux, F.; Mathet, V.; et al. Planar patterned magnetic media obtained by ion irradiation. Science 1998, 280, 1919. [CrossRef] [PubMed] Devolder, T.; Chappert, C.; Chen, Y.; Cambril, E.; Bernas, H.; Jamet, J.P.; Ferre, J. Sub-50 nm planar magnetic nanostructures fabricated by ion irradiation. Appl. Phys. Lett. 1999, 74, 3383. [CrossRef] Devolder, T.; Ferre, J.; Chappert, C.; Bernas, H.; Jamet, J.-P.; Mathet, V. Magnetic properties of He+-irradiated Pt/Co/Pt ultrathin films. Phys. Rev. B 2001, 64, 064415. [CrossRef] Vieu, C.; Gierak, J.; Launois, H.; Aign, T.; Meyer, P.; Jamet, J.P.; Ferré, J.; Chappert, C.; Devolder, T.; Mathet, V.; et al. Modifications of magnetic properties of Pt/Co/Pt thin layers by focused gallium ion beam irradiation. J. Appl. Phys. 2002, 91, 3103. [CrossRef] Aziz, A.; Bending, S.J.; Roberts, H.; Crampin, S.; Heard, P.J.; Marrows, C.H. Artificial domain structures realized by local gallium focused ion-beam modification of Pt/Co/Pt trilayer transport structure. J. Appl. Phys. 2006, 98, 124102. [CrossRef] Mironov, V.L.; Ermolaeva, O.L.; Skorohodov, E.V.; Klimov, A.Y. Field-controlled domain wall pinning-depinning effects in ferromagnetic nanowire-nanoislands system. Phys. Rev. B 2012, 85, 144418. [CrossRef] Sapozhnikov, M.V.; Vdovichev, S.N.; Ermolaeva, O.L.; Gusev, N.S.; Fraerman, A.A.; Gusev, S.A.; Petrov, Y.V. Artificial dense lattice of magnetic bubbles. Appl. Phys. Lett. 2016, 109, 042406. [CrossRef] Tryputen, L.; Guo, F.; Liu, F.; Anh Nguyen, T.N.; Mohseni, M.S.; Chung, S.; Fang, Y.; Akerman, J.; McMichael, R.D.; Ross, C.A. Magnetic structure and anisotropy of [Co/Pd]5/NiFe multilayers. Phys. Rev. B 2015, 91, 014407. [CrossRef] Breitkreutz, S.; Kiermaier, J.; Karthik, S.V.; Csaba, G.; Schmitt-Landsiedel, D.; Becherer, M. Controlled reversal of Co/Pt dots for nanomagnetic logic applications. J. Appl. Phys. 2012, 111, 07A715. [CrossRef] Fowley, C.; Diao, Z.; Faulkner, C.C.; Kally, J.; Ackland, K.; Behan, G.; Zhang, H.Z.; Deac, A.M.; Coey, J.M.D. Local modification of magnetic anisotropy and ion milling of Co/Pt multilayers using a He+ ion beam microscope. J. Phys. D Appl. Phys. 2013, 46, 195501. [CrossRef] Mironov, V.L.; Gribkov, B.A.; Vdovichev, S.N.; Gusev, S.A.; Fraerman, A.A.; Ermolaeva, O.L.; Shubin, A.B.; Alexeev, A.M.; Zhdan, P.A.; Binns, C. Magnetic force microscope tip induced remagnetization of CoPt nanodiscs with perpendicular anisotropy. J. Appl. Phys. 2009, 106, 053911. [CrossRef] Mironov, V.L.; Fraerman, A.A.; Gribkov, B.A.; Ermolayeva, O.L.; Klimov, A.Y.; Gusev, S.A.; Nefedov, I.M.; Shereshevskii, I.A. Control of the magnetic state of arrays of ferromagnetic nanoparticles with the aid of the inhomogeneous field of a magnetic-force-microscope probe. Phys. Met. Metallogr. 2010, 110, 708–734. [CrossRef] © 2017 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
Magnetic Force Microscopy of Nanostructured Co/Pt Multilayer Films with Perpendicular Magnetization O. L. Ermolaeva 1, * and V. L. Mironov 1 1
2
*
ID
, N. S. Gusev 1 , E. V. Skorohodov 1 , Yu. V. Petrov 2 , M. V. Sapozhnikov 1
ID
Institute for Physics of Microstructures RAS, GSP-105, 603950 Nizhny Novgorod, Russia; [email protected] (N.S.G.); [email protected] (E.V.S.); [email protected] (M.V.S.); [email protected] (V.L.M.) Saint Petersburg State University, University Embankment, 7/9, 199034 St. Petersburg, Russia; [email protected] Correspondence: [email protected]; Tel.: +10-910-3963589
Received: 29 June 2017; Accepted: 31 August 2017; Published: 5 September 2017
Abstract: We present the results of magnetic force microscopy investigations of domain structures in multilayer [Co (0.5 nm)/Pt (1 nm)]5 thin film structures (denoted hereafter as Co/Pt) modified by additional Co capping layers and by ion irradiation. It is demonstrated that a Co capping layer essentially changes the domain structure and decreases the threshold of magnetization reversal, due to the formation of noncollinear magnetization in Co/Pt. It is shown that local irradiation with a focused He+ ion beam enables the formation of regions with decreased easy-axis anisotropy (magnetic bubbles) that have the inverse magnetization direction in the demagnetized state of Co/Pt. The experimental results demonstrate that the magnetic bubbles can be switched using a probe of a magnetic force microscope. The possible application of these effects for the development of magnetic logic and data storage systems is discussed. Keywords: perpendicular anisotropy; magnetic domains; magnetic nanostructures; magnetic nanowires; magnetic force microscopy
1. Introduction The engineering of magnetic states in ferromagnetic nanosystems is one of the topical problems closely connected with the development of modern magnetic data processing systems and element base of spin electronics [1]. One effective solution to this problem is the creation of planar patterned ferromagnetic nanostructures using lithographic methods. The control of shape and size of the patterned elements allows ferromagnetic nanostructures with various distributions of magnetization to be obtained [2–7]. Additional possibilities are opened with the use of magnetic films with perpendicular (easy-axis) anisotropy. The fabrication of nanostructures based on multilayer systems consisting of films with easy-axis and easy-plane anisotropy enables the realization of structures with essentially noncollinear magnetization distribution [8–10] for magnetic devices with vertical architectures. Additionally, in recent years, new approaches to modifying the properties of magnetic films by focused ion beams have been developed [11–15]. These methods enable the local modification of the magnetic properties of thin-film structures without changing the surface relief. The combination of all these methods is a powerful tool for micromagnetic engineering. In the present study, we investigate the possibilities of local modification of the domain structure in Co/Pt multilayer films with perpendicular anisotropy through the growth of additional patterned Co layers, and by local modification of the Co/Pt anisotropy parameter using a focused He+ ion beam.
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Multilayer thin film structures [Co (0.5 nm)/Pt (1 nm)]5 were grown by DC magnetron sputtering on Si(100) substrate with Ta(10 nm)/Pt(10 nm) buffer sublayer. Additionally, we deposited Co capping 2. Experiment layers with variable thickness. The Co capping layers were patterned using lithographic methods Multilayer thin film structures [Co (0.5 nm)/Pt (1 nm)]5 were grown by DC magnetron sputtering to locally changesubstrate the domain The Carl 50VP” scanning microscope on Si(100) with structure. Ta(10 nm)/Pt(10 nm) Zeiss buffer “Supra sublayer. Additionally, weelectron deposited Co with “ELPHY Plus” exposure system (Carl Zeiss company, Jena, Germany) was used for the e-beam capping layers with variable thickness. The Co capping layers were patterned using lithographic lithography [4,16]. Additionally, wedomain studied the Co/Pt modified by a focused methods to locally change the structure. Thestructures Carl Zeiss when “Supraspatially 50VP” scanning electron + ion microscope, microscope with “ELPHY Plus”He exposure system (Carl equipped Zeiss company, Germany) was used generator for ion beam. The Carl Zeiss “Orion” withJena, “Nanomaker” pattern the e-beam lithography Additionally, we studied the Co/Pt structures when spatially (Carl Zeiss company, Jena, [4,16]. Germany), was used to fabricate arrays of modified areas inmodified Co/Pt [17]. by a focused ion beam. The Carl Zeiss “Orion” He+ ion microscope, equipped with “Nanomaker” The magnetic properties of the samples were investigated by home-made magneto-optical Kerr pattern generator (Carl Zeiss company, Jena, Germany), was used to fabricate arrays of modified effect (MOKE) technique. Domain structure was studied by magnetic force microscope (MFM) areas in Co/Pt [17]. “Solver HV” company, Moscow, Russia) equippedmagneto-optical by DC magnet with 500 The(NT-MDT magnetic properties of the samplesZelenograd, were investigated by home-made Kerr Oe perpendicular the sample) magnetic field. by The MFM probes were standard effect (MOKE)(relatively technique.toDomain structure was studied magnetic force microscope (MFM)NSG-11 “Solvercoated HV” (NT-MDT Moscow, Zelenograd, by DC magnet with 500 were cantilevers with Cocompany, with a thickness of 30 nm. Russia) Before equipped the measurements, the probes Oe perpendicular (relatively to the sample) magnetic field. The MFM probes were standard NSG-11 magnetized along the tip axes in a 1T external magnetic field. The MFM measurements were performed with Co with of 30constant-height nm. Before the measurements, the probes both incantilevers two-passcoated tapping mode and ainthickness noncontact mode. The phase shift ofwere cantilever magnetized along the tip axes in a 1T external magnetic field. The MFM measurements were oscillations under the gradient of the sample magnetic force was registered as the MFM contrast. performed both in two-pass tapping mode and in noncontact constant-height mode. The phase shift The MFM measurements were performed in a vacuum chamber with the rest gas pressure of 10−4 of cantilever oscillations under the gradient of the sample magnetic force was registered as the MFM Torr that improved the MFM signal due an increase cantilever quality contrast. The MFM measurements weretoperformed in aof vacuum chamber with factor. the rest gas pressure of 10−4 Torr that improved the MFM signal due to an increase of cantilever quality factor.
3. Results and Discussion
3. Results and Discussion
The typical hysteresis loop (the dependence of perpendicular component of magnetization on The typical hysteresis loop field) (the dependence of perpendicular on curve an out-of-plane external magnetic of the initial Co/Pt film iscomponent presentedofinmagnetization Figure 1a. This an out-of-plane external magnetic field) of the initial Co/Pt film is presented in Figure 1a. This curve has a practically rectangular shape with the remanent magnetization equal to the magnetization in has a practically rectangular shape with the remanent magnetization equal to the magnetization in saturation M0 . The coercive field is about 200 Oe. The MFM image of Co/Pt labyrinth domain structure saturation M0. The coercive field is about 200 Oe. The MFM image of Co/Pt labyrinth domain structure in a demagnetized statestate (demagnetizing 200Oe) Oe)isisshown shown in Figure in a demagnetized (demagnetizingfield field was was 200 in Figure 1b. 1b.
Figure 1. The normed MOKE hysteresis curves and MFM images of the samples’ domain structure in
Figure 1. The normed MOKE hysteresis curves and MFM images of the samples’ domain structure demagnetized state. (a,b) are for Co/Pt structure without capping layer; (c,d) are for the Co/Pt in demagnetized (a,b) are for Co/Pt without capping are forlayer. the Co/Pt structure withstate. 1 nm Co capping layer; (e,f) structure are for the Co/Pt structure withlayer; 1.3 nm(c,d) Co capping structure nm Co (e,f)MFM are for the Co/Pt structure with 1.3 nm Co capping layer. The with MFM1frame sizecapping is 5 μm ×layer; 5 μm. The contrast is normalized to the maximum. The MFM frame size is 5 µm × 5 µm. The MFM contrast is normalized to the maximum. As can be seen from Figure 1b, the typical lateral size of magnetic domains is about 1 μm. The additional Co capping layer changes the domain structure considerably. The hysteresis curve As can be seen from Figure 1b, the typical lateral size of magnetic domains is about 1 µm. and MFM image for Co/Pt structure with 1 nm Co capping layer are represented in Figure 1c,d. The additional Cowe capping changes of thethe domain structure considerably. The hysteresis and In this case, observelayer the narrowing hysteresis loop (the remanent magnetization is 0.8curve M0
MFM image for Co/Pt structure with 1 nm Co capping layer are represented in Figure 1c,d. In this case, we observe the narrowing of the hysteresis loop (the remanent magnetization is 0.8 M0 and the
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coercive field is 50 Oe) and a reduction in the size of the domains (typical width is about 350 nm). In the field is 50layer, Oe) and reduction in the size of the is domains width isfield about case ofand the the 1.3 coercive nm Co capping thearemanent magnetization 0.2 M0(typical , the coercive is 10 Oe 350 nm).2017, In the case of the 1.3 nm Co capping layer, the remanent magnetization is 0.2 M0, the coercive Materials 10, 1034 3 of 7 and the typical domain width is 150 nm. The decrease of the domain structure scales with increasing field is 10 Oe and the typical domain width is 150 nm. The decrease of the domain structure scales thickness ofincreasing the Co capping layer cancapping be explained bybeeffects of exchange interaction. The Co-Co/Pt with thickness of the layer can explained effects of(typical exchange interaction. and the coercive field is 50 Oe)Co and a reduction in the size of thebydomains width is about exchange coupling leads to a decrease in the effective perpendicular anisotropy, and to an increase The Co-Co/Pt to alayer, decrease in the effective perpendicular anisotropy, 350 nm). In the exchange case of thecoupling 1.3 nm Coleads capping the remanent magnetization is 0.2 M0, the coercive in the density ofOe theand domain in of thethe Co/Pt subsystem. Similar effects of changes in the domain and increase in typical thewalls density domain walls in the Co/Pt of subsystem. Similar effects of fieldtois an 10 the domain width is 150 nm. The decrease the domain structure scales changes inmentioned the domain structure were mentioned in [18], Co/Pd structures withlayer a NiFe capping structure were in [18], Co/Pd structures with aby NiFe capping were studied. with increasing thickness of thewhere Co capping layer can bewhere explained effects of exchange interaction. were studied. The Co-Co/Pt exchange coupling leads to astructures decrease ininthe perpendicular anisotropy, Tolayer create laterally inhomogeneous domain theeffective Co/Pt sample, we fabricated spatially To create laterally inhomogeneous domain structures in the Co/Pt sample, we fabricated and to an increase in the density of the domain walls in the Co/Pt subsystem. Similar effectsstructure of inhomogeneous Co coatings. As an example, Figure 2 shows the MFM image of the Co/Pt-Co spatially Co coatings. an example, Figure 2 shows MFM image the Co/Pt-Co changes inhomogeneous in the domain structure were As mentioned in [18], where Co/Pdthe structures withof a NiFe capping in the region near the edge of the 1 nm Co capping layer. structure instudied. the region near the edge of the 1 nm Co capping layer. layer were To create laterally inhomogeneous domain structures in the Co/Pt sample, we fabricated spatially inhomogeneous Co coatings. As an example, Figure 2 shows the MFM image of the Co/Pt-Co structure in the region near the edge of the 1 nm Co capping layer.
Figure 2. The MFM image of domain structure in demagnetized state of Co/Pt-Со (1 nm) sample near
Figurethe 2. The image of domain in layer demagnetized state of The Co/Pt-Co (1 nm) sample edgeMFM of Со (1 nm) capping layerstructure (Co capping is on the right side). border between Co/Pt near the edge Co (1 nm) capping layerThe is on theframe rightsize side). between andofCo/Pt-Co structures is layer shown(Co by acapping dashed line. MFM is 4The μm ×border 4 μm. The MFM Co/Pt and Co/Pt-Co structures is shown by a dashed line. The MFM frame size is 4 µm × 4 µm. The MFM contrast is normalized to the maximum. Figure 2. The MFM image of domain structure in demagnetized state of Co/Pt-Со (1 nm) sample near contrastthe isedge normalized to the maximum. of Со (1 nm) capping layer (Co capping layer is on the right side). The border between Co/Pt Itand can clearlystructures be seen is that the by deposition of a The thinMFM Co film lateral Co/Pt-Co shown a dashed line. framesubstantially size is 4 μm × changes 4 μm. Thethe MFM domain structure of the Co/Pt. From this point of view, the patterning of the upper Co layer enables contrast is normalized to the maximum. It the cancontrol clearlyofbe thatstructure the deposition a thin film substantially changes the lateral domain theseen domain of Co/Ptof layers onCo small spatial scales. structure ofWe Co/Pt. this point view, the thesubstantially upperCo Coislands layer enables the investigated change of of magnetic domains caused by circular covering thecontrol Itthe can clearlyFrom be the seen that the deposition ofpatterning a thin Co of film changes the lateral Co/Pt. Instructure Figures 3of and 4 Co/Pt. we show thethis sequence MFM during of magnetization of the of the domain structure of layers on point smallofspatial scales. domain theCo/Pt From view, images the patterning the upper Coreversal layer enables Co/Pt film with Cothe circular discs (1.5magnetic nm of thickness and 2 μm in diameter) in an external perpendicular theinvestigated control of the domain structure of Co/Pt layers on small spatial scales. We change of domains caused by circular Co islands covering the magnetic field. We investigated the change of magnetic domains caused by circular Co islands covering the Co/Pt. In Figures 3 and 4 we show the sequence of MFM images during magnetization reversal Co/Pt. In Figures 3 and 4 we show the sequence of MFM images during magnetization reversal of the of the Co/Pt film with Co circular discs (1.5 nm of thickness and 2 µm in diameter) in an external Co/Pt film with Co circular discs (1.5 nm of thickness and 2 μm in diameter) in an external perpendicular perpendicular magnetic field. magnetic field.
Figure 3. The sequential MFM images of first stage of magnetization reversal for Co/Pt sample with Co (1.5 nm) discs. (a) Initial state; (b) The remanent state after applying a 100 Oe reversed magnetic field; (c) The state after applying a 150 Oe magnetic field. The MFM frame size is 7 μm × 7 μm. The MFM contrast is normalized to the of maximum. Figure 3. The sequential MFM images first stage of magnetization reversal for Co/Pt sample with
Figure Co 3. The sequential MFM images of The firstremanent stage of magnetization reversal Co/Pt sample with Co (1.5 nm) discs. (a) Initial state; (b) state after applying a 100 for Oe reversed magnetic (1.5 nm) discs. state;applying (b) Thearemanent state after a 100 Oesize reversed field; (c) (a) TheInitial state after 150 Oe magnetic field.applying The MFM frame is 7 μmmagnetic × 7 μm. field; (c) The The state after applying a 150 Oetomagnetic field. The MFM frame size is 7 µm × 7 µm. The MFM MFM contrast is normalized the maximum. contrast is normalized to the maximum.
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(a)
(b)
Figure 4. The MFM images of the second stage of magnetization reversal for Co/Pt sample with Co
Figure (1.5 4. The MFM images of the second stage of magnetization reversal for Co/Pt sample with Co nm) discs. (a) The remanent state after applying a 200 Oe reversed magnetic field; (b) The state (1.5 nm) discs. (a) The remanent state after applying a 200 magnetic field; (b) The after applying a 250 Oe magnetic field. The MFM frame sizeOe is 7reversed μm × (b) 7 μm. The MFM contrast is state (a) after applying a 250 Oemaximum. magnetic field. The MFM frame size is 7 µm × 7 µm. The MFM contrast is normalized to the Figureto4.the Themaximum. MFM images of the second stage of magnetization reversal for Co/Pt sample with Co normalized (1.5 nm) discs. (a) The remanent state after applying ain200 reversed magnetic (b) The state Initially, the sample was uniformly magnetized anOe external field of 500field; Oe. The MFM image applying a 250 Oe magnetic The contours MFM frame 7 μm μm. The contrast is of of theafter initial state is shown in Figurefield. 3a. The of size the is discs are× 7visible dueMFM to the influence Initially, the sample uniformly inreversal an external of occurred 500 Oe. The MFM image of normalized to the thewas maximum. the edge relief on MFM image. Themagnetized magnetization of thisfield sample in two stages. the initial state is shown in Figure 3a. The contours of the discs are visible due to the influence In the first stage (in the fields less than the CoPt coercive field of 200 Oe), we observed the of the Initially, theof sample was uniformly magnetized an of external field 500 The image edge relief on the MFM image. The magnetization this sample inMFM two stages. remagnetization the regions of Co/Pt under thereversal Coindisks (Figure 3b,c).ofAtoccurred theOe. second stage, in the In the of the exceeding initial state200 is shown in Figure 3a. The contours of outside the discsthe arediscs visible due to the influence of fields Oe, the magnetization was reversed (Figure 4a,b). first stage (in the fields less than the CoPt coercive field of 200 Oe), we observed the remagnetization the edge relief on these the MFM image. The magnetization reversal of this sample occurred in two stages. We of applied effects to (Figure control the domain structure instage, planar Co/Pt nanowires of the regions Co/Pt under theinCoorder disks 3b,c). At the second in the fields exceeding In the first stage (in the fields less than the CoPt coercive field of 200 Oe), we observed (NWs). The NWs were fabricated by e-beam lithography and Ar+ ion etching [16]. The lateral sizesthe of 200 Oe,remagnetization the magnetization was reversed outsidethe theCodiscs 4a,b). of the of Co/Pt disks(Figure (Figure(AFM) 3b,c). At the second stage, in the the NWs were 100 nm regions × 3000 nm. The under atomic force microscopy image of the NW array is Wefields applied effects order to control the domain in planar exceeding 200 Oe, thein magnetization was reversed outsidestructure the discs (Figure 4a,b). Co/Pt nanowires presented inthese Figure 5a. applied effects inby order to control the domain planar Co/Pt nanowires (NWs). TheWe NWs werethese fabricated e-beam lithography andstructure Ar+ ioninetching [16]. The lateral sizes + (NWs). The NWs were fabricated by e-beam lithography and Ar ion etching [16]. The lateral sizes of the NWs were 100 nm × 3000 nm. The atomic force microscopy (AFM) image of the NWofarray is the NWs were 100 nm × 3000 nm. The atomic force microscopy (AFM) image of the NW array is presented in Figure 5a. presented in Figure 5a.
Figure 5. The sequential stages of remagnetization for Co/Pt NWs partly covered by Co capping layer. (a) The AFM image of the sample with Co/Pt NWs. The area below line A-B is covered by Co (1.3 nm) capping layer. The border of Co capping layer is shown by arrows and dashed line; (b) The MFM image of up-magnetized sample; (c) The MFM image of partly remagnetized NWs after applying of Figure The sequential stages remagnetization forofCo/Pt NWs partly covered bythe Cofield capping layer. 150 Oe 5. reversed magnetic field;of(d) The MFM image down-magnetized NWs in of 200 Oe. Figure (a) 5. The sequential stages of remagnetization for Co/Pt NWsline partly covered by Co(1.3 capping The AFM image of the sample with Co/Pt NWs. The area below A-B is covered by Co nm) layer. The frame sizes are 1.5 μm × 1.5 μm. The MFM contrast is normalized to the maximum. (a) The capping AFM image the sample with Co/Pt NWs. The area belowand linedashed A-B isline; covered byMFM Co (1.3 nm) layer. of The border of Co capping layer is shown by arrows (b) The capping layer. The border ofsample; Co capping layer image is shown by arrows and dashed line; (b) The image of up-magnetized (c) The MFM of partly remagnetized NWs after applying of MFM 150up-magnetized Oe reversed magnetic field;(c) (d)The The MFM of down-magnetized NWs in the fieldafter of 200 Oe. image of sample; MFMimage image of partly remagnetized NWs applying of frame sizes are 1.5field; μm × 1.5 MFM contrast is normalized to the maximum. 150 Oe The reversed magnetic (d)μm. TheThe MFM image of down-magnetized NWs in the field of 200 Oe.
The frame sizes are 1.5 µm × 1.5 µm. The MFM contrast is normalized to the maximum.
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The NW array was partly covered by a Co (1.3 nm) layer (the border of Co layer is indicated Materialsand 2017, a 10, dashed 1034 5 of 7of this by arrows line in Figure 5a). We investigated the magnetization reversal sample in an external perpendicular magnetic field. The sequential stages of remagnetization are The NW array was partly covered by a Co (1.3 nm) layer (the border of Co layer is indicated by presented in Figure 5b–d. In the initial state, all NWs were magnetized uniformly (Figure 5b) in arrows and a dashed line in Figure 5a). We investigated the magnetization reversal of this sample in the upward directed 500 Oe magnetic magnetic field. we applied a 150 Oeare reversed field an external perpendicular field. The Afterward sequential stages of remagnetization presented in (H1 ), and observed a partial remagnetization of thewere NWsmagnetized in remanent state (Figure 5c).5b) This state is stabilized Figure 5b–d. In the initial state, all NWs uniformly (Figure in the upward by magnetostatic interaction between partswe of applied the NWs having opposite When we directed 500 Oe magnetic field. Afterward a 150 Oe reversed fieldmagnetization. (H1), and observed a partial remagnetization of the NWs in remanent state (Figure 5c). This state is stabilized by applied a 200 Oe magnetic field (H2 ) all NWs were remagnetized completely. The magnitude of the magnetostatic of is thequite NWsenough having for opposite magnetization. Whenofwe gap ∆H = H2 − H1 interaction was equal between to 50 Oe,parts which applications in a variety devices applied a 200 Oe magnetic field (H2) all NWs were remagnetized completely. The magnitude of the based on NW remagnetization. gap ΔH = H2 − H1 was equal to 50 Oe, which is quite enough for applications in a variety of devices Another method for local modification of the magnetic properties of Co/Pt films is their irradiation based on NW remagnetization. by focusedAnother ion beams [17,19,20]. Depending of onthe themagnetic dose ofproperties the irradiation, perpendicular easy-axis method for local modification of Co/Ptthe films is their irradiation magnetic anisotropy can be reduced or even transformed into easy-plane anisotropy. This method by focused ion beams [17,19,20]. Depending on the dose of the irradiation, the perpendicular easyallowsaxis themagnetic formation of submicron with different topologies [19,20]. anisotropy. This anisotropy can bestructures reduced or even transformed into easy-plane method allows theCo/Pt formation of submicron structures with different topologies We investigated film with an array of locally irradiated spots[19,20]. of 100 nm in diameter, We investigated Co/Pt film with an array of locally irradiated spots of 100 in Co/Pt diameter, ordered in a square lattice with a 200 nm period. The domain structure ofnmthe sample ordered in a square lattice with a 200 nm period. The domain structure of the Co/Pt sample (in demagnetized state after applying a 200 Oe reversed magnetic field) near the boundary between (in demagnetized state after applying a 200 Oe reversed magnetic field) near the boundary between the irradiated and unirradiated regions is shown in Figure 6. the irradiated and unirradiated regions is shown in Figure 6.
Figure 6. The MFM image of the Co/Pt film locally irradiated by focused He+ ion beam. The irradiated
+ ion beam. The irradiated Figurearea 6. The image the Co/Pt area film islocally bythe focused HeThe is onMFM the left side,of unirradiated on theirradiated right side of picture. MFM frame size is area is5 on left The side,MFM unirradiated area is on to the side of the picture. The MFM frame size is μmthe × 5 μm. contrast is normalized theright maximum. 5 µm × 5 µm. The MFM contrast is normalized to the maximum. It can clearly be seen that periodic ion irradiation essentially changes the spatial structure of magnetization, and allows the realization of high-density lattices of inverted domains (bubbles). It can clearly be seen that periodic ion irradiation essentially changes the spatial structure of These objects can be used for application in data storage systems. magnetization, and ofallows the realization high-density lattices of bubbles inverted domains (bubbles). The effects magnetization reversal inofthe arrays of Co/Pt magnetic under an external These magnetic objects can used for application dataHere, storage systems. fieldbe were investigated earlier in in [17]. we studied the switching of bubbles under the The effectsofofanmagnetization in the arrays Co/Pt magnetic bubbles under of anthe external influence inhomogeneousreversal stray field of the MFMofprobe. The successive MFM images same region of bubble array are presented in Figure 7. Initially, the sample was magnetized magnetic field were investigated earlier in [17]. Here, we studied the switching of bubbles under the downward in the 500 Oe external we applied reversed magnetic field tooflead influence of an inhomogeneous strayfield. fieldAfterward of the MFM probe. 200 TheOe successive MFM images the same the sample in demagnetized state. All bubbles had upward-directed magnetic moments (Figure 7a), region of bubble array are presented in Figure 7. Initially, the sample was magnetized downward in while the MFM probe had magnetic moments directed in the opposite direction. Nonperturbing the 500 Oe external field. Afterward we applied 200 Oe reversed magnetic field to lead the sample MFM imaging was carried out in constant-height mode with a scanning height of about 30 nm (the in demagnetized All was bubbles had magnetic moments (Figure 7a), amplitude of state. oscillation 10 nm). Atupward-directed the second stage, we reduced the scanning height to 15 while nm the MFM to probe had magnetic moments directed in the opposite direction. Nonperturbing MFM imaging observe the switching of bubble magnetization under the MFM probe field. The corresponding was carried out inisconstant-height with a scanning ofofabout nm (the amplitude of MFM image shown in Figure mode 7b. Some cases of sharp height changing MFM30 contrast during the scanning Asstage, a result, observed thescanning switchingheight of contrast oscillation wasare 10indicated nm). At by thearrows. second wewe reduced the to 15(disappearance nm to observe the of white dots) in magnetization some areas of theunder MFM the image (Figure 7c). field. The mechanism of bubble switching is switching of bubble MFM probe The corresponding MFM image is similar to the mechanism for changing the magnetization of Co/Pt discs described in Ref. [21]. shown in Figure 7b. Some cases of sharp changing of MFM contrast during the scanning are indicated
by arrows. As a result, we observed the switching of contrast (disappearance of white dots) in some
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areas of the MFM image (Figure 7c). The mechanism of bubble switching is similar to the mechanism Materials 2017, 10, 1034 6 of 7 for changing the magnetization of Co/Pt discs described in Ref. [21]. Through manipulation of the probe Through positionmanipulation over the sample [22], it is possible tosample realize[22], theitselective reversal of of the probe position over the is possiblemagnetization to realize the selective individual bubbles.reversal of individual bubbles. magnetization
Figure 7. The successive MFM images of magnetic bubble array in Co/Pt. (a) The initial state of
Figureuniformly 7. The successive MFM images of magnetic bubble array in Co/Pt. (a) The initial state of magnetized array; (b) The MFM imaging with low scanning height. The places of sharp uniformly magnetized array; The MFM with(c)low height. The places of sharp switching of MFM contrast (b) are indicated byimaging white arrows; Thescanning final state of magnetic bubble array switching of MFM contrast are indicated by white arrows; (c) The final state of magnetic bubble after the MFM probe switching. The switched bubbles are indicated by white arrows. The MFM frame array after the probe switching. switched bubbles to are by white arrows. The MFM frame sizeMFM is 2 μm × 2 μm. The MFMThe contrast is normalized theindicated maximum. size is 2 µm × 2 µm. The MFM contrast is normalized to the maximum. 4. Conclusions
4. Conclusions We have studied the peculiarities of domain structure in multilayer Co/Pt thin-films modified by patterned Co capping layers and by a focused He+ ion beam. It was shown that the application of
We have studied the peculiarities of domain structure in multilayer Co/Pt thin-films modified an additional Co layer allows a reduction in the characteristic scale of the domain structure and the by patterned Co capping layers and by a focused He+ ion the beam. It was shownofthat thefilms application of magnitude of magnetization reversal fields. Therefore, nanostructuring Co/Pt and an additional layer allows a reduction characteristic of theand domain structure and the covering Co Co layers enables the possibilityin tothe control domain wallscale nucleation pinning/depinning magnitude of magnetization reversaloffields. Therefore, films and covering processes for the development magnetic devices.the Onnanostructuring the other hand, of theCo/Pt application of local irradiation ofthe Co/Pt films by ion beams opens up significant opportunities for creating highly dense Co layers enables possibility to control domain wall nucleation and pinning/depinning processes of controllable magnetic devices. domains that can be usedhand, to develop novel and promising data for thearrays development of magnetic On the other the application of local irradiation storage systems. of Co/Pt films by ion beams opens up significant opportunities for creating highly dense arrays of controllable magnetic domains that used to develop promising storage systems. Acknowledgments: The authors arecan verybe thankful to S. A. Gusevnovel and S.and N. Vdovichev for data their assistance in sample preparation. This research is supported by the Russian Science Foundation (project No. 16-12-10254).
Acknowledgments: The authors arebeam verywas thankful to S. and S. N. Vdovichev their assistance in performed at A. the Gusev Saint Petersburg State University for in the framework The sample irradiation by He+ ion sampleofpreparation. This research is supported by the Russian Science Foundation (project No. 16-12-10254). project No. 15-02-10254 of the Russian Foundation for Basic Researches The sample irradiation by He+ ion beam was performed at the Saint Petersburg State University in the framework Author and O.L.E. conceived designed the experiments; O.L.E., M.V.S., N.S.G., of project No. Contributions: 15-02-10254 ofV.L.M. the Russian Foundation forand Basic Researches. Y.V.P. performed the experiments; V.L.M. and O.L.E. analyzed the data; N.S.G. and E.V.S. contributed in
Authorsamples Contributions: V.L.M. and O.L.E. conceived and designed the experiments; O.L.E., M.V.S., N.S.G. and preparations; V.L.M. and O.L.E. wrote the paper. Authorship must be limited to those who have Y.V.P. performed the experiments; V.L.M. and O.L.E. analyzed the data; N.S.G. and E.V.S. contributed in samples contributed substantially to the work reported. preparations; V.L.M. and O.L.E. wrote the paper. Authorship must be limited to those who have contributed substantially to of theInterest: work reported. Conflicts The authors declare no conflict of interest. Conflicts of Interest: The authors declare no conflict of interest. References 1. Stamps, R.L.; Breitkreutz, S.; Akerman, J.; Chumak, A.V.; Otani, Y.; Bauer, G.E.W.; Thiele, J.-U.; Bowen, M.; References Majetich, S.A.; Klaui, M.; et al. The 2014 magnetism roadmap. J. Phys. D Appl. Phys. 2014, 47, 333001.
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