All-optical helicity dependent magnetic switching in Tb-Fe thin films with a MHz laser oscillator Alexander Hassdenteufel,1 Christian Schubert,1,4 Birgit Hebler,1,4 Helmut Schultheiss,2,3 Jürgen Fassbender,2,3 Manfred Albrecht,1,4 and Rudolf Bratschitsch1,5,* 2
1 Institute of Physics, Chemnitz University of Technology, D-09107 Chemnitz, Germany Helmholtz Zentrum Dresden Rossendorf, P.O. Box 510119, D-01314 Dresden, Germany 3 Technische Universität Dresden, D-01062 Dresden, Germany 4 Institute of Physics, University of Augsburg, D-86135 Augsburg, Germany 5 Institute of Physics, University of Münster, D-48149 Münster, Germany * [email protected]
Abstract: We demonstrate all-optical magnetic switching (AOS) in an amorphous Tb30Fe70 thin film, triggered by a 5.1 MHz laser oscillator. The magnetic layer is grown on SiO2/Si substrate. An identical magnetic film deposited on a microscope glass slide reveals no AOS but solely thermally induced demagnetization. This effect is due to heat accumulation by multiple laser pulses because of the low thermal conductivity of the glass substrate. In contrast, the use of a proper heat sink (e.g. SiO2/Si) avoids the need for low repetitive laser amplifier systems to induce AOS and paves the way for a cheap and simple technical implementation using conventional laser oscillators. ©2014 Optical Society of America OCIS codes: (320.7160) Ultrafast technology; (260.7120) Ultrafast phenomena; (320.7130) Ultrafast processes in condensed matter, including semiconductors; (210.0210) Optical data storage; (210.3820) Magneto-optical materials; (210.4810) Optical storage-recording materials.
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1. Introduction The quest for high density and fast magnetic data storage devices stimulates applied as well as basic research. In 2007, it was demonstrated that a circularly polarized femtosecond laser pulse is able to reverse the orientation of magnetization in a thin ferrimagnetic GdFeCo film [1]. This so called all-optical magnetic switching (AOS) constitutes a new route for an ultrafast and entirely optically controlled data storage technology, without the necessity of an external magnetic write-head. Up to now, regenerative or chirped pulse laser amplifier systems with repetition rates in the kHz regime had to be used to induce the AOS process [2– 15]. They generate high energy laser pulses, and their low repetition rate prevents overheating of the magnetic film. However, the disadvantages of these amplifiers such as high cost, complexity, and their large dimensions [16] render them impractical for mass use. It has soon been recognized that heating by laser pulses is necessary for AOS to occur [5, 10]. Increasing the laser repetition rate even lowers the AOS threshold fluence and therefore facilitates AOS [5]. However, at high laser repetition rates exceeding hundreds of kHz heat accumulation in the magnetic layer demagnetizes the sample completely and eventually leads to its destruction [17–19]. Here, we demonstrate AOS in a ferrimagnetic Tb30Fe70 thin film with a conventional MHz laser oscillator. The Tb-Fe layer is deposited onto a SiO2/Si substrate instead of the commonly used glass substrate. Due to its high thermal conductivity, SiO2/Si serves as an efficient heat sink and prevents laser-induced overheating of the sample. This strategy paves the way to compact mass data storage devices operated exclusively by light. 2. Sample preparation and experimental techniques Recently, it has been demonstrated that ferrimagnetic TbxFe100-x films deposited on glass substrate exhibit AOS in the composition range x = 22 - 34 at.%, when excited with a 250 kHz laser amplifier system [10]. In our present study we chose a sample inside this composition range (Tb30Fe70) and another one, which is outside (Tb19Fe81). The 16-nm-thick magnetic films were deposited by DC magnetron co-sputtering from element targets at room temperature simultaneously on a 1 mm thick glass microscope slide and a 375 µm thick pSi(100) substrate (p/boron 10-20 Ω) with a 100 nm thick thermally oxidized SiO2 layer on top (Fig. 1). The film is sandwiched between a 5 nm thick Pt buffer to initiate growth and a 3 nm thick Pt capping to prevent oxidation. The integral magnetic properties are measured by a superconducting quantum interference device – vibrating sample magnetometer (SQUIDVSM). The samples exhibit a strong out-of-plane magnetic anisotropy (MR / MS ≅ 1) and a remanent sample magnetization of MR = (270 ± 41) emu/cc for Tb19Fe81 and MR = (162 ± 12) emu/cc for Tb30Fe70, respectively. Further details concerning the sample fabrication and properties may be found in [10].
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Received 14 Jan 2014; revised 31 Mar 2014; accepted 5 Apr 2014; published 18 Apr 2014 21 April 2014 | Vol. 22, No. 8 | DOI:10.1364/OE.22.010017 | OPTICS EXPRESS 10019
Fig. 1. Schematic drawing of the experimental setup and the samples. Top: Both switching and imaging pulses are provided by a laser oscillator with a repetition rate of 5.1 MHz. See main text for a detailed description of the optical elements. Bottom: Schematic drawing of the samples. ”AOS” denotes helicity-dependent all-optical switching and “PTD” stands for pure thermal demagnetization.
A long cavity laser oscillator (Femtolasers XL 500) with a pulse repetition rate of 5.1 MHz and a center wavelength of 795 nm is used to provide both switching and imaging pulses. The pulse duration at sample position is 90 fs for the pump pulses and 100 fs for the probe pulses. The desired circular polarization of the switching pulses is created by a zero order quarter waveplate (QWP) and attenuated by a combination of a zero order half waveplate (HWP) and a polarizer (Pol). The beam is steered onto the sample by a dichroic mirror (DCM) and focused by an achromatic lens doublet (ACL, f = 75 mm) to a spot size of 31 µm full width at half maximum (FWHM). The imaging pulses are frequency-doubled from 795 nm to 397.5 nm by a Ba(BO2)2 crystal, linearly polarized and focused onto the sample by the same achromatic lens doublet to a spot size of 5 µm (FWHM). After reflection from the sample surface, the imaging beam is collimated again and steered to the detector by a beam splitting mirror (BSM). To quantify the orientation of sample magnetization, we use a scanning Kerr imaging setup [20]. Kerr rotation θ is measured with a Wollaston prism (WoP), which splits the incoming light into two beams with perpendicular linear polarization. Both beams are focused
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Received 14 Jan 2014; revised 31 Mar 2014; accepted 5 Apr 2014; published 18 Apr 2014 21 April 2014 | Vol. 22, No. 8 | DOI:10.1364/OE.22.010017 | OPTICS EXPRESS 10020
by lenses (FL) onto a balanced dual-channel photodetector (BD). The resulting photocurrents are subtracted from each other and measured using a lock-in amplifier. The measured voltage scales linearly with Kerr rotation θ. The two photocurrents are equalized before each measurement while the pump beam is blocked [21]. A lateral map of sample magnetization is constructed by a pointwise measurement, where the sample is laterally moved by a xy-piezo stage below the stationary laser focus of the imaging beam, while the switching beam is blocked. At each position the sample magnetization is recorded. 3. All-optical magnetic switching measurements In general, AOS can only be established by the correct combination of sample magnetization and light helicity [1]. For instance, if the sample magnetization is pointing out of the sample plane (i. e. M+), AOS is witnessed only after illumination with left handed circularly polarized light σ-. In this case, the exposed area reverses to the opposite magnetization M-. Therefore, the all-optical switching experiment is performed as follows: At the beginning of each measurement, the sample is homogeneously magnetized in one direction (e. g. M+, i. e. out of the sample plane) by an applied external magnetic field. A reference image of the sample magnetization (e.g. M+) is obtained as described in the previous paragraph (Figs. 2(a) and 2(d)). Subsequently, the sample is irradiated with pulses of circularly polarized light (switching pulses), e.g. σ-, at a repetition rate of 5.1 MHz. Again, an image of the sample magnetization is recorded. This image acquisition and illumination procedure is repeated at the same sample position while steadily increasing the fluence of the switching pulses, until a structure with opposite contrast as in the initial image appears (e.g. M- in Figs. 2(b) and 2(e)). Afterwards, the helicity of the switching pulses is changed to the opposite direction (e.g. right handed polarization, σ+) and the sample is again irradiated at the same fluence. If the written area may be “erased”, all-optical magnetic switching is witnessed. Clearly, AOS occurs for the Tb30Fe70 film deposited on SiO2/Si substrate (Figs. 2(a)–2(c)). In contrast, only helicity independent pure thermal demagnetization (PTD) occurs, if the written area remains the same. A multidomain state forms, where in some parts of the irradiated area the magnetization points upwards and in other parts downwards, which is below our spatial resolution. This state cannot be “erased” by light. Erasing attempts only lead to an increase of the multidomain area, as observed for the Tb19Fe81 sample on SiO2/Si substrate (Figs. 2(d)– 2(f)).
Fig. 2. Helicity controlled all-optical magnetic switching (AOS) in a Tb30Fe70 film and pure thermal demagnetization (PTD) in a Tb19Fe81 film, both deposited on a SiO2/Si substrate and excited by a 5.1 MHz laser oscillator. Scanning Kerr images (45 x 45) µm2 for (a) – (c) Tb30Fe70 and (d) – (f) Tb19Fe81. (a) and (d) show a homogeneous sample magnetization before laser excitation. For Tb30Fe70 AOS occurs for a threshold laser fluence of (1.7 ± 0.1) mJ/cm2. Helicity independent PTD is observed in Tb19Fe81 after exposure with a threshold fluence of (1.3 ± 0.1) mJ/cm2.
To elucidate the influence of the substrate on the AOS process, we repeated the above experiment for a Tb30Fe70 film deposited on a microscope glass slide. This film has the same
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Received 14 Jan 2014; revised 31 Mar 2014; accepted 5 Apr 2014; published 18 Apr 2014 21 April 2014 | Vol. 22, No. 8 | DOI:10.1364/OE.22.010017 | OPTICS EXPRESS 10021
magnetic properties as the one deposited on SiO2/Si. At first, the sample is homogeneously magnetized in M- direction (Fig. 3(a)). Subsequently, it is illuminated with right circularly polarized light (σ+). An irregular structure emerges in the Kerr image at a threshold fluence of F = (1.3 ± 0.1) mJ/cm2 (Fig. 3(b)), which cannot be erased after illumination with opposite helicity (σ-) using the same fluence (Fig. 3(c)). Obviously, AOS does not occur and is replaced by PTD.
Fig. 3. Pure thermal demagnetization (PTD) in Tb30Fe70 deposited on a glass substrate. (a) Scanning Kerr images of the homogeneously magnetized sample before illumination. (b) After excitation with σ+ polarized laser pulses at (1.3 ± 0.1) mJ/cm2, and (c) excitation with σpolarized laser pulses at the same fluence and the same sample position as in (b). White circles mark the area of exposure with switching pulses.
This observation on an identical amorphous Tb-Fe film demonstrates the decisive role of the substrate via its thermal conductivity for AOS. To corroborate this finding, we theoretically investigate the thermal transport in laser-heated thin metallic films on different dielectric substrates. 4. Heat dissipation due to different substrates We estimate the heat dissipation from the laser-heated metal film into the substrate by solving the heat conduction equation Eq. (1) by the finite difference method (FDM) in forward time and centred space, as described in [22, 23]: ∂T (r , t ) = ∇(k ∇T (r , t )), ρ ⋅c⋅ (1) ∂t where the volumetric heat capacity ρ·c is the product of the mass density ρ and the heat capacity c, k refers to the thermal conductivity, and T( r ,t) is the space and time-dependent temperature. We will first address the question how much heat is deposited into the metallic film by laser irradiation. Afterwards, the intralayer and interlayer heat flow will be discussed. Finally, the difference between glass and silicon substrates concerning heat accumulation will be illustrated. Within the two temperature model [24], the laser-induced temperature rise of a thin film is directly related to an increase of the electronic temperature, which in turn increases the lattice temperature of the metal via electron-phonon coupling. For timescales much longer than the electron-phonon coupling time, which is typically between 1 and 10 ps [22], the two temperatures equilibrate. Furthermore, earlier work has demonstrated that AOS is observed in GdFeCo samples also by using 10 ps laser pulses, where the temporal evolution of the electronic and phonon temperature is more or less similar [3]. Therefore, it is reasonable to use a single steady state temperature to model the metallic film, because at 5.1 MHz repetition rate the cycle duration is 0.196 µs. The temperature rise ΔT of the film after laser irradiation may be estimated by: Eabs Fabs ΔT = = , (2) cf ⋅ ρf ⋅ V cf ⋅ ρ f ⋅ d with Eabs = A·Fabs, where A is the area of the laser focus and Fabs is the absorbed fluence. Fabs may be calculated from the incident laser fluence F according to Fabs = F·a, where a is the absorbance of the entire film, which amounts to (34.0 ± 1.5) % [10]. V is the heated volume. The variables cf, ρf, d, denote the specific heat, the mass density, and the thickness of the
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Received 14 Jan 2014; revised 31 Mar 2014; accepted 5 Apr 2014; published 18 Apr 2014 21 April 2014 | Vol. 22, No. 8 | DOI:10.1364/OE.22.010017 | OPTICS EXPRESS 10022
entire metallic film, which are calculated as a weighted average from bulk values at T = 300 K. With cPt = 133 J·kg−1·K−1, cFe = 449 J·kg−1·K−1, cTb = 182 J·kg−1·K−1, ρPt = 21.5·103 kg·m−3, ρFe = 7.87·103 kg·m−3, and ρTb = 8.23·103 kg·m−3 [25], we get cTb19Fe81 = 398 J·kg−1·K−1 and cTb30Fe70 = 369 J·kg−1·K−1, which is close to literature values for comparable compositions [26]. For the entire film stack with platinum capping and bottom layer, the values of the specific heat capacity are cf = 310 J·kg−1·K−1 and cf = 290 J·kg−1·K−1 for Tb19Fe81 and Tb30Fe70, respectively. The mass density for the entire film stack is ρf = 12.5·103 kg·m−3 for both samples. The temperature rise due to laser heating using Eq. (2) is: ΔT (Tb19Fe81) = (48 ± 8) K and ΔT (Tb30Fe70) = (67 ± 10) K for the samples on SiO2/Si, and ΔT (Tb30Fe70) = (51 ± 8) K for the sample on glass substrate. The substrate temperature is assumed to be unchanged by the laser [22]. For the glass substrate the optical absorbance is negligible at a wavelength of 800 nm (a = 1.57 m−1 [27]) compared to the metallic film (a = 72·106 m−1, determined by ellipsometry). For the Si substrate the absorbance amounts to a = 0.079·106 m−1, but the assumption of an unchanged substrate temperature is still valid due to the high thermal conductivity of bulk crystalline Si, as discussed in the next paragraph. Furthermore, the reflected laser intensity from SiO2/Si may also be neglected, because at 800 nm the reflectance of the SiO2/Si interface amounts to 10%. We now turn to the question how the stored heat dissipates. In metals heat is primarily carried by electrons [28], while in semiconductors and insulators by phonons [29]. Heat transport across a metal non-metal interface can therefore be ascribed to two mechanisms. First, the coupling between electrons in the metal and phonons in the non-metal through electron interface scattering and second, by the coupling of phonons from the metal and phonons from the non-metal, which leads to an interfacial thermal resistivity [30]. For a Pt SiO2 interface this resistivity amounts to RK = 3.83·10−8 m2·K·W−1 [31]. This value can easily rise by a factor of three to RK = 1.15·10−7 m2·K·W−1 by taking into account interfacial roughness and grain boundaries at the interface [32]. In contrast, the thermal resistivity of the SiO2 Si interface is nearly fifty times smaller compared to the resistivity between the metallic Pt layer and the SiO2 interface, RK = 3.7·10−9 m2·K·W−1 [31]. Furthermore, the thermal conductivities for each single layer have to be taken into account, which are significantly lower for thin films compared to bulk material, although the heat capacity and density are close to the properties of bulk material [33]. At room temperature the thermal conductivity for a 10 nm thick Pt layer amounts to k = 18 W·m−1·K−1 [31]. The thermal conductivity for the amorphous Tb-Fe layer is taken as the average of the reported literature values of the thermal conductivity for TbFeCo thin films (k = 5 W·m−1·K−1 [34] and k = 8.1 W·m−1·K−1 [35]) to be kTb-Fe = 6.6 W·m−1·K−1. For the entire metallic film stack the weighted average due to different thicknesses of the Pt and Tb-Fe layers is calculated to be k = 8.9 W·m−1·K−1. This low value for the metallic film, compared to bulk Fe (k = 80.2 W·m−1·K−1 [25]) or bulk Pt (k = 71.6 W·m−1·K−1 [25]) can be understood by taking the mean free path of the main energy carriers into account. The amorphous structure of the metallic layer leads to a much shorter phonon mean free path compared to that of a crystal, which is at room temperature on the order of an interatomic spacing [31]. In contrast, the mean free path for the electrons can easily reach the layer thickness. Scattering of electrons at the boundaries of the film becomes increasingly dominant, which affects the decrease of thermal conductivity [36]. For SiO2 the phonon mean free path reaches only Λ = 2 – 5 nm [37], and for Si Λ = 250 nm [37], which is in both cases well below the particular thicknesses of the SiO2 layer and Si substrate, respectively. As a result, the thermal conductivity for a 100 nm thermally oxidized SiO2 layer amounts to k = 1.1 W·m−1·K−1 [38]. This is close to the bulk value: k = 1.06 W·m−1·K−1 for borosilicate crown glass [39]. For the Si substrate, we use the bulk value for crystalline Si, k = 156 W·m−1·K−1 [40]. Next, we study the interplay between interfacial thermal resistivity and thermal conductivity with respect to the temporal thermal behavior of the film heated by the laser. For #204784 - $15.00 USD (C) 2014 OSA
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film thicknesses d smaller than the Kapitza length lK = RK·k [22], the thermal decay is governed by heat transport across the interface. In contrast, if d is larger than lK the decay happens by heat diffusion in the film itself: d < lK :τ K = ρ ⋅ c ⋅ RK ⋅ d , (3) 4 ρ ⋅c 2 d > lK :τ diff = 2 ⋅d . π k For the metal film the condition d > lK = 4 nm, and hence τdiff = 7.1 ns (with ρ = 2.5·103 kg·m−3, and c = 769 J·kg−1·K−1 [39]). Furthermore, we have to consider the thermal response after an external perturbation by an ultrashort laser pulse, which can be well described by the characteristic time [41],
τc =
2 d layer
2⋅k
⋅ ρ ⋅ c.
(4)
This yields τc = 126 ps and τc = 113 ps for Tb19Fe81 and Tb30Fe70, respectively. A comparison of the interfacial cooling time constant τK and the characteristic time τc shows that the intralayer diffusion is more than 80 times faster than the interlayer heat dissipation. Based on this finding and the optical transmission of the metallic film at a wavelength of 800 nm, we assume a homogeneous temperature distribution across the entire thickness of the film, which will be represented in the numerical analysis as a von-Neumann type boundary condition: ∇T = 0. For the second boundary (back side of the sample), a Dirichlet condition is applied (constant ambient temperature). In addition, lateral heat diffusion will be neglected, because the diameter of the laser focus of 31 µm is more than 1000 times larger than the layer thickness [23]. Therefore Eq. (1) may be solved in one dimension, where heat flow is perpendicular to the sample surface. The von-Neumann boundary condition, which is governed by the conservation of the energy at the interface, may be treated as [22, 37]:
ρ ⋅c⋅
d 1 ⋅ (Tn − Tn −1 ) , Tn = − dt RK
(5)
where n refers to each individual layer, which contributes to the interface. For our numerical considerations, we use the material properties as summarized in Table 1. The spatial step size is chosen well below the structure size to 10 nm. The temporal increment is 0.587 ps for SiO2/Si and 90.686 ps for glass substrates, to fulfill the stability criterion of a Fourier number Fo ≤ 0.5 [42] for the one-dimensional case. Table 1. Material constants used for the numerical calculation described in the text. Layer
Metal film
Thermal conductivity k (W·m−1·K−1)
Volumetric heat capacity ρ·c (J·m−3·K−1)
Tb-Fe: 6.6
Tb19Fe81 3.9·106
Pt: 18
Microscope glass slide
Layer thickness d (nm) 24
average: 8.9
Tb30Fe70 3.6·106
1.1 [38]
1.9·106 [39]
Thermally oxidized SiO2 Silicon
Interfacial thermal resistivity RK (m2·KW−1)
1.15·10−7 [31,32] 100 3.70·10−9 [31]
156 [40] 1.06 [39]
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6
375·103
1.7·10 [22] 1.9·106 [39]
1.15·10−7 [31,32]
106
Received 14 Jan 2014; revised 31 Mar 2014; accepted 5 Apr 2014; published 18 Apr 2014 21 April 2014 | Vol. 22, No. 8 | DOI:10.1364/OE.22.010017 | OPTICS EXPRESS 10024
The results of the simulation of the transient temperature for the metallic layer on the two different substrates after ultrafast laser heating are depicted in Fig. 4.
Fig. 4. Calculated transient temperature of a Tb30Fe70 film on glass or SiO2/Si substrate for laser pulses at a repetition rate of 5.1 MHz.
The transient temperature for the magnetic film on SiO2/Si strongly differs from the film on the glass substrate. For the latter the temperature rise after several laser pulses is substantially higher. Due to heat accumulation, the steady-state temperature rise is (93 ± 15) K and about twice as large compared to the SiO2/Si case ((48 ± 6) K). Errors are estimated by varying the volumetric heat capacity and thermal resistivity of the metal-glass-interface by ± 10%. We believe this error range to be justified, because the exact thermal properties of the metallic films, the substrates, and in particular the interface thermal resistivity have been estimated from literature values. In addition, a detailed theoretical understanding of the thermal transport mechanisms is still lacking and subject of current research [43]. However, the distinct result of our rough estimation clearly demonstrates that the use of a proper heat sink (SiO2/Si substrate) prevents heat accumulation in the magnetic film. In this way, AOS was established with a common laser oscillator instead of a complex and expensive laser amplifier system for the first time. 5. Conclusions
In summary, we have demonstrated all-optical helicity dependent magnetic switching in a Tb30Fe70 thin film using a laser oscillator with 5.1 MHz repetition rate. To overcome laserinduced heat accumulation in the metallic film, it was deposited on a SiO2/Si substrate, which acts as an effective heat sink. In contrast, an identical film grown on a glass microscope slide reveals no AOS. This work paves the way for the implementation of AOS with robust and easy to use laser oscillators, e.g. with compact fiber lasers for future ultrafast data storage. Acknowledgments
We thank M. Daniel for RBS characterization, J. von Borany for access to and R. Wilhelm for technical support at the RBS beam line at Helmholtz-Zentrum Dresden Rossendorf, and A. Neudert for initial experimental assistance. M. Radek, R. Frieling, H. Bracht, S. Fähler, and M. Heidernätsch are gratefully acknowledged for valuable discussions. We acknowledge support by Deutsche Forschungsgemeinschaft and Open Access Publication Fund of University of Muenster.
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Received 14 Jan 2014; revised 31 Mar 2014; accepted 5 Apr 2014; published 18 Apr 2014 21 April 2014 | Vol. 22, No. 8 | DOI:10.1364/OE.22.010017 | OPTICS EXPRESS 10025
1 Institute of Physics, Chemnitz University of Technology, D-09107 Chemnitz, Germany Helmholtz Zentrum Dresden Rossendorf, P.O. Box 510119, D-01314 Dresden, Germany 3 Technische Universität Dresden, D-01062 Dresden, Germany 4 Institute of Physics, University of Augsburg, D-86135 Augsburg, Germany 5 Institute of Physics, University of Münster, D-48149 Münster, Germany * [email protected]
Abstract: We demonstrate all-optical magnetic switching (AOS) in an amorphous Tb30Fe70 thin film, triggered by a 5.1 MHz laser oscillator. The magnetic layer is grown on SiO2/Si substrate. An identical magnetic film deposited on a microscope glass slide reveals no AOS but solely thermally induced demagnetization. This effect is due to heat accumulation by multiple laser pulses because of the low thermal conductivity of the glass substrate. In contrast, the use of a proper heat sink (e.g. SiO2/Si) avoids the need for low repetitive laser amplifier systems to induce AOS and paves the way for a cheap and simple technical implementation using conventional laser oscillators. ©2014 Optical Society of America OCIS codes: (320.7160) Ultrafast technology; (260.7120) Ultrafast phenomena; (320.7130) Ultrafast processes in condensed matter, including semiconductors; (210.0210) Optical data storage; (210.3820) Magneto-optical materials; (210.4810) Optical storage-recording materials.
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1. Introduction The quest for high density and fast magnetic data storage devices stimulates applied as well as basic research. In 2007, it was demonstrated that a circularly polarized femtosecond laser pulse is able to reverse the orientation of magnetization in a thin ferrimagnetic GdFeCo film [1]. This so called all-optical magnetic switching (AOS) constitutes a new route for an ultrafast and entirely optically controlled data storage technology, without the necessity of an external magnetic write-head. Up to now, regenerative or chirped pulse laser amplifier systems with repetition rates in the kHz regime had to be used to induce the AOS process [2– 15]. They generate high energy laser pulses, and their low repetition rate prevents overheating of the magnetic film. However, the disadvantages of these amplifiers such as high cost, complexity, and their large dimensions [16] render them impractical for mass use. It has soon been recognized that heating by laser pulses is necessary for AOS to occur [5, 10]. Increasing the laser repetition rate even lowers the AOS threshold fluence and therefore facilitates AOS [5]. However, at high laser repetition rates exceeding hundreds of kHz heat accumulation in the magnetic layer demagnetizes the sample completely and eventually leads to its destruction [17–19]. Here, we demonstrate AOS in a ferrimagnetic Tb30Fe70 thin film with a conventional MHz laser oscillator. The Tb-Fe layer is deposited onto a SiO2/Si substrate instead of the commonly used glass substrate. Due to its high thermal conductivity, SiO2/Si serves as an efficient heat sink and prevents laser-induced overheating of the sample. This strategy paves the way to compact mass data storage devices operated exclusively by light. 2. Sample preparation and experimental techniques Recently, it has been demonstrated that ferrimagnetic TbxFe100-x films deposited on glass substrate exhibit AOS in the composition range x = 22 - 34 at.%, when excited with a 250 kHz laser amplifier system [10]. In our present study we chose a sample inside this composition range (Tb30Fe70) and another one, which is outside (Tb19Fe81). The 16-nm-thick magnetic films were deposited by DC magnetron co-sputtering from element targets at room temperature simultaneously on a 1 mm thick glass microscope slide and a 375 µm thick pSi(100) substrate (p/boron 10-20 Ω) with a 100 nm thick thermally oxidized SiO2 layer on top (Fig. 1). The film is sandwiched between a 5 nm thick Pt buffer to initiate growth and a 3 nm thick Pt capping to prevent oxidation. The integral magnetic properties are measured by a superconducting quantum interference device – vibrating sample magnetometer (SQUIDVSM). The samples exhibit a strong out-of-plane magnetic anisotropy (MR / MS ≅ 1) and a remanent sample magnetization of MR = (270 ± 41) emu/cc for Tb19Fe81 and MR = (162 ± 12) emu/cc for Tb30Fe70, respectively. Further details concerning the sample fabrication and properties may be found in [10].
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Fig. 1. Schematic drawing of the experimental setup and the samples. Top: Both switching and imaging pulses are provided by a laser oscillator with a repetition rate of 5.1 MHz. See main text for a detailed description of the optical elements. Bottom: Schematic drawing of the samples. ”AOS” denotes helicity-dependent all-optical switching and “PTD” stands for pure thermal demagnetization.
A long cavity laser oscillator (Femtolasers XL 500) with a pulse repetition rate of 5.1 MHz and a center wavelength of 795 nm is used to provide both switching and imaging pulses. The pulse duration at sample position is 90 fs for the pump pulses and 100 fs for the probe pulses. The desired circular polarization of the switching pulses is created by a zero order quarter waveplate (QWP) and attenuated by a combination of a zero order half waveplate (HWP) and a polarizer (Pol). The beam is steered onto the sample by a dichroic mirror (DCM) and focused by an achromatic lens doublet (ACL, f = 75 mm) to a spot size of 31 µm full width at half maximum (FWHM). The imaging pulses are frequency-doubled from 795 nm to 397.5 nm by a Ba(BO2)2 crystal, linearly polarized and focused onto the sample by the same achromatic lens doublet to a spot size of 5 µm (FWHM). After reflection from the sample surface, the imaging beam is collimated again and steered to the detector by a beam splitting mirror (BSM). To quantify the orientation of sample magnetization, we use a scanning Kerr imaging setup [20]. Kerr rotation θ is measured with a Wollaston prism (WoP), which splits the incoming light into two beams with perpendicular linear polarization. Both beams are focused
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Received 14 Jan 2014; revised 31 Mar 2014; accepted 5 Apr 2014; published 18 Apr 2014 21 April 2014 | Vol. 22, No. 8 | DOI:10.1364/OE.22.010017 | OPTICS EXPRESS 10020
by lenses (FL) onto a balanced dual-channel photodetector (BD). The resulting photocurrents are subtracted from each other and measured using a lock-in amplifier. The measured voltage scales linearly with Kerr rotation θ. The two photocurrents are equalized before each measurement while the pump beam is blocked [21]. A lateral map of sample magnetization is constructed by a pointwise measurement, where the sample is laterally moved by a xy-piezo stage below the stationary laser focus of the imaging beam, while the switching beam is blocked. At each position the sample magnetization is recorded. 3. All-optical magnetic switching measurements In general, AOS can only be established by the correct combination of sample magnetization and light helicity [1]. For instance, if the sample magnetization is pointing out of the sample plane (i. e. M+), AOS is witnessed only after illumination with left handed circularly polarized light σ-. In this case, the exposed area reverses to the opposite magnetization M-. Therefore, the all-optical switching experiment is performed as follows: At the beginning of each measurement, the sample is homogeneously magnetized in one direction (e. g. M+, i. e. out of the sample plane) by an applied external magnetic field. A reference image of the sample magnetization (e.g. M+) is obtained as described in the previous paragraph (Figs. 2(a) and 2(d)). Subsequently, the sample is irradiated with pulses of circularly polarized light (switching pulses), e.g. σ-, at a repetition rate of 5.1 MHz. Again, an image of the sample magnetization is recorded. This image acquisition and illumination procedure is repeated at the same sample position while steadily increasing the fluence of the switching pulses, until a structure with opposite contrast as in the initial image appears (e.g. M- in Figs. 2(b) and 2(e)). Afterwards, the helicity of the switching pulses is changed to the opposite direction (e.g. right handed polarization, σ+) and the sample is again irradiated at the same fluence. If the written area may be “erased”, all-optical magnetic switching is witnessed. Clearly, AOS occurs for the Tb30Fe70 film deposited on SiO2/Si substrate (Figs. 2(a)–2(c)). In contrast, only helicity independent pure thermal demagnetization (PTD) occurs, if the written area remains the same. A multidomain state forms, where in some parts of the irradiated area the magnetization points upwards and in other parts downwards, which is below our spatial resolution. This state cannot be “erased” by light. Erasing attempts only lead to an increase of the multidomain area, as observed for the Tb19Fe81 sample on SiO2/Si substrate (Figs. 2(d)– 2(f)).
Fig. 2. Helicity controlled all-optical magnetic switching (AOS) in a Tb30Fe70 film and pure thermal demagnetization (PTD) in a Tb19Fe81 film, both deposited on a SiO2/Si substrate and excited by a 5.1 MHz laser oscillator. Scanning Kerr images (45 x 45) µm2 for (a) – (c) Tb30Fe70 and (d) – (f) Tb19Fe81. (a) and (d) show a homogeneous sample magnetization before laser excitation. For Tb30Fe70 AOS occurs for a threshold laser fluence of (1.7 ± 0.1) mJ/cm2. Helicity independent PTD is observed in Tb19Fe81 after exposure with a threshold fluence of (1.3 ± 0.1) mJ/cm2.
To elucidate the influence of the substrate on the AOS process, we repeated the above experiment for a Tb30Fe70 film deposited on a microscope glass slide. This film has the same
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magnetic properties as the one deposited on SiO2/Si. At first, the sample is homogeneously magnetized in M- direction (Fig. 3(a)). Subsequently, it is illuminated with right circularly polarized light (σ+). An irregular structure emerges in the Kerr image at a threshold fluence of F = (1.3 ± 0.1) mJ/cm2 (Fig. 3(b)), which cannot be erased after illumination with opposite helicity (σ-) using the same fluence (Fig. 3(c)). Obviously, AOS does not occur and is replaced by PTD.
Fig. 3. Pure thermal demagnetization (PTD) in Tb30Fe70 deposited on a glass substrate. (a) Scanning Kerr images of the homogeneously magnetized sample before illumination. (b) After excitation with σ+ polarized laser pulses at (1.3 ± 0.1) mJ/cm2, and (c) excitation with σpolarized laser pulses at the same fluence and the same sample position as in (b). White circles mark the area of exposure with switching pulses.
This observation on an identical amorphous Tb-Fe film demonstrates the decisive role of the substrate via its thermal conductivity for AOS. To corroborate this finding, we theoretically investigate the thermal transport in laser-heated thin metallic films on different dielectric substrates. 4. Heat dissipation due to different substrates We estimate the heat dissipation from the laser-heated metal film into the substrate by solving the heat conduction equation Eq. (1) by the finite difference method (FDM) in forward time and centred space, as described in [22, 23]: ∂T (r , t ) = ∇(k ∇T (r , t )), ρ ⋅c⋅ (1) ∂t where the volumetric heat capacity ρ·c is the product of the mass density ρ and the heat capacity c, k refers to the thermal conductivity, and T( r ,t) is the space and time-dependent temperature. We will first address the question how much heat is deposited into the metallic film by laser irradiation. Afterwards, the intralayer and interlayer heat flow will be discussed. Finally, the difference between glass and silicon substrates concerning heat accumulation will be illustrated. Within the two temperature model [24], the laser-induced temperature rise of a thin film is directly related to an increase of the electronic temperature, which in turn increases the lattice temperature of the metal via electron-phonon coupling. For timescales much longer than the electron-phonon coupling time, which is typically between 1 and 10 ps [22], the two temperatures equilibrate. Furthermore, earlier work has demonstrated that AOS is observed in GdFeCo samples also by using 10 ps laser pulses, where the temporal evolution of the electronic and phonon temperature is more or less similar [3]. Therefore, it is reasonable to use a single steady state temperature to model the metallic film, because at 5.1 MHz repetition rate the cycle duration is 0.196 µs. The temperature rise ΔT of the film after laser irradiation may be estimated by: Eabs Fabs ΔT = = , (2) cf ⋅ ρf ⋅ V cf ⋅ ρ f ⋅ d with Eabs = A·Fabs, where A is the area of the laser focus and Fabs is the absorbed fluence. Fabs may be calculated from the incident laser fluence F according to Fabs = F·a, where a is the absorbance of the entire film, which amounts to (34.0 ± 1.5) % [10]. V is the heated volume. The variables cf, ρf, d, denote the specific heat, the mass density, and the thickness of the
#204784 - $15.00 USD (C) 2014 OSA
Received 14 Jan 2014; revised 31 Mar 2014; accepted 5 Apr 2014; published 18 Apr 2014 21 April 2014 | Vol. 22, No. 8 | DOI:10.1364/OE.22.010017 | OPTICS EXPRESS 10022
entire metallic film, which are calculated as a weighted average from bulk values at T = 300 K. With cPt = 133 J·kg−1·K−1, cFe = 449 J·kg−1·K−1, cTb = 182 J·kg−1·K−1, ρPt = 21.5·103 kg·m−3, ρFe = 7.87·103 kg·m−3, and ρTb = 8.23·103 kg·m−3 [25], we get cTb19Fe81 = 398 J·kg−1·K−1 and cTb30Fe70 = 369 J·kg−1·K−1, which is close to literature values for comparable compositions [26]. For the entire film stack with platinum capping and bottom layer, the values of the specific heat capacity are cf = 310 J·kg−1·K−1 and cf = 290 J·kg−1·K−1 for Tb19Fe81 and Tb30Fe70, respectively. The mass density for the entire film stack is ρf = 12.5·103 kg·m−3 for both samples. The temperature rise due to laser heating using Eq. (2) is: ΔT (Tb19Fe81) = (48 ± 8) K and ΔT (Tb30Fe70) = (67 ± 10) K for the samples on SiO2/Si, and ΔT (Tb30Fe70) = (51 ± 8) K for the sample on glass substrate. The substrate temperature is assumed to be unchanged by the laser [22]. For the glass substrate the optical absorbance is negligible at a wavelength of 800 nm (a = 1.57 m−1 [27]) compared to the metallic film (a = 72·106 m−1, determined by ellipsometry). For the Si substrate the absorbance amounts to a = 0.079·106 m−1, but the assumption of an unchanged substrate temperature is still valid due to the high thermal conductivity of bulk crystalline Si, as discussed in the next paragraph. Furthermore, the reflected laser intensity from SiO2/Si may also be neglected, because at 800 nm the reflectance of the SiO2/Si interface amounts to 10%. We now turn to the question how the stored heat dissipates. In metals heat is primarily carried by electrons [28], while in semiconductors and insulators by phonons [29]. Heat transport across a metal non-metal interface can therefore be ascribed to two mechanisms. First, the coupling between electrons in the metal and phonons in the non-metal through electron interface scattering and second, by the coupling of phonons from the metal and phonons from the non-metal, which leads to an interfacial thermal resistivity [30]. For a Pt SiO2 interface this resistivity amounts to RK = 3.83·10−8 m2·K·W−1 [31]. This value can easily rise by a factor of three to RK = 1.15·10−7 m2·K·W−1 by taking into account interfacial roughness and grain boundaries at the interface [32]. In contrast, the thermal resistivity of the SiO2 Si interface is nearly fifty times smaller compared to the resistivity between the metallic Pt layer and the SiO2 interface, RK = 3.7·10−9 m2·K·W−1 [31]. Furthermore, the thermal conductivities for each single layer have to be taken into account, which are significantly lower for thin films compared to bulk material, although the heat capacity and density are close to the properties of bulk material [33]. At room temperature the thermal conductivity for a 10 nm thick Pt layer amounts to k = 18 W·m−1·K−1 [31]. The thermal conductivity for the amorphous Tb-Fe layer is taken as the average of the reported literature values of the thermal conductivity for TbFeCo thin films (k = 5 W·m−1·K−1 [34] and k = 8.1 W·m−1·K−1 [35]) to be kTb-Fe = 6.6 W·m−1·K−1. For the entire metallic film stack the weighted average due to different thicknesses of the Pt and Tb-Fe layers is calculated to be k = 8.9 W·m−1·K−1. This low value for the metallic film, compared to bulk Fe (k = 80.2 W·m−1·K−1 [25]) or bulk Pt (k = 71.6 W·m−1·K−1 [25]) can be understood by taking the mean free path of the main energy carriers into account. The amorphous structure of the metallic layer leads to a much shorter phonon mean free path compared to that of a crystal, which is at room temperature on the order of an interatomic spacing [31]. In contrast, the mean free path for the electrons can easily reach the layer thickness. Scattering of electrons at the boundaries of the film becomes increasingly dominant, which affects the decrease of thermal conductivity [36]. For SiO2 the phonon mean free path reaches only Λ = 2 – 5 nm [37], and for Si Λ = 250 nm [37], which is in both cases well below the particular thicknesses of the SiO2 layer and Si substrate, respectively. As a result, the thermal conductivity for a 100 nm thermally oxidized SiO2 layer amounts to k = 1.1 W·m−1·K−1 [38]. This is close to the bulk value: k = 1.06 W·m−1·K−1 for borosilicate crown glass [39]. For the Si substrate, we use the bulk value for crystalline Si, k = 156 W·m−1·K−1 [40]. Next, we study the interplay between interfacial thermal resistivity and thermal conductivity with respect to the temporal thermal behavior of the film heated by the laser. For #204784 - $15.00 USD (C) 2014 OSA
Received 14 Jan 2014; revised 31 Mar 2014; accepted 5 Apr 2014; published 18 Apr 2014 21 April 2014 | Vol. 22, No. 8 | DOI:10.1364/OE.22.010017 | OPTICS EXPRESS 10023
film thicknesses d smaller than the Kapitza length lK = RK·k [22], the thermal decay is governed by heat transport across the interface. In contrast, if d is larger than lK the decay happens by heat diffusion in the film itself: d < lK :τ K = ρ ⋅ c ⋅ RK ⋅ d , (3) 4 ρ ⋅c 2 d > lK :τ diff = 2 ⋅d . π k For the metal film the condition d > lK = 4 nm, and hence τdiff = 7.1 ns (with ρ = 2.5·103 kg·m−3, and c = 769 J·kg−1·K−1 [39]). Furthermore, we have to consider the thermal response after an external perturbation by an ultrashort laser pulse, which can be well described by the characteristic time [41],
τc =
2 d layer
2⋅k
⋅ ρ ⋅ c.
(4)
This yields τc = 126 ps and τc = 113 ps for Tb19Fe81 and Tb30Fe70, respectively. A comparison of the interfacial cooling time constant τK and the characteristic time τc shows that the intralayer diffusion is more than 80 times faster than the interlayer heat dissipation. Based on this finding and the optical transmission of the metallic film at a wavelength of 800 nm, we assume a homogeneous temperature distribution across the entire thickness of the film, which will be represented in the numerical analysis as a von-Neumann type boundary condition: ∇T = 0. For the second boundary (back side of the sample), a Dirichlet condition is applied (constant ambient temperature). In addition, lateral heat diffusion will be neglected, because the diameter of the laser focus of 31 µm is more than 1000 times larger than the layer thickness [23]. Therefore Eq. (1) may be solved in one dimension, where heat flow is perpendicular to the sample surface. The von-Neumann boundary condition, which is governed by the conservation of the energy at the interface, may be treated as [22, 37]:
ρ ⋅c⋅
d 1 ⋅ (Tn − Tn −1 ) , Tn = − dt RK
(5)
where n refers to each individual layer, which contributes to the interface. For our numerical considerations, we use the material properties as summarized in Table 1. The spatial step size is chosen well below the structure size to 10 nm. The temporal increment is 0.587 ps for SiO2/Si and 90.686 ps for glass substrates, to fulfill the stability criterion of a Fourier number Fo ≤ 0.5 [42] for the one-dimensional case. Table 1. Material constants used for the numerical calculation described in the text. Layer
Metal film
Thermal conductivity k (W·m−1·K−1)
Volumetric heat capacity ρ·c (J·m−3·K−1)
Tb-Fe: 6.6
Tb19Fe81 3.9·106
Pt: 18
Microscope glass slide
Layer thickness d (nm) 24
average: 8.9
Tb30Fe70 3.6·106
1.1 [38]
1.9·106 [39]
Thermally oxidized SiO2 Silicon
Interfacial thermal resistivity RK (m2·KW−1)
1.15·10−7 [31,32] 100 3.70·10−9 [31]
156 [40] 1.06 [39]
#204784 - $15.00 USD (C) 2014 OSA
6
375·103
1.7·10 [22] 1.9·106 [39]
1.15·10−7 [31,32]
106
Received 14 Jan 2014; revised 31 Mar 2014; accepted 5 Apr 2014; published 18 Apr 2014 21 April 2014 | Vol. 22, No. 8 | DOI:10.1364/OE.22.010017 | OPTICS EXPRESS 10024
The results of the simulation of the transient temperature for the metallic layer on the two different substrates after ultrafast laser heating are depicted in Fig. 4.
Fig. 4. Calculated transient temperature of a Tb30Fe70 film on glass or SiO2/Si substrate for laser pulses at a repetition rate of 5.1 MHz.
The transient temperature for the magnetic film on SiO2/Si strongly differs from the film on the glass substrate. For the latter the temperature rise after several laser pulses is substantially higher. Due to heat accumulation, the steady-state temperature rise is (93 ± 15) K and about twice as large compared to the SiO2/Si case ((48 ± 6) K). Errors are estimated by varying the volumetric heat capacity and thermal resistivity of the metal-glass-interface by ± 10%. We believe this error range to be justified, because the exact thermal properties of the metallic films, the substrates, and in particular the interface thermal resistivity have been estimated from literature values. In addition, a detailed theoretical understanding of the thermal transport mechanisms is still lacking and subject of current research [43]. However, the distinct result of our rough estimation clearly demonstrates that the use of a proper heat sink (SiO2/Si substrate) prevents heat accumulation in the magnetic film. In this way, AOS was established with a common laser oscillator instead of a complex and expensive laser amplifier system for the first time. 5. Conclusions
In summary, we have demonstrated all-optical helicity dependent magnetic switching in a Tb30Fe70 thin film using a laser oscillator with 5.1 MHz repetition rate. To overcome laserinduced heat accumulation in the metallic film, it was deposited on a SiO2/Si substrate, which acts as an effective heat sink. In contrast, an identical film grown on a glass microscope slide reveals no AOS. This work paves the way for the implementation of AOS with robust and easy to use laser oscillators, e.g. with compact fiber lasers for future ultrafast data storage. Acknowledgments
We thank M. Daniel for RBS characterization, J. von Borany for access to and R. Wilhelm for technical support at the RBS beam line at Helmholtz-Zentrum Dresden Rossendorf, and A. Neudert for initial experimental assistance. M. Radek, R. Frieling, H. Bracht, S. Fähler, and M. Heidernätsch are gratefully acknowledged for valuable discussions. We acknowledge support by Deutsche Forschungsgemeinschaft and Open Access Publication Fund of University of Muenster.
#204784 - $15.00 USD (C) 2014 OSA
Received 14 Jan 2014; revised 31 Mar 2014; accepted 5 Apr 2014; published 18 Apr 2014 21 April 2014 | Vol. 22, No. 8 | DOI:10.1364/OE.22.010017 | OPTICS EXPRESS 10025
