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Nanoscale layer-selective readout of magnetization direction from a magnetic multilayer using a spin-torque oscillator
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Nanotechnology Nanotechnology 25 (2014) 245501 (8pp)
doi:10.1088/0957-4484/25/24/245501
Nanoscale layer-selective readout of magnetization direction from a magnetic multilayer using a spin-torque oscillator Hirofumi Suto, Tazumi Nagasawa, Kiwamu Kudo, Koichi Mizushima and Rie Sato 1
Corporate Research and Development Center, Toshiba Corporation, Komukai-Toshiba-cho, Saiwai-ku, Kawasaki 212-8582, Japan E-mail: [email protected] Received 16 January 2014, revised 3 April 2014 Accepted for publication 17 April 2014 Published 28 May 2014 Abstract
Technology for detecting the magnetization direction of nanoscale magnetic material is crucial for realizing high-density magnetic recording devices. Conventionally, a magnetoresistive device is used that changes its resistivity in accordance with the direction of the stray field from an objective magnet. However, when several magnets are near such a device, the superposition of stray fields from all the magnets acts on the sensor, preventing selective recognition of their individual magnetization directions. Here we introduce a novel readout method for detecting the magnetization direction of a nanoscale magnet by use of a spin-torque oscillator (STO). The principles behind this method are dynamic dipolar coupling between an STO and a nanoscale magnet, and detection of ferromagnetic resonance (FMR) of this coupled system from the STO signal. Because the STO couples with a specific magnet by tuning the STO oscillation frequency to match its FMR frequency, this readout method can selectively determine the magnetization direction of the magnet. Keywords: spin-torque oscillator, three-dimensional magnetic recording, ferromagnetic resonance (Some figures may appear in colour only in the online journal) 1. Introduction
recording density achievable by this scheme is approaching its physical limit, where further reducing the size of data bits will result in spontaneous magnetic switching due to thermal fluctuation, preventing the data from being stably stored [1–3]. Three-dimensional magnetic recording has been proposed to surpass this limit [4–12]. This method employs multiple vertically stacked magnetic recording layers as the media. Such recording is the only way to increase recording density while keeping the size of data bits large enough to store data stably. As for the writing procedure, several simulation studies on layer-selective methods have been reported [11, 12], triggered by the emergence of microwaveassisted recording [13–15]. In addition to these writing methods, a new readout method with layer selectivity is necessary for realizing three-dimensional magnetic recording.
Detecting the magnetization direction of nanoscale magnetic material has various applications. Among them, a read head element of magnetic recording devices such as hard disk drives (HDDs) is of great importance because these devices underpin information technologies, owing to their ability to store and access massive amounts of data. In a conventional magnetic recording device, data are stored as the magnetization directions of data bits composed of small magnetic particles fabricated on magnetic storage media. For reproducing data, a magnetoresistive (MR) sensor that changes its resistivity in response to the direction of the stray field from these data bits is used. To meet growing demand for higher data capacity, reduction in the size of data bits and improvements to the head elements have been explored. However, the 0957-4484/14/245501+08$33.00
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enables layer selectivity. To distinguish the magnetization direction, an additional magnetic field along the easy axis of data bits (readout field) is applied. This field modulates the FMR frequency of a data bit, which causes it to depend on whether the field and the magnetization direction are parallel or antiparallel and allows the STO signal to distinguish the magnetization direction. By tuning the oscillation frequency, the STO couples only with recording layer 1 and the STO signal changes in accordance with the magnetization direction of data bits in recording layer 1, as shown in figure 1(b). By changing the oscillation frequency, the STO couples with a recording layer 2 that has a different FMR frequency from recording layer 1. In this instance, the STO signal changes in accordance with the magnetization direction of data bits in recording layer 2. Instead of tuning the STO frequency, the use of multiple STOs, each designed to couple with a specific recording layer, may be possible. Detailed analysis of how the STO signal changes and the experimental demonstration of this are the focus of this paper and are discussed in later sections. Several research studies have already explored STOs as a read head element in HDDs and a magnetic field sensor [20–22]. Their principle is different from that presented in this paper and is worth mentioning to avoid confusion. The method reported in the previous research detects a static stray field from a bit magnet in the same manner as a conventional MR sensor, and use of an STO can bring about several improvements such as enhancement of the signal-to-noise ratio. On the other hand, the STO resonance readout presented in this paper uses the FMR phenomenon instead of stray-field detection and, owing to this novel principle, attains layer selectivity.
Figure 1. (a) Schematics of a read head element and a multilayer
medium with two recording layers. Recording layers 1 and 2 have FMR frequencies of f1 and f2, respectively. (b) Output signals from the STO during the readout procedure of the data bit shown in (a). Under operation condition 1, the oscillation frequency changes in accordance with the magnetization direction of data bits in recording layer 1 that stores ‘1010’. Under operation condition 2, the oscillation signal reflects the magnetization direction of recording layer 2 and reproduces ‘1100’.
This is because the conventional readout method based on the detection of stray fields senses the superposition of stray fields from the data bits in all recording layers, and thus it cannot selectively read the data stored in a specific recording layer. In an attempt to solve this problem, we recently reported studies on ferromagnetic resonance (FMR) excitation of magnetic multilayer films through using a vector network analyzer (VNA) [16, 17]. By providing each recording layer with a different FMR frequency, microwave signal matching to one of these frequencies can excite FMR in a specific recording layer; subsequently, detecting the absorption of microwave signals by FMR excitation can selectively reveal the magnetization direction of the layer. The absorption of the microwave signal is, however, proportional to the volume of the magnet. Nanoscale data bits in magnetic storage media are therefore too small for a VNA to detect absorption of a microwave signal strong enough to reveal the magnetization direction. In this paper, we introduce and experimentally demonstrate a novel method for reading out magnetization direction from a nanoscale magnet by using a spin-torque oscillator (STO) [18, 19]. Figure 1(a) illustrates its readout principle. An STO is employed in a read head element and placed so close to a data bit in a multilayer medium that magnetization motions of the STO and the data bit influence each other through dynamic dipolar interaction. As a result, coupled FMR mode emerges and is manifested in the output signal from the STO. Because an STO is a nanoscale device, it couples with a nanoscale magnet that is too small to be detected by VNA-FMR measurement, while utilizing FMR
2. Experimental details We prepared a nanoscale stack consisting of an STO, a spacer, and two perpendicularly magnetized magnetic films, as shown in figure 2. This setup imitates a three-dimensional magnetic recording device, and the STO, the spacer, and the set of perpendicular magnets respectively correspond to a read head element, an air gap, and a multilayer recording medium. The upper perpendicular magnet is designed to have a higher FMR frequency than the lower perpendicular magnet. Hereafter, they are respectively referred to as the hard layer and the soft layer. These two layers have nearly the same magnetic moment and are antiferromagnetically coupled (AFC) by insertion of a Ru layer between them [23, 24], allowing the static stray fields from the two magnets to cancel each other. The purpose of employing the AFC structure is to suppress the influence of its static stray fields on the magnetization dynamics of the STO, which helps to clearly detect the dynamic coupling through dipolar interaction. As an initial step to fabricate this sample, a continuous film having this stacking structure was deposited on a sapphire substrate by using a magnetron sputtering system and subsequently annealed at 300 °C for 1 h in a vacuum under a magnetic field of +6000 Oe. Then the film was patterned into a nanoscale 2
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Because the antiferromagnetic coupling establishes a spontaneous antiferromagnetic configuration, it shows almost zero magnetization near H z = 0 Oe, indicating that the net stray field from the AFC magnets is also suppressed. In addition to the switching of the AFC magnets, the magnetic hysteresis loop shows slope over the measured H z range. This is attributed to in-plane magnetized layers of the STO film, in which the magnetization gradually aligns toward the +z direction as H z increases. The slope changes around H z = ±9000 Oe, where the free-layer magnetization is saturated along the +z direction, whereas magnetizations of the in-plane magnetized pinned-layer films still point in a tilted direction. The free layer has perpendicular anisotropy induced at the interface with a MgO barrier layer [25, 26], which reduces the H z necessary to align the magnetization along the +z direction. On the other hand, the pinned layers require larger H z due to the bias field induced by the IrMn and Ru layers. This saturation field can be interpreted as effective perpendicular magnetic anisotropy of a film HeffPMA = −9000 Oe for the free layer. Here HeffPMA is given by H PMA − 4πM sat , where H PMA is perpendicular magnetic anisotropy and M sat is saturation magnetization. Although the hard and soft layers have the same total Co thickness, slight magnetization exists at H z = 0 Oe. This is explained by the contribution of the interfacial Pt atoms, polarized by the neighbouring Co atoms [27–29]. The hard layer has larger magnetization because it possesses more Co/Pt interfaces. This also accounts for the higher perpendicular magnetic anisotropy of the hard layer. Figures 3(b) and (c) show VNA-FMR measurement results for sample A obtained by sweeping H x (magnetic field along the x-axis) and H z , respectively. The background spectra measured by applying H y (magnetic field along the yaxis) = +2000 Oe and H z = −10000 Oe, respectively, were subtracted. In figure 3(b), signals corresponding to the freelayer FMR are observed. The FMR frequency of the pinned layers is higher than the shown frequency range due to the bias field induced by the IrMn and Ru layers. Also, FMR signals from the AFC magnets cannot be identified because they have little H x dependence. In contrast, signals corresponding to the hard-layer and soft-layer FMR with linear H z dependence are observed in figure 3(c). Discontinuities and sign switching in the slope of the FMR frequency correspond to switching of the AFC magnets, which occurs at almost the same H z as in the VSM measurements. A system composed of two magnets coupled through exchange bias has two coupled FMR modes whose frequencies are modified from their intrinsic FMR frequencies [30, 31]. This modification becomes evident when the intrinsic FMR frequencies of two magnets are close and the exchange coupling field is strong. In our AFC magnets, however, the hard-layer anisotropy field is much larger than both the soft-layer anisotropy field and the exchange coupling field. The FMR frequencies can therefore be treated independently and fitted by the Kittel equation. From the fitting, magnetic characteristics such as HeffPMA , the bias field of antiferromagnetic coupling H bias (for the hard and soft layers only), and the Néel coupling field H Néel [32] (for
Figure 2. Sample structure and experimental setup. The sample
consists of an STO, a spacing layer, and AFC perpendicular magnets. Two samples having the following film structure for AFC magnets were fabricated: sample A [(Pt6/Co15.5)2/Pt6/Ru8.5/ Co4.43/(Pt6/Co8.86)3/Pt6], and sample B [(Pt6/Co14)2/Pt6/Ru8.5/ Co4/(Pt6/Co8)3/Pt6]. All thicknesses are given in angstroms. Blue arrows denote the direction of H1ext and H2ext , which are applied during STO measurements. A red arrow denotes the current direction (positive current flows from the pinned layer to the free layer).
elliptic pillar by electron-beam lithography and ion milling. The longer axis of the pillar and the field direction during the annealing process were both along the x-axis. The milling process removed the AFC magnets and the spacing layer, together with the MR film, until the etched surface reached the middle of the IrMn layer. We prepared two samples (A and B) with different film structures for AFC magnets. Both the hard and soft layers of sample A were designed to have lower perpendicular anisotropy than those of sample B. Prior to pillar formation, the magnetic characteristics of the films were investigated by magnetic hysteresis loop measurements using a vibrating sample magnetometer (VSM) and by FMR measurements using a VNA. Details of the VNA-FMR measurement setup are given in [17]. As for the pillar sample, signals from an STO were obtained by applying a direct current I DC and a magnetic field. Microwave signals generated by resistance oscillation of the STO were separated and amplified by a conventional circuit consisting of a bias tee and an amplifier, and measured by a spectrum analyzer. All measurements were carried out at room temperature.
3. Results and discussion 3.1. Film characterization
In this section, we present magnetic characteristics of the continuous films before fabrication into nanoscale pillars. Figure 3(a) shows a magnetic hysteresis loop for sample A. Sweeping H z (magnetic field along the z-axis) from −15 to +15 kOe shows the sequential magnetization switching of the soft layer followed by the hard-layer switching, changing from saturation along the −z direction to a head-to-head antiferromagnetic configuration and then to saturation along the +z direction. The tail-to-tail antiferromagnetic configuration is also obtained for the opposite field sweep direction. 3
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Figure 3. (a) Perpendicular M-H loop for sample A, reflecting magnetization switching of the perpendicular magnetized hard and soft layers, and gradual magnetization tilt of the in-plane magnetized free and pinned layers toward the perpendicular-to-plane direction. Because the soft-layer magnetization switches due to the antiferromagnetic coupling field induced by the Ru layer, a spontaneous antiferromagnetic configuration is stable in the remanent state. The inset shows a minor loop. VNA reflection spectra for sample A as a function of (b) H x and (c) H z and (d) for sample B as a function of H z . The field sweep direction is from left to right (also in figures 4(a), (d), 6(a), and (b)). Broken lines denote fitting by the Kittel equation. Table 1. Magnetic characteristics of film samples evaluated by VNA-
We carried out an identical measurement for sample B, as shown in figure 3(d). Because the STO films are identical in both samples, only the AFC magnets were evaluated. The FMR frequencies of the AFC magnets are higher than those of sample A, which is consistent with the film structures. Table 1 also shows the magnetic characteristics of sample B.
FMR measurement. Sample A
PMA eff
H H
bias
Soft layer
Hard layer
300 Oe 1400 Oe
8800 Oe 1100 Oe
Free layer PMA Heff N éel H
3.2. Coupling between the STO free layer and soft layer through dipolar interaction
−9000 Oe −75 Oe
In experiments on the film samples, dipolar fields emitted at the edges of the film are negligible because the lateral size is much larger than the thickness. By patterning the film into a nanoscale pillar, this dipolar field influences magnetization motions. We next use pillar samples and apply H x and I DC to excite free-layer magnetization within the modest amplitude in which the trajectory of the free-layer magnetization evolves from that of its FMR mode. The H x dependence of the oscillation frequency is expected to coincide with the freelayer FMR frequency in figure 3(b), which increases up to
Sample B
PMA Heff
H
bias
Soft layer
Hard layer
2000 Oe 1700 Oe
13000 Oe 1200 Oe
the free layer only) are estimated as shown in table 1. The value of HeffPMA for the free layer is in agreement with that estimated by the VSM measurement. 4
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direction suppresses higher harmonics of the STO signal as the magnetization trajectory becomes asymmetric with respect to the pinned-layer magnetization direction. In addition, the output power is enhanced because the trajectory utilizes larger resistance change. The sample is initialized at H z = −10 kOe prior to the measurement so that the AFC magnets have a head-to-head configuration. Strong STO signals are obtained in the range of H1ext = +200 to +750 Oe, where the sample resistance is high. Considering the current direction, this signal corresponds to the magnetization excitation of the free layer. The STO frequency increases as H1ext increases, and this is accompanied by a gap near H1ext = + 450 Oe and 4 GHz. This gap originates from dynamic coupling of the free layer and the soft layer and corresponds to the condition where the oscillation frequency crosses the soft-layer FMR frequency [33, 34]. Because the free layer has its easy axis along the x-axis and H1ext is applied almost parallel to the x-axis, the precessional axis of the freelayer magnetization is near the x-axis. In this case, the ycomponent of the magnetization oscillates due mainly to the elliptical trajectory of an in-plane magnetized film. On the other hand, the precessional axis of the soft layer is near the zaxis, and the x- and y-components of the magnetization oscillate, although H1ext tilts its precessional axis to some extent. Therefore, the two magnetizations couple through the y-component of their magnetization. A coupled mode below the gap can be assigned to an acoustic oscillation mode in which the magnetizations have anti-phase y-components, and a coupled mode above the gap can be assigned to an optical oscillation mode in which the magnetizations have in-phase ycomponents. In the field range H1ext = +500 to +1500 Oe, where strong optical mode signals are observed, very weak acoustic mode signals appear around 4 GHz. This can be attributed to the thermally excited FMR of the soft layer reflected in the STO signals through dipolar interaction. This soft-layer FMR frequency of the pillar is slightly lower than that of the unpatterned film (figure 3(c)), which might be explained by an additional stray field from the hard layer and the change in the self-demagnetizing field of the soft layer introduced by pillar formation. In addition, damage during the milling process may have some effect. Under the assumption that these factors can be included in HeffPMA , it is estimated to be 1050 Oe. Figure 4(d) shows H1ext dependence of the PSD for sample B, obtained by applying I DC = −0.8 mA. In contrast with the case of sample A, no gap is observed because the STO frequency does not cross the soft-layer FMR frequency. The FMR frequency of the unpatterned soft layer estimated from the VNA-FMR measurements is approximately 10 GHz, which is too high to cross the STO oscillation frequency. Comparing the results from the two samples provides evidence that the gap appears when the STO oscillation frequency matches the FMR frequency of an adjacent magnet. Although stray fields from the AFC magnets cancel each other, there are remaining stray fields acting on the STO because the STO and the AFC magnets are placed so close
ext
Figure 4. (a) PSDs of STO signals for sample A as a function of H1 ,
obtained by applying I DC = −0.8 mA. A broken line denotes the calculated FMR frequencies of the soft layer. The STO signal shows a gap where the STO frequency crosses the soft-layer FMR frequency. (b) Corresponding sample resistance. (c) Typical PSDs representing an acoustic mode, an optical mode, and their transitions. (d) PSDs of STO signals for sample B obtained for the same measurement conditions as (a).
10 GHz as H x increases. The FMR frequency of the soft layer (figure 3(c)) is in this frequency range, so these frequencies cross where the coupling between the free layer and the soft layer is expected to occur. Figures 4(a) and (b) show in-plane magnetic field H1ext dependence of the power spectrum density (PSD) for sample A, obtained by applying I DC = −0.8 mA and the corresponding sample resistance, respectively. Figure 4(c) shows typical PSDs in figure 4(a). Here H1ext is tilted from the +x direction by 20° in the x-y plane (figure 2). This field 5
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agreement with the experimental results and reproduce the characteristic gap of the STO signal, showing that a gap in STO signals appears when at least dipolar interaction acts between the STO and the adjacent magnet, although contribution from other interactions cannot be excluded. This result indicates that the readout method based on the coupling of magnetization dynamics between an STO and an adjacent magnet is applicable to a read head element and a medium in HDDs. 3.3. Detection of magnetization direction by STO measurements with in-plane and perpendicular-to-plane external fields
As mentioned in the previous section, switching in AFC magnets does not change STO signals because the STO free layer couples equally with the soft layer regardless of its magnetization direction. To use this coupling in distinguishing the magnetization direction, we apply a perpendicular-toplane readout field in combination with an in-plane magnetic field and conduct STO measurements. The in-plane magnetic field is necessary to establish the magnetization oscillation in the STO, whereas the readout field modulates the FMR frequency of the soft layer. The FMR frequency increases when the readout field and the soft-layer magnetization direction are parallel and decreases in the antiparallel case. Figures 6(a) and (b) show H2ext dependence of PSD for sample A. Here H2ext is tilted from the +x direction by 20° in the x-y plane and from the x-y plane by 25° (figure 2). The two results are obtained after initializing the sample with H z = −10 and +10 kOe, respectively. The gaps shift as a result of modulation in the soft-layer FMR frequency as compared with the results obtained by applying the in-plane field H1ext (figure 4(a)), and the soft-layer magnetization direction can be distinguished from the STO signal. The STO signals either jump up to the optical mode or stay in the acoustic mode at H2ext = +475 Oe, depending on the magnetization direction of the soft layer. This change in oscillation modes provides drastic change in the STO signal, as shown in figure 6(c), where two spectral peaks show no overlap. When an STO operates under such a condition, the STO signal reproduces data without field sweeping and behaves as depicted in figure 1(b). In figure 6(a), another gap is observed at H2ext = + 650 Oe and 5.5 GHz, where the oscillation frequency becomes twice as high as the soft-layer FMR frequency. This might be explained by parametric coupling between the STO free layer and the soft layer. In the sample used in this study, a current flows perpendicular-to-plane through an STO, a spacing layer, and AFC magnets. However, it may be difficult to maintain this configuration in a flying head because an electrode needs to be fabricated on its air-bearing surface. The thickness of this electrode increases the distance between the read head and the media, resulting in significant deterioration of the readout performance. From a practical viewpoint, the current direction and the stacking direction of the STO film must be parallel to the media surface. Figure 1(a) illustrates one example that
Figure 5. Simulated PSDs of a scalar product of the free-layer and
the pinned-layer magnetization as a function of H1ext , by applying approximately I DC = −0.3 mA. Free-layer H PMA and H N éel and softlayer H bias are estimated from VNA-FMR measurements (table 1), and soft-layer H PMA is estimated from thermally excited FMR of the patterned soft layer (figure 4(a)). Other parameters are as follows: α = 0.02 in the free layer and the soft layer, M sat = 850 emu cm−3 in the free layer, and M sat = 900 emu cm−3 in the soft layer. These are typical values except for free-layer M sat , which is smaller than the value reported for the bulk material. This accounts for the inhomogeneity of the magnetization in the free layer. PSDs are obtained by averaging 100 Fourier transforms for a 26.5 ns time trace.
that the distance between the hard and soft layers is nonnegligible. To estimate the influence of the remaining static stray fields on the STO, the same measurements as in figure 4(a) are carried out after initializing the sample at H z = +10 kOe, which establishes the tail-to-tail configuration in the AFC magnets. The results (not shown) are almost the same as those for the head-to-head configuration in the AFC magnets, showing that the remaining stray fields from the AFC magnets have little influence on the magnetization dynamics of the STO. Thus far, we have focused on dipolar interaction as the origin of gaps in STO signals. Other interactions originating from spin-torque and electrical signals, however, might have some influence because in our samples AFC magnets as well as STOs are patterned into the same pillar and a current flows through both. For employing the readout method based on dynamic coupling in a read head element in HDDs, it is necessary for the coupling to be established dominantly by dipolar interaction. Because a read head element and a medium are separated by a thin air gap and no current flows between them, only dipolar interaction acts between them. To evaluate this issue, we performed macrospin simulations and calculated the time evolution of the magnetization motions of a free layer and a soft layer. In this simulation model, only the dipolar field was taken into account as interaction between the two layers. Figure 5 shows calculated PSDs of a scalar product of the free-layer and the pinnedlayer magnetization. These calculation results show overall 6
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measure it experimentally, simulation studies on microwaveassisted switching aided the estimation. These studies reported the time duration required for magnetization switching as typically less than 1 ns, within which magnetization excitation by a microwave magnetic field occurs [14, 15]. This also implies that the microwave field emitted by the STO excites magnetization precession in a data bit within the same order of time, resulting in change in the STO signal.
4. Conclusion We have introduced a novel readout method to detect the magnetization direction of a nanoscale magnet by using an STO and presented both theoretical aspects and experimental demonstration of the method. This is a promising readout method for realizing three-dimensional magnetic recording with multiple recording layers because the method can selectively determine the magnetization direction of data bits in a specific recording layer. For the experimental demonstration, we prepared a nanoscale pillar comprising an STO and two adjacent perpendicular magnets and measured the STO signal. The STO and perpendicular magnets respectively correspond to a read head element and a multilayer recording medium. Because the two perpendicular magnets have different FMR frequencies, the STO free layer selectively couples with one of them by tuning the oscillation frequency to match its FMR frequency. This coupling appears as a gap in the STO signal, which corresponds to the transition from an acoustic oscillation mode to an optical oscillation mode. Macrospin simulations show that, among several interactions between the STO and the perpendicular magnets, it is mainly dipolar interaction that contributes to the coupling. This means that the readout method based on the coupling of magnetization dynamics between an STO and an adjacent magnet is applicable to a read head element and a medium separated by a thin air gap in an HDD. By combining the STO measurements and a readout field, the gap shifts in accordance with the magnetization direction of the perpendicular magnet, allowing the STO signals to distinguish the magnetization direction. By choosing the conditions that emphasize the difference in the STO signals, the spectral peaks obtained for the different magnetization directions are separated completely, thereby showing that this method can clearly distinguish the magnetization direction.
ext
Figure 6. PSDs of STO signals for sample A as a function of H2 ,
obtained by applying I DC = −0.8 mA for the AFC magnets having (a) the head-to-head and (b) the tail-to-tail configuration. Broken lines denote the calculated FMR frequencies of the soft layer. The STO signals show a gap where the STO frequency crosses the soft-layer FMR frequency. Switching of the soft layer occurred around H2ext = +700 Oe in (a), as shown by a white dotted line. (c) Comparison of PSDs obtained for the two magnetization states in the AFC magnets.
satisfies this condition. Similar to a write head element proposed for microwave-assisted recording, an STO with perpendicularly magnetized material is employed, allowing excitation of the magnetization oscillation around the stacking direction [35–38]. This magnetization trajectory shares an oscillating component with FMR precession of a perpendicular magnet in a medium, enabling them to couple. Finally, we would like to comment on data transfer rate, which is an important performance index for recording devices as well as recording density. In three-dimensional recording, the data transfer rate is determined by the time duration required for an STO signal to cause detectable change due to coupling. Although we were not able to
Acknowledgments We thank H Kubota, H Imamura, and S Okamoto for helpful discussions. This work is partially supported by Strategic Promotion of Innovative Research and Development from Japan Science and Technology Agency, JST. 7
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Appendix. Macrospin simulation Magnetization dynamics of the free layer and the soft layer are calculated using the Landau–Lifshitz–Gilbert– Slonczewski (LLGS) equation dmj dt
(
)
= −γ mj × H jeff + αjmj ×
dmj dt
+ T jspin − torque, (A.1)
for j = FL (free layer), SL (soft layer). Here γ is the gyromagnetic ratio, mj is the normalized magnetization vector, and αj is the Gilbert damping constant. The third term represents spin torque, which acts only on the free layer, spin − torque spin − torque = 0. TFL = σI DCmFL × ( mFL × mPL ) and TSL σ The spin-transfer efficiency is defined as sat σ = γ ℏ (2 e MFL VFL ) × P (1 + P 2mFL ⋅ mPL ), where ℏ is the Planck constant, e is the elementary electric charge, VFL is the volume of the free layer, and P = 0.65 is the polarization factor. The pinned-layer magnetization mPL is fixed to the −x direction. The effective magnetic field Hjeff is the sum of the external field H ext , bias field H bias (soft layer only), Néel coupling field H N éel (free layer only), anisotropy field Hjani , demagnetizing field Hjdemag , and thermal random field Hjthermal at 300 K. The demagnetizing field is expressed as H jdemag =
mk . ∑ −4πMksatN jdemag ,k
(A.2)
k
Here k = FL, SL, HL (hard layer) denotes the layer creating the field. N jdemag is the demagnetizing tensor calculated by a ,k three-dimensional model of 1 × 1 × 1 nm cells. This term represents a self-demagnetizing field for j = k and a dipolar field for j ≠ k. The hard-layer magnetization mHL is fixed to the −z direction.
References [1] Sharrock M P 1994 J. Appl. Phys. 76 6413–8 [2] Charap S H, Lu P-L and He Y 1997 IEEE Trans. Magn. 33 978–83 [3] Richter H J, Brockie R M and Pressesky J L 2002 IEEE Trans. Magn. 38 260–70 [4] Yuan Z and Liu B 2001 J. Magn. Magn. Mater. 235 481–6 [5] Albrecht M, Hu G, Moser A, Hellwig O and Terris B D 2005 J. Appl. Phys. 97 103910 [6] Baltz V, Landis S, Rodmacq B and Dieny B 2005 J. Magn. Magn. Mater. 290–291 1286–9 [7] Khizroev S, Hijazi Y, Amos N, Chomko R and Litvinov D 2006 J. Appl. Phys. 100 063907 [8] Baltz V, Bollero A, Rodmacq B, Dieny B, Jamet J-P and Ferré J 2007 Eur. J. Appl. Phys. 39 33–8 [9] Sato R, Mizushima K, Nagasawa T and Kudo K 2011 Threedimensional magnetic recording and playback device Patent Specification WO/2011/030449
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Nanoscale layer-selective readout of magnetization direction from a magnetic multilayer using a spin-torque oscillator
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Nanotechnology 25 245501 (http://iopscience.iop.org/0957-4484/25/24/245501) View the table of contents for this issue, or go to the journal homepage for more
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Nanotechnology Nanotechnology 25 (2014) 245501 (8pp)
doi:10.1088/0957-4484/25/24/245501
Nanoscale layer-selective readout of magnetization direction from a magnetic multilayer using a spin-torque oscillator Hirofumi Suto, Tazumi Nagasawa, Kiwamu Kudo, Koichi Mizushima and Rie Sato 1
Corporate Research and Development Center, Toshiba Corporation, Komukai-Toshiba-cho, Saiwai-ku, Kawasaki 212-8582, Japan E-mail: [email protected] Received 16 January 2014, revised 3 April 2014 Accepted for publication 17 April 2014 Published 28 May 2014 Abstract
Technology for detecting the magnetization direction of nanoscale magnetic material is crucial for realizing high-density magnetic recording devices. Conventionally, a magnetoresistive device is used that changes its resistivity in accordance with the direction of the stray field from an objective magnet. However, when several magnets are near such a device, the superposition of stray fields from all the magnets acts on the sensor, preventing selective recognition of their individual magnetization directions. Here we introduce a novel readout method for detecting the magnetization direction of a nanoscale magnet by use of a spin-torque oscillator (STO). The principles behind this method are dynamic dipolar coupling between an STO and a nanoscale magnet, and detection of ferromagnetic resonance (FMR) of this coupled system from the STO signal. Because the STO couples with a specific magnet by tuning the STO oscillation frequency to match its FMR frequency, this readout method can selectively determine the magnetization direction of the magnet. Keywords: spin-torque oscillator, three-dimensional magnetic recording, ferromagnetic resonance (Some figures may appear in colour only in the online journal) 1. Introduction
recording density achievable by this scheme is approaching its physical limit, where further reducing the size of data bits will result in spontaneous magnetic switching due to thermal fluctuation, preventing the data from being stably stored [1–3]. Three-dimensional magnetic recording has been proposed to surpass this limit [4–12]. This method employs multiple vertically stacked magnetic recording layers as the media. Such recording is the only way to increase recording density while keeping the size of data bits large enough to store data stably. As for the writing procedure, several simulation studies on layer-selective methods have been reported [11, 12], triggered by the emergence of microwaveassisted recording [13–15]. In addition to these writing methods, a new readout method with layer selectivity is necessary for realizing three-dimensional magnetic recording.
Detecting the magnetization direction of nanoscale magnetic material has various applications. Among them, a read head element of magnetic recording devices such as hard disk drives (HDDs) is of great importance because these devices underpin information technologies, owing to their ability to store and access massive amounts of data. In a conventional magnetic recording device, data are stored as the magnetization directions of data bits composed of small magnetic particles fabricated on magnetic storage media. For reproducing data, a magnetoresistive (MR) sensor that changes its resistivity in response to the direction of the stray field from these data bits is used. To meet growing demand for higher data capacity, reduction in the size of data bits and improvements to the head elements have been explored. However, the 0957-4484/14/245501+08$33.00
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© 2014 IOP Publishing Ltd Printed in the UK
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enables layer selectivity. To distinguish the magnetization direction, an additional magnetic field along the easy axis of data bits (readout field) is applied. This field modulates the FMR frequency of a data bit, which causes it to depend on whether the field and the magnetization direction are parallel or antiparallel and allows the STO signal to distinguish the magnetization direction. By tuning the oscillation frequency, the STO couples only with recording layer 1 and the STO signal changes in accordance with the magnetization direction of data bits in recording layer 1, as shown in figure 1(b). By changing the oscillation frequency, the STO couples with a recording layer 2 that has a different FMR frequency from recording layer 1. In this instance, the STO signal changes in accordance with the magnetization direction of data bits in recording layer 2. Instead of tuning the STO frequency, the use of multiple STOs, each designed to couple with a specific recording layer, may be possible. Detailed analysis of how the STO signal changes and the experimental demonstration of this are the focus of this paper and are discussed in later sections. Several research studies have already explored STOs as a read head element in HDDs and a magnetic field sensor [20–22]. Their principle is different from that presented in this paper and is worth mentioning to avoid confusion. The method reported in the previous research detects a static stray field from a bit magnet in the same manner as a conventional MR sensor, and use of an STO can bring about several improvements such as enhancement of the signal-to-noise ratio. On the other hand, the STO resonance readout presented in this paper uses the FMR phenomenon instead of stray-field detection and, owing to this novel principle, attains layer selectivity.
Figure 1. (a) Schematics of a read head element and a multilayer
medium with two recording layers. Recording layers 1 and 2 have FMR frequencies of f1 and f2, respectively. (b) Output signals from the STO during the readout procedure of the data bit shown in (a). Under operation condition 1, the oscillation frequency changes in accordance with the magnetization direction of data bits in recording layer 1 that stores ‘1010’. Under operation condition 2, the oscillation signal reflects the magnetization direction of recording layer 2 and reproduces ‘1100’.
This is because the conventional readout method based on the detection of stray fields senses the superposition of stray fields from the data bits in all recording layers, and thus it cannot selectively read the data stored in a specific recording layer. In an attempt to solve this problem, we recently reported studies on ferromagnetic resonance (FMR) excitation of magnetic multilayer films through using a vector network analyzer (VNA) [16, 17]. By providing each recording layer with a different FMR frequency, microwave signal matching to one of these frequencies can excite FMR in a specific recording layer; subsequently, detecting the absorption of microwave signals by FMR excitation can selectively reveal the magnetization direction of the layer. The absorption of the microwave signal is, however, proportional to the volume of the magnet. Nanoscale data bits in magnetic storage media are therefore too small for a VNA to detect absorption of a microwave signal strong enough to reveal the magnetization direction. In this paper, we introduce and experimentally demonstrate a novel method for reading out magnetization direction from a nanoscale magnet by using a spin-torque oscillator (STO) [18, 19]. Figure 1(a) illustrates its readout principle. An STO is employed in a read head element and placed so close to a data bit in a multilayer medium that magnetization motions of the STO and the data bit influence each other through dynamic dipolar interaction. As a result, coupled FMR mode emerges and is manifested in the output signal from the STO. Because an STO is a nanoscale device, it couples with a nanoscale magnet that is too small to be detected by VNA-FMR measurement, while utilizing FMR
2. Experimental details We prepared a nanoscale stack consisting of an STO, a spacer, and two perpendicularly magnetized magnetic films, as shown in figure 2. This setup imitates a three-dimensional magnetic recording device, and the STO, the spacer, and the set of perpendicular magnets respectively correspond to a read head element, an air gap, and a multilayer recording medium. The upper perpendicular magnet is designed to have a higher FMR frequency than the lower perpendicular magnet. Hereafter, they are respectively referred to as the hard layer and the soft layer. These two layers have nearly the same magnetic moment and are antiferromagnetically coupled (AFC) by insertion of a Ru layer between them [23, 24], allowing the static stray fields from the two magnets to cancel each other. The purpose of employing the AFC structure is to suppress the influence of its static stray fields on the magnetization dynamics of the STO, which helps to clearly detect the dynamic coupling through dipolar interaction. As an initial step to fabricate this sample, a continuous film having this stacking structure was deposited on a sapphire substrate by using a magnetron sputtering system and subsequently annealed at 300 °C for 1 h in a vacuum under a magnetic field of +6000 Oe. Then the film was patterned into a nanoscale 2
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Because the antiferromagnetic coupling establishes a spontaneous antiferromagnetic configuration, it shows almost zero magnetization near H z = 0 Oe, indicating that the net stray field from the AFC magnets is also suppressed. In addition to the switching of the AFC magnets, the magnetic hysteresis loop shows slope over the measured H z range. This is attributed to in-plane magnetized layers of the STO film, in which the magnetization gradually aligns toward the +z direction as H z increases. The slope changes around H z = ±9000 Oe, where the free-layer magnetization is saturated along the +z direction, whereas magnetizations of the in-plane magnetized pinned-layer films still point in a tilted direction. The free layer has perpendicular anisotropy induced at the interface with a MgO barrier layer [25, 26], which reduces the H z necessary to align the magnetization along the +z direction. On the other hand, the pinned layers require larger H z due to the bias field induced by the IrMn and Ru layers. This saturation field can be interpreted as effective perpendicular magnetic anisotropy of a film HeffPMA = −9000 Oe for the free layer. Here HeffPMA is given by H PMA − 4πM sat , where H PMA is perpendicular magnetic anisotropy and M sat is saturation magnetization. Although the hard and soft layers have the same total Co thickness, slight magnetization exists at H z = 0 Oe. This is explained by the contribution of the interfacial Pt atoms, polarized by the neighbouring Co atoms [27–29]. The hard layer has larger magnetization because it possesses more Co/Pt interfaces. This also accounts for the higher perpendicular magnetic anisotropy of the hard layer. Figures 3(b) and (c) show VNA-FMR measurement results for sample A obtained by sweeping H x (magnetic field along the x-axis) and H z , respectively. The background spectra measured by applying H y (magnetic field along the yaxis) = +2000 Oe and H z = −10000 Oe, respectively, were subtracted. In figure 3(b), signals corresponding to the freelayer FMR are observed. The FMR frequency of the pinned layers is higher than the shown frequency range due to the bias field induced by the IrMn and Ru layers. Also, FMR signals from the AFC magnets cannot be identified because they have little H x dependence. In contrast, signals corresponding to the hard-layer and soft-layer FMR with linear H z dependence are observed in figure 3(c). Discontinuities and sign switching in the slope of the FMR frequency correspond to switching of the AFC magnets, which occurs at almost the same H z as in the VSM measurements. A system composed of two magnets coupled through exchange bias has two coupled FMR modes whose frequencies are modified from their intrinsic FMR frequencies [30, 31]. This modification becomes evident when the intrinsic FMR frequencies of two magnets are close and the exchange coupling field is strong. In our AFC magnets, however, the hard-layer anisotropy field is much larger than both the soft-layer anisotropy field and the exchange coupling field. The FMR frequencies can therefore be treated independently and fitted by the Kittel equation. From the fitting, magnetic characteristics such as HeffPMA , the bias field of antiferromagnetic coupling H bias (for the hard and soft layers only), and the Néel coupling field H Néel [32] (for
Figure 2. Sample structure and experimental setup. The sample
consists of an STO, a spacing layer, and AFC perpendicular magnets. Two samples having the following film structure for AFC magnets were fabricated: sample A [(Pt6/Co15.5)2/Pt6/Ru8.5/ Co4.43/(Pt6/Co8.86)3/Pt6], and sample B [(Pt6/Co14)2/Pt6/Ru8.5/ Co4/(Pt6/Co8)3/Pt6]. All thicknesses are given in angstroms. Blue arrows denote the direction of H1ext and H2ext , which are applied during STO measurements. A red arrow denotes the current direction (positive current flows from the pinned layer to the free layer).
elliptic pillar by electron-beam lithography and ion milling. The longer axis of the pillar and the field direction during the annealing process were both along the x-axis. The milling process removed the AFC magnets and the spacing layer, together with the MR film, until the etched surface reached the middle of the IrMn layer. We prepared two samples (A and B) with different film structures for AFC magnets. Both the hard and soft layers of sample A were designed to have lower perpendicular anisotropy than those of sample B. Prior to pillar formation, the magnetic characteristics of the films were investigated by magnetic hysteresis loop measurements using a vibrating sample magnetometer (VSM) and by FMR measurements using a VNA. Details of the VNA-FMR measurement setup are given in [17]. As for the pillar sample, signals from an STO were obtained by applying a direct current I DC and a magnetic field. Microwave signals generated by resistance oscillation of the STO were separated and amplified by a conventional circuit consisting of a bias tee and an amplifier, and measured by a spectrum analyzer. All measurements were carried out at room temperature.
3. Results and discussion 3.1. Film characterization
In this section, we present magnetic characteristics of the continuous films before fabrication into nanoscale pillars. Figure 3(a) shows a magnetic hysteresis loop for sample A. Sweeping H z (magnetic field along the z-axis) from −15 to +15 kOe shows the sequential magnetization switching of the soft layer followed by the hard-layer switching, changing from saturation along the −z direction to a head-to-head antiferromagnetic configuration and then to saturation along the +z direction. The tail-to-tail antiferromagnetic configuration is also obtained for the opposite field sweep direction. 3
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Figure 3. (a) Perpendicular M-H loop for sample A, reflecting magnetization switching of the perpendicular magnetized hard and soft layers, and gradual magnetization tilt of the in-plane magnetized free and pinned layers toward the perpendicular-to-plane direction. Because the soft-layer magnetization switches due to the antiferromagnetic coupling field induced by the Ru layer, a spontaneous antiferromagnetic configuration is stable in the remanent state. The inset shows a minor loop. VNA reflection spectra for sample A as a function of (b) H x and (c) H z and (d) for sample B as a function of H z . The field sweep direction is from left to right (also in figures 4(a), (d), 6(a), and (b)). Broken lines denote fitting by the Kittel equation. Table 1. Magnetic characteristics of film samples evaluated by VNA-
We carried out an identical measurement for sample B, as shown in figure 3(d). Because the STO films are identical in both samples, only the AFC magnets were evaluated. The FMR frequencies of the AFC magnets are higher than those of sample A, which is consistent with the film structures. Table 1 also shows the magnetic characteristics of sample B.
FMR measurement. Sample A
PMA eff
H H
bias
Soft layer
Hard layer
300 Oe 1400 Oe
8800 Oe 1100 Oe
Free layer PMA Heff N éel H
3.2. Coupling between the STO free layer and soft layer through dipolar interaction
−9000 Oe −75 Oe
In experiments on the film samples, dipolar fields emitted at the edges of the film are negligible because the lateral size is much larger than the thickness. By patterning the film into a nanoscale pillar, this dipolar field influences magnetization motions. We next use pillar samples and apply H x and I DC to excite free-layer magnetization within the modest amplitude in which the trajectory of the free-layer magnetization evolves from that of its FMR mode. The H x dependence of the oscillation frequency is expected to coincide with the freelayer FMR frequency in figure 3(b), which increases up to
Sample B
PMA Heff
H
bias
Soft layer
Hard layer
2000 Oe 1700 Oe
13000 Oe 1200 Oe
the free layer only) are estimated as shown in table 1. The value of HeffPMA for the free layer is in agreement with that estimated by the VSM measurement. 4
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direction suppresses higher harmonics of the STO signal as the magnetization trajectory becomes asymmetric with respect to the pinned-layer magnetization direction. In addition, the output power is enhanced because the trajectory utilizes larger resistance change. The sample is initialized at H z = −10 kOe prior to the measurement so that the AFC magnets have a head-to-head configuration. Strong STO signals are obtained in the range of H1ext = +200 to +750 Oe, where the sample resistance is high. Considering the current direction, this signal corresponds to the magnetization excitation of the free layer. The STO frequency increases as H1ext increases, and this is accompanied by a gap near H1ext = + 450 Oe and 4 GHz. This gap originates from dynamic coupling of the free layer and the soft layer and corresponds to the condition where the oscillation frequency crosses the soft-layer FMR frequency [33, 34]. Because the free layer has its easy axis along the x-axis and H1ext is applied almost parallel to the x-axis, the precessional axis of the freelayer magnetization is near the x-axis. In this case, the ycomponent of the magnetization oscillates due mainly to the elliptical trajectory of an in-plane magnetized film. On the other hand, the precessional axis of the soft layer is near the zaxis, and the x- and y-components of the magnetization oscillate, although H1ext tilts its precessional axis to some extent. Therefore, the two magnetizations couple through the y-component of their magnetization. A coupled mode below the gap can be assigned to an acoustic oscillation mode in which the magnetizations have anti-phase y-components, and a coupled mode above the gap can be assigned to an optical oscillation mode in which the magnetizations have in-phase ycomponents. In the field range H1ext = +500 to +1500 Oe, where strong optical mode signals are observed, very weak acoustic mode signals appear around 4 GHz. This can be attributed to the thermally excited FMR of the soft layer reflected in the STO signals through dipolar interaction. This soft-layer FMR frequency of the pillar is slightly lower than that of the unpatterned film (figure 3(c)), which might be explained by an additional stray field from the hard layer and the change in the self-demagnetizing field of the soft layer introduced by pillar formation. In addition, damage during the milling process may have some effect. Under the assumption that these factors can be included in HeffPMA , it is estimated to be 1050 Oe. Figure 4(d) shows H1ext dependence of the PSD for sample B, obtained by applying I DC = −0.8 mA. In contrast with the case of sample A, no gap is observed because the STO frequency does not cross the soft-layer FMR frequency. The FMR frequency of the unpatterned soft layer estimated from the VNA-FMR measurements is approximately 10 GHz, which is too high to cross the STO oscillation frequency. Comparing the results from the two samples provides evidence that the gap appears when the STO oscillation frequency matches the FMR frequency of an adjacent magnet. Although stray fields from the AFC magnets cancel each other, there are remaining stray fields acting on the STO because the STO and the AFC magnets are placed so close
ext
Figure 4. (a) PSDs of STO signals for sample A as a function of H1 ,
obtained by applying I DC = −0.8 mA. A broken line denotes the calculated FMR frequencies of the soft layer. The STO signal shows a gap where the STO frequency crosses the soft-layer FMR frequency. (b) Corresponding sample resistance. (c) Typical PSDs representing an acoustic mode, an optical mode, and their transitions. (d) PSDs of STO signals for sample B obtained for the same measurement conditions as (a).
10 GHz as H x increases. The FMR frequency of the soft layer (figure 3(c)) is in this frequency range, so these frequencies cross where the coupling between the free layer and the soft layer is expected to occur. Figures 4(a) and (b) show in-plane magnetic field H1ext dependence of the power spectrum density (PSD) for sample A, obtained by applying I DC = −0.8 mA and the corresponding sample resistance, respectively. Figure 4(c) shows typical PSDs in figure 4(a). Here H1ext is tilted from the +x direction by 20° in the x-y plane (figure 2). This field 5
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agreement with the experimental results and reproduce the characteristic gap of the STO signal, showing that a gap in STO signals appears when at least dipolar interaction acts between the STO and the adjacent magnet, although contribution from other interactions cannot be excluded. This result indicates that the readout method based on the coupling of magnetization dynamics between an STO and an adjacent magnet is applicable to a read head element and a medium in HDDs. 3.3. Detection of magnetization direction by STO measurements with in-plane and perpendicular-to-plane external fields
As mentioned in the previous section, switching in AFC magnets does not change STO signals because the STO free layer couples equally with the soft layer regardless of its magnetization direction. To use this coupling in distinguishing the magnetization direction, we apply a perpendicular-toplane readout field in combination with an in-plane magnetic field and conduct STO measurements. The in-plane magnetic field is necessary to establish the magnetization oscillation in the STO, whereas the readout field modulates the FMR frequency of the soft layer. The FMR frequency increases when the readout field and the soft-layer magnetization direction are parallel and decreases in the antiparallel case. Figures 6(a) and (b) show H2ext dependence of PSD for sample A. Here H2ext is tilted from the +x direction by 20° in the x-y plane and from the x-y plane by 25° (figure 2). The two results are obtained after initializing the sample with H z = −10 and +10 kOe, respectively. The gaps shift as a result of modulation in the soft-layer FMR frequency as compared with the results obtained by applying the in-plane field H1ext (figure 4(a)), and the soft-layer magnetization direction can be distinguished from the STO signal. The STO signals either jump up to the optical mode or stay in the acoustic mode at H2ext = +475 Oe, depending on the magnetization direction of the soft layer. This change in oscillation modes provides drastic change in the STO signal, as shown in figure 6(c), where two spectral peaks show no overlap. When an STO operates under such a condition, the STO signal reproduces data without field sweeping and behaves as depicted in figure 1(b). In figure 6(a), another gap is observed at H2ext = + 650 Oe and 5.5 GHz, where the oscillation frequency becomes twice as high as the soft-layer FMR frequency. This might be explained by parametric coupling between the STO free layer and the soft layer. In the sample used in this study, a current flows perpendicular-to-plane through an STO, a spacing layer, and AFC magnets. However, it may be difficult to maintain this configuration in a flying head because an electrode needs to be fabricated on its air-bearing surface. The thickness of this electrode increases the distance between the read head and the media, resulting in significant deterioration of the readout performance. From a practical viewpoint, the current direction and the stacking direction of the STO film must be parallel to the media surface. Figure 1(a) illustrates one example that
Figure 5. Simulated PSDs of a scalar product of the free-layer and
the pinned-layer magnetization as a function of H1ext , by applying approximately I DC = −0.3 mA. Free-layer H PMA and H N éel and softlayer H bias are estimated from VNA-FMR measurements (table 1), and soft-layer H PMA is estimated from thermally excited FMR of the patterned soft layer (figure 4(a)). Other parameters are as follows: α = 0.02 in the free layer and the soft layer, M sat = 850 emu cm−3 in the free layer, and M sat = 900 emu cm−3 in the soft layer. These are typical values except for free-layer M sat , which is smaller than the value reported for the bulk material. This accounts for the inhomogeneity of the magnetization in the free layer. PSDs are obtained by averaging 100 Fourier transforms for a 26.5 ns time trace.
that the distance between the hard and soft layers is nonnegligible. To estimate the influence of the remaining static stray fields on the STO, the same measurements as in figure 4(a) are carried out after initializing the sample at H z = +10 kOe, which establishes the tail-to-tail configuration in the AFC magnets. The results (not shown) are almost the same as those for the head-to-head configuration in the AFC magnets, showing that the remaining stray fields from the AFC magnets have little influence on the magnetization dynamics of the STO. Thus far, we have focused on dipolar interaction as the origin of gaps in STO signals. Other interactions originating from spin-torque and electrical signals, however, might have some influence because in our samples AFC magnets as well as STOs are patterned into the same pillar and a current flows through both. For employing the readout method based on dynamic coupling in a read head element in HDDs, it is necessary for the coupling to be established dominantly by dipolar interaction. Because a read head element and a medium are separated by a thin air gap and no current flows between them, only dipolar interaction acts between them. To evaluate this issue, we performed macrospin simulations and calculated the time evolution of the magnetization motions of a free layer and a soft layer. In this simulation model, only the dipolar field was taken into account as interaction between the two layers. Figure 5 shows calculated PSDs of a scalar product of the free-layer and the pinnedlayer magnetization. These calculation results show overall 6
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measure it experimentally, simulation studies on microwaveassisted switching aided the estimation. These studies reported the time duration required for magnetization switching as typically less than 1 ns, within which magnetization excitation by a microwave magnetic field occurs [14, 15]. This also implies that the microwave field emitted by the STO excites magnetization precession in a data bit within the same order of time, resulting in change in the STO signal.
4. Conclusion We have introduced a novel readout method to detect the magnetization direction of a nanoscale magnet by using an STO and presented both theoretical aspects and experimental demonstration of the method. This is a promising readout method for realizing three-dimensional magnetic recording with multiple recording layers because the method can selectively determine the magnetization direction of data bits in a specific recording layer. For the experimental demonstration, we prepared a nanoscale pillar comprising an STO and two adjacent perpendicular magnets and measured the STO signal. The STO and perpendicular magnets respectively correspond to a read head element and a multilayer recording medium. Because the two perpendicular magnets have different FMR frequencies, the STO free layer selectively couples with one of them by tuning the oscillation frequency to match its FMR frequency. This coupling appears as a gap in the STO signal, which corresponds to the transition from an acoustic oscillation mode to an optical oscillation mode. Macrospin simulations show that, among several interactions between the STO and the perpendicular magnets, it is mainly dipolar interaction that contributes to the coupling. This means that the readout method based on the coupling of magnetization dynamics between an STO and an adjacent magnet is applicable to a read head element and a medium separated by a thin air gap in an HDD. By combining the STO measurements and a readout field, the gap shifts in accordance with the magnetization direction of the perpendicular magnet, allowing the STO signals to distinguish the magnetization direction. By choosing the conditions that emphasize the difference in the STO signals, the spectral peaks obtained for the different magnetization directions are separated completely, thereby showing that this method can clearly distinguish the magnetization direction.
ext
Figure 6. PSDs of STO signals for sample A as a function of H2 ,
obtained by applying I DC = −0.8 mA for the AFC magnets having (a) the head-to-head and (b) the tail-to-tail configuration. Broken lines denote the calculated FMR frequencies of the soft layer. The STO signals show a gap where the STO frequency crosses the soft-layer FMR frequency. Switching of the soft layer occurred around H2ext = +700 Oe in (a), as shown by a white dotted line. (c) Comparison of PSDs obtained for the two magnetization states in the AFC magnets.
satisfies this condition. Similar to a write head element proposed for microwave-assisted recording, an STO with perpendicularly magnetized material is employed, allowing excitation of the magnetization oscillation around the stacking direction [35–38]. This magnetization trajectory shares an oscillating component with FMR precession of a perpendicular magnet in a medium, enabling them to couple. Finally, we would like to comment on data transfer rate, which is an important performance index for recording devices as well as recording density. In three-dimensional recording, the data transfer rate is determined by the time duration required for an STO signal to cause detectable change due to coupling. Although we were not able to
Acknowledgments We thank H Kubota, H Imamura, and S Okamoto for helpful discussions. This work is partially supported by Strategic Promotion of Innovative Research and Development from Japan Science and Technology Agency, JST. 7
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Appendix. Macrospin simulation Magnetization dynamics of the free layer and the soft layer are calculated using the Landau–Lifshitz–Gilbert– Slonczewski (LLGS) equation dmj dt
(
)
= −γ mj × H jeff + αjmj ×
dmj dt
+ T jspin − torque, (A.1)
for j = FL (free layer), SL (soft layer). Here γ is the gyromagnetic ratio, mj is the normalized magnetization vector, and αj is the Gilbert damping constant. The third term represents spin torque, which acts only on the free layer, spin − torque spin − torque = 0. TFL = σI DCmFL × ( mFL × mPL ) and TSL σ The spin-transfer efficiency is defined as sat σ = γ ℏ (2 e MFL VFL ) × P (1 + P 2mFL ⋅ mPL ), where ℏ is the Planck constant, e is the elementary electric charge, VFL is the volume of the free layer, and P = 0.65 is the polarization factor. The pinned-layer magnetization mPL is fixed to the −x direction. The effective magnetic field Hjeff is the sum of the external field H ext , bias field H bias (soft layer only), Néel coupling field H N éel (free layer only), anisotropy field Hjani , demagnetizing field Hjdemag , and thermal random field Hjthermal at 300 K. The demagnetizing field is expressed as H jdemag =
mk . ∑ −4πMksatN jdemag ,k
(A.2)
k
Here k = FL, SL, HL (hard layer) denotes the layer creating the field. N jdemag is the demagnetizing tensor calculated by a ,k three-dimensional model of 1 × 1 × 1 nm cells. This term represents a self-demagnetizing field for j = k and a dipolar field for j ≠ k. The hard-layer magnetization mHL is fixed to the −z direction.
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