Microwave Assisted Magnetic Recording Utilizing Perpendicular Spin

IEEE TRANSACTIONS ON MAGNETICS, VOL. 46, NO. 3, MARCH 2010

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Microwave Assisted Magnetic Recording Utilizing Perpendicular Spin Torque Oscillator With Switchable Perpendicular Electrodes Jian-Gang Zhu, Fellow, IEEE, and Yiming Wang Data Storage Systems Center, Department of ECE, Carnegie Mellon University, Pittsburgh, PA 15213 USA In this paper, we present a systematic micromagnetic modeling study on microwave assisted magnetic recording (MAMR) utilizing a perpendicular spin torque oscillator (PSTO) to generate localized circular ac field. The PSTO at narrow track width naturally produces a circular field rotating at desired high frequency within the plane of the media, resulting in a substantially enhanced effective write field gradient. With the STT oscillator generated ac field waveform, recording simulations show that area recording density exceeding 3 Tbits/in2 can be readily achieved with a medium signal-to-noise ratio above 13 dB (1T-SNR) and thermal stability at u B 300 K .

K V=

60k T

Index Terms—Magnetic recording, MAMR, PSTO, spin transfer torque, STT, STT oscillator.

I. INTRODUCTION

AGNETIC recording assisted by a localized ac field at ferromagnetic resonance frequency of the medium magnetic grains has been proposed [1]. By using a perpendicular spin torque oscillator (PSTO) to generate very localized ac field, the microwave assisted magnetic recording (MAMR) should enable a significant increase of area data storage density in hard disk drives (HDD) [1], [2]. If successful, the scheme would only require very minimal change on current HDD technology in contrast to some other proposed technologies, such as heat assisted magnetic recording (HAMR) and bit patterned media (BPM). Microwave magnetic field assisted magnetization reversal has -Co particles [3] and been demonstrated experimentally for a variety of magnetic thin films with various coercivities [4], [13]–[15]. Most notably, magnetization switching of a perpendicular Co/Pd multilayer film with coercivity value approaching that of the perpendicular thin film media used in present hard disk drives has been demonstrated with the assist of an external ac field at ferromagnetic resonance frequency [5]. In this paper, we present a micromagnetic modeling study of MAMR with a narrow track width PSTO. Dynamic spin configurations of the field generating layer (FGL) in the PSTO are simulated. The generated ac field, measured in recording media, is used in recording modeling. The area recording density capability is assessed in terms of the recording performance and medium signal-to-noise ratio of small grain size and high coercivity perpendicular thin film media.

M

II. EFFECT OF CIRCULAR AC FIELD Microwave assisted magnetic recording relies on an ac field with its frequency matching the ferromagnetic resonance frequency of a recording medium so that its magnetization can

Manuscript received September 06, 2009; accepted October 08, 2009. Current version published February 18, 2010. Corresponding author: J.-G. Zhu (e-mail: [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TMAG.2009.2036588

Fig. 1. Calculated switching field threshold of a single domain medium grain for a 1 ns pulsed external field applied at 30 angle w.r.t. the easy axis. The ac field is applied in the plane normal to the easy axis.

be switched by a recording field significantly below the coercivity of the medium. Using a circular ac field instead of a linear one, the switching becomes significantly more effective, provided the chirality of the ac field matches that of the medium magnetization precession [6]–[8]. Fig. 1 shows a calculation for switching a single domain medium grain with a uniaxial magnetic anisotropy. The switching field threshold plotted is the minimum magnitude of a pulsed recording field that is applied at 30 angle w.r.t. the easy axis of the grain. The field pulse duration is 1 ns with 0.2 ns rise time. Two types of ac fields are utilized here: a linear (blue curve) one and circular (red curve) , where one, both having the same amplitude: is the anisotropy field of the grain. Negative frequency indicates that the chirality of the circular ac field is opposite to that of the grain natural magnetization precession. As shown in the figure, the switching field threshold is significantly lower for the circular ac field. The results also show little dependence on the phase of the ac field. The circular ac field yields faster switching comparing to the linear ac field with the same ac field amplitude. Fig. 2 shows the comparison of the easy axis magnetization component as a function of time for the linear and circular ac field cases along with the corresponding magnetization trajectory for each case. The ac

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Fig. 4. Mapping of the initial magnetization orientation for irreversible reversal = 0:13H . (red) with a 1 ns pulsed recording field having a magnitude of H and an angle of 30 w.r.t. the easy axis (z-axis, out of the paper here). The ac field amplitude is 0:1H for both cases. Fig. 2. Left: calculated magnetization reversal for the circular (red) and linear (blue) ac fields. The amplitude of the both ac fields is 0:1H while the recording field is 0:13H . pulsed field of a duration of 1 ns and a rise time of 0.2 ns, applied with a 30 angle w.r.t. the easy axis. Right: corresponding magnetization trajectories.

The nature of precessional switching leads to the fact that the switching becomes initial magnetization orientation sensitive, especially in the case of linear ac field. Fig. 4 shows the mapping of the initial magnetization orientation for irreversible magnetization reversal with a recording field of a pulse width 1 ns and a . The map for the circular ac field case is submagnitude stantially denser comparing with that for the linear ac field one and it virtually ensure switching within the recording field pulse considering addition of any thermal excitation. The observed probabilistic switching at recording field exceeding switching field threshold [9] should not be a concern for the circular ac field case in practice. III. CIRCULAR AC FIELD AND CHIRALITY SWITCHING

Fig. 3. Calculate switching field thresholds as a function of the Gilbert damping constant of a single domain medium grain for circular and linear ac fields. The recording field is applied at an angle of 30 w.r.t. the easy axis. The ac field amplitude is 0:1H where H is the anisotropy field of the medium grain.

fields had been on prior to the application of the pulsed reversal field. Here the switching time is defined as the time to reach the onset of the irreversible switching. At the onset, the magnetization precession reverses chirality and the perpendicular reversal will continue even if the recording field is removed after this point. With this definition, the switching time for the circular ac field case is significantly shorter than that for the linear ac field. It is important to note that the reversal shows very little dependence on the phase of the ac fields relative to the rise of the recording field, namely no synchronization between the ac field and the recording field is needed. With a linear ac field, to yield a speedy switching with recording field magnitude significantly below the medium nominal coercivity requires the Gilbert damping constant of medium grains to be relatively small [1], [6]. Such requirement is significantly relaxed with a circular ac field. Fig. 3 shows the switching field threshold of the 1 ns pulsed field, as a function of the Gilbert damping constant of a single domain medium grain. With the circular ac field, a switching can occur within a for an individual medium grain. nanosecond even with

Fig. 5 shows a PSTO, for generating the ac field, placed in between the write pole and the trailing shield of a perpendicular recording head. In the field generating layer (FGL) of the PSTO, the magnetization should be essentially in the film plane [6] for an injected dc current at optimal current density. The magnetization rotation in the FGL would generate a circularly rotating field in the media along the center of the track due to the corresponding magnetic poles resulted on the bottom and side edges of the FGL, as illustrated in the figure. The ellipticity of the generated circular ac field by the FGL is calculated and mapped in the middle of medium plane, assuming the FGL has a track width of 30 nm and a thickness of 15 nm. The different colors indicate opposite chirality of the rotating ac field. In the calculation, a 5 nm head medium magnetic spacing and a 15 nm thick medium is assumed. The detailed structure of the PSTO is shown in Fig. 6. The PSTO consists of a current polarization layer (reference layer), a non-magnetic interlayer which could be an either metallic layer or tunnel barrier, a high moment oscillating layer (FGL), and a perpendicular layer [1]. Both the polarization and the perpendicular layers should have relatively strong perpendicular anisotropy [1], [10] and their magnetization should be switchable by the stray field of the recording head which is essentially perpendicular to the plane of the PSTO. The stray field of a present perpendicular recording head at the oscillator should have a magnitude greater than 1 Tesla and perpendicular to the plane of the oscillator. The direction of injected DC current remains unchanged while the magnetic electrodes are switched.

ZHU AND WANG: MAMR UTILIZING PSTO WITH SWITCHABLE PERPENDICULAR ELECTRODES

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Fig. 7. Top: calculated in-plane component of the magnetization in a FGL, oscillating at a frequency of 48 GHz. Bottom: the corresponding head field profile (thick) and the instantaneous frequency of the FGL magnetization oscillation as a function of time. The FGL is assumed to have a emu/cm and a thickness of 15 nm. The stray field at the oscillator is approximately 11 kOe. The erg/cm , perpendicular layer has an anisotropy energy constant of emu/cm , and a thickness of 5 nm.

Ms = 500

Fig. 5. Top: illustration of the integration of a PSTO ac field generator with a present perpendicular recording head. The detailed structure of the generator is shown in Fig. 6. The oscillating layer, also referred to as the field generating layer (FGL) is assumed to have a thickness of 10 nm and track width of 25 nm. Middle: illustration for a magnetization rotation in the FGL of the circular ac field generated in the middle of media plane. Bottom: a calculated mapping of the ellipticity of the generated rotating ac field inside medium.

Fig. 6. Schematic view of a PSTO with the perpendicular electodes switchable by the recording head stray field. The polarization layer is also referred to as the reference layer. The interlayer can be either metallic or tunnel barrier. Both the polarization and the perpendicular layers are of intrinsic perpendicular anisotropy.

To switch the chirality of magnetization rotation in the oscillating layer requires magnetization switching of both the spin polarization layer and the perpendicular layer, which takes on the order of 100 picoseconds. Fig. 7 shows a calculation for a STT oscillator integrated with a recording head as shown in Fig. 5. The recording head field switches its polarity every nanosecond with a rise time of 0.2 ns and the magnetization rotation in the FGL clearly switches its chirality in responding to the head field. The calculated instantaneous frequency of the

Ms = 1700 K = 4 2 10

FGL magnetization rotation clearly lags behind the recording field in terms of reaching the final stable oscillation frequency, 48 GHz in this case. The major cause of the delay is a combination of the transient switching of the perpendicular electrodes and the time taken to establish a stable oscillation in the FGL layer after the switching is completed. Fig. 8 shows magnetization profile for all three magnetic layers in the oscillator at a recording rate of 1 Gbits/second. The three rows, counting from the top, show the perpendicular magnetization component (w.r.t. the film plane of the oscillator) for the perpendicular layer (P), FGL (F), and polarization/reference layer (R), respectively. The bottom three rows show the corresponding in-plane magnetization component. Clearly, the magnetization of the perpendicular layer rotates with that of the FGL due to the intimate interlayer exchange coupling. The magnetization of the reference layer (spin polarization layer) follows the recording head field while the spin transfer torque causes both the perpendicular layer and the FGL to have a small perpendicular component opposite to that of the reference layer, against recording head field. It should be noted here that the Gilbert damping constant of the polarization layer is assumed to be on purpose for suppressing any spin transfer torque induced oscillation of its magnetization since any magnetization oscillation of the polarization layer will cause irregular and magnetization oscillation in FGL. In addition, are assumed for the FGL and the perpendicular layer, A/cm respectively. The dc current density is and polarization factor is assumed for spin polarized current. Fig. 9 shows a sequence of four transient magnetization configurations, four snap shots, of a FGL during a chirality switching. In this particular case, the FGL layer is 10 nm emu/cm . The perpendicular in thickness with layer is assumed to be 5 nm in thickness and with an intrinsic erg/cm . All three magnetic anisotropy constant layers in the oscillator are discretized laterally with mesh cells, nm thickness in size in the microeach cell of nm magnetic simulation. The lateral cross-section of the oscillator

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Fig. 8. Magnetization components of the three magnetic layers in a STT ac field generator with (R) denoting the polarization layer, (F) the field generating layer, and (P) the perpendicular layer. The recording head field data rate is at 1 Gbits/second with a rise time of 0.2 ns.

Fig. 9. A sequence of magnetization configurations in the FGL of an STT oscillator over several periods of oscillation during the chirality switching of the magnetization rotation.

is nm . Picture 1 shows the beginning of a chirality reverse from counterclockwise to clockwise. As the chirality of the magnetization rotation reverses, the rotation also become spatially non-uniform and the magnetic poles at the edges of the layer becomes out-of-phase. As the magnetization switching of the polarization layer completes, the spatial uniformity of the magnetization rotation quickly restores. Fig. 10 shows the dynamic magnetization configurations of the same FGL layer shown in Fig. 9 when steady oscillation is reached. The eight snap shots shown in the figure is over a single complete cycle. The magnetization rotation shows excellent spatial uniformity, even though the in-plane demagnetization field within the FGL Oe, and has a thickness of 10 nm and is on the order of emu/cm . One of the a saturation magnetization reasons for the spatial uniformity of the magnetization within the FGL during oscillation is the strong perpendicular field from the perpendicular layer and the strong stray field within the trailing shield gap. IV. EFFECTIVE HEAD FIELD GRADIENT One of the potential advantages of using the PSTO to produce a circular ac field in assisting recording is the resulting enhancements of effective write field gradient in both cross-track and down track directions. As shown in Fig. 5, the generated ac field is well circular at the center of the track and becomes essentially linear at the track edges in the medium. Considering the fact that the write efficiency of a circular ac field is essentially twice as the linear one in terms of the ac field amplitude, the recording track width can be effectively defined by the width of the ac field generator while the physical width of the recording head field can be substantially wider and no tapering of the pole in the down track direction is needed. The strong dependence of effective write field on the ellipticity of the rotating ac field also yield significant enhancement of effective write field gradient in the down track direction.

Fig. 10. A sequence of simulated magnetization configurations over a complete cycle of magnetization rotation in the FGL layer when steady oscillation is reached. The color indicates the magnetic pole densities. The FGL is 10 nm in thickness and has a saturation magnetization of emu/cm .

M = 1750

Fig. 11 shows calculated two dimensional mapping of effective switching field of a single domain medium grain placed at the middle plane of a medium. The magnetic head-medium spacing nm. The switching field is calcuis assumed to be lated utilizing the directional distribution of the head field with


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Fig. 12. Calculated maximum down track gradient of the effective field, measured at the center of the recording track, for a spatial uniform applied linearly polarized ac field (left), an ac field generated by an infinite wide STT oscillator (middle), and an narrow track (25 nm wide) STT oscillator (right) which generates a circular ac field pattern similar to the one shown in Fig. 5. The recording head field is the same one shown in Fig. 11.

Fig. 11. Top: calculated mapping of the effective recording head field alone. (a): Effective write field with a spatially uniform applied ac field; (b): with an STT ac field generator of a wide track width; (c): with an STT ac field generator of a 25 nm track width. For all the three cases, the ac field amplitude are the same with the same frequency f = 50 GHz. The effective write field is calculated using an isolated single domain grain with the same recording shown at the top.

a track width of 120 nm and a trailing shield gap of 35 nm, calculated using a finite element method (FEM) based commercial software with saturation effect included. The medium grain is assumed to be 17 nm in height (thickness of the medium) and a soft underlayer with relatively high permeability is placed 20 nm below the bottom surface of the medium. For a spatially uniformly ac field, case (a) in the figure, the write field is the strongest at the perimeter of the write pole where the recording head field vector is tilted at relatively large angles w.r.t. the easy axis of the medium grain. For a STT oscillator with infinitely wide track width, case (b), the strongest effective field occurs near the trailing side of the write pole where the horizontal component of the ac field maximizes (only on the side towards the write pole). Note with the infinitely wide FGL of the oscillator, the ac field in the media is purely linear and the horizontal component of the ac field peaks at both sides of the FGL in the down track direction. For a narrow track FGL, case (c) where the track width is 25 nm, the resulting on-track ac field is circular. Since

the chirality of the rotating ac field in the medium is opposite on the opposite sides of the FGL in the down track direction (as shown in Fig. 5), only the side towards the write pole provides the switching field reduction. The result is a well confined effective field in both down-track and cross-track directions. The enhancement of the effective write field gradient is substantial in the case of narrow track FGL producing circular on-track ac field. Fig. 12 shows the effective write field gradient at the center of the track, corresponding to the case (c) shown in Fig. 11. For the recording head, the write current is set at such that the maximum head field magnitude (recording head only) in the medium is 12 kOe while the amplitude of the ac field is at 1500 Oe. With the ac field frequency at 50 GHz, the resulting maximum effective write field gradient is 3250 Oe/nm, assuming an anisotropy field of 30 kOe for the medium is assumed for grain Note a Gilbert damping constant the medium grain. The on-track effective write field gradient for the cases (a) and (b) are also plotted for comparison. V. RECORDING SIMULATIONS In this section, we present a systematic recording simulation. A trailing shield perpendicular recording head of a 200 nm track width without downtrack tapering (as shown in Fig. 11) is employed. The trailing shield gap is 35 nm. The maximum head field in the medium is 12,000 Oe. The head field rise time (i.e. the same for the stray field on the oscillator) is 0.2 ns. A headnm is assumed. Squaremedium magnetic spacing of wave 1T recording is simulated with a data rate of 500 Mbits/ second. The integration of the recording head and a STT oscillator shown in Fig. 5 is assumed. The anisotropy and saturation magnetization for the polarization and the perpendicular layers erg/cm and emu/cm . are the same: emu/cm with a thickThe FGL is assumed to have ness of 15 nm and a track width of 25 nm and the steady state GHz. The calculation shows oscillating frequency is that the time for the ac field generator to reach stable oscillation after a recording head field switching is around 0.4 ns.

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W = 27 nm

Fig. 14. Calculated 1T medium signal-to-noise ratio (SNR) as a function of . linear density. The actual recording track width is

for MAMR, smaller medium damping constant means higher switching speed [1], [6]. Fig. 13. Recording simulation at 3 MFCI linear density. The resulting track width is approximately 27 nm. Top: resulting medium magnetization pattern averaged over 20 cases. Bottom: a typical individual run.

VI. CONCLUSION AND REMARKS

The medium is modeled by a single layer of magnetic grains with uniaxial anisotropy with the easy axes randomly distributed within a 6 cone angle perpendicular to medium plane. The center-to-center distance between adjacent grains is 5.5 nm with a grain boundary thickness of 0.5 nm. The saturation magnetiemu/cm . A zation of the grains is assumed to be Gaussian distribution is assumed for the grain anisotropy field with a mean of 30 kOe and a standard deviation of 3%. Full magnetostatic interactions among the grains are included along with a spatial uniform intergranular exchange coupling [1], [2]. An intermediate layer of 20 nm in thickness is assumed in between the medium and a SUL. The energy barrier of each for . grain in the media is estimated exceed . The Gilbert damping constant of the medium is Fig. 13 shows the recorded magnetization pattern averaged over 20 runs (top) and a representative individual run (bottom) at a linear density of 3MFCI. The resulting track width is only slightly greater than that of the FGL width and no notable erase band with a squeeze test simulation. The track edge is relatively sharp and there is no observable extension of the writing beyond the recording track. Fig. 14 shows the calculated medium signalnoise-ratio (SNR) as a function of linear density (Note that the SNR is calculated at 1T linear density.) As shown in the figure, an SNR of 15 dB is obtained corresponding to an area density of Tbits/in , 2.5 Tbits/in . At 3.5 MFCI, which correspond to dB is obtained. an For the recording simulation results presented above, we purposely assumed the relatively large Gilbert damping constant for the medium, although it is unclear that the actual medium constant would be as large as the value used here. Ferromagnetic resonance based experimental measurements have reported for CoPtCr perpendicular media [11] and for FePt perpendicular film [12], both significantly smaller than that used in the above recording simulations. Note that

We have presented a systematic micromagnetic modeling investigation on microwave assisted magnetic recording. In particular, the study focuses on the effect of the circular ac field generated by a narrow track width PSTO with magnetic electrodes switched by the stray field of the recording head. We found that a circular ac field is much more effective for reducing the switching field of medium grains. It is also found that the recording track width can be well defined by the width of the FGL of the oscillator, largely due to the circular ac field produced. The resulting track width is essentially the same as the physical width of the FGL. No phase synchronization between the ac field and recording field are necessary. The circular ac field produced by the oscillator design also yields extremely high on-track write field gradient. Recording simulation shows that a 1T medium SNR of 13 dB can be obtained at 3 Tbits/in area density on a medium with an anisotropy field of kOe and 5 nm size grains of sufficient thermal magnetic stability. It is concluded that with most practical considerations, the technology should enable the recording density to . reach beyond 3 It is important to note that the term MAMR could be misleading since the recording is merely assisted by ac field at a frequency matching the ferromagnetic resonance (FMR) frequency of the recording media. The localized ac field is generated by a PSTO is near field in nature and no electromagnetic (EM) wave should be radiated since the dimension of the FGL is many orders of magnitude smaller than the wavelength of the corresponding EM wave. In another word, the energy transfer from the oscillator to the medium grains is strictly in a near field sense, involving no radiation or wave propagation. Since the ferromagnetic exchange coupling between the magnetic grains is much weaker than the anisotropy field, no spin wave propagation in the medium has been observed in the recording simulation. At the required ac field frequencies, eddy current in the medium layers could be significant. For media with recording


layer consisting of oxide grain boundaries, the lateral conductivity could be sufficiently small. Whereas the eddy current induced in intermediate layer(s) and SUL could generate additional ac field in the recording layer on top. Such effect could be favorable to the recording since the field produced by the eddy current would partially cancel the perpendicular component while strengthen the horizontal component of the ac field produced by the FGL. For a more exact assessment of the effect, a quantitative study focusing on the effect is certainly needed. The most significant advantage of MAMR is that it requires very minimal technological change. It could be a good intermediate technology to succeed the present conventional perpendicular recording while the other more sophisticated ones, such as HAMR and BPM, are to be developed into practice. The key to the success of the proposed MAMR scheme resides on the validity of the simulated PSTO performance. A successful practical realization of the PSTO ac field generator could essentially guarantee the viability of the MAMR technology.

ACKNOWLEDGMENT This research was supported in part by the industrial sponsors of the Data Storage Systems Center at Carnegie Mellon University. In particular, the authors would like to thank Dr. K. Gao for helping with the project.

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