Microwave-assisted Magnetic Recording Research Articles

Optimisation of microwave-assisted magnetic recording media for spin-torque oscillators with dual field generating layers

Optimisation of microwave-assisted magnetic recording media for spin-torque oscillators with dual field generating layers

Micromagnetic simulation of microwave-assisted magnetization switching and signal recording characteristics for exchange-coupled composite media with layer anisotropy structure

Microwave-assisted magnetic recording (MAMR) is a promising candidate for ultra-high-density recording devices. In this study, microwave-assisted magnetization switching (MAS) and MAMR characteristics of exchange-coupled composite media with layer anisotropy structures were numerically estimated. MAS characteristics showed that the order and duration of applied magnetic fields (head and microwave fields) could significantly affect the switching characteristics, depending on the layer anisotropy structure. The optimization of the layer anisotropy structure significantly improved microwave assistance for signal recording, resulting in a signal-to-noise ratio of 17 dB without increasing the magnetic write width.

Demonstration of substantial improvements in recording process with MAMR

Frequency matching is the most important key point for designing microwave-assisted switching (MAS) and microwave-assisted magnetic recording (MAMR). It is necessary to quantitatively compare an assist effect by MAS (MAS effect) of some media when designing and optimizing a medium from the viewpoint of frequency matching experimentally. However, it is difficult to directly compare the MAS effect between media because the differences in the resolution and saturation characteristics of the medium strongly influence the recording properties. In this study, we propose a new method for appropriately removing these effects other than the MAS effect by evaluating the relationship of recording characteristics between media. By using this method, the difference of the MAS effect between any media can be quantitatively compared and used as an index for medium design to optimize MAMR.

Micromagnetic model analysis of dual field generation layer (FGL) spin torque oscillators (STO) for microwave-assisted magnetic recording (MAMR)

Dual field generation layer (FGL) spin torque oscillators (STO) were investigated for use in microwave-assisted magnetic recording. The STOs were integrated into the gaps of write heads for stable oscillation. According to preliminary calculations, a FGL thickness of 5 nm was suitable, while the volume of 7.5 nm and 10 nm thick FGLs was too large and they did not oscillate stably. The most dominant factor influencing the FGL rotation was found to be the strength of the antiferromagnetic coupling between the FGLs. Other parameters were also varied, e.g., the exchange coupling between the negative spin injection layer (nSIL) and the trailing shield (TS), however, no significant effect was found. Unlike single FGL STOs, the oscillation frequency changed drastically, from 38 GHz to 73 GHz, on varying the injected current density to the STO. Recording simulations showed that the signal-to-noise ratio was maximized for an STO oscillation frequencies between 34 GHz and 50 GHz, depending on the recording media used in the calculations. Therefore, the dual FGL STO may be suitable for a wide range of recording media. Alternatively, the dual FGL STO may be used for dual-layer recording, where different resonance frequencies are needed for each of the layers, obviating the need for a second STO.

Hysteresis Loops of Recording Media Grains Under the Influence of High-Frequency Fields

Hysteresis loops of single grains and recording media were calculated under the influence of high-frequency (HF) magnetic fields, similar to those produced by a spin-torque oscillator (STO) in microwave-assisted magnetic recording (MAMR). The effect of the HF field ellipticity and the strength of magnetostatic interactions between grains were investigated. Elliptical HF fields with a major:minor axis ratio of 6:1 were almost as effective as circular HF fields in reducing the media coercivity. For media with a small grain pitch, i.e., strong magnetostatic interactions between grains, the HF field was able to reduce the coercivity by much more than when the grains were far apart, and magnetostatic interactions were weak.

Spin-torque oscillator with coupled out-of-plane oscillation layers for microwave-assisted magnetic recording: Experimental, analytical, and numerical studies

Spin-torque oscillators (STOs) are unique spintronic devices in which spin current induces the precession of magnetization in the oscillation layer. In this study, we demonstrate coupled out-of-plane oscillation between two oscillation layers within an STO. The coupled oscillation state is observed in the magnetoresistance spectra as well as its degradation due to the mixing of the free oscillation state depending on the bias current. These behaviors are analytically described and numerically reproduced based on the Landau–Lifshitz–Gilbert equation. Since the STO is fabricated in the write head of a hard disk drive, the coupled oscillation state and the subsequent microwave magnetic field are applicable for microwave-assisted magnetic recording for high-density recording.

Microwave-assisted switching in granular media with continuous magnetic overlayers for interaction control

Optimizing switching behavior under a microwave field is a key issue for microwave-assisted magnetic recording. We study the effect of inter-grain exchange coupling induced by a continuous magnetic layer on microwave-assisted switching of CoCrPt-SiO2 granular media. The microwave assistance effect is improved in the low-frequency region by introducing the inter-grain exchange coupling, but the coupling reduces the critical frequency. The maximum assistance effect is almost independent of the film structure since the dual effect cancels each other. Micromagnetic simulations also show similar results on exchange coupling between grains. These results indicate that the inter-grain exchange coupling can be used to optimize the critical frequency of the media in practical applications.

Multiple Spin Injection Into Coupled Field Generation Layers for Low Current Operation of MAMR Heads

Microwave-assisted magnetic recording (MAMR) is a promising technique for improving the recording density of hard disk drives (HDDs). We propose a spin-torque oscillator (STO) structure with two field generation layers (FGLs) and two spin-injection layers (SILs) for microwave generation. By using a macrospin model and micromagnetic simulations, we numerically demonstrate the low current operation of the STO. The nearly antiparallel oscillations of the FGLs, which make suitable microwave distribution for MAMR, are stabilized by slight modulation of the in-plane phases of the FGLs and synchronization of the SILs. The enlarged spacing between the FGLs enhances the microwave amplitude at the media plane. These results indicate that, by using the dual-SIL dual-FGL STO, write heads for MAMR can operate using little energy to achieve high recording density.

A Study of Channel Time-Domain Response on Equalization for Reproducing a Double-Layer Magnetic Recording Medium

To increase the recording density of hard disk drives (HDD), 3-D magnetic recording using microwave-assisted magnetic recording (MAMR) with multiple recording layers is attracting much attention as a next-generation recording method. Developing a signal processing method for 3-D magnetic recording using the multiple conventional recording layers and an magnetoresistive (MR) head reading system is desired. In this article, we evaluate 2-D partial response maximum likelihood (PRML) channels by the bit error rate (BER) performance, where the transfer function of the PR channel is determined by the response of the in-phase or differential-phase bits between layers in a double-layer magnetic recording medium. The results show that a BER of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$10^{-4}$ </tex-math></inline-formula> can be achieved by the PRML channel determined by the differential-phase response with the signal-to-noise ratio (SNR) greater than 26.0 dB for system noise.

Micromagnetic Simulation of L10-FePt-Based Exchange-Coupled-Composite-Bit-Patterned Media with Microwave-Assisted Magnetic Recording at Ultrahigh Areal Density.

In this work, we propose exchange-coupled-composite-bit-patterned media (ECC-BPM) with microwave-assisted magnetic recording (MAMR) to improve the writability of the magnetic media at a 4 Tb/in2 recording density. The suitable values of the applied microwave field’s frequency and the exchange coupling between magnetic dots, Adot, of the proposed media were evaluated. It was found that the magnitude of the switching field, Hsw, of the bilayer ECC-BPM is significantly lower than that of a conventional BPM. Additionally, using the MAMR enables further reduction of Hsw of the ECC-BPM. The suitable frequency of the applied microwave field for the proposed media is 5 GHz. The dependence of Adot on the Hsw was additionally examined, showing that the Adot of 0.14 pJ/m is the most suitable value for the proposed bilayer ECC-BPM. The physical explanation of the Hsw of the media under a variation of MAMR and Adot was given. Hysteresis loops and the magnetic domain of the media were characterized to provide further details on the results. The lowest Hsw found in our proposed media is 12.2 kOe, achieved by the bilayer ECC-BPM with an Adot of 0.14 pJ/m using a 5 GHz MAMR.

Magnetization dynamics of a flux control device fabricated in the write gap of a hard-disk-drive write head for high-density recording

The design concept of microwave-assisted magnetic recording (MAMR) using the flux control (FC) effect has been proposed as a technology for hard disk drives (HDDs). In this type of MAMR, the magnetization of an in-gap device (FC device) is reversed against the gap field by spin-transfer torque, enhancing the amplitude and gradient of the recording field. In this paper, we study the magnetization dynamics of an FC device fabricated in the write gap of an HDD write head. The operation of the FC device is analyzed by measuring the temporal resistance change in the sub-nanosecond region. Reversal of the FC device becomes faster as the bias current is increased and can be completed by 0.5 ns after the transition of the write current. The experimental results are reproduced by micromagnetic simulations using a head model, confirming that the simulations correctly describe the magnetization dynamics of the actual device. The simulations show that the recording field gain by the FC device appears with little delay after the rise of the recording field and that the FC device operates effectively even at a fast write rate of approximately 3 Gbit/s. Furthermore, we demonstrate the effectiveness of boosting the bias current, which can realize both fast and reliable operation of the FC device. These results indicate that the FC device operates as designed and that MAMR using the FC effect is promising for extending the recording density of HDDs.

MAMR writability and signal-recording characteristics on granular exchange-coupled composite media

Exchange-coupled composite (ECC) media present a possible layered structure for microwave-assisted magnetic recording (MAMR) systems. In this paper, MAMR writability and read/write characteristics were computed using a micromagnetic simulation, which assumed ECC media with graded and notched anisotropy structures. Writability was estimated from the switchable maximum averaged anisotropy field for footprint images. The calculation indicated 40–50% enhanced writability by microwave assistance for both graded and notched anisotropy structure media with suitable magnetic parameters. A read/write simulation indicated that notched-structure media with strongly coupled magnetic layers tended to be advantageous for signal-to-noise ratio and indicated well-confined recorded bits with a narrow magnetic write width.

Performance Improvement of Dual STO Reading With AM–FM Combined Detection for 3-D Magnetic Recording

3-D magnetic recording using microwave-assisted magnetic recording (MAMR) and a spin-torque oscillator (STO) for reading has been proposed as a candidate for prospective recording technologies. In 3-D magnetic recording using STO as a reading, the temporal magnetization dynamics of the STO has the fluctuations for frequency and amplitude which depend on the recorded pattern. In our previous work, we have constructed the STO signal model considering both amplitude and frequency fluctuations to develop a combined signal processing using the amplitude and frequency fluctuation detections. We have applied the STO signal model to the low-density parity-check (LDPC) coding and iterative decoding system with the individual detectors for amplitude modulation (AM) and frequency modulation (FM). Furthermore, we have reported that the dual STO reading system with only AM detection provides the improvement of the bit error rate (BER) performance. In this study, we propose the dual STO reading system with AM-FM combined detection in the hope that the finite impulse response (FIR) filter output distribution will be easier to detect with the decoder. We evaluate the BER performances of the proposed system for carrier-to-noise ratio (CNRS) of the system noise under an areal recording density of 1 Tbits/in <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> specification with bit-pattern medium stricture by computer simulation. The results show that the FIR filter output distribution for our proposed dual STO reading system at an FM detector output has become sharper, and the proposed system achieves the “No errors” with CNRS less by about 3.5 dB than the single STO reading system.

Design and Numerical Study of Flux Control Effect Dominant MAMR Head: FC Writer

Flux control effect (FC effect) is one of the field modification effects in microwave-assisted magnetic recording heads by locating the spin torque oscillator in the write gap (WG). In this article, we propose a flux control writer (FC writer) that maximizes the FC effect by flipping the magnetization in the WG with respect to the gap field. In the FC writer, the field gradient gain is emphasized when compared with the amplitude gain. The bit error rate is also improved by about −0.2 compared with a conventional writer at the same erase width. These results indicate that the improvement of recording properties may be possible on any media using FC writer.

Optimisation of dual structure recording media for microwave-assisted magnetic recording

Dual structure recording media with a “notched” anisotropy configuration were optimised for use in a microwave-assisted magnetic recording system. First of all the effective head field gradient was maximised by tuning the anisotropy of the layers in each structure. Next, intergranular and interlayer exchange coupling was adjusted to maximise the signal to noise ratio of written tracks. A grain switching probability model was trained and used to generate written bit sequences for read bit error rate calculations. For two structures with different ferromagnetic resonance frequencies the total amount of user data that could be stored on the two structures was estimated to be 2.98 Tbit/in2.

Effect of FGL Cone Angle on Recording Performance in Microwave-Assisted Magnetic Recording

Microwave-assisted magnetic recording (MAMR) uses the field from a spin torque oscillator (STO) to assist the switching of recording medium magnetization. In an ideal STO, the magnetization of the field generating layer (FGL) would rotate in the plane of the FGL. In practice, the FGL magnetization usually has an out-of-plane component and the FGL magnetization vector describes a cone as it rotates. The cone angle $\theta _{c}$ is the angle between the FGL magnetization and the axis normal to the plane of the FGL. This article investigates the effect of the cone angle on the recording performance and compares two types of recording media: notched and exchange-coupled composite. The results show that for FGL cone angles between 45° and 90°, the recording performance was mainly determined by the strength of the high-frequency magnetic field from the FGL. For smaller cone angles, the signal-to-noise ratio (SNR) rapidly decreased.

Design Concept of MAS Effect Dominant MAMR Head and Numerical Study

We summarize several key factors in microwave-assisted magnetic recording (MAMR) design, particularly interactions between spin-torque oscillators (STOs) and write poles, frequency matching, the negative flux-control (FC) effect, and the STO vertical field effect. To satisfy these key factors, we study several configurations with longer gap lengths of 30 nm using a simulation based on the Landau–Lifshitz–Gilbert equation with a spin torque (ST) term including both the write head and STO. This simulation compared a dual spin-injection layer (SIL) structure, a dual flux-generation layer (FGL) structure, and two types of all-in-plane STO. While the dual SIL and dual FGL structures indicate sufficient oscillation behavior to satisfy the examined key factors, the dual SIL structure demonstrates poorer signal-to-noise ratio (SNR) performance than even conventional magnetic recording (CMR). We conclude that this poor performance is due to the STO vertical field effect. We newly propose a dual FGL structure with no vertical field in the in-plane field peak, leading to much better performance than that of the dual SIL structure. While this structure requires a much longer write gap than does a CMR writer, it retains an erase width comparable with CMR writers. These results indicate that MAMR with the proposed dual FGL structure will be usable at over 2800 kFCI without TPI loss.

Investigation of heat transfer across a nanoscale air gap between a flying head and a rotating disk

Understanding nanoscale heat transfer at the head–disk interface (HDI) is necessary for thermal management of hard disk drives (HDDs), especially for heat-assisted magnetic recording and microwave-assisted magnetic recording. To accurately model the head temperature profile in HDDs, it is imperative to employ a spacing-dependent heat transfer coefficient due to the combined effects of pressurized air conduction and wave-based phonon conduction. Moreover, while flying at near-contact, the fly height and heat transfer are affected by adhesion/contact forces in the HDI. In this study, we develop a numerical model to predict the temperature profile and the fly height for a flying slider over a rotating disk. We compare our simulations with touchdown experiments performed with a flying Thermal Fly-Height Control (TFC) slider with a near-surface Embedded Contact Sensor (ECS), which helps us to detect the temperature change. We incorporate the effects of disk temperature rise, adhesion/contact forces, air and phonon conduction heat transfer, and friction heating in our model. As the head approaches the disk with increasing TFC power, enhanced nanoscale heat transfer leads to a drop in the ECS temperature change vs TFC power curve. We find that the exclusion of the disk temperature rise causes the simulation to overestimate the ECS cooling drop. The incorporation of adhesion force results in a steeper ECS cooling drop. The addition of phonon conduction in the model causes a larger ECS cooling drop. The simulation with friction heating predicts a larger ECS temperature slope beyond contact. The simulation with these features agrees with the experiment.

A Study on AM–FM Combined Detection for Magnetic Recording Using STO Reading

A 3-D magnetic recording using microwave-assisted magnetic recording (MAMR) and a spin-torque oscillator (STO) as a reading sensor has been proposed as a candidate for the prospective recording technologies. In the STO reading system, the temporal magnetization dynamics of the STO vary not only in amplitude but also in frequency, depending on the recorded pattern. Therefore, we have constructed the STO signal model which considers both amplitude and frequency fluctuations to develop a combined signal processing using the detection of amplitude and frequency fluctuations. Furthermore, we have applied the STO signal model to the low-density parity-check (LDPC) coding and iterative decoding system with the detectors for amplitude modulation (AM) and frequency modulation (FM) and have proposed the AM-FM combined detection. Then, we have evaluated the bit error rate (BER) performances by computer simulation. The results show that the system with AM-FM combined detection improves the performance of the LDPC coding and iterative decoding system, which achieves “no errors” at the carrier-to-noise ratio for system noise CNR <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S</sub> = 15.9$ dB, whereas the AM and FM detections require CNR <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S</sub> = 21.2 and 17.2 dB.

A Study of the Nanoscale Heat Transfer in the HDD Head–Disk Interface Based on a Static Touchdown Experiment

Understanding the heat transfer behavior at the nanoscale head-disk interface (HDI) in hard disk drives is crucial for head design, media design, and failure analysis of the current hard disk drive (HDD) industry, especially for the emerging technologies including heat-assisted magnetic recording (HAMR), microwave-assisted magnetic recording (MAMR), and 2-D magnetic recording (TDMR). Previous experimental studies of both static touchdown technique and theoretical developments of the wave-based phonon conduction have shown enhanced heat transfer at the HDI. To better understand the heat transfer behavior across the HDI, a series of the simulation is necessary to connect the theory and the detailed geometric model of the HDI. In this article, we developed a finite-element model to explain the temperature change of the head during a static touchdown experiment. The wave-based phonon conduction theory is integrated into the simulation through iteration. The simulation results trend in agreement with the theoretical development and the experimental results. This simulation strategy can also be implemented for flying heads to predict the heat transfer behavior under HAMR conditions.