Microwave-assisted Magnetic Recording Research Articles

Performance Limitation of Microwave Assisted Magnetic Recording Combined With Exchange Coupled Composite Media Explored by Genetic Algorithm

A simple spin model combined with genetic algorithm showed the performance limitation of microwave assisted magnetic recording (MAMR) with multilayer media (MLM). Designs of MLM and MAMR were optimized so as to maximize the media thermal stability and the response of media magnetization to the external recording field. Tradeoff relationships between the two design objectives showed that higher AC-field is required for MLM than for the case in single-layer media. This is mainly because the condition for obtaining frequency matching between AC field and precession in each layer is difficult to achieve with the exchange coupling composite effect.

Oscillation Stability of a Small Size Spin Torque Oscillator for MAMR

Oscillation behavior of a spin torque oscillator (STO) less than 40 nm in size was investigated by using micromagnetic simulations for microwave-assisted magnetic recording (MAMR) over 5 Tb/in2. Decreasing the STO size from 40 to 20 nm deteriorates the stability of a reference layer (REF) and it shrinks the stable oscillation region of a field generation layer (FGL). Enhancing the magnetic anisotropy energy of the REF Ku_REF stabilizes the magnetization motion of the REF against the spin transfer torque (STT) effect and the external magnetic field Hz, and the stable oscillation region of the FGL expands within a relative lower applied voltage Vsto region, but random oscillation with fluctuating elevation angles appears at a higher Hz and Vsto. Similar phenomenon is observed when increasing the thickness of the REF tREF. Enhancing both Ku_REF × tREF and the thickness of FGL are found to stabilize oscillation of the FGL against the STT effect from a stable REF at a larger strength of Hz and a higher Vsto without the appearance of random oscillation, making it possible to apply about 20-nm-square STO to a magnetic head for over 5 Tb/in2.

Effect of Effective Field Distribution on Recording Performance in Microwave Assisted Magnetic Recording

We investigated the effect of effective field distribution on the recording performance in microwave-assisted magnetic recording (MAMR) through calculations. We calculated the effective field distribution and the recording characteristics for different media that had similar grain sizes, but different anisotropy fields (Hk). We found that the effective field gradient near the trailing edge of the main pole was drastically enhanced because of the localized AC-field (Hac) near the spin-torque oscillator (STO) and the field angle distribution of writer, which affects on assist effect on the reducing the switching field (Hsw) of the media with applying the (Hac). It was also clarified that the SNR at high linear density drastically increased as the (Hk) of the media increased. This is because the effective field gradient at the (Hsw) of media increases as (Hk) of media is increased. Furthermore, SNR can be increased by reducing the grain size of media with high (Hk). Therefore, increasing both the (Hk) of media and the (Hac) of the STO is effective for achieving high areal density from view point of utilizing small grain media as well as high field gradient.

Switching Phase Diagram of Two Frequencies MAMR for ECC Media

A two-particle model is utilized to study the switching phase diagram of ECC media in the microwave-assisted magnetic recording, by applying an in-plane AC magnetic field for microwave and a DC magnetic field for write head field. An interesting result is found that, by introducing two mixed linearly polarized AC fields when the ratio of frequencies f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> is in a certain range, the minimum switching DC field can be almost zero when H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ac</sub> is 20% of H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k1</sub> of hard magnetic layer. This phase diagram is also checked by a 3D micromagnetic model of ECC media with polycrystalline structure included; with two linearly polarized H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ac</sub> applied, the coercivity H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> can be reduced to almost zero when H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ac</sub> = 2.0 H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k1</sub> , and H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> still decreases from 10.5 kOe to 7.7 kOe when H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ac</sub> = 0.02 H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k1</sub> .

Thin Spin-torque Oscillator With High AC-Field for High Density Microwave-Assisted Magnetic Recording

A spin-torque oscillator (STO) with perpendicularly magnetized reference layer and a field generating layer (FGL) with high saturation moment was compared with an STO having a perpendicular anisotropy layer (PL) on the FGL using micro-magnetic simulation. The STO without PL was more suitable for obtaining areal-density over 5 Tb/in2 from the viewpoint of achieving a high ac magnetic field and thin stack thickness required for an MAMR head. The FGL stayed in-plane against a strong trailing gap field, and stable high-frequency oscillation was achieved over 10 kOe of perpendicular external field. Magneto-resistance measurement with high applied current was carried out in order to distinguish the stable oscillation with large FGL oscillation trajectories. Qualitative correspondence between the simulation and the experimental results indicated that stable oscillation with large FGL oscillation trajectories was realized in the structure without PL.

Optimal Design of MAMR and HAMR by Applying Response Surface Methodology

In this paper, we systematically perform optimal design of microwave-assisted magnetic recording (MAMR) and heat-assisted magnetic recording (HAMR) by applying response surface methodology (RSM) to optimize the signal-to-noise ratio (SNR) of the recording process. We evaluate the performance of design of MAMR and HAMR through micromagnetic modeling. Design of experiments (DoE) is conducted by using composite central design (CCD), and the influence of the design parameters on the SNR is investigated. The optimal designs of MAMR and HAMR systems are obtained by solving for the best-fit quadratic response-surface to the sampled points and solving for the parameters that maximize this cost function.

Feasibility of Bit Patterned Magnetic Recording With Microwave Assistance Over 5 Tbitps

Microwave assisted magnetic recording (MAMR) on bit-patterned media (BPM) with areal recording density over 5 Tbit/in2 was investigated using micromagnetic simulation and an analytical method. It was found that a planar magnetic anisotropy material is indispensable for a narrow spin torque oscillator to obtain in-plane oscillation with the required frequency. It was also found that the MAMR on BPM with an areal recording density of 6.2 Tbit/in2 can be obtained by superimposing the write magnetic field gradient from main pole and the effective assist field gradient from field generation layer. These head and media requirements are likely to be achieved by improving the present technology.

Spin dynamics of magnetic nanostructures investigated by micromagnetic simulations

Micromagnetic simulations were performed to investigate the spin dynamics of cylindrical magnetic nanostructures of various sizes down to 20 nm. The presented simulation technique provides information on the equilibrium states, magnetization precession, and spatial distributions of excited spin wave modes of individual nanostructures. Larger cylindrical nanostructures reveal a flower state magnetization configuration and show rather complex edge and center modes, which depend strongly on the saturation magnetization. This behavior allows controlling the precession frequencies, which is important in microwave-assisted three-dimensional magnetic recording, where layers of different resonance frequency need to be addressed and switched individually.

Soft layer driven switching of microwave-assisted magnetic recording on segmented perpendicular media

In this work, we use micromagnetic modeling to investigate microwave-assisted magnetic recording on segmented perpendicular media. Two types of microwave fields, sinusoidal and finite bandwidth square microwaves, have been chosen for our study. We show that microwave fields, when applied to the soft segment, can assist the magnetization reversal of the hard segment. The calculated assisted frequencies are insensitive to the variation of inter-segment exchange coupling of segmented perpendicular media. Our results suggest that sinusoidal microwave fields are practically more feasible than square microwave fields for the soft layer driven switching mechanism of segmented perpendicular media.

Microwave-assisted magnetic recording simulation on exchange-coupled composite medium

The potential of microwave-assisted magnetic recording on exchange-coupled composite (ECC) media is investigated by numerically simulating read/write processes with respect to the physical and magnetic properties of the soft section of ECC media. To obtain desirable recording performance, the thickness of the soft section must be comparable to the exchange length. Under such condition, the optimal microwave frequency decreases owing to the non-uniformity of the reversal mode. A relatively high anisotropy field in the soft section produces high signal-to-noise ratios. Moreover, large saturation magnetization of the soft section enhances the magnetization reversal of ECC grains but excessive enhancement is likely to induce wide writing, which decreases the high track density.

Frequency tuning the ultrafast magnetization dynamics of FexPt100-x alloys

We report frequency tuning of magnetic oscillations in both planar films and individual patterned nano-sized dots by varying the iron content of disordered FexPt100-x alloys from 42 at. % to 100 at. %. It is seen that as the Fe content is increased, the magnetic oscillation frequency rises by up to 50% in both the films and individual dots due to the change in the saturation magnetization. Micromagnetic simulations and analytic calculations are in agreement with the data. These tunable alloys could be used in microwave-assisted magnetic recording and especially for developing a 3D storage technology where layers of different resonance frequency are switched individually.

Micromagnetic study on microwave-assisted magnetic recording in perpendicular medium with intergrain exchange coupling

The influence of intergrain exchange coupling on the properties of microwave-assisted magnetization reversal has been investigated. The results of micromagnetic simulation suggest that the microwave frequency realizing the magnetization reversal is increased as the exchange stiffness constant among the magnetic grains becomes larger than 1 × 10−7 erg/cm if an inhomogeneous magnetization reversal occurred. The numerically expected increase in the microwave frequency was observed experimentally in perpendicularly magnetized Co/Pd multilayer with an exchange stiffness constant of ∼1 × 10−6 erg/cm. The simulations of signal recording and reproducing processes in a granular medium with assistance of a microwave field also suggest that an adequate intergrain exchange coupling is required to inhibit the nucleation of small island domains that will suppress the amplitude of the signal-noise ratio of the readback signal.

Experimental feasibility of spin-torque oscillator with synthetic field generation layer for microwave assisted magnetic recording

We investigated the oscillation characteristics of a fabricated spin-torque oscillator (STO) with a synthetic field generation layer (FGL) composed of the FGL and perpendicular anisotropy magnetic layer in microwave assisted magnetic recording (MAMR). The fabricated STO was composed of a perpendicularly magnetized reference layer/interlayer/synthetic-FGL. The STO with synthetic-FGL showed very narrow peaks caused by spin-torque oscillation. We also found oscillating frequency of the fabricated STO increased as the external field increased. This implies that the FGL magnetization rotates out-of-plane. Calculations by micromagnetic simulation were qualitatively consistent with the STO measurements. The calculated magnetization configurations in the synthetic-FGL formed a single domain but the one in the single-FGL formed multiple domains. This is one reason that the synthetic-FGL is estimated to generate a larger ac-field. In sum, we experimentally confirmed the effective oscillation of fabricated STO with a synthetic-FGL even with a thick FGL and its feasibility for high areal density in MAMR.

Damping Constant Influence on Spin Dynamics in Field Generating Layer of STO for MAMR Writing Head

For Microwave Assisted Magnetic Recording (MAMR), large amplitude of ac stray field and coherent motion of magnetization and also low driving power are necessary as a performance of spin torque oscillator for writing head. The damping constant is characterized by both the dynamical motion of magnetization and critical current for the precession-motion. The damping constants for FexCo1−x dot were investigated by a network analyzer-ferromagnetic resonance measurement. From these results, suitable material for the oscillator and the effect of finite magnetic element size are discussed.

Dynamic Fly Performance of Air Bearing Sliders on Patterned Media

It is anticipated that a radical shift in design paradigm will be required in order to enable perpendicular magnetic recording (PMR) areal densities beyond 1 Tb/in <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . The shift in question will involve the use of advanced magnetic recording assist technologies such as patterned media, heat-assisted magnetic recording (HAMR), or microwave-assisted magnetic recording (MAMR). This paper is concerned with one of the challenges associated with the practical implementation of patterned media technology, namely, the impact of non-planarity, and shifts in pattern orientation (at data-servo transitions, in particular) on the dynamic fly performance of air bearing sliders. The nature of the latter challenge becomes apparent when considering that key to realizing recording areal densities exceeding 1 Tb/in <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> will be air bearing sliders with superior dynamic fly stability at sub-2 nm clearances. Of the two types of patterned media technologies in development in the disk drive industry, namely, discrete track media (DTM) and bit patterned media (BPM), the former was chosen as a case study in this work.

Oscillation Feature of Planar Spin-Torque Oscillator for Microwave-Assisted Magnetic Recording

The mechanism of oscillation or magnetization rotation of a field generation layer (FGL) in a spin torque oscillator (STO) under the large gap field between main pole and trailing shield for microwave assisted magnetic recording (MAMR) was investigated using an analytical model. The following results were obtained. The oscillation frequency at the maximum magnetic field output is decided by the vertical component of the magnetic field from the main pole. When there is a strong external magnetic field, it is difficult to restrain the FGL magnetization in the plane of FGL and the oscillation becomes unstable. A FGL with zero (or positive) anisotropy will not be suitable for use in the strong external magnetic field because it will not oscillate properly. It can be expected that the spin torque oscillator with a planar FGL with negative perpendicular magnetocrystalline anisotropic material is likely to be suitable for MAMR.

Spin Torque Oscillator With Negative Magnetic Anisotropy Materials for MAMR

Microwave assisted magnetic recording (MAMR) has recently emerged as a candidate to solve the trilemma in magnetic recording. In MAMR the magnetizations of recording medium can be switched with much lower magnetic field by magnetic resonance, which however requires a higher frequency oscillator than 20 GHz. In this paper, it is described that spin torque oscillator (STO) using a magnetic material with a large negative uniaxial anisotropy has a possibility to generate higher frequency and stable magnetic field and it might be preferable to a conventional oscillator with soft magnetic materials.

Effective Write Field for Microwave Assisted Magnetic Recording

The effective write field of microwave assisted magnetic recording (MAMR) was investigated using the Landau Lifshitz Gilbert (LLG) simulation and an analytical method. An effective component of high frequency (HF) field was obtained by considering the angle between the magnetization of recording media and the HF field generated from field generation layer (FGL). An analytical method to obtain the effective write field was proposed by using the switching properties of the recording medium graphically. The obtained effective write field has shown that the value of the maximal effective field gradient of 1850 Oe/nm is three times larger than that for Stoner-Wohlfarth effective write field.

Microwave Assisted Magnetic Recording Utilizing Perpendicular Spin Torque Oscillator With Switchable Perpendicular Electrodes

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/in <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> can be readily achieved with a medium signal-to-noise ratio above 13 dB (1T-SNR) and thermal stability at K <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">u</sub> V = 60 k <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</sub> T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">300°K</sub> .

Investigation of a Spin-Torque Oscillator using Material with Negative Magnetic Anisotropy

Microwave-assisted magnetic recording (MAMR) has recently emerged as a candidate approach to solving the trilemma in magnetic recording. In MAMR, the magnetic recording medium can be switched with a much lower magnetic field by magnetic resonance, which requires an oscillator with a higher frequency than 20 GHz. This study, describes a spin-torque oscillator (STO) using a material with a large negative anisotropy that is capable of generating a higher frequency and a stable magnetic field, and shows that it is preferable to a conventional oscillator using soft magnetic materials.