Optimization of the Spin-Torque Oscillator Response for Microwave-Assisted Magnetic Recording

Kheong Sann Chan,Susanto Rahardja,Ryo Itagaki,Yasushi Kanai

doi:10.1109/access.2019.2940741

Abstract

Microwave assisted magnetic recording (MAMR) is one of the promising next generation recording technologies that is expected to carry the magnetic recording industry beyond today's perpendicular magnetic recording (PMR) limit. MAMR involves the introduction of a high frequency oscillating magnetic field in the microwave frequency (GHz) range at the time of writing. This assisting energy at a specific frequency helps to make the magnetic grains more malleable and thereby reverse more easily which permits the usage of smaller, higher coercivity grains in the media, supporting a higher recording density. The generation of a stable oscillating magnetic field is expected to come from a spin-torque oscillator (STO) that has a rotating field-generating layer (FGL) when a current is injected. The STO must be properly designed in order to generate the stable oscillating magnetic field of the desired frequency and with sufficient amplitude to assist in the media reversal. In this work, we optimize the STO to produce a large and stable oscillating field with the desired parameters. Micromagnetic simulations of the STO are used to determine the STO's behavior and we propose various metrics to be used as a measure of the performance of the STO's response. Subsequently the response surface methodology (RSM) is employed to optimize our selected performance metric, producing the highest performing STO.

Highlights

Microwave Assisted Magnetic Recording (MAMR), [1] is a candidate for the generation technology being developed to continue the areal density growth of the magnetic recording industry that has been saturating over the past decade due to the superparamagnetic limit [2]
On computing the eigenvalues of the C matrix in each case, it was found that the response surface for PM1 had a positive eigenvalue, whereas for PM2, PM3 and PM4, the eigenvalues were all negative as required for a global maximum
The positive eigenvalue associated with C for PM1 indicates that in one of the response surface’s principal directions, the parabola opens upwards indicating that the fit to the underlying data points may not be so good

Summary

INTRODUCTION

Microwave Assisted Magnetic Recording (MAMR), [1] is a candidate for the generation technology being developed to continue the areal density growth of the magnetic recording industry that has been saturating over the past decade due to the superparamagnetic limit [2]. MAMR contributes towards the areal density growth by injecting a high frequency (HF) oscillating magnetic field during the writing. These parameters are θ , the angle of the STO, φ, the angle of the trailing shield, STOSize, the length of one side of the STO (the STO having a square cross-section), and J , the injected current density driving the STO. Φ is the trailing shield angle that affects the angle of the fields from the main pole that pass through the STO, influencing the stability of its rotation, and the second chosen controlling parameter. The STO size and injected current density, J , obviously play a strong role in determining the stability and amplitude of the STO rotation that we wish to optimize

OPTIMIZATION OF THE STO

THE RESPONSE SURFACE METHODOLOGY

RESULTS

CONCLUSION