Micromagnetic Model Analysis of Spin-Torque Oscillator (STO) Integrated Into Recording Head for Microwave-Assisted Magnetic Recording—Oscillation of STO Versus Rise Time of In-Gap Field

Abstract

Microwave-assisted magnetic recording (MAMR) [1] is one candidate for next-generation perpendicular magnetic recording [2]. Stable oscillation is one of the most important factors for spin-torque oscillators (STOs) used in a MAMR system. We performed micromagnetic simulations and found that stable STO oscillations were hard to obtain when the STO was inserted into the main pole - trailing shield (MP-TS) gap, primarily due to the strong magnetostatic interactions between the STO and write head [3]. We also showed that the rise time of the field applied to an isolated STO greatly affected the STO oscillation [4], i.e., a shorter rise time gave better, more consistent STO oscillation. In this paper, we show that the rise time of the in-gap field acting on the STO is critical to stable STO oscillation. We also show that the combination of a tilted STO and a tilted main pole - trailing shield gap results in stable STO oscillation due to weaker magnetostatic interactions between the STO and write head. Calculation Model A micromagnetic model analysis was carried out considering a double-layered STO utilizing transmission spin torque. We used commercial micromagnetic software (Fujitsu, EXAMAG v.2.1) [5]. The thickness of the field generation layer (FGL) was 10 nm, whilst the spin injection layer (SIL) was 2 nm thick. A 2 nm thick, non-magnetic inter layer was located between the FGL and SIL. The saturation magnetization $(4 \pi M_{s})$ was 20 kG for the FGL and 6 kG for the SIL. The anisotropy fields $(H_{k})$ of both the FGL and SIL were 31.4 Oe. The exchange constants, A, were $2.5 \times 10 ^{-6}$ erg/cm3 for the FGL and $0.75 \times 10 ^{-6}$ erg/cm3 for the SIL. The Gilbert damping factor, $\alpha $, was 0.02 for both the FGL and SIL. The write head model had overall dimensions close to those of commercial write heads $(3.25 \mu \mathrm {m}\times 2.55 \mu \mathrm {m}\times 4.5 \mu \mathrm {m})$.