Microwave assisted magnetic recording technologies and related physics

Microwave-assisted magnetic recording (MAMR) is one of the promising candidate technologies for next-generation hard disk drives. In MAMR, the high-frequency field from a spin-torque oscillator (STO) in the recording head is utilized to reduce the magnetization switching field of the medium. Micromagnetic modeling has shown the potential of MAMR; however, in terms of experimental results with actual heads and media, only little evidence of media magnetization reversal and improvement on overwrite has been reported. In this paper, we focus upon the experimental results of MAMR gain defined as a difference in the read-write performance when the STO is activated compared with when not activated. In conclusion, we show clear bit error rate improvement by activating the STO, giving an areal density increase of around 5–10% after considering the penalty associated with the magnetic core width increase. We also confirm that the magnetization switching in the STO is fast enough up to 2.3 GHz, and that there is no negative impact on the adjacent track interference nor on the far track interference.

Hard disk drives (HDDs) have been investigated to determine how to increase their areal recording density. Recently, however, HDDs have been having problems with noise, thermal fluctuations, and writability simultaneously, commonly known as a trilemma. Microwave-assisted magnetic recording (MAMR) is a promising recording method to solve this trilemma [1-4]. In MAMR, a spin torque oscillator (STO) is placed between a conventional write head and a trailing shield. Because the AC field from the STO decreases the switching magnetic fields of the medium, the writability can be improved. As a result, the trilemma can be solved. Therefore, a high frequency AC field with strong amplitude is needed. In this study, we examined methods to increase the AC-field frequency by using a micro-magnetic simulator that takes into account magnetic interactions between the write head, the medium, and the STO.

We investigated spin torque induced magnetization dynamics in the mag-flip spin torque oscillators (STOs) of diameters D from 29 to 96 nm comprising of an in-plane magnetized field generation layer (FGL) Fe67Co33 (7 nm) with high saturation magnetization, μ0Ms ∼ 2.3 T, and perpendicular FePt(10 nm)/Co2FeGa0.5Ge0.5(3 nm) highly spin polarized spin injection layers. Out-of-plane high frequency, f ∼ 21–26 GHz, spin torque induced oscillation with a large cone angle in FGL was observed under nearly perpendicular external magnetic field μ0Hext of 1.1 T for the pillar D of 29 and 42 nm. Our micromagnetic simulation results indicated that ac magnetic fields of about 0.15 to 0.2 T are obtainable from the STOs having the same stacking structure and size as the experiment, which is large enough for the applications to microwave assisted magnetic recording technology.

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.

Microwave-assisted magnetic recording (MAMR) on single layer and exchange coupled composite (ECC) media was simulated. The high-frequency field was modeled with a macrospin model and also micromagnetically using a spin-torque oscillator integrated into the write head. ECC media with a range of hard/soft layer thicknesses were optimized for MAMR, and the gain over a non-MAMR system was established. The results show a bigger gain from using MAMR when the hard/soft layer thickness ratio was small. MAMR systems were also less susceptible to changes in magnetostatic interactions between medium grains.

To overcome superparamagnetic limit, microwave assisted magnetic recording (MAMR) is one interesting magnetic recording technology. Therefore, the effect of microwave on magnetization reversal in media should be analyzed. In this work, we propose the MAMR to decrease switching field (coercivity, Hsw) in exchange coupled composite (ECC) media by using the micromagnetic simulation based on the Landau - Lifshitz - Gilbert equation. The Hsw of single layer and ECC media without microwave field is 110.90 and 7.7 kOe, respectively. When the oscillating microwave field is added, Hsw of single layer media with microwave frequency of 2.5 - 40 GHz is lower than 110.90 kOe. Likewise, Hsw of ECC media with microwave frequency of 5 - 16 GHz is lower than 7.7 kOe and has the lowest value of 4.9 kOe at frequency of 10 GHz. The results from this work lead to solve superparamagnetic limit and increase areal density in hard disk drive.

A potential technology for use in future hard disk drives is microwave-assisted magnetic recording (MAMR). In a MAMR drive a spin-torque oscillator (STO) is integrated into the write head and applies a high frequency (HF) magnetic field to the recording medium along with the field from the write head. The HF field reduces the switching field of the medium, allowing media with higher uniaxial anisotropy to be used. As the write head moves over the recording medium the HF field seen by the medium grains changes from elliptical to circular to linear and back again. In this work the influence of circular, elliptical and linear HF fields on recording media was examined by calculating hysteresis loops for single and multiple grains.

A micromagnetic model analysis was carried out considering two types of write heads and three types of spin-torque oscillators (STOs). For write heads, we considered a small-sized head to reduce calculation time and a large-sized head with dimensions close to commercial heads. For STOs, we used a double-layered STO with reflection spin torque, a double-layered STO with transmission spin torque, and a tri-layered STO with two spin injection layers. We also carried out micromagnetic simulations of the STOs without the write heads (isolated STO) to compare the oscillation states. It was found that, compared with isolated STOs in a uniform applied field, a larger applied field was necessary for STOs integrated into a write head. This result shows that the write head should be designed to generate a large, well-shaped gap field to ensure stable STO oscillation.

The signal-to-noise ratio (SNR) gain of microwave-assisted magnetic recording (MAMR) with a spin-transfer oscillator is analyzed by a micromagnetic model simulating the write process on a CoX/Pt media, by including the Voronoi polycrystalline microstructure built upon a regular mesh of micromagnetic cells. With a fixed head of 40 nm-width main pole and a 10.5 kOe maximum perpendicular field, a 2 dB SNR gain is found for low-coercivity media (0 K $H_{c} kOe) and a 4 dB gain is obtained for high-coercivity media (0 K $H_{c}>14$ kOe). Besides, the MAMR performance considering the finite rise time of single-pole-type head field in a bit is also calculated. It is found that the MAMR SNR gain is obtained in the media with lower anisotropy because of the weak writing capacity during the rise time.

Microwave assisted switching (MAS) of magnetization has been intensively studied as an alternative technique for ultrahigh density magnetic recording. In this paper, comparison between the Landau–Lifshitz–Gilbert simulation and the analytical model in the rotating frame {Bertotti et al., [Phys. Rev. Lett. 86, 724 (2001)]} reveals that the switching behaviors of MAS can be clearly divided into two groups, that is, stable and unstable switching regions, depending on the frequency and amplitude of the ac field. The stable switching exhibits small switching field and narrow switching field distribution, and perfectly coincides with by the analytical model. Furthermore, in this region, the Sharrock type thermal fluctuation formula can be applicable to the MAS at finite temperature. On the other hand, for the unstable switching, the switching field is rather large and the SDF becomes very broad. Obviously, the former is preferable for the practical application of MAS. The critical frequency of the ac field for the limit of stable switching is almost proportional to the ac field amplitude.

Microwave-assisted magnetization switching MAS has been expected as one of the prominent future ultra-high-density magnetic recording technologies. In this paper, we have studied the radio frequency RF field dependence of the switching behavior of Co/Pt nanodot arrays with various dot diameters. While the critical frequency increases and the minimum switching field decreases with increasing the RF field, MAS behavior critically depends on the dot size. For small dots less than 130i¾źnm in diameter, the critical frequency and the minimum switching field exhibit linear variation with the RF field. On the other hand, for the larger dots dot diameter i¾ź 230i¾źnm, they exhibit not only profound MAS effect but also obvious change in the behavior at a certain RF field. These changes of MAS behaviors are explained as a consequence of the change of magnetization precession mode. On the other hand, the dispersions of MAS effect exhibit no remarkable variation on the RF field.

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.

Simulation of stress-modulated magnetization precession frequency in Heusler-based spin torque oscillator

For most of the past 50 years, the areal density of hard disks-a measure of how many bits of data that engineers can squeeze into a given area-increased by an average of nearly 40 percent each year. Lately, though, that rate has slowed to around 10 percent. Everyone who works on magnetic storage is well aware of this problem, but only in the past year or so have executives from Seagate Technology and Western Digital, the leading manufacturers of hard drives, very publicly split on how to solve it. In back-to-back announcements in October 2017, Western Digital pledged to begin shipping drives based on what is known as microwave-assisted magnetic recording (MAMR) in 2019, and Seagate said it would have drives that incorporate heat-assisted magnetic recording (HAMR) on the market by 2020.

Spin torque oscillator and magnetization switching in double-barrier Rashba Zeeman magnetic tunnel junction