Advantage of MAMR Read-Write Performance

Microwave assisted magnetic recording (MAMR) has been proposed as one of the prospective recording technologies for forthcoming tera-bit-class magnetic recording. In this paper, the technological and related physical issues of MAMR are reviewed. MAMR is the combined technology of microwave assisted switching (MAS) and spin torque oscillator (STO). MAS is a magnetization switching technology in which magnetization switches through excitation of its precessional motion by a radio frequency (rf) field. The MAS behavior in which nonlinear large angle precession is excited has been a challenging issue from scientific and technological viewpoints. Recently developed theories successfully predict the MAS behavior of a single macrospin in which all spins in a magnet are assumed to behave coherently. For a magnetic element with a size larger than the exchange length, spatially non-uniform magnetization precession modes become dominant instead of the coherent precession mentioned above, resulting in significant enhancement of the MAS effect. This size dependent MAS effect is experimentally very well verified for nanosized dots with perpendicular magnetization. Meanwhile, the STO is used as an rf field generator in MAMR technology. The aforementioned large angle magnetization precession dynamics are also a crucial problem for the stable operation of STO. While various types of STOs have been proposed so far, the optimum structure of STO for MAMR is still under investigation. The combination of the MAS and STO may provide frequency domain recording architecture which enables us to realize a new recording scheme, such as multilevel recording.

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.

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.

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.

The role of exchange interactions and thermal relaxation in advanced three-layer exchange-coupled composite perpendicular recording media with graded anisotropy was investigated through studies on magnetic viscosity, magnetic hysteresis reptation, and spin-stand adjacent track interference. For this purpose, thickness series in the NiW seed and the three magnetic layers were made and studied. For each sample, both magnetic viscosity and reptation were evaluated using a magnetometer over a series of initial magnetic states. Magnetic viscosity, which is the change in the magnetization of the sample with time, showed a distinct second-order dependence on a logarithmic time-scale. In general, viscosity appeared to be more strongly influenced by exchange-coupling interactions in the media than the thermal stability. Magnetic reptation, which is the change in the magnetization when the external field was repeatedly cycled between a nonzero field and zero field, i.e., remanence, showed two types of distinct reptation phenomena—field reptation and remanence reptation—depending on whether the magnetic state was evaluated with the external field present or removed, respectively. Both field and remanence reptation showed a second-order logarithmic dependence on the number of cycles. A comparison of viscosity and field reptation revealed a correlation, indicating that the origin of field reptation in perpendicular recording media can be associated with viscosity. Such a correlation could not be established between viscosity and remanence reptation. Spin-stand signal amplitude change of written tracks, due to adjacent track interference, also followed a second-order logarithmic dependence on the number of adjacent writes. When compared appropriately, the signal amplitude change showed an excellent correlation with remanence reptation across all media samples. This suggests that adjacent track interference in magnetic recording can be associated with remanence reptation evaluated using a magnetometer, despite their vast difference in time-scales. Moreover, this correlation was observed irrespective of the thermal stability of the media, which corroborates the hypothesis that in composite media, exchange interaction effects have a more dominant role than thermal relaxation effects in influencing switchability vis-à-vis adjacent track interference. In consequence, remanence reptation can be a useful technique to study the interplay of exchange and thermal effects in magnetic media.

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.

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.

In this work, the performance of an antisymmetric dual magnetoresistive (A-DMR) head, having a track-width of W/sub t/=2 /spl mu/m, a stripe height of h=2 /spl mu/m and a gap length of 80 nm, is presented and compared to theory. A thin film inductive head that incorporates high magnetization FeAlN poles, with a gap length of 0.25 /spl mu/m, was used to record thin film disk media with an areal magnetic moment of 0.5 memu/cm/sup 2/ and a coercivity of 3 kOe. The A-DMR roll-off and spectral curves that are fit in amplitude and shape with a 2-D analytical curve, indicated D50=190 kfci. The theoretical curves fit the experimental data with two fitting parameters, the spacing parameter of the spacing loss term and a parameter /spl zeta/=/spl zeta/ (W/sub t//h) that can be calculated with a 3-D micromagnetic model. The cross-track profile, off-track response and adjacent track interference are analyzed in the time domain and compared with previously reported theoretical results, where in this case the write head track-width is 2.5 /spl mu/m. The head showed some adjacent track interference and pulse amplitude asymmetry at lower linear densities, mainly due to a lower magnetic transverse bias level of one of the sensor layers. In spite of the good linear resolution of these heads, they offer limited sensitivity in comparison to spin valve heads.

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.

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

The analytical model for the intrinsic coercive squareness parameter, Sint* was applied to hard–soft stacked exchange-coupled composite media, and correlations with the microstructure and switching were studied. Thickening the hard magnetic layer in the composite stack led to a decrease in Sint*, as did thickening the NiW seed layers. However, this decrease was masked by thermal effects at the normal magnetometry time-scales of measurement. Upon thickening the soft layer in the composite stack, Sint* increased sharply at first and then only slightly. In contrast, the extent of incoherent switching, estimated from the peak value of the minor loop slope, increased slowly at first, and then sharply. The changes in Sint* and switching are correlated to the microstructure, particularly, grain size effects for the NiW series and growth effects for the hard–soft composite media series. Media signal-to-noise ratio at low recording frequencies, and adjacent track interference also show correlations with Sint*.

Stable oscillation is one of the most important factors for spin-torque oscillators (STO) used in microwave-assisted magnetic recording (MAMR) [1].

As the areal density of future magneto-optical (MO) disk drives is increased, adjacent track interference on readout will become a more significant engineering problem. Many issues are involved in the analysis of this problem, including tracking servo misregistration, the transverse dimension of written domains, the reading laser beam intensity profile (especially aberrations), the nature of the optical-transfer function for reading information to each side of the track center, and diffraction effects involving disk grooves and the linearly polarized read beam. The change from pulse-position to pulse-width modulation significantly increases the recording system sensitivity to adjacent track crosstalk (ATC). We conducted a variety of experimental tests of ATC to separate and understand the above effects. Two numerical scalar diffraction models of MO readout have been used to make quantitative predictions of signal amplitude as a function of off-track displacement and written frequency. Sensitive measures of read-beam aberration have been developed from conventional disk tests involving ATC and on-track signal analysis. In this paper, we report on the closure of readout models with experimental measurements and the development of methods for choosing the appropriate balance of bit and track density for future MO drives.

Traditionally, channel generated bit error rates (BERs) at a certain signal to noise ratio (SNR) have been modeled by a Gaussian distribution function. This approach has been suitable for a peak detection channel as the analysis is performed in only one bit. In the case of a partial response maximum likelihood (PRML) channel, one data bit is determined by multiple code bits. There is a burst of errors detected if any one of the involved code bits gives an error. The chi-square distribution function, which is a sum of several independent Gaussian-squared distributions, is another method to model BERs of a PRML channel. Particularly, different PRML codes could be modeled by a chi-square distribution with different degrees of freedom. In this paper, the relationship of SNR to BER is generated using the chi-square method. The measured on-track rms SNR, read sensitivity function, and media magnetization are the input parameters. Old data and adjacent track interference are considered as random variables. At specified track pitch and read write misregistration, the probability distributions of these random variables are defined by the read sensitivity function and media magnetization. For a given interference, the off-track SNR is calculated. Error rate at a given interference is then derived from off-track SNR by using the SNR to BER relationship defined by a chi-square distribution. The BER at a specific track pitch and read–write misregistration is computed by integrating the error rate at a given interference with the probability distribution of the interference. This model describes the statistical nature of the BER in magnetoresistance head applications and provides very good correlation to experimental data.