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Uniform and stable oscillation of magnetization is one of the most important factors for the spin-torque oscillator (STO) in microwave assisted magnetic recording (MAMR) [1], [2]. Impact of magneto-static interaction between STO and recording head have been discussed often [3], [4], but the interaction including spin torque effect is not understood well even just between the field generation layer (FGL) and the spin injection layer (SIL) in STO. In this paper, we focused upon an effect of SIL magnetic design, i.e., the saturation magnetization and layer thickness, on STO optimization for uniform and stable oscillation and tried to understand the magnetic interaction with spin torque. A commercial micromagnetics software (Fujitsu Examag v2.1.1) was used, in which the effective field due to spin transfer torque is considered by the equation shown in Fig.1. In this analysis, a uniform alternative field was applied to STO and magneto-static interaction between STO and head was neglected as shown in Fig.1. The dimensions and magnetic properties are as the followings; FGL&SIL width and height $=30 \mathrm {x}30$ nm, FGL thickness $=6$ nm, FGL Bs $=2$ or 1 T, FGL&SIL Hk $=1$ Oe, Polarization $=0.5$, Exchange $=1.0\mathrm {E} -6$ erg/cm, Damping $=0.02$, SIL thickness and SIL Bs are variables. An alternative rectangular field with 1 GHz frequency and 14 kOe magnitude was applied and the injection current density was set at $4.0\mathrm {E} +8\mathrm {A} /$cm 2 with a direction to SIL from FGL. Results are shown in Fig.2 for (a) FGL Bs $=2\mathrm {T}$ and (b) FGL Bs $=1\mathrm {T}$, respectively. Here, since the reduced crosstrack component (My/Ms) of averaged FGL magnetization alternates at a microwave frequency, the root mean square (rms) value was calculated and multiplied by $\surd 2$ to get 1 at the perfect sine-wave oscillation. In Fig.2(a) for the FGL Bs $=2\mathrm {T}$, we can see that the optimum Bs of SIL is around at 1.5, 0.8, 0.6 and 0.4 T for the SIL thickness of 6, 3, 2 and 1, respectively. It's very interesting that the optimum Bs of SIL decreases simply according as SIL thickness increases, and these SIL design impacts significantly to FGL oscillation. In the case for FGL Bs $=1\mathrm {T}$, as shown in Fig.2(b), all optimum points have moved to larger Bs positions compared to Fig.2(a). Mechanism for these characteristics is not fully understood yet, but it should relate to both the magneto-static effect and the spin transfer effect, which interact each other. From these figures, once FGL Bs is given, the optimum Bs and the thickness for SIL can be designed. We also confirmed that these optimum positions for the SIL doesn't change even if FGL thickness was changed, and it depends only upon the FGL Bs.

The HDD industry is at a critical technology cross roads and it is paramount that we quickly establish comprehensive paths to push beyond the superparamagnetic limit. Several promising technology options have been explored to increase the areal density beyond the limit such as bit patterned magnetic recording (BPMR), heat assisted magnetic recording (HAMR), and microwave assisted magnetic recording (MAMR). In addition to these three technology options, there is recent interest in a fourth approach that has the advantage of staying with a relatively conventional perpendicular medium and head. This combines shingled write recording (SWR) and/or 2-D read back and signal processing. Either technique can be used separately to give large gains and the combination of the two, which is referred to as 2-D magnetic recording (TDMR), promises particularly large gains. Recording with energy assist on BPM or 2-D signal processing will enable the areal density beyond around 5 Tb/in2, although none of them have shown a clear problem-free solution. Precompetitive research projects and programs have recently started targeting 2 Tb/in2 by 2010 for SRC, 5 Tb/in2 by 2013 for NEDO, and 10 Tb/in2 by 2015 for INSIC. This paper reviews the technology options ahead, and the pros & cons for each option. We include a brief introduction to SWR and TDMR.

We have fabricated a new layered structure, named "U-mag," for perpendicular magnetic recording media, in terms of stacked films including a very thin (2 nm) ferromagnetic (Co) intermediate layer and lattice spacing control layers. The formed medium having a 100-nm soft magnetic underlayer with high coercivity over 6 kOe and sufficient nucleation field (-1.5 kOe) shows higher signal-to-noise ratio (SNR) than a medium with a conventional Ru underlayer. The detailed microstructures of the U-mag medium were also revealed by a transmission electron microscope with an energy dispersive spectrometer. A double Co intermediate layer gave a significant improvement in the SNR compared to a single Co layer.

Henkel plots arelinearized when the field −αM compensating for the interparticle meaninteraction field is applied during VSM measurements. By this method, the mean field constants α were estimated to be about 0 and 0.2 for longitudinally oriented Baferrite particulate media with squarenesses of 0.5 and 0.8, respectively. To consider the interparticle magnetic interaction in the recording simulation, the mean field was introduced into our simulator based on a finite element method and the curling magnetization switching model. By simulation analysis, it was shown that the reproducing voltage atlow recording densities was increased by the positive mean field in a longitudinally oriented Ba-ferrite medium. On the other hand, in a perpendicularly oriented medium, the high-density performance was greatly improved by the mean field.

The perpendicular recording media are thermally stable in high recording density. We tried to build thermal decay model of waveform and noise for perpendicular recording media based on theoretical and practical approach, in order to estimate BER degradation using channel simulation.

Thermal relaxation in perpendicular double-layered media was examined by simulation and experiments, using a single-pole writing head and an MR flying head. A new FEM simulation with a curling switching model is expanded in association with the exponential relaxation theory. The simulation indicates that the relaxation is less at high densities than at low densities. Only a 2% output deterioration due to thermal relaxation was actually confirmed experimentally for about 105 seconds at 5 kFRPI, and there was almost no deterioration at 130 kFRPI. In contrast to longitudinal recording, less output reduction is observed for higher linear densities; this is well explained by the nature of the demagnetization field in perpendicular magnetic recording.

The paper points out that soft magnetic underlayer (SUL) domain wall causes wide-band erasure in perpendicular recording media, and that laminated or multi-layered SUL can avoid this problem. Additionally, the spike noise produced by SUL causes bit error rate (BER) degradation. The types of SUL required to solve neighboring track erasure are similar to the approaches proposed to solve the spike noise problem. Thus, controlling the SUL domain is very important, not only for BER performance, but also for the reliability of the recorded information.