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Micromagnetic model simulations were performed to investigate spin–torque oscillators (STOs) integrated into the main pole–trailing shield (MP–TS) gap of the write head (integrated STO). First, the oscillation states of a tilted STO were investigated in a model comprising the write head, STO, and medium soft underlayer (SUL). We found that inserting the STO into a non-parallel MP–TS gap resulted in more stable oscillation. Second, we performed full micromagnetic simulations of the write head, STO, SUL, and recording layer (RL), and the oscillation of the tilted STO was investigated. We found that the RL affected the STO oscillation to some extent; however, the effect was smaller than that from the write head. We also discuss the calculation times for various structures with and without the RL, and with/without medium motion.

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 $(3.25 \mu \mathrm {m}\times 2.55 \mu \mathrm {m}\times 4.5 \mu \mathrm {m})$.

In recent years, shingled magnetic recording (SMR) has attracted attention as a new recording system of the hard disk drives. Although SMR can realize narrow track by overwriting data track like tile roofing, it has remarkable performance deterioration due to the influence of inter-track interference. Therefore, the performance improvement of the low-density parity-check (LDPC) coding and iterative decoding system using the sum-product decoding is required. In this paper, we propose the LDPC encoding and iterative decoding system with the log-likelihood ratio modulator and show the error floor is eliminated and improvement of signal-to-noise ratio to achieve the error free is obtained in the SMR under an areal recording density of 4 Tbits/in2 specification for the channel bit sequence coded by run-length limited and LDPC encoders by computer simulation. Therefore, the channel bit is composed bit about five magnetic grains.

Multiple layer magnetic recording may be possible with the advent of microwave-assisted magnetic recording (MAMR) [1], [2]. In a MAMR system, recording on a medium with high uniaxial anisotropy is made possible by applying a high frequency (HF) magnetic field at the resonance frequency of the medium, leading to a reduction in the switching field. If two discrete recording layers, RL1 and RL2, have different resonance frequencies, selective recording on one or other of the layers may be possible by adjusting the frequency of the spin torque oscillator (STO) used to generate the HF field. The magnitude of the coercivity reduction is related to the strength of the HF field in the medium. As STOs typically have dimensions from 20 – 40 nm the HF field strength decreases rapidly with distance from the STO. For this reason, if multiple recording layers are to be used each layer must be as thin as possible and the separation between the layers cannot be large. However, if the separation between the recording layers is small, magnetostatic interactions between them are large. This can make it difficult to implement selective recording due to the magnetostatic interaction favouring parallel alignment of the magnetisation of both recording layers. It may be possible to counteract the influence of the magnetostatic field by introducing antiferromagnetic exchange coupling between the two recording layers, e.g., by using Ru or Ir spacers [3], and this is the topic of this work. The aim is to optimise the antiferromagnetic exchange coupling and to determine the effect on recording performance. The Model Dual layer recording media with a 7 nm RL1 / 2 nm Spacer / 5 nm RL2 / 3.5 nm Air / Write Head structure were modelled. Each recording layer was an exchange coupled composite (ECC) structure with $M_{s} =750$ emu/cm 3 in all layers and the hard layers below the soft layers. In the model the medium grains were discretised into 1 nm-thick layers with the exchange coupling constant between layers of the same material being 10 erg/cm 2. After optimisation of the layer thicknesses and other parameters RL1 had a 3 nm hard layer and a 4 nm soft layer. The soft layer had $K_{u}$ of 3 × 106 erg/ cm 3 and the exchange coupling between the hard and soft layers was 4 erg/ cm 2. RL2 was formed of a 2 nm hard layer and a 3 nm soft layer. The soft layer had $K_{u}$ of 5 × 106 erg/cm 3 and the exchange coupling between the hard and soft layers was 6 erg/cm 2. The hard layer $K_{u}$ of both recording layers was a variable parameter. The field distribution from a write head with a 36 nm wide main pole and a 30 nm gap between the main pole and the trailing shield was calculated using a finite element model. A STO was placed 5 nm behind the main pole to generate the HF field in the media. The STO was modelled as a 36 nm wide, 36 nm high and 14 nm thick field generating layer (FGL) with $M_{s}$ of 1591 emu/cm 3 and coherent rotation of the FGL magnetisation was assumed. Results The effect of the magnetostatic interaction on the switching of the magnetisation of RL1 is shown in Fig. 1. In the calculation a 4 ns field pulse was applied to an 8 nm $\times 8$ nm grain containing two discrete recording layers, RL1 and RL2. The hard layer $H_{k} ( = 2 K_{u}/ M_{s})$ was varied and the maximum $H_{k}$ that could be switched by the combination of the head field and STO field was calculated as a function of HF field frequency. The magnetisation of RL2 was fixed either “up” or “down” and, as Fig. 1 shows, the result was to change the maximum switchable $H_{k}$ of RL1 by about 17 kOe: this difference was due to the magnetostatic field from RL2 acting on RL1 and is measured by the quantity $\Delta H_{k}$. Next, antiferromagnetic exchange coupling with a strength $J_{IL}$ was introduced between RL1 and RL2. The inset to Fig. 1 shows the effect of $J_{IL}$ on $\Delta H_{k}$. When $J_{IL}$ was about -0.17 erg/cm 2 the influence of the magnetostatic field was eliminated and $\Delta H_{k}$ was zero. Tracks were written on media with two recording layers using HF field frequencies of 20 GHz and 50 GHz. The bit length was 30 nm (847 kfci) and the head velocity was 5 m/s. The temperature was 300 K and distributions of grain size (16.5%), $M_{s}$ (4.25%), $K_{u}$ (8.5%) and easy axes $(3^{\circ })$ were included. The signal to noise ratio (SNR) was calculated for tracks recorded on each layer using the sensitivity function of a 20 nm wide magnetoresistive (MR) read head. Fig. 2 shows the results as a function of $J_{IL}$. For RL1 the SNR increased by about 1.2 dB when $J_{IL}$ was optimised, compared with the case when $J_{IL}$ was zero. A small amount of ferromagnetic coupling between RL1 and RL2, e.g., $J_{IL} =0.1$ erg/cm 2, caused a sharp decrease in the SNR of RL1. The SNR of RL2 was higher due to the layer being nearer to the write head and consequently experiencing higher head field gradients and HF fields. The SNR of RL2 was almost unchanged by $J_{IL}$ until $J_{IL}$ reached -0.3 erg/cm 2. Reversing the order of the hard and soft layers in RL1 to put the hard layer next to the non-magnetic interlayer made RL1 more resistant to the magnetostatic fields from RL2, but reduced the SNR. This was because magnetisation reversal was initiated in the soft layer and this layer experienced a smaller HF field when it was further from the STO. The authors would like to thank the ASRC for their support of this work.

With the increasing need for effective storage management due to ever-growing content-generation over the Internet, Distributed Storage Systems (DSS) has arisen as a valuable tool. Although DSS has considerably improved in the past years, it still leverages legacy techniques in its networking. To cope with the demanding requirements, Software Defined Networking (SDN) has revolutionized the way we manage networks and can significantly help in improving DSS network management. In this paper, we propose an SDN-based network control method that is capable of handling DSS network management and improving its performance. This paper presents the design, implementation, and evaluation using an emulated environment of a typical Data Center Network (DCN) deployment. The experiment results show that by applying the proposed method, DSS can increase the performance and service resilience compared to existing solutions.

Antiferromagnetically coupled (AFC) media were modeled as a part of a microwave-assisted magnetic recording system. Recording media grains with magnetically hard and soft layers were AFC by a Ru interlayer. The magnetostatic field arising from such grains has a smaller influence on neighboring grains and recording layers during recording. Compared with single-layer recording media, the SNR of AFC media decreased much more slowly with increasing linear density, and AFC media SNR was superior at high densities. An example of an AFC medium with two recording layers is also presented.

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.

Composite media grains consisting of two discrete magnetic layers with different uniaxial anisotropies, $K_{u}$ , and Curie temperatures, $T_{c}$ , were investigated for heat assisted magnetic recording. The maximum switching probability of optimized composite grains exceeded that of single phase grains. As $K_{u}$ of one layer increased the optimum $K_{u}$ of the other layer became smaller, tending toward zero in many cases. Leading and trailing edge recording were also compared. Trailing edge recording was superior when the maximum temperature in the grain, $T_{\text {max}}$ , was similar to $T_{c}$ , but when $T_{\text {max}}$ exceeded $T_{c}$ leading edge recording performed better.

The shingled magnetic recording (SMR) system is influenced by not only jitter-like medium noise from adjacent bits in the down-track direction but also intertrack interference (ITI) from the adjacent tracks. In this paper, we investigate the relationship between the recording pattern and the decoding reliability by classifying the log-likelihood ratio at the a posteriori probability decoder output according to the recording patterns under an SMR R–W channel specification of 4 Tb/in2 by computer simulation. The result shows that the decoding reliability for the recording pattern “010” heavily affected by transition jitter is the lowest and little improved even if the influence of ITI is mitigated by a 2-D finite-impulse response filter.

Two-dimensional magnetic recording (TDMR), which combines shingled magnetic recording (SMR) with 2-D signal processing attracts attention from the hard disk drive industry. SMR is a magnetic recording system in order to increase the track density by writing tracks without the guard band. Therefore, the influences of writing and reading intertrack interferences (ITIs) deteriorate the quality of reproducing waveforms. Additionally, degradation of track due to overwriting and crosstalk from the adjacent tracks due to the width of reader sensitivity. The equalization using 2-D finite impulse response filter for the waveforms from three adjacent tracks is proposed as a method to reduce the influence of reading ITIs. In this paper, we evaluate the bit aspect ratio (BAR) in TDMR read/write channel under a specification of 4 Tb/in2 using bit error rate of the low-density parity-check (LDPC) coding and iterative decoding system obtained by computer simulation. The results show that the suitable BAR in array head reading is 2.0.

Simulations of recording on media with two and three distinct recording layers are presented. A high frequency field is used to select the layer on which data are recorded. Layer-selective recording was confirmed for two layer granular media with narrow distributions. For three layer media magnetostatic interactions between the layers led to a loss of selectivity in the bottom recording layer. Readout of data is possible by staggering transitions in different layers or by using a spin-torque oscillator.

A micromagnetic model analysis was used to investigate spin-transfer torque oscillators (STOs) and magnetic write heads for microwave-assisted magnetic recording (MAMR). Our goal was to propose STOs, the Gilbert damping constant of the write head, and write head configurations that allow the STO element to operate at a low injected current density J. First, we derived the write head field and field generation layer rotations for various models with various main pole (MP) widths and MP throat heights. Second, we used the write head model and tried to reduce J by optimizing the Gilbert damping constant of the write head. Then, we used various STOs and write head structures and derived STO fields, clockwise and counterclockwise components, and FGL angles versus J. Finally, we performed media recording simulations and noted some challenges in the MAMR simulations.

This paper describes how microwave-assisted magnetic recording can be used to reduce adjacent track erasure and increase areal density in dual-thickness and dual-layer bit-patterned media. By tuning the high-frequency magnetic field, selective switching of dots in different layers or dots with different thicknesses can be achieved. Offsetting arrays of dots in different layers makes it easier to detect the signal from each layer, and areal densities of 5 Tb/in2, or more, may be possible.

The spin-torque oscillator (STO) is the most important component in microwave-assisted magnetic recording (MAMR). To realize MAMR, a small injected STO current is mandatory, as are large amplitude and stable oscillations. In this paper, an isolated STO element (without the write head) and an STO integrated into a write head were analyzed using micromagnetic analysis. First, the integrated STO was used and the optimum injected currents ( $J_{{\textrm {opt}}}\text{s}$ ) that excite high-frequency magnetic fields were derived for various Gilbert damping factors ( $\alpha \text{s}$ ) and saturation magnetization ( $4\pi M_{s}$ ) in the field generating layer. Second, $J_{{\textrm {opt}}}\text{s}$ were derived for various isolated STO structures. The effect of the material characteristics on $J_{{\textrm {opt}}}$ was also investigated.

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.

Ultrashort magnetic field pulses of tens of picoseconds duration are able to reverse the magnetization of a particle if the applied field is perpendicular to the initial magnetization vector. Here, a dual write head structure combining ring and single pole type (SPT) heads is proposed to generate such fields in a recording medium in order to increase the rate at which data can be written. Using such a head, the magnetization reversal time can be reduced to 1/2–1/3 of that when using an SPT head.

The 2-D magnetic recording (TDMR) by shingled magnetic recording (SMR) draws attention as a technology to increase the recording densities in a hard disk drive. The magnetization pattern is curved by the write field at the corner of the writer in SMR, and the information to read/write (R/W) channel performance is feared. Therefore, it is possible that the bit error rate performance is improved by fitting the reader sensitivity to the bit shape. In this paper, we evaluate the effect of reader sensitivity rotation in the TDMR R/W channel with head skew under a specification of 4 Tb/in2 by computer simulation, applying the low-density parity-check coding and the iterative decoding system with the 2-D finite impulse response filter. The results show that the rotated reader sensitivity fits the recorded bit magnetization shape, and the error-free skew range increases to $-3^{\circ }\sim 6^{\circ }$ . Therefore, the reader sensitivity rotation is considered to be an effective way to improve the skew margin in the TDMR system.

2-D magnetic recording (TDMR) exploits 2-D signal processing using the waveforms reproduced by array head from the intended and adjacent tracks to mitigate the effects of intertrack interference. However, an array head has the issue on the degradation of bit error rate (BER) performance due to head skew. In this paper, we evaluate the BER performance of TDMR R/W channel with head skew under a specification of 4 Tbit/in $^{2}$ employing low-density parity check coding and iterative decoding system using a 2-D finite-impulse response filter by computer simulation. The results show that our system can achieve the reliable BER performances by appropriately setting the reader scanning positions at the head skew angle between 0° and 3°.

In order to avoid the requirement for a narrow track reader in high track density shingled recording, decoding schemes for two-track simultaneous reading, which allows a wide-track reader to be used, are investigated. The strong intertrack interference (ITI) is a major difficulty to be overcome. To avoid ITI, the recorded magnetization in the two tracks should be staggered. Two possible 2-D decoding techniques are provided: 1) recursive decoding by oversampling and 2) Partial Response Maximum Likelihood (PRML) detection. Because data in two track are read simultaneously, the data transfer rate is two times higher than in regular single-track reading. Despite the high data rate, the requirement on the reader for high readback resolution, such as a small shield gap length, magnetic spacing, can be mitigated.

This paper presents a method to solve the ‘replication site decision problem’ (RSDP) in a short computation time in the case of multiple replicas. RSDP is a problem of finding which combination with pairs of primary-replication sites is the safest when an assumed disaster such as an earthquake affects hundreds or thousands of sites. The existing representation of RSDP is solvable, but it frequently takes much computation time to seek an optimal solution because numerous replicas cause a rapid increase in the number of primary-replication site combinations. The proposed heuristic method, derived from redundancy-based problem partitioning and iterative parameter update techniques, reduces the number of combinations at the slight cost of data availability in the disaster-affected area. Computation time evaluation shows that the proposed method with two or three replicas costs at most twice or thrice, respectively, as much time as that of the original RSDP with one replica, independently of the number of sites. However, the original RSDP with two replicas costs 5 times as much time as that of the original RSDP at 10 sites and 3036 times at 80 sites. Moreover, the data availability cost of the proposed method is only 0.1%. © 2015 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

Due to the increasing need for timely and reliable access to user-generated content, Distributed Storage Systems (DSS) became more relevant in the past years. However, since they have more interconnections among layers compared to traditional network applications, load imbalance issues arise. In this paper, we propose a hybrid approach combining server and link load balancing for multipath routing in DSS. The approach is Software Defined Networking (SDN) based, and uses a process we call on-demand inverse multiplexing. Preliminary results show that, by applying the proposal, the overall throughput considerably increases and resource usage remain balanced.

Micromagnetic simulations were carried out to obtain stable oscillation of a spin transfer torque oscillator (STO) inserted into the gap between the main pole and trailing shield of a write head. We assumed a spin injection layer with in-plane anisotropy. Results show that reducing the magnetostatic interaction between the write head and the STO is the key to obtaining stable STO oscillation.

The use of Distributed Storage Systems (DSS) has considerably increased in the past years, alongside the need for effective data transfer from storage to storage. Although current network infrastructure can reliably handle large amounts of traffic, networking techniques have not changed for several years, leading to an under-use of resources, i.e. most routing solutions still use single-path routing. In this paper, we present a pragmatic approach for multipath routing in DSS, which is based on Software Defined Networking (SDN) that uses parallel links at the edge-side. Path discovery is calculated by finding the k-maximum disjoint paths in a multigraph. Preliminary results show that, by using our multipath solution, not only the overall throughput increases but also the efficiency of resources usage.

Models of microwave-assisted magnetic recording are used to investigate the potential of the technology. With both single- and double-layer [exchange coupled composite (ECC)] media, the higher effective head field gradient led to reduced transition jitter and higher SNR in written tracks. To reduce the high-frequency field to a realistic frequency range, the use of ECC media is required. However, the ECC medium design must consider interactions between the spin torque oscillator and the medium.

The two-dimensional magnetic recording (TDMR) by shingled magnetic recording needs two-dimensional (TD) signal processing using the multiple read-back waveforms. In this paper, to allocate dual readers (reader 1 and 2) for the intended tracks, we evaluate the bit error rate (BER) performance of a low-density parity-check coding and iterative decoding system with a TD-finite impulse response equalizer under TDMR R/W channel specifications of 4 Tb/in2. Here, the TDMR R/W channel is modeled using a discretized granular medium and the writing process based on a slope model considering the media switching field angle given by the Stoner-Walfarth reversal mechanism. The result shows that to affect the good BER performance, the reader 1 should be placed slightly toward the next track from the center of the home track, and the reader 2 should be placed around the boundary between the home and following tracks.