Bingjin Chen

Write Failure Analysis for Bit-Patterned-Media Recording and Its Impact on Read Channel Modeling

In this paper, we present a write failure analysis for bit-patterned media recording (BPMR) with formulas derived to calculate its probability for a given set of system parameters. The write failure model derived in this work includes the effect of the recording head and media characteristics and that of a hard disk spindle motor speed variations. Such a model can help assess the performance of the writing strategy for BPMR. It is also found that since the media local parameters associated with each bit directly affect both the write failure rate and read-back signal, the read-back signal actually contains information regarding whether a bit is correctly recorded. This may be exploited by coding and detection schemes to reduce the impact of write failure on bit-error-rate performance.

Channel Modeling and Equalizer Design for Staggered Islands Bit-Patterned Media Recording

In this paper, we first present a recording physics based analytical channel model for bit-patterned media recording (BPMR) systems with staggered islands configuration. We further propose an analytical approach to jointly design the equalizer with partial response (PR) target for BPMR systems, by using the minimum mean-squared error (MMSE) criterion with monic constraint, taking into account inter-track interference (ITI) and media noise. Simulation results show that the proposed approach performs better than the system designed without considering ITI and media noise. We further investigate the performance of staggered array against regular array islands with bit-aspect ratios (BAR) of 1 and 2, and with different amount of media noise, inter-symbol interference (ISI), and ITI. We found that staggered array islands with BAR of 2 offer better bit error rate (BER) performance and better tolerance to media noise.

Electric Field Assisted Switching in Magnetic Random Access Memory

Electric field (EF)-assisted magnetization reversal is investigated in both top-pinned and bottom-pinned CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJs). EF modulation in coercivity (Hc) shows an increasing dependence with the thickness of the free layer (tFL) at small tFL values. This abnormal variation was attributed to possible Ta diffusion through the free layer to the interface with MgO barrier. It is found that the bidirectional switching is not achievable using unipolar EF only in our MTJ devices, which is due to small spin-transfer torque (STT) effect and switching uncertainty due to unipolar feature of EF-induced Hc modulation. A bipolar external magnetic field should be applied to realize EF-controlled magnetic random access memory (MRAM). For quasistatic magnetization reversal, the required field can be as low as 10 Oe when Hc is effectively modulated to nearly zero. Switching field as a function of switching time shows that the magnetization switching is dominated by thermal activation. Simulation results show that switching time is mainly determined by the damping constant and the EF efficiency. Different from STT-MRAM, a large damping constant is desired to achieve fast switching. Using a sweeping field method, we show that MTJs with a thermal stability factor as high as 58 can be reliably switched using an EF-modulated anisotropy scheme with a bipolar magnetic field as low as 10 Oe.

Channel capacity and soft-decision decoding of LDPC codes for spin-torque transfer magnetic random access memory (STT-MRAM)

Spin-torque transfer magnetic random access memory (STT-MRAM) has emerged as a promising non-volatile memory (NVM) technology, featuring compelling advantages in scalability, speed, endurance, and power consumption. In this paper, we focus on large-capacity stand-alone STT-MRAM, and investigate the channel capacity and the viability of applying low-density parity-check (LDPC) codes with soft-decision decoding to correct the memory cell errors and improve the storage density of STT-MRAM. We propose to use LDPC codes with short codeword lengths, with the reliability-based min-sum (RB-MS) algorithm for decoding. Furthermore, we propose to use the capacity-maximization criterion to design the quantizer and minimize the number of quantization bits. Simulation results demonstrate the potential of applying short-block-length LDPC codes with soft-decision decoding to improve the yield and push the scaling limitation of STT-MRAM.

Channel Capacity and Soft-Decision Decoding of LDPC Codes for Spin-Torque Transfer Magnetic Random Access Memory (STT-MRAM)

Spin-torque transfer magnetic random access memory (STT-MRAM) has emerged as a promising non-volatile memory (NVM) technology, featuring compelling advantages in scalability, speed, endurance, and power consumption. In this paper, we focus on large-capacity stand-alone STT-MRAM, and investigate the channel capacity and the viability of applying low-density parity-check (LDPC) codes with soft-decision decoding to correct the memory cell errors and improve the storage density of STT-MRAM. We propose to use LDPC codes with short codeword lengths, with the reliability-based min-sum (RB-MS) algorithm for decoding. Furthermore, we propose to use the capacity-maximization criterion to design the quantizer and minimize the number of quantization bits. Simulation results demonstrate the potential of applying short-block-length LDPC codes with soft-decision decoding to improve the yield and push the scaling limitation of STT-MRAM.

Effect of Read Head Scaling on Servo and Data Signal Characteristics for Staggered Two-Row-per-Track Bit-Patterned-Media Recording

In this paper, we investigate the effect of read head profile on the playback signal from data and servo sector in a staggered bit-patterned-media recording system. The scaling of the read head is targeted to achieve better data signal quality while such scaling is shown to have negative effect on the servo signal. A secondary servo burst that uses data samples to generate supplementary PES is then proposed and its feasibility is demonstrated.

Field-Assisted Switching of Free-Layer Magnetization in Magnetic Tunnel Junctions

We investigate the switching of electric-field (EF)-controlled magnetic tunneling junction free layer (FL) magnetization assisted by Oersted fields using micromagnetic simulations. The effects of several physical parameters, such as damping constant, magnetic anisotropy, as well as the EF efficiency and the applied Oersted fields on the switching of the FL magnetization are examined. Successful and reliable magnetization switching depends on all these parameters. The switching time is mainly determined by the time required for the magnetization of the FL to turn in-plane due to the demagnetizing field created after applying the EF.

A portable dynamics switching model for perpendicular magnetic tunnel junctions considering both thermal and process variations

Spin-transfer torque magnetic random access memory (STT-MRAM) has become a promising candidate for future nonvolatile and universal memory because it features non-volatility, fast access time, almost unlimited programming endurance and zero standby power [1-2]. The magnetic tunneling junction (MTJ) is the fundamental building element of STT-MRAM. An MTJ consists of two ferromagnetic layers (a free layer, FL and a reference layer, RL) separated by a thin tunneling dielectric film. The magnetization of the FL can be set as parallel or anti-parallel with the RL by a spin polarized current, which leads to low or high resistance state of the MTJ. The applications of STT-MRAM have been successfully demonstrated [3-4]. However, like all the other nanotechnologies, STT-MRAM suffers from process variations and environment fluctuations, such as thermal fluctuations, which significantly affect the performance and stability of MTJ devices. Several studies have been performed to address the impact of the process variations on the reliability of STT-MRAM and the thermal fluctuation effects on the magnetization switching [5-6]. These works either need costly Monte-Carlo simulations with complex macro-magnetic and SPICE models or do not integrate both effects of the thermal fluctuation and process variations. Wang et al [7] developed a compact MTJ switching model for MTJs derived from the macro-magnetic modeling to simulate the statistical electrical and magnetic properties of MTJ due to both thermal fluctuation and process variations; however it is restricted for in-plane MTJs.

Magnetization Switching of a Thin Ferromagnetic Layer by Spin-Orbit Torques

We conducted micromagnetic simulations to investigate magnetization switching dynamics in ferromagnet/nonmagnet bilayers driven by an in-plane current for 1) free layer with out-of-plane anisotropy, 2) free layer with in-plane anisotropy, and 3) free layer with both in-plane anisotropy and electric-field-controlled out-of-plane anisotropy. The effects of various material parameters on the switching performance are discussed. Based on the simulations, a fast and reliable switching scheme may be realized by using in-plane magnetization switching assisted by an electric-field-controlled out-of-plane magnetic anisotropy. This fast and deterministic switching does not require any external magnetic fields.

Modeling and information theoretic analysis of spin-torque transfer magnetic random access memory (STT-MRAM)

Spin-torque transfer magnetic random access memory (STT-MRAM) has emerged as a promising non-volatile memory (NVM) technology, which has compelling advantages in scalability, speed, endurance, and power consumption. In this paper, we first propose a resistance distribution based generic channel model for STT-MRAM. We then apply information theory and propose approaches to compute the mutual information and the capacity of the STT-MRAM channels. The presented information theoretic analysis provides valuable guideline for the design of practical channel coding and quantization schemes for STT-MRAM.