Xiaochun Zhu

Microwave Assisted Magnetic Recording

In this paper, we present a novel mechanism for recording at a head held significantly below the medium coercivity in a perpendicular recording geometry. By applying a localized ac field at adequate frequency to the perpendicular recording medium, saturation recording can be achieved with recording field amplitudes significantly below the medium coercivity, or the medium perpendicular anisotropy field. A scheme utilizing spin torque to generate a localized ac field at high frequency (tens of gigahertz) with kilo-oersted field amplitude in the medium is proposed along with a systematic modeling analysis. Recording simulations at high linear densities are presented.

Magnetic bistability and controllable reversal of asymmetric ferromagnetic nanorings.

Magnetization reversals through the formation of a vortex state and the rotation of an onion state are two processes with comparable probabilities for symmetric magnetic nanorings with a radius of about 50 nanometers. This magnetic bistability is the manifestation of the competition between the exchange energy and the magnetostatic energy in nanomagnets. The relative probability of the two processes in symmetric nanorings is dictated by the ring geometry and cannot be altered after fabrication. In this work, we report a novel type of nanorings--asymmetric nanorings. By tuning the asymmetry, we can control the fraction of the vortex formation process from about 40% to nearly 100% by utilizing the direction of the external magnetic field. The observed results have been accounted for by the dependence of the domain-wall energy on the local cross-section area for which we have provided theoretical calculations.

Bias-Field-Free Microwave Oscillator Driven by Perpendicularly Polarized Spin Current

In this paper, we propose a novel design of spin-torque-driven microwave oscillator that is free of bias magnetic field. The oscillator consists of a perpendicularly magnetized spin-polarization layer and an oscillating bilayer. The oscillating bilayer comprises a spin-torque-driven layer and a magnetic layer of perpendicular anisotropy with adequate interlayer-exchange coupling. The perpendicularly spin-polarized current can yield a sustained stable magnetization oscillation in the oscillating bilayer around the perpendicular easy axis. The oscillating frequency can be tuned from zero to tens of gigahertz by varying the inject current density. The underlying physical mechanism of the oscillation is discussed, and a micromagnetic analysis on the characteristics of the oscillator is reported

Spin Torque and Field-Driven Perpendicular MRAM Designs Scalable to Multi-Gb/Chip Capacity

In this paper, we present a micromagnetic analysis of two novel magnetoresistive memory designs, both of which utilize the material-intrinsic perpendicular uniaxial magnetic anisotropy for retaining memory states. The analysis shows that such perpendicular memory element design allows the utilization of thick magnetic film, thereby enabling downsize scalability of each memory element while maintaining sufficient thermal stability. One of the designs is to utilize direct current injection for switching the memory states via the effect of spin momentum transfer. The other design utilizes current-generated field for switching. The performance characteristics of both designs are reported

Spin transfer induced noise in CPP read heads

In this paper, spin transfer induced magnetic noise in current-perpendicular-to-plane (CPP) spin valve heads is investigated via micromagnetic modeling with spin transfer torque included. It is found that there exists of a critical sense current density beyond which the spin transfer effect yields a substantial noise with a pronounced (1/f/sup n/) spectral content at low frequencies. This magnetic noise arises from the essentially chaotic magnetization precessions excited by the spin transfer effect that can effectively acts as energy pumping to the spin system. The onset of the noise occurs when the pumping rate becomes equal to the rate of energy dissipation. The critical current density is found to be proportional to the damping constant /spl alpha/ as well as the longitudinal bias field.

A vertical MRAM free of write disturbance

Write disturbance in the cross-point addressing scheme employed in most of todays magnetoresistive random access memories (MRAM) designs presents practical limitations in memory element down-size scaling. In this paper, we present a new vertical MRAM design that is free of write disturbance. Its performance is analyzed via micromagnetic modeling. A memory element in this design is of an annular shape and consists of two ferromagnetic layers with a nonmagnetic interlayer. The interlayer can be either a tunnel barrier for a magnetic tunnel junction or metal layer for a current-perpendicular-to-plane/giant magnetoresistive stack. Injecting a pulsed current in a nanosecond time scale vertically through the memory element performs the switching between the two memory states with relatively low current threshold. Each memory element is connected to a corresponding transistor that performs both write and read addressing.

Spin transfer excited regular and chaotic spin waves in current perpendicular to plane spin valves

Spin polarized current induced spin wave excitations have been studied with utilizing the spin transfer modified Gilbert equation in micromagnetic modeling. An elliptical shaped spin valve at deep submicron dimension is modeled. It is found that stable spin waves with extremely narrow linewidth are excited in the free layer when the perpendicularly injected direct current is slightly above a threshold. The spin transfer effect yields self leveling of the generated spin waves and results in spin waves with stable amplitude for each excited mode. At high current level, the excited spin waves become chaotic in nature, causing a pronounced 1/f-like spectral content in the magnetoresistive output.

Current induced noise in CPP spin valves

In this paper, an experimental investigation of current-induced noise in current perpendicular-to-plane spin-valve heads is reported. It is found that the sense current yields excitations of coherent spin waves as well as substantial 1/f-like noise when current exceeds a critical value. The noise is magnetic in nature. The criticality and current directional dependence of the noise excitation clearly show that the spin transfer is the cause.

On the shape optimization of magnetic random access memory element design

In designing the shape of magnetic random access memory elements, a sufficient difference in switching current threshold between a full-select element and a half-select memory element is critical. In this article, we present a systematic micromagnetic study of the margin of switching threshold for two specific shapes: eye shaped and ellipse. It is found that at small magnetic thickness, the eye-shaped element exhibits a higher switching threshold margin than the ellipse. However, for relatively thick storage layers, the opposite becomes true. The switching threshold is also a strong function of the initial magnetization state for the ellipse.

Effect of Damping Constant on Magnetic Switching in Spin Torque Driven Perpendicular MRAM

In this paper, we present a spin torque included dynamic micromagnetic modeling investigation on the perpendicular magnetoresistive random access memory design. A small inject current always generates a steady magnetization precession of the composite storage layer around perpendicular axis. For a given current density, the precession frequency is lower when the Gilbert damping constant of the perpendicular layer is higher. It is found that magnetization reversal of the perpendicular storage layer occurs when the lateral precession frequency reaches the ferromagnetic resonance condition. To reach the required precession frequency, a higher current density is required for a great value of damping constant of the perpendicular layer