Jian-Gang Zhu

Ultrahigh density vertical magnetoresistive random access memory (invited)

In this paper, we present the vertical magnetoresistive random access memory (VMRAM) design based on micromagnetic simulation analysis. The design utilizes the vertical giant magnetoresistive effect of the magnetic multilayer. By making the memory element into a ring-shaped magnetic multilayer stack with orthogonal paired word lines, magnetic switching of the memory device becomes very robust. The design also adopts the readback scheme in pseudo spin valve MRAM so that only one transistor is needed for each bit line which can connect hundreds of memory elements, yielding a very high area density. It is estimated that the ultimate area density for the VMRAM is 400 Gbits/in.2. It is suggested that this memory design has the potential to not only replace the present semiconductor memory devices, such as FLASH, but also the potential to replace DRAM, SRAM, and even disk drives.

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 tunnel junctions

Fueled by the ever-increasing demand for larger hard disk drive storage capacities, extensive research over the past decade has resulted in the development of AlO x - and TiO x -based magnetic tunnel junctions that exhibit a large magnetoresistive effect at room temperature. As their commercialization in various applications begins, a new type of magnetic tunnel junction with a crystalline MgO tunnel barrier has emerged that shows a much larger room-temperature magnetoresistive effect. We present a brief overview of the development of magnetic tunnel junctions, introducing the underlying physics. We also discuss two important commercial applications: read sensors in hard disk drives and memory elements in magnetoresistive random access memory. An emphasis is placed on the material aspects of magnetic tunnel junctions.

Micromagnetic studies of thin metallic films (invited)

A computer simulation model has been developed to conduct micromagnetic studies of thin magnetic films. Thin‐film media are modeled as a planar hexagonal array of hexagonally shaped grains. Each grain is a single domain particle whose magnetization reverses by coherent rotation. The computation utilizes coupled gyromagnetic dynamic equations with phenomenological Landau–Lifshitz damping. In particular, the effects of particle interactions are investigated. The effect of media microstructure on magnetic hysteresis is examined as well as the effect of intergranular exchange coupling. The difference between planar and completely random orientation of the crystalline anisotropy axes is discussed. Recorded transitions are simulated by allowing a pair of perfect transitions to relax. With no intergranular exchange coupling, the transitions show profound irregularity and zig‐zag structure. Intergranular exchange coupling produces more uniform transitions with increased zig‐zag structure amplitude. For a closely ...

Magnetoresistive Random Access Memory: The Path to Competitiveness and Scalability

This paper provides an in-depth review of the magnetoresistive random access memory technology and its developments over the past decade. Both the traditional field-driven and more recent spin torque transfer driven designs are discussed. By pointing out key technical challenges, important aspects and characteristics of various designs are used to illustrate mechanisms that overcome the technical obstacles. A significant portion of this paper is devoted to the principles of various designs based on spin torque transfer effect, including memory elements with in-plane and perpendicular magnetic electrodes.

Switching field variation in patterned submicron magnetic film elements

In this article, a micromagnetic study of magnetic switching properties on submicron scale single layer and multilayer thin film elements is presented. Even at deep submicron scale, there exist various edge domain configurations at the saturation remanent state. It is found that the switching field of these patterned film elements can strongly depend on these edge domain configurations. If the edge domains are not controlled, switching field of a patterned magnetic film element can vary significantly during repeated switching processes.

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.

Co∕Pt multilayer based magnetic tunnel junctions using perpendicular magnetic anisotropy

Magnetic tunnel junctions that utilize perpendicular magnetic anisotropy have attracted growing attention due to their potential for higher storage densities in future high capacity magnetic memory applications. In this study, we present an experimental demonstration of magnetic tunnel junctions composed of perpendicularly magnetized Co∕Pt multilayer electrodes and an AlOx tunnel barrier. The emphasis has been on how to maximize the thickness of the Co layers adjacent to the tunnel barrier while still magnetized perpendicularly for possible spin torque utilization in future applications. It is found that the thickness ratio between the Co and Pt layers and the number of bilayers were significant parameters to customize the magnetic properties. The difference between the switching fields of the soft and the hard layers can be adjusted by the number of repeats of the Co∕Pt bilayers. The measured hysteresis shows virtually zero exchange coupling between the two layers through the tunnel barrier. Measured tun...

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