James G. Deak

Spin injection in thermally assisted magnetic random access memory

An integrated thermal, micromagnetic, spin-momentum-transfer (SMT) model was developed to study the effect of SMT on the programming current required for thermally assisted magnetic random access memory (MRAM). The thermal portion of the model is used to compute Joule heating by the spin-polarized current, and it is based on a Crank–Nicolson inhomogeneous heat equation solver. The magnetic portion of the model is based on a micromagnetic Langevin dynamic Landau–Lifshitz–Gilbert solver including SMT torque. Simulations of thermally assisted magnetization reversal of 0.09-μm MRAM elements, heated by passing current through the barrier separating the pinned and free layers, were performed. The free layer of the MRAM elements was switched using a magnetic field at fixed heating-SMT current bias. Results suggest that a spin-polarized heating current can be used to lower the programming current required to write thermally assisted MRAM if the direction of the heating current is properly synchronized with the re...

Finite temperature micromagnetics and magnetic measurements of submicron patterned permalloy thin films

Micromagnetic simulation using the deterministic Landau–Lifshitz–Gilbert (LLG) equation is inadequate for predicting the coercivity of submicron patterned thin films. The discrepancy results because the deterministic LLG equation only provides a zero-temperature description of the magnetization processes of a ferromagnetic material. In order to properly simulate the coercivity, the stochastic LLG equation, which includes thermal effects through a fluctuating magnetic field, must be used. Direct comparison of measurements of the coercivity of arrays of submicron patterned permalloy thin films with simulation has been used to show that the second-order Heun scheme is adequate for this purpose. In addition, the simulated temperature dependence of the magnetization of the patterned bits tracks the measured dependence.

Thermally activated vortex nucleation as a function of thickness in submicron patterned permalloy thin films

Arrays of submicron patterned magnetic thin films often show tails in their hysteresis loops, which are usually attributed to either the coercivity distribution in the array or vortex formation/domain wall trapping in individual bits. In arrays of submicron-sized elliptical patterned bits, the tails can appear at less than the critical thickness for vortex formation, suggesting that vortices are unlikely. Thermal fluctuations, however, allow small magnetic systems to sample magnetic states that are normally inaccessible at zero temperature, which can result in different switching processes than would be expected based on zero-temperature simulations. In order to study the effect of thermal fluctuations on switching processes, arrays of submicron patterned magnetic films of different sizes and over a range of thickness were measured using the magnetooptic Kerr effect (MOKE) and compared with Langevin micromagnetic simulations. The comparison indicates that the presence of hysteresis loop tails in arrays of patterned bits is a result of vortex formation in individual bits.

Influence of Pinned-Layer Dispersion on Magnetic Tunnel Junction Switching Distributions

The influence of pinning field dispersion (PFD) on the coercivity (Hc) and offset field (Ho) of IrMn/NiFeCo/AlOx/NiFeCo magnetic tunnel junctions (MTJ) has been studied by repeated in situ measurement of the resistance as a function of magnetic field, R(H), hysteresis loops of the MTJs after resetting the IrMn/NiFeCo pinned layer. Magneto-thermal magnetic random access memory, MT-MRAM, cells were used to perform this task. Here, the pinned layer is reset by pulsing a current through the tunnel barrier of the MT-MRAM MTJ in the presence of an external magnetic field. This permits rapid field cooling of the pinned layer through the blocking temperature of the IrMn. Repeated measurements of an R(H) loop after a pinned layer reset cycle show only minor variation. R(H) loops measured for different pinned layer reset cycles however, can show large changes in Hc , Ho, and R(H) loop shape. The effect is believed to be due to the random magnetostatic field distribution resulting from the frozen-in PFD of the IrMn/NiFeCo pinning layer. In order to test this conclusion, the evolution of the R(H) loop is studied as a function of reset pulse amplitude. Because of the intrinsic distribution of blocking temperatures of the IrMn layer, PFD should decrease as reset pulse amplitude increases. Results show that Hc increases, while the R(H) loops become more square as the reset pulse amplitude is increased. Ho shows a more complex dependence, which is a competition between ferromagnetic magnetic-roughness-induced Neel coupling and anti-ferromagnetic stray field coupling. A micromagnetic model for studying the dependence of the R(H) loops on the alignment of the IrMn crystallites was developed, and simulated results are in good agreement with the measurements. This work provides a simple explanation for large switching field distributions that can result in arrays of seemingly identical MRAM bits

Langevin micromagnetic simulation and MOKE measurements of vortex formation as a function of thickness in submicron patterned permalloy thin films

In this paper, Langevin micromagnetic simulation and MOKE measurements of vortex formation as a function of thickness in submicron patterned permalloy thin films have been investigated.