Micromagnetic instabilities associated with domain wall dynamics in perpendicular magnetized magnetic tunnel junction nanopillars

Abstract

Magnetoresistive random access memories (MRAM) based on spin-transfer torque are a scalable low-power nonvolatile memory solution being actively developed by industry, where a perpendicularly magnetized thin disk is reversed by a spin polarized current. During the switching process domain walls (DWs) can form [1-4] and understanding their dynamics in a disk geometry becomes of great interest. Here we show that DW surface tension always leads to oscillatory DW motion and instabilities that affect the switching dynamics. In a disk, a DW generally takes a circular form to minimize the sum of the surface and volume energies of the reversed domain, with the domain boundary perpendicular to the element boundary [1]. In a collective coordinate model the domain wall has two degrees of freedom, its position relative to the center of the disk q and the angle of the spins relative to the normal to the DW Φ. Here, Φ=0 is a Neel and Φ=90 deg. is a Bloch DW. We analyze the dynamics with micromagnetic and the collective coordinate model. Fig. 1a shows micromagnetic simulations of the evolution of the average perpendicular magnetization (m_z) with fixed initial DW position q, such that m_z<0, and different initial angles Φ, with no field or current applied. Whereas both Neel and Bloch DWs states relax towards a reversed state (m_z=-1), an intermediate initial angle DWs (Φ=45 deg) switches. Fig. 1b shows the (q,Φ) phase diagram indicating that the dynamics of domains close to the disk center is sensitive to small changes in the initial phase. Fig. 2 shows the same phase diagram as in Fig. 1b, now driven by a fixed spin-polarized J/Jc= 0.1. The results from both relaxation and switching simulations indicate that the DW phase Φ plays a critical role in the dynamics. A collective coordinate model will be presented which captures aspects of these instabilities and illustrates their physical origin.