Multiscale modeling of current-induced switching in magnetic tunnel junctions using ab initio spin-transfer torques

Abstract

There exists a significant challenge in developing efficient magnetic tunnel junctions with low write currents for nonvolatile memory devices. With the aim of analyzing potential materials for efficient current-operated magnetic junctions, we have developed a multi-scale methodology combining ab initio calculations of spin-transfer torque with large-scale time-dependent simulations using atomistic spin dynamics. In this work we introduce our multiscale approach, including a discussion on a number of possible schemes for mapping the ab initio spin torques into the spin dynamics. We demonstrate this methodology on a prototype Co/MgO/Co/Cu tunnel junction showing that the spin torques are primarily acting at the interface between the Co free layer and MgO. Using spin dynamics we then calculate the reversal switching times for the free layer and the critical voltages and currents required for such switching. Our work provides an efficient, accurate, and versatile framework for designing novel current-operated magnetic devices, where all the materials details are taken into account.

Quantum spin transfer torque induced nonclassical magnetization dynamics and electron-magnetization entanglement

“…The origin of STT is transfer of spin angular momentum from electrons to local magnetic moments of the FM layer, so it is fundamentally a nonequilibrium quantum many-body physics effect. Nevertheless, local magnetic moments are typically treated as classical vectors of fixed length [1, 4] whose dynamics is governed by the Landau-Lifshitz-Gilbert (LLG) equation [5] extended by adding the STT term [6][7][8] T ∝ ŝ e × S(r).(1)Thus, the nonequilibrium spin density ŝ e caused by flowing electrons must be noncollinear to the direction of local spin S(r) [i.e., to the local magnetization proportional to local spin], to drive magnetization dynamics in such a classical picture. The dynamics can include oscillations or complete reversal, whose conversion into resistance variations has emerged as a key resource for next generation spintronic technologies, such as nonvolatile magnetic random access memories, microwave oscillators, microwave detectors, spin-wave emitters, memristors and artificial neural networks [9][10][11].For example, passing current through a spin valve trilayer fixed-FM/normal-metal/free-FM, as employed in early experiments on standard STT [12,13], causes first FM layer with fixed magnetization to spin-polarize the current which then impinges onto the second FM layer with free magnetization that fluctuates in the classical picture due to a random magnetic field caused by thermal motion.…”

“…The origin of STT is transfer of spin angular momentum from electrons to local magnetic moments of the FM layer, so it is fundamentally a nonequilibrium quantum many-body physics effect. Nevertheless, local magnetic moments are typically treated as classical vectors of fixed length [1,4] whose dynamics is governed by the Landau-Lifshitz-Gilbert (LLG) equation [5] extended by adding the STT term [6][7][8] T ∝ ŝ e × S(r).…”

“…port theories, such as the nonequilibrium Green function formalism [7,8,24,25] or the scattering matrix approach [26,27], are routinely used to compute ŝ e in Eq. (1) for a given single-particle Hamiltonian, but this serves only as an input [7,8] for the LLG equation describing classical dynamics of magnetization. The LLG equation can be justified under the assumptions [5] of large spin S → ∞, → 0 (while S × → 1) and in the absence of entanglement.…”

“…Instead of classical micromagnetics [4,14] or quantumclassical [7,8] description of standard-STT-induced magnetization dynamics, here we introduce a quantum manybody picture of both flowing-electron-spin-local-spins interactions and the ensuing time evolution of local spins at zero temperature. For this purpose, we employ a system depicted in Fig.…”

Microwave Oscillator Based on a Single Straintronic Magnetotunneling Junction

“…4, we show how the charge current flowing through the MTJ varies with time. The peak current density is 6.5× 10 11 A/m 2 which is high, but not completely unreasonable since current densities of this order have been used in MTJs [6,14,15]. Reducing it would have required reducing the power supply voltage Vs.…”