Half-metallic magnetism and the search for better spin valves

TL;DR

This paper analyzes how thermal fluctuations affect spin valve performance and suggests that half-metallic ferromagnets with high transition temperatures can improve spin valve efficiency.

Contribution

It introduces a theory explaining temperature effects on tunneling magnetoresistance and proposes material criteria for optimizing spin valves.

Findings

Thermal fluctuations limit spin valve performance at finite temperatures.

High spin polarization and transition temperatures enhance spin valve performance.

Half-metallic ferromagnets outperform current systems if their transition temperatures exceed ~950 K.

Abstract

We use a previously proposed theory for the temperature dependence of tunneling magnetoresistance to shed light on ongoing efforts to optimize spin valves. First we show that a mechanism in which spin valve performance at finite temperatures is limited by uncorrelated thermal fluctuations of magnetization orientations on opposite sides of a tunnel junction is in good agreement with recent studies of the temperature-dependent magnetoresistance of high quality tunnel junctions with MgO barriers. Using this insight, we propose a simple formula which captures the advantages for spin-valve optimization of using materials with a high spin polarization of Fermi-level tunneling electrons, and of using materials with high ferromagnetic transition temperatures. We conclude that half-metallic ferromagnets can yield better spin-value performance than current elemental transition metal ferromagnet/MgO systems only if their ferromagnetic transition temperatures exceed $\sim 950~\mathrm{K}$.