Half-metallic magnetism and the search for better spin valves

We propose a simple formula for the temperature dependence of tunneling magnetoresistance to shed light on ongoing efforts to optimize spin valves. It captures 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. Furthermore, it directly reveals the advantages for spin-valve optimization by using materials with a high spin polarization of Fermi-level tunneling electrons, and by using materials with high ferromagnetic transition temperatures. We show that our theory is in good agreement with recent experimental studies of the temperature-dependent magnetoresistance of high-quality tunnel junctions with MgO barriers. 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 $\sim 950~\mathrm{K}$.\\[0.2em] \noindent [1] K.\ Everschor-Sitte, M.\ Sitte and A.\ H.\ MacDonald, J.\ Appl.\ Phys.\ \textbf{116}, 083906 (2014).