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We use a previously proposed theory for the temperature dependence oftunneling magnetoresistance to shed light on ongoing efforts to optimize spinvalves. First we show that a mechanism in which spin valve performance atfinite temperatures is limited by uncorrelated thermal fluctuations ofmagnetization orientations on opposite sides of a tunnel junction is in goodagreement with recent studies of the temperature-dependent magnetoresistance ofhigh quality tunnel junctions with MgO barriers. Using this insight, we proposea simple formula which captures the advantages for spin-valve optimization ofusing materials with a high spin polarization of Fermi-level tunnelingelectrons, and of using materials with high ferromagnetic transitiontemperatures. We conclude that half-metallic ferromagnets can yield betterspin-value performance than current elemental transition metal ferromagnet/MgOsystems only if their ferromagnetic transition temperatures exceed $\sim950~\mathrm{K}$.

Half-metallic magnetism and the search for better spin valves