  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 $\sim
950~\mathrm{K}$.
