  The effect of spin relaxation on tunnel magnetoresistance (TMR) in a
ferromagnet/superconductor/ferromagnet (FM/SC/FM) double tunnel junction is
theoretically studied. The spin accumulation in SC is determined by balancing
of the spin-injection rate and the spin-relaxation rate. In the superconducting
state, the spin-relaxation time becomes longer with decreasing temperature,
resulting in a rapid increase of TMR. The TMR of FM/SC/FM junctions provides a
useful probe to extract information about spin-relaxation in superconductors.
