  The spin relaxation time (tau) on the spin transport in thermally evaporated
thin films of a naphthyl diamine derivative: N, N'-Bis(naphthalen-1-yl)-N,
N'-bis(phenyl)-2,2'-dimethylbenzidine (a-NPD) was evaluated with the
spin-pump-induced spin transport properties and the electrical current-voltage
properties in a-NPD films. The zero-bias mobility and the diffusion constant of
charges in a-NPD films were obtained to be about 1.2*10-3 cm2/Vs and about
3.0*10-5 cm2/s, respectively. Using these values and the previously evaluated
spin diffusion length in a-NPD films of about 62 nm, the tau in a-NPD films was
estimated to be about 1.9 micro-second at room temperature, under an assumption
of diffusive transport of the spin current in a-NPD films. This estimated tau
in a-NPD films is long enough for practical use as a spintronic molecular
material.
