Large and inverted spin signals in nonlocal spin valves

A nonlocal spin valve (NLSV) is a nanoscale planar heterostructure, consisting of a spin injector, a spin detector, and a nonmagnetic channel. A pure spin current can be induced in the nonmagnetic channel by electrical spin injection. We report large but inverted spin signals in a set of NLSV structures. The interface between the ferromagnetic spin detector and the nonmagnetic channel is found to be a break junction formed by electrostatic discharge. A large (> 80%) but negative tunneling spin polarization across the break junction is inferred. The large magnitude is due to a strong coupling between the charges and spins at the low-conductance break junction. The inverted sign results from the specific interfacial atomic structures and bonding states, which influence the spin-dependent tunneling matrices.