Detecting and switching magnetization of Stoner nanograin in nonlocal spin valve

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The magnetization detection and switching of an ultrasmall Stoner nanograin in a nonlocal spin valve (NLSV) device is studied theoretically. With the help of the rate equations, a unified description can be presented on the same footing for the NLSV signal that reads out the magnetization, and for the switching process. The setup can be viewed as that the grain is connected to two nonmagnetic leads via sequential tunneling. In one lead, the chemical potentials for spin-up and -down electrons are split due to the spin injection in the NLSV. This splitting (or the spin bias) is crucial to the NLSV signal and the critical condition to the magnetization switching. By using the standard spin diffusion equation and parameters from recent NLSV device, the magnitude of the spin bias is estimated and found large enough to drive the magnetization switching of the cobalt nanograin reported in earlier experiments. A microscopic interpretation of NLSV signal in the sequential tunneling regime is thereby raised, which show properties due to the ultrasmall size of the grain. The dynamics at the reversal point shows that there may be a spin-polarized current instead of the anticipated pure spin current flowing during the reversal due to the electron accumulation in the floating lead used for the readout of NLSV signal.