Energy and momentum conservation in spin transfer*

Spin transfer (ST) effect is a consequence of angular momentum conservation, one of the three fundamental conservation laws in nature. We use fully quantum simulations of scattering of spin-polarized electron wavepacket by a ferromagnet modelled as a 1d spin-1/2 chain, to show that energy and momentum conservation laws impose strong restrictions on the characteristics of magnetic excitations generated by spin transfer. We found that two groups of magnons are generated by ST, forward-propagating magnons with large energy, and backward-propagating magnons with small energy. Analysis shows that the backward-propagating magnons are generated by the reflected electron component, while the forward-propagating magnons - by the transmitted electron component, with the values of energy and momenta of the generated magnons determined by the changes of the corresponding characteristics of the electron wave. We also show a converse effect – dependence of the scattered electron’s characteristics on the dynamical characteristics of the magnetic system. Our results demonstrate that quantum magnetism plays an important role in magnetoelectronic phenomena.