Ultra-thin magnetic film with giant phonon-drag for heat to spin current conversion.

A tightly confined 2D electron gas with good carrier mobility and large spin-polarization is an essential ingredient for the implementation of spin-caloritronic conversion device technology. Here we give evidence that the SrTiO/EuTiO/LaAlO heterostructure is a prototype material for this purpose. The presence of Eu induces strong spin-polarization in the 2D electron gas spontaneously formed at the interface and ferromagnetic order at low temperature. Furthermore, tight 2D confinement and spin-polarization can be highly enhanced upon charge depletion, in turn generating huge thermopower associated with the phonon-drag mechanism. Most importantly, the remarkable difference in the population of the two spin channels results in the giant spin-polarized Seebeck effect and in turn, giant spin voltages of mV K order at the two ends of an applied thermal gradient. Our results represent a strong assessment to the capabilities of this interface for low-temperature spin-caloritronic applications.