Initiation of spin-transfer torque by thermal transport from magnons

As now practiced in experimental nanomagnetic spintronics, spin-transfer torque acting on a free metallic moment is driven by electric current flowing serially through it and a metallic reference magnet. I propose driving spin-transfer torque by flow of heat serially through the free magnet and an insulating reference ferrite. The needed spin current initiates from magnons present in the ferrite. A quantum yield of heat-driven in-plane spin-transfer torque can be substantially greater, in principle, than that achievable using electric current in a magnetic tunnel junction. A Bloch-type dynamical equation for the conduction-electron-spin polarization excited by a paramagnetic-monolayer model of the ferrite/metal interface predicts the dependence of this yield on material parameters. In practice, achieving a high yield beneficial to applications will require strong exchange coupling of the local $3d$-electron atomic spins in this monolayer to both to the ferrite moment with ferromagnetic sign and also with either sign to the conduction s electrons in a normal metallic spacer. Advantageous will be suppression of the interfacial heat flow diverted to phonons within the ferrite. If a nonmagnetic electrically insulating layer additionally adjoins the free magnet, the theory also predicts a perpendicular spin-transfer torque component whose angular dependence mimics conventional uniaxial magnetic anisotropy.