Quantum Spin Transfer of Spin-Correlated Electron Pairs

We theoretically investigate quantum spin transfer from spin-correlated conduction-electron pairs to localized spins in a ferromagnet, given that electrons are correlated intrinsically. We show that even spin-singlet pairs and triplet pairs with $m=0$, both carrying no net spin, can transfer finite angular momentum through the quantum fluctuation term inherent to the $sd$ exchange interaction. The amount of transferred spin differs between the singlet and triplet $m=0$ states due to quantum interference. The difference is such that the independent-electron approximation remains valid for spin transfer when injected spin currents are completely incoherent. However, in partially coherent systems, like superconductor/ferromagnet junctions, coherent spin-singlet currents can directly convert into equal-spin triplet currents in generic ferromagnets, without requiring magnetic inhomogeniety or spin-orbit coupling.