Spintronic magnetic anisotropy

Superparamagnetism of magnetic adatoms and molecules-preferential alignment of their spins along an easy axis-is a useful effect for nanoscale applications as it prevents undesired spin reversal. The underlying magnetic anisotropy barrier-a quadrupolar energy splitting-originates from spin-orbit interaction and can nowadays be probed by electronic transport measurements. Here we predict that in a much broader class of systems, quantum dots with spins larger than 1/2, superparamagnetism can arise without spin-orbit interaction: by attaching them to ferromagnets, a quadrupolar spintronic exchange field is generated locally. It is observable by means of conductance measurements and leads to enhanced spin filtering even in a state with zero average spin. Analogously to the spintronic dipolar exchange field, giving rise to a local spin torque, the effect is susceptible to electric control and increases with tunnel coupling as well as with spin polarization.