Spintronic magnetic anisotropy

TL;DR

This paper predicts a new form of superparamagnetism in quantum-dot systems with larger spins, induced by ferromagnetic exchange fields rather than spin-orbit interaction, enabling electric control and enhanced spin filtering.

Contribution

It introduces a novel mechanism for superparamagnetism in quantum dots via spintronic exchange fields, expanding understanding beyond spin-orbit-based anisotropy.

Findings

Superparamagnetism can occur without spin-orbit interaction in quantum dots.

Ferromagnetic contacts generate a quadrupolar exchange field observable in conductance.

The effect enhances spin filtering and is controllable by electric means.

Abstract

An attractive feature of magnetic adatoms and molecules for nanoscale applications is their superparamagnetism, the preferred alignment of their spin along an easy axis preventing undesired spin reversal. The underlying magnetic anisotropy barrier --a quadrupolar energy splitting-- is internally generated by spin-orbit interaction and can nowadays be probed by electronic transport. Here we predict that in a much broader class of quantum-dot systems with spin larger than one-half, superparamagnetism may arise without spin-orbit interaction: by attaching ferromagnets a spintronic exchange field of quadrupolar nature is generated locally. It can be observed in conductance measurements and surprisingly leads to enhanced spin filtering even in a state with zero average spin. Analogously to the spintronic dipolar exchange field, responsible for a local spin torque, the effect is susceptible to electric control and increases with tunnel coupling as well as with spin polarization.