Fluctuations of the inverted magnetic state and how to sense them

TL;DR

This paper investigates the fluctuations of the inverted magnetic state induced by spin current injection, highlighting their quantum nature and potential for experimental detection and applications in spintronics.

Contribution

It provides a theoretical analysis of antimagnon fluctuations in the inverted magnetic state and suggests methods for their experimental sensing.

Findings

Fluctuations are dominated by spin current shot noise.

Inverted state exhibits larger quantum fluctuations than equilibrium.

Proposes using qubits to detect antimagnon fluctuations.

Abstract

Magnons are the low-energy excitations of magnetically ordered materials. While the magnetic moment of a ferromagnet aligns with an applied magnetic field, it has been experimentally shown that the magnetic order can be inverted by injecting spin current into the magnet. This results in an energetically unstable but dynamically stabilized state where the magnetic moment aligns antiparallel to an applied magnetic field, called the inverted magnetic state. The excitations on top of such a state have negative energy and are called antimagnons. The inverted state is subject to fluctuations, in particular, as shot noise in the spin current, which are different from fluctuations in equilibrium, especially at low temperatures. Here, we theoretically study the fluctuations of the inverted magnetic state and their signatures in experimental setups. We find that the fluctuations from the injection of spin current play a large role. In the quantum regime, the inverted magnetic state exhibits larger fluctuations compared to the equilibrium position, which can be probed using a qubit. Our results advance the understanding of the fundamental properties of antimagnons and their experimental controllability, and they pave the way for applications in spintronics and magnonics, such as spin wave amplification and entanglement.