Size-dependence of Strong-Coupling Between Nanomagnets and Photonic Cavities

TL;DR

This paper investigates how the size of nanomagnets influences their strong coupling with photonic cavities, revealing size-dependent eigenstates and the limits of the macrospin approximation for creating coherent states.

Contribution

It demonstrates the size-dependent behavior of nanomagnet-photon coupling and identifies the optimal nanomagnet size for strong coupling without domain separation.

Findings

Eigenstates involve highly entangled photon and spin states at strong coupling.

Coupling strength and involved states increase with nanomagnet size.

Crystalline anisotropy localizes eigenstates in small nanomagnets, hindering coherence.

Abstract

The coherent dynamics of a coupled photonic cavity and a nanomagnet is explored as a function of nanomagnet size. For sufficiently strong coupling eigenstates involving highly entangled photon and spin states are found, which can be combined to create coherent states. As the size of the nanomagnet increases its coupling to the photonic mode also monotonically increases, as well as the number of photon and spin states involved in the system's eigenstates. For small nanomagnets the crystalline anisotropy of the magnet strongly localized the eigenstates in photon and spin number, quenching the potential for coherent states. For a sufficiently large nanomagnet the macrospin approximation breaks down and different domains of the nanomagnet may couple separately to the photonic mode. Thus the optimal nanomagnet size is just below the threshold for failure of the macrospin approximation.