Effects of Finite Material Size On Axion-magnon Conversion

TL;DR

This paper investigates how the size of magnetic materials influences the efficiency of axion detection via magnon excitation, especially when the axion's de Broglie wavelength is comparable to the material size.

Contribution

It provides a systematic analysis of finite-size effects on axion-magnon conversion, extending beyond the commonly assumed zero-momentum mode approximation.

Findings

Finite material size enables excitation of finite-momentum magnon modes.

Size dependence significantly affects axion detection sensitivity.

Relevance for detecting relativistic axions and heavy axion dark matter.

Abstract

Magnetic materials are particularly favorable targets for detecting axions interacting with electrons because the collective excitation of electron spins, the magnon, can be excited through the axion-magnon conversion process. It is often assumed that only the zero-momentum uniformly precessing magnetostatic (Kittel) mode of the magnon is excited. This is justified if the de Broglie wavelength of the axion is much longer than the size of the target magnetic material. However, if the de Broglie wavelength is shorter, finite-momentum magnon modes can also be excited. We systematically analyze the target material size dependence of the axion-magnon conversion rate. We discuss the importance of these effects in the detection of relativistic axions as well as in the detection of axion dark matter of relatively heavy mass with large material size.