Magnon transport through microwave pumping

TL;DR

This paper develops a microscopic theory of magnon transport in ferromagnetic insulators driven by microwave pumping, revealing how to generate and enhance magnon currents, including persistent currents influenced by the Aharonov-Casher phase.

Contribution

It introduces a microscopic framework for microwave-induced magnon transport and demonstrates how to enhance magnon currents in hybrid ferromagnetic insulator junctions.

Findings

Microwave pumping can generate both ac and dc magnon currents.

FMR significantly enhances magnon current amplitudes.

Persistent magnon currents can flow at finite temperature due to magnon-magnon interactions.

Abstract

We present a microscopic theory of magnon transport in ferromagnetic insulators (FIs). Using magnon injection through microwave pumping, we propose a way to generate magnon dc currents and show how to enhance their amplitudes in hybrid ferromagnetic insulating junctions. To this end focusing on a single FI, we first revisit microwave pumping at finite (room) temperature from the microscopic viewpoint of magnon injection. Next, we apply it to two kinds of hybrid ferromagnetic insulating junctions. The first is the junction between a quasi-equilibrium magnon condensate and magnons being pumped by microwave, while the second is the junction between such pumped magnons and noncondensed magnons. We show that quasi-equilibrium magnon condensates generate ac and dc magnon currents, while noncondensed magnons produce essentially a dc magnon current. The ferromagnetic resonance (FMR) drastically increases the density of the pumped magnons and enhances such magnon currents. Lastly, using microwave pumping in a single FI, we discuss the possibility that a magnon current through an Aharonov-Casher phase flows persistently even at finite temperature. We show that such a magnon current arises even at finite temperature in the presence of magnon-magnon interactions. Due to FMR, its amplitude becomes much larger than the condensed magnon current.