Spin pumping from a ferromagnet into a hopping insulator: the role of resonant absorption of magnons

TL;DR

This paper theoretically investigates spin pumping into a hopping insulator, revealing that magnon absorption at localized states enhances spin current generation, especially at resonance conditions where Zeeman splitting matches ferromagnetic resonance frequency.

Contribution

It introduces a resonant absorption mechanism for magnons in hopping insulators, predicting enhanced spin pumping efficiency at specific magnetic field conditions.

Findings

Resonant magnon absorption significantly boosts spin current.

Spin polarization spreads via resistor network without charge flow.

Maximum efficiency occurs when Zeeman splitting equals FMR frequency.

Abstract

Motivated by recent experiments on spin pumping from a ferromagnet into organic materials in which the charge transport is due to hopping, we study theoretically the generation and propagation of spin current in a hopping insulator. Unlike metals, the spin polarization at the boundary with ferromagnet is created as a result of magnon absorption within pairs of localized states and it spreads following the current-currying resistor network (although the charge current is absent). We consider a classic resonant mechanism of the ac absorption in insulators and adapt it to the absorption of magnons. A strong enhancement of pumping efficiency is predicted when the Zeeman splitting of the localized states in external magnetic field is equal to the frequency of ferromagnetic resonance. Under this condition the absorption of a magnon takes place within individual sites.