Temperature Evolution of Magnon Propagation Length in Tm$_3$Fe$_5$O$_{12}$ Thin Films: Roles of Magnetic Anisotropy and Gilbert Damping

TL;DR

This study experimentally confirms that the magnon propagation length in Tm3Fe5O12 thin films decreases with increasing magnetic damping and anisotropy, providing insights for optimizing spin caloritronic devices.

Contribution

First experimental validation of the inverse relationship between magnon propagation length, Gilbert damping, and magnetic anisotropy in TmIG/Pt bilayers.

Findings

Magnon propagation length drops below 200K in TmIG/Pt bilayers.

Correlation between increased magnetic anisotropy, damping, and reduced magnon propagation length.

Temperature dependence of magnon propagation length linked to magnetic properties.

Abstract

The magnon propagation length ($\langle\xi\rangle$) of a ferro/ferrimagnet (FM) is one of the key factors that controls the generation and propagation of thermally-driven spin current in FM/heavy metal (HM) bilayer based spincaloritronic devices. Theory predicts that for the FM layer, $\langle\xi\rangle$ is inversely proportional to the Gilbert damping ($\alpha$) and the square root of the effective magnetic anisotropy constant ($K_{\rm eff}$). However, direct experimental evidence of this relationship is lacking. To experimentally confirm this prediction, we employ a combination of longitudinal spin Seebeck effect (LSSE), transverse susceptibility, and ferromagnetic resonance experiments to investigate the temperature evolution of $\langle\xi\rangle$ and establish its correlation with the effective magnetic anisotropy field, $H_K^{\rm eff}$ ($\propto K_{\rm eff}$) and $\alpha$ in Tm$_3$Fe$_5$O$_{12}$ (TmIG)/Pt bilayers. We observe concurrent drops in the LSSE voltage and $\langle\xi\rangle$ below 200$^\circ$K in TmIG/Pt bilayers regardless of TmIG film thickness and substrate choice and attribute it to the noticeable increases in $H_K^{\rm eff}$ and $\alpha$ that occur within the same temperature range. From the TmIG thickness dependence of the LSSE voltage, we determined the temperature dependence of $\langle\xi\rangle$ and highlighted its correlation with the temperature-dependent $H_K^{\rm eff}$ and $\alpha$ in TmIG/Pt bilayers, which will be beneficial for the development of rare-earth iron garnet-based efficient spincaloritronic nanodevices.