Criteria for accurate determination of the magnon relaxation length from the nonlocal spin Seebeck effect

TL;DR

This paper establishes criteria for accurately measuring the magnon relaxation length in magnetic materials using nonlocal spin Seebeck effect experiments, emphasizing the importance of focusing on the exponential decay regime.

Contribution

It introduces a method to correctly extract magnon relaxation length by analyzing the exponential decay regime and provides experimental and modeling evidence across different temperatures and film thicknesses.

Findings

Magnon relaxation length is about 15 μm at room temperature.

The relaxation length increases to approximately 40 μm at 3.5 K.

Nonlocal SSE signals deviate from exponential decay at long distances, following a 1/d^2 decay.

Abstract

The nonlocal transport of thermally generated magnons not only unveils the underlying mechanism of the spin Seebeck effect, but also allows for the extraction of the magnon relaxation length ($\lambda_m$) in a magnetic material, the average distance over which thermal magnons can propagate. In this study, we experimentally explore in yttrium iron garnet (YIG)/platinum systems much further ranges compared with previous investigations. We observe that the nonlocal SSE signals at long distances ($d$) clearly deviate from a typical exponential decay. Instead, they can be dominated by the nonlocal generation of magnon accumulation as a result of the temperature gradient present away from the heater, and decay geometrically as $1/d^2$. We emphasize the importance of looking only into the exponential regime (i.e., the intermediate distance regime) to extract $\lambda_m$. With this principle, we study $\lambda_m$ as a function of temperature in two YIG films which are 2.7 and 50 $\mu$m in thickness, respectively. We find $\lambda_m$ to be around 15 $\mu$m at room temperature and it increases to 40 $\mu$m at $T=$ 3.5 K. Finite element modeling results agree with experimental studies qualitatively, showing also a geometrical decay beyond the exponential regime. Based on both experimental and modeling results we put forward a general guideline for extracting $\lambda_m$ from the nonlocal spin Seebeck effect.