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

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 ($λ_m$) 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 $λ_m$. With this principle, we study $λ_m$ as a function of temperature in two YIG films which are 2.7 and 50 $μ$m in thickness, respectively. We find $λ_m$ to be around 15 $μ$m at room temperature and it increases to 40 $μ$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 $λ_m$ from the nonlocal spin Seebeck effect.

9 pages, 7 figures