Sharp structures in magnetic field-dependent spin Seebeck effect (SSE) voltages of Pt/Y$_{3}$Fe$_{5}$O$_{12}$ (YIG) at low temperatures are attributed to the magnon-phonon interaction. Experimental results are well reproduced by a Boltzmann theory that includes the magnetoelastic coupling (MEC). The SSE anomalies coincide with magnetic fields tuned to the threshold of magnon-polaron formation. The effect gives insight into the relative quality of the lattice and magnetization dynamics.Comment: 6 pages, 4 figures, 1 tabl

We theoretically study the effects of strong magnetoelastic coupling on the transport properties of magnetic insulators. We develop a Boltzmann transport theory for the mixed magnon-phonon modes ("magnon polarons") and determine transport coefficients and the spin diffusion length. Magnon-polaron formation causes anomalous features in the magnetic field and temperature dependence of the spin Seebeck effect when the disorder scattering in the magnetic and elastic subsystems is sufficiently different. Experimental data by Kikkawa et al. [Phys. Rev. Lett. 117, 207203 (2016)] on yttrium iron garnet films can be explained by an acoustic quality that is much better than the magnetic quality of the material. We predict similar anomalous features in the spin and heat conductivity and nonlocal spin transport experiments.

We investigate coupled spin and heat transport in easy-plane magnetic insulators. These materials display a continuous phase transition between normal and condensate states that is controlled by an external magnetic field. Using hydrodynamic equations supplemented by Gross-Pitaevski phenomenology and magnetoelectric circuit theory, we derive a two-fluid model to describe the dynamics of thermal and condensed magnons, and the appropriate boundary conditions in a hybrid normal-metal-magneticinsulator-normal-metal heterostructure. We discuss how the emergent spin superfluidity can be experimentally probed via a spin Seebeck effect measurement.

Prototypes of quantum impurities (QI), such as NV and SiV centers in diamond, have been recently growing in popularity due to their minimally invasive and high-resolution magnetic field sensing. Here, we focus on quantum-impurity relaxometry as a method to probe collective excitations in magnetic insulators. We develop a general framework that relates the experimentally-measurable quantum-impurity relaxation rates to the properties of a magnetic system via the noise emitted by the latter. We suggest that, when the quantum-impurity frequency lies within the spin-wave gap, quantum-impurity relaxometry can be effectively deployed to detect signatures of the coherent spin dynamics, such as magnon condensation, both in ferromagnetic and antiferromagnetic systems, as well as open prospects to nonintrusively probe spin-wave transport regimes in magnetic insulators.

Temperature-dependent spin-Seebeck effect data on Pt|YIG (Y3Fe5O12)|GGG (Gd3Ga5O12) are reported for YIG films of various thicknesses. The effect is reported as a spin-Seebeck resistivity (SSR), the inverse spin-Hall field divided by the heat flux, to circumvent uncertainties about temperature gradients inside the films. The SSR is a non-monotonic function of YIG thickness. A diffusive model for magnon transport demonstrates how these data give evidence for the existence of two distinct length scales in thermal spin transport, a spin diffusion length and a magnon energy relaxation length. 2Since the discovery of the (longitudinal) spin-Seebeck effect (SSE) 1 , much work has been done to identify the length scales involved in the phenomenon. 2 Using nonlocal detection, it has been shown that relaxation of thermal magnons in YIG is governed by a spin diffusion length. 3The latter is reported to be around 10 μm, and, in some studies, increases to up to 70 μm at low temperatures. 4,5 . This has led to a consensus that the micron-scale dependence of SSE observed in planar geometries corresponds to the generation and accumulation of the nonequilibrium magnondensity gradients in the bulk. These experiments have been modeled theoretically in terms of magnon spin transport only, while assuming that the magnon-phonon processes leading to the relaxation of the magnon energy occur over very short length scales; hence, their effects can be disregarded. 3,5 Nevertheless, it is clear that magnon energy relaxation mechanisms by the phononic environment must be invoked generally for a complete understanding of thermal spin transport, and particularly for the physics underlying the SSE. Indeed, while heaters and thermometers couple to phonons, these must in turn couple to magnons in order to give rise to the SSE in a magnon-based system like YIG. These relaxation processes can be parameterized by the length over which magnon-to-phonon thermalization occurs, and the latter is expected to be a much smaller length scale than magnon spin-diffusion lengths; a theoretical argument can be found in Ref. [6]. Early work 7 defines an energy relaxation length similar to the one invoked here, but without quantifying it. Additionally, phonon-magnon drag has been put into evidence in previous SSE experiments 8,9 , which, again, points to the importance of interactions between magnons and phonons. To the best of our knowledge, no explicit evidence for the effect of this length scale on SSE measurements has been reported to date.Previous articles on thin films using various growth techniques 10,11,12 have shown the SSE signal to increase with increasing YIG film thickness. In this study, we grow a series of 3 Pt|YIG|GGG heterostructures, with YIG thickness varying from 10 nm to 1 μm, using the same growth technique for all films. We measure the temperature-dependent spin-Seebeck effect on these structures and of bulk single-crystal Pt|YIG. The spin-Seebeck signal increases for film thicknesses from 10 to 250 nm and again for the bulk Y...