We use the Boltzmann transport theory in the relaxation time approximation to describe the thermal transport of spin waves in a ferromagnet. By treating spin waves as magnon excitations we are able to compute analytically and numerically the coefficients of the constitutive thermomagnetic transport equations. As a main result, we find that the absolute thermo-magnetic power coefficient M , relating the gradient of the potential of the magnetization current and the gradient of the temperature, in the limit of low temperature and low field, is a constant M = −0.6419 kB/µB. The theory correctly describes the low-temperature and magnetic-field dependencies of spin Seebeck experiments. Furthermore, the theory predicts that in the limit of very low temperatures the spin Peltier coefficient ΠM , relating the heat and the magnetization currents, tends to a finite value which depends on the amplitude of the magnetic field. This indicates the possibility to exploit the spin Peltier effect as an efficient cooling mechanism in cryogenics.
In the present paper, we review the recent research on the physics of magnetocaloric materials aiming to define a coherent theoretical framework in which hysteresis and kinetic effects can be appropriately discussed and interpreted in relation to intrinsic and extrinsic factors. We dedicate our efforts to introduce a thermodynamic description of the material, including the out-of-equilibrium aspects which are necessary to understand hysteresis, heat flux avalanches and thermal relaxation effects. In particular we show how intrinsic and extrinsic factors, contributing to define the energy landscape of the system, influence the resulting hysteresis and how different kinetic effects are expected depending on the phase transformation mechanisms, here described either as an out-of-equilibrium domain boundary motion or as a thermally activated process associated to energy barriers. Several applications of the theoretical models are discussed in relation with experiments on La(Fe-Si) 13 -based compounds and Mn-Bi.
We investigate the kinetics of first order magnetic phase transitions by measuring and modelling the heat flux avalanches corresponding to the irreversible motion of the phase boundary interface separating the coexisting low-and high-temperature stable magnetic phases. By means of out-ofequilibrium thermodynamics we encompass the damping mechanisms of the boundary motion in a phenomenological parameter αs. By analyzing the time behaviour of the heat flux signals measured on La(Fe-Mn-Si)13-H magnetocaloric compounds through Peltier calorimetry temperature scans performed at low rates, we relate the linear rise of the individual avalanches to an intrinsic thermal resistance associated to αs.
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