We study the problem of magnetization and heat currents and their associated thermodynamic forces in a magnetic system by focusing on the magnetization transport in ferromagnetic insulators like YIG. The resulting theory is applied to the longitudinal spin Seebeck and spin Peltier effects. By focusing on the specific geometry with one Y3Fe5O12 (YIG) layer and one Pt layer, we obtain the optimal conditions for generating large magnetization currents into Pt or large temperature effects in YIG. The theoretical predictions are compared with experiments from the literature permitting to derive the values of the thermomagnetic coefficients of YIG: the magnetization diffusion length lM ∼0.4 μm and the absolute thermomagnetic power coefficient εM∼10−2 TK−1

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In this paper we present the results of an experimental investigation of the magnetocaloric properties of hydrogenated La(Fe-Mn-Si) 13 -H with Mn substituting Fe to finely tune the transition temperature. We measured the specific heat under magnetic field c p (H, T ) and the magnetic field induced isothermal entropy change ∆s(H, T ) of a series of compounds by direct Peltier calorimetry. Results show that increasing Mn from 0.06 to 0.46 reduces the transition temperature from 339 K to 270 K whilst the total entropy change due to a 1.5 T field is depressed from 18.7 Jkg −1 K −1 to 10.2 Jkg −1 K −1 and the thermal hysteresis similarly is reduced from 1.5 K to zero. In the paper we interpret the results in terms of a magnetic phase transition changing from the first to the second order with increasing Mn content and we discuss the value of the results for magnetic cooling applications.

The determination of the longitudinal spin Seebeck effect (LSSE) coefficient is currently plagued by a large uncertainty due to the poor reproducibility of the experimental conditions used in its measurement. In this work we present a detailed analysis of two different methods used for the determination of the LSSE coefficient. We have performed LSSE experiments in different laboratories, by using different setups and employing both the temperature difference method and the heat flux method. We found that the lack of reproducibility can be mainly attributed to the thermal contact resistance between the sample and the thermal baths which generate the temperature gradient. Due to the variation of the thermal resistance, we found that the scaling of the LSSE voltage to the heat flux through the sample rather than to the temperature difference across the sample greatly reduces the uncertainty. The characteristics of a single YIG/Pt LSSE device obtained with two different setups was (1.143 ± 0.007) 10−7 Vm/W and (1.101 ± 0.015) 10−7 Vm/W with the heat flux method and (2.313 ± 0.017) 10−7 V/K and (4.956 ± 0.005) 10−7 V/K with the temperature difference method. This shows that systematic errors can be considerably reduced with the heat flux method.

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.

We investigate the temperature induced ferromagnetic to paramagnetic phase transition in Co substituted La(FexCoySi1- x-y)13 with x = 0.9 and low Co content of y = 0.015 (Tc?200 K) by means of magneto-optical imaging with indicator film and by calorimetry at very low temperature rates. We were able to visualize the motion of the ferromagnetic (FM)/paramagnetic (PM) front which is forming reproducible patterns independently of the temperature rate. The average velocity of the FM/PM front was calculated to be 10-4 m/s during the continuous propagation and 4×10-3 m/s during an avalanche. The heat flux was measured at low temperature rates by a differential scanning calorimeter and shows a reproducible sequence of individual and separated avalanches which occurs independently of the rate. We interpret the observed effects as the result of the athermal character of the phase transition

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In the framework of the longitudinal spin Seebeck effect (LSSE), we developed an experimental\ud setup for the characterization of LSSE devices. This class of device consists in a layered structure\ud formed by a substrate, a ferrimagnetic insulator (YIG) where the spin current is thermally\ud generated, and a paramagnetic metal (Pt) for the detection of the spin current via the inverse spinHall\ud effect. In this kind of experiments, the evaluation of a thermal gradient through the thin YIG\ud layer is a crucial point. In this work, we perform an indirect determination of the thermal gradient\ud through the measurement of the heat flux. We developed an experimental setup using Peltier cells\ud that allow us to measure the heat flux through a given sample. In order to test the technique, a\ud standard LSSE device produced at Tohoku University was measured. We find a spin Seebeck SSSE\ud coefficient of 2.8 10^-7 V K^-1\ud