Kaspar Kirstein Nielsen

The influence of hysteresis on the determination of the magnetocaloric effect in Gd5Si2Ge2

We present a non-equilibrium Preisach-type hysteresis model based on the first order magnetocaloric material Gd5Si2Ge2. The model is developed from isofield magnetization measurements and first order reversal curves, both of which constitute a new and detailed approach to characterizing and modelling magnetocaloric materials. It is shown that the model reproduces the magnetization data, directly measured adiabatic temperature changes and provides a good description of the material behavior under application conditions. We find that the material settles in an area of metastability under continuous magnetization cycles, which effectively limits the adiabatic temperature change by the amount of thermal hysteresis present. We suggest a straightforward method for realistic estimation of the magnetocaloric effect from indirect measurements.

Direct measurements of the magnetic entropy change

An experimental device that can accurately measure the magnetic entropy change, Δs, as a function of temperature, T, and magnetic field, H, is presented. The magnetic field source is in this case a set of counter-rotating concentric Halbach-type magnets, which produce a highly homogeneous applied field with constant orientation. The field may be varied from 0 to 1.5 T in a continuous way. The temperature stability of the system is controlled to within ±10 mK and the standard range for the current setup is from 230 K to 330 K. The device is under high vacuum and we show that thermal losses to the ambient are negligible in terms of the calorimetric determination of the magnetic entropy change, while the losses cannot be ignored when correcting for the actual sample temperature. We apply the device to two different types of samples; one is commercial grade Gd, i.e., a pure second-order phase transition material, while the other is Gd5Si2Ge2, a first order magnetic phase transition material. We demonstrate the devices ability to fully capture the thermal hysteresis of the latter sample by following appropriate thermal resetting scheme and magnetic resetting scheme.