Michaela Küpferling

A Peltier cells differential calorimeter with kinetic correction for the measurement of cp(H,T) and Δs(H,T) of magnetocaloric materials

In this paper we describe and test a setup for the characterization of the magnetocaloric effect around room temperature. The setup is a differential calorimeter able to measure both the specific heat c(p)(H,T) under constant magnetic field H and the isothermal entropy change induced by changing H, Δs(H,T), in the room temperature range. The setup uses miniaturized Peltier cells to measure the heat flux, with resolution of about 1 μW, and power Peltier cells to regulate the temperature in the range from 243 K (-30 °C) to 343 K (+70 °C). The kinetic effects due to the heat capacity of the measuring cells are taken into account by a simple model of the heat flux diffusion in the calorimetric cell. As measurement examples, we show the characterization of the magnetocaloric effect in magnetic materials with a second order transition [without latent heat and without hysteresis, as in the La(1)(Fe(1-x-y)Co(y)Si(x))(13) alloy with x=0.077 and y=0.079] and with a first order transitions (with latent heat and hysteresis as in Ni(50)Mn(36)Co(1)Sn(13)). As a result we compare the entropy change Δs(H,T) derived from (i) the integration of the specific heat c(p)(H,T) and (ii) the direct isothermal measurements, obtaining an excellent agreement.

Rare earth substitutions in M-type ferrites

In this paper, the effects of a partial substitution of Sr by either Sm or Nd but also by La in SrFe/sub 12/O/sub 19/ on the fundamental magnetic properties is investigated. Hysteresis loop measurements were performed using a pulsed field magnetometer. The anisotropy fields of the samples was measured using the SPD method.

Magnetic and structural characterization of nanosized BaCoxZn2−xFe16O27 hexaferrite in the vicinity of spin reorientation transition

Numerous applications of hexagonal ferrites are related to their easy axis or easy plane magnetocrystalline anisotropy configurations. Certain W-type ferrites undergo spin reorientation transitions (SRT) between different anisotropy states on magnetic field or temperature variation. The transition point can be tuned by modifying the chemical composition, which suggests a potential application of hexaferrites in room temperature magnetic refrigeration. Here we present the results of structural and magnetic characterization of BaCo_xZn_{2-x}Fe_{16}O_{27} (0.7 ≤ x ≤ 2) doped barium ferrites. Fine powders were prepared using a sol-gel citrate precursor method. Crystal structures and particle size distributions were examined by X-ray diffraction and transmission electron microscopy. The optimal synthesis temperature ensuring complete formation of single W-phase with limited grain growth has been determined. Spin reorientation transitions were revealed by thermomagnetic analysis and AC susceptibility measurements.

Magnetocaloric Properties of MnBi Compound as a Function of Grain Size

MnBi is a magnetic material with many peculiar properties, interesting for different applications such as rare-earth-free permanent magnets and magnetic refrigeration. In this work the entropy change and the transformation kinetics of the magneto-structural transition are investigated, for two sets of samples with different grain size (“as-annealed” and “compacted pwd”). Both sets show a logarithm dependence on the temperature scan rate of the transition temperature upon cooling. This suggests that thermal activation effects are present. The estimated activation volume v0 of the magneto-structural transition corresponds to around 120 atoms.