Magnetocaloric Effect Parameters Research Articles

Influence of Eu doping on magnetocaloric behavior of La0.67Sr0.33MnO3

Abstract Colossal Magneto Resistance (CMR) materials with the compositional formula, La0.67 −xEuxSr0.33MnO3 (where x = 0.3 , 0.33 , 0.36 and 0.39 ) were prepared by the sol–gel method. After characterizing the samples structurally, a systematic investigation of magnetization has been undertaken, over a temperature range 70–300 K under 0.05 T magnetic field. An effort has been made to calculate the magnetocaloric behavior of all the samples theoretically using the experimental magnetization data. Magnetocaloric effect parameters such as maximum entropy change, full width at half-maximum and relative cooling power under 0.05 T were computed. The observed behavior has been explained qualitatively.

Influence of thermal hysteresis and field cycling on the magnetocaloric effect in LaFe11.6Si1.4

The adiabatic temperature change ΔTad at a field-induced first-order transition can be different when a magnetic field H is applied the first time compared to measurements under further applications of H. This can lead to overestimation of the magnetocaloric effect (MCE) if only the first field cycle is considered. In polycrystalline LaFe11.6Si1.4, for Δμ0H=1.93T, field cycling reduces the achieved value of ΔTad from 7K to about 5.8K. A metastable entropy-temperature diagram has been constructed from experimental isofield heat capacity data, which is used to comprehensively analyze results of direct measurements of ΔTad(T)0→H and isothermal magnetization measurements. It is shown that the reduction in ΔTad by field cycling is due to the presence of two-phase states that are stabilized by thermal hysteresis. Relations between optimum operational MCE parameters of a magnetic refrigerant and the maximum available field Havl have been obtained by the approach of an idealized first-order transition.

Influence of sample geometry on determination of magnetocaloric effect for Gd60Co30Al10 glassy ribbons using direct and indirect methods

Rare-earth based metallic glasses with high saturation magnetization show a sizeable magnetocaloric effect (MCE) and are subject of extensive research concerning magnetic refrigeration materials. In this work, the magnetic phase transition from paramagnetic to ferromagnetic of Gd60Co30Al10 metallic glass has been characterized and three different methods were applied for the determination of its magnetocaloric specific parameters: (a) direct measurement of the adiabatic temperature change by exposing the material to an adiabatically applied magnetic field; (b) determination of the magnetization M(H,T) and calculation of the temperature dependent magnetic field induced entropy change ΔSm by application of the Maxwell relation and (c) measuring the total heat capacity Cp(H,T) in zero and non-zero magnetic field. Gd60Co30Al10 glassy ribbons were prepared by melt spinning, a technique that offers very high cooling rates due to the low dimensionality of the sample. Depending on the particular method of measurement, pieces of these glassy ribbons form samples with different appropriate total volume and dimensions. We show that the combination of the pronounced two-dimensionality of the ribbon pieces (aspect ratio ∼100) together with the very high magnetic permeability principally can cause strong internal demagnetizing fields that cannot be neglected when evaluating the intrinsic MCE parameters obtained from different methods.

Transdisciplinary aspects of diffusion and magnetocaloric effect

The modeling of magnetocaloric effect (MCE) based on quasiparticle formalism was used to show a possibility to increase our knowledge of diffusion and phase transitions by a transdisciplinary scientific approach. Generalized understanding of diffusion and of some phase transitions was suggested. New relations between MCE and elastic parameters of materials, obtained from modeling procedure, were presented. The paper contains also the short discussion of a necessity to use hypercomplex mathematics in modeling of magnetic processes and specifically in a modeling of MCE. Some remarks concerning nanomaterials are added.