Maximal Entropy Change Research Articles

Surface effects on the magnetocaloric properties of perovskites ferromagnetic thin films: A Monte Carlo study

We have used the Monte Carlo simulations to study the surface effects on the magnetocaloric properties of perovskites ferromagnetic thin films. We have determined the magnetization, the transition temperatures, magnetic entropy change, the relative cooling power (RCP), and the magnetic hysteresis cycle temperature as functions of the film thickness, and surface exchange coupling. The reduced critical temperature tC of the perovskites ferromagnetic thin films is studied as a function of film thickness L and the exchange interactions in the bulk JB, in the surface JS and between surface. We have shown that maximal entropy change in thin film systems can be observed at temperatures well below the magnetic phase transition temperature. The maximal entropy change increases with increasing the external magnetic field. RCP increases with increasing the external magnetic field and it is dependent of thickness film. The RCP decreases with increasing the value of surface exchange coupling. The magnetic coercive field decreases with increasing the temperatures values when the surface exchange coupling is inferior to bulk exchange coupling. The magnetic coercive field hC of the perovskites ferromagnetic thin films is studied as a function of reduced surface exchange coupling.

Magnetocaloric effect and magnetoelastic properties of NiMnGa and NiMnSn Heusler alloy thin films

Series of NiMnSn and NiMnGa Heusler alloys thin films of different compositions were obtained by DC magnetron sputtering in order to achieve strong magnetocaloric effect in the temperature range close to the room temperature. The maximal magnetocaloric effect was found in the temperature range 320–350 K, with the maximal entropy change -ΔS ranges from 1.4×104 erg/gK to 2.7×104 erg/gK. Strain modulated ferromagnetic resonance technique was used to determine magnetoelastic properties of the films. Magnetoelastic constants of an order of 106 erg/cm3 were found.

Excellent mechanical properties and age stability of hydrogenated La0.8Ce0.2Fe12.5Mn0.2Si1.3Hδ plates with extra Fe

Herein, hydrogenated La0.8Ce0.2Fe12.5Mn0.2Si1.3Hδ plates as thin as 0.4–0.6 mm with extra Fe were prepared, whereupon the microstructure, mechanical properties, age stability, and magnetocaloric effect (MCE) were systematically investigated. It was demonstrated that the introduction of extra α-Fe largely enhanced the mechanical properties. The bending strength was about 61 MPa for a single 0.6 mm-thick plate, while the compressive strength was as much as 352 MPa for the stacked plates. Meanwhile, the hydride plates showed considerable MCE around room temperature. The maximal entropy change was 14.2 J/kgK around its Curie temperature TC = ∼298 K and the effective refrigeration capacity was 284 J/kg under a magnetic field change of 0–5 T. Moreover, the hydride plates exhibited excellent age stability and good thermal conductivity. No age splitting occurred, even when the plates were held at room temperature for more than one year, owing to full hydrogenation. Finally, room-temperature thermal conductivity was as large as 9.5 Wm−1K−1. All of these results demonstrate that the hydride plates with extra Fe are desirable magnetic refrigerants, which is beneficial to increase the working efficiency in a device.

Influence of modified surface effects on the magnetocaloric properties of ferromagnetic thin films

We have studied the influence of surface effects on the magnetocaloric properties of ferromagnetic thin films. Using the effective field theory, we have computed the refrigerant capacity, isothermal entropy change, and the magnetic phase transition temperature as functions of the film thickness, and surface exchange coupling. We have shown that reduced and enhanced surfaces due to the presence of modified surface exchange couplings may drastically affect the magnetocaloric properties of the system. According to our results, in contrast to the conventional bulk ferromagnetic materials, depending on the film thickness and surface exchange coupling strength, maximal entropy change in thin film systems can be observed at temperatures well below the magnetic phase transition temperature. This observation can be beneficial from view point of technological applications.

Rotating Magnetocaloric Effect in an Anisotropic Two-Dimensional CuII[WV(CN)8]3- Molecular Magnet with Topological Phase Transition: Experiment and Theory.

Conventional (MCE) and rotating (RMCE) magnetocaloric effects have been explored in the two-dimensional (2D) coordination polymer {(tetren)H5)0.8CuII4[WV(CN)8]4·7.2H2O}n (WCu-t; tetren = tetraethylenepentamine). The unusual magnetostructural properties were exploited, including the bilayered Prussian Blue like coordination skeleton and the XY easy-plane magnetic anisotropy based on the in-plane correlation between WV and CuII spins of 1/2, underlying the Berezinskii-Kosterlitz-Thouless (BKT) topological phase transition to the long-range-ordered state at TC = 33 K. The magnetic properties were studied on single crystals along the H∥ac easy plane and H∥b hard axis. The maximal entropy change for MCE for easy-plane geometry at 38.0 K and the magnetic field change μ0ΔH = 7.0 T reached ∼4.01 J K-1 kg-1. The strong magnetic anisotropy was used to study the RMCE in which the maximal entropy change was observed at 35.5 K for 7.0 T, attaining 1.81 J K-1 kg-1. Moreover, easy-plane anisotropy introduces the inverse magnetocaloric effect for H∥b, which enhances the RMCE by up to 47%. This observation was confirmed by a theoretical investigation considering the XY model using a molecular field and cluster variational method in the pair approximation approach, dedicated to the bilayered systems with the adequate nearest neighbor number z = 5 and spin S = 1/2.

Magnetic and structural phase diagram of antiperovskites ZnCFe3−xCox (0 ≤ x ≤ 3): The combined negative magnetoresistance and large room-temperature magnetocaloric effect in x = 0.5

Effects of Co doping on the crystal structure, magnetic, electrical transport properties, and magnetocaloric effect (MCE) of antiperovskite ZnCFe3−xCox (0 ≤ x ≤ 3) were investigated. The value of lattice constant decreases with increasing x. The Curie temperature (TC) decreases from 358 K in ZnCFe3 to 5 K in ZnCCo3 and the saturated magnetization declines monotonously. The reduced ferromagnetic state is explained by the band filling effect resulted from the Co substitution. For x ≥ 2.3, the negative magnetoresistance (MR) below TC starts to increase as x or magnetic field increases, reaching −16% for x = 2.9 under H = 45 kOe, which is three times larger than that of GaCMn3 at 250 K. MCE was investigated for selected compositions. The temperature range of MCE is tuned covering 5–350 K as a function of Co-level. Although the maximal entropy change decreases a little with increasing x, the refrigeration ranges across room temperature. At the optimal composition x = 0.5, TC is 312 K with a magnetic entropy change of 2.09 J/kg K and relative cooling power (RCP) of 283 J/kg (ΔH = 45 kOe), make it a room-temperature MCE candidate.

ChemInform Abstract: Magnetic Properties and Magnetocaloric Effects in NaZn13‐Type La(Fe,Al)13‐Based Compounds

AbstractReview: 58 refs.

On the determination of the magnetic entropy change in materials with first-order transitions

An accurate method to determine the magnetic entropy change in materials with hysteretic first-order transitions is presented, which is needed to estimate their potential for applications. We have investigated the effect of the maximal entropy change derived from magnetization measurements performed in different measurement processes. The results show that the isothermal entropy change can be derived from the Maxwell relations even for samples with large thermal hysteresis. In the temperature region with hysteresis, overestimating the entropy change can be avoided by measuring the isothermal magnetization of the sample after it is cooled from the paramagnetic state to the measurement temperature. In this way the so-called peak effect is not observed as shown here for a few compounds.