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A 100 micron fragment of a b-axis oriented single crystal Gd5Si2Ge2 has been studied using microcalorimetry, enabling the separate measurement of the heat capacity and the latent heat. The sample was taken from the same crystal previously studied with Hall probe imaging, which showed that the phase transition is seeded by a second phase of Gd5Si1.5Ge1.5 nanoplatelets on the increasing field sweep direction only. The multiple transition features observed in the latent heat signature suggests a nucleation size of approximately 20 μm, consistent with the lengthscale suggested by Hall imaging. The difference in nucleation and growth process with field sweep direction is clearly identified in the latent heat. We show that the latent heat contribution to the entropy change is of the order of 50% of the total entropy change and unlike other systems studied, the transition does not broaden (and the latent heat contribution does not diminish significantly) as magnetic field and temperature are increased within the parameter range explored in these experiments.
A simple theoretical five-state Potts model for the investigation of magnetocaloric effect in systems with competing ferromagnetic and antiferromagnetic interactions has been proposed. It is shown that this simple model can be applied to the description of the origin of the negative and positive magnetocaloric effect in systems with competing interactions, for example, Heusler alloys.
The primary objective of this study is to discuss the optimum operating conditions of magnetocaloric heat pumps according to the fundamental heat transfer characteristics of an active magnetic regenerator (AMR) bed. The AMR cycle has four sequential processes: magnetization, heat exchange fluid flow, demagnetization, and heat exchange fluid blow. The fundamental heat transfer characteristics of each process of the AMR cycle is investigated minutely. Moreover, the cooling power and the overall system performance are evaluated when the system is running continuously.
In addition to the aforementioned investigation, we have developed a prototype rotational magnetocaloric heat pump having a compact component arrangement and an uncomplicated control system. A performance evaluation has been conducted to obtain the optimum conditions for practical operation. The operation parameters such as the heat transfer fluid flow rate, rotational frequency, and initial temperature of the heat transfer fluid are examined, and the variations of the maximum temperature span between the inlet and outlet for the heat transfer fluid are discussed. As a result, the values of the optimum rotational frequency and flow rate are obtained to obtain the maximum temperature span between the inlet and outlet of the present magnetocaloric heat pump.
Hysteresis is unattractive for magnetocaloric applications because it introduces loss in the cooling cycle. It is however usually associated with a first order transition and large entropy change. In this paper we review the sources of hysteresis in magnetocaloric materials and in particular in manganite systems where the nature of the transition in terms of whether it is indeed a first order transition remains elusive.
In present work we propose a theoretical model for investigation of the exchange bias effect in Ni50Mn37.5Sb12.5 alloy. In the model, we use a three-dimensional cubic lattice with periodic boundary conditions. Also we take into account the magnetic interactions between atoms in 1st, 2nd and 3rd coordination spheres and the ferromagnetic and antiferromagnetic anisotropy terms. It is shown that the obtained theoretical temperature dependence of the exchange bias field for Ni50Mn37.5Sb12.5 alloy is close to the experimental data.
In order to realize the magnetic refrigeration system, it is necessary to develop a 100 W class refrigerator with COP > 7.5. This requires us to find new magnetic refrigerant materials, of which cooling capacity is 2.5 times higher than that of Gd. In this paper, first we discuss the cooling capacity of magnetic refrigerant materials to achieve COP = 7.5. Then, we compare the experimental results of MnAsSb, MnFe(PGe) and La(FeCoSi)13 compounds with the calculated cooling capacity. It is suggested that a composite layer material of MnFe(PGe) would show excellent cooling capacity in the temperature span of 20 K.
A magnetic refrigeration test was performed using a test device filled with spherical GdN material synthesized by the hot isostatic pressing (HIP) method. Refrigeration with an active magnetic regenerator cycle was tested in the temperature range between 48 and 66 K, with the field changing from 1.2 to 3.7 T and 2.0 to 4.0 T at upper and lower sides of the regenerator bed filled with the GdN spheres, respectively. Temperature spans about of 2 K were obtained at both sides, and the total temperature span in each cycle attained about 5 K. The specific heat of the material was measured to calculate the magnetic entropy change ΔS and the adiabatic temperature change ΔT induced by the magnetic field change ΔH. It was suggested that for a given ΔH, larger ΔS and ΔT can be exploited when demagnetized to lower H, especially, to zero field.
A direct calorimetry method was developed and used to measure the electrocaloric effect (ECE). A temperature change ΔT of over 20 °C and an entropy change ΔS of over 95 J/(kgK) were procured at 33 °C and 160 MV/m in the high-energy electron irradiated poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) 68/32 mol% copolymers, which were larger than those of terpolymer blends (ΔT = 9 °C, ΔS=46 J/(kgK) at 180 MV/m and room temperature) and our earlier report on P(VDF-TrFE) 55/45 mol% normal ferroelectric copolymer (12 °C and 55 J/(kgK) at 80 °C). We observed that the β value ((8.7±0.6)×107 JmC-2K-1) in the equation of ΔS=1/2βΔD2 derived from ΔS - ΔD2 relation for irradiated copolymers was larger than that of the terpolymer blends ((5.4±0.5)×107 JmC-2K-1). It was also found that the irradiated copolymer showed a sharp depolarization peak at Td < Tm (maximum permittivity temperature), which is frequency independent, in the dielectric constant - temperature characteristics, a larger depolarization value at Td in the thermally stimulated depolarization current (TSDC) - temperature relationship, and a larger volume strain/longitudinal strain ratio over terpolymer blends. The giant ECE in irradiated copolymer is regarded as due to the greater randomness present in the relaxor state. In irradiated copolymers, the long all-trans chains are broken by the high-energy electrons, which make the small sized all-trans sequences more easily reorient along the electric field, more remarkably affecting the permittivity, TSDC, and volume strain.
In this paper we review the phase diagram and derive the entropy change for spin reorientation transitions by considering first order magnetization process theory with temperature dependent magneto-crystalline anisotropy constants. We derive the magnetic field-induced entropy change Δs for a transition between easy axis and easy plane, showing that for alternating magnetic field, Δs has a change of sign at the reorientation temperature, while for rotating magnetic field its sign is definite. We apply the model to CoZn W-type barium ferrite.
In this work a microscopic Hamiltonian is investigated using the Hubbard model for a ferromagnet with two degenerate bands, taking into account the Jahn-Teller effect. A macroscopic free energy is obtained from the microscopic Hubbard Hamiltonian. All free energy coefficients depend on microscopic parameters: temperature T and composition x. As a result of analytical minimization of free energy, phase diagrams are numerically constructed. It is shown that at certain values of parameters on the phase diagrams there are thermodynamic paths which correspond to experimentally observed sequences of phase transitions. Using density of states spectra for different compositions x the T-x phase diagram is numerically constructed. This phase diagram can theoretically explain experimentally observed behavior of the temperatures of phase transitions.