Anisotropy of the magnetocaloric effect in DyNiAl

We present study of the anisotropic magnetocaloric effect in DyNiAl. This compound crystallizes in the hexagonal ZrNiAl-type structure, orders magnetically below and undergoes a further magnetic phase transition at . The Dy-moments are aligned ferromagnetically along the hexagonal c-axis below T_C, the additional antiferromagnetic component develops within the basal plane below T₁. The magnetocaloric effect was evaluated from the magnetization measurements with field applied along the c-axis and perpendicular to it. Our data reveal a strong anisotropy of the magnetocaloric effect. The large effect occurs for field applied along the c-axis whereas the entropy change is small for the perpendicular field direction.     

Weak amplification of the magnetocaloric effect in manganites

The magnetocaloric effect (MCE) has been measured by direct method in La_0.8Ag_0.15MnO₃ and La_0.85Ag_0.15MnO₃ before and after coating of Fe–Co layer on the surfaces of manganites. An evaporated film thickness has been 500 nm. The measurements have shown the MCE to be increased by 7%–8% under 26 kOe after Fe–Co coating on the flat surfaces.     

Review of the magnetocaloric effect in manganite materials

A thorough understanding of the magnetocaloric properties of existing magnetic refrigerant materials has been an important issue in magnetic refrigeration technology. This paper reviews a new class of magnetocaloric material, that is, the ferromagnetic perovskite manganites (R_1−xM_xMnO₃, where R=La, Nd, Pr and M=Ca, Sr, Ba, etc.). The nature of these materials with respect to their magnetocaloric properties has been analyzed and discussed systematically. A comparison of the magnetocaloric effect of the manganites with other materials is given. The potential manganites are nominated for a variety of large- and small-scale magnetic refrigeration applications in the temperature range of 100–375 K. It is believed that the manganite materials with the superior magnetocaloric properties in addition to cheap materials-processing cost will be the option of future magnetic refrigeration technology.     

Oscillating magnetocaloric effect in quantum nanoribbons

We investigate the oscillating magnetocaloric effect on a diamagnetic nanoribbon, using the model of a quasi-one-dimensional electron gas (Q1DEG) made with a parabolic confinement potential. We obtained analytical expressions for the thermodynamic potential and for the entropy change. The entropy change exhibits the same dependence on field and temperature observed for other diamagnetic systems. The period of the field-oscillating pattern is ~0.1 mT and the temperature of maximum entropy change is ~0.1 K with an applied field of the order of 1 T. An interesting feature of the results is the dependence of the oscillations with the strength of the confinement potential, as well as the possibility to provide a relationship among this last with nanoribbon width. In the limit of null confinement potential our expressions match those for the 2D diamagnetic system.     

Pressure-induced colossal magnetocaloric effect in MnAs

To present day, the maximum magnetocaloric effect (MCE) at room temperature for a magnetic field change of 5 T is 40 J/(kg K) for MnAs. In this Letter we present colossal MCE measurements on MnAs under pressure, reaching values up to 267 J/(kg K), far greater than the magnetic limit arising from the assumption of magnetic field independence of the lattice and electronic entropy contributions. The origin of the effect is the contribution to the entropy variation coming from the lattice through the magnetoelastic coupling.     

磁热效应材料的研究进展

磁制冷技术的发展取决于具有大磁热效应磁制冷材料的研发进展.经过长期的工作积累,特别是近20年来的努力,许多新型磁制冷材料的探索和研究极大地促进了磁制冷技术的进步.本文介绍了磁热效应的基本原理和磁制冷研究的发展历史,系统综述了低温区和室温区具有大磁热效应的磁制冷材料的研究进展,重点介绍了一些受到较为关注的磁热效应材料的最新研究成果.低温区磁制冷材料主要包括具有低温相变的二元稀土基金属间化合物(RGa,RNi,RZn,RSi,R_3Co以及R_(12)Co_7)、稀土-过渡金属-主族金属三元化合物(RTSi,RTAl,RT_2Si_2,RCo_2B_2,RCo_3B_2)以及四元化合物RT_2B_2C等,其中R代表稀土元素,T代表过渡金属.这些材料一般都具有二级相变,具有良好的热、磁可逆性,也因其合金属性具有良好的导热性.室温区磁制冷材料主要包括Gd-Si-Ge,La-Fe-Si,Mn As基,Mn基Husler合金,Mn基反钙钛矿,Mn-Co-Ge,Fe-Rh以及钙钛矿氧化物等系列.这些材料一般都具有一级相变,多数在室温具有巨大的磁热效应而受到国内外的极大关注.其中,La-Fe-Si系列是国际上普遍认为具有重要应用前景的磁制冷工质之一,也是我国具有自主知识产权的材料.本文还对磁制冷材料的发展方向进行了展望.     

Review of magnetocaloric effect in perovskite-type oxides

We survey the magnetocaloric effect in perovskite-type oxides (including doped ABO 3-type manganese oxides, A3B2O7-type two-layered perovskite oxides, and A2B'B'O6-type ordered double-perovskite oxides). Magnetic entropy changes larger than those of gadolinium can be observed in polycrystalline La1-xCaxMnO3 and alkali-metal (Na or K) doped La0.8Ca0.2MnO3 perovskite-type manganese oxides. The large magnetic entropy change produced by an abrupt reduction of magnetization is attributed to the anomalous thermal expansion at the Curie temperature. Considerable magnetic entropy changes can also be observed in two-layered perovskites La1.6Ca1.4Mn2O7 and La2.5-xK0.5+xMn2O7+δ (0 x 0.5), and double-perovskite Ba2Fe1+xMo1-xO6 (0 ≤ x ≤ 0.3) near their respective Curie temperatures. Compared with rare earth metals and their alloys, the perovskite-type oxides are lower in cost, and they exhibit higher chemical stability and higher electrical resistivity, which together favor lower eddy-current heating. They are potential magnetic refrigerants at high temperatures, especially near room temperature.     

Optimization of the magnetocaloric effect in Ni-Mn-In alloys: A theoretical study

Based on ab initio and Monte Carlo simulations, we study the influence of the strength of the magnetic exchange parameters on the inverse and conventional magnetocaloric effect in the Ni₅₀Mn₃₄In₁₆ Heusler alloy using the mixed Potts and Blume-Emery-Griffiths model Hamiltonian. Within the proposed model, the temperature dependences of the magnetization, tetragonal deformation, and adiabatic temperature changes for magnetic field variation are obtained. It is first shown that a decrease in the magnetic exchange interactions leads to increased values of the magnetocaloric effect. We suppose that a reduction of the exchange interactions in the Ni-Mn-In alloy can be realized by the doping with nonmagnetic atoms such as B, Si, Zn, Cu, etc.     

On the theory of magnetocaloric effect in cooperative paramagnets

The theory developed in [1] is used as a basis for analyzing the magnetocaloric effect observed experimentally in cooperative paramagnets [2, 3]. Even though the degenerate ground state of such magnetic systems is strongly correlated, the final formulas describing the magnetocaloric effect are found to be very similar to those for a normal paramagnet. The order of magnitude of the effect is discussed, and theoretical results are compared with experimental data.     

Monte Carlo calculations of the magnetocaloric effect in gadolinium

In this work we discuss the magnetocaloric effect in metallic gadolinium. We use a model Hamiltonian of interacting 4f spins and treat the 4f spin–spin interaction both in the mean field approximation and in the Monte Carlo simulation. The calculations show that the mean field approximation yields reasonable results for the magnetocaloric potentials ΔS$\mathrm{\Delta }S$ and Δ_Tad$\mathrm{\Delta }{T}_{\mathrm{ad}}$ but it fails in explaining the experimental data of specific heat at the magnetic ordering temperature. On the other hand, our theoretical results show that the Monte Carlo calculation describes well not only the magnetocaloric potentials ΔS$\mathrm{\Delta }S$ and Δ_Tad$\mathrm{\Delta }{T}_{\mathrm{ad}}$ but also the specific heat capacity.     

Strain engineering of the magnetocaloric effect in MnAs epilayers

By using heteroepitaxy on two different GaAs templates, we have investigated the impact of anisotropic strain on the magnetocaloric effect (MCE) of MnAs. The temperature range, spread around room temperature, and the maximal MCE position are markedly different in the two epitaxial systems. Simulated MCE curves, obtained from a model based on the mean-field approximation, are in good agreement with the experimental data, indicating that the entropy variation is magnetic in origin. These results illustrate how strain can be used to tune the MCE in materials with coupled structural and magnetic phase transition and suggest that the MCE of MnAs may find applications in microelectronic circuitry.     

Anisotropic magnetocaloric effect in nanostructured magnetic clusters

We report the first experimental observation of anisotropic magnetocaloric effect (MCE) in the Fe8 clusters. It is found that the magnetic anisotropy plays a very important role in the determination of the magnetocaloric effect. The maximum and minimum MCE's are observed when the applied magnetic fields are parallel and perpendicular to the easy axis, respectively. The quantum spin Hamiltonian of a Fe8 cluster is used to calculate the partition function and the magnetization in a range of temperature and magnetic field. Excellent quantitative agreement between the experimental data and calculation is observed.     

Anisotropic magnetocaloric effect in thin magnetic films

We consider the magnetocaloric effect in thin magnetic films. The experimental method is based on heat transfer in a layered structure containing a magnetic film and a film of material with a metal-dielectric phase transition. The anisotropy of the magnetocaloric effect is studied.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 40–43, November, 1989.The authors thank G. A. Petrakovskii and V. G. Pyn'ko for useful discussions.     

Anisotropy of the magnetocaloric effect in a single cobalt crystal

A calculation of the anisotropy of the magnetocalorlc effect caused by rotation of the spontaneous magnetization vectors, was carried out for single-crystal specimens of the hexagonal system. Comparison of the theoretical formulas with experimental data obtained on a spherical single-crystal cobalt specimen, made it possible to determine the derivatives of the cobalt anisotropy constants with respect to temperature.The authors are deeply indebted to I. M. Puzeyu for donating the single-crystal cobalt specimen. We are also indebted to Prof. E. I. Kondorskii for discussion of this work.     

Measuring the effect of demagnetization in stacks of gadolinium plates using the magnetocaloric effect

The effect of demagnetization in a stack of gadolinium plates is determined experimentally by using spatially resolved measurements of the adiabatic temperature change due to the magnetocaloric effect. The number of plates in the stack, the spacing between them and the position of the plate on which the temperature is measured are varied. The orientation of the magnetic field is also varied. The measurements are compared to a magnetostatic model previously described. The results show that the magnetocaloric effect, due to the change in the internal field, is sensitive to the stack configuration and the orientation of the applied field. This may have significant implications for the construction of a magnetic cooling device.     

Magnetic transitions in delafossite CuFeO2: A magnetocaloric effect study

CuFeO₂ was synthetized by a solid-state reaction and its low temperature magnetic properties were investigated using the magnetocaloric effect. Magnetic susceptibility measurements show that there are two magnetic transition temperatures at about 16 and 11 K. Measurement of isothermal magnetization curves for different applied magnetic fields near these temperatures show a reversal in the magnetization trend around 16 K, and Arrott plots indicate they are accompanied by second- and first-order magnetic phase transitions, respectively. Both normal and inverse magnetocaloric effects are observed, and the maximum magnetic entropy change is obtained at 11 K.     

Experimental validation of the largest calculated isospin-symmetry-breaking effect in a superallowed Fermi decay

A precision measurement of the γ yields following the β decay of (32)Cl has determined its isobaric-analogue branch to be (22.47(-0.18)(+0.21))%. Since it is an almost pure-Fermi decay, we can also determine the amount of isospin-symmetry breaking in this superallowed transition. We find a very large value, δ(C) = 5.3(9)%, in agreement with a shell-model calculation. This result sets a benchmark for isospin-symmetry-breaking calculations and lends support for similarly calculated, yet smaller, corrections that are currently applied to 0+ → 0 + transitions for tests of the standard model.     

Phase diagram and magnetocaloric effect of alloys

We propose the phase diagram of a new pseudo-ternary compound, CoMnGe_1-xSn_x, in the range x0.1. Our phase diagram is a result of magnetic and calorimetric measurements. We find that the separate magnetic and structural transitions in CoMnGe are tuned together as the level of Sn substitution is increased. We demonstrate the appearance of a hysteretic magnetostructural phase transition in the range x=0.04–0.055, similar to that observed in CoMnGe under hydrostatic pressure. From magnetisation measurements, we show that the isothermal entropy change associated with the magnetostructural transition can be as high as in a field of 1 T. However, the large thermal hysteresis in this transition () will limit its straightforward use in a magnetocaloric device.