Magnetocaloric Materials with Multiple Instabilities

Abstract

the cases of H = 0 and H ≠ 0. In the zero field, the entropy exhibits an anomaly at a ferromagnetic (or ferrimagnetic) transition temperature (T C ), where the slope changes. As a magnetic field is applied to the material at around T C , the fluctuation is suppressed and the entropy is reduced. By exploiting this effect and combining several processes to form a thermodynamic cycle, e.g., Brayton cycle (i)→(ii)→(iii)→(iv)→(i), [7] heat can be extracted from the magnetic material to the thermal bath.While there are several prerequisites (e.g., magnetic and thermal hysteresis properties, mechanical stability, etc.) for materials to be applied to magnetic refrigeration, the enhanced magnitude of the magnetocaloric effect is of primary importance. The magnitude of the magnetocaloric effect is usually evaluated as the isothermal entropy change ΔS = S(H, T) -S(H = 0, T), or adiabatic temperature change ΔT, which are illustrated in Figure 1a, for a given magnetic field change ΔH. In many cases, the isothermal entropy change ΔS is adopted to argue the magnetocaloric effect as it is easier to experimentally evaluate than the adiabatic temperature change ΔT. Here, ΔS is calculated from a series of MH curves with a fine temperature interval using the Maxwell relationRecent research on magnetocaloric materials has focused on first-order magnetic transition systems, rather than secondorder ones because of the enhanced magnitude of the magnetocaloric effect in the former class of materials. [1] In second-order transition systems, the entropy value itself is continuous as a function of temperature, and only its temperature derivative is discontinuous, as shown in the upper panel of Figure 1b. Thus, the entropy change -ΔS displays a triangular-like shape with broad tails at both the high-and low-temperature sides. In this case, the maximum value of -ΔS is generally not very large. On the other hand, in the case of the first-order transition, the entropy itself changes discontinuously at the transition temperature, and therefore, the isothermal entropy change -ΔS tends to be large in magnitude for a rather narrow range of temperatures, as illustrated in Figure 1c. At the same time, the temperature profile of -ΔS indicates a rectangular-like shape rather than the triangular shape in the case of the secondorder transition. Based on these arguments, first-order transition systems have more widely been investigated than secondorder transition systems. Representative examples showing the The magnetocaloric effect is a well-known phenomenon where the temperature of a magnetic material varies upon application or removal of a magnetic field. This effect is anticipated to be applied to magnetic refrigeration technology, which is environmentally benign. For practical applications, it is essential to explore and expand the materials horizon of novel magnets that exhibit giant magnetocaloric effects to achieve sufficient cooling efficiency. In this article, several attempts to enhance the magnetocaloric effect are reviewed from the viewpoint...

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