The recent literature concerning the magnetocaloric effect (MCE) has been reviewed. The MCE properties have been compiled and correlations have been made comparing the behaviours of the different families of magnetic materials which exhibit large or unusual MCE values. These families include: the lanthanide (R) Laves phases (RM2, where M = Al, Co and Ni), Gd5(Si1−xGex)4 ,M n(As1−xSbx), MnFe(P1−xAsx), La(Fe13−xSix) and their hydrides and the manganites (R1−xMxMnO3, where R = lanthanide and M = Ca, Sr and Ba). The potential for use of these materials in magnetic refrigeration is discussed, including a comparison with Gd as a near room temperature active magnetic regenerator material. (Some figures in this article are in colour only in the electronic version)

https://iopscience.iop.org/article/10.1088/0034-4885/68/6/R04/pdf

Recent developments in magnetocaloric materials

In this chapter, we will restrict our attention to the ferrites and a few other closely related materials. The great interest in ferrites stems from their unique combination of a spontaneous magnetization and a high electrical resistivity. The observed magnetization results from the difference in the magnetizations of two non-equivalent sub-lattices of the magnetic ions in the crystal structure. Materials of this type should strictly be designated as “ferrimagnetic” and in some respects are more closely related to anti-ferromagnetic substances than they are to ferromagnetics in which the magnetization results from the parallel alignment of all the magnetic moments present. We shall not adhere to this special nomenclature except to emphasize effects, which are due to the existence of the sub-lattices.

Ferromagnetic materials

The itinerant-electron metamagnetic (IEM) transition and magnetocaloric effects (MCE's) in the $\mathrm{La}({\mathrm{Fe}}_{x}{\mathrm{Si}}_{1\ensuremath{-}x}{)}_{13}$ and $\mathrm{La}({\mathrm{Fe}}_{x}{\mathrm{Si}}_{1\ensuremath{-}x}{)}_{13}{\mathrm{H}}_{y}$ compounds have been investigated. The $\mathrm{La}({\mathrm{Fe}}_{x}{\mathrm{Si}}_{1\ensuremath{-}x}{)}_{13}$ compounds exhibit large values of both the isothermal entropy change $\ensuremath{\Delta}{S}_{\mathrm{m}}$ and the adiabatic temperature change $\ensuremath{\Delta}{T}_{\mathrm{ad}}$ around the Curie temperature ${T}_{\mathrm{C}}$ in relatively low magnetic fields. Such large MCE's are explained by a large magnetization change at ${T}_{\mathrm{C}}$ and a strong temperature dependence of the critical field ${B}_{\mathrm{C}}$ for the IEM transition. By hydrogen absorption into the compounds, ${T}_{\mathrm{C}}$ is increased up to about 330 K, keeping the metamagnetic transition properties. Accordingly, the extension of the working temperature range having the large MCE's in relatively low magnetic fields is demonstrated by controlling y in the $\mathrm{La}({\mathrm{Fe}}_{x}{\mathrm{Si}}_{1\ensuremath{-}x}{)}_{13}{\mathrm{H}}_{y}$ compounds. The correlation between the increase of ${T}_{\mathrm{C}}$ and the large MCE's in the $\mathrm{La}({\mathrm{Fe}}_{x}{\mathrm{Si}}_{1\ensuremath{-}x}{)}_{13}{\mathrm{H}}_{y}$ compounds is discussed by taking the magnetovolume effects into consideration.

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Itinerant-electron metamagnetic transition and large magnetocaloric effects in La(FexSi1-x)13 compounds and their hydrides

Magnetocaloric effect: From materials research to refrigeration devices

Modern society relies on readily available refrigeration. Magnetic refrigeration has three prominent advantages compared with compressor-based refrigeration. First, there are no harmful gases involved; second, it may be built more compactly as the working material is a solid; and third, magnetic refrigerators generate much less noise. Recently a new class of magnetic refrigerant-materials for room-temperature applications was discovered. These new materials have important advantages over existing magnetic coolants: they exhibit a large magnetocaloric effect (MCE) in conjunction with a magnetic phase-transition of first order. This MCE is larger than that of Gd metal, which is used in the demonstration refrigerators built to explore the potential of this evolving technology. In the present review we compare the different materials considering both scientific aspects and industrial applicability. Because fundamental aspects of MCE are not so widely discussed, we also give some theoretical considerations.

https://iopscience.iop.org/article/10.1088/0022-3727/38/23/R01/pdf

Developments in magnetocaloric refrigeration

Magnetic properties of the 5f itinerant electron metamagnet UCoAl under high pressure

By making use of the Landau-Ginzburg energy expanded up to the sixth power of magnetization density, the spontaneous volume magnetostriction and dependence on pressure P of the Curie temperature Tc of an itinerant-electron ferromagnet with negative mode-mode coupling among spin fluctuations are discussed. The spontaneous moment is found to show first-order and second-order transitions at Tc, according to the values of the Landau coefficients. It is shown that the difference between the spontaneous volume magnetostrictions at T=0 and Tc is large and the P-dependence of Tc becomes anomalously large when a certain condition is satisfied by the Landau coefficients. The results obtained in the present theory could explain the anomalous T-dependence of the thermal expansion and the strong P-dependence of Tc observed in Invar alloys, some Laves phase Co compounds and others.

Negative mode-mode coupling among spin fluctuations and the magneto-volume effect in an itinerant-electron ferromagnet

Electronic structure and magnetism of YCo4B and YCo3B2

Dependences of the magnetic moment on the lattice constant for binary and pseudo-binary compounds , and with a hexagonal -type structure are studied by an ab initio calculation on a self-consistent linear muffin-tin orbital method within the atomic sphere approximation. It is found that Co moments in decrease abruptly at a critical lattice constant smaller than the observed one. On the other hand, Ni moments in are shown to appear at a lattice constant a little larger than the observed one. Similar calculations are carried out for the ordered compound , where Co and Ni atoms occupy the 3g and 2c sites, respectively. It is shown that Co and Ni atoms in this compound lose their moments at a lattice constant slightly smaller than the observed one. It is also found that the Co atom on the 3g site in and has two magnetic states, a high-moment state and a low-moment or non-magnetic state, depending on the lattice constant. The present results explain the concentration dependence of the the bulk moment observed for and .

Electronic structure and magnetism of ?, ? and ?

Electronic structures of the pseudo-binary compound with the -type hexagonal structure are calculated in the self-consistent linear muffin-tin orbital method within the atomic sphere approximation, by taking into account the preferential occupation sites for Ni. The calculated bulk moment disappears abruptly near 60% Ni concentration. Above the critical concentration of Ni, a metamagnetic transition from the paramagnetic to the ferromagnetic state is found to occur. These calculated results are consistent with the recently observed ones. It has been shown that the anomalous magnetic behaviours of can be explained by the detailed shape of the local density-of-states curve of the 3d-transition-metal atoms at the 3g site near the Fermi level.

https://iopscience.iop.org/article/10.1088/0953-8984/11/2/013/pdf

H. Yamada

Papers

Magnetic properties of the 5f itinerant electron metamagnet UCoAl under high pressure

Negative mode-mode coupling among spin fluctuations and the magneto-volume effect in an itinerant-electron ferromagnet

Electronic structure and magnetism of YCo4B and YCo3B2

Electronic structure and magnetism of ?, ? and ?

Itinerant-electron metamagnetism of