J. Appl. Cryst.の発刊に際して

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 magnetocaloric effect (MCE) in paramagnetic materials has been widely used for attaining very low temperatures by applying a magnetic field isothermally and removing it adiabatically. The effect can also be exploited for room-temperature refrigeration by using giant MCE materials. Here we report on an inverse situation in Ni-Mn-Sn alloys, whereby applying a magnetic field adiabatically, rather than removing it, causes the sample to cool. This has been known to occur in some intermetallic compounds, for which a moderate entropy increase can be induced when a field is applied, thus giving rise to an inverse magnetocaloric effect. However, the entropy change found for some ferromagnetic Ni-Mn-Sn alloys is just as large as that reported for giant MCE materials, but with opposite sign. The giant inverse MCE has its origin in a martensitic phase transformation that modifies the magnetic exchange interactions through the change in the lattice parameters.

Inverse magnetocaloric effect in ferromagnetic Ni-Mn-Sn alloys.

Magnetocaloric effect and magnetic refrigeration

We present direct evidence that the giant magnetocaloric effect recently discovered in the ${\mathrm{Gd}}_{5}{(\mathrm{S}\mathrm{i}}_{1.8}{\mathrm{Ge}}_{2.2})$ alloy is associated with a field-induced first-order structural transition from a ${P112}_{1}/a$ monoclinic (paramagnetic) to a Pnma orthorhombic (ferromagnetic) structure. A large volume contraction of $\ensuremath{\Delta}V/V\ensuremath{\cong}0.4%$ takes place spontaneously at the transition temperature, ${T}_{C}\ensuremath{\cong}240\mathrm{K}.$ The reported structural transition can be induced reversibly by application of an external magnetic field, producing strong magnetoelastic effects.

Magnetic-field-induced structural phase transition in Gd 5 ( S i 1.8 Ge 2.2 )

Commensurate and incommensurate “5M” modulated crystal structures in Ni–Mn–Ga martensitic phases

We have studied the isothermal entropy change around a first-order structural transformation and in correspondence to the second-order Curie transition in the ferromagnetic Heusler alloy Ni2.15Mn0.85Ga. The results have been compared with those obtained for the composition Ni2.19Mn0.81Ga, in which the martensitic structural transformation and the magnetic transition occur simultaneously. With a magnetic field span from 0 to 1.6 T, the magnetic entropy change reaches the value of 20 J/kg K when transitions are co-occurring, while 5 J/kg K is found when the only structural transition occurs.

Giant entropy change at the co-occurrence of structural and magnetic transitions in the Ni2.19Mn0.81Ga Heusler alloy

Crystal structure of 7M modulated Ni–Mn–Ga martensitic phase

The magnetic behavior of the perpendicular exchange-spring bilayer and multilayer, constituted of a hard and a soft phase that are exchange-coupled on a nanometric scale, is analyzed by a one-dimensional micromagnetic model leading to a complete magnetic phase diagram in terms of layer thicknesses. The validity of the one-dimensional assumption for the perpendicular situation is demonstrated. The phase diagram provides information on the type of demagnetization processes and the critical fields at which nucleation and reversal take place, depending on the intrinsic properties of the chosen soft and hard materials. An analytical expression of the reversal field is deduced for relatively large thicknesses. Moreover, the effect of a reduced interlayer coupling is also taken into account, leading to slight modifications of both the magnetic phase diagram and the hysteresis loops. A series of $\mathrm{Fe}∕\mathrm{Fe}\mathrm{Pt}$ bilayers, prepared by sputtering, has been used to evaluate the predictions of the model, which has also been tested with the available literature data on $\mathrm{Fe}\mathrm{Rh}∕\mathrm{Fe}\mathrm{Pt}$ bilayers. Both systems have a particular relevance for potential applications in magnetic recording as well as magnetic microelectromechanical systems.

Franca Albertini

Papers

Magnetic-field-induced structural phase transition in Gd 5 ( S i 1.8 Ge 2.2 )

Commensurate and incommensurate “5M” modulated crystal structures in Ni–Mn–Ga martensitic phases

Giant entropy change at the co-occurrence of structural and magnetic transitions in the Ni2.19Mn0.81Ga Heusler alloy

Crystal structure of 7M modulated Ni–Mn–Ga martensitic phase

Magnetic phase diagram and demagnetization processes in perpendicular exchange-spring multilayers