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

The magnetocaloric effect is the change in temperature of a material as a result of the alignment of its magnetic spins that occurs on exposure to an external magnetic field. The phenomenon forms the basis for magnetic refrigeration, a concept purported to be more efficient and environmentally friendly than conventional refrigeration systems. In 1997, a 'giant' magnetocaloric effect, between 270 K and 300 K, was reported in Gd5Ge2Si2, demonstrating its potential as a near-room-temperature magnetic refrigerant. However, large hysteretic losses (which make magnetic refrigeration less efficient) occur in the same temperature range. Here we report the reduction (by more than 90 per cent) of these hysteretic losses by alloying the compound with a small amount of iron. This has the additional benefit of shifting the magnetic entropy change peak (a measure of the refrigerator's optimal operating temperature) from 275 K to 305 K, and broadening its width. Although the addition of iron does not significantly affect the refrigerant capacity of the material, a greater net capacity is obtained for the iron-containing alloy when the hysteresis losses are accounted for. The iron-containing alloy is thus a much-improved magnetic refrigerant for near-room-temperature applications.

Reduction of hysteresis losses in the magnetic refrigerant Gd5Ge2Si2 by the addition of iron.

Theoretical aspects of the magnetocaloric effect

We show the key role of the elastic local strain (or micro-strain) e of the CuO2 lattice in the phase diagram of cuprate superconductors. The superconducting critical temperature Tc(δ, e )i s shown tobe a function of two variables, the doping δ and the microstrain e. PACS number: 74.72.−h (Some figures in this article are in colour only in the electronic version)

https://iopscience.iop.org/article/10.1088/0305-4470/36/35/302/pdf

The strain of CuO2 lattice: the second variable for the phase diagram of cuprate perovskites

MnFeP0.595Si0.33B0.075 has recently been presented as a top class magnetocaloric material combining a large magnetocaloric entropy change, a large temperature change, limited thermal hysteresis, and an enhanced mechanical stability. By providing practical rules to control the transition temperature in the MnFe(P,Si,B) system, we demonstrate that this new material was not a single composition and that a giant magnetocaloric effect (MCE) can be observed over a broad temperature range, a point of great interest for applications. As important prerequisite is the cyclability of the MCE. The thermal hysteresis and the recovery of the MCE during field oscillations have been addressed for MnFe(P,Si,B) materials. It is found that when the thermal hysteresis becomes about as large as the field induced shift of the transition, the MCE becomes partially irreversible and a strong decrease in the cyclic temperature change occurs. For an intermediate field change, typically 1 T, the limit for thermal hysteresis is about...

Magnetocaloric effect, cyclability and coefficient of refrigerant performance in the MnFe(P, Si, B) system

It is well known that the intermetallic compound ${\mathrm{DyAl}}_{2}$ is ferromagnetic and exhibits a change in the easy direction of magnetization from 〈100〉 to 〈111〉, for a critical applied magnetic field in the 〈111〉 direction. Using a simple model which includes the crystalline electric field and the exchange interaction we have theoretically investigated its magnetic entropy along the three main crystallographic directions. An anomalous behavior was theoretically predicted to exist in the magnetic entropy, which consists in the increase of entropy when the magnetic field is applied in the 〈111〉 direction. Comparisons between the theoretically predicted anomaly in ${\mathrm{DyAl}}_{2}$ with the recent discovery of a magnetic anomaly in ${\mathrm{PrNi}}_{5}$ are also presented.

Anomaly in the magnetocaloric effect in the intermetallic compound DyAl 2

We report a theoretical investigation that predicts the existence of anomalous magnetocaloric effect in the YbAs compound. The YbAs cool upon magnetizing and warm upon demagnetizing, in the temperature range between 33 and 84 K for external magnetic field change from 0 to 23.5 T. The anomalous effect can be observed on the negative part of magnetic entropy changes, upon variation of the external magnetic field, in an isothermic process. This study was carried out using a model Hamiltonian, which takes into account the crystalline electrical field and the quadrupolar interactions within the molecular-field approximation.

Anomalous magnetocaloric effect in YbAs associated with the giant quadrupolar interaction

Pseudopotential Approach for Dilute Alloys. I. Nontransition and Non-Noble Hosts

The combined role of intraband and electron-localized moment exchange in determining the magnetic behaviour of a system composed of itinerant electrons and localized magnetic moments is investigated. Having in mind rare-earth–transition metal and rare-earth–normal metal intermetallic compounds, the critical temperature versus de Gennes factors and the temperature dependence of the magnetizations and susceptibilities of the two magnetic species are studied. Results are obtained for two cases, a) a delta-like band (narrow band limit), and b) a parabolic density of states, using the molecular field approximation both in the treatment of intraband interaction (Stoner-like description) and electron-localized spin exchange. Some comments are made on the parallel between computed and measured magnetic quantities in the systems RAl2, RFe2 and R(Fe1–xAlx)2 (0 < x < 1 and R heavy rare-earth).



On etudie le rǒle combine d'echange intrabande electron-electron et d'echange electron-ion dans la determination du comportement magnetique d'un systeme dans lequel des electrons de conduction se couplent avec des moments magnetiques localises. Ce modele peut ětre interessant pour expliquer a) la dependance avec la temperature des magnetisations et des susceptibilites des deux contributions magnetiques (ionique et de bande); b) la temperature critique en fonction des facteurs de de Gennes dans les composes intermetalliques terre rare-metal normal et terre rare-metal de transition. On calcule ces quantites physiques pour deux cas: a) une bande en delta (limite de bande etroite) et b) une densite d'etats parabolique. On fait un parallele entre les quantites magnetiques calculees et mesurees dans les systemes RAl2, RFe2 et R(Fe1–xAlx)2 (0 < x < 1, R terre rare lourde).

A simple model approach to localized-itinerant magnetism application to rare-earth intermetallics

The magnetic behavior of a model in which van Vleck ions, under the action of a crystal field, interacting by exchange with an electron gas is investigated. The condition of onset of ferromagnetism and the behavior of the critical temperature, band and ionic magnetizations (and susceptibilities) versus temperature, as a function of the band width, exchange interaction, and the crystal field splitting energy parameters are obtained within an approximation equivalent to a molecular field formulation.



On a etudie le comportement magnetique d'un modele dans lequel des ions van Vleck, sous I'action d'un champ cristallin, interagent par echange avec un gas d'electrons. On a obtenu la condition d'existence d'ordre ferromagnetique, la dependence avec la temperature de l'aimantation et de la susceptibilitye ainsi que la temperature critique en fouction de la largeur de bande et des parametres du champ cristallin et d'echange dans une approximation de champ moleculaire.

X. A. da Silva

Papers

Anomaly in the magnetocaloric effect in the intermetallic compound DyAl 2

Anomalous magnetocaloric effect in YbAs associated with the giant quadrupolar interaction

Pseudopotential Approach for Dilute Alloys. I. Nontransition and Non-Noble Hosts

A simple model approach to localized-itinerant magnetism application to rare-earth intermetallics

Magnetic Behavior of van Vleck Ions and an Electron Gas Interacting by Exchange