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

Magnetic refrigeration techniques based on the magnetocaloric effect (MCE) have recently been demonstrated as a promising alternative to conventional vapour-cycle refrigeration1. In a material displaying the MCE, the alignment of randomly oriented magnetic moments by an external magnetic field results in heating. This heat can then be removed from the MCE material to the ambient atmosphere by heat transfer. If the magnetic field is subsequently turned off, the magnetic moments randomize again, which leads to cooling of the material below the ambient temperature. Here we report the discovery of a large magnetic entropy change in MnFeP0.45As0.55, a material that has a Curie temperature of about 300 K and which allows magnetic refrigeration at room temperature. The magnetic entropy changes reach values of 14.5 J K-1 kg-1 and 18 J K-1 kg-1 for field changes of 2 T and 5 T, respectively. The so-called giant-MCE material Gd5Ge2Si2 (ref. 2) displays similar entropy changes, but can only be used below room temperature. The refrigerant capacity of our material is also significantly greater than that of Gd (ref. 3). The large entropy change is attributed to a field-induced first-order phase transition enhancing the effect of the applied magnetic field.

Transition-metal-based magnetic refrigerants for room-temperature applications

Magnetocaloric effect and magnetic refrigeration

M Agnetocaloric M Aterials

Applying an electrical field to a polar polymer may induce a large change in the dipolar ordering, and if the associated entropy changes are large, they can be explored in cooling applications. With the use of the Maxwell relation between the pyroelectric coefficient and the electrocaloric effect (ECE), it was determined that a large ECE can be realized in the ferroelectric poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] copolymer at temperatures above the ferroelectric-paraelectric transition (above 70°C), where an isothermal entropy change of more than 55 joules per kilogram per kelvin degree and adiabatic temperature change of more than 12°C were observed. We further showed that a similar level of ECE near room temperature can be achieved by working with the relaxor ferroelectric polymer of P(VDF-TrFE-chlorofluoroethylene).

http://science.sciencemag.org/content/sci/321/5890/821.full.pdf

Large electrocaloric effect in ferroelectric polymers near room temperature.

Magnetic refrigeration, an emerging new technology for cooling and gas liquefaction, needs magnetic materials with specific thermomagnetic behavior. Depending on the thermodynamic cycle selected, the isothermal magnetic entropy change or the adiabatic temperature change upon field application needs to be a preselected function of temperature. To obtain these properties, most designers rely on calorimetry, an expensive and time consuming technique. The present article describes that, classical magnetic measurements, when evaluated within the framework of the Landau theory for the second order phase transition or the thermodynamics in magnetic fields, are able to provide the preliminary information needed for the design of magnetic refrigerators. After reviewing the theory, experimental results on ferromagnetic gadolinium (Gd) and helimagnetic dysprosium (Dy) are analyzed and compared to direct experimental results as well as to those obtained using the molecular field model. The results demonstrate the reproducibility of entropy calculations and the good agreement between the experimental and the calculated specific heat anomalies. While the molecular field theory which assumes simple ferromagnetic order clearly fails for helimagnetic dysprosium, an analysis of the experimental data based on the Landau theory gives reliable results. Besides, the field and temperature dependencies of the isothermal magnetic entropy change allows one to characterize the magnetic structure (nature of the magnetic order) of the sample. Furthermore, magnetic measurements define the useful field range and provide information on transitions that influence the thermal behavior and magnetic losses.

Magnetic measurements: A powerful tool in magnetic refrigerator design

Direct adiabatic temperature change as well as magnetic measurements were carried out on two different samples of ${\mathrm{Gd}}_{5}{\mathrm{Si}}_{2}{\mathrm{Ge}}_{2}$ composition for which calculations predicted a ``giant'' magnetocaloric effect (MCE). While magnetic measurements well reproduce published values, serving the basis for the predictions, direct adiabatic temperature change measurements show a significantly smaller MCE. The discrepancy can be interpreted on the basis of the thermodynamics of first order magnetic transitions.

Direct Measurement of the “Giant” Adiabatic Temperature Change in Gd 5 Si 2 Ge 2

The magnetic properties and field‐dependent specific heat of melt‐spun amorphousRE70TM30 (RE=Gd, Tb, Dy, Ho and Er; TM=Fe and Ni) and Gd65Co35 alloys were investigated as potential magnetic refrigerants. Essentially zero magnetic hysteresis was observed in all the Gd–TM alloys at temperatures from 5 K up to the ordering temperatures. The coercive force of the RE70TM30 alloys depended mainly on the RE species and increased according to the order of RE=Gd<Ho<Er<Dy<Tb. The magnetic susceptibility of most of the alloys showed apparently normal Curie–Weiss behavior above the ordering temperatures. The heat capacitymeasurements in zero field and applied fields of 4 and 8 T indicated that the magnetic transition in these alloys are significantly broadened. The maximum adiabatic temperature changes for Er70Fe30, Gd70Ni30 and Gd65Co35amorphous alloys in a field change of 8 T are 4.0, 3.4, and 3.0 K, respectively. Mossbauer spectroscopy revealed that Fe atoms in the amorphousRE70Fe30 alloys carry a small magnetic moment that may complicate the magnetic ordering in the alloys. A simple model assuming a Gaussian distribution of ordering temperatures around the apparent Curie temperature was constructed to attempt to reconcile the differences in the observed magnetic properties of these amorphous alloys. The broad magnetic transition is attributed to the fluctuation of the exchange integral caused by the structural disorder in amorphous alloys. The calculated susceptibility, magnetization, and heat capacity agreed reasonably well with the experimental data and show that the magnetic susceptibility and magnetization are only weakly affected by the distribution of ordering temperatures, but the heat capacity is much more sensitive to such a distribution. To effectively screen out magnetic refrigerants with sharp magnetic transitions and correspondingly large adiabatic temperature changes from those with broadened transitions and small adiabatic temperature changes, the field‐dependent heat capacitymeasurement technique is a powerful tool to use.

Thermomagnetic properties of amorphous rare‐earth alloys with Fe, Ni, or Co

The magnetization MH(T) and the specific heat capacity cP,H(T) of the ErCo2 intermetallic compound were measured in the temperature range 5 - 100 K and in 0, 7 or 14 T applied field, respectively. A clear first-order phase transition is found at the magnetic ordering of the Er sublattice. While for order-disorder transitions in simple ferromagnets there is a good agreement between magnetocaloric performance predicted on the basis of magnetization measurements compared to calorimetric measurements, it is necessary to investigate whether the agreement is still present for materials with more complex transitions (e.g. order - order, metamagnetic, first order etc). From the magnetization data the magnetic entropy change at the transition was calculated using the Maxwell relations. From the cP,H(T) measurements both the magnetic entropy change and the adiabatic temperature change were calculated and compared to values obtained from MH(T) and to the values calculated by the usual approximative expressions. The agreement is less good than in the case of second-order phase transitions. The discrepancy is interpreted in terms of the theory of first-order/metamagnetic transitions showing that the boundary conditions used in the derivation of the approximative formulae for simple ferromagnetic materials are not appropriate for more complex transitions as in ErCo2.

https://iopscience.iop.org/article/10.1088/0953-8984/11/36/313/pdf

Metamagnetic transition and magnetocaloric effect in ErCo2

The isothermal change of the magnetic entropy of a magnetically ordered material upon application of external magnetic field can be calculated from the temperature and field dependence of the magnetization or of the specific heat. The adiabatic temperature change, i.e., the magnetocaloric effect (MCE) can be measured directly or can be calculated via different methods using the field-dependent specific heat values, or a combination of data obtained via magnetization and thermal measurements. In the present study, magnetic and thermal measurements were carried out on Gd75Y25(TC=232 K) and Gd48Y52(TC=161 K) samples, for applied fields ranging between 0 and 7 T. From both datasets, the magnetic entropy change and MCE values were calculated and compared, in order to assess the mutual reliability of the methods applied. The magnetically or thermally deduced specific heat discontinuities show a reasonable agreement within experimental error. Similar comparison of the calculated magnetic entropy changes reveals ...

http://www.personal.ceu.hu/staff/Botond_Koszegi/magnetocaloric.pdf

M. Foldeaki

Papers

Magnetic measurements: A powerful tool in magnetic refrigerator design

Direct Measurement of the “Giant” Adiabatic Temperature Change in Gd 5 Si 2 Ge 2

Thermomagnetic properties of amorphous rare‐earth alloys with Fe, Ni, or Co

Metamagnetic transition and magnetocaloric effect in ErCo2

Comparison of magnetocaloric properties from magnetic and thermal measurements