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

Itinerant electron metamagnetism and magnetocaloric effect in RCo2-based Laves phase compounds

Differential scanning calorimetry experiments have been performed under magnetic field in RCo2 Laves phases compounds R = Pr, Nd, Tb, Dy, Ho, and Er. The thermodynamical nature of the ferromagnetic transitions in PrCo2 and NdCo2 is clarified, after the controversy present in the literature about the character of their ordering transitions. The magnetocaloric effect in the compounds showing a first-order magnetostructural transition R = Dy, Ho, and Er is characterized. The latent heat, L, and the entropy change at the transition, S, have been studied by inducing the transition sweeping the temperature at a constant field and sweeping the field at a constant temperature. Results from calorimetric data show that L is essentially temperature independent, suggesting that the first order transitions in RCo2 are dominated by the structural effects. The magnetocaloric effect has been also characterized from magnetization data and the results are in excellent agreement with those from calorimetric data. However, the corresponding values of S for isothermal and isofield experiments differ significantly. We interpret the differences in terms of the broadness of the transitions and prove that the various S values reported in literature for first-order transitions are not always directly comparable.

/pdf/nature-and-entropy-content-of-the-ordering-transitions-in-4zbcq35drx.pdf

Nature and entropy content of the ordering transitions in RCo2

Rare-earth (RE) rich intermetallics crystallizing in orthorhombic Ho6Co2Ga-type crystal structure exhibit peculiar magnetic properties that are not widely reported for their magnetic ordering, order of magnetic phase transition, and related magnetocaloric behavior. By tuning the type of RE element in RE6Co2Ga (RE = Ho, Dy or Gd) compounds, metamagnetic anti-to-paramagnetic (AF to PM) phase transitions could be tuned to ferro-to-paramagnetic (FM to PM) phase transitions. Furthermore, the FM ground state for Gd6Co2Ga is confirmed by density functional theory calculations in addition to experimental observations. The field dependence magnetocaloric and Banerjee’s criteria demonstrate that Ho6Co2Ga and Dy6Co2Ga undergo a first-order phase transition in addition to a second-order phase transition, whereas only the latter is observed for Gd6Co2Ga. The two extreme alloys of the series, Ho6Co2Ga and Gd6Co2Ga, show maximum isothermal entropy change (∣ΔS
 iso
 max
 (5 T)∣) of 10.1 and 9.1 J kg−1K−1 at 26 and 75 K, close to H2 and N2 liquefaction, respectively. This outstanding magnetocaloric effect performance makes the RE6Co2Ga series of potential for cryogenic magnetic refrigeration applications.

/pdf/first-and-second-order-phase-transitions-in-re6co2ga-re-ho-4w90ead2dx.pdf

First- and second-order phase transitions in RE6Co2Ga (RE = Ho, Dy or Gd) cryogenic magnetocaloric materials

Magnetism in ErCo 2 under high pressure

Magnetism in HoCo2 and ErCo2 under high pressure

We have studied electrical resistivity anomalies connected with magnetism in RECo2 (RE=Nd, Tb, Er, Ho) compounds in pressures up to 8 GPa. At ambient pressure the former two compounds exhibit a second order magnetic phase transition (SOMPT) at TC, whereas a first order magnetic phase transition (FOMPT) is observed in the latter two. Although TC decreases with pressure in all four compounds, the TC(P) dependence for NdCo2 and TbCo2 differs considerably from this for HoCo2 and ErCo2. For the latter two, Tc vs P data deviate dramatically from the initial linear dependence above a critical pressure Pc to become almost pressure independent at higher pressures. We propose that this is reflecting the loss of Co metamagnetism that is also indicated by the vanishing resistivity drop at Tc for P>4 GPa and by the change from a FOMPT to a SOMPT. A scenario is discussed assuming that for P>Pc the projected Co 3d density of states at EF decreases. Hence the Co moment collapses because the RE-Co-RE exchange channel beco...

Magnetism in rare earth Co2 compounds under high pressures

Magnetic and transport properties of Er6Ni2Sn

We report on low-temperature measurements of magnetization, magnetoresistivity, Hall resistivity, magnetostriction, and elastic moduli on a UNiAl single crystal in magnetic fields up to 14 T applied along the c axis of the hexagonal structure. UNiAl is an itinerant 5f-electron antiferromagnet below TN=19.3 K. At temperatures below 7 K (∼0.3 TN), it exhibits a first-order metamagnetic transition (MT) at a critical field μ0Hc≈11.35 T between the low-field antiferromagnetic state and the high-field ferromagnetic one. The transition is accompanied by a sharp anomaly in transport and lattice properties. A striking observation is that the “virgin” curves expressing the field dependence of the various electronic properties clearly differ from those observed on a sample that has once appeared in the high-field state. This irreversibility in bulk properties corresponds to an irreversible change of the magnetic-structure propagation vector within the basal plane observed by neutron diffraction. The different shapes...

O. Syshchenko

Papers

Magnetism in ErCo 2 under high pressure

Magnetism in HoCo2 and ErCo2 under high pressure

Magnetism in rare earth Co2 compounds under high pressures

Magnetic and transport properties of Er6Ni2Sn

Magnetic field induced irreversibility in UNiAl