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

Drosophila melanogaster is emerging as one of the most effective tools for analyzing the function of human disease genes, including those responsible for developmental and neurological disorders, cancer, cardiovascular disease, metabolic and storage diseases, and genes required for the function of the visual, auditory and immune systems. Flies have several experimental advantages, including their rapid life cycle and the large numbers of individuals that can be generated, which make them ideal for sophisticated genetic screens, and in future should aid the analysis of complex multigenic disorders. The general principles by which D. melanogaster can be used to understand human disease, together with several specific examples, are considered in this review.

Drosophila, the golden bug, emerges as a tool for human genetics.

Theoretical aspects of the magnetocaloric effect

Tetralogy of Fallot (TOF), the most common severe congenital heart malformation, occurs sporadically, without other anomaly, and from unknown cause in 70% of cases. Through a genome-wide survey of 114 subjects with TOF and their unaffected parents, we identified 11 de novo copy number variants (CNVs) that were absent or extremely rare (<0.1%) in 2,265 controls. We then examined a second, independent TOF cohort (n = 398) for additional CNVs at these loci. We identified CNVs at chromosome 1q21.1 in 1% (5/512, P = 0.0002, OR = 22.3) of nonsyndromic sporadic TOF cases. We also identified recurrent CNVs at 3p25.1, 7p21.3 and 22q11.2. CNVs in a single subject with TOF occurred at six loci, two that encode known (NOTCH1, JAG1) disease-associated genes. Our findings predict that at least 10% (4.5-15.5%, 95% confidence interval) of sporadic nonsyndromic TOF cases result from de novo CNVs and suggest that mutations within these loci might be etiologic in other cases of TOF.

http://www.med.upenn.edu/timm/documents/Greenwayetal.2009NG.pdf

De Novo Copy Number Variants Identify New Genes and Loci in Isolated, Sporadic Tetralogy of Fallot

Magnetisation and magnetoresistance isotherms were measured for a number of (R,R′)Co2, (R,Y)Co2 and R(Co,Si)2 (R,R′=rare earth) compounds. A metamagnetic transition is observed just above the Curie temperature (TC) of compounds having a first order phase transition, i.e. ErCo2-, HoCo2-, and DyCo2-based ones. Both 4f- and 3d-sublattice magnetic moments contribute to a sharp change of the magnetisation at this transition. The concurring suppression of the magnetoresistance can be considered to be due to quenching of spin fluctuations. In addition, the magnetic entropy change ΔSm is estimated from the magnetisation data by using a Maxwell equation. The resulting giant magnetocaloric effects are discussed in terms of the 4f(R)-localised spin and the 3d(Co)-spin fluctuations as well as the nature of the phase transition.

Metamagnetism, giant magnetoresistance and magnetocaloric effects in RCo2-based compounds in the vicinity of the Curie temperature

The magnetic behaviour of rare-earth—transition metal compounds

Inoue and Shimizu [1] give the free-energy expansion for a system consisting of a set of identical local (rare-earth) magnetic moments interacting with an itinerant electron subsystem (Co2). In this paper the free-energy expansion is given for a generalized system consisting of an arbitrary number of interacting magnetic contributions. This generalization is necessary for a discussion of the type of magnetic transition in quasi-binary RE-Co2 compounds such as TbxHo1−xCo2.

A generalization of the Inoue-Shimizu model

Ferromagnetic compounds of the series ErxY1-xCo2 (x=1, 08, 07, 06) have been investigated in specific-heat, thermal-expansion, magnetisation, AC susceptibility and resistivity experiments The magnetic moment on the Co atoms decreases with decreasing Er content, as does the Curie temperature The character of the magnetic transition changes from first order to second order around x=07 This change is compared with the similar one observed in other RExY1-xCO2 compounds with RE-Gd, Tb, Dy and is discussed in terms of the Inoue-Shimizu model by taking into account the strong temperature dependence of the magnetic susceptibility of the 3d electrons The spontaneous volume magnetostriction at zero temperature, the magnetic contribution to the specific heat and the magnetic entropy below the magnetic ordering temperature are determined The experimental results for these quantities confirm the induced character of the cobalt magnetic moment in these compounds

https://iopscience.iop.org/article/10.1088/0305-4608/18/2/010/pdf

Electronic and magnetic properties of ErxY1-xCo2 compounds

In a series of (Nd, Dy)Co 2 and (Pr, Dy)Co 2 compounds, the type of the magnetic phase transition was studied by magnetization and electrical-resistivity measurements. In both systems, a cchange of type was observed from second order (for NdCo 2 and PrCo 2 ) to first order (for compounds containing more than about 20% Dy). The results are discussed in the Inoue-Shimizu model, generalized in such a way that the presence of both light and heavy rare-earth atoms is taken into account. The volume dependence of the parameter a 3 (i.e. of the free-energy contribution a 3 M 2 Co /4) is supposed to play a decisive role in the determination of the order of the magnetic transition.

P.E. Brommer

Papers

Metamagnetism, giant magnetoresistance and magnetocaloric effects in RCo2-based compounds in the vicinity of the Curie temperature

The magnetic behaviour of rare-earth—transition metal compounds

A generalization of the Inoue-Shimizu model

Electronic and magnetic properties of ErxY1-xCo2 compounds

Magnetic phase transitions in (Nd, Dy)Co2 and (Pr, Dy)Co2 compounds