E.H. Brück

Developments in magnetocaloric refrigeration

Modern society relies on readily available refrigeration. Magnetic refrigeration has three prominent advantages compared with compressor-based refrigeration. First, there are no harmful gases involved; second, it may be built more compactly as the working material is a solid; and third, magnetic refrigerators generate much less noise. Recently a new class of magnetic refrigerant-materials for room-temperature applications was discovered. These new materials have important advantages over existing magnetic coolants: they exhibit a large magnetocaloric effect (MCE) in conjunction with a magnetic phase-transition of first order. This MCE is larger than that of Gd metal, which is used in the demonstration refrigerators built to explore the potential of this evolving technology. In the present review we compare the different materials considering both scientific aspects and industrial applicability. Because fundamental aspects of MCE are not so widely discussed, we also give some theoretical considerations.Modern society relies on readily available refrigeration. Magnetic refrigeration has three prominent advantages compared with compressor-based refrigeration. First, there are no harmful gases involved; second, it may be built more compactly as the working material is a solid; and third, magnetic refrigerators generate much less noise. Recently a new class of magnetic refrigerant-materials for room-temperature applications was discovered. These new materials have important advantages over existing magnetic coolants: they exhibit a large magnetocaloric effect (MCE) in conjunction with a magnetic phase-transition of first order. This MCE is larger than that of Gd metal, which is used in the demonstration refrigerators built to explore the potential of this evolving technology. In the present review we compare the different materials considering both scientific aspects and industrial applicability. Because fundamental aspects of MCE are not so widely discussed, we also give some theoretical considerations.

Magnetic and X-ray diffraction measurements for the determination of retained austenite in TRIP steels

The accurate determination of the volume fraction of retained austenite is of great importance for the optimization of transformation induced plasticity (TRIP) steels. In this work, two aluminium-containing TRIP steels are studied by means of magnetization and X-ray diffraction (XRD) measurements. By fitting the field dependence of the approach to saturation in the magnetization curves, the saturation magnetization is determined, which is linearly related to the volume fraction of retained austenite. Moreover, information with respect to the microstructure can be obtained from the fitting parameters and the demagnetizing factor for the magnetization curve. The volume fractions obtained from the magnetization measurements are compared with data from XRD measurements. A discussion of the data suggests that magnetization measurements lead to more reliable results and a more sensitive detection of the retained austenite than XRD measurements.

Magnetocaloric effect in MnFe(P,Si,Ge) compounds

We have studied the magnetocaloric effect in MnFe(P,Si,Ge) compounds. The structural properties of the compounds were determined by x-ray diffraction. The homogeneity and the stoichiometry of the compounds were checked by electron probe microanalysis. The Curie temperature is found to be near room temperature. Specific-heat measurements made on these compounds show a first-order ferromagnetic—paramagnetic phase transition. The magnetocaloric effect derived from magnetization data shows that this effect in the MnFe(P,Si,Ge) compounds is as large as that in Gd-based compounds and MnFeP1−xAsx compounds. This means that we have succeeded in totally replacing As by (Ge,Si) in the latter compounds without losing the favorable magnetic properties. The upshot is that we have found relatively low cost and nontoxic materials for room-temperature cooling applications.

Structure, magnetism, and magnetocaloric properties of MnFeP1−xSix compounds

MnFeP1−xSix compounds with x=0.10,0.20,0.24,0.28,…,0.80,1 were prepared by high-energy ball milling and solid-state reaction. The structural, magnetic, and magnetocaloric properties are investigated as a function of temperature and magnetic field. X-ray diffraction studies show that the samples in the range from x=0.28 to 0.64 adopt the hexagonal Fe2P-type structure with a small amount of second phase which increases with increasing Si content. The samples with lower Si content show the orthorhombic Co2P-type structure. Magnetic measurements show that the paramagnetic-ferromagnetic transition temperatures range from 214to377K. Of much importance is the fact that these compounds do not contain any toxic components and exhibit excellent magnetocaloric properties.

Structural and Magnetic Properties of MnFe

The structural and magnetic properties of MnFe1-xCoxGe compounds with x=0,0.1,0.2,0.3,0.4,0.5,0.6,0.7,0.8,0.85,0.9, and 1.0 were investigated by means of X-ray diffraction (XRD) and magnetization measurements. XRD shows that the MnFe1-xCoxGe compounds crystallize in the hexagonal Ni2In-type crystal structure for xles0.8 and in the orthorhombic NiTiSi-type structure for x>0.8. The magnetization measurements show that the MnFe1-xCoxGe compounds exhibit a complex magnetic behavior. The Curie temperature increases with increasing of x. The saturation magnetization of the compounds with Ni2In type structure increase with increasing of x and the saturation of the magnetization in the NiTiSi-type structure also increases with increasing x. We investigated the magnetocaloric effects in these compounds by means of magnetization measurements. The maximum magnetic-entropy change observed in these compounds reaches 9 J/kgK for x=0.8 in a field change from 0 to 5 T at around 289 K

_1 - rm x

Magnetic transitions in UNiGa were found to cause pronounced anomalies in temperature and magnetic‐field dependencies of the electrical resistivity. The resistivity values are drastically reduced by the field induced transition from an antiferromagnetic to a ferromagnetic phase. The giant magnetoresistance (as large as Δρ/ρ=−87% at T=1.2 K for i∥c) observed in UNiGa is typical for materials characterized by a strong coupling of conduction electrons with highly correlated f electrons. The observed anisotropy of transport properties is closely connected with the strong uniaxial magnetic anisotropy of this compound.

Co

Abstract Investigations of magnetic and related electronic properties of UTX compounds have revealed a variety of magnetic phenomena ranging from weak paramagnetism to various types of magnetic ordering. The observed systematics of ground-state properties is discussed with respect to the development of the electronic structure related to the expected variations of the 5f-ligand hybridization. The huge magnetic anisotropy observed in all studied materials is tentatively attributed mainly to the hybridization-mediated anisotropic two-ion interaction.

_rm x

In this article we report on the magnetic and transport properties of FexRh1−x thin films, prepared by evaporation in high vacuum, in the composition range 0.41<xFe<0.59. Upon annealing (at a temperature of 870 K or higher) a certain volume fraction transforms to the ordered CsCl-type (α′) FeRh phase. Close to room temperature this phase shows a hysteretic transition between the antiferromagnetic (AF) and the ferromagnetic (F) state for samples with xFe<0.5, which gives rise to a magnetoresistance (MR) effect. Although the magnetic transition was never found to be complete, it is shown that the full MR ratio can be obtained by extrapolation of the measured MR ratio as a function of the relative change of the magnetization at the transition. The AF→F transition is only observed for films with xFe<0.505±0.015, for which the α′ phase with this (fixed) composition is present together with a nonmagnetic Rh-rich fcc-type phase, as is shown from a combination of x-ray diffraction, Mossbauer spectroscopy, and mag...

Ge Compounds

Abstract We have studied the magnetic properties of Mn 5 Ge 3− x Sb x compounds with x =0, 0.1, 0.2 and 0.3 by means of magnetisation measurements at different temperatures and fields up to 5 T. The compounds crystallize in the hexagonal Mn 5 Si 3 -type structure with space group P6 3 /mcm . The compounds show enhanced Curie temperatures but decreasing average magnetic Mn moments with increasing Sb content. The magnetic entropy changes in these compounds are determined from the temperature and field dependence of the magnetisation using the thermodynamic Maxwell relation. Mn 5 Ge 3 exhibits considerably large magnetic entropy changes that are comparable with that of Gd metal. The Sb substitution has two kinds of effects on the magnetocaloric effect (MCE) of Mn 5 Ge 3− x Sb x . One is the magnetic entropy change that decreases with increasing Sb content, the other is the MCE peak that becomes broadened.

Giant Magnetoresistance Effects in UNiGa

Abstract UPdSn orders antiferromagnetically below 40 K and undergoes another magnetic phase transition at about 27 K, which is connected with a re-arrangement of the antiferromagnetic structure. The magnetic properties of UPdSn are strongly anisotropic with the c-axis as the hard-magnetization direction. For UAuSn, the maximum at 36 K in the χ(T) curve may also indicate the onset of antiferromagnetism. However, the very broad anomaly in the specific heat, the poorly resolved 119Sn Mossbauer spectra and the magnetic history phenomena at lower temperatures do not point to long-range magnetic order. The possibility of a frozen non-periodic arrangement of U magnetic moments is discussed in connection with atomic disorder within the Au-Sn sublattice. The distinction between the electron properties of UAuSn and UPdSn is reflected in the different values of the γ coefficient of the low-temperature specific heat, which is remarkably small for UPdSn (5 mJ/mol K2) in contrast to UAuSn (80 mJ/mol K2). A possible localization of the 5f states in UPdSn is envisaged.