J.C.P. Klaasse

Superconductivity on the Border of Weak Itinerant Ferromagnetism in UCoGe

We report the coexistence of ferromagnetic order and superconductivity in UCoGe at ambient pressure. Magnetization measurements show that UCoGe is a weak ferromagnet with a Curie temperature T(C)=3 K and a small ordered moment m(0)=0.03 micro(B). Superconductivity is observed with a resistive transition temperature T(s)=0.8 K for the best sample. Thermal-expansion and specific-heat measurements provide solid evidence for bulk magnetism and superconductivity. The proximity to a ferromagnetic instability, the defect sensitivity of T(s), and the absence of Pauli limiting, suggest triplet superconductivity mediated by critical ferromagnetic fluctuations.

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.

Enhancement of spin fluctuations by alloying heavy-fermion UPt3 with Pd

Abstract In the course of our investigation of the low-temperature properties of heavy-fermion UPt 3 , a number of pseudobinary U(Pt 1- x Pd x ) 3 compounds has been prepared and their specific heat measured in the temperature range 1.2–30 K. For low Pd concentrations the linear term in the specific heat increases, pointing to an enhancement of the spin-fluctuation effects at low temperatures. Superconductivity has not been observed down to 40 mK in the U(Pt 0.99 Pd 0.01 ) 3 sample.

A specific-heat study of RMn4Al8 compounds (R=Y, La, Pr, Nd, Dy, Er)

Abstract Specific-heat measurements have been performed on the tetragonal ThMn 12 -type RMn 4 Al 8 compounds with R=Y, La, Pr, Nd, Dy and Er in the temperature range 1.5 to 200 K. The compound PrMn 4 Al 8 gives rise to a λ-type anomaly at 14 K. No anomalies indicative of magnetic ordering were found above 1.5 K in the specific-heat curves of the RMn 4 Al 8 compounds with R=Y, La, Nd, Dy, and Er. From the temperature dependence of the magnetic entropy, it is concluded that the overall crystal-field splitting in the compounds with R=Pr, Nd, Dy, and Er is of the order of 200 K or even less.

Low-temperature specific heat of REBa2Cu3O7 in magnetic fields up to 5 T (RE = Y, Pr, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb and Lu)

Abstract Specific heat measurements have been performed on a series of REBa 2 Cu 3 O 7 -compounds (RE = Y, Pr, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb and Lu) in magnetic fields up to 5 T in a temperature range between 1.4 and 40 K. The electronic and lattice contributions to the specific heat of GdBa 2 Cu 3 O 7 have been taken as a reference for the heavy rare-earth compounds in order to deduce the magnetic and crystal-field contributions to the specific heat. The low-temperature data in field of the Dy-, Er-, and Yb-compounds confirm a doublet ground state, that is splitted by the applied field. The crystal-field contributions to the specific heat in these compounds deviate from Schottky behaviour due to anisotropy effects. Values for gJ amount to 2.3, 3.2 and 1.6, respectively.

Tuning the giant inverse magnetocaloric effect in Mn2−xCrxSb compounds

Structural, magnetic, and magnetocaloric properties of Mn2-xCrxSb compounds have been studied. In these compounds, a first order magnetic phase transition from the ferrimagnetic to the antiferromagnetic state occurs with decreasing temperature, giving rise to giant inverse magnetocaloric effects that can be tuned over a wide temperature interval through changes in substitution concentration. Entropy changes as high as 7.5 J/kg K have been observed, and a composition independent entropy change is obtained for several different concentrations/working temperatures, making these compounds suitable candidates for a composite working material.

Systematics in intermetallic compounds containing intermediate-valent ytterbium

Abstract Results are presented on the magnetic susceptibility of a number of Yb intermetallic compounds, where Yb is either divalent or, as will be argued, in an intermediate-(mixed)-valence state. Low-temperature upturns in the susceptibility are ascribed to impurities, for which corrections are made under the assumption that only Yb 2 O 3 is responsible for the impurity contribution. The parameters that describe the magnetic behaviour of the I.V. compounds - the temperature-independent low-temperature susceptibility, χ 0 , and the temperature, T max , where the susceptibility reaches its maximum - are correlated with each other and also with the valence, υ, as determined from lattice-parameter data. The only simple relationship that can be established is that T max depends linearly on υ. Linear specific heat coefficients, γ, do not scale with χ 0 over the whole range of intermediate valences.

Ambiguous magnetic behaviour of some Yb-compounds

Abstract For the intermetallic compounds YbIn3, YbCu2 and YbAl2 we observed the susceptibility to increase with temperature at high temperatures. After correction for small amounts of Yb2O3 the susceptibility was found to be independent of temperature at low temperatures. The increase of the susceptibility suggests a description in terms of a non magnetic ground state and a magnetic excited state.

Magnetic phase transition and magnetocaloric effect in Mn5−xFexSi3

Abstract The magnetic phase transitions and the magnetocaloric properties of the pseudo binary system Mn 5− x Fe x Si 3 with x =0, 1, 2, 3, 4, 5 have been investigated by measuring the magnetisation as a function of temperature and magnetic field. With increasing Fe content, the antiferromagnetic ordering temperature shifts to higher temperatures. The compound Mn 5 Si 3 is an antiferromagnet with a field-induced transition. At 4.2 K, the Mn 5− x Fe x Si 3 compounds with x =1 and 2 display antiferromagnetic behavior up to 38 T. For the compounds with x =4 and 5 we find ferromagnetic order. The magnetic-entropy changes in this system are derived from the temperature and field dependence of the magnetization by means of the thermodynamic Maxwell relation. For the MnFe 4 Si 3 compound, enhanced but modest magnetic-entropy changes are observed.