V. Sechovsky

Systematics across the UTX series (T = Ru, Co, Ni; X = Al,Ga, Sn) of high-field and low-temperature properties of non-ferromagnetic compounds

Abstract Results are presented of magnetic and specific-heat studies performed on some equiatomic compounds of the series U TX with a special emphasis on non-ferromagnetic UNiAl, UCoAl, URuA1 and URuGa. The occurrence of magnetism in these compounds is attributed mainly to the 5f electrons of uranium. The systematics of the ground-state properties across the series is discussed with respect to delocalization mechanisms of the 5f electrons. The possible role of spin fluctuations in the low-temperature behaviour of UCoAl, URuAl and URuGa is indicated by magnetic measurements. The compound with the highest γ value, UNiAl (γ = 167 mJ/mol K 2 ), behaves as an itinerant antiferromagnet.

Thermal properties of metamagnetic transition in heavy-fermion systems

Abstract In order to investigate the metamagnetic anomaly (MA) observed in several heavy-fermion compounds, field dependence of specific heat C(B) and magnetocaloric effect have been measured in the prototypical 4f-electron compound CeRu2Si2, and compared to that of a 5f-electron metamagnet UCoAl. In the C/T versus B curves, an unexpected double-peak structure is found in CeRu2Si2, suggesting that MA is closely related to a sharp peak in the density of states of the hybridized quasiparticle bands. On the other hand, for UCoAl, only a small decrease in C/T is observed, although both compounds exhibit similar bulk magnetic properties.

Giant Magnetoresistance Effects in UNiGa

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.

5f-Ligand Hybridization and Magnetism in UTX Compounds

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.

Magnetic, Electrical, and Specific-Heat Properties of UPdSn and UAuSn

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.

Heavy fermion behavior of U2T2X compounds

Magnetic and specific-heat studies of U[sub 2][ital T][sub 2][ital X] compounds show a frequent occurrence of the [gamma] enhancement in conjunction with the onset of antiferromagnetic ordering. The largest value of 830 mJ/mol K[sup 2] was observed in U[sub 2]Pt[sub 2]In, which is nonmagnetic down to 1.2 K. Variations of electronic structure are documented by optimized relativistic LCAO calculation.Magnetic and specific‐heat studies of U2T2X compounds show a frequent occurrence of the γ enhancement in conjunction with the onset of antiferromagnetic ordering. The largest value of 830 mJ/mol K2 was observed in U2Pt2In, which is nonmagnetic down to 1.2 K. Variations of electronic structure are documented by optimized relativistic LCAO calculation.

Specific-Heat and Magnetic-Behavior of UTGe Compounds

The temperature dependence of the magnetic susceptibility, electrical resistivity, and specific heat, as well as the magnetization in fields up to 35 T, of polycrystalline samples of UTGe (T=transition metal) compounds has been studied. The development from nonmagnetic UCoGe and URuGe through the weak ferromagnet URhGe, which displays the highest γ value, to complicated types of the local 5f‐moment ordering for T=Pd, Ni, and Pt is in agreement with the expected trends of gradually reduced 5f‐d hybridization.

Magnetic anisotropy in UTX compounds

Abstract The huge magnetic anisotropy in UTX compounds is found to be closely connected with the coordination of U atoms in the crystal lattice. Generally, the easy magnetization direction is oriented perpendicular to the highest coordination plane or direction. The magnetization behaviour in ordered state yields effective anisotropy fields in the range 100–400 T. The anisotropy persists in the paramagnetic state where the values of the paramagnetic Curie temperature for the easy and hard magnetization directions differ by 100–400 K. The results are discussed in the framework of a model considering the anisotropic two-ion exchange interaction mediated by the 5f-ligand hybridization.

Low temperature magnetic structures of UPdSn

Abstract The intermetallic compound UPdSn has been studied by means of dc magnetic susceptibility and neutron diffraction. While it has been assumed in previous work that the space group is P6 3 /mmc and there is disorder between the Pd and Sn atoms, we find a significantly better fit to our data with the related space group P6 3 mc which allows Pd/Sn ordering. There are two magnetic phase transitions at approximately 25 and 40 K. Above 40 K, there is no long-range magnetic order, but between 25 and 40 K, a canted antiferromagnetic structure forms, in which the moments lie in, or close to, the y-z -plane of a double-sized orthorhombic cell, with an angle of approximately 54° away from the z -axis. Below 25 K, the moments rotate out of the y-z -plane by approximately 45° to form a second related non-collinear structure. At the lowest temperature measured, the uranium moment is (2.05±0.13)μ B .

Magnetic transitions in UNiGa

Magnetization, magnetic susceptibility, and electrical resistivity studies of UNiGa in the single‐crystalline form revealed a strong uniaxial anisotropy both in the paramagnetic and in magnetically ordered state below Tc = 38 K. At T=35 K the system undergoes an antiferromagnetic transition, but by applying a field of about 1 T ferromagnetic alignment can be achieved.