L. Havela

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.

A new family of actinide ternary intermetallic compounds

Abstract We have prepared an extended group of compounds of the general formula An 2 T 2 X (AnU, Np; T Ni, Co, Rh, Pd, Pt, Ir; XIn, Sn), which crystallize in the tetragonal structure (U 3 Si 2 type). The lattice parameters and atomic positions were determined using X-ray diffraction on single crystals. Preliminary magnetic and Mossbauer spectroscopy data show that we can follow the onset of magnetism within this group, which can be correlated with the Hill criterion and the development of 5f-d hybridization.

Coulomb-U and magnetic-moment collapse in δ-Pu

The around-the-mean-field version of the LDA+U method is applied to investigate electron correlation effects in δ-Pu. It yields a non-magnetic ground state of δ-Pu, and provides a good agreement with experimental equilibrium volume, bulk modulus and explains important features of the photoelectron spectra.

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.

Study of orbital and spin magnetism in UCoAl by polarized neutron diffraction

Abstract The induced magnetization density in a single crystal of UCoAl has been studied by polarized neutron diffraction in fields of 1.7 and 5.0 T along the c -axis. The field dependence of the moments on the U and Co sites, which has been extracted from an atomic multipole expansion of the magnetization density using 5f and 3d orbitals, show that the meta-magnetic transition seen in the bulk magnetization takes place on the U sublattice.

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.

Magnetic behaviour of RCuAl compounds

Abstract We studied structural and magnetic properties of the RCuAl (R=rare earth) intermetallic compounds by means of X-ray diffraction, susceptibility, magnetization and specific heat measurements. Magnetic ordering at low temperatures was observed in most of these materials. Results provide indications of ferromagnetic coupling in the heavy rare-earth compounds (R=Gd–Er), while PrCuAl and NdCuAl show antiferromagnetic behaviour. Additional magnetic phase transition in the ordered state was found in GdCuAl and DyCuAl. A metamagnetic transition was observed in DyCuAl.

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.