A. Purwanto

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.

Magnetism in U2T2X compounds

Abstract Magnetic and other electronic properties are presented for the U 2 T 2 X compounds (T = late transition metal, X = Sn or In), that crystallize in the tetragonal U 3 Si 2 -type structure. They show the formation of 5f magnetic moments and antiferromagnetic ordering in compounds where weak 5f-d hybridization is expected. Near the onset of magnetic ordering strongly enhanced γ-values are found. Neutron-diffraction experiments on U 2 Pd 2 Sn and U 2 Pd 2 In point to a connection between the mutual coordination of U atoms and the type of magnetic anisotropy.

Magnetic phases in UNiGe

Original measurements of 4.2 K magnetization curves and temperature dependence of magnetic susceptibility were completed by an extended study of magnetization, specific heat, and electric resistivity as a function of temperature and magnetic field. Neutron diffraction experiments were also performed on powder and single crystal. The specific heat shows a sharp peak at 41.5 K, corresponding to the first-order phase transition, and a broad anomaly around 50 K. The magnetic structures in UNiGe confirm that the strong bonding of 5f orbitals along the a axis leads to a huge magnetic anisotropy with U magnetic moments perpendicular to this direction.

Structure, transport and thermal properties of UCoGa

By means of neutron powder diffraction, we find that UCoGa crystallizes in the hexagonal ZrNiAl structure and orders ferromagnetically at low temperatures with magnetic moments stacked along the c axis. The magnetic‐ordering temperature is reflected in anomalies in the temperature dependencies of the electrical resistivity and the specific heat at TC =47 K. Furthermore, the strong anisotropy in the electrical resistivity for i∥c and i⊥c indicates a significant contribution of the magnetic anisotropy to the electrical resistivity.

Magnetic properties of single-crystalline UCu/sub 3/Al/sub 2/

UCu/sub 3/Al/sub 2/ crystallizes in an ordered variant of the hexagonal CaCu/sub 5/ structure. By neutron powder-diffraction, the U atoms were found to occupy the 1a sites, while the 2c sites are occupied by Cu atoms only and a random occupation of the 3g sites by the remaining Cu and Al is found. The magnetic susceptibility, measured on a single crystal grown by the Czochralski triarc technique, is found to be maximal within the hexagonal basal plane with a maximum at about 10 K. For fields applied within the basal plane, the magnetization at 4.2 K exhibits a slight S-shape starting slightly below 15 T. No such anomalies are found for fields applied along the c-axis where the magnetic response is found to be much lower. No additional magnetic peaks, which could be related with long-range antiferromagnetic ordering, were detected in the neutron powder-patterns at low temperatures. >

Anisotropic exchange interactions in UNiGe

Abstract UNiGe crystallizing in the orthorhombic TiNiSi-type of structure orders magnetically below T N = 50 K with an additional magnetic phase transition at 42 K. Both structures (below 42 K commensurate antiferromagnetic, between 42 K and T N incommensurate) are non-collinear with a significant a -axis component (μ x = 0.35μ B /U at 20 K). The magnetic properties are highly anisotropic both in the ordered and the paramagnetic state. There are two metamagnetic transitions both with the field applied along the b - and the c -axis. While the magnetic structure above the second metamagnetic transition is forced ferromagnetic for both field orientations, for the field applied along the a -axis the magnetization curve at 4.2 K is linear up to 38 T and no change in magnetic structrure is observed. UNiGe provides strong evidence for anisotropic exchange interaction. The fact that the a -axis component cannot be aligned by the highest magnetic field used indicates that the antiferromagnetic interaction is much stronger between the a -axis components than between the others. This behaviour cannot be due to single-ion anisotropy because in that case the free energy does not depend on the sign of the a -axis component.

Electronic properties of UCuSn

Crystallographic analysis shows that UCuSn does not form in the hexagonal CaIn2 structure as reported previously, but is an ordered ternary compound and forms in an orthorhombic structure (space group: P21cn). Bulk and neutron‐diffraction measurements reveal that UCuSn orders antiferromagnetically below 62 K. At 4.2 K, high‐field magnetization reveals a complex magnetization process with two metamagnetic transitions. Furthermore, bulk investigations show an additional anomaly at 25 K, but a smooth temperature dependence of various magnetic peaks down to the lowest temperature gives no evidence for a second magnetic transition. Possible scenarios responsible for the drastic changes in the electronic properties around 25 K are discussed.

Crystallographic and magnetic structure of UCu1.5Sn2

We report on the crystallographic and magnetic structures of the antiferromagnet UCu1.5Sn2, as determined by x‐ray and neutron powder diffraction. It crystallizes in the tetragonal CaBe2Ge2 structure type, with space group P/4nmm, and we find no site disorder between two different Sn 2c sites, in contrast with a previous report. UCu1.5Sn2 orders antiferromagnetically with a Neel temperature of about 110 K. This is unusually high among uranium intermetallics. The uranium moments align along the c axis in a collinear arrangement but alternating along the c axis. The low‐temperature uranium moment is 2.01μB.

Incommensurate antiferromagnetic phase in UNiGe

By specific‐heat, magnetization, electrical resistivity, and neutron‐diffraction measurements on a single crystal we have confirmed that UNiGe orders antiferromagnetically below 50.5 K into an incommensurate phase with q=(0,1/2,−1/2)±(0,δ,δ), δ∼0.15. δ decreases continuously with decreasing temperature to ∼0.123 at 41.5 K, where the incommensurate phase vanishes in a first‐order phase transition and a commensurate antiferromagnetic structure with q=(0,1/2,1/2) sets in and remains stable down to the lowest temperatures. If a magnetic field sufficient to induce a metalmagnetic transition (1–5 T) is applied along the c axis, both antiferromagnetic phases are transformed to an uncompensated AF phase with q=(0,1/3,1/3) yielding a nonzero magnetization M≊1/3×MS. The latter structure is destroyed and a complete alignment of U moments is achieved in fields above 10 T. The strikingly different B‐T diagrams observed for a magnetic field applied along different crystallographic directions reflect strongly anisotropi...

Magnetic structures in UCuSn

We report neutron-diffraction results on UCuSn which crystallizes in the orthorhombic P21cn structure. This compound undergoes two magnetic transitions around TN=62K and TM=25K. At T=52K, UCuSn exhibits a noncollinear arrangement of uranium moments of ∼1.5μB∕U-atom with components in all three principal directions. Our diffraction data provide further evidence that the transition around 25K is indeed magnetic in origin, although our refinements did not lead to a conclusive answer on its exact nature.