Magnetic structure of URhSi single crystal

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Abstract

A single crystal of URhSi has been grown and subsequently investigated by magnetization and neutron-diffraction measurements. The neutron-diffraction experiment confirms that URhSi crystallizes in the orthorhombic TiNiSi-type of structure. Low-temperature experimental data point to ferromagnetic order in URhSi below 10.5 K. Similar to previous powder neutron-diffraction experiments, our single-crystal data are consistent with a collinear ferromagnetic ordering of U moments of 0.58±0.09 μB/U oriented along the crystallographic c-axis. No superconductivity has been detected at temperatures down to 40 mK.

Introduction

Materials containing 5f-electrons show a large variety of intriguing physical properties (for review see Ref. [1]). It has been well-documented by the evolution of electronic properties over isostructural series of compounds that the hybridization of 5f-electron states with valence electron states of ligands surrounding the 5f-ion (5f-ligand hybridization) governs the degree of the 5f-electron localization and thus has a strong influence on the type of magnetic ground state. The 5f-ligand hybridization plays an important role in magnetism also by mediating exchange interactions between magnetic moments born on the 5f-ions. Anisotropy of the hybridization, which is intimately connected with the spatial distribution of 5f-wavefunctions and the 5f orbital moments of actinide ions cause a strong magnetic anisotropy. The type of anisotropy thus strongly depends on the geometry of the layout of nearest-neighbor actinide ions and surrounding ligands. Besides the experimental evidences, also results of ab initio electronic structure calculations corroborate the scenario of magnetism in actinide compounds involving the role of the 5f-ligand hybridization.

Among the 5f systems, the U-based compounds are the most widely studied. The systematic investigation of isostructural groups of UTX compounds (T—a late transition metal, X—a p-electron element) revealed clear tendencies in the type of magnetic ordering, direction of magnetic moments, thermal and transport properties with respect to T and/or X species [1]. The UTX compounds containing transition metals from the right end of the transition element series, like UPdSn, exhibit by rule relatively large U magnetic moments and magnetic ordering at low temperatures. When moving to compounds containing the transition metals with less populated d-electron states, in which the 5f–d hybridization is enhanced, a stronger delocalization of the 5f-electron states leading finally to the 5f-electron moment destabilization is the general finding. The Pauli paramagnet UFeAl is a good representative of the strong 5f-ligand hybridization limit.

URhSi may be considered as a material that is situated in the crossover region of the two limits. This compound has been studied to date experimentally by several authors but only in polycrystalline form [2], [3], [4], [5], [6], [7]. The important issues connected with the electronic structure of this compound were also topic of theoretical studies [8], [9]. This compound was originally reported to adopt the orthorhombic CeCu2-type of structure (space group Imma) [2]. Later, however, it had been recognized to crystallize in the ordered ternary version, the TiNiSi-type of structure (space group Pnma) [3]. Recently, a great attention was paid to the isostructural compound, URhGe, which is reported to order ferromagnetically (F) at 9.6 K [10] and become superconducting well below 1 K while preserving ferromagnetism [11].

Bulk properties that have been studied so far on URhSi point to a ferromagnetic (F) order in URhSi with nearly the same Curie temperature (9.5 K) as for URhGe. Application of magnetic field has been found to have profound effects on the physical properties at low temperatures, e.g. the electrical resistivity was found to be reduced by 40% near TC [6]. A spontaneous moment of about 0.3 μB/f.u. has been derived from the free-powder magnetization at 4.2 K [4]. A clear difference between the free-powder and fixed-powder magnetization was attributed to the easy-plane magnetocrystalline anisotropy, which was corroborated by results of ab initio electronic-structure calculations [9]. The neutron powder-diffraction data obtained at low temperatures have been interpreted in terms of a collinear ferromagnetic structure with U moments oriented along the c-axis. While Prokeš et al. [3] found strongly reduced moments of μU=0.11 μB, Tran et al. [7] determined much larger value of μU=0.50 μB. Such a strong disagreement together with necessity to study magnetocrystalline anisotropy issues in URhSi prompted us to determine the magnitude of U moments in a single crystal that become available recently.

Section snippets

Experimental details

A single crystal of URhSi has been grown from a stoichiometric melt by means of a modified Czochralski technique in a tetra-arc furnace with continuously gettered Ar atmosphere. The starting element materials of at least 99.9% were used. No subsequent heat treatment was given to the crystal. The crystal quality was checked by the Laue X-ray technique and by the electron probe microanalysis (EPMA). The structural parameters of URhSi were verified by neutron diffraction on the single crystal. The

Results

The top and bottom parts of the crystal were checked by means of electron-probe microanalysis (EPMA). Both parts were found to be single phase and homogeneous with the composition U 35.5 at%, Rh 34.4 at% and Si 30.1 at%. This corresponds to the Si deficiency expressed by the U1.06Rh1.03Si0.91 with 3 atoms per formula unit.

Neutron diffraction investigation of the crystal structure has been performed at 15 K, well above the magnetic phase transition, confirmed that URhSi forms in the orthorhombic

Discussion and conclusions

The structure investigation has confirmed that URhSi forms in the orthorhombic TiNiSi-type of structure. The uranium atoms form zigzag chains extended along the a-axis. Usually, for the UTX compounds crystallizing in this type of structure, the shortest U–U distance is found within these chains and the separation of the chains determines the next-nearest U–U distance. However, in URhSi, the shortest U–U distance, which amounts using our structural data to d1=339.4 pm, is found not within but

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1

On leave from: Department of Electronic Structures, Charles University, 12116 Prague 2, The Czech Republic.

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