Role of disorder in magnetic and conducting properties of U–Mo and U–Mo–H thin films

Highlights

• Preparation of U–Mo and UH₃–Mo thin films by sputter deposition.

• Superconductivity in U_0.79Mo_0.21 below T_c = 0.55 K.

• UH₃–Mo hydride films are ferromagnets; the Curie temperature of (UH₃)_0.74Mo_0.26 is 165 K.

• Negative temperature coefficient of resistivity, TCR = dρ/dT in all films attributed to the randomness on atomic scale.

Abstract

Crystal structure, magnetic and electrical properties of thin U–Mo films and their hydrides prepared by reactive sputter deposition from metallic targets were studied. The films are metallic but with high values of electrical resistivity. Both bcc U_0.79Mo_0.21 thin film and the hydride (UH₃)_0.74Mo_0.26 (β-UH₃ structure type) have a very strong texture. Superconductivity is identified below T_c = 0.55 K for U_0.79Mo_0.21. The critical field of 1.0 T is exceeding that of α-U (0.3 T). The hydride film (UH₃)_0.74Mo_0.26 is a ferromagnet with the Curie temperature T_C = 165 K, higher concentration of Mo somewhat reduces the T_C value. Deposition on a cooled substrate reduces the grain size of the hydride, approaching the limit of amorphous state. The U–Mo films and hydrides exhibit the net negative temperature coefficient of resistivity, TCR = dρ/dT, attributed to the randomness on atomic scale, yielding very strong scattering of electrons and weak localization.

Introduction

In quest for low-enriched fuels, the required combination of high uranium density and mechanical integrity was found in U–Mo alloys. These are formed with the bcc structure. Typically 18 at.% Mo gives a single-phase bcc material, corresponding to the γ-phase of U metal [1], stable above 1049 K [2]. At lower Mo concentrations, the γ-phase is metastable and alloys contain a significant fraction of the orthorhombic α-U-like phase [1]. Both α- and γ-U are superconducting. In the former case, however, the superconductivity is not a bulk property and it is eliminated when the charge density wave state is realized at low temperatures [3]. The bulk superconducting ground state is apparently one of general characteristics of the bcc U-M alloys, formed with a number of transition metals M. The superconducting transition temperature T_c of bcc U–Mo alloys can reach ≈2 K [4,5]. Recently, the low-temperature conductivity of the U–Mo alloys was studied on samples obtained by ultrafast cooling (splat-cooling) [6,7]. This technique allowed stabilization of the pure, but weakly distorted γ⁰ phase down to room temperature at Mo concentrations as low as 11–12 at.% while the bcc γ-U phase was observed for 13–15 at.% of Mo. The splat-cooled U–Mo alloys demonstrated the increase of T_c from 1.24 K to 2.1 K as the Mo content increased from 0 to 15 at.%.

Superconductivity of the γ-phase is very robust, with the upper critical field μ₀H_c2 > 5 T, which is much higher than μ₀H_c2 = 0.3 T found for α-U superconductivity [6.7]. Another general feature of γ-U-Mo is an enhancement (in comparison to α-U with 10 mJ/mol K²) of the Sommerfeld coefficient of electronic heat capacity [7]. In the normal state, a striking enhancement of electrical resistivity and gradually developing negative slope (TCR) dρ/dT < 0 was observed and attributed to strong impurity scattering due to random distribution of alloying atoms, giving the potential strongly varying between the U and Mo ions [8]. As the α-U phase can evidently incorporate only very low concentrations of the alloying elements, the γ-phase purity can be actually proved by an absence of any low-temperature ρ(T) decrease.

Although the 5f states of U could in principle give rise to the Kondo effect, at which a strong exchange coupling of conduction electrons to local magnetic moments can provide dρ/dT < 0 due to conduction electrons condensing around local magnetic moments, such phenomena are naturally impossible if the broad 5f band in weak Pauli paramagnetic γ-U phases does not yield local U moments. It is, however, less obvious whether the Kondo effect can be in principle operable in materials where U magnetic moments are formed and ordered at low temperatures. The possibility to prepare hydrides of the U–Mo alloys, which are ferromagnetic due to a pronounced lattice expansion (ΔV/V₀ > 60%), and which preserve the randomness of the Mo distribution, led to the recognition that magnetic fluctuations in the paramagnetic state with the chemical randomness both contribute to the weak localization, which enhances resistivity with decreasing temperature. The weak localization can be disrupted both by electron-phonon scattering at elevated temperatures and/or by external magnetic field reducing the magnetic disorder in the nanocrystalline ferromagnet.

The sputter deposition technique can yield systems far from thermodynamic equilibrium, being even more diverse than in the ultrafast-cooled materials. Moreover, it is in principle possible, by means of reactive sputtering, to prepare hydrides of the U alloys, having the same concentrations of metal atoms. Therefore it is tempting to explore the possibilities of the sputtering technique for the U-M alloys and related hydrides for a broad concentration range of M. Here we present results of the study of crystal structure and physical properties of selected films, discussed from the point of view of occurrence of superconductivity and magnetism. The possibility to probe metastable structures was demonstrated in the successful synthesis of UH₂ with the CaF₂ structure type [9], known so far for most of 4f and 5f elements except uranium, for which UH₃ represents the first stable hydride. Recently there has been also an effort to synthesize epitaxially grown single crystalline γ-U-Mo films [10]. However, no data on the conducting properties of the materials was presented. The structure, electrical, and magnetic properties of U–Mo and U–Mo–H films are here compared with available bulk data. We demonstrate how the structure and properties are affected by the substrate temperature. While the hydride films are ferromagnets, the superconductivity was observed for the U–Mo films.