Magnetism in Hydrides of -Uranium Alloys

Magnetism in hydrides of 훾-Uranium alloys.

M. Paukov, I. Tkach, Z. Matěj, L. Havela

Department of Condensed Matter Physics, Faculty of Mathematics and Physics, Charles University, Ke Karlovu 5, 121 16, Prague 2, Czech Republic e-mail: [email protected]

The bcc form of Uranium metal, its alloys, and their hydrides have been used in various applications, such as low-enriched nuclear fuel. The 훾-Uranium metal is stable at elevated temperatures. The only possibility to retain this phase to low temperatures is to combine doping (by Mo, Zr, Ti, Nb, Re, Ru, Pd, or Pt) with quenching. It opens the possibility to study the ground state properties in this form of U. We have been testing the possibility to reduce the necessary amount of the dopant using ultrafast cooling [1]. As next, we decided to test the interaction with hydrogen. The common α-U phase is known to react readily with H at low pressures already, producing a fine pyrophoric powder of β- UH3, which is ferromagnetic with the Curie temperature around 170 K. Experiments revealed that the Mo stabilized 훾-U absorbs H only at elevated pressure. The product is surprisingly not powder. We obtained brittle but compact pieces of hydrides denoted as UH3Mox with practically amorphous crystal structure, most likely related to β-UH3 [2]. Varying the Mo concentration, we found that the Curie temperatures increase up to 200 K. The Curie temperature has a maximum for UH3Mo0.18 (see Fig. 1a).

a) b) 180

175

170 (K) (K) C T 165

160

155 0.1 0.2 0.3 0.4 x, Zr concentration

Fig. 1. The dependence of Curie temperature on a) Mo, b) Zr concentration.

The magnetic moment per U atom increases, as well [2]. The saturation magnetization of the U-Mo hydrides is higher than that in β-UH3 (see Fig. 2a). The type of hysteresis loops at T = 2 K can be associated with pinning of domain walls in highly disordered systems with high anisotropy (see Fig. 2b). There exists extended evidence of similar behavior among such materials, called as High Anisotropy Random Distribution, for example on the basis of SmCo5 or amorphous TbFe2. With increasing concentration of Mo the coercive force increases extremely up to 5 Tesla. For comparison with U-Mo, we undertook the same type of study with bcc U doped by Zr. The U-Zr alloys absorb hydrogen (pressures above 5 bar H2 are necessary) up to the stoichiometry UH3Zrx, forming a crystalline material with the cubic structure corresponding to α- UH3, which represents a bcc U lattice filled with H.

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UH3Mo0.18 a) b) 1.2 1.0

0.5 T = 2 K 60 K

/U) 0.8 / U)

B 20 K B

( 1.8 K, PPMS 0.0 M

1.5 K, pulsed field (

UH , (4.2 K, paper-1998) M 3 6 K 0.4 -0.5

UH Mo 3 0.22 -1.0

0.0 0 20 40 60 -10 -5 0 5 10 H 0H (T) 0 (T)

Fig. 2. a) Approach to saturation in high magnetic fields for UH3Mo0.22 compared with -UH3 b) temperature variations of hysteresis loop for UH3Mo0.18.

We assume that Zr occupies randomly the U lattice sites, as the atomic radius of Zr and U are similar. TC  175 K slightly exceeds that of β-UH3 for low Zr concentrations, but decreases with increasing Zr concentration (see Fig. 1b). U moments remain on the level of 1 μB. The study reveals a surprising fact that magnetic properties of α-UH3 and β-UH3 are similar, despite different atomic spacing. A striking feature of UH3Zrx systems is enormous magnetic coercivity, with the width of hysteresis loop exceeding 11 T at low temperatures (see Fig. 3).

UH3Zr0.43 1.0

0.5 / U)

B 0.0

( ZFC 1.8 K

M 60 K 30 K -0.5 12 K 6 K 3 K 1.8 K -1.0 -10 -5 0 5 10 H 0 (T)

Fig. 3. Hysteresis loops of UH3Zr0.43 measured at various temperatures.

It can be again attributed to the statistical distribution of Zr leading to randomness of easy-magnetization direction. Our results indicate that interestingly large variability can be achieved for the UH3 hydrides. The amorphous phase may allow for diverse other dopants tuning magnetic properties. At present we try to dope U by both Mo and Zr.

[1]. I. Tkach, N.-T. H. Kim-Ngan, S. Mašková, M. Dzevenko, L. Havela, A. Warren, C. Stitt, T. Scott, J.Alloys Comp. 534 (2012) 101-109. [2]. I. Tkach, S. Mašková, Z. Matěj, N.-T. H. Kim-Ngan, A.V. Andreev and L. Havela, Phys. Rev. B 88 (2013) 060407(R).

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