| AVS 59th Annual International Symposium and Exhibition | |
| Actinides and Rare Earths Focus Topic | Monday Sessions |
| Session AC+TF+SS+MI-MoA |
| Session: | Actinides and Rare Earths: Thin Films and Surface Science |
| Presenter: | L. Havela, Charles University, Czech Republic |
| Authors: | L. Havela, Charles University, Czech Republic A. Gonçalves, ITN Sacavém, Portugal J.-C. Waerenbogh, ITN Sacavém, Portugal L. Pereira, ITN Sacavém, Portugal I. Tkach, Charles University, Czech Republic N.-T. Kim-Ngan, Pedagogical University Cracow, Poland T.B. Scott, University of Bristol, UK |
| Correspondent: | Click to Email |
The splat cooling technique is one of the methods of ultrafast cooling of a melt, particularly suitable for small amounts of material. In particular cases it can help to overcome constraints imposed by thermodynamics. One of them was the issue of magnetic properties of non-stoichiometric Laves phase UFe2. This compound has a ferromagnetic ground state (with both U and Fe magnetic moments). An excess of U, achieved by quenching, led to the decrease of TC from 162 K for pure compound to 112 K for UFe1.7 [1]. Quenching was, however, unable to provide Fe-rich material, with expected increase of TC. Using splat cooling of a series of materials with various off-stoichiometry from UFe2 to UFe6 we found that the cubic Laves phase structure (with sum-micron grains) can absorb excessive Fe up to the stoichiometry UFe2.3. Additional excess leds to the segregation of α-Fe [2]. The increase of TC up to 230-240 K was the impact on magnetic properties. The spontaneous magnetization also increases from 1.0 μB/f.u. in UFe2 to 1.9 μB/f.u. in UFe2.3. 57Fe Mössbauer spectroscopy reveals that the excessive Fe atoms enter the U sublattice and develop higher magnetic moments (approx. 1.0 μB/Fe).
Applying the splat cooling technique on pure and doped U metal had the aim to stabilize the high temperature bcc phase (γ-U) to low temperatures, to be able to establish its basic electronic properties. In particular, changes in magnetic characteristics and electronic specific heat can be expected due to the modest volume expansion comparing to orthorhombic α-U. We found that splat cooling reduces the necessary concentration of dopants and U with 12 at.% Mo has no traces of α-U. The Sommerfeld coefficient γ = 19 mJ/mol K2 estimated for pure γ-U is enhanced comparing to 11 mJ/mol K2 for pure U splat, which is close to values given in literature [3] for the U metal. The splats exhibit a superconducting ground state with Tc ranging from 1.24 K for pure U to 2.11 K for 15 % Mo. The γ-U superconductivity is characterized by a large critical field exceeding 6 T and a sharp λ-type anomaly in specific heat Cp(T) with the size corresponding to the BCS theory. The superconductivity of a pure U splat, which contains only traces of γ-U, has much lower critical field (0.33 T) and the weak anomaly in Cp(T) does not convince about the bulk character of superconductivity.
This work was supported by Grant Agency of the Czech Republic under the grant No. P204/10/0330.[1] A.T. Aldred, J.Magn.Magn.Mater. 10, 42 (1979).
[2] L. Havela et al., Intermetallics 19, 113 (2011).
[3] J.C. Lashley et al., Phys. Rev. B 63, 224510 (2001).