Origin of negative resistivity slope in U-based ferromagnets L. Havela * , M. Paukov, V. Buturlim, I. Tkach, S. Maskova, M. Dopita Charles University, Faculty of Mathematics and Physics, Ke Karlovu 3, Prague 2, Czech Republic ARTICLE INFO Keywords: Uranium Hydride Resistivity Nanocrystalline Magnetoresistance ABSTRACT Ultra-nanocrystalline UH 3 -based ferromagnets with T C  200 K exhibit a flat temperature dependence of electrical resistivity with a negative slope both in the ferromagnetic and paramagnetic range. The ordered state with randomness on atomic scale, equivalent to a non-collinear ferromagnetism, can be affected by magnetic field, supressing the static magnetic disorder, which reduces the resistivity and removes the negative slope. It is deduced that the dynamic magnetic disorder in the paramagnetic state can be conceived as continuation of the static disorder in the ordered state. The experiments, performed for (UH 3 ) 0.78 Mo 0.12 Ti 0.10 , demonstrate that the negative resistivity slope, observed for numerous U-based intermetallics in the paramagnetic state, can be due to the strong disorder effect on resistivity. The resulting weak localization, as a quantum interference effect which increases resistivity, is gradually suppressed by enhanced temperature, contributing by electron-phonon scat- tering, inelastic in nature and removing the quantum coherence. 1. Introduction The effect of negative resistivity slope, i.e. electrical resistivity decreasing with increasing temperature, is at metallic systems tradi- tionally attributed, at least within the SCES community, to the Kondo effect, occurring typically at anomalous f-elements, with Ce as the most transparent example. Strongly correlated electron systems can indeed exhibit the Kondo effect, yielding, besides fluctuating f-moments, a gradual confinement of conduction electrons around f-sites, which amounts to a logarithmic increase of resistivity with decreasing tem- perature T. The Kondo regime is located between the stronger f-insta- bility labelled as valence fluctuations, in which the spin fluctuations are driven by charge fluctuations, and magnetically ordered regime, with spin fluctuations suppressed due to the possibility to interact with neighbours via the RKKY interaction. Although this conventional Kondo or Kondo lattice picture was extended to multichannel Kondo [1], quadrupolar Kondo [2], or underscreened Kondo [3] models, their applicability to actinide systems has never been generally accepted. In the case of antiferromagnets, the negative resistivity slope could be due to a superzone boundary effect, which is induced by additional magnetic periodicity multiplying the basic crystallographic unit cell, gapping possibly the Fermi surface. This concept was suggested for Cr by Slater [4] and applied for rare-earths by Mackintosh [5] and Elliott and Wedgwood [6]. Surveying electrical resistivities ρ(T) of U and other light-actinide compounds, we cannot leave unnoticed that relatively large fraction of them exhibits the negative resistivity slope. In particular it applies for narrow 5f-band compounds, the resistivities of which reach or even substantially exceed 200 μΩcm, assumed as an upper limit for con- ventional metallic systems by Mott (known also as Mott-Ioffe-Regel limit) [7]. There are numerous such binary and ternary compounds, as USb 2 [8],U 3 Au 3 Sn 4 [9], UCu 2 Sn, UAu 2 Al, UPt 2 Sn [10], or URuGa [11]. In some cases, as antiferromagnets, the reason for the negative slope could be in the superzone boundary effect, or possibly due to the Kondo effect. There are, however, cases where the two mechanisms are hardly appli- cable. For example, UGa 2 has a ferromagnetic ground state, so there are no superzones, and large magnetic moment of 3.0 μ B /U leaves no space for spin compensation due to the Kondo effect [12,13]. Nevertheless, as the Kondo effect remains almost a synonym for negative resistivity slope, there has been persisting discussion about its applicability even in such case. One of reasons is that the Kondo prediction of ρ proportional to –lnT is rather non-specific, and such term can be used successfully to fit over certain T range almost any decreasing function with a saturating ten- dency. As the negative slope for UGa 2 occurs in the paramagnetic state and the ferromagnetic state has a regular drop of ρ(T) below the Curie temperature T C to very low values testifying a good quality of single-crystal sample, we have to ask what is the effect of fluctuating 5f moments on electronic transport. Main features distinguishing uranium and other light actinides from all other types of magnetic materials is the very strong spin-orbit * Corresponding author. E-mail address: havela@mag.mff.cuni.cz (L. Havela). Contents lists available at ScienceDirect Physica B: Physics of Condensed Matter journal homepage: www.elsevier.com/locate/physb https://doi.org/10.1016/j.physb.2017.10.084 Received 18 June 2017; Received in revised form 20 September 2017; Accepted 18 October 2017 Available online xxxx 0921-4526/© 2017 Elsevier B.V. All rights reserved. Physica B: Physics of Condensed Matter xxx (2017) 1–5 Please cite this article in press as: L. Havela, et al., Origin of negative resistivity slope in U-based ferromagnets, Physica B: Physics of Condensed Matter (2017), https://doi.org/10.1016/j.physb.2017.10.084