Properties and Collapse of the Ferromagnetism in UCo_1−xRu_xAl Studied in Single Crystals

Abstract

We have investigated the ferromagnetic (FM) phase which suddenly develops in UCo1-xRuxAl and is isolated by paramagnetic regions on both sides from the parent UCoAl and URuAl. For that purpose we have grown high quality single crystals with x = 0.62, 0.70, 0.74, 0.75 and 0.78. The properties of the FM phase have been investigated by microscopic and macroscopic methods. Polarized neutron diffraction on a single crystal with x = 0.62 revealed the gradual growth of the hybridization between U and T-site in the U -T plane with increasing x. Hybridization works here as a mediator of the strong indirect interaction, while the delocalized character of the 5 f states is still conserved. As a result very weak spontaneous magnetic moments are observed for all alloys with magnitude nearly in proportion to the TC for x < 0.62, while an enormous disproportion exists between them near xcrit.. The magnetization, specific heat, electrical resistivity, and Hall effect measurements confirmed that the FM transition is suppressed continuously at the critical concentration xcrit.  0.77. Two quantum critical points are then expected on both sides of the FM dome. We propose a scenario that the order of the FM/PM transition differs at opposite boundaries of the FM dome. We conclude that both criticalities are influenced by disorder. Criticality on the UCoAl side has the character of a clean FM metal, while on the Ru rich side it has the character of a magnetically inhomogeneous system involving a Griffiths phase. IntroductionThe physics of uranium 5f electron systems is a subject of continuing interest because many uranium compounds reveal strong electron correlations leading to phenomena like non-Fermi Liquid (NFL) behavior or unconventional superconductivity.In the UTX series recent interest focuses on two ground state paramagnets (PM) UCoAl 1-6) and URuAl 7) with features differing significantly from ordinary PMs. UCoAl has attracted because of the very low critical field of the metamagnetic transition (Bc  0.6 T), which occurs only in fields parallel to the c axis 8) . This Ising-type anisotropy arises from ferromagnetic fluctuations which lack any transversal component 9, 10) . The paramagnetic ground state of URuAl is also unusual. Magnetic susceptibility is characterized by a broad maximum around 50 K and band structure calculations suggest an anomalous cancelation of the U orbital -and spin -momentum finally responsible for the lack of magnetic ordering 7) . Despite PM ground state, magnetocrystalline anisotropy is conserved in URuAl in contrast to the true Pauli PM UFeAl, which is magnetically isotropic 11) .A spectacular feature of the UCo1-xRuxAl 12) solid solutions, the subject of our research, is that they do not reflect the magnetic ground states of the parent compounds. Instead, a robust FM phase emerges isolated on both sides by a PM phase. Already a very small concentration of Ru (x  0.01) in UCoAl suddenly gives rise to stable FM with an enormous TC jump from zero up to almost 20 K 13,14) . Further substitution by R...

Colossal anomalous Nernst effect in a correlated noncentrosymmetric kagome ferromagnet

“…Magnetic x-ray and photoemission measurements also certify the presence of electronic correlations (25,26). Furthermore, previous work has shown substantial anomalous Hall conductivity (AHC) for dopings close to the quantum-phase transition (21). Here, we examine x = 0.2 with a critical temperature (T c) of 56 K, close to the optimal value in the series.…”

“…In our exploration of materials we found that the layered distorted kagome lattice system UCo 1−x Ru x Al in the ZrNiAl structure type (shown in Fig. 1A) satisfies all the above crite- (20,21). Members of the entire alloy series show relatively large Sommerfeld coefficients γ of ∼50 mJ/mol K 2 (21-24), indicating that charge carriers are moderately heavy.…”

“…Both parent compounds UCoAl and URuAl are paramagnetic, but much of their solid solution UCo 1−x Ru x Al is ferromagnetic (20,21). Members of the entire alloy series show relatively large Sommerfeld coefficients  of ∼50 mJ/mol K 2 (21)(22)(23)(24), indicating that charge carriers are moderately heavy. Magnetic x-ray and photoemission measurements also certify the presence of electronic correlations (25,26).…”

“…Hall conductivity (AHC) for dopings close to the quantum-phase transition (21). Here we examine x=0.2 with a T C of 56 K, close to the optimal value in the series.…”

Element-specific observation of the ferromagnetic ordering process in UCoAl via soft x-ray magnetic circular dichroism

“…It is considered here that the spin-fluctuation behavior should be derived dominantly from the U 5f electrons as a plausible interpretation. However, the magnetic stability of UCoAl is very sensitive to the elemental substitution for Co and Al atoms [17][18][19][20][21]. For example, in the case of the UCo 1−x Ru x Al system [19,21], the ferromagnetism already appears for very low Ru content with x = 0.01 and persists up to x ∼ 0.75, although both of the parent compounds, UCoAl and URuAl, are in a PM ground state.…”