The suppression of hidden order and the onset of ferromagnetism in URu₂Si₂via Re substitution

Abstract

Abstract. Substitution of Re for Ru in the heavy fermion compound URu 2 Si 2 suppresses the hidden order transition and gives rise to ferromagnetism at higher concentrations. The hidden order transition of URu 2−x Re x Si 2 , tracked via specific heat and electrical resistivity measurements, decreases in temperature and broadens, and is no longer observed for x > 0.1. A critical scaling analysis of the bulk magnetization indicates that the ferromagnetic ordering temperature and ordered moment are suppressed continuously towards zero at a critical concentration of x ≈ 0.15, accompanied by the additional suppression of the critical exponents γ and δ − 1 towards zero. This unusual trend appears to reflect the underlying interplay between Kondo and ferromagnetic interactions, and perhaps the proximity of the hidden order phase.PACS numbers: 71.27.+a,75.40.Cx,75.60.Ej ‡ Present address: University of Maryland, College Park, MD 20742 2 For well over two decades, the identity of the ordered phase found in URu 2 Si 2 at temperatures below 17 K has eluded researchers. This hidden order (HO) phase coexists with a heavy fermion state, but yields to a superconducting ground state below 1.5 K. In order to develop a better understanding of the HO phase, URu 2 Si 2 has been extensively studied by various techniques, and these efforts have uncovered fascinating behavior under magnetic field, applied pressure, and chemical substitution. We describe the novel phase changes induced by Re substitution for Ru, including the suppression of the HO transition, the nearby emergence of ferromagnetic (FM) order, and the unique critical behavior of this phase.Chemical substitution studies have been used primarily to investigate the robustness of the HO phase, and substitutions have been made for all three elements in URu 2 Si 2 . Replacing U with Th or rare earths quickly suppresses the HO transition temperature [1,2], emphasizing the critical role of the U f -electrons to the HO phase. In contrast, substitution for Si has no dramatic effects [3]. Most of the neighboring elements of Ru: Mn, Tc, Th, Re, Os, Rh, and Ir have been substituted onto the Ru site, and in all cases except Os, the HO transition is suppressed by about 5% substituent concentration [4,5,6].For a few elements, the phase diagrams at higher concentrations have been studied. In the Rh case, in addition to the HO phase, there are two other nominally antiferromagnetic (AFM) phases, which are separated by nonmagnetic regions [7]. In contrast to the HO, these AFM phases are characterized by a larger moment [8,9], and a complex magnetic structure exists at intermediate Rh concentrations [10]. A closer examination of the HO phase found that for almost half of the range of Rh concentration over which the HO phase exists, the order is actually AFM [11].Substituting into URu 2 Si 2 with Re, Tc, or Mn leads instead to ferromagnetic (FM) order [6], whose discovery constituted the first experimental realization of a heavy fermion FM instability. In URu 2−x Re x Si 2 , a maximum T C ≈ ...

Symmetry and correlations underlying hidden order inURu2Si2

“…First, Q = 0 phases that are not ferromagnets are generally difficult to detect experimentally. Second, there are incipient Q = 0 correlations in URu 2 Si 2 that maintain strongly correlated behavior [102][103][104] , as evidenced by proximity of the HO phase to a chemically-tuned ferromagnetic (FM) instability 92,[105][106][107] and the presence of a minority-volume FM phase in some samples 108 . Third, a Q = 0 order parameter has no associated change in electronic structure.…”

“…12b (magenta) is calculated. The measured specific heat data are taken from Reference 92 . Figure 12c shows the subtracted specific heat δC, divided by temperature, which is the electronic/magnetic contribution that accounts for the entropy change due to the HO transition.…”

Single Crystal Growth of URu2Si2 by the Modified Bridgman Technique

“…This is an important advance in crystal growth for these materials, because it provides an alternative way to produce single crystals that are needed for a variety of experiments: e.g., not only is it possible to cleave them, but their quality is also high enough to observe quantum oscillations, as seen in other specimens of similar quality [16,17]. Furthermore, previous studies of URu 2 Si 2 have benefitted from investigation of chemical substitution series [16,[18][19][20][21][22][23]. Most such work has focused on the substitution of elements with low vapor pressures that are easily dissolved in a stoichiometric melt.…”

Non-Fermi Liquid Regimes and Superconductivity in the Low Temperature Phase Diagrams of Strongly Correlated d- and f-Electron Materials

“…The most extensively studied examples have been antiferromagnetism in the case of Rh substitution [113] and ferromagnetism in the case of Re substitution [114]. Details of the phase diagrams of these two systems were recently compared [115]. In fact URu 2−x Re x Si 2 , studied extensively in our laboratory, is the first known example of a ferromagnetic heavy fermion compound, with a Fig.…”