Switching of magnetic ground states across the UIr1−xRhxGe alloy system

Abstract

We investigated the evolution of magnetism in the UIr 1-x Rh x Ge system by the systematic study of high-quality single crystals. Lattice parameters of both parent compounds are very similar resulting in almost identical nearest interatomic uranium distance close to the Hill limit. We established the x-T phase diagram of the UIr 1-x Rh x Ge system and found a discontinuous antiferromagnetic/ferromagnetic boundary at x crit = 0.56 where a local minimum in ordering temperature and maximum of the Sommerfeld coefficient   175 mJ/mol K 2 occurs in the UCoGe-URhGe-UIrGe system, signaling an increase in magnetic fluctuations. However, a quantum critical point is not realized because of the finite ordering temperature at x crit . A magnon gap on the antiferromagnetic side abruptly suppresses magnetic fluctuations. We find a fieldinduced first order transition in the vicinity of the critical magnetic field along the b axis in the entire UIr 1-x Rh x Ge system including the ferromagnetic region UCo 0.6 Rh 0.4 Ge -URhGe. *E-mail address: jiri.pospisil@centrum.cz 2 the b axis becomes the easy magnetization axis, similar to the magnetic behavior of FM URhGe. Here the so-called intermediate b axis is characterized by a magnetic moment re-orientation at a critical magnetic field H R = 12 T which restores the SC state 5, 15 . Recent papers revealed strong tricritical fluctuations in the vicinity of H R 16, 17 accompanied by a Lifshitz-type transition and enhancement of the coefficient  18-20 .We studied the magnetic properties and quantum critical phenomena in the UIr 1-x Rh x Ge system which has an interesting FM/ antiferromagnetic (AFM) boundary at low temperature. This AFM/FM boundary is of interest because the AFM and FM are separated at this point throughout the whole orthorhombic TiNiSi-type UTGe system by the Hill limit 4 . Many studies have been conducted to determine the delicate balance of magnetic interactions in UTGe alloy systems, but they have been primarily on polycrystalline samples where the crucially important magnetocrystalline anisotropy remains hidden 4, 21-26 . Our investigation of single crystal study has allowed us to develop a general picture of the magnetism in the AFM part of the UIr 1-x Rh x Ge system which surprisingly preserves many of the magnetic features of the FM parent compounds URhGe and UCoGe. Our discussion and conclusions are based on a detailed analysis of the crystal structure, magnetization, and heat capacity.

Consecutive magnetic phase diagram of UCoGe-URhGe-UIrGe system

“…The ferromagnetic phase was detected between UCoGe and UIr0.43Rh0.57Ge. UIr0.44Rh0.55Ge -UIrGe region orders antiferromagnetically [22]. TC of the ferromagnetic phases were estimated as an inflection point of the temperature dependent magnetization and as an onset of the -anomaly in heat capacity data ( Fig.…”

“…We have prepared, for the purpose of our research, single crystals of compositions x = 0.1, 0.2, 0.3, 0.4 and 0.8 in UCo1−xRhxGe system and x = 0.42, 0.55, 0.57 and 0.86 in UIr1−xRhxGe system by Czochralski pulling in a tetra arc furnace. UCo1−xRhxGe single crystals were thermally treated by conventional procedure at 900°C [20,21] and UIr1−xRhxGe at 1000°C [22]. An Electron-probe microanalyzer EPMA JXA-8900 (JEOL) has been used for the chemical analysis.…”

High-field moment polarization in the itinerant ferromagnet URhSi

“…In the case of U-based ferromagnets in a field applied along a hard magnetic axis, no microscopic picture has been proposed yet to describe the maximum in the magnetic susceptibility and its relation with metamagnetism. Interestingly, the ferromagnet URhGe is in the vicinity of an antiferromagnetic instability, which can be induced by Ir-doping on the Rh-site [35,36]). Further studies are now needed to identify how the physics, including superconductivity, of the U-based ferromagnets or nearly-ferromagnets is affected by their proximity to ferromagnetic and antiferromagnetic quantum phase transitions.…”

Alloying-driven transition between ferromagnetism and antiferromagnetism in UTGe compounds: UCo1−xIrxGe

“…This confirms the fact that the microscopic origin of the magnetism in these compounds is more complex than determined only by the overlaps of the 5f-orbitals of the nearest-neighbor U ions. The strong role of competing FM and AFM interactions mediated by the 5fligand hybridization determining the magnetic ground state was predicted for these compounds ADDIN EN.CITE [13,14] as well as for the recently discovered nonmagnetic heavy-fermion superconductor UTe 2 ADDIN EN.CITE [15,16].…”

“…It is, of course, of natural interest to see the effect of alloying UIrGe with the two ferromagnetic superconductors. The URh 1-x Ir x Ge case has been studied and results published earlier [14]. The key feature of the system is the discontinuity at x crit = 0.56 in all the magnetic parameters between the ferromagnetic and antiferromagnetic phase typical for the first-order transition.…”