Alloying driven transition between ferro- and antiferromagnetism in UTGe compounds: the UCo1-xIrxGe case (original) (raw)
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Physical review, 2022
The evolution of magnetic properties of isostructural and isoelectronic solid solutions of the superconducting itinerant 5 f-electron ferromagnet (FM) UCoGe with antiferromagnet (AFM) UIrGe was studied by magnetization, AC susceptibility, specific heat, and electrical resistivity measurements of a series of UCo 1−x Ir x Ge compounds in polycrystalline and single-crystalline form at various temperatures and magnetic fields. Both the weak FM and superconductivity in UCoGe were found to have vanished already for very low Ir substitution for Co (x = 0.02). The AFM of UIrGe is gradually suppressed. This is documented by a rapid decrease in both Néel temperature and the critical field of the metamagnetic transition with decreasing Ir concentration, which both tend to vanish just above x = 0.8. The section of the T-x phase diagram in the range 0.02 x 0.8 is dominated by a correlated paramagnetic phase (CPM) exhibiting very broad bumps in temperature dependencies of b-axis magnetization and specific heat developing with increasing x. For x 0.24, wide peaks appear in the c-axis thermomagnetic curves due to AFM correlations which may eventually lead to frozen incoherent spin configurations at low temperatures. The CPM is also accompanied by specific electrical resistivity anomalies. The T-x phase diagram determined for the UCo 1−x Ir x Ge compounds contrasts with the behavior of the related URh 1−x Ir x Ge system, which was reported to exhibit an extended concentration range of stable FM in Rh-rich compounds and a discontinuous transformation between the FM and AFM phases at a critical Rh-Ir concentration. The striking difference is tentatively attributed to (a) the instability of tiny U moment in the weak itinerant FM UCoGe induced by substitutional disorder at already very low Ir doping, (b) the rather stable U moment in URhGe formed by much less delocalized 5 f electrons assisted by weakly varying lattice parameters of URh 1−x Ir x Ge compounds.
Intriguing behavior of UCo1−xRhxGe ferromagnets in magnetic field along the b axis
Physical Review B
Single crystals of solid solutions of ferromagnetic superconductors UCoGe and URhGe were investigated by magnetization and specific-heat measurements. These compounds behave as anisotropic ferromagnets with the easy-magnetization direction along the orthorhombic c axis. The maximum Curie temperature T C = 20 K has been observed in UCo 1−x Rh x Ge for x = 0.4. The main interest of the study was the intriguing behavior in magnetic fields applied along the b axis. The temperature dependence of the b-axis magnetic susceptibility exhibits a maximum at a characteristic temperature T max. For low Rh concentrations (x < 0.4) T max > T C and the magnetic entropy S mag at T C is very low, whereas T max = T C and the S mag at T C is much higher for x 0.4. A metamagneticlike anomaly in magnetization is observed in all compounds, at a critical field which appears on a similar energy scale with the value of the corresponding T max. The character of three paramagnetic regimes which can be distinguished in the T-x and H-x phase diagrams, normal, correlated, and field polarized, is discussed in relation to the field-induced reentrant superconductivity in the parent compounds.
Ferromagnetism and Superconductivity in Uranium Compounds
Journal of the Physical Society of Japan, 2012
Recent advances on ferromagnetic superconductors, UGe2, URhGe and UCoGe are presented. The superconductivity (SC) peacefully coexists with the ferromagnetism (FM), forming the spin-triplet state of Cooper pairs. The striking new phenomena, such as SC reinforced by the magnetic field, are associated with Ising-type ferromagnetic fluctuations. A variety of ferromagnetic ordered moments between UGe2, URhGe and UCoGe affords to understand the relation between FM, tricriticality and SC.
Superconductivity on the border of itinerant electron ferromagnetism in UGe2
Journal of Magnetism and Magnetic Materials, 2001
diagram has for many years cast doubt on the validity of conventional models of magnetically mediated superconductivity. On closer examination, however, very few systems have been studied in the extreme conditions of purity, proximity to the ferromagnetic state and very low temperatures required to test the theory de®nitively. Here we report the observation of superconductivity on the border of ferromagnetism in a pure system, UGe 2 , which is known to be qualitatively similar to the classic d-electron ferromagnets. The superconductivity that we observe below 1 K, in a limited pressure range on the border of ferromagnetism, seems to arise from the same electrons that produce band magnetism. In this case, superconductivity is most naturally understood in terms of magnetic as opposed to lattice interactions, and by a spin-triplet rather than the spin-singlet pairing normally associated with nearly antiferromagnetic metals. ³ Present address: MPI Chemische Physik fester Stoffe,
Evolution of Magnetism in UCo 1− x Ru x Ge
Proceedings of the International Conference on Strongly Correlated Electron Systems (SCES2013), 2014
UCoGe is an archetypal 5f-electron ferromagnet (T C ~ 3K) which becomes superconducting at ambient pressure (T SC ~ 0.6K). The low values of T C and the spontaneous magnetic moment of 0.03μ B /f.u. indicate nearness of a ferromagnetic instability. We have prepared a series of UCo 1-x Ru x Ge polycrystalline samples and studied development of the magnetism and superconductivity by measuring the magnetization, specific heat and resistivity as functions of temperature and magnetic field. We have observed that the Ru doping leads to a rapid increase of the T C up to 8.5K for x = 0.1 and simultaneous suppression of superconductivity. Further doping reduces T C towards a ferromagnetic quantum critical point at x ≈ 0.3. The expected quantum criticality is reflected in specific scaling in the temperature dependence of magnetization, specific heat and electrical resistivity. We have also grown an UCo 0.88 Ru 0.12 Ge single crystal. It exhibits strong magnetocrystalline anisotropy similar to UCoGe but the spontaneous magnetization in the easy magnetization direction is considerably higher. To shed more light on the microscopic background of these findings we have performed a polarized neutron diffraction experiment at D3 in ILL. In contrast to the antiparallel orientation of the Co and U moments in UCoGe we have observed parallel orientation in UCo 0.88 Ru 0.12 Ge. This result rises a question whether the magnetic state of Co plays a momentous role for the presence of superconductivity in the U(Co,Ru)Ge compounds.
Field-induced Phenomena in Ferromagnetic Superconductors UCoGe and URhGe
2012
We review our recent studies on ferromagnetic superconductors, UGe 2 , URhGe and UCoGe, where the spin-triplet state with the so-called equal spin pairing is realized. We focus on experimental results of URhGe and UCoGe in which the superconductivity occurs already at ambient pressure. The huge upper critical field H c2 on UCoGe for the field along the hard magnetization axis (b-axis) is confirmed by the AC susceptibility measurements by the fine tuning of field angle. Contrary to the huge H c2 along the hard-magnetization axis, H c2 along the easy-magnetization axis (c-axis) is relatively small in value. However, the initial slope of H c2 , namely dH c2 /dT (H → 0) both in UCoGe and in URhGe indicates the large value, which can be explained by the magnetic domain effect detected in the magnetization measurements. The specific heat measurements using a high quality single crystal of UCoGe demonstrate the bulk superconductivity, which is extended under magnetic field for the field along c-axis.
Simultaneous suppression of ferromagnetism and superconductivity in UCoGe by Si substitution
Physical Review B, 2008
We investigate the effect of substituting Si for Ge in the ferromagnetic superconductor UCoGe. Dc-magnetization, ac-susceptibility and electrical resistivity measurements on polycrystalline UCoGe1−xSix samples show that ferromagnetic order and superconductivity are progressively depressed with increasing Si content and simultaneously vanish at a critical concentration xcr ≃ 0.12. The non-Fermi liquid temperature variation in the electrical resistivity near xcr and the smooth depression of the ordered moment point to a continuous ferromagnetic quantum phase transition. Superconductivity is confined to the ferromagnetic phase, which provides further evidence for magnetically mediated superconductivity.
Superconductivity on the Border of Weak Itinerant Ferromagnetism in UCoGe
Physical Review Letters, 2007
We report the coexistence of ferromagnetic order and superconductivity in UCoGe at ambient pressure. Magnetization measurements show that UCoGe is a weak ferromagnet with a Curie temperature TC= 3 K and a small ordered moment m0= 0.03 µB. Superconductivity is observed with a resistive transition temperature Ts = 0.8 K for the best sample. Thermal-expansion and specific-heat measurements provide solid evidence for bulk magnetism and superconductivity. The proximity to a ferromagnetic instability, the defect sensitivity of Ts, and the absence of Pauli limiting, suggest triplet superconductivity mediated by critical ferromagnetic fluctuations.
The Superconducting Ferromagnet UCoGe
Journal of Low Temperature Physics, 2010
The correlated metal UCoGe is a weak itinerant ferromagnet with a Curie temperature T C = 3 K and a superconductor with a transition temperature T s = 0.6 K. We review its basic thermal, magnetic on the macro and microscopic scale and transport properties, as well as the response to high pressure. The data unambiguously show that superconductivity and ferromagnetism coexist below T s = 0.6 K and are carried by the same 5f electrons. We present evidence that UCoGe is a p-wave superconductor and argue that superconductivity is mediated by critical ferromagnetic spin fluctuations.
Magnetism and unconventional superconductivity in isostructural cerium and plutonium compounds
Journal of Magnetism and Magnetic Materials, 2007
The heavy-fermion antiferromagnet CeRhIn 5 exhibits a complex interplay between magnetism and unconventional superconductivity (SC) as a function of applied pressure and magnetic field. This interplay leads to a line of magnetic quantum-critical points within the superconducting state. A comparison of nuclear spin-relaxation measurements on CeRhIn 5 to those made on CeCoIn 5 , PuCoGa 5, and PuRhGa 5 suggests that SC and magnetism also may be related closely in these isostructural superconductors.