This work has been supported by the National Science Foundation under grant numbers NSF DMR-09-29966, DMR-09-01907, DMR-1401410, and DMR-1401449, and by the Deutsche Forschungsgemeinschaft under grant number FOR-960. Part of this work has been supported by the National Science Foundation under Grant. No. PHYS-1066293 and the hospitality of the Aspen Center for Physics.

Metallic Quantum Ferromagnets

With only a few exceptions that are well understood, conventional superconductivity does not coexist with long-range magnetic order (for example, ref. 1). Unconventional superconductivity, on the other hand, develops near a phase boundary separating magnetically ordered and magnetically disordered phases. A maximum in the superconducting transition temperature Tc develops where this boundary extrapolates to zero Kelvin, suggesting that fluctuations associated with this magnetic quantum-critical point are essential for unconventional superconductivity. Invariably, though, unconventional superconductivity masks the magnetic phase boundary when T < Tc, preventing proof of a magnetic quantum-critical point. Here we report specific-heat measurements of the pressure-tuned unconventional superconductor CeRhIn5 in which we find a line of quantum–phase transitions induced inside the superconducting state by an applied magnetic field. This quantum-critical line separates a phase of coexisting antiferromagnetism and superconductivity from a purely unconventional superconducting phase, and terminates at a quantum tetracritical point where the magnetic field completely suppresses superconductivity. The T → 0 K magnetic field–pressure phase diagram of CeRhIn5 is well described with a theoretical model developed to explain field-induced magnetism in the high-Tc copper oxides, but in which a clear delineation of quantum–phase boundaries has not been possible. These experiments establish a common relationship among hidden magnetism, quantum criticality and unconventional superconductivity in copper oxides and heavy-electron systems such as CeRhIn5.

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Hidden magnetism and quantum criticality in the heavy fermion superconductor CeRhIn$_{5}$

Effect of chemical disorder on Griffiths phase in weak itinerant ferromagnetic Ni92-xCuxCr8 alloy

In the present work we report low temperature magnetic properties of Ni 92−x Al x Cr 8 alloys. It is found that the Curie temperature and spontaneous magnetization ( M SP) get suppressed around xC ≈ 7. From the analysis of the temperature and field dependent magnetization data we have obtained high value of Rhodes–Wohlfarth ratio, which indicates the weak itinerant ferromagnetic behavior for samples below x C. This is further supported by analysing the temperature variation of M SP in the framework of spin fluctuations, SCR theory. Furthermore, the Ginzburg-Landau formalism-based on spin fluctuation theory revealed that the contribution of spin fluctuations at low temperature increases as we approach x C.

Weak itinerant magnetic behaviour in Al substituted Ni92Cr8 alloys

Contrary to prior calculations, the Ni-rich ordered structures of the Cr-Ni alloy system are found to be antiferromagnetic under semi-local density-functional theory. The optimization of local magnetic moments signiﬁcantly increases the driving force for the formation of CrNi 2 , the only experimentally observed intermetallic phase. This structure’s ab initio magnetism appears well described by a Heisenberg Hamiltonian with longitudinal spin ﬂuctuations; itinerant Cr moments are induced only by the strength of exchange interactions. The role of magnetism at temperature is less clear and several scenarios are considered based on a review of experimental literature, speciﬁcally a failure of the theory, the existence of an overlooked magnetic phase transition, and the coupling of antiferromagnetism to chemical ordering. Implications for related commercial and high-entropy alloys are discussed for each case.

Theoretical antiferromagnetism of ordered face-centered cubic Cr-Ni alloys

Detailed magnetic, electrical, and thermal property measurements have been carried out on Ni100−xCrx binary alloys, mainly to study the effect of Cr. The following points emerge from this study: with the increase in Cr concentration, magnetic moment and Curie temperature linearly decreased and the ferromagnetic order is completely suppressed at the critical concentration (xcr ≈ 12.16 ± 0.03). The Rhodes–Wohlfarth ratio increases as the concentration approaches xcr, suggesting a weak itinerant ferromagnetic character of NiCr compositions (x < xcr). Analysis of low-temperature electrical resistivity and specific heat data suggests that the spin fluctuation’s contribution increases and the Fermi-liquid behavior breaks down as the concentration approaches xcr. For x ∼ xcr, the dc susceptibility χ(T) deviates from the Curie–Weiss law reminiscent to that of the Griffiths phase. The low-temperature magnetic isotherms of Ni–Cr follow power law, M ( H ) = M 0 + d λ H λ, and the non-universal exponent (λ) shows a minimum at xcr ∼ 12. Further, temperature dependence of magnetization studies also support the presence of the quantum Griffiths phase, similar to that reported in the Ni–V alloy system. The temperature dependencies of the electrical resistivity, magnetization, and specific heat follow the theoretical predictions of a quantum critical point within experimental uncertainties.

Non-Fermi liquid behavior and signature of Griffiths phase in Ni–Cr binary alloy

We report a comprehensive study on the magnetic, electrical and thermal properties of Ni100-xTMx(TM= V, Cr, Nb,) alloys around their critical concentration Analysis of field and temperature dependence magnetization data suggests a weak itinerant ferromagnetic behavior inx= 8 and 10 compositions and the ferromagnetic ordering suppresses in the concentration range 10 <x< 12 Further, the temperature dependence of specific heat shows an unusual low temperature variation with an enhanced Sommerfeld coefficient,γ, with a signature of non-Fermi-liquid (NFL) behavior close to critical concentration Further, the enhancement in Kadowaki-Woods ratio suggests it to be a strongly correlated electron system near critical concentration Present analysis of experimental data consistently revealed that the NFL behavior is caused by spin fluctuations near critical concentration The temperature dependencies of the electrical resistivity, the magnetization and linear term of the electronic specific heat appear to follow the theoretical predictions of a quantum phase transition and it is tempting to suggest that the presently studies Ni-rich alloys can be candidates for the observation of Griffith phase

https://iopscience.iop.org/article/10.1088/1361-648X/abe514/pdf

Sonu Vishvakarma

Papers

Non-Fermi liquid behavior and signature of Griffiths phase in Ni–Cr binary alloy

Weak itinerant ferromagnetism and non-Fermi liquid behavior in Ni-TM (TM=Cr, Nb) alloys near critical concentration.

Effect of chemical disorder on Griffiths phase in weak itinerant ferromagnetic Ni92-xCuxCr8 alloy

Weak itinerant magnetic behaviour in Al substituted Ni92Cr8 alloys

Anomalous magnetic behavior in Ni80Cr20 nanoparticles prepared by physical and chemical methods