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Register a new userEvidence for magnetic clusters in Ni$_{1-x}$V$_{x}$ close to the quantum critical concentration
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The d-metal alloy Ni$_{1-x}$V$_{x}$ undergoes a quantum phase transition from a ferromagnetic ground state to a paramagnetic ground state as the vanadium concentration $x$ is increased. We present magnetization, ac-susceptibility and muon-spin relaxation data at several vanadium concentrations near the critical concentration $x_c approx11.6%$ at which the onset of ferromagnetic order is suppressed to zero temperature. Below $x_c$, the muon data reveal a broad magnetic field distribution indicative of long-range ordered ferromagnetic state with spatial disorder. We show evidence of magnetic clusters in the ferromagnetic phase and close to the phase boundary in this disordered itinerant system as an important generic ingredient of a disordered quantum phase transition. In contrast, the temperature dependence of the magnetic susceptibility above $x_c$ is best described in terms of a magnetic quantum Griffiths phase with a power-law distribution of fluctuation rates of dynamic magnetic clusters. At the lowest temperatures, the onset of a short-range ordered cluster-glass phase is recognized by an increase in the muon depolarization in transverse fields and maxima in ac-susceptibility.
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Zero (ZF) and longitudinal field (LF) muon spin relaxation data of the {it d}-metal alloy Ni$_{1-x}$V$_{x}$ are presented at several vanadium concentrations $x$ below and above the critical $x_c approx 11$% where long-range ferromagnetic (FM) order is suppressed. The clear single precession frequency observed for Ni, as expected for a homogeneous FM, changes to rather damped oscillation with small V substitution at $x=4$%, confirming magnetic inhomogeneities caused by the less magnetic V environments in the magnetic Ni matrix. Furthermore, local fields and spatial field distributions can be estimated to characterize different inhomogeneous regimes developing with $x$ in the FM phase of Ni$_{1-x}$V$_{x}$. In the regime of $x=7-10$% a Kubo Toyabe function well describes the low temperature ZF and LF asymmetry data supporting a static Gaussian field distribution. Closer to the quantum critical concentration a single scale static Kubo Toyabe function with one field distribution is not sufficient to describe the muon relaxation. These data indicate that further changes in spatial distributions and dynamics are evolving as expected within the critical regime of a disordered quantum critical point.
We report a chemical substitution-induced ferromagnetic quantum critical point in polycrystalline Ni$_{1-x}$Rh$_x$ alloys. Through magnetization and muon spin relaxation measurements, we show that the ferromagnetic ordering temperature is suppressed continuously to zero at $x_{crit} = 0.375$ while the magnetic volume fraction remains 100% up to $x_{crit}$, pointing to a second order transition. Non-Fermi liquid behavior is observed close to $x_{crit}$, where the electronic specific heat $C_{el}/T$ diverges logarithmically, while immediately above $x_{crit}$ the volume thermal expansion coefficient $alpha_{V}/T$ and the Gruneisen ratio $Gamma = alpha_{V}/C_{el}$ both diverge logarithmically in the low temperature limit, further indication of a ferromagnetic quantum critical point in Ni$_{1-x}$Rh$_x$.
Electron-doped Sr(Co{1-x}Ni{x})2As2 single crystals with compositions x = 0 to 0.9 were grown out of self-flux and SrNi2As2 single crystals out of Bi flux. The crystals were characterized using single-crystal x-ray diffraction (XRD), magnetic susceptibility chi(H,T), isothermal magnetization M(H,T), heat capacity Cp(H,T), and electrical resistivity ho(H,T) measurements versus applied magnetic field H and temperature T. The chi(T) data show that the crystals exhibit an antiferromagnetic (AFM) ground state almost immediately upon Ni doping on the Co site. Ab-initio electronic-structure calculations for x = 0 and x = 0.15 indicate that a flat band with a peak in the density of states just above the Fermi energy is responsible for this initial magnetic-ordering behavior on Ni doping. The Curie-Weiss-like T dependence of chi in the paramagnetic (PM) state indicates dominant ferromagnetic (FM) interactions. The small ordered moments ~0.1 muB per transition metal atom and the values of the Rhodes-Wohlfarth ratio indicate that the magnetism is itinerant. The Cp(T) at low T exhibits Fermi-liquid behavior for 0 < x < 0.15 whereas an evolution to a logarithmic non-Fermi-liquid (NFL) behavior is found for x = 0.2 to 0.3. The logarithmic dependence is suppressed in an applied magnetic field. The low- T rho(H = 0,T) data show a T^2 dependence for 0 < x < 0.20 and a power-law dependence with n < 2 for x = 0.20 and 0.30. These low-T NFL behaviors observed in the Cp and rho measurements are most evident near the quantum-critical concentration x ~ 0.3 at which a T = 0 composition-induced transition from the AFM phase to the PM phase occurs.
We have measured de Haas-van Alphen oscillations of Cr$_{1-x}$V$_x$, $0 le x le 0.05$, at high fields for samples on both sides of the quantum critical point at $x_c=0.035$. For all samples we observe only those oscillations associated with a single small hole band with magnetic breakdown orbits of the reconstructed Fermi surface evident for $x<x_c$. The absence of oscillations from Fermi surface sheets most responsible for the spin density wave (SDW) in Cr for $x>x_c$ is further evidence for strong fluctuation scattering of these charge carriers well into the paramagnetic regime. We find no significant mass enhancement of the carriers in the single observed band at any $x$. An anomalous field dependence of the dHvA signal for our $x=0.035$ crystal at particular orientations of the magnetic field is identified as due to magnetic breakdown that we speculate results from a field induced SDW transition at high fields.
We investigate the structural and critical properties of CrTe1-xSbx with 0.0 leq x leq 0.2. The XRD patterns revealed that Sb-substitution resulted in a pure NiAs-hexagonal structure with P63/mmc (194) space-group. Lattice refinement of the structure revealed little changes in the a-lattice parameter, along with a more pronounced reduction in the c-axis. The critical behavior in CrTe1-xSbx has been investigated using the magnetization isotherms near the ferromagnetic transition. The obtained critical exponents ( beta, gamma ,and delta) revealed that all samples (with 0.0 leq x leq 0.2) closely follow a mean field-like behavior with ferromagnetic Curie temperature (Tc) near room temperature. The results from Widom scaling relation indicating self-consistency of the acquired values. Moreover, the magnetization isotherms near the Curie temperature follow a universal scaling behavior, giving further support for the obtained critical exponents.
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