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Register a new userEvolution of spin relaxation processes in LiY$_{1-x}$Ho$_x$F$_4$ with increasing x studied via AC-susceptibility and muon spin relaxation
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We present measurements of magnetic field and frequency dependences of the low temperature (T = 1.8 K) AC-susceptibility, and temperature and field dependences of the longitudinal field positive muon spin relaxation ({mu}SR) for LiY$_{1-x}$Ho$_x$F$_4$ with x = 0.0017, 0.0085, 0.0408, and 0.0855. The fits of numerical simulations to the susceptibility data for the x = 0.0017, 0.0085 and 0.0408 show that Ho-Ho cross-relaxation processes become more important at higher concentrations, signaling the crossover from single-ion to correlated behavior. We simulate the muon spin depolarization using the parameters extracted from the susceptibility, and the simulations agree well with our data for samples with x = 0.0017 and 0.0085. The {mu}SR data for samples with x = 0.0408 and 0.0855 at low temperatures (T < 10 K) cannot be described within a single-ion picture of magnetic field fluctuations and give evidence for additional mechanisms of depolarization due to Ho$^{3+}$ correlations. We also observe an unusual peak in the magnetic field dependence of the muon relaxation rate in the temperature interval 10 - 20 K that we ascribe to a modification of the Ho$^{3+}$ fluctuation rate due to a field induced shift of the energy gap between the ground and the first excited doublet crystal field states relative to a peak in the phonon density of states centered near 63 cm$^{-1}$.
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We present zero-field {mu}SR measurements for LiY$_{1-x}$Ho$_{x}$F$_{4}$ samples with x = 0.0017, 0.0085, 0.0406, and 0.0855. We characterize the dynamics associated with the formation of the (F-{mu}-F)$^{-1}$ complex by comparing our data to Monte Carlo simulations to determine the concentration range over which the spin dynamics are determined primarily by the Ho$^{3+}$-{mu} interaction rather than the F-{mu} interaction. Simulations show that F-{mu}-F oscillations should evolve into a Lorentzian Kubo-Toyabe decay for an increasing static magnetic field distribution {Gamma} (i.e., increasing x), but the data do not show this behavior, consistent with the recently reported existence of strong magnetic fluctuations in this system at low temperatures. Anisotropy in the field distribution is shown to cause small errors of order 10% from behavior predicted for an isotropic distribution. Finally, numerical calculations show that values of {Gamma} calculated in the single ion limit greatly exceed the values extracted from curve fits, suggesting that strong correlations play an important role in this system.
Positive-muon ($mu^+$) Knight shifts have been measured in the paramagnetic states of Pr$_{1-x}$Nd$_x$Os$_4$Sb$_{12}$ alloys, where $x =$ 0, 0.25, 0.45, 0.50, 0.55, 0.75, and 1.00. In Pr-substituted NdOs$_4$Sb$_{12}$ ($x le$ 0.75), but not in NdOs$_4$Sb$_{12}$, Clogston-Jaccarino plots of $mu^+$ Knight shift~$K$ versus magnetic susceptibility~$chi$ exhibit an anomalous saturation of $K(chi)$ at $sim-$0.5% for large susceptibilities (low temperatures), indicating a reduction of the coupling strength between $mu^+$ spins and $4f$ paramagnetism for temperatures $lesssim$ 15~K. We speculate that itinerant Pr$^{3+}$ quadrupolar excitations, invoked to mediate the superconducting Cooper-pair interaction, might modify the $mu^+$-$4f$ ion indirect spin-spin interaction.
Muon spin rotation and relaxation ($mu$SR) experiments have been carried out to characterize magnetic and superconducting ground states in the Pr$_{1-x}$Nd$_x$Os$_4$Sb$_{12}$ alloy series. In the ferromagnetic end compound NdOs$_4$Sb$_{12}$ the spontaneous local field at positive-muon ($mu^+$) sites below the ordering temperature $T_C$ is greater than expected from dipolar coupling to ferromagnetically aligned Nd$^{3+}$ moments, indicating an additional indirect RKKY-like transferred hyperfine mechanism. For 0.45 $le x le$ 0.75, $mu^+$ spin relaxation rates in zero and weak longitudinal applied fields indicate that static fields at $mu^+$ sites below $T_C$ are reduced and strongly disordered. We argue this is unlikely to be due to reduction of Nd$^{3+}$ moments, and speculate that the Nd$^{3+}$-$mu^+$ interaction is suppressed and disordered by Pr doping. In an $x$ = 0.25 sample, which is superconducting below $T_c$ = 1.3 K, there is no sign of spin freezing (static Nd$^{3+}$ magnetism), ordered or disordered, down to 25 mK. Dynamic $mu^+$ spin relaxation is strong, indicating significant Nd-moment fluctuations. The $mu^+$ diamagnetic frequency shift and spin relaxation in the superconducting vortex-lattice phase decrease slowly below $T_c$, suggesting pair breaking and/or possible modification of Fermi-liquid renormalization by Nd spin fluctuations. For 0.25 $le x le$ 0.75, the $mu$SR data provide evidence against phase separation; superconductivity and Nd$^{3+}$ magnetism coexist on the atomic scale.
The quantum-spin S = 1=2 chain system Cs$_2$CuCl$_4$ is of high interest due to competing anti-ferromagnetic intra-chain J and inter-chain exchange J interactions and represents a paramount example for Bose-Einstein condensation of magnons [R. Coldea et al., Phys. Rev. Lett. 88, 137202 (2002)]. Substitution of chlorine by bromine allows tuning the competing exchange interactions and corresponding magnetic frustration. Here we report on electron spin resonance (ESR) in single crystals of Cs$_2$CuCl$_{4-x}$Br$_x$ with the aim to analyze the evolution of anisotropic exchange contributions. The main source of the ESR linewidth is attributed to the uniform Dzyaloshinskii-Moriya interaction. The vector components of the Dzyaloshinskii-Moriya interaction are determined from the angular dependence of the ESR spectra using a high-temperature approximation. The obtained results support the site selectivity of the Br substitution suggested from the evolution of lattice parameters and magnetic susceptibility dependent on the Br concentration.
We present the results of muon-spin relaxation (muSR) measurements on the hexagonal manganite HoMnO3. Features in the temperature-dependent relaxation rate, lambda, correlate with the magnetic transitions at 76 K, 38 K and 34 K. The highest temperature transition, associated with the ordering of Mn3+ moments has the largest effect on lambda. The application of a static electric field of E=10^4 Vm^-1 below T=50 K causes a small reduction in lambda which is suggestive of coupling between ferroelectric and magnetic domain walls in the ordered state of the material.
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