Muon Relaxation Rate Research Articles

The positive muon relaxation rate at low temperature in a Heisenberg ferromagnet

The authors show that the positive muon spin depolarization should be controlled at low temperature by a magnon Raman scattering for a Heisenberg ferromagnet if the degree of symmetry at the muon site is not too high. The data recently reported for the intermetallic ferromagnet GdNi5 support their conclusion.

Anomalous anisotropies and field dependencies of the muon Knight shift and the relaxation rate in monocrystalline Bi

The angular dependence of the muon Knight shift,Kμ, and the muon relaxation rate in Bi at 11 K were measured in external magnetic fields up to 1 T. BothKμ and the second moment,M2, are field dependent and involveP40(cos θ) andP43(cos θ) terms in the angular dependence. The Knight shift behaviour is discussed in terms of the dipole-dipole interaction and the de Haas-van Alphen effect, a consistent interpretation was not achieved in either case. The field dependence ofM2 is in complete contrast to the second moment calculations and points to a field dependent redistribution of the charge distribution around the interstitial site.

Study of La2CuO4 and related compounds by μSR

The spontaneous muon precession frequency and relaxation rate were measured in several La-based Cu-oxide samples of La2CuO4 with different stoichiometry in order to study the influence of La-vacancy, Sr-doping and O-doping on magnetism and super-conductivity. From the measured frequency at low temperatures in the magnetically ordered samples and the relaxation rate in the normal state of superconducting samples the muon site was determined. We find the muon to be on the face of the unit cell of La2CuO4. 1 A away from the apical oxygens ions O(2), favoring the typical bond length of 1 A for O−H.

Angle dependence of the muon relaxation rate in the mixed state for single crystals of YBa2Cu3O7-δ

Angle dependence of the muon relaxation rate in the mixed state for single crystals of YBa2Cu3O7-δ

Magnetic rare-earth contributions to muon relaxation rates and frequency shifts in RBa2Cu3O7- delta.

Contributions of rare-earth (R) magnetic moments to muon frequency shifts and relaxation rates are obtained for superconducting Gd- and Er-based barium cuprates by comparison with a (nonmagnetic) Eu-based sample. The R-related contributions approximately scale with the paramagnetism. A model of R-spin-flip-induced local supercurrents is introduced, which is consistent with very short coherence lengths and qualitatively reproduces the enhanced frequency shifts and reduced R-related relaxation terms observed below ${\mathit{T}}_{\mathit{c}}$.

Study of the 1-K phase transition in the heavy-electron compound UCu5 by muon spin resonance and neutron scattering.

The 1-K phase transition in ${\mathrm{UCu}}_{5}$, showing up in specific-heat data within the antiferromagnetic state below ${\mathit{T}}_{\mathit{N}}$\ensuremath{\simeq}15 K, was investigated. Neither the average internal fields seen by the ${\mathrm{\ensuremath{\mu}}}^{+}$ nor the magnetic and nuclear Bragg reflections in the neutron-diffraction data reflect the phase transition while the muon relaxation rates increase drastically below 1.2 K. These results are interpreted in terms of some additional small-moment magnetic order or spin-density-wave phenomenon, implying the coexistence of two rather independent electronic subsystems, one involving ``heavy'' electrons, associated with the weak magnetism, and the other associated with the ``conventional'' antiferromagnetic order.

Comparison of the paramagnetic fluctuations in GdM2 and DyM2 (M=Al, Ni)

The muon spin relaxation in the paramagnetic states of GdNi2 and DyNi2 compounds has been measured, as a complement to earlier studies of the rare earth dialuminides (REAl2). In addition, improved data have been taken on a GdAl2-sample, up to a temperature of 4Tc. At high temperatures the Moriya molecular field model predicts a rare earth spin fluctuation rate proportional to the Curie temperature of the compund, which would lead to a Tc−1 dependence of the muon relaxation rate. This relation is fulfilled for the pair GdNi2/GdAl2, but not for the DyNi2/DyAl2 ratios. Possible reasons for this discrepancy are discussed.

Magnetic interactions, bonding, and motion of positive muons in magnetite.

Positive-muon behavior in magnetite is investigated by the muon-spin-rotation technique. The ob­ served muon relaxation rate in zero applied field, in conjunction with the measured local field, al­ lows us to separate muon-motion effects from phase transitions associated with magnetite. The lo­ cal magnetic field is observed to be 4.02 kOe directed along the ( 111 ) axis, the easy axis f magneti­ zation. Possible origins of this field are discussed in terms which include local muon diffusion and a supertransfer hyperfine interaction resulting from muon-oJ!,ygen bonding. An anomaly in the muon hyperfine interactions is observed at 247 K.

Effects of superconductivity on rare-earth ion dynamics in (HoxLu1−x)Rh4B4

Zero-field muon spin-lattice relaxation rates have been measured in magnetically diluted HoxLu1−xRh4B4 ternary compounds, primarily for x=0.02. For temperatures below ∼11 K a characteristic two-component structure of the muon depolarization function is observed, which is consistent with slow (quasistatic) Ho3+-moment fluctuations. We have reported similar behavior for x=0.7, which indicates that the slow fluctuations are due to crystal-field isolation of the Ho3+ ground state. The observed relaxation rates in this temperature regime depend little on x, which is consistent with conduction-electron (Korringa) exchange scattering as the dominant mechanism for the fluctuations. The observed temperature dependence of muon spin-lattice relaxation in the superconducting state is not presently understood.

Positive muon Knight shift in graphite and Grafoil

Positive muon Knight shifts and relaxation rates were measured at room temperature in a graphite crystal and in a stack of Grafoil sheets. The Knight shift was ≈ 500 ppm in the single crystal and reduced by 0.702 in Grafoil. Both have the same (large) fractional anisotropy relative to theĉ axis or to the normal to the Grafoil sheets, respectively. The (isotropic) relaxation rates were 0.024(4)μs−1 in the crystal and 0.194(6)μ −1 in the Grafoil. Apparently theμ + in Grafoil “sees” highly aligned bulk crystalline graphite, and does not reach the surfaces of the sheets.

Muon spin relaxation measurements of spin-correlation decay in spin-glassAgMn

The field (H) dependence of the muon longitudinal spin-lattice relaxation rate well below the spin glass temperature inAgMn is found to obey an algebraic form given by (H) v −1 withv=0.54±0.05. This suggests that Mn spin correlations decay with time ast −v , in agreement with mean field theories of spin-glass dynamics which yieldv≲0.5. Near the glass temperature the agreement between the data and theory is not as good.

Muon spin relaxation in a random ferromagnet:PdMn

Transverse and longitudinal muon spin-relaxation experiments have been carried out in the ferromagnetic and paramagnetic states of a Pd--2.0 at. % Mn alloy to determine effects of spin-configuration disorder on the distribution of local (dipolar) fields at positive muon (..mu../sup +/) sites, and on dynamic-relaxation processes. The giant-moment polarization cloud appears to make no contribution to the local-field distribution in the paramagnetic state (T>T/sub c/ = 5.8 K). The Walstedt-Walker formula for the ..mu../sup +/ linewidth is in good agreement with the data. In the ferromagnetic phase the distribution width is less than predicted by a mean-field model of a uniform spin system. This ''shortfall'' is accounted for qualitatively by the Sherrington-Kirkpatrick (replica-symmetric) theory for disordered ferromagnets. Spin-lattice relaxation rates in zero applied field give clear evidence for critical slowing down of Mn spin fluctuations over a wide range of temperatures. Muon-relaxation rates well below T/sub c/ are consistent with a two-magnon (Raman) scattering mechanism.

Evidence for Power-Law Spin-Correlation Decay from Muon Spin Relaxation inAgMnSpin-Glass

Muon spin relaxation measurements have been carried out below the "glass" temperature ${T}_{g}$ in $\mathrm{Ag}\mathrm{Mn}$ spin-glasses. The muon spin-lattice relaxation rate varies with field $H$ as ${H}^{\ensuremath{-}0.46\ifmmode\pm\else\textpm\fi{}0.05}$ for $0.30<~\frac{T}{{T}_{g}}<~0.66$. This suggests that impurity-spin correlations decay with time as ${t}^{\ensuremath{-}\ensuremath{\nu}}$, $\ensuremath{\nu}\ensuremath{\simeq}0.54\ifmmode\pm\else\textpm\fi{}0.05$, in contrast to the more usual exponential decay. The present data therefore agree quantitatively with the prediction $\ensuremath{\nu}\ensuremath{\simeq}\frac{1}{2}$ of mean-field dynamic theories.

Master-equation approach to muonium depolarization in solids

The theory of master equations for open systems is applied to describe the spin relaxation of muonium in solids. An equation of motion for the density matrix of muonium is derived which takes into account the interaction of the bound electron with the host. The calculated muon relaxation rates depend strongly on the applied magnetic field. The range of validity of the Markov approximation is investigated and the results are compared with those of previous theories.

Spin Fluctuations of Itinerant Electrons in MnSi Studied by Muon Spin Rotation and Relaxation

The static and the dynamic character of the hyperfine field in weak itinerant helimagnet MnSi has been probed by using positive muon spins. The Knight shift has been measured by the transverse-field muon spin rotation method, which revealed a large negative hyperfine field at interstitial sites, even larger compared with other ferromagnetic metals. The hyperfine coupling constant A hf has been determined to be -4.8 kOe/µ B . The divergence of the muon spin-lattice relaxation rate near magnetic ordering has been observed for the first time by using the longitudinal-field muon spin relaxation method. The observed temperature dependence was found to be in agreement with the prediction of the self-consistent renormalization theory of itinerant electron spin fluctuations developed by Moriya and his collaborators.