ALBERT

Notes: Abstract We have carried out a comprehensive study of the static and dynamic spin-spin correlations of Mn x Zn1−x F2 in a magnetic field. Samples withx=0.75 andx=0.5 have been studied. This system exhibits behavior closely related, if not identical, to that of the Random Field Ising Model (RFIM). An additional feature of Mn x Zn1−x F2 is that it exhibits an easily accessible bicritical point; thus one can study the changeover from the RFIM to the uniformXY model with a transverse random field. Quite generally, the instantaneous spin-spin correlations in a field are described by a combination of Lorentzian, Lorentzian-squared and delta function terms the latter corresponds to the long range order (LRO) component. In the Ising phase one finds history dependent behavior as discussed previously. In theXY phase, except very near the spin-flop boundary, one finds ergodic behavior withXY LRO and Lorentzian squared Ising fluctuations. Rather complicated instability effects are found all along the spin-flop boundary. Further, when one establishes LRO in theXY phase and lowers the field through the spin-flop value, one obtains a LRO Ising state in thex=0.75 sample whereas one obtains the field-cooled domain state in thex=0.50 sample. This dramatic difference in behavior is not understood. Our results on the RFIM aspects of the problem are consistent with our previous studies. The transition is dominated by the metastability effects with an underlying equilibrium transition which is either first order or weakly second order (β≈0). The underlying transition manifests itself directly in measurements of the dynamic response nearT N (H). From the data above the metastability boundary we deduce for the static correlation length exponentv=1.4±0.3 in good agreement with theory. We find for the RFIM crossover exponent φRF=1.5±0.2 where the errors represent the spread in values obtained from different techniques. Finally, we have determined in detail the field-temperature phase diagram of thex=0.5 sample including the critical behavior along the spin-flop line; the latter transition appears to be second order for an extended region.

Notes: Abstract Positive muon spin rotation and relaxation measurements have been carried out on the antiferromagnets, pure MnF2 and site-diluted (Mn0.5Zn0.5)F2, above and below the Néel temperature TN using single-crystal specimens. Two different muon signals have been found in the pure MnF2; with the precession frequency υA for the site A and υB for the site B measured in zero external magnetic field at T=5 K. We propose a picture that the signal from the A site represents the “muonium” state, and discuss the characteristic features of muonium in magnetic materials. The spin relaxation rate 1/T1, measured in zero external field, decreases rapidly with decreasing temperature below TN. The mechanism of the spin relaxation above TN is explained by the exchange fluctuations of the Mn moments, while below TN by the Raman scattering of spin waves. At the same normalized temperature T/TN, 1/T1 observed in the diluted (Mn0.5Zn0.5)F2 is significantly larger than that in the pure MnF2 below TN. The difference between the pure and diluted systems is related to the large spectral weight of low-energy magnons in (Mn0.5Zn0.5)F2 found by neutron scattering.

Notes: Abstract We combine the results from muon spin relaxation (μSR) and neutron scattering measurements performed on the same specimen (or system) of magnetic materials. The example on a spin glassCuMn (5%) shows that the two techniques have complementary time windows for studying dynamic spin fluctuations. In combining the results, one should note that muons and neutrons probe dynamic phenomena with different wavevectors. The results on antiferromagnetic La2CuO4−y illustrate the difference in the spatial range of static spin correlations reflected in the μSR precession frequency and the neutron Bragg peak intensity. With the examples of CeCu2.1Si2, YBa2Cu3Ox and Bi2Sr2YCu2O8+y , we point out that μSR is a superb tool for discovering static magnetic order while neutron scattering is the unique method to determine the spin structure. We emphasize that it is very fruitful to perform μSR and neutron experiments on the same specimen and to compare and combine the results for the better understanding of magnetism of various system.

Notes: Abstract The zero-field muon spin relaxation functionG zz (t) has been measured as a function of reduced temperaturet=T/T g in the amorphous metallic spin glasses Pd75Fe5Si20 and Pd75Fe5P20. The results are in qualitative agreement with earlier measurements on dilute alloy spin glasses, including an onset of static order belowT g and a [t/(t-1)]2 dependence of the correlation time τc aboveT g. Both samples have the same τc (t) aboveT g and almost identical static width ΔS→Δo≃43 μS−1) asT»0, but thet-dependence of Δs nearT g differs markedly.

Notes: Abstract Positive muon spin precession has been observed in various heavy-fermion systems in the transverse external magnetic field. In the superconductor CeCu2.1Si2, the relaxation rate of muon spins increases rapidly with decreasing temperature below TC. This is interpreted as the results of the inhomogeneous fields due to the imperfect penetration of the external field into the type-II superconducting state. The magnetic-field penetration depth λ is derived from the observed muon spin relaxation rate. λ is about 1200 ∢ at T∼0.5TC, and the temperature dependence of λ is consistent with the relation expected for a BCS superconductor. We have also measured the muon Knight shift Kμ in the normal (or paramagnetic) state of various heavy-fermion systems. Kμ is large and negative (about −1000∼−3000 ppm at T=10 K) for CeCu2Si2, UPt3 and CeAl3, while more complicated signals are measured in CePb3 and CeB6. The negative muon Knight shift in the non-magnetic heavy-fermion systems is discussed in terms of the Kondo-coupling between the conduction- and f-electrons.

Notes: Abstract We report the observation of μ+ Level-Crossing Resonances (LCR's) in the ordered phase of an antiferromagnetic material. Two LCR's were observed in MnF2 as a function of longitudinal magnetic field in the temperature range between 10K and 65K. Both are attributed to a muon in an interstitial octahedral-like site. The low field resonance is attributed to a muon-nuclear spin flip-flop transition involving the two nearest neighbour19F nuclei. The high field resonance occurs when the applied field cancels the local hyperfine field on the muon. The positions and widths of the LCR's were seen to scale with the sublattice magnetization.