μ+SR study of Ising spin-glass Fe0.5Mn0.5TiO3

A typical Ising spin-glass Fe_0.5Mn_0.5TiO₃ has been investigated by ⁺ spin relaxation measurements. It is found that the local magnetic fields at the ⁺ stopping sites appear at a temperature twice as high asT _SG. The directional distribution of the local magnetic fields is nearly isotropic, indicating the existence of the transverse spin components. It is clearly demonstrated that the spins fluctuate belowT _SG and even at 4 K. Taking into account the results obtained by other methods, the main part of the fluctuations is inferred to be due to the transverse spin components and the fluctuation limes at 4 K are driven into a range between 10^–5 and 10^–7 s.     

μ +SR study of some magnetic superconductors

The relaxation of the ⁺SR signal has been investigated for the three compounds YRh₄B₄, ErRh₄B₄ and SmRh₄B₄. In the non-magnetic superconducting (T _c 11 K) YRh₄B₄, the data display a Kubo-Toyabe (gaussian) shape for zero (transverse) magnetic fields. ErRh₄B₄ (superconducting below 8.7 K and ferromagnetic below 1 K) shows a dominant signal with very slow relaxation. In contrast SmRh₄B₄ (superconducting below 2.7 K and antiferromagnetic-superconducting below 0.87 K) shows a change in relaxation from gaussian above 60 K to exponential between 1 K and 4 K to two exponential signals (fast and slow) belowT _N=0.9 K. In the region 0.9 K <T < 4.5 K, the relaxation time and the asymmetry both increase withT.supported by the U.S. Department of Energy.Supported by NSERC of Canada.supported by the U.S. Department of Energy.We are grateful to Drs. H. Umezawa and H. Matsumoto for interesting discussions regarding the persistent current screening and the results of self-consistent calculations.     

μ+SR in imperfect solids

The use of ⁺SR as a probe of point defects in metals is discussed. The importance of separating the effects due to the presence of the muon from those intrinsic to the defect-host system is stressed. It is suggested that studies of magnetic impurities in non-magnetic hosts may yield data which are relatively probe-independent. A theoretical analysis of the depolarization of muons coupled to magnetic impurities is outlined. It is pointed out that measurements of the field dependence of the depolarization rate can give information about impurity spin correlation times.Work performed under the auspices of the U. S. Department of Energy.     

μSR study of UTr2Si2, Tr=Co,Rh OR Pt

We have studied bySR spectroscopy the intermetallicsUTr ₂Si₂ where Tr=Co, Rh or Pt. Whereas for the Co compound we only get information on the muon localisation site, from the other two materials we obtain information on their electronic properties. We compare the characteristics of theirSR response.     

μSR magnetic response in frustrated antiferromagnets of type RMn2 (R = rare earth)

Zero longitudinal and transverse fieldSR was carried out in the antiferromagnets YMn₂, Y_0.95 Tb_0.15 Mn₂, Y_0.9Tb_0.1Mn₂, Y_0.99 Sc_0.01 Mn₂, Y_0.98Sc_0.02Mn₂ and TbMn₂. The dynamics of Mn magnetic moments above T _N is typical for an itinerant antiferromagnet. Within a certain temperature range above T _N part of the material enters a randomly ordered (spin glass like) magnetic state as an outcome of frustration. At temperatures above 150 K the muon spin relaxation rate indicates that the muon has become mobile.This work was supported by the German Federal Minister for Research and Technology under Contract Nr. 03KA2-TUM-4.     

RF-μSR study of muonium charge states and dynamics in Si

The radio frequencySR technique developed at TRIUMF was used to measure the temperature dependence of the diamagnetic muon, Mu, and Mu^* amplitudes in silicon between 10 K and 500 K. Six samples doped with phosphorus (n-type) and boron (p-type) in the concentration range 10¹¹ to 10¹⁵ cm^–3 were studied. In pure Si a very good fit over the whole temperature range is obtained from a model that includes the ionization of Mu^* and Mu to a bond centered ⁺ followed at high temperature by charge exchange involving Mu.     

μSR investigation of cupric oxide

TF and ZFSR-investigations were performed on high purity CuO powder. By TF measurements a phase transition to the ordered state was observed at 227K. A commensurate-incommensurate phase transition was detected at 213K by ZF measurements. In the commensurate phase we observed the Larmor precession. Four signals were detected below 55K, but by increasing temperature above 190K, precession became having only one component. This fact may be explained by muons tunneling between equivalent sites. In the incommensurate phase the Larmor precession was not detected because of too large damping.     

μSR studies of YFeMnO4

Muon spin relaxation (SR) studies were carried out on YFeMnO₄. Two interesting phenomena were found in measurements of longitudinal relaxation time spectra atT>T _cusp for a single crystal: (1) difference of the relaxation rates between the directions parallel with and perpendicular to thec-axis, and (2) suppression of local field fluctuations in a longitudinal external field. The spectra atT >T _cusp were reproduced with the function exp[-(₀ t)]. The results are discussed under a picture of the magnetic spin fluctuations of triangular antiferromagnets.     

μ+ Diffusion in copper below 4 K reconfirmed by LF-μ+SR

Muon spin relaxation in longitudinal magnetic field (LF-μ⁺SR) reveals separately the static width of the local dipolar field distribution at the μ⁺ and the μ⁺ hopping rate in the copper lattice at 4.2 and 0.7 K, confirming that muon hopping between octahedral interstitial sites increases at lower temperatures.     

μSR study of the properties of Fe3O4-based nanostructured magnetic systems

A ferrofluid based on Fe₃O₄ nanoparticles dispersed in heavy water D₂O is studied using the μSR method. The experiment has been carried out at temperatures 26–300 K. It is found that the diamagnetic (muon) fraction is formed in the ferrofluid in about the same amount as in D₂O, but the muon-spin relaxation rate in the ferrofluid is much higher than in D₂O. A significant shift of the muon-spin precession frequency in the ferrofluid is observed. It is shown that the shift of the muon precession frequency as a function of the external magnetic field is described by the Langevin function typical of paramagnetic magnetization. The mean magnetic field in the medium due to magnetic-nanoparticle polarization in an external field is experimentally determined. The nanoparticle sizes are estimated.     

Spin dynamics of diluted antiferromagnets MnxMg1-xTiO3 by μSR method

The diluted antiferromagnet Mn_xMg_1-xTiO₃ transforms from the antiferromagnet (AF) to the spin-glass (SG) system at x ∼ 0.6. In order to study the microscopic nature of spin dynamics and ordering, we performed muon spin relaxation measurements for the powder samples of x=1.0∼ 0.55 and examined the concentration dependence. From these results, a drastic change in the spin dynamics was found to occur at x ∼ 0.6. This revised version was published online in August 2006 with corrections to the Cover Date.     

Maximum-entropy TFμSR study of vortex states in 1/2/3 cuprates

A maximum-entropy (ME) method has been applied to TFSR vortex data of RBa₂Cu₃O₇ (R1237; R=Er, Gd and Eu). The obtained information on the field distribution is of much better quality than that resulting from Fourier transformation and curve-fitting of the same data. ME application to R1237 TFSR data taken below T_c at LAMPF show asymmetrical, nearly triangular, field distributions, which broaden with decreasing temperature. Significant deviations from Abrikosov predictions have been observed for the estimated R1237-vortex field distributions. Below 10 K, for Er1237 and Gd1237, the non-Abrikosov features appear to be influenced by the rare-earth magnetism. The low-field tail in the field distribution and the ME-intensity anomaly near T_c suggest glassy features in the 1/2/3 cuprate vortex states.     

μSR study of the S=1/2 triangular lattice antiferromagnet AgNiO2

AgNiO₂, a model compound of an S=1/2 triangular lattice, was studied by muon spin relaxation in addition to ac, dc susceptibility, electrical resistivity and neutron diffraction. The relaxation rate shows a sharp peak at around T_N=28 K followed by a sudden decrease of initial asymmetry indicating a magnetic ordering. Three internal fields ranging from 0.2 to 0.3 T were obtained from the muon precession period. However, a neutron diffraction experiment failed to detect any magnetic order at low temperatures. From these results, it was concluded that magnetic coherence is confined to small domain compared with the coherence length of neutron diffraction due to spin frustration. This revised version was published online in August 2006 with corrections to the Cover Date.     

Applications of μSR to the study of fullerenes

A review is given of the current status of SR results on muonium states in the pure fullerenes C₆₀ and C₇₀ as well as on the alkali doped series of C₆₀. In particular the results of studies of the hyperfine interaction of the muonated radicals are discussed in order to determine their electronic distribution and the rotational ordering and dynamics of the fullerenes. Unique information on the alkali doped C₆₀ species is obtained from studies of the endohedral muonium species.     

Néel and Spin-Peierls ground states of two-dimensional SU(N) quantum antiferromagnets

The two-dimensional SU(N) quantum antiferromagnet, a generalization of the quantum Heisenberg model, is investigated by quantum Monte Carlo simulations. The ground state for Nor=5 with broken lattice translational invariance. Our computation of the magnetization and the dimerization order parameter shows the absence of the intermediate spin-liquid phase.     

μSR Investigation of ethyl radicals adsorbed on silica

SR spectra of the ethyl radical adsorbed on porous silica were observed in transverse and in longitudinal magnetic fields in the temperature range 190–298 K. The line widths reflect the dynamic partial averaging of the hyperfine anisotropy due to reorientation and surface diffusion.     

Interplays ofμSR,susceptibility and neutron studies on dilute-alloy spin glasses

Static spin polarization of Fe moments in a spin glassAuFe, determined by zero-fieldSR, is compared to an ac-susceptibility measurement below the cusp temperatureT _g, and a rather uniform amplitude of the static polarization is pointed out for frozen spins. Completely random orientation of frozen spins is revealed by neutron scattering in a dilute 1%CuMn, and an importance of comparing results of these different methods is demonstrated.     

μ+SR and Mössbauer studies of low dimensional magnetic ordering in ilvaite

Ilvaite reveals a quasi one dimensional ferromagnetic order along Fe chains between 50–120K. Both Mössbauer spectroscopy and ⁺SR reveal the onset of long range order below 120K with spin dynamics in the MHz range which is interpreted with relatively slowly moving wide domain boundaries. Below 50K the individual chains couple antiferromagnetically, yet a considerable fraction of dynamical spins is detected from ⁺SR down to low temperature.Work supported by the German Federal Minister of Research and Technology (BMFT) under contract Nr. 03-LI3-BRA.     

Non-Born-Oppenheimer study of the muonic molecule ion 4Heμ^+

Accurate non-Born-Oppenheimer variational calculations of all bound states of the positive muon molecular ion 4Heμ^+ have been performed using explicitly correlated Gaussian functions in conjunction with the global vectors.All the energies obtained are accurate in the order of 10-6 Hartree(1 Hartree=27.2114 eV).Compared with the binding energies obtained from calculations based on the Born-Oppenheimer potential with the mass-weighted adiabatic corrections(Chem.Phys.Lett.110487(1984)),the largest relative deviation is up to 15%.By analyzing the average interparticle distances and possibility distributions of interparticle distances of this system,it is confirmed that the Born-Oppenheimer approximation is reasonable for this system and that 4Heμ^+ can be regarded as a system of positive muon bound to a slightly distorted helium atom.