P.J.C. King

First-order transition in the spin dynamics of geometrically frustrated Yb2Ti2O7

Using neutron diffraction, 1 7 0 Yb Mossbauer and muon spin relaxation spectroscopies, we have examined the pyrochlore Yb 2 Ti 2 O 7 , where the Yb 3 + S = ½ ground state has planar anisotropy. Below ∼0.24 K, the temperature of the known specific-heat A transition, there is no long range magnetic order. We show that the transition corresponds to a first-order change in the fluctuation rate of the Yb 3 + spins. Above the transition temperature, the rate, in the GHz range, follows a thermal excitation law, whereas below, the rate, in the MHz range, is temperature independent, indicative of a quantum fluctuation regime.

Spin dynamics and magnetic order in magnetically frustrated Tb2Sn2O7

We report a study of the geometrically frustrated magnetic material Tb2Sn2O7 by the positive muon-spin relaxation technique. No signature of a static magnetically ordered state is detected while neutron magnetic reflections are observed in agreement with a published report. This is explained by the dynamical nature of the ground state of Tb2Sn2O7: the Tb3+ magnetic moment characteristic fluctuation time is approximately 10(-10) s. The strong effect of the magnetic field on the muon-spin-lattice relaxation rate at low fields indicates a large field-induced increase of the magnetic density of states of the collective excitations at low energy.

Measuring small samples at the ISIS muon source

Studying samples appreciably smaller than the muon beam spot size, using conventional masked holders, is complicated by the detection of positrons from the muons stopped in the masking material. A technique of suspending samples in the beam as a method of overcoming these inherent difficulties has been developed; muons not implanted in the sample are now able to fly-past and their contribution is removed from the measured signal. The implementation of this method on the EMU spectrometer at ISIS is described in this paper. Details of modifications to equipment to enable this type of operation and the results obtained from several test experiments are reported. Practical details associated with carrying out experiments using suspended samples are also considered. Although greatly reduced, background counts are still present in the data; experiments investigating the source of these counts and future improvements to the apparatus are considered.

Characterization of hydrogen-like states in bulk Si1−xGex alloys through muonium observations

Much of our knowledge of the charge states, lattice site and behaviour of hydrogen in bulk semiconductors comes from observation of its muonium analogue. Here we present studies of muonium behaviour across the composition range in bulk, Czochralski-grown Si1?xGex alloy material, focusing in particular on the muonium hyperfine parameters. For the bond-centred muonium species, a broad distribution of parameters is observed, consistent with a variety of bonding environments. The average value of the isotropic component of the bond-centred hyperfine parameter shows a linear variation with alloy composition, which might be expected based on the linear variation with composition of alloy bond lengths. In contrast, the hyperfine parameter of the tetrahedral-site muonium species (MuT) appears to vary non-linearly with alloy composition, and an explanation of this in terms of MuT mobility is provided. The temperature dependence of the MuT hyperfine parameter observed in several alloy compositions is compared with that seen in pure Si. Previous descriptions of the low-temperature behaviour of the MuT parameter in Si are discussed in the light of results from Si1?xGex material.

Nd2Sn2O7: an all-in-all-out pyrochlore magnet with no divergence-free field and anomalously slow paramagnetic spin dynamics

We report measurements performed on a polycrystalline sample of the pyrochlore compound Nd2Sn2O7. It undergoes a second order magnetic phase transition at T-c approximate to 0.91 K to a noncoplanar all-in-all-out magnetic structure of the Nd3+ magnetic moments. The thermal behavior of the low temperature specific heat fingerprints excitations with linear dispersion in a three-dimensional lattice. The temperature independent spin-lattice relaxation rate measured below T-c and the anomalously slow paramagnetic spin dynamics detected up to approximate to 30T(c) are suggested to be due to magnetic short-range correlations in unidimensional spin clusters, i.e., spin loops. The observation of a spontaneous field in muon spin relaxation measurements is associated with the absence of a divergence-free field for the ground state of an all-in-all-out pyrochlore magnet as predicted recently.

Probing the shallow-donor muonium wave function in ZnO and CdS via transferred hyperfine interactions

Abstract The assignment of muon spin rotation spectra to muonium counterparts of hydrogen shallow–donor states is reviewed in four II–VI widegap semiconductors, CdS, CdSe, CdTe and ZnO. The existence of extended electronic orbitals is argued from the muon–electron hyperfine parameters and supported by the new muon spin repolarization data for CdS and ZnO, characterizing the superhyperfine parameters on the sparse Cd and Zn dipolar nuclei. The possibility of a more tightly bound electron occupying a compact orbital is reasonably excluded in these materials, contrasting with the muonium state in HgO.

Magnetic frustration in the disordered pyrochlore Yb2GaSbO7

In the pyrochlore Yb(2)GaSbO(7), the Yb(3+) sublattice forms a network of corner sharing tetrahedra and the second sublattice is made up of disordered, non-magnetic Ga(3+) and Sb(5+) ions. We have examined this compound using magnetic susceptibility, (170)Yb Mössbauer spectroscopy (down to 0.03 K) and muon spin relaxation (μSR) (down to 0.02 K) measurements. We establish the size of the Yb(3+) magnetic moments and that of the Yb(3+)-Yb(3+) coupling. At low temperatures, the correlated moments fluctuate between directions that are well tilted relative to the local [111] axis. The lattice disorder does not quench the frustration induced low temperature spin fluctuations but it does remove the first order dynamic transition that is present in the crystallographically ordered counterpart Yb(2)Ti(2)O(7). Below 1.0 K, the fluctuation rate of the correlated moments decreases progressively as the temperature is reduced and the moments remain dynamic down to 0.02u2009K where the rate is 7 × 10(7) s(-1). Magnetic frustration is operative in Yb(2)GaSbO(7) where the Yb(3+)-Yb(3+) interaction is antiferromagnetic as it is in Yb(2)Ti(2)O(7) where the interaction is ferromagnetic.

Absence of Magnetic Order in Yb3Ga5O12: Relation between Phase Transition and Entropy in Geometrically Frustrated Materials

From muon spin relaxation spectroscopy experiments, we show that the sharp peak (lambda-type anomaly) detected by specific heat measurements at 54 mK for the ytterbium gallium garnet compound, Yb3Ga5O12, does not correspond to the onset of a magnetic phase transition, but to a pronounced building up of dynamical magnetic pair correlations. Beside the lambda anomaly, a broad hump is observed at higher temperature in the specific heat of this garnet and other geometrically frustrated compounds. Comparing with other frustrated magnetic systems we infer that a ground state with long-range order is reached only when at least 1/4-1/3 of the magnetic entropy is released at the lambda transition.

Modelling hydrogen in the group-III nitrides by its pseudo-isotope, muonium

Abstract Muon and muonium states in the wide-bandgap semiconductors BN, AlN, and GaN are characterised by various types of μSR measurement on polycrystalline samples. The muonium fractions range from 80% in hexagonal BN to zero in GaN. The hyperfine constants estimated from repolarization curves are 80% of the free muonium value in BN and 95% in AlN, with superhyperfine interactions to the host nuclei is evident. The electronically diamagnetic states show strong level-crossing resonances in AlN and GaN (although none is detectable in BN). These have the signature of cross-relaxation to 14 N in AlN and to 69 Ga and 71 Ga in GaN, suggesting that the diamagnetic states are Mu + and Mu − in these naturally p- and n-type materials, respectively. Mu − diffusion in GaN sets is only above 600 K, with an activation energy of 1 eV.

Anomalously slow spin dynamics and short-range correlations in the quantum spin ice systemsYb2Ti2O7andYb2Sn2O7

We report a positive muon spin relaxation and rotation (muSR) study of the quantum spin ice materials Yb2Ti2O7 and Yb2Sn2O7 focusing on the low field response. In agreement with earlier reports, data recorded in small longitudinal fields evidence anomalously slow spin dynamics in the microsecond range below the temperature T_c at which the specific heat displays an intense peak, namely T_c = 0.24 K and 0.15 K, respectively, for the two systems. We found that slow dynamics extends above T_c up to at least 0.7 K for both compounds. The conventional dynamical Gaussian Kubo-Toyabe model describes the muSR spectra recorded above T_c. At lower temperatures a published analytical extension of the Gaussian Kubo-Toyabe model provides a good description, consistent with the existence of short-range magnetic correlations. While the physical response of the two systems is qualitatively the same, Yb2Ti2O7 exhibits a much larger local magnetic susceptibility than Yb2Sn2O7 below T_c. Considering previously reported ac susceptibility, neutron scattering and muSR results, we suggest the existence of anomalously slow spin dynamics to be a common physical property of pyrochlore magnetic materials. The possibility of molecular spin substructures to be associated to the slow dynamics and therefore the short-range correlations is mentioned. The slow spin dynamics observed under field does not exclude the presence of much faster dynamics detected in extremely low or zero field.