E. Schreier

Magnetic order in NpO2 and UO2 studied by muon spin rotation

Abstract Muon spin rotation/relaxation (μSR) measurements were carried out in zero applied field on NpO 2 and UO 2 above and below their transition temperatures of 25 and 30.8 K, respectively. In NpO 2 , a spontaneous spin precession pattern is observed for T 2 where the 30.8 K transition is well established as the Neel temperature for antiferromagnetic order. The μSR patterns are, in their basic features, quite alike for the two compounds. This establishes unambiguously that the transition at 25 K in NpO 2 is basically of magnetic origin. The ordered moment on Np is estimated to be smaller than about 0.15 μ B , i.e. less than 10% of the moment on U in antiferromagnetic UO 2 .

Valence fluctuations in ternary europium compounds

We outline the possibility to study europium valence fluctuations with the μSR method and report on μSR experiments on the intermetallic compounds EuPdAs and NdPdAs. Above a magnetic transition at 15 K the temperature dependence of the relaxation rate in the trivalent neodymium system behaves like a typical localized moment system. In the valence fluctuating europium compound the zero field relaxation rate levels off at 1.0\ μs-1 above 40 K. Furthermore, the relaxation enhancement in transverse field experiments is much smaller than expected for a pure dipolar coupling. Therefore an isotropic hyperfine coupling of typical strength is assumed and a valence fluctuation rate of 0.8 μs-1 at 200 K is derived. Below the magnetic transition at 5 K a disordered spin freezing is concluded in EuPdAs.

High pressure mu SR studies on single crystalline gadolinium

Muon spin rotation data on a single crystalline gadolinium sample have been obtained as function of temperature and hydrostatic external pressure up to 0.6 GPa.In the ferromagnetic state the application of pressure has a strong influence on the whole spin‐turning‐process of the spontaneous magnetization: The onset of the spin‐turning is shifted towards lower temperatures with a rate of approximately dTst/ dp\approx -50 K/GPa. Higher values of the turning angle up to \varthetaext=90\circ can be reached and stabilized over a wider temperature range. At low temperatures the magnetization turns back again.In the paramagnetic regime the data show a similar behaviour as under ambient pressure: both the Knight‐shift and the muon spin relaxation show deviations from their high temperature behaviour in the critical regime below (T-TC) < 10 K. So there is no indication of pressure induced effects in this temperature range.

Magnetic behavior of YFexAl12−x

Abstract Previous μSR studies on the RFe6Al6 (R=Tb, Ho, Er) ferrimagnets (T N ≈340 K ) showed effects of frustration due to competing exchange between the R and Fe sublattices. This puts the compounds on the borderline between spin glass and long-range order. In continuation of this work, μSR and 57 Fe Mossbauer spectroscopy were carried out on YFe6Al6 and YFe7Al5. In these compounds, no moments exist on the normally strongly magnetic R sublattice. Neutron diffraction was unable to detect magnetic Bragg peaks, but the μSR and Mossbauer spectra clearly reveal a ferrimagnetic transition near 340 K in both materials. Not all of the Fe ions order at this temperature. The Fe ions on the Fe sublattice (8f) order only below ∼70 K . Since competing exchange is absent, the Fe sublattices possess inherent frustration reflected in distributions of moment size and orientation, short correlation lengths and strong spin fluctuations.

Magnetic correlations in frustrated LiV2O4 and ZnV2O4

We report on NMR, μSR and neutron diffraction studies of (Li:Zn)V 2 O 4 . Both compounds crystallize in the geometrically-frustrated cubic spinel structure. 7 Li NMR was performed for 100 mK< T<280 K and at applied magnetic fields of 4.6, 10, 44, and 83 kOe: inherent geometric frustration in LiV 2 O 4 results in a dynamic magnetic ground state. The μSR studies on ZnV 2 O 4 covered the temperatures 1.7 K ≤ T ≤125 K. It is concluded that the trigonal distortion does not fully relieve frustration, leading to a spin-glass-like precursor state and a magnetic ground state with a high degree of spin disorder which suppresses true long-range order.

μSR spectroscopy of the Kondo insulators Lu1-xYbxB12

Abstract Single crystals of Yb 1−x Lu x B 12 (x=0,0.125,0.5,1) were measured between 1.8 and 300 K . We found similar spectral shapes in all compounds studied over the whole temperature range scanned. In zero field the spectra alter their appearance around 20, 100 and 150 K . In a longitudinal field of 100 G , which largely suppresses the contribution from 11 B nuclear moments the relaxation rate remained constant up to ∼150 K where it suddenly peaks. These findings exclude magnetic correlations as the origin of muon spin relaxational behavior. The dominant features of the μSR spectra at low temperatures arise from lattice dynamics, probably involving the B 12 clusters, rather than from spin dynamics. Higher temperature features are most likely muon related.

High-pressure μSR studies on elemental rare earth metals

Abstract Muon-spin rotation data on single-crystalline samples of the heavy rare-earth metals Dy and Ho in their antiferromagnetic and ferromagnetic temperature regimes (20 K N ) have been obtained as function of hydrostatic external pressure up to 0.9 GPa using the He-gas high-pressure system installed at the decay muon beam μE1 at PSI. Due to the absence of pressure-dependent changes of the moment orientation (spin-turning) as observed in previous high-pressure measurement on ferromagnetic Gd, the pressure coefficients (∂Bμ/∂p)T in Dy and Ho are comparatively small, but nevertheless temperature-dependent. The volume dependence of the contact field is extracted.

μSR magnetic response in UPdSn

Abstract Muon spin relaxation measurements have been performed on a powder sample of UPdSn. μSR verifies the existence of the two separate antiferromagnetic states (denoted AFM 1 and AFM 2) found by neutron diffraction, with transitions near 40 and 24 K , and with different magnitudes of their local fields. Both phase transitions extend over a region of ∼5 K width where two magnetic phases (para/AFM 1 and AFM 1/AFM 2) coexist. No evidence for a fluctuating and/or disordered spin component could be detected in AFM 1. It is concluded that the spins in AFM 2 are about 30% larger in magnitude than those in AFM 1. The absence of partial LRO ordering in UPdSn in contrast to the μSR results for the magnetic ground state of isostructural CeCuSn can be taken as another indicator that Cu–Sn disorder in the latter may be decisive, since similar disorder in the Pd–Sn sublattices is definitely absent. The spontaneous frequency in AFM 1 attains its highest value around TN and then decreases on cooling. A similar behavior was observed in CeCuSn for the LRO portion of its magnetic ground state. This striking similarity in non-standard behavior most likely indicates that spin-structural properties of the LRO portion in CeCuSn are similar to the ones of AFM 1 in UPdSn.

Magnetic ordering in UPdSn and CeCuSn

Abstract Zero-field muon spin relaxation (μSR) has been measured in a powder sample of UPdSn. Magnetic ordering below ∼40 K generates coherent oscillation of the muon polarization. Around the second magnetic transition (nominally 25 K ) there is an ∼4 K range of inhomogeneous re-ordering, with the lower-temperature ordered state generating a higher oscillation frequency. In the higher-temperature ordered state, the frequency rises slightly with temperature, contrary to the usual order parameter behavior, which is to drop toward zero as TN is approached. A similar increase of the coherent oscillation frequency with temperature was observed by μSR in isostructural CeCuSn.

Internal fields in magnetically ordered dysprosium, holmium and erbium

Muon spin rotation data on single-crystalline samples of the heavy rare earth metals Dy, Ho and Er have been obtained as function of temperature in both the antiferromagnetic (afm) and the ferromagnetic (fm) state. In the afm state the temperature dependence of the spontaneous muon spin precession frequency consistently exhibits Brillouin-like behavior. In the fm state we observe, in all three metals, a decrease of frequency on cooling, while one expects a nearly temperature-independent saturation (T→0) behavior. Although the origin of this feature is not clear, it definitely cannot be connected to a spin reorientation. It is suggested that spontaneous bulk magnetization caused by the strong magnetic anisotropy might be responsible. In Dy and Ho only minor irregularities are seen at TC. In contrast, Er shows a huge drop of the spontaneous frequency at the fm transition temperature, which can be directly traced to the behavior of the dipolar field component at the muon site. Saturation values for the dipolar and the contact field at the muon site for the three metals are given.