W. Wagener

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 .

Fast paramagnetic relaxation in Mössbauer spectra of ferrocenium salts

We performed 57Fe Mössbauer spectroscopy on differently substituted ferrocenium salts. The spectra exhibit a temperature dependent non-Lorentzian absorption line due to fast paramagnetic relaxation processes. χ2 fits using a two-level stochastic fluctuation theory revealed an activated temperature behaviour of the relaxation rate interpreted in terms of an Orbach process.

μSR on the novel heavy‐fermion system Nd2-yCeyCuO4

The magnetic correlations in Nd2-yCeyCuO4 have been studied by zero and longitudinal field μSR. Nd1.8Ce0.2CuO4 reveals a saturation of the muon relaxation rate below 0.5 K. Even down to 70 mK LF spectra evidence spin fluctuations in the dynamic regime with a rate of \sim 109 s-1 excluding long range magnetic order or spin glass freezing. The average Nd moment is estimated to be \approx 0.2\muB, i.e., strongly reduced from the value determined for the ground state Kramers doublet of Nd2CuO4. Extending former μSR measurements on Nd2CuO4, a gradual enhancement of the internal field has been detected below 10 K which is accompanied by an increase of the relaxation rate. These results are attributed to the development of ordered Nd moments.

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.

Magnetic stripe order in La1.8−xEu0.2SrxCuO4

Abstract The magnetic order in La1.8−xEu0.2SrxCuO4 (x⩽0.2) with low-temperature tetragonal (LTT) structure has been investigated with muon spin relaxation. The magnetic phase diagram of the LTT phase resembles the generic phase diagram of the cuprates where superconductivity is replaced by a second antiferromagnetic phase. A crossover from magnetic order to superconductivity is observed at a charge doping of x=0.2. Changing the degree of the LTT lattice distortion via a variation of the Eu content the ground state at x=0.2 can be tuned from magnetism to superconductivity.

μ+SR on La 2 − x−yRExSryCuO4

Abstract Magnetic order is found in La 1.85 − xNdxSr0.15CuO4 with 0.3 ⩽ x ⩽ 0.6 and in La 1.8 − yEu0.2SryCuO4 with 0.12 ⩽ y ⩽ 0.18. The onset temperature Tc depends on the Sr concentration but no dependence on the Nd concentration is detectable. The spectra of the Nd doped samples show certain deviations from the spectra of the Eu doped samples which are caused by the interactions of the Nd spins with the Cu spins. These interactions are clearly visible below 5 K.

μ+SR on ( La_1.85-x Ndx) Sr0.15 CuO_4

Below 30 K, magnetic order was found in La1.85-x Nd_x Sr0.15 CuO_4 with x ranging between 0.30 and 0.60. This order occurs in the low temperature tetragonal phase. Below 5 K, the interaction of Nd and Cu moments modifies the magnetic order in the Cu–O planes leading to an increase of the muon spin rotation frequency and the transverse damping rate. Even down to 0.1 K, considerable spin dynamics is observable.

The magnetic phases and structures of Ce(Ru1-xFex)2Ge2 studied by μ-SR

We report on the structures of the magnetic phases of CeRu2Ge2 and their evolution in the series Ce(Ru1-xFex)2Ge2. The crossover behavior between the ferromagnetic ground state of CeRu2Ge2 to the paramagnetic ground state of CeFe2Ge2 is compared to the behavior in the series CeRu2(Ge1-xSix)2. A profound difference is observed between these two series, which is in contradiction to the uniform behavior predicted by the Doniach-necklace model.

μSR on an induced‐moment spin glass system: Hg1-xFexSe

Zero and longitudinal field μSR have been used to examine the singlet ground state magnetism of the diluted magnetic semiconductor Hg1-xFexSe. For x\geq 0.05\ μSR experiments indicate spin glass behaviour which is limited to sample regions with locally enhanced Fe2+ content. Longitudinal field spectra in the glassy state reveal a static local field \sim 50 mT due to induced moments of Fe2+. These are proposed to originate from a distribution of bound magnetic polaron energies.

Local magnetic field and muon site in CeAs

We report on zero field and longitudinal field μSR experiments on a CeAs single crystal between 3.3 and 12 K. Below the antiferromagnetic transition at 7.5 K a spontaneously precessing signal with a saturation frequency of \approx25 MHz representing the full sample amplitude has been found. From an analysis of the field dependence of the relaxation rate of this signal in \langle 100\rangle and \langle 110\rangle crystal orientation parallel to the muon spin and the applied longitudinal field, a \langle 100\rangle orientation of the local field at the muon site is concluded. This supports an AF‐I single‐\veck magnetic ordering.