A. Yaouanc

Mossbauer spectroscopy of R2Fe14B

The authors present results obtained by 57Fe Mossbauer spectroscopy on R2Fe14B, where R represents any of the rare earth elements except Pm, Eu and Yb. Additional measurements have been performed on oriented powder for R=Y, Pr and Ho. For the case of Y the measurements at 4.2 K were performed in applied magnetic fields up to 4 T. From these measurements it is deduced that the only compounds with an easy c axis at room temperature which show a spin reorientation of the iron sublattice at a lower temperature are the Nd and Ho alloys. Iron hyperfine fields are anisotropic and the anisotropy has different signs on different sites. An analysis of the non-4f contribution to the electric field gradient at the 161Dy or 166Er nuclei in Dy2Fe14B and Er2Fe14B is consistent with published results on 155Gd in Gd2Fe14B. In order to explain that a spin rotation is found only for R=Nd, Ho the authors invoke the relative small quadrupole moment of the 4f shell for these two ions and crystal field terms of higher than second order.

Muon spin rotation and relaxation in magnetic materials

A review of the muon spin rotation and relaxation studies on magnetic materials published from July 1993 is presented. It covers the investigation of magnetic phase diagrams, that of spin dynamics and the analysis of the magnetic properties of superconductors. We have chosen to focus on selected experimental works in these different topics. In addition, a list of published works is provided.

Structural and magnetic properties of RE2Fe14BH(D)x; REY, Ce, Er☆

Abstract The structural and magnetic characteristics of RE 2 Fe 14 BH x hydrides with RE ≡ Y, Ce, Er have been studied as a function of x . Both Curie temperature and magnetization are increased on hydriding the ternary bondes. Neutron diffraction experiments performed on deuterated samples have permitted the location of the host sites and the measurement of the magnetic moments of iron and RE. Different anomalies reported in the structural and magnetic behaviour of Ce 2 Fe 14 BH x are related to the change of electronic state on cerium.

Effects of hydrogen absorption on the 3d and 4f anisotropies in RE2Fe14B (RE ≡ Y, Nd, Ho, Tm)

Abstract The effects of hydrogen absorption on the iron and rare earth sublattice anisotropies in RE2Fe14B (RE ≡ Y, Nd, Ho, Tm) have been investigated using the singular point detection technique. Hydrogen is found to substantially depress both the iron and rare earth contributions to the magnetocrystalline anisotropy. However, such a reduction appears to be very large for holmium, and particularly for neodymium, which both give a resultant axial anisotropy. It is less important in the case of thulium which gives a resultant planar anisotropy. The observed hydrogen composition dependence of the holmium anisotropy implies the existence of a particular hydrogen filling sequence.

Magnetic density of states at low energy in geometrically frustrated systems

Using muon-spin-relaxation measurements we show that the pyrochlore compound Gd(2)Ti(2)O(7), in its magnetically ordered phase below approximately 1 K, displays persistent spin dynamics down to temperatures as low as 20 mK. The characteristics of the induced muon relaxation can be accounted for by a scattering process involving two magnetic excitations, with a density of states characterized by an upturn at low energy and a small gap depending linearly on the temperature. We propose that such a density of states is a generic feature of geometrically frustrated magnetic materials.

Positive muon data analysis at ISIS

The pulsed surface muon beam of the Rutherford Appleton Laboratory is well suited to performing μSR measurements in zero and low magnetic fields with the longitudinal set-up. In this paper we describe our data analysis procedure and the effect of the collimation on the spectra. The determination of the efficiency ratio of the telescopes is discussed. We point out that for some measurements it is important to take into account the muon beam structure properly.

μSR study of RNi5 intermetallics where R=La, Gd or Tb

Zero-field μSR studies of some hexagonal intermetallic compounds are described. Our LaNI5 data provide information on the muon localisation site. This site could be one of the deuterium sites deduced from neutron diffraction. A comparison of the temperature dependence of the exponential damping rate, λ(T), of our samples shows that λ(T) is strongly influenced by crystal field effects. The TbNi5 spectra exhibit two components forT<60 K. We discuss the possible explanations of this result. We see a μSR signal below the magnetic phase transition in GdNi5. This makes the study of the spin-lattice relaxation rate possible in the ordered magnetic state.

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.

Spin dynamics in geometrically frustrated compounds

Abstract We have probed the low temperature spin dynamics in two pyrochlore lattice compounds R2Ti2O7, where the rare earth (R3+) form a sublattice of tetrahedra linked by their corners such that geometrically derived magnetic frustration is possible. In Yb2Ti2O7 (Yb3+: S′=1/2, XY anisotropy), we show that below 0.24 K , the temperature of the known specific heat λ transition, there is no long range order. The transition, in fact, corresponds to a first-order change in the fluctuation rate of the Yb3+ spins. Above the transition temperature, the rate is in the GHz range and it follows a thermal excitation law, whereas below the transition, the rate is in the MHz range and it is temperature independent. In Gd2Ti2O7, where the S-state Gd3+ ions are antiferromagnetically coupled, there is a magnetic phase transition near 1 K . As the temperature is lowered through the critical region, λZ, the spin-lattice relaxation increases. On lowering the temperature to 21 mK , λZ tends to a temperature independent value. Concomitant with the persisting electronic spin fluctuations, we observe oscillating components evidencing long-range spin correlations. Therefore, temperature independent spin dynamics is observed even after passing through a magnetic phase transition and even when the magnetic correlations are long range.

Transitions and Spin Dynamics at Very Low Temperature in the Pyrochlores Yb2Ti2O7 and Gd2Sn2O7

The very low temperature properties of two pyrochlore compounds, Yb2Ti2O7 and Gd2Sn2O7, were investigated using an ensemble of microscopic and bulk techniques. In both compounds, a first order transition is evidenced, as well as spin dynamics persisting down to the 20 mK range. The transition however has a quite different character in the two materials: whereas that in Gd2Sn2O7 (at 1 K) is a magnetic transition towards long range order, that in Yb2Ti2O7 (at 0.24 K) is reminiscent of the liquid–gas transition, in the sense that it involves a 4 orders of magnitude drop of the spin fluctuation frequency, with no long range order. We attribute these unusual features to the frustration of the antiferromagnetic exchange interaction in the pyrochlore lattice.