Origin of the Suppression of Magnetic Order in MnSi under Hydrostatic Pressure.

We experimentally study the evolution of the magnetic moment m and exchange interaction J as a function of hydrostatic pressure in the zero-field helimagnetic phase of the strongly correlated electron system MnSi. The suppression of magnetic order at ≈1.5  GPa is shown to arise from the J collapse and not from a quantum fluctuations induced reduction of m.

A novel type of splayed ferromagnetic order observed in Yb2Ti2O7.

The pyrochlore insulator Yb2Ti2O7 has attracted the attention of experimentalists and theoreticians alike for about 15 years. Conflicting neutron diffraction data on the possible existence of magnetic Bragg reflections at low temperature have been published. Here we report the observation of magnetic Bragg reflections by neutron powder diffraction at 60 mK.

Dynamical splayed ferromagnetic ground state in the quantum spin ice Yb(2)Sn(2)O(7).

From magnetic, specific heat, (170)Yb Mössbauer effect, neutron diffraction, and muon spin relaxation measurements on polycrystalline Yb(2)Sn(2)O(7), we show that below the first order transition at 0.15 K all of the Yb(3+) ions are long-range magnetically ordered and each has a moment of 1.1 μ(B) which lies at ≃ 10° to a common fourfold cubic axis.

Crystal electric field in the R2Ti2O7 pyrochlore compounds.

Using a simple scaling law, a unique set of crystal-field parameters for the rare-earth ions R in the R(2)Ti(2)O(7) pyrochlore series is shown to provide a proper description of the crystal-field excitations previously observed by neutron scattering. The two spectroscopic g factors are given for the compounds with doublet ground states, as well as the ground-state wavefunctions for all the compounds.

Magnetic frustration in the disordered pyrochlore Yb(2)GaSbO(7).

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.

CdEr2Se4: a new erbium spin ice system in a spinel structure.

Here we present a detailed study of the spinel CdEr2Se4 and show it to be a new instance of spin ice, the first one in an erbium material and the first one in a spinel. Definitive experimental evidence comes from the temperature dependence of the magnetic entropy, which shows an excellent agreement with the predicted behavior for a spin ice state. Crystal field calculations demonstrate that the change in the local environment from that of the titanates completely alters the rare-earth anisotropy giving rise, in the case of Er3+, to the required Ising anisotropy, when Er2Ti2O7 behaves as an XY antiferromagnet.

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.

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.

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

Using neutron diffraction, 170Yb Mössbauer and muon spin relaxation spectroscopies, we have examined the pyrochlore Yb2Ti2O7, where the Yb3+S' = 1/2 ground state has planar anisotropy. Below approximately 0.24 K, the temperature of the known specific-heat lambda transition, there is no long range magnetic order.

Evidence for a two component magnetic response in UPt3.

The magnetic response of the heavy fermion superconductor UPt3 has been investigated on a microscopic scale by muon Knight shift studies. Two distinct and isotropic Knight shifts have been found for the field in the basal plane. While the volume fractions associated with the two Knight shifts are approximately equal at low and high temperatures, they show a dramatic and opposite temperature dependence around T(N).