Biomedical subjects

P C M Gubbens

Publications and source records attributed to P C M Gubbens.

5 recordsLinked to original sources

PubMed2006

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.

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PubMed2005

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.

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PubMed2003

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.

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PubMed2002

Evidence for weak itinerant long-range magnetic correlations in UGe2.

Positive muon spin relaxation measurements performed on the ferromagnet UGe2 reveal, in addition to the well-known localized 5f-electron density responsible for the bulk magnetic properties, the existence of itinerant quasistatic magnetic correlations. Their critical dynamics is well described by the conventional dipolar Heisenberg model. These correlations involve small magnetic moments.

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PubMed2002

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. We show that the transition corresponds to a first-order change in the fluctuation rate of the Yb3+ 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.

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