Polarized Neutron studies of Spin Dynamics of Functional Materials

Giovanni Pasquino

Research output: ThesisDoctoral Thesis

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Three technologically important materials were studied using polarised neutron techniques. The first, INVAR, is widely used in devices that exploit its low thermal expansion. The second, sodium-cobaltate is an example of a new class of correlated thermoelectric materials. The third, manganese vanadate has potential for devices that exploit the couple between its spin and orbital degrees of freedom. Spin dynamics in INVAR have been studied using the quasi-elastic neutron spin echo technique. Spin dynamics with a characteristic time scale of nanoseconds has been found. Additional muon spin relaxation investigations have confirmed this result and, in addition, revealed the presence of second time scale in the range 0.1 microseconds. The origin of the nanosecond scale may be associated with individual spin excitations, while the slower scale may be associated with the dynamics of magnetic in-homogeneities, in agreement with previous polarised neutron results in the literature. This second component of spin dynamics may be important in understanding the INVAR effect. The spin-wave dispersion of sodium cobaltate was measured by inelastic neutron scattering using XYZ polarization analysis in order to separate the magnetic signal from the structural scattering. Excellent agreement with the low energy excitations was obtained with calculations using the McPhase program for this itinerant system. In particular, it was possible to explain the energy gap via the anisotropy of the exchange coupling. McPhase calculations were performed for manganese vanadate, and it was possible to understand the magnetic excitations observed previously from this system. We obtained a new magnetic structure that differs from the existing literature. The calculated magnetic excitations from this ground state are in good agreement with the experimental data. We are able to rule out the previously published model, and our exchange constants are consistent theoretical predictions on the basis of the proposed orbital ordering.
Original languageEnglish
Awarding Institution
  • Royal Holloway, University of London
  • Goff, Jon, Supervisor
Award date1 Dec 2015
Publication statusUnpublished - 2015


  • Polarized neutrons

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