Magnetic exchange interactions in compressed CrO2 and Co/CuMn multilayers studied with polarised x-rays and neutrons

Nicholas Loh

    Research output: ThesisDoctoral Thesis

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    [Truncated abstract] In this thesis the nature of the exchange interactions which underlie ordered phases in magnetic materials is investigated by measuring, with polarised x-ray and neutron scattering techniques, changes in atomic and mesoscopic scale magnetic order which occur as temperature or pressure is varied. Two systems in particular have been studied; double- and super-exchange interactions in the complex ferromagnetic oxide CrO2 under increasing hydrostatic pressure and the temperature dependence of interlayer exchange interactions in Co/Cu0.94Mn0.06 multilayers. In the case of CrO2 under pressure, changes in several bond related aspects including bandwidth, crystal field splitting and the exchange coupling constants are expected to a affect electronic correlations responsible for long range magnetic ordering. Indeed polarised X-ray spectroscopy measurements from CrO2 powder compressed within a diamond anvil cell presented in this thesis indicate a loss of spontaneous magnetisation that can be attributed to a gradual reduction of the ferromagnetic ordering temperature, TC. Conservative extrapolation indicates that the critical pressure for the complete disappearance of ferromagnetic ordering, PC, is in the range 45±11 GPa which is towards the lower end of what has been predicted previously by band structure calculations. Arguments based on the Goodenough-Kanamori exchange rules suggest that enhanced antiferromagnetic superexchange interactions at pressure due to changes in bond geometry are responsible for the suppression of ferromagnetism in CrO2.
    Original languageEnglish
    QualificationDoctor of Philosophy
    Publication statusUnpublished - 2011


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