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

[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.