Domains, phase coexistence and extinction phenomena in helical and modulated antiferromagnets

Using neutron diffraction and topography techniques the phase transitions, domain structures and phase coexistence of a range of modulated antiferromagnetic materials have been studied. (i) In chromium-0.6 atomic percent silicon (a modulated antiferromagnet) an inhomogeneous silicon content was detected by analysing backscattered x-rays from an electron beam. The variation of the magnetic intensity detected on a series of neutron topographs and the broadening of the first order transitions of Cr-0.6 at.% Si (as observed by neutron diffraction and elastic constant measurements), were attributed to this variation of silicon concentration within the sample. (ii) The helimagnetic-ferromagnetic phase boundaries of terbium and manganese phosphide have been imaged using neutron and synchrotron topography under both zero and applied field conditions. Their resultant shapes are explained using a dimensionless parameter (e), representing the ratio of the competing energy terms of the system, and taking into account the applied field, the ferromagnetic volume and sample dimensions. The nucleation and growth of ferromagnetic regions isolated in the helimagnetic phase of a single crystal MnP sample have been studied under the application of a temperature gradient. With an additional applied field gradient the coexistence of three magnetic phases together with the associated triple point were observed. The third (fan) phase was nucleated from within the helimagnetic-ferromagnetic interface on the application of a b axis field. The Ferro-Fan and Ferro-Heli interface shapes can be understood in terms of magnetostatic and elastic energy considerations whilst the Heli- Fan interface is as yet not understood. (iii) An anomalous increase, at around 20K in the integrated intensity of the 002±t magnetic reflections of holmium has been modelled by the RED extinction theory. The resultant parameters are discussed in relation to the coexistence of regions of different interplanar turn angle as seen near the commensurate lock-in structures described in the spin slip model. The magnetisation data of this sample shows a field dependent, relative maximum of the c axis component of the magnetic moment below 20K. This is explained by the stabilisation of commensurate states as observed in the neutron diffraction data. (iv) On cycling in temperature within the helimagnetic phase of MnP the reduction of the magnetic integrated intensity is interpreted (through a secondary extinction treatment) as a result of an increase in the average chirality domain size (from 8 to 10 μm). A mechanism is proposed for the temperature driven movement of chirality domain walls, which also accounts for the possibility of domain wall annihilation, leading to an increase in the average chirality domain size of the sample.