Evidence for non-collinear magnetic structure at low temperature in M-type hexaferrite

The field of magnetoelectric (ME) multiferroics is driven by the promise of controlling magnetism using applied electric fields, offering the possibility of a new generation of ultra-low power devices. Among the few room temperature single-phase ME multiferroics reported, M-type hexaferrite shows potential for device applications as they exhibit a low field ME effect at room temperature. The observed magnetoelectric properties in Co-Ti doped strontium M-type hexaferrrite are associated with the presence of the non-collinear magnetic structure that induces an electric polarisation through the inverse Dzyaloshinskii-Moriya interaction. Many studies reported the presence of the non-collinear magnetic structure with different elements and substitution rates. Hence, the non-collinear magnetic structure is the key to understanding the magnteoelectric properties in M-type doped hexaferrrites. The work presented in this thesis had two main objectives. The first was to investigate the role of variation (Co-Ti) substitution in strontium ferrite polycrystalline hexaferrite on magnetic properties and their temperature dependence. The second was to characterise the structural and magnetic properties of doped strontium ferrite single crystal, where the substitution induced magnetic anisotropy changes from uniaxial to planar anisotropy. In addition, to investigate the role of the strain of doped strontium ferrite epitaxial thin films on magnetic properties. A spontaneous transition from a collinear ferrimagnetic structure to a noncollinear magnetic structure has been detected in temperature dependent magnetisation measurements. This transition temperature, Tp is sensitive to the substitution rate as well as the measuring field. XMCD analysis reveals that the spin orientation of the magnetic ions (Fe, Co) at specific site symmetries vary with the thickness of the thin film.