Microscopic understanding of disorder induced ferromagnetism in B2-alloy thin films II (MUMAGI II)

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Magnetic phase transitions that are sensitive to small structural variations can be used to directly probe the correlation between magnetic ordering and the atomic arrangement. In materials where such transitions can be spatially confined, there is the additional possibility to investigate the magnetic behaviour at the interfaces of the spatial confinement. The above opportunities exist in certain alloys of the chemically ordered B2 phase that can be disordered to the A2 phase. In paramagnetic B2-Fe60Al40 and in antiferromagnetic B2-Fe50Rh50, subtle switching of the Fe site with the Al or Rh sites respectively is sufficient to induce a large ferromagnetic saturation magnetization. The induced magnetization has been attributed to increased Fe-Fe nearest-neighbour interactions. This phenomenon of disorder induced ferromagnetism has been known for several decades; however its investigations have largely been restricted to bulk materials. There still remain open questions on its precise mechanism, for example, a change in the lattice parameter with disorder has been observed, however its contribution to the induced magnetization has not been clarified. Furthermore, the magnetic structure at the interfaces of locally disorder regions is unclear. We will approach these problems using light ion-irradiation (e.g. Ne+) as an effective tool to systematically disorder thin film materials. Furthermore, ion-beams can be focused to nm dimensions for generating ferromagnetic nanostructures and order/disorder (B2/A2) phase boundaries. In the proposed 36-month period, with the resource of two PhD students, we will deploy ion-irradiation to investigate disorder induced ferromagnetism along two intertwined approaches: at the HZDR, we will investigate the interfaces of ion-induced ferromagnetic regions by characterizing their magneto-resistive properties. Dipolar fields of the ferromagnetic A2-structures will be investigated as a means to influence parameters such as susceptibility and transition temperature of the surrounding B2-ordered material. At the UDE systematically disordered thin films of B2 -Fe60Al40 and -Fe50Rh50 and track their magnetic and structural changes using X-ray and Mössbauer spectroscopy, thereby achieving a microscopic understanding of the effect of atomic rearrangements on ferromagnetism emerging respectively from paramagnetic and antiferromagnetic precursors. This project will lay the foundation for future investigations on the mechanisms and interface effects in materials exhibiting disorder induced ferromagnetism.