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Theoretical and experimental investigations on atomic and magnetic ordering in full heusler alloys

Topbaşı, Cem
The aim of the study, which was carried out in two main parts, was to investigate the atomic and magnetic ordering in various full Heusler alloy systems.In the theoretical part of the thesis, investigations based on the statiscothermodynamical theory of ordering by means of Bragg-Williams-Gorsky (BWG) method combined with electronic theory in the pseudopotential approximation have been conducted to model order-order (L21↔B2) and order-disorder (B2↔A2) phase transformations in Ni-Mn-C (C=Ga, In, Sb) and A2BGa (A=Fe, Ni, Co; B=Ni, Mn, Co, Fe) Heusler alloys. The partial ordering energies, calculated according to the electronic theory of alloys in pseudopotential approximation for the first two coordination spheres were utilized as input data for the theoretical superlattice formation models based on BWG approximation. Furthermore, the trends of L21↔B2 transition temperatures with electron concentration at A, B and C atomic sites were determined. In the experimental part of the thesis, structural and magnetic properties of Ni-Mn-Ga and Ni-Mn-Al Heusler alloys were investigated. For the Ni-Mn-Ga Heusler alloy system, it was found that the martensitic (TM) and Curie temperatures (TC) merge for Ni54Mn20Ga26 and Ni56Mn18Ga26 alloys by compositional tuning. For the Ni-Mn-Al Heusler alloy system, it was found that ferromagnetism was introduced into these alloys by low temperature aging, as a result of the stabilization of the L21 phase. In addition to that, magnetocaloric effect (MCE) was determined in the vicinity of the first order magneto-structural transition for Ni-rich Ni-Mn-Ga alloys and near the second order magnetic transition for the Ni2MnAl alloy.