Ni-Mn-In Alaşımında Atomik Kompozisyonun Malzemenin Kristal Yapısı Ve Manyetik Özelliklerine Etkisi
Özet
Recent studies have revealed a new magnetic property in Ni-Mn based Heusler alloy, especially for the composition of Ni50Mn45In5. It has been observed that two distinct phases with different crystal symmetries and magnetic properties are formed in the Ni50Mn45In5 alloy when annealed in temperature range of 600-750 K under a magnetic field in the range 0,5-4 T Formation of these magnetic and structural changes can be attributed to martensitic and inter-martensitic transitions. The cubic structure (L21), highly symmetrical at high temperatures, turns into a non-symmetrical tetragonal L10 structure (or 5M, 7M) by lowering the temperature, and this mechanism is known as martensitic transition. The Heusler alloys can be characterized in terms of martensitic and inter-martensitic transformations.
In this study, the martensitic and inter-martensitic transformations have been investigated experimentally in polycrystalline NixMnyInz alloys prepared at different constituent compositions, 49,39 ≤ x ≤ 50,26, 32,60 ≤ y ≤ 42,85 and 7,63 ≤ z ≤ 17,20. A full understanding of martensitic and inter-martensitic transformations, and the phases formed before and after the transition have been characterized with x-ray diffraction (XRD), resistance (R), magnetization (M) measurements as a function of temperature. Lattice parameters of crystal structure of NixMnyInz specimens was analyzed with Rietveld refinements applied to XRD patterns collected in the temperature range 110-300 K. The characteristic hysterical martensitic phase transition was observed in field-cooled and field-warmed magnetization measurements (M(T)) and electrical resistance measurements (R(T)) carried out in the temperature range 10-380 K. The martensitic start (MS) and martensitic final (MF) temperatures, and ferromagnetic (FM) ¬¬ anti-ferromagnetic (AFM) phase transition temperatures were determined from experimental data obtained. Hysteretic phase transitions were interpreted in terms of AFM and FM interactions in the alloys.
The phase diagram of NixMnyInz alloys showing the L10, 5M, 7M phases as a function of valence electron concentrations (e/a value) formed by combining the results obtained from the M(T) and R(T) measurements. These alloys, cubic at high temperatures, have been transformed to the 5M, 7M meta-stable states and L10 (tetragonal) structures with decreasing temperature. The phase diagram for the NixMnyInz Heusler alloy studied in this thesis was compared with the phase diagrams of Ni50Mn50-xGax and Ni50Mn50-xSnx published previously in the literature.
