Journal of Advanced Materials in Engineering (Esteghlal)

In this investigation, Ni₄₇Mn₄₀Sn₁₃ ferromagnetic shape memory alloy was prepared by mechanical alloying. The metal powders were ball milled in argon atmosphere for 20 hours. X-ray diffraction pattern confirmed formation of crystalline structure of Heusler alloy. As-milled powder samples were sealed in quartz tubes under high vacuum and subjected to heat treatments at 950°C for different time durations. Then, the effect of isothermal ageing on structural, magnetic and electrical properties of samples was investigated. Results of electrical resistance displayed a metal-like behavior around martensitic transformation. The results showed that 16 hours of annealing was the optimal time for producing this alloy which could be an appropriate candidate for magnetic refrigerant.

In this paper, the phase formation process of Mn_2.5Ge samples, prepared by mechanical alloying of Mn and Ge metal powders and annealing, has been studied. Results showed that in the milled samples the stable phase is Mn₁₁Ge₈ compound with orthorhombic structure and Pnam space group. The value of saturation magnetization increases by increasing milling time from 0.2 up to 1.95 (Am²Kg^-1). The remanece of the samples increases by increasing the milling time while the coercivity decreases. Annealing of 15-hour milled sample results in disappearance of Mn and Ge and the formation of new phases of Mn₃Ge, Mn₅Ge₂, Mn₅Ge₃ and Mn_2.3Ge. Mn₃Ge is the main phase with Do₂₂ tetragonal structure and I4/mmm space group which is stable and dominant. The enhancement of saturation magnetization in the annealed sample is related to the formation of three new magnetic phases and the increase of coercivity is due to the presence of Mn₃Ge compound with tetragonal structure. Studies were replicated on samples made by arc melting method to compare the results and to investigate the effect of the preparation method on phase formation and structural and magnetic properties of the materials. In these samples the saturation value was in range of 0.2 up to 1.95 (Am²Kg^-1) depending on preparation methods. Rietveld refinement shows that Mn_2.3Ge sample prepared from arc melted under 620^oC anealing is single phase. Magnetic analysis of this sample show a saturation magnetization of 5.252(Am²Kg^-1) and 0.005 T coercive field.

In this work, cobalt ferrite nanocrystallites were synthesized by air annealing of milled Co-Fe compound. Effect of annealing temperature on phase formation of cobalt ferrite and structural and magnetic properties of the product was studied. Analysis of annealed sample in 450 ^oC showed that around 46 weight percent of the specimen was changed to Co₂FeO₄. This value increased to 95 and 90% for 800 ^oC and 900 ^oC annealed samples respectively. Reduction of saturation magnetization under annealing was related to transformation of Co-Fe to cobalt ferrite. Increasing the value of saturation magnetization in 900 ^oC annealed sample compared to 800 ^oC one was attributed to decreased surface to volume ratio and crystallite size. The main reason of occurrence of maximum coercivity in 800  ^oC annealed sample was its low crystallite size.