Studies On HEUSLER Ferromagnetic Shape Memory Alloys

In my thesis, the Ferromagnetic Shape Memory Alloys CoNiGaX(X=Si, Sb, Fe) have been studied. Structure, magnetic properties and the martensitic transformation behaviors of the alloys have been investigated in detail using XRD, A. C. susptibility, Vibrating Sample Magnetometer and Superconducting Quantum Interference Device.From the studies on Co50Ni20Ga30-xSix(x=0~10) and Co50Ni20Ga30-xSbx(x=0~4) alloys, we found from the XRD pattern that all the alloys mentioned above can form B2 structure. The traditional report about dimension effect for CoNiGa alloys that the martensitic transformation temperature decreses with the increasing of the doped atoms radius is not appropriate in the doped CoNiGa alloys. Dimension effect can be considered from the proportion of the doped atom dimension and the matrix atom dimension. When the proportion of the doped atom dimension and the matrix atom dimension is larger than 1, the martensitic transformation temperature will increase with the decreasing of the mean atom dimension. Otherwise, when the proportion is smaller than 1, the martensitic transformation temperature will increase with the increasing of the mean atom dimension.Comparing Co50Ni20Ga30-xSbx(x=0~4) with Co50Ni22Ga28-xSbx(x=0~6) series alloys, we found that the martensitic transformation temperature of the alloys increase with the increase of Ni content, that is the Ni element can decrease the stability of the austenite. From the XRD patterns we also can find that the compounds with more Ni content are easier to formγphase. From the investigation of CoNiGaFe alloys, we found that the Fe dopants can make B2 phase more stable. Much Ni content will inducedγphase in the compounds. From the magnetization curve we found that compared with Co, Ni and Ga, Fe can increase the saturation magnetiation of the alloy because of the higher magnetic momnet.