Study On Phase Transition And Magnetization Of Ferromagnetic Shape Memory Alloys Ni_51.5Mn₂₅Ga_23.5 And Ni₂FeGa

Highly chemical ordered Heusler alloy Ni2MnGa is one kind of Ferromagnetic Shape Memory Alloy (FSMA), and has received much attention as high performance magnetically controlled actuator material. The application to industry necessitates the various kinds of alloys with various phase transition temperatures. In fact, we can tune the phase transition temperature through changing the content of Ni and Mn in the alloy, however, this way will lead to some other unfavorable things such as the too much changes of the component, or the decreases of the magnetization and Curie temperature. Therefore in this work, we have studied the effects of the doping of IVA, VA and VIA nonmagnetic transition metals on the phase transition properties and magnetization of the parent alloys. Ni_51.5Mn₂₃M₂Ga_23.5(M denotes Ti, Zr, Hf, V, Nb, Ta and W) alloys were prepared by the arc-melting method, and the as-cast ingots were subsequently annealed at 1000℃for 24 h and then at 800℃for 72 h. The A.C. susceptibility measurements reveal that: 1) The phase transition temperature is decreased by the doping of Ti, Zr, Hf, V, Nb and Ta, while increased by the doping of W. 2) By the doping of the same group of elements such as Group IVA(Ti, Zr and Hf) and Group VA(V, Nb and Ta), the phase transition temperature decreases with the increasing period of elements, namely, the increasing radius of atoms. and 3) Compared with the doping of Group VA elements, the doping of Group IVA elements lead to much more decreases of valence electronic concentration of the parent alloys, and as a result, the change of atoms'radius affects the phase transition temperature much more intensively in the latter case. This indicates that the effects of the doping of the nonmagnetic transition metals on the phase transition temperature involve not only the change of valence electronic concentration but also the change of radius of atoms. The measurement of magnetization indicates that: 1) Owing to the dilute effect of the doping, the saturation magnetization and the molecular magnetic moments of doped alloys decrease in all cases. 2) However, not all the doping decreases the magnetic moment of Mn atom. Ignoring the moments of Ni atoms, the doping of Nb, Ta and W enlarge apparently the magnetic moment of Mn atom, which indicates that the doping of nonmagnetic transition metals is not bound to weaken the coupling between the Mn atoms, and will enhances it in some cases. This finding will help to improve the magnetic performance of this kind of material. and 3) The change of magnetic moment of Mn atom and the Curie temperature have no simple relation with the changing of valence electronic concentration and radius of atoms, which may be caused by the structure difference between the austenitic phase in higher temperature and the martensitic phase in the lower temperature, and indicates thatthe effects of doping on magnetic moment of Mn atom should be studied more precisely by electronic structure calculation. Heusler alloy Ni2FeGa is another kind of FSMA. Different from Ni2MnGa, highly chemical ordered Heusler alloy Ni2FeGa cannot be prepared by conventional method (arc-melting + annealing) which can produce only the fcc γphase alloy. Previous work reveals that only by non-conventional method such as melt-spinning we can obtain the highly chemical ordered Heusler alloy Ni2FeGa. This means that in spite of the same chemical formula, the fcc γphase structure is much stabler than the bcc βphase structure, which in this work is interpreted from the point of view of electronic states and energy by the electronic structure calculation and first principle calculation of Ni2FeGa βphase austenitic structure and martensitic structure and γphase structure. The electronic density of state (DOS) calculation indicates that: 1) The ground states of the three structure are ferromagnetic due to the enough DOS near the Fermi energy which meets the Stoner criterion. 2) Comparing the non-magnetic and spin-polarized DOS curves of βphase austenitic structure with those of βphase martensitic structure, we find that as the token of martensitic phase transition, the splitting of eg and t2g peaks in the DOS of Ni atom occurs only in the spin-polarized case, which indicates that the matensitic phase transition takes place below the Curie temperature. and 3) Comparing the DOS curves of bcc βphase austenitic structure and those of fcc γphase structure, we find that in the latter case the Fermi energy is situated in the so-called pseudogap, which indicates the lower energy...