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The changes of crystal structure, magnetic structure, electronic structure in the martensitic phase transition for magnetic shape memory alloy Mn2NiGe are calculated by first-principles method. The results show that in the martensitic phase transition for Mn2NiGe, there is produced a Jahn-Teller distortion, in which the c-axis becomes longer but a-axis and b-axis turn shorter, forming an elongated octahedral geometry. There is a significant change in magnetic moment for Mn ion in the centre of octahedron, but a little change happens to the Ni and Ge ions that are regarded as a ligand. The energy levels of eg and t2g are split by redistributing the density of states for d electrons and so opening a pseudogap near the fermi energy due to lattice distortion.

The change rules of electronic structure and magnetism in the process of transform from a cubic structure to a tetragonal structure and their responses to pressure for Hg2CuTi-type Mn2NiAl are studied by first principles method based on the density functional theory. The study shows that in the process of transform from a cubic austenite phase to a tetragonal martensite phase, the shift of the density of states of occupied state to ward lower energy in the martensitic phase results from the enhanced Ni-Mn(A) hybridization caused by a decrease in the Ni-Mn(A) distance, and it is the reason for the stabilization of the martensitic phase. In the process of transform from the austenite phase to the martensite phase, the bonding interaction becomes stronger, owing to energy band broadening, and it can improve the stabilization in the martensitic phase. In the process of tetragonal distortions, the change of total magnetic moment is determined by the change of magnetic moment of Ni atom. The bulk modulus at zero pressure of Mn2NiAl is calculated to be 125.69 GPa, so that we expect Mn2NiAl to be the more compressible material than the familiar Heusler alloys.

Tetragonal distortions,crystal structure,magnetism,electronic structure and pressure response of Hg2CuTi-type Mn2NiGe are calculated by the first-principles method based on the density functional theory. The calculations show that 1)in the process of transform from cubic to a tetragonal in structure,Mn2NiGe alloys exhibit a stable martensitic phase at c/ a≈1.34; 2)in austenite and martensite,Mn atoms are the main contributors to the magnetism in Mn2NiGe,and Mn2NiGe alloys show ferrimagnetism due to antiparallel but unbalanced magnetic moments of Mn(A) atom and Mn(B) atom; 3)in austenite and martensite,the direct d-d exchange interaction between Mn(A) atom and Mn(B) atom is weak because of small overlap of d-projected DOS of Mn(A) atom and Mn(B) atom nearby the Fermi level. This is the reason why the Mn2NiGe alloys show ferrimagnetism. We can predict the magnetic shape memory effect (MSME) in Mn2NiGe alloys based on the the results of calculation above.

Considering the electrostrictive，magnetostrictive effects in the ferroelectric （FE） phase and ferromagnetic （FM） phase and the elastic stress arising from FE/FM and film/substrate interfaces，using the Landau-Devonshire thermodynamic theory，we investigate the magnetoelectric coupling effect in epitaxial 1-3 type multiferroic nanocomposite thin films. We renormalize the dielectric and magnetic coeffcients twice and obtain the quasi-intrinsic magnetoelectric coupling in Landau free energy function of this multiferroic system. Then，the electrical-field-induced magnetization and polarization for films of different thickness in 1-3 type BaTiO3-CoFe2O4 nanocomposite system are analyzed. Our results show that the relaxation of the in-plane compressive stress of the thin film makes magnetization and polarization both decrease with the increasing film thickness，and an applied electric field can result in the reversal of magnetization in step with the switching of the electric polarization due to the out-of-plane elastic coupling between the interface of the FE and FM phases. Our results agree well with the experimental results.