We study the electronic structures and magnetic properties of Mn 2 CoZ ͑Z =Al,Ga,In,Si,Ge,Sn,Sb͒ compounds with Hg 2 CuTi-type structure using first-principles full-potential linearized-augmented plane-wave calculations. It is found that the compounds with Z = Al, Si, Ge, Sn, and Sb are half-metallic ferrimagnet. Experimentally, we successfully synthesized the Mn 2 CoZ ͑Z =Al,Ga,In,Ge,Sn,Sb͒ compounds. Using the x-ray diffraction method and Rietveld refinement, we confirm that these compounds form Hg 2 CuTi-type structure instead of the conventional L2 1 structure. Based on the analysis on the electronic structures, we find that there are two mechanisms to induce the minority-spin band gap near the Fermi level, but only the d-d band gap determines the final width of the band gap. The magnetic interaction is quite complex in these alloys. It is the hybridization between the Mn͑C͒ and Co atom that dominates the magnitude of magnetic moment of the Co atom and the sign of the Mn͑B͒-Co exchange interaction. The Mn 2 CoZ alloys follow the Slater-Pauling rule M H = N V − 24 with varying Z atom. It was further elucidated that the molecular magnetic moment M H increases with increasing valence concentration only by decreasing the antiparallel magnetic moment of Mn͑C͒, while the magnetic moments of Mn͑B͒ and Co are unaffected.

It is shown that a temperature window between the Curie temperatures of martensite and austenite phases around the room temperature can be obtained by a vacancy-tuning strategy in Mn-poor Mn 1-x CoGe alloys (0 ≤ x ≤ 0.050). Based on this, a martensitic transformation from paramagnetic austenite to ferromagnetic martensite with a large magnetization difference can be realized in this window. This gives rise to a magnetic-field-induced martensitic transformation and a large magnetocaloric effect in the Mn 1-x CoGe system. The decrease of the transformation temperature and of the thermal hysteresis of the transformation, as well as the stable Curie temperatures of martensite and austenite, are discussed on the basis of the Mn-poor Co-vacancy structure and the corresponding valence-electron concentration.

With a high Curie temperature and low entropy change, the magnetic-field-induced martensitic transformation has been obtained in ferromagnetic shape memory alloys MnNiGa by doping a small amount of Co. Due to the ferromagnetic activation effect of Co, a large amount of antiferromagnetically aligned Mn moments are turned into ferromagnetic ordering, which is verified by our electronic structural calculation and experimental observation. Consequently, the magnetization rises up to 70emu∕g and the magnetization difference between two phases increases about ten times, resulting in a considerable dT∕dH of 4K∕T and a well-defined reversed transformation induced by a magnetic field.

Quaternary Heusler alloy Ni2(Mn,Fe)Ga has been studied systematically for the structure, martensitic transformation, and magnetic properties in two systems of Ni50.5Mn25−xFexGa24.5 and Ni50.4Mn28−xFexGa21.6. Substituting Fe for Mn up to about 70%, the pure L21 phase and the thermoelastic martensitic transformation still can be observed in these quaternary systems. Iron doping dropped the martensitic transformation temperature from 220 to 140 K, increased the Curie temperature from 351 to 429 K, and broadened the thermal hysteresis from about 7 to 18 K. Magnetic analysis revealed that Fe atoms contribute to the net magnetization of the material with a moment lower than that of Mn. The temperature dependence of magnetic-field-induced strains has been improved by this doping method.

A two-way magnetic field controlled shape memory effect has been observed in single crystals of CoNiGa with martensitic transformation temperature ranging from 205 to 341 K. Two-way shape memory with −2.3% strain has been obtained in free samples. By applying a bias field of up to 2 T, the shape memory strain can be continuously controlled from negative 2.3% to positive 2.2% giving it a total strain of 4.5%. The magnetic properties of CoNiGa show that it is a good shape memory material working at relatively high temperature of up to 450 K, and has a lower magnetic anisotropy than NiMnGa.