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.

A kind of ferromagnetic shape memory alloy with off-stoichiometric composition of Heusler alloy Fe 2 MnGa has been synthesized. By optimizing composition, the martensitic transformation has been modified to occur at about 163 K accompanying spontaneous magnetization, which enables a magnetic field-induced structural transition from a paramagnetic parent phase to a ferromagnetic martensite with high magnetization of 93.8 emu/g. The material performs a quite large lattice distortion through the transformation, ͑c−a͒ / c = 33.5%, causing a shape memory strain upto 3.6%. Such large lattice distortions strongly influence the electron structures, and thus some special physical behavior related to the transport and conductive properties is investigated.

The ferromagnetic exchange interaction between Co and Mn in Heusler alloys has been phenomenologically investigated by analyzing the composition dependence of the magnetic moment and the Curie temperature in a series of quaternary CuCoMnAl alloys. The curves of the composition dependence of the magnetic moment show an interesting valleylike profile and their minima are positioned at different Co contents for different Mn concentrations. The ferromagnetic Co-Mn exchange interaction is a short-range effect which is only effective at the nearest-neighbor distance. At this distance, the exchange interaction can be further enhanced by a Mn-rich composition, but it might be destroyed by the lattice distortion due to the martensitic transformation.