Extraordinary magnetic and structural properties of the off-stoichiometric and the Co-doped Ni2MnGa Heusler alloys under high pressure

To show and to describe a contrast between the magnetic and structural properties of the stoichiometric Ni2MnGa alloy and the off-stoichiometric and the Co-doped Ni-Mn-Ga alloys, we report the effect of high hydrostatic pressure (up to 1.2 GPa) on magnetization and the critical temperatures of structural and magnetic transitions in the off-stoichiometric Ni50−xMn25+x+yGa25−y and the Co-doped Ni50−xCoxMn25+yGa25−y Heusler alloys. A huge decrease in their saturation magnetization MM(5 K) under pressure was observed with an extreme value of pressure parameter d ln MM(5 K)/dP = −56 × 10−3 GPa−1 in the case of the Ni39Co11Mn34Ga16 alloy. The Curie temperature TCA increases under pressure in all the studied alloys. Almost identical values of dTCA/dP = +4.7 ÷ +5.9 K GPa−1 point to a normalized steepness of a dependence of exchange interactions on interatomic distances. The values of paramagnetic effective moments, meff, are identical in both structural phases of the alloys and they lie in a narrow interval from 4.6 μB to 5.7 μB. Furthermore, the Co-doped alloys show a very strong pressure dependence on the structural critical temperature TM-A, with values of dTM-A/dP up to +34 K GPa−1, the highest reported values for the Ni-Mn-based Heusler alloys. The results are discussed in light of the fundamental magnetic interactions in the Heusler alloys and from the point of view of a further exploration of their multifunctional applications.