Magnetic and Structural Transitions Tuned through Valence Electron Concentration in Magnetocaloric Mn(Co_1–xNi_x)Ge

The structural and magnetic properties of magnetocaloric Mn(Co 1– x Ni x )Ge compounds have been studied. Two responses to the increase of valence electron concentration on substitution of Ni (3d 8 4s 2 ) for Co (3d 7 4s 2 ) in the orthorhombic phase ( Pnma ) are proposed: expansion of unit-cell volume and redistribution of valence electrons. We present experimental evidence for electronic redistribution associated with the competition between magnetism and bonding. This competition in turn leads to complex dependences of the reverse martensitic transformation temperature T M (orthorhombic to hexagonal ( P6 3 /mmc )) and the magnetic structures on the Ni concentration. Magnetic transitions from ferromagnetic structures below x = 0.50 to noncollinear spiral antiferromagnetic structures above x = 0.55 at low temperature (e.g., 5 K) are induced by modification of the density of states at the Fermi surface due to the redistribution of valence electrons. T M is found to decrease initially with increasing Ni content and then increase. Both direct and inverse magnetocaloric effects are observed.