Influence of boron addition on the microstructure, structural and magnetic properties of Ni53.5Mn26.0Ga20.5 alloy

Investigations on Ni53.5Mn26.0Ga20.5Bx system with x = 0, 0.5 and 1.0 show that boron addition does not alter the monoclinic crystal structure, but modifies the superstructural ordering of the boron-free parent compound. Observed reduction in the martensitic transformation temperature by nearly 30 K differs from the usual trend expected with a rise in electron concentration. Contrary to the reports on Ni-Mn-Ga alloys with boron substitution, saturation magnetization at low temperatures increases substantially (similar to 19% per formula unit) on boron addition. The observed large rise in magnetization is correlated to the compositional shift in the majority matrix phase caused by the secondary phase segregation at the grain boundaries, promoted by addition of B. Formation of Ni-rich secondary phase and the associated reduction in Ni content in the matrix phase results in redistribution of Mn atoms at Mn and Ga sites. This causes an effective change in the balance between Mn moments with ferro- and anti-ferromagnetic order in the unit cell, thus explaining the magnetic properties. (C) 2012 Elsevier Ltd. All rights reserved.