Structural and magnetic investigations in the vicinity of first-order transformations in Ni–Mn–Ga–Co ferromagnetic shape memory alloys

Abstract

Among the series of alloys derived from Ni₅₀Mn₂₉Ga₂₁ on selective substitution of Co for Ni and Mn, two alloys Ni_49.8Mn_27.2Ga_21.2Co_1.8 and Ni_46.9Mn_28.8Ga₂₁Co_3.3 referred to as CoMn-1.8 and CoNi-3.3, respectively, are found to exhibit an additional first-order transformation below their martensitic transformation temperatures. Systematic studies on temperature and field dependence of magnetic properties of these alloys are carried out, through the transformations, to understand their origin. An examination of these results in conjunction with those from structural investigations reveals that the transformation in the CoMn-1.8 alloy is an intermartensitic transformation and has a structural origin, while that in the CoNi-3.3 alloy is not of the structural origin and is attributed to local spin inversion of Co moments, which is of the magnetic origin.

Keywords:

• ferromagnetic shape memory alloys
• martensitic transformations
• magnetostructural transformation
• Ni–Mn–Ga–Co alloys

Acknowledgements

Funding of research projects ADA/AF/DISMAS/163/2005 from Aeronautical Development Agency (ADA), Bangalore, and 03(1065)/06/EMR-II from CSIR are acknowledged. M. Ramudu acknowledges the UGC-RFSMS for financial support in the form of fellowship. Authors acknowledge funding from DST under Centre for Nanotechnology for PPMS measurements and UGC-CAS for low-temperature cryostat.