Phase separation and superparamagnetism in the martensitic phase of Ni50-xCoxMn40Sn10

Ni50-xCoxMn40Sn10 shape memory alloys in the approximate range 5 <= x <= 8 display desirable properties for applications as well as intriguing magnetism. These off-stoichiometric Heusler alloys undergo a martensitic phase transformation at a temperature T-M of 300-400 K, from ferromagnetic (FM) to nonferromagnetic, with unusually low thermal hysteresis and a large change in magnetization. The low temperature magnetic structures in the martensitic phase of such alloys, which are distinctly inhomogeneous, are of great interest but are not well understood. Our present use of spin echo nuclear magnetic resonance in the large hyperfine fields at Mn-55 sites provides compelling evidence that nanoscale magnetic phase separation into FM and antiferromagnetic (AFM) regions occurs below T-M in alloys with x in the range 0 to 7. At finite Co substitution, the FM regions are found to be of two distinct types, corresponding to high and low local concentrations of Co on Ni sites. Estimates of the size distributions of both the FM and AFM nanoregions have been made. At x = 7, the AFM component is not long-range ordered, even below 4 K, and is quite different from the AFM component found at x = 0; by x = 14, the FM phase is completely dominant. Of particular interest, we find for x = 7 that field cooling leads to dramatic changes in the AFM regions. These findings provide insight into the origins of magnetic phase separation and superparamagnetism in these complex alloys, particularly their intrinsic exchange bias, which is of considerable current interest.