Phase Separation and Superparamagnetism in the Martensitic Phase of $Ni_{50-x}Co_{x}Mn_{40}Sn_{10}$

TL;DR

This study investigates nanoscale magnetic phase separation and superparamagnetism in Ni-Co-Mn-Sn shape memory alloys using spin echo NMR, revealing how Co substitution influences magnetic structures and properties.

Contribution

It provides new insights into the magnetic phase separation and superparamagnetic behavior in Ni-Co-Mn-Sn alloys, highlighting the effects of Co content on magnetic nanoregions.

Findings

Nanoscale ferromagnetic and antiferromagnetic regions coexist below T_M.

Co substitution results in distinct high and low Co concentration F regions.

Field cooling significantly alters AF regions at x=7.

Abstract

$Ni_{50-x}Co_{x}Mn_{40}Sn_{10}$ shape memory alloys in the approximate range $5 \le x \le 10$ 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 (F) 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 NMR, in the large hyperfine fields at $^{55}Mn$ sites, provides compelling evidence that nanoscale magnetic phase separation into F and antiferromagnetic (AF) regions occurs below $T_{M}$ in alloys with x in the range 0 to 7. At finite Co substitution the F 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 F and AF nanoregions have been made. At x = 7 the AF component is not long-range ordered, even below 4 K, and is quite different to the AF component found at x = 0; by x = 14 the F phase is completely dominant. Of particular interest, we find, for x = 7, that field cooling leads to dramatic changes in the AF 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.