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

摘要

$Ni_{50-x}Co_{x}Mn_{40}Sn_{10}$ shape memory alloys in the approximate range$5 \le x \le 10$ display desirable properties for applications as well asintriguing magnetism. These off-stoichiometric Heusler alloys undergo amartensitic phase transformation at a temperature $T_{M}$ of 300 - 400 K, fromferromagnetic (F) to nonferromagnetic, with unusually low thermal hysteresisand a large change in magnetization. The low temperature magnetic structures inthe martensitic phase of such alloys, which are distinctly inhomogeneous, areof great interest but are not well understood. Our present use of spin echoNMR, in the large hyperfine fields at $^{55}Mn$ sites, provides compellingevidence 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. Atfinite 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. Estimatesof 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 quitedifferent to the AF component found at x = 0; by x = 14 the F phase iscompletely dominant. Of particular interest, we find, for x = 7, that fieldcooling leads to dramatic changes in the AF regions. These findings provideinsight into the origins of magnetic phase separation and superparamagnetism inthese complex alloys, particularly their intrinsic exchange bias, which is ofconsiderable current interest.