Superparamagnetism in the martensitic phase of the magnetic shape-memory alloys Ni$_{50-x}$Co$_{x}$Mn$_{40}$Sn$_{10}$

The Ni-Mn based shape magnetic memory alloys have attracted considerable attention because of their interesting magnetic properties, including intrinsic superparamagnetism and intrinsic exchange-bias effects, which are found in the martensitic phase of these materials. Here, we report on the results of zero-field $^{55}$Mn NMR measurements made on the alloys Ni$_{50-x}$Co$_{x}$Mn$_{40}$Sn$_{10}$ with x=7 and x=14. The results show that Co substitution not only changes the electronic configuration of a fraction of the Mn ions but also alters the magnetic interaction amongst these ions, leading to marked changes in the low temperature antiferromagnetic and ferromagnetic components compared to x=0. For both x=7 and x=14 our analysis shows that the Mn ions form a new ferromagnetic nanoclusters in small Co rich regions of the sample. Based on the temperature dependence of the NMR spectral features, we propose a method to estimate the superparamagnetic cluster size distribution.