Magnetically nanostructured state in a Ni-Mn-Sn shape-memory alloy

For certain compositions Ni-Mn-Sn and related magnetic shape-memory alloys undergo a martensitic transition at temperatures in the range 300--400 K, with the emergence of novel magnetic properties below the transition. While $\mathrm{N}{\mathrm{i}}_{50}\mathrm{M}{\mathrm{n}}_{50}$ is an antiferromagnet, substitution of Sn on some fraction of the Mn sites in $\mathrm{N}{\mathrm{i}}_{50}\mathrm{M}{\mathrm{n}}_{50\ensuremath{-}x}\mathrm{S}{\mathrm{n}}_{x}$ leads to competing ferromagnetic (F) and antiferromagnetic (AF) phases at low temperatures. Details of this magnetic phase coexistence are, however, significantly lacking, particularly with respect to the AF phase. The present investigations use zero applied magnetic field $^{55}\mathrm{Mn}$ NMR as a local probe of the magnetic properties of the alloy $\mathrm{N}{\mathrm{i}}_{50}\mathrm{M}{\mathrm{n}}_{50\ensuremath{-}x}\mathrm{S}{\mathrm{n}}_{x}$ with $x=10$. Rich multipeak spectra are observed, and the various components are definitively assigned to nanoscale F or AF regions. Measurements of the static nuclear hyperfine field distributions as a function of temperature, and in small applied fields, together with nuclear relaxation rates provide detailed information on the size distributions, relative concentrations, and physical natures of these F and AF regions. The results show that the nanoscale magnetic features of the $x=10$ system are substantially more complex than previous studies have suggested. We argue that the general approach used in these experiments is applicable to other such complex metal alloys, and could yield many additional insights.