The Ni2MnSn-derived Ni50-xCoxMn25+ySn25-y alloys are premier examples of a class of off-stoichiometric Heusler alloys recently discovered to exhibit attractive magnetic properties in tandem with extraordinarily reversible martensitic phase transformations. Multiferroicity, magnetic phase competition and separation, field-induced martensitic transformations, magnetic shape memory behavior, and sizable magneto-, elasto-, and barocaloric effects result, generating substantial interest and application potential. In this work we expand on a prior small-angle neutron scattering (SANS) study at a single composition (Ni44Co6Mn40Sn10) by exploring all three main regions of the recently established Ni50-xCoxMn40Sn10 phase diagram, i.e., at the representative y=15 composition. Wide temperature and scattering wave-vector range (20-500K, 0.004-0.2Å-1) SANS data on x=2, 6, and 14 polycrystals provide a detailed picture of the evolution in magnetic order and inhomogeneity. Consistent with recent studies with a variety of techniques, phase separation into short-range coexisting ferromagnetic and antiferromagnetic regions is deduced below the martensitic transformation at x=2 and 6, with average ferromagnetic cluster spacing of ∼13 nm. Remarkably, at x=14, where the martensitic transformation is suppressed and ferromagnetic austenite is stabilized to low temperatures, nanoscopic magnetic inhomogeneity nevertheless persists. Distinct ferromagnetic clusters (∼36-nm average spacing) in a ferromagnetic matrix are observed at intermediate temperatures, homogenizing into a uniform long-range ordered ferromagnet only at low temperatures. This unusual ferromagnet cluster/ferromagnet matrix inhomogeneity, as well as x-dependent subtleties of the superparamagnetic freezing of ferromagnetic clusters, are discussed in light of Mn55 nuclear magnetic resonance data, and the recent observation of annealing-induced core/shell nanoprecipitates. The origins of nanoscale magnetic inhomogeneity are discussed in terms of statistical variations in local composition and structure, tendency to chemical phase separation, and other forms of disorder.
Bibliographical noteFunding Information:
Work supported primarily by the US Department of Energy through the University of Minnesota Center for Quantum Materials under Grant No. DE-SC-0016371. The contribution of R.D.J. was supported by a Vannevar Bush Fellowship. We acknowledge the support of the National Institute of Technology, US Department of Commerce, in providing the neutron facilities used in this work. S.E.-K. acknowledges travel support from AUS (Grants No. FRG-2014 and No. FRG-2015).
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