  $Ni_{50-x}Co_{x}Mn_{40}Sn_{10}$ 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.
