  We studied Ni50+xTi50-x with compositions up to x = 2, performing 47Ti and
49Ti nuclear magnetic resonance (NMR) measurements from 4 K to 400 K. For large
x in this system, a strain-glass appears in which frozen ferroelastic
nano-domains replace the displacive martensite structural transition. Here we
demonstrate that NMR can provide an extremely effective probe of the strain
glass freezing process, with large changes in NMR line-shape due to the effects
of random strains which become motionally narrowed at high temperatures. At the
same time with high-resolution x-ray diffraction we confirm the lack of
structural changes in x >= 1.2 samples, while we show that there is little
change in the electronic behavior across the strain glass freezing temperature.
NMR spin-lattice relaxation time (T1) measurements provide a further measure of
the dynamics of the freezing process, and indicate a predominantly thermally
activated behavior both above and below the strain-glass freezing temperature.
We show that the strain glass results are consistent with a very small density
of critically divergent domains undergoing a Vogel-Fulcher-type freezing
process, coexisting with domains exhibiting faster dynamics and stronger
pinning.
