NMR test for glass-freezing dynamics in NiTi strain-glass materials

We studied Ni_50+xTi_50-x with compositions between x = 0.1 and 2.0, by performing ⁴⁷Ti and ⁴⁹Ti nuclear magnetic resonance (NMR) measurements over a temperature range from 4 K to 400 K. Substitutional disorder has been shown to interrupt the displacive phase transition in these materials and induce a reversible freezing process akin to that of spin glasses or relaxor ferroelectrics. The unfrozen state is characterized by cooperative nanoscale dynamical displacements. Our work probes the local dynamics of this system by NMR spin-lattice relaxation time (T₁) measurements, and points to large fluctuation in the parent system as well as in the substituted strain glasses. The Vogel-Fulcher relation has been extensively used to parameterize the relaxation dynamics in glassy systems. However, our results point instead to more standard Arrhenius behavior, implying a nonstandard glass freezing process. High-resolution x-ray measurements indicate no change in the structure across the freezing transition in our samples, even though changes in the NMR shifts show the local difference in electronic behavior. This implies the importance of the distribution of local dynamical behavior in the response of these materials.