Determination of the positions of impurity centers in a dislocation core in a NaCl crystal from magnetoplasticity spectra

The spectra of the mean free paths $l(\nu)$ of edge dislocations have been studied in NaCl crystals exposed in the electron paramagnetic resonance scheme to the crossed magnetic fields: the Earth's field ($50$ $\mu$T) and the pump field ($2.5$ $\mu$T, $5$–$440$ kHz). The spectra have been measured for a series of angles $\theta=0^\circ{-}5^{\circ}$ of rotation of the sample around its edge [100] with respect to the Earth’s field. The fine structure of the spectra contains a series of peaks whose resonance frequencies are described by the empirical expression $\nu^{\pm}_{i} = A\sin(\theta \pm \Delta\theta_{i}) \approx A(\theta \pm \Delta\theta_{i})$. The parameters $\Delta \theta_i$ are independent of the angle $\theta$ within the experimental errors. Within the model of “frozen” magnetic moments associated with impurity center Ca$^+$–Cl$^0$, the angles $\Delta \theta_i$ characterize the deviation of the axis of the center from the $\langle 100\rangle$ direction in the core of a dislocation. These angles can be expressed in terms of the spectra obtained: $\Delta \theta_i = (\nu^{+}_{i} - \nu^{-}_{i})/2A$. The computer simulation of the edge dislocation core provides the set of the angles $\Delta \theta_i$ close to the measured values. The spin-lattice relaxation time of the center on dislocation has been estimated from the low-frequency edge of the spectrum $l(\nu)$ as $\tau_{s-l}\sim10^{-4}$ s.