Anisotropic resonant magnetoplasticity of NaCl crystals in the Earth’s magnetic field

Resonant relaxation of the dislocation structure under the action of crossed magnetic fields, i.e., constant magnetic field of the Earth $({\mathbf B}_{\mathrm{Earth}})$ and alternating radio-frequency field $(\widetilde{\mathbf B})$, has been experimentally studied in a series of dielectric (NaCl) crystals with various compositions of impurities under variations in the frequency, direction of the pumping field $\widetilde{\mathbf B}$, and orientation of the samples in the Earth’s magnetic field. The frequency dependence of the dislocation path length $l(\nu)$ exhibits peaks with various heights $(l_{\mathrm{\max}})$ and resonant frequencies $\nu_{\mathrm{res}}$. The maximum resonant effect has been observed for dislocations with the direction $\mathrm L$ orthogonal to the plane of crossed magnetic fields in a configuration of mutually perpendicular vectors $\{{\mathbf L},\widetilde{\mathbf B},{\mathbf B}_{\mathrm{Earth}}\}$ belonging, together with sample edges $\{\mathbf a,\mathbf b,\mathbf c\}$, to the $\langle$100$\rangle$ system. Variation of the concentration $C$ of calcium impurity in crystals of the $\mathrm{NaCl}_{\mathrm{Ca}}$ series only influenced the resonant peak height as $l_{\max}\propto 1/\sqrt{C}$. Rotation of the magnetic field $\widetilde{\mathbf B}$ in the $(\mathbf b,\mathbf c)$ plane from direction $\widetilde{\mathbf B}\perp{\mathbf B}_{\mathrm{Earth}}$ to $\widetilde{\mathbf B}\parallel{\mathbf B}_{\mathrm{Earth}}$ also did not influence the frequency of the resonance but changed its amplitude. Depending on the crystal type, this influence changed from rather insignificant (in crystals of the $\mathrm{NaCl}_{\mathrm{LOMO}}$ series) to complete suppression of the effect for $\widetilde{\mathbf B}\parallel{\mathbf B}_{\mathrm{Earth}}$ (in the $\mathrm{NaCl}_{\mathrm{Nik}}$ series). The resonant frequency $\nu_{\mathrm{res}}$ is sensitive to orientation of the sample with respect to ${\mathbf B}_{\mathrm{Earth}}$. Upon rotation of the crystal by the angle $\theta=\angle(\mathbf c,{\mathbf B}_{\mathrm{Earth}})$ about the $\mathbf a \perp{\mathbf B}_{\mathrm{Earth}}$ edge, the initial peak for dislocations $\mathbf L\parallel \mathbf a$ at the crystal orientation $\theta=0$ and the frequency $\nu^0_{\mathrm{res}}$ is replaced by a pair of peaks at frequencies $\nu_{1,2}\approx \nu^0_{\mathrm{res}}\cos\theta_{1,2}$, where $\theta_1=90^\circ-\theta$ and $\theta_2=\theta$. Previously, these peaks were observed separately in $\mathrm{NaCl}_{\mathrm{Nik}}$ crystals for $\widetilde{\mathbf B}\parallel \mathbf c$ and $\widetilde{\mathbf B}\parallel \mathbf b$. In the present study, these peaks have been observed simultaneously for both orientations of $\widetilde{\mathbf B}$ in $\mathrm{NaCl}_{\mathrm{LOMO}}$ and $\mathrm{NACl}_{\mathrm{Ca}}$ crystals, where the resonance is not completely suppressed for $\widetilde{\mathrm B}\parallel{\mathbf B}_{\mathrm{Earth}}$.