Anisotropy of the resonance transformation of the microhardness of crystals after their exposure in the EPR scheme in the Earth’s magnetic field

It is shown that the preliminary exposure of ZnO, triglycine sulfate, and potassium hydrogen phthalate crystals in ultralow crossed magnetic fields—Earth’s magnetic field and ac pump field — leads to a resonance change in their microhardness. The resonance frequency of microhardness peaks is determined by the classical condition of electron paramagnetic resonance only at certain orientations of the crystals with respect to the Earth’s magnetic field $B_{\text{Earth}}$. Rotations of all samples with respect to the direction $B_{\text{Earth}}$ by angle $\theta$ reduce the resonance frequency in proportion to $\cos\theta$. The observed anisotropy has been attributed to the presence of their own local magnetic fields $B_{\text{loc}}\gg B_{\text{Earth}}$ in the crystals.