Magnetically induced resonance change in the microhardness of NaCl crystals

A resonance decrease in the microhardness of Ni-doped NaCl crystals exposed in the EPR scheme to ultralow mutually perpendicular magnetic fields including the Earth's static magnetic field $B_{\text{Earth}}\approx50$ $\mu$T and an alternating field with an amplitude of about $3$ $\mu$T at frequencies $1.1$–$2.2$ MHz is discovered. A series of softening peaks separating into two groups each consisting of nine peaks are identified. Resonant frequencies in the groups are related by the empirical relationship $\nu_n^{\text{II}}\approx\nu_n^{\text{I}}/\sqrt{2}$ ($n = 1,...,9$). The maximum decrease in the microhardness is observed $2$–$3$ h after exposure, and then it slowly relaxes to its initial value. The interpretation of the observed effects, including the relation between the frequencies $\nu _{n}^{{\text{I}}}$ and $\nu _{n}^{{{\text{II}}}}$, is based on the features of resonance magnetoplasticity, when the external field ($B_{\text{Earth}}$) is several orders of magnitude lower than the local magnetic fields in the crystal.