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 $${{B}_{{{\text{Earth}}}}} \approx 50$$ μT and an alternating field with an amplitude of about 3 μT 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 $$\nu _{n}^{{{\text{II}}}} \approx \nu _{n}^{{\text{I}}}{\text{/}}\sqrt 2 $$ ( $$n = 1,\;...,\;9$$ ). 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.