Change in the microhardness of nonmagnetic crystals after their exposure to the Earth’s magnetic field and AC pump field in the EPR scheme

Changes in the microhardness of ZnO, triglycine sulfate (TGS), and potassium acid phthalate (KAP) crystals after their exposure to crossed ultralow magnetic fields, i.e., the Earth’s field B Earth ≈ 50 μT and the alternating-current field $$\tilde B \approx 3 \mu {\rm T}$$ orthogonal to it, have been revealed. In ZnO crystals, the microhardness increases, whereas in TGS and KAP, it decreases. A maximum change (10–15%) is reached within 1–3 h after magnetic treatment; then, the microhardness gradually recovers to its initial value for the first day. After a sufficient pause, the effect is completely reproduced under the same conditions. The resonant frequency of the pump field $$\tilde B$$ corresponds to the EPR condition with a g-factor close to two. The magnetic memory exhibits a strong anisotropy: for each of the crystals, a direction is found, which, being coincident with the Earth’s magnetic field vector B Earth, causes complete or partial suppression of the effect. In ZnO and TGS crystals, these are symmetry axes 6 and 2, respectively. In the KAP crystal, it is the direction in the cleavage plane orthogonal the 2 axis. Possible physical mechanisms of the observed phenomena have been discussed.