Magnetic memory effect in ZnO single crystals

Hardening of ZnO single crystals is found to occur after their treatment in a constant magnetic field (B = 1–2 T). A maximum increase in the microhardness of the crystals is observed within 3–4 h after magnetic treatment. Then, the effect gradually decreases and the microhardness regains its initial value in 2–3 days. It is revealed that the sensitivity of the microhardness to a variation in the magnetic field has a threshold character: the effect appears at a magnetic induction higher than a critical value, rapidly increases in a narrow magnetic field range ΔB (～0.3 T), and then reaches saturation. It is shown that the magnitude of the effect depends on the orientation of the magnetic field with respect to the polar axis of symmetry of the crystal and is independent of the crystallographic orientation of the measurement plane. The maximum increase in the microhardness (～20%) is observed for all the (0001), (11\(\bar 2\)0), and (10\(\bar 1\)0) faces studied in the magnetic field B ∥ 10\(\bar 1\)0]. No change in the microhardness is found to occur in the magnetic field direction B ∥ 0001]. A physical model related to the spin-dependent variations in the impurity subsystem of the crystal in the magnetic field is proposed.