Threshold effects of the magnetic influence on the microhardness of KDP crystals

The threshold of the magnetic effect on the microhardness of potassium dihydrogen phosphate (KDP) crystals has been found. It was previously shown that the preliminary exposure of such crystals to a magnetic field $B$ for time $t_{\mathrm{m}}$ changes their microhardness reversibly. This effect is well described by the quantity $B^2t_{\mathrm{m}}$; therefore, a decrease in the field $B$ can be compensated by increasing the exposure time. In this work, it has been shown that such a compensation is possible only for fields above the threshold $B_{\mathrm{th}}\approx 0.45$ T, below which the effect is absent even at large exposure times. Both change in the microhardness and the lifetime of the meta-stable state of the crystal are suppressed near the threshold. At $B > B_{\mathrm{th}}$, the effect occurs only when the exposure time $t_{\mathrm{m}}$ exceeds a certain minimum time $t_{\mathrm{m}}^{\min}$ depending on $B$. A physical interpretation of the observed effects has been proposed.