Relaxation kinetics of the microhardness of KDP crystals after their exposure to a magnetic field

It has been shown that potassium dihydrogen phosphate crystals change their microhardness reversibly after their exposure to a magnetic field of $B=0.8$ or $1.2$ T for $t_{\mathrm{m}}=7{-}90\,$ min. It has been found that the magnetic effect can be conveniently characterized by the quantity $B^2t_{\mathrm{m}}$, because the variation of the parameters conserving $B^2t_{\mathrm{m}}=\mathrm{const}$ does not change the result. At $B^2t_{\mathrm{m}}<10\,$T$^2$ min, the effect is almost absent. Above this threshold, the amplitude of changes in the microhardness increases and approaches a constant value of $\sim10\%$ at $B^2t_{\mathrm{m}}\approx19\,$T$^2$ min. The responses of samples of the same crystal from the faces of the prismatic and pyramidal growth sectors to exposure are different. In the former case, they soften; in the latter case, the hardening stage follows the softening stage. However, in both cases, the microhardness returns to the initial value. At $B^2t_{\mathrm{m}}$ values from $19$ to $37$ T$^2$ min, the amplitudes and durations of the effect do not change, but in the narrow range of $37$–$43$ T$^2$ min, the lifetime of the modified state increases sharply with transition to a new level: “sharp” peaks with a half-width of $\sim2$ days are transformed to trapezoids with the width of the horizontal side of $\sim1$–$2$weeks. A physical scheme of the observed effects has been proposed.