INFLUENCE OF THE CONCENTRATION OF CA IMPURITY ON THE MAGNETIC THRESHOLD OF THE MAGNETOPLASTIC EFFECT IN NACL-CRYSTALS

It is found experimentally that the threshold magnetic field B-c for t he magnetoplastic effect, i.e., the field at which the depinning of di slocations from paramagnetic impurities in an external magnetic field begins to be observed, increases with increasing concentration C of Ca impurity in NaCl crystals in the range C = (0.5-100) ppm. It is shown that the dependence B-c(C) exhibits a distinct tendency toward satura tion. The physical interpretation of the observed dependence rests on the notion that as the impurity concentration C increases, the average size of the impurity complexes increases and, accordingly, the local atomic configuration around the impurity atoms changes according to a definite pattern. In particular, the average number <(n)over bar (v)> of cation vacancies among the nearest neighbors increases from 1 to 6 as the number N of Ca atoms in the complex increases, and this trend, in turn, should cause the thermal vibration amplitude of the Ca atoms to increase. In other words, the phenomenon in question appears to be physically analogous in its microscopic mechanisms to the previously o bserved increase of B-c with increasing temperature. The proposed inte rpretation is further supported by good correlation of the experimenta l dependence B-c(C) with the calculated function <(n)over bar (c)>(N). (C) 1998 American Institute of Physics.