A decrease in microhardness along the (010) cleavage in potassium acid phthalate single crystals by 15–18% after the application of a permanent magnetic field is revealed for the first time. It is shown that the effect revealed is of the volume character. The role of interlayer water in the processes stimulated by a magnetic field is studied. Interlayer water does not cause the observed changes; it only plays the part of an indicator of these changes in potassium acid phthalate crystals in a magnetic field. It is established that microhardness in the (100) plane of the crystal in an applied a magnetic field first increases by 12–15% and then remains constant in time within the accuracy of the experiment. The possibility of varying the crystal structure of potassium acid phthalate crystals by applying magnetic fields inducing rearrangement in the system of hydrogen bonds or in the defect structure is discussed.

The influence of a magnetic field on the microhardness of potassium acid phthalate has been studied for different magnetic inductions, exposure times, sample orientations in a magnetic field, and impurity compositions of the crystals. It was shown that the magnetic field effect is multiply repeated on the (010) face after relaxation. The influence of magnetic treatment on ammonium, rubidium, thallium, and cesium acid phthalate crystals is analyzed. The reasons for the observed changes in the crystal microhardness in the magnetic field are discussed.

It is shown that the criterion of dislocation mobility resonance in NaCl crystals in the electron spin resonance scheme in the Earth’s magnetic field depends on both the sample orientation in this field and the location of impurity atoms in the dislocation core. The measured low-frequency spectrum of dislocation paths makes it possible to resolve a series of peaks corresponding to different positions of a Ca impurity that correlate with the results from computer simulations of the dislocation core structure.

Dislocation motions are studied in NaCl crystals exposed to an EPR impact under crossed ultralow magnetic fields, the Earth field BEarth ∼ 50 µT, and the AC pumping field B̃(ν) of the amplitude 2.5 µT in the frequency band of 6 kHz to 2.1 MHz. Mean dislocation paths form a spectrum of multiple peaks at definite resonance frequencies. Dislocation motions are supposed to be caused by spin-dependent transformation of impurity centers, which, in turn, provide depinning of dislocations and relaxation of their structure. The observed spectrum is attributed to specific features of the hyperfine EPR at the applied field BEarth small as compared with the crystalline local fields Bloc created by nuclei of Cl ligands surrounding the Ca pinning centers. The developed theory well describes peak positions in the observed frequency spectrum.