Magnetic properties of lattice imperfections in alkali halide single crystals

Measurements have been made on the magnetic properties of high purity, optical quality, single crystals of three alkali halides -- lithium fluoride, potassium chloride, and sodium chloride. A cylindrical sample was suspended along its cylindrical axis from a torsion fiber forming a torsion pendulum. This pendulum system was placed so that the crystal was in the uniform and constant magnetic field of 12 inch diameter pole-face magnet. A small torque accompanied by a change in the angular frequency of the pendulum was observed. The torque and change in angular frequency were found to depend on the magnetic field and on the orientation of the magnetic field relative to the crystalline axes. It was also observed that this effect could be increased by x-irradiation and could be decreased by heat annealing. A phenomenological interpretation of these experiments invokes an induced magnetization and a type of ferromagnetism involving a cooperative interaction among the magnetic centers. The ferromagnetic magnetization is temperature dependent. The magnetic centers are attributed to lattice imperfections such as dislocations and vacancies