Influence of Grain Size on Square-Loop Properties of Lithium Ferrites

Abstract

Pure lithium ferrite can be obtained as a very good square‐loop material if certain precautionary steps are taken in material preparation, firing, and cooling. Due to its simple composition and high Curie temperature, this material is of practical as well as of theoretical interest. In a series of memory cores prepared under various conditions, grain size was found to be the determining factor for switching time and threshold field. The series covered a range of 2 to 30 microns in average grain size. Below 2 microns good squareness was not obtained even in samples with uniform grain size. Cores with 2.3‐μ average grain size proved to have as little as 120‐nsec switching times under coincident current test conditions. Results indicate that there is little magnetic interaction between radially adjacent grains. Therefore, the best guarantee to obtain a square hysteresis loop requires a relatively narrow distribution of grain sizes. Cores fired under conditions which resulted in a wide distribution of grain sizes exhibited poor squareness. The change of threshold field with temperature of lithium ferrite is only 0.15% per °C compared with 1.5% per °C in some extreme cases of commonly used MgO–MnO ferrites with high ZnO content. The procedures used for obtaining grains of uniform size are discussed. Adjusting the powder particle size to a certain fraction of the grain size desired in the finished core was found to be the best physical method of controlling grain growth without introducing complications through use of oxide additions acting as fluxes or grain growth inhibitors. Powder particle size was measured by use of x‐ray diffraction line broadening techniques. As a very practical relative measure of the powder particle size in the submicron range, the intrinsic coercive force can be used.