Dense ferrites and their applications

The art of making ferrites with very low porosity resulted from thorough investigations of the different processing steps in ferrite fabrication. This has been applied, with various substitutions, to different compositions in the nickel-zinc (Ni-Zn) system. At high frequencies, such as 100–3,000 mc (megacycles), dense ferrites show small losses if the permeability is measured in a steady magnetic field perpendicular to the measuring field and the whole sample is saturated; the permeability can be altered by varying the steady field. The high resistivity and the small line width suggest applications in nonreciprocal devices. Values of 0.50 for the extensional piezomagnetic coupling coefficient and of over 0.60 for the torsional coupling coefficient were obtained for low-porosity nickel-cobalt (Ni-Co) ferrites. Low-porosity ferrites with glass-bonding techniques allow the construction of long-life tape-contact heads for high-frequency recording, and some typical constructions are discussed. Dense Ni-Zn ferrites with a small Co content show rectangular hysteresis loops with small coercive forces, but switching times are long. Developments in rectangular loop ferrites have been concentrated on different systems and have resulted in a high temperature stability, in very fast switching cores, and and, in addition, in 30-mil cores with low driving current.