Fine-Grained Ferrites. II. Ni1−xZnxFe2O4

Abstract

A series of fine‐grained single‐phase ferrites was prepared according to the formula Ni_1−xZn_xFe₂O₄, where x took the values 0, 0.10, 0.33, 0.50, and 0.67. A previously‐described process combining the so‐called flame‐spraying and hot‐pressing techniques was used to obtain grain sizes of approximately 0.1 micron in the densified bodies. μ′ and μ″ were measured to 1000 Mc, with grain size and composition as parameters. Measurements were also made to 3800 Mc on a ferrite annealed through the critical size for multi‐domains, confirming previously‐reported theory that magnetic poles on the domain walls are the source of the microwave peak at about 2000 Mc. Wall displacement appears to contribute to the static μ₀ in large‐grained ferrites when x>0. In addition, the temperature dependence of initial permeability μ₀ was studied. The temperature coefficient of μ₀ was found to increase with grain size. While the μ₀ vs temperature curves were of various shapes generally, still a set of ferrites made up of differing compositions and grain sizes were obtained having linear temperature dependences. Of practical interest are those with slopes of −220 ppm/°C, 0 ppm/°C (NPO), and 220 ppm/°C. Microwave properties were studied also. Because zero‐field (H_dc=0) measurements showed that fine‐grains (i.e., below the critical size for multidomains) eliminated the so‐called microwave loss peak in the rf dispersion, a decrease in the low‐field loss follows. Also, microwave measurements of the main resonance loss susceptibility vs rf power were made at X band using a cavity technique. Although normal critical fields h_c were generally found, one sample with x=0.67 and ΔH=655 oe showed an anomalous rise in χ″/χ₀″ at high powers, significantly expanding its power‐handling capability.