New developments in the synthesis and processing of magnetostrictive materials that are highly piezomagnetic in the polarized state are briefly described and interpreted in terms of the relationship for reversible stress sensitivity derived by Bozorth and Williams. In the system MnO–CoO–Fe2O3, a coupling coefficient 0.39 has been obtained for Mn0.970Co0.068Fe1.962O4 and 0.33 for Mn0.932Co0.068Fe2O4. Solid solutions of the nickel cobalt and manganese cobalt ferrites are also piezomagnetic. These materials require atmosphere control during the firing process. When similar processing is applied to the commercially established nickel cobalt ferrites, a coupling coefficient 0.42 is obtained for Ni0.974Co0.026Fe2O4 without requiring quenching from high temperatures. It is suggested that the high piezomagnetic activity of the manganese cobalt ferrites can be understood if the appropriate strain term in the expression derived by Bozorth and Williams is better identified by the easy direction magnetostriction instead of the polycrystalline magnetostriction. This interpretation leads to the prediction that magnetite with the correct cobalt ferrite addition is highly piezomagnetic.

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Rectangular hysteresis loops have been found in ferrite systems containing copper during an inv estiga~ tion based on the proposal by Baltzer that a zero or near-zero value of the effective magnetocrystalline domain anisotropy is a necessary condition for loop squareness.Data on hysteresis loop squareness of polycrystalline bodies as a function of composition, firing conditions and magnetostrictive effects arc presented for the system: copper ferrite-magnesium ferrite . This system is characterized in general by large grains and long switching times. Abrupt flux changes occur in the hysteresis loop at low temperatures. Simultaneously, a decrease in the coercive force with decreasing temperature and squareness values approaching unity are observed. No crystallographic transitions were detected at low temperatures.Magnetostrictive measurements do not unequivocally show whether anisotropy goes through zero at compositional region of maximum squareness. The saturation magnetostriction and probably the All! go through a minimum in this region .

Iron powders with Hc between 350 Oe and 1000 Oe were prepared by borohydride reduction of ferrous sulfate. The powders were stabilized by a chromating procedure. This paper discusses the effect of the chromating as observed by: TEM, changes in surface area, O2 content, ESCA/Auger and adsorption. Chromating coats the surface of the particles with a layer of FeCr2O4. Oxygen content in chromated powders is generally ∼8% while in the unchromated powders, it varies between 11 and ≳16%. This increase in O2 can be correlated with a decrease in σs. Sheet-like impurities are observed in unchromated powders which have been identified as FeOOH. Chromating results in a reduction in surface area of the particles as measured by N2 adsorption. The stability of the powders is greatly improved with chromating. Powders subjected to 80% R.H. for 112 days showed that the unchromated powder lost 25% of its original magnetization, while the chromated powder lost only 3%. Simultaneously, a band of FeO and Fe2O3 formed around the particles. Adsorption studies of reference polymers have shown that chromating reduces the acidic surface sites of the powders.

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