Hexamminecobalt(II1) sa.lts may be prepared by any of three methods that depend on oxidation of cobalt(I1) ion * Procedures for preparing most of these salts were originally submitted by Dr. McReynolds, who was impressed by the simplicity of Dr. Bjerrum's method. Shortly after the manuscript for Vol. I1 had been sent to the publisher, similar procedures for three of these salts were submitted by Dr. Bjerrum. Inasmuch as the method was developed by Dr. Bjerrum and war conditions prevented his submitting the procedures a t an earlier date, the editors have taken the liberty of combining the two sets of directions and presenting them as a joint effort of the two men.

Air-stable, single-domain Fe and Fe-Co particles prepared by the reaction of the metal salts with aqueous borohydride have higher coercivities and axial ratios if prepared in a magnetic field. Oxide and boride impurities are indicated by elemental analyses and lowered saturation magnetizations. Studies of coercivitiesand magnetizations as a function of temperature, remanence ratios, and rotational hysteresis measurements indicate single-domain behavior with coercivity controlled by shape and somewhat by crystalline anisotropy in approximate agreement with the predictions of the Jacobs-Bean “chain of spheres” model.

Ferromagnetic chromium oxides of high purity have been synthesized by thermal decomposition of chromium trioxide in the presence of water at elevated pressure. These materials consist entirely of a rutile-type crystalline phase with cell constants a0 = 4.41±0.01 A and c0 = 2.91±0.01 A and have compositions that are very close to the exact stoichiometry of the dioxide, CrO2. Saturation specific magnetizations of these oxides are in the range of 98–100 emu/g at room temperature and their Curie temperatures are about 126°C. By using different reaction conditions, it is possible to obtain either microcrystalline particles from 3–10 μ in length and from 1–3 μ in width or single crystals of fractional millimeter size that are suitable for physical studies. In addition, elongated single-domain crystals only 0.5–1.5 μ in length and with length-to-width ratios of 8:1 or higher can also be obtained merely by adding catalytic amounts of Sb2O3 or RuO2 under hydrothermal synthesis conditions.

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