Chromium oxide, magnetic

Electrobalances suitable for thermogravimetry are readily adapted for measurements of magnetic susceptibility [333—336] by the Faraday method, with or without variable temperature [337] and data processing facilities [338]. This approach has been particularly valuable in determinations of the changes in oxidation states which occur during the decompositions of iron, cobalt and chromium oxides and hydroxides [339] and during the formation of ferrites [340]. The method requires higher concentrations of ions than those needed in Mossbauer spectroscopy, but the apparatus, techniques and interpretation of observations are often simpler. 

The high hardness of chromium oxide resulting from its crystal structure is exploited in polishing agents for metals and in brake linings. Addition of a small amount of chromium oxide to magnetic materials of audio and video tapes imparts a self-cleaning effect to the sound heads. 

Oxides. E.s.r. spectra and the temperature dependence of the magnetic susceptibility of chromium oxide powders show differences from the properties found for the bulk material. These differences are proportional to both the increase in surface area and to the concentration of lattice defects. ... 

The effect will be clear from a comparison of the magnetic properties of chromium oxide gel with those of massive crystalline chromic oxide. Chromium oxide gel may be made by precipitation of the hydroxide from a nitrate solution, followed by slow dehydration. Several other processes are available, of which slow reduction from a basic chromate solution is one. On ignition, these gels generally undergo the glow-phenomenon during which they revert to Crystalline chromic oxide. 

Table II shows the magnetic susceptibility of a chromium oxide gel compared with that of crystalline chromic oxide. The gels always contain some water, hence a more striking comparison is made by calculating the susceptibility of the chromium ions in each substance.

Chromium oxide has a rather complicated electronic structure. It is antiferromagnetic. The so-called spinwaves are difficult to take into account in periodic calculations. The band gap, approximately 3.4 eV, is a bit larger than the one of Ti02. The very complex magnetic properties make the choice of the functional for DFT calculations very delicate. 

Particulate Materials. There are three principal classes of particulate magnetic materials y-ferric oxide, y-Fe202, and its modifications chromium dioxide [12018-01 -8] and iron [7439-89-6]. A comparison of the remanent magnetization, and coercivity, for several y-Fe202... 

Mag netic Tapes. Chromium dioxide, Cr02, is used as a ferromagnetic material in high fidelity magnetic tapes (qv). Chromium dioxide has several technical advantages over the magnetic iron oxides generally used (58,246). 

A 5% solution of chromium trioxide-pyridine complex in dry methylene chloride is prepared. The alcohol (0.01 mole) is dissolved in dry methylene chloride and is added in one portion to the magnetically stirred oxidizing solution (310 ml, a 6 1 mole ratio) at room temperature. The oxidation is complete in 5-15 minutes as indicated by the precipitation of the brownish black chromium reduction products. The mixture is filtered and the solvent is removed (rotary evaporator) leaving the crude product, which may be purified by distillation or recrystallization. Examples are given in Table 1.1. 

Tris(bipyridine)ehromium(II) perchlorate is stable for long periods in dry air but slowly oxidizes when wet. It has a room-temperature magnetic moment of 3.0 B.M., typical of a low-spin chromium(II) compound.18... 

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