Magnetite

About: Magnetite is a research topic. Over the lifetime, 10277 publications have been published within this topic receiving 278071 citations.

Magnetic after-effects in Fe3O4and vacancy-doped magnetite

Abstract: The magnetic relaxation spectrum of stoichiometric and vacancy-doped magnetite has been investigated in the temperature range between 4·2 and 130 K. The observed relaxation processes are classified as short- and long-range rearrangements of Fe2+ and Fe3+ ionic states within domain walls. It is proposed that below 35 K a coherent tunnelling process governs the mobility of electrons, but that above 50 K is replaced by a thermally activated hopping process leading to the Verwey transition at 123 K.

Interacting single-domain properties of magnetite intergrowths

Abstract: Recognition that magnetite intergrowths are a common carrier of stable remanence in igneous rocks has prompted a detailed investigation of their magnetic properties. A Tertiary basalt containing homogeneous titanomagnetite grains was oxidized in air to form magnetite/ilmenite intergrowths, the morphology of which has been studied by electron microscopy. The intergrown grains are shown to contain arrays of interacting single-domained magnetites with uniaxial shape anisotropy. Interactions between single domains cause reduction of coercivity from the very large values associated with isolated single domains to values which are, however, many times larger than those measured in multidomained grains. The intergrown grains have an observed IRS/IS value of 0.30, whereas the value of this ratio obtained experimentally under nondemagnetizing conditions is found to be 0.51. The reduction is shown to be entirely explicable in terms of localized array demagnetizing fields arising from surface poles. A similar effect is seen in magnetite/ulvospinel intergrowths. Qualitative evidence that this reduction proceeds mainly by rotation of the spontaneous magnetization from easy axes is seen in a measured anisotropy of low field axial susceptibility having a value 11% higher in a direction perpendicular to the saturation remanence than that parallel to it. This phenomenon may be used to discriminate between rocks containing interacting single domains and those containing either multidomains or noninteracting single domains. These results also apply to magnetic powder experiments reported in the literature, in which reduction in IRS/IS horn 0.5 will occur if single-domained particles of uniaxial anisotropy are clumped together, a situation which is difficult to avoid. Magnetic intergrowth structures commonly observed in subaerial basalts should be regarded as arrays of interacting single-domain magnetite particles capable of carrying a strong remanent magnetization stable over geological intervals of time.

The Oxidation of Iron at 175 to 350 degrees C

Abstract: The oxidation of iron, previously freed from oxide by hydrogen treatment, has been studied at 175 to 350 degrees C; five methods (gravimetric, electrometric, film-transfer followed by chemical or microscopic examination, X-rays and electron diffraction) have been used to identify and estimate the different oxides. If specimens are exposed to air at room temperature without subsequent hydrogen treatment, complications are introduced, magnetite being formed where otherwise it would be absent. On hydrogen-reduced iron powder, ferrous oxide, magnetite and $\alpha $-ferric oxide appear when oxygen is admitted, but the oxidation is not isothermal, since a glow appears on admission of oxygen even when the initial temperature is only 40 degrees C. On hydrogen-reduced Swedish sheet, $\alpha $-ferric oxide alone appears at 175 and 225 degrees C, while at 300, 325 and 350 degrees C, a duplex film of magnetite (not $\gamma $-ferric oxide) overlaid with $\alpha $-ferric oxide is formed; at 250 degrees C, the film consists of a single layer ($\alpha $-ferric oxide) for 8 h, but then magnetite appears below it at certain places (distinguishable by the more advanced interference colours) and spreads laterally, the rate of magnetite formation increasing rapidly with time. Most of the films studied display interference tints in the usual sequence, when they are on the metal; after transfer to glass, they show different colours, which in the case of the duplex films are themselves different according as they are viewed from the magnetite side or the $\alpha $-ferric oxide side. This can easily be explained. Some experiments on pure iron give results similar to those on Swedish iron. The growth law on sheet is parabolic (W$^{2}$ = Kt + K$^{\prime}$) at 325 and 350 degrees C but logarithmic (W = K$^{\prime}$ ln (Kt + K$^{\prime \prime}$)) at 175, 225, 250, 275 and 300 degrees C. The parabolic law is generally recognized to be connected with the growth of a continuous oxide film, the thickening of which is controlled by migration or diffusion through the film, giving dW/dt = k/W or $\frac{1}{2}$W$^{2}$ = kt + k$^{\prime}$. At low temperature such movement through the film substance will be negligibly slow, but leakage of oxygen through definite discontinuities can continue; if the volume increase accompanying oxygen at each discontinuity exerts a certain chance of blocking other leakage points in the neighbourhood, the logarithmic law is arrived at.