Theoretical single-domain grain size range in magnetite and titanomagnetite

Abstract

A theoretical model of single‐domain (SD) grain sizes is applied to magnetite and titanomagnetite. In this model, transition to a two‐domain configuration takes place at the SD threshold d0. This two‐domain configuration is shown to be more applicable to fine‐grained magnetites in igneous rocks than previous models involving transition to a circular spin configuration at d0. Calculations of the stable SD grain size range were accomplished by calculating the superparamagnetic threshold size ds by Néel's relaxation equation and calculating the SD threshold d0 at which SD to two‐domain transition occurs. For cubic magnetite particles the SD range is extremely narrow and occurs at very small grain size. At room temperature, ds ≃ 0.05 μm, and d0 ≃ 0.076 μm. For cubic magnetite particles just above d0 a two‐domain configuration is predicted in which a 180° domain wall occupies ∼60% of the particle volume. No SD range exists for cubic magnetites at T > 450°K. These results are in good agreement with experimental determinations of SD limits in equant magnetites and also agree with experimental observations of thermoremanent magnetization in submicron pseudo‐single‐domain (PSD) magnetites. The SD range increases rapidly with particle elongation. For a length : width ratio of 5 : 1, SD limits of ds ≃ 0.05 μm and d0 ≃ 1.4 μm are calculated. Both d0 and the SD range for titanomagnetites (Fe3−x Tix04) increase with Ti content. For cubic titanomagnetites of x = 0.6, ds ≃ 0.08 μm, and d0 ≃ 0.3 μm. Comparison of the calculated SD range with the available high‐resolution grain size distributions of opaque grains in igneous rocks suggests that elongated SD grains or submicron PSD grains are the major carriers of stable natural remanence in igneous rocks.

Structure and morphology of magnetite anaerobically-produced by a marine magnetotactic bacterium and a dissimilatory iron-reducing bacterium

“…The species-specific crystals produced by MV-1 are matched to their func tional requirement as organelles responsible for magnetotactic behaviour. In this respect, the par ticle size range of MV-1 crystals falls within that required for single domain remanent magnetiza tion (35-100 nm) [14], whereas the smaller GS-15 crystals are superparamagnetic. The latter be haviour results in the spectral broadening, caused by relaxation effects, observed in the M6ssbauer spectra (Fig.…”

Magnetofossil spike during the Paleocene‐Eocene thermal maximum: Ferromagnetic resonance, rock magnetic, and electron microscopy evidence from Ancora, New Jersey, United States

“…Length and shape factor (width/length ratio) of magnetite particles from 168.81 m depth. The lower, SD/ superparamagnetic boundary is determined for rectangular parallelepipeds with unblocking times of 100 s (bottom dashed line) and 4.5 Ga (bottom solid line) [Butler and Banerjee, 1975;Diaz-Ricci and Kirschvink, 1992]. The top solid line and top dashed line show the upper boundary of the metastable single-domain field and stable single-domain field, respectively [Witt et al, 2005].…”

Electron Holography of Magnetic Materials

“…The effect of particle size and shape on the magnetic state of an isolated magnetite particle was discussed theoretically by Butler & Banerjee (1975), who suggested that the transition from single-domain to superparamagnetic behavior occurs below a particle size of 25-30 nm. Figure 9 shows local magnetic structures of magnetite particle assemblies on smectite (clay) platelets, which provide an ideal geometry for the experimental study of the critical threshold size for superparamagnetic behavior in the presence of strong interactions between adjacent particles (Galindo-Gonzalez et al, 2009).…”