A direct comparison of the properties of CRM and VRM in the low-temperature oxidation of magnetite

We report results of a collaborative investigation of the experimental properties of chemical remanent magnetization (CRM) acquired when nearly single-domain size magnetite undergoes low-temperature oxidation to maghemite and of viscous remanent magnetization (VRM) acquired by the daughter phase maghemite. Magnetite grains 100–200 nm in diameter were oxidized for varying lengths of time at a temperature of 220°C. The acquisition of CRM in fields of 0.2, 0.5 and 1 Oe was monitored at frequent intervals during heating runs lasting 30 hr. Zero-field decay of this remanence was then monitored for periods of 160–430 hr. Subsequently VRM acquisition and decay were measured for similar time periods for the daughter phase maghemite (z = 0.96). The principal results are as follows. (1) CRM and VRM have similar quasi-log t time dependences, in acquisition as well as decay of remanence. (2) After a time 3–9 times the acquisition time, there is no further decay and a true CRM residual is isolated. (3) Residual (time-independent) CRM forms about 45 per cent and viscous CRM forms about 55 per cent of the total CRM, which is appropriately called a chemico-viscous remanent magnetization (CVRM). (4) The alternating field (AF) stability of residual CRM is significantly greater than that of either VRM or total CVRM. However, the coercivity spectra are sufficiently similar that AF cleaning would not be an effective means of separating CRM formed in the direction of the primary remanence of parent magnetite from VRM formed in the applied field direction.