Partial anhysteretic remanent magnetization in magnetite 1. Additivity

Abstract

[1] We have tested the additivity of partial anhysteretic remanent magnetization (pARM) for suites of eight synthetic magnetites with mean grain sizes from 65 nm to 18 mm and 18 natural samples including lake sediments, oceanic and continental volcanic rocks, gabbros, and granites. In both synthetic and natural sample suites, domain states inferred from hysteresis and other magnetic properties vary from single-domain (SD) through pseudosingle-domain (PSD) to multidomain (MD). For each sample, total ARM intensity was compared with sums of partial ARMs of three different types: four conjugate pairs of parallel pARMs; four pairs of perpendicular pARMs; and one set of five neighboring parallel pARMs. In each case, the intervals of alternating field (AF) over which a steady field H was applied to produce the partial ARMs are nonoverlapping and cover the entire AF range (0-100 mT) used to produce the total ARM. Additivity of partial ARMs was verified to better than ±3% for all the samples, whatever the domain state (SD, PSD, and MD) or composition (ranging from pure magnetite to x = 0.6 titanomagnetite). The universality of pARM additivity is unexpected because its analog, partial thermoremanent magnetization (pTRM), deviates from ideal behavior as the grain size increases and the domain structure becomes MD. The different behaviors probably result from the fact that pARM is produced at ordinary temperatures over short times, whereas the most intense pTRM is produced at temperatures approaching the Curie point with significant dwell times, promoting such processes as isothermal remanence acquisition, domain nucleation, and domain wall reequilibration. Verification of the law of additivity of pARMs is an encouraging first step toward validating pseudo-Thellier and other methods of paleointensity determination that use ARM in place of, or in addition to, TRM.

Particle Size-Specific Magnetic Measurements as a Tool for Enhancing Our Understanding of the Bulk Magnetic Properties of Sediments

“…This may not always be a valid assumption in natural samples [99], particularly given the strong interacting nature of some components [40,53,101], that can promote strong non-linearity. However, the generally low concentration of magnetic minerals-and therefore greater dispersal of potentially interacting particles-in sediments compared to synthetic mixtures [99,100] or concentrated magnetic extracts often makes this less of a concern in natural samples [50,115,116]. Interpretation of identified components often assumes a link between sediment and magnetic granulometry.…”

Effects of internal stress on remanence intensity jumps across the Verwey transition for multi-domain magnetite

“…MD magnetites with two different origins were studied: synthetic polycrystalline magnetite produced by Wright Industries, and a natural magnetite crushed from a (Yu et al, 2002). Both samples fall in the MD range according to the criteria of Day et al (1977).…”

“…Upon passing through T v , thermomagnetic curves commonly change abruptly, although detailed remanence variation is strongly dependent on the type of initial room temperature magnetization. SIRM appears either to stop demagnetizing on reaching T v , and undergoes little or no variation with temperature below T v , or a small proportion of remanence associated with the high-coercivity fraction displays abrupt increases or "jumps" at T v that are partially reversible upon warming (Muxworthy & McClelland, 2000b;Özdemir et al, 2002;Muxworthy et al, 2003;Yu et al, 2003a). For example, for fine-grained magnetites, the SIRM cooling curve changed little with temperature (Yu et al, 2003a).…”

“…This change in symmetry strongly affects the magnetocrystalline anisotropy of magnetite, which is an order of magnitude larger in the monoclinic phase than in the cubic phase (Kakol et al, 1992(Kakol et al, , 1994Muxworthy & McClelland, 2000a). In addition to the Verwey transition, the cubic magnetocrystalline anisotropy of magnetite has an isotropic point T i at 130 K, which also influences its low-temperature magnetic behaviour (Muxworthy & McClelland, 2000a;Özdemir et al, 2002).…”

“…For example, upon cooling through T v the coercive force increases and the susceptibility decreases (Muxworthy & Williams, 1999;Özdemir et al, 2002). Similarly, the change in sign and magnitude of anisotropy constants causes demagnetization of a saturation isothermal remanent magnetization (SIRM) induced in MD magnetite on either heating or cooling through T v (Özdemir & Dunlop, 1999).…”