Cooling behaviour of partial thermoremanences induced in multidomain magnetite

Section: Discussionsupporting

confidence: 85%

Section: Introductionmentioning

confidence: 99%

Magnetic hysteresis and rotational hysteresis properties of hydrothermally grown multidomain magnetite

Muxworthy¹

2002

“…The transdomain changes (Moon & Merrill 1984 invoked as a consequence of temperature variation have been reported for (titano)magnetite above room temperature, although domain observations yield contrasting results. Also, in acquisition experiments, patterns of partial TRM suggest temperature dependence of domain structure in MD magnetite (Muxworthy 2000). In hydrothermally recrystallized magnetite Heider et al (1988) observed that the domain structure was temperature-dependent below 100 uC, while for natural crystals no major changes occurred up to y400 uC followed by a gradual decrease up to the Curie temperature (Ambatiello et al 1999).…”

Section: Low-field Susceptibility Measurementsmentioning

confidence: 95%

Low-temperature behaviour of haematite: susceptibility and magnetization increase on cycling through the Morin transition

Boer¹,

Mullender²,

Dekkers³

2001

Summary It has been realized previously (e.g. Borradaile 1994) that cycling through the Morin transition (Tm, occurring in ideal α‐Fe2O3 at −10 °C) may have implications for the NRM of some haematite‐bearing rocks. We investigate the behaviour of the low‐field susceptibility (χlf), several magnetizations (in fields of 5, 25, 100 and 1600 mT) and SIRM on cycling through Tm of several well‐characterized haematite types of varying crystallinity and particle shape. Before low‐temperature treatment, χlf of the haematites varied between ∼ 40 and ∼ 235 × 10−8 m3 kg−1. Below Tm, where only haematite's defect moment resides, χlf was much more uniform at ∼ 19 to ∼ 28×10−8 m3 kg−1. After return to room temperature, increases in χlf of up to ∼ 50 per cent were observed (when cycling in the Earth's magnetic field as well as in a field‐free space), inferred to be a function of the domain state of the haematite. This was shown for one of the haematites (LH2 which occurs as small platelets and is particularly well crystalline) where a relation y = (8.60 ± 1.01) ln(x)−2.98 was obtained, where x is the grain size (µm) and y is the percentage susceptibility increase. We suggest that transdomain changes induce the change in χlf. The nucleation of (additional) domain walls in ‘metastable’ single‐domain (SD) to pseudo‐single‐domain (PSD) grains is made possible by the low anisotropy at the Morin transition. In view of this mechanism, small stable SD haematite particles would not be affected and the grain size corresponding to y = 0 (∼ 1.5 µm for LH2) would represent the ‘real’ SD threshold size. Thermal cycling to over the Curie temperature (680 °C) is needed to return to the original domain state before the LT treatment, as expressed by a return to the original χlf values. Measuring χlf between alternating field (AF) demagnetization steps shows that AF demagnetization gradually removes the χlf increase, which appears to be soft; 30 mT is sufficient to erase 90 per cent. Thermal cycling in a 5 mT field between temperatures above Tm showed that irreversible changes in domain structure are noticeable before the isotropic point is passed. After cycling, magnetization is added to PSD and multidomain (MD) grains that intriguingly appears to be remanence, probably induced by the broadening and subsequent irreversible displacement of loosely pinned domain walls. Complete cycling through the isotropic point considerably enhances the new remanence component in ‘metastable’ SD to MD particles by an increase in the number of domains. If this behaviour can be extrapolated to the intensity of the Earth's magnetic field, this would imply that large ‘metastable’ SD to MD specularite crystals with a well‐developed Morin transition are susceptible to acquiring geologically irrelevant remanence components when subjected to low ambient temperatures. Fine‐grained haematite pigment, on the other hand, would not be affected. Thermal demagnetization alone would not be able to separate these two remanences as the new domain structure persists up to close to ...

“…Repeated microscopic observations of the same TRM ac-quisition process leave the particle in different domain structures at room temperature (Halgedahl 1991). This also indicates that, besides domain wall pinning, domain nucleation is of major importance during TRM acquisition (Moon & Merrill 1985), especially in MD particles with low internal stress (Muxworthy 2000). This also indicates that, besides domain wall pinning, domain nucleation is of major importance during TRM acquisition (Moon & Merrill 1985), especially in MD particles with low internal stress (Muxworthy 2000).…”

Section: Introductionmentioning

confidence: 91%

Statistical theory of weak field thermoremanent magnetization in multidomain particle ensembles

Fabian

2003

SUMMARY A non‐equilibrium statistical theory of multidomain thermoremanent magnetization (TRM) is developed, which describes thermal magnetization changes as continuous inhomogeneous Markov processes. The proposed theory relies on three very general physical properties of TRM: (a) The probability that a magnetization state Sj is transformed during an infinitesimal temperature change into state Si depends only on external conditions and on Sj, but not on previously assumed states. (b) Due to time inversion symmetry of the Maxwell equations, the magnetic energies are invariant with respect to inversion of all spins in zero field. (c) The probability that an energy barrier between two magnetization states is overcome during a thermal process is governed by Boltzmann statistics. From these properties, the linearity of TRM with field is derived for generic multidomain particle ensembles. The general validity of Thellier's law of additivity of partial TRM's in weak fields is established and a method for proving a large class of similar additivity laws is developed. The theory allows consistent treatment of blocking and unblocking of remanence in multidomain particle ensembles and naturally explains apparent differences between blocking and unblocking temperatures.