Multidomain Remanence Research Articles

Analyzing absolute paleointensity determinations: Acceptance criteria and the software ThellierTool4.0

The ThellierTool4.0 is an intuitive and easy‐to‐use software which provides the possibility to analyze a wide range of different modifications of the Thellier absolute paleointensity experiment (available at http://earthref.org/tools/). Besides the Arai plot for paleointensity determination, orthogonal projections of the direction, decay of NRM during thermal demagnetization, and additional plots regarding alteration and multidomain checks enable the user to visualize the quality of individual determinations. Experimental checks for magnetomineralogical changes, either in‐field or zero‐field pTRM* checks, are evaluated regarding their differences to the corresponding pTRM* acquisition in two most commonly used ways. Furthermore, a measure for the cumulative alteration differences beginning at room temperature is calculated, and the possibility to correct for magnetomineralogical changes is provided. Two different experimental methods to check for multidomain bias are supported and analyzed by the software. Intensity differences recorded by pTRM*‐tail checks are calculated. Accounting for the directional difference between applied laboratory field and magnetization of the sample, the effective pTRM*‐tail is determined, and thus failures of Thellier's law of independence are monitored. Failures of the law of additivity, experimentally observed by additivity checks, are also evaluated by the software. The vectorial character of individual measurements is fully considered for all calculations. Uniform selection criteria for acceptance and rejection of determinations can be applied, and a set of such criteria with emphasis on minimal bias due to alteration, multidomain remanence, and analysis/experimental inaccuracies is suggested.

Multidomain behavior during Thellier paleointensity experiments: a phenomenological model

The most commonly used method for paleointensity determination, the Thellier method, requires the absence of multidomain magnetic behavior. The influence of multidomain remanence on paleointensity determination and its recognition during the experiment are, however, only partially understood. Here, we present a phenomenological model of TRM acquisition and its application to the Thellier experiment. This model is an extension of the approach of [Earth Planet. Sci. Lett. 188 (2001) 45] and is used to investigate, beside other multidomain characteristics, the effect of pTRM*-tails. Furthermore, the appropriateness of typically applied checks during paleointensity determination like alteration checks, pTRM*-tail checks and additivity checks is scrutinized. It is shown that pTRM*-tails significantly affect the shape of the Arai diagram. Concave curvature of the Arai plot for samples containing multidomain particles is related to remanences with unblocking temperatures below their respective blocking temperatures. pTRM*-tails, however, which affect unblocking temperatures above the respective blocking temperatures, lead to a convex shape. An overlap of both convex and concave characteristics can produce s-shaped Arai diagrams [Phys. Earth Planet. Int.]. The effect of pTRM*-tails is strongly dependent on the angle and intensity difference between applied field and NRM of the sample. The commonly used in-field alteration check is biased by pTRM*-tails and shows deviations from the expected value even if alteration is absent. pTRM*-tail checks show the presence of tails and thus give evidence for multidomain behavior, only if the laboratory field is applied in a different direction than the NRM. The larger the angle between NRM and laboratory field, the larger the value of the pTRM*-tail check. A new method for calculation of the pTRM*-tail check is proposed which accounts for such angular difference, as well as intensity differences. Conducting the Thellier experiment with alteration checks, pTRM*-tail checks, and additivity checks at an angle between NRM and laboratory field > 0, provides the possibility to unequivocally identify multidomain bias.

Experimental procedure to detect multidomain remanence during Thellier–Thellier experiments

Reliability checks in Thellier-type experiments usually focus on the detection of chemical alteration of the magnetic mineral component or the formation of new magnetic phases during the heating process. However, a major problem in Thellier-type experiments is the presence of multidomain (MD) particles which can lead either to complete failure of palaeointensity determinations or to serious misinterpretation. We present a modification of the Thellier–Thellier experiment that detects the presence of MD particles by verifying the law of additivity of pTRMs. The law of additivity is valid for regular pTRMs (i.e. the upper temperature of pTRM acquisition is reached by cooling from the Curie temperature) for both SD and MD particles, whereas it is not valid for pTRM * (the upper temperature of pTRM acquisition is reached by heating from room temperature) in the case of MD particles. As the partial thermoremanences imparted in Thellier–Thellier experiments are of the pTRM * type, additivity as a prerequisite for the validity of the obtained result is not given if the remanence is carried predominantly by MD particles. The proposed method is applied to seven synthetic samples covering a grain size range of 23 nm to 12.1 μm. The obtained palaeointensity estimates show a significant error for all samples with grain sizes >0.7 μm due to the failure of the law of additivity. Our experiment is able to detect this failure.

The Lowrie–Fuller test: single-domain and micromagnetic theory

Lowrie and Fuller [J. Geophys. Res. 76 (1971) 6339–6349] proposed a test to distinguish between single-domain (SD) and multi-domain remanence using alternating field (AF) demagnetization. However, their criterion for SD grains, derived from experiments on nominally SD samples, seems to contradict the SD theory. A new, more general SD theory is presented that includes the effects of variations in grain shape and volume, applied stress and composition. One factor is varied at a time while the others are held fixed. If the variable is stress or grain shape, the AF demagnetization curves do not satisfy the Lowrie–Fuller criterion for SD grains, as noted in earlier work. The same is true for variable composition if the magnetic hysteresis is controlled by shape anisotropy. Grain volume affects the AF demagnetization curve through thermal fluctuations. When the variable is volume, the sample passes the Lowrie–Fuller test. If the variable is grain orientation, the outcome depends on the distribution of orientations and the direction of the field. A highly anisotropic sample will pass the Lowrie–Fuller test in one direction and fail it in another. Thus, depending on the dominant variable, an ensemble of SD grains can pass or fail the Lowrie–Fuller test. However, a full analysis of the Lowrie–Fuller test must also take into account the critical sizes for transition from SD to non-SD properties. In particular, above the critical volume V SD coerc, the magnetization reverses by non-uniform rotation. In samples that have been used to test the SD theory, the range of volumes is broad and most of the remanence is carried by grains larger than V SD coerc. The SD theory will only apply to samples with grain volumes concentrated near the superparamagnetic–SD boundary. Such samples will have many characteristics usually attributed to MD grains. The shapes of their AF demagnetization curves will be determined mainly by the volume distribution. Therefore, they will satisfy the Lowrie–Fuller criterion for SD grains.

Review of the low-temperature magnetic properties of magnetite from a rock magnetic perspective

The low-temperature magnetic properties of magnetite are reviewed, and implications for rock magnetism considered. The behaviour of fundamental properties of magnetite at low temperatures near the Verwey transition (Tv ) are documented, and attention is given to various Verwey transition theories. The low-temperature behaviour of the magnetic energies that control domain structure is reviewed in detail. For the first time in rock magnetic literature, the low-temperature anomaly in spontaneous magnetization (Ms ) is documented and the differences between the saturation magnetization and Ms near the Verwey transition are discussed. It is argued that the low-temperature behaviour of the magnetocrystalline anisotropy, and in particular the anomaly at Tv , is most likely to affect multidomain remanence during low-temperature cycling. For multidomain crystals it is calculated that the large increase in magnetocrystalline anisotropy intensity and reduction in symmetry on cooling through Tv is likely to reduce the stability of closure domains.

Paleomagnetism and paleothermometry of the Sydney Basin 2. Origin of anomalously high unblocking temperatures

The Milton Monzonite of southeastern Australia was thermoviscously remagnetized as a result of Cretaceous burial and uplift. Thermal demagnetization separates the low unblocking temperature (LT) overprint from the high unblocking temperature (HT) primary remanence, with a relatively sharp junction between LT and HT components in vector projections. For single‐domain grains, the junction temperature TL between two such vectors corresponds to the maximum blocking temperature Tr reactivated in nature, apart from a correction for the difference between natural and laboratory timescales. However, measured TL values are distributed over an implausibly wide range (>250°C) for burial remagnetization of an untilted intrusion like the Milton Monzonite. Furthermore, many TL values are anomalously high compared to the predictions of single‐domain theory. Multidomain grains are the cause of these anomalies. Samples pretreated before thermal demagnetization by zero‐field cycling to liquid nitrogen temperature, so as to erase multidomain remanence and isolate single‐domain remanence, do have the theoretically expected TL values. In these samples, realistic remagnetization time and temperature (tr, Tr) conditions in nature are predicted using the t‐T contours of Pullaiah et al. [1975]. The anomalously high TL values before low‐temperature treatment are due to multidomain grains, which carry ≥50% of the LT overprint. The LT thermal demagnetization curve in samples dominated by multidomain grains is quasi‐exponential in shape with a high‐temperature tail extending almost to the Curie point, as predicted by multidomain theory. These high LT unblocking temperatures, which are much greater than plausible remagnetization temperatures reached in nature, overlap and mask the lower part of the HT unblocking temperature spectrum, driving up TL values and leading to inflated estimates of Tr. Although multidomain remanence is a sufficient explanation of anomalously high unblocking temperatures of thermoviscous overprints in the Milton Monzonite, chemical overprinting may be a factor in other lithologies and tectonic settings.

Separating multidomain and single‐domain‐like remanences in pseudo‐single‐domain magnetites (215–540 nm) by low‐temperature demagnetization

It has long been hypothesized that the pseudo‐single‐domain properties of submicron magnetites are a result of coexisting multidomain and single‐domain‐like remanences. We have separated these two types of remanence by low‐temperature demagnetization (LTD), in which samples are cooled through the low‐temperature magnetite isotropic point and rewarmed to room temperature, all in zero field. At the isotropic point, the first crystalline anisotropy constant vanishes, and multidomain remanence due to displaced domain walls will be unpinned. We measured stepwise alternating field (AF) and thermal demagnetization curves of weak‐field thermoremanent magnetization (TRM) and anhysteretic remanent magnetization and of saturation isothermal remanent magnetization (IRMs), both with and without prior LTD, for magnetites with mean particle sizes of 215, 390, and 540 nm. The memory fraction of remanence recovered after LTD, which represents the single‐domain‐like component, decreased from 44% for 215‐nm grains to 23% for 540‐nm grains in the case of IRMs. Memory of TRM was approximately twice as large. AF decay curves were soft and multidomain‐like before LTD but became harder and single‐domain‐like in shape after LTD. LTD removed most low unblocking temperature TRM and IRMs. Distributed low unblocking temperatures are therefore a multidomain phenomenon, probably resulting from successive adjustments of domain walls to changing internal fields and pinning strengths during heating.

Domain wall resonance and stability in magnetite

An understanding of the factors governing domain wall stability is crucial to any attempt to model remanence in multidomain materials. This paper introduces a technique to the study of multidomain remanence which has not previously seen application in rock magnetism: domain wall resonance (DWR). We show that the variation of the DWR spectra with different RF power levels gives useful, and hitherto unavailable, information about the nature of the potentials which govern domain wall motion. In particular, we demonstrate that these potentials must rise less rapidly than a harmonic potential, contrary to previous models. The possible mechanisms for domain wall pinning are discussed and it is concluded that the likeliest candidates for pinning sites in our samples, at least, are large nonuniform dislocation arrays. Finally, we suggest a possible method for selectively demagnetizing multidomain grains based on DWR techniques.

Discrimination of TRM and CRM by blocking-temperature spectrum analysis

Overlap of blocking-temperature spectra can be caused by three factors: chemical overprinting, the presence of partial thermoremanences (PTRM's) in two coexisting magnetic minerals, and the presence of PTRM's in coexisting multidomain and single-domain states of the same mineral. Multidomain relaxation mechanisms are not well understood and therefore quantitative estimates of overlap due to the coexistence of single-domain and multidomain states of the same mineral cannot be made, although it is evident that such overlap may be large. The maximum overlaps which would be expected at various temperatures due to coexisting single-domain magnetite and hematite are calculated. Where it can be shown that multidomain remanence is negligible, blocking-temperature overlaps which exceed these limits are, therefore, indicative of CRM overprinting.

The demagnetization field of multidomain grains.

The demagnetization ‘constant’, N, enters into all theoretical considerations of multidomain remanence. Although most workers believe N to be related simply to the shape of the grain, no theoretical study of N has previously been completed. It is shown in this paper thatN monotonically increases with increase in remanence in multidomain grains. Although N can be approximated by a constant in the limit of small displacements of domain walls from their demagnetized sites, this constant critically depends on the domain geometry.