Weak-field Thermoremanent Magnetization Research Articles

High‐Temperature Observation of Transdomain Transitions in Vortex States in Intermediate Titanomagnetite

AbstractNatural materials contain small grains of magnetic iron oxides that can record information about the magnetic field of the Earth when they form and can be used to document changes in the geomagnetic field through time. Thermoremanent magnetization is the most stable type of remanent magnetization in igneous rocks and can be carried by particle sizes above the upper size limit for single‐domain behavior. To better understand thermoremanent magnetization in particles larger than single domain, we imaged the thermal dependence of magnetic structures in ~1.5‐μm grains of titanomagnetite (Fe2.46Ti0.54O4) using variable‐temperature magnetic force microscopy. At room temperature, grains displayed single‐vortex and multivortex states. Upon heating, the single‐vortex state was found to be stable up to the Curie temperature (~215 °C), whereas multivortex states unblocked between 125 and 200 °C by transitioning into single‐vortex states. During cooling in a weak field (~0.1 mT), single‐vortex states nucleated just below the Curie temperature and remained unchanged to room temperature. The single‐vortex state was the only magnetic state observed at room temperature after weak field thermoremanent magnetization acquisition experiments. These observations indicate that single‐vortex states occur in titanomagnetite and, like single‐domain particles, have high thermal stability necessary for carrying stable paleomagnetic remanence.

Acquisition of thermoremanent magnetization in weak magnetic fields

Summary By means of a phenomenological model of weak-field thermoremanent magnetization (TRM) it is possible to reproduce the complex behaviour of TRM and partial TRM (pTRM) to a good approximation. The model depicts a sample as a collection of statistically independent magnetization elements, each of which possesses a definite blocking and a definite unblocking temperature. Moreover, for weak fields, linear dependence of remanence on applied field is assumed for each magnetization element. These assumptions are sufficient to derive to first order the experimentally found relations between the different types of pTRMs. The kinematic equation, which is assumed to govern the cooling process in multidomain (MD) material, is obtained from the model when mean-field interaction is taken into account. The phenomenological model is supported by two physical approaches. Using magnetic phase theory the coefficients that occur in the kinematic equation as derived from the phenomenological model can be physically interpreted for an isotropic ensemble of uniaxial MD particles. A rigorous micromagnetic analysis of the thermal equilibrium state proves the linearity of equilibrium remanence as a function of a weak applied field. A statistical argument extends this linearity to weak-field TRM, which may be carried by non-equilibrium states.

Revisiting the Lowrie–Fuller test: alternating field demagnetization characteristics of single-domain through multidomain glass–ceramic magnetite

This paper reports the alternating field demagnetization characteristics of glass–ceramic magnetite assemblages carrying weak-field thermoremanent magnetization (TRM), weak-field anhysteretic remanent magnetization (ARM), and saturation remanence ( J rs). Average grain sizes vary from less than 0.1 μm to approximately 100 μm, and hysteresis parameters indicate that these assemblages encompass single-domain (SD) through truly multidomain (MD) behavior. In all assemblages, weak-field TRM and weak-field ARM are more stable to alternating field demagnetization than is ( J rs). This response is especially remarkable in the 100 μm assemblage, which otherwise displays truly MD behavior. Although the SD samples pass the Lowrie–Fuller test for SD behavior, calculations presented here show that populations of noninteracting, uniaxial SD grains should behave in just the opposite sense to that reported originally by Lowrie and Fuller. This discrepancy could indicate that SD, glass–ceramic magnetite populations are more affected by magnetic interactions than would be expected for magnetite crystals that nucleated individually from a silicate matrix. This interpretation is supported by the SD assemblages failing the `Cisowski' test: that is, the curves for acquisition and AF demagnetization of ( J rs) intersect well below the 50% mark. However, a second and intriguing explanation of the SD-like results obtained from all samples is that alternating field demagnetization characteristics reflect a strong dependence of local energy minimum domain state, and its associated stability, on the state of magnetization.

High-temperature hysteresis and thermoremanence of single-domain maghemite

Magnetic properties have been measured for acicular maghemite crystals of single-domain (SD) size between room temperature and the Curie temperature TC of 645°C. The transformation of maghemite to hematite takes place at 752°C, far above TC of the present γ-Fe2O3. The observed high-temperature stability of this material made it possible to study the thermoremanent magnetization (TRM) and high-temperature hysteresis properties of γ-Fe2O3. The temperature dependence of coercive force shows that the main contribution to the coercivity is from shape anisotropy. Magnetization reversal is dominated by incoherent rotation. The experimental results are consistent with the fanning mode of reversal in a chain of spheres. The weak-field TRM was linear with applied field, as predicted from Néel SD theory.

A synthetic TRM induction curve for fine particles generated from domain observations

A synthetic thermoremanent magnetization (TRM) induction curve is produced from domain observations of an ensemble of pseudosingle domain (PSD) titanomagnetite particles in oceanic basalts. The observations were made after the samples were given a TRM in fields of 0.1, 2, 4, 6, and 8mT. An observation was also made of the domain state for saturation isothermal remanent magnetization (IRMs). The number and maximum size of grains which remain in a metastable single domain (SD) state increases with increasing field. This in turn gives rise to a field dependence of the total moment carried by such particles in the ensemble. By observing the number of metastable SD grains and estimating their volume one can construct a TRM induction curve. This curve approximates the known TRM induction curve for fine particles, demonstrating that the magnitude of TRM in fine particles of titanomagnetites is controlled by nucleation. This nucleation model also explains why weak field TRM is more stable against alternating field (AF) demagnetization than IRMs.

Domain observations of titanomagnetites from room temperature to Curie point and the nature of thermoremanent magnetism in fine particles

Single-domain theory1 has been remarkably successful as a theoretical basis for rock magnetism and hence palaeomagnetism. However, the grain size of much of the magnetic material present in rocks is too large to be single-domain according to classical theory2. The resolution of this paradox may lie in the recognition that difficulties in domain wall nucleation3–10 can preclude the multi-domain state even though it is energetically favoured. Thus, multi-domain particles are observed in a single-domain state in rocks carrying saturation remanence magnetization (IRMs). Inhibition of domain wall nucleation has also been shown to produce high-coercivity materials suitable for permanent magnets11–13. Thermo-remanent magnetization (TRM) is one of the most important mechanisms of remanent magnetization for the study of palaeomagnetism, being acquired for example by lavas as they cool on the Earth's surface. We present here domain studies of titanomagnetites showing the increasing importance of the magnetostatic energy in the external field as temperature is increased, and that rocks carrying weak-field TRM, like IRMs, contain multi-domain grains in a single-domain state. These particles contribute very large moments to TRM; indeed, particles of this nature may carry a substantial part of the palaeomagnetic signal. The results also suggest that weak-field cooling of dispersions of suitable fine particles might leave them in a particularly hard magnetic state of some utility.

Coercive forces and coercivity spectra of submicron magnetites

Remanent coercivity spectra have been determined for four dispersions of magnetite with mean particle sizes of 0.04–0.22 μm using four different methods: (1) acquisition of isothermal remanent magnetization (IRM) in continuous fields; (2) stepwise reversal of saturation IRM by opposite-polarity continuous fields; (3) stepwise alternating-field (AF) demagnetization of saturation IRM; and (4) AF demagnetization of weak-field thermoremanent magnetization (TRM). Alternating-field methods gave narrower distributions with lower median coercivities than continuous-field (CF) methods. The two CF spectra are offset: the remanent coercive force, H R , which is the median coercivity determined by CF reversal of saturation remanence, is always less than the median coercivity, H′ R , determined from IRM acquisition. The median destructive field of saturation IRM, H˜ 1/2 , the median coercivity determined by AF demagnetization, is less than either H R or H′ R . The relation reported by Dankers [1], H′ R +H˜ 1/2 ≈ 2H R , is obeyed for these samples and for a companion suite of single-domain magnetites and maghemites. Ordinary coercive force. H C , is substantially affected by reversible magnetization and is a poor indicator of the median coercivity of remanence. H C is much smaller than H˜ 1/2 , H R or H′ R and varies in a different way with particle size.

Reversed thermoremanent magnetization in synthetic titanomagnetites as a consequence of high temperature oxidation.

Synthetic single crystals of titanomagnetite (Fe2.4T0.6O4) were progressively oxidized at 1, 275°C, under controlled CO/CO2 atmospheres to simulate the “deuteric oxidation” process which occurs in nature. For moderate degrees of oxidation the resultant phase assemblage comprised two titanomagnetite phases intergrown with hemoilmenite lamellae. During the acquisition of weak-field thermoremanent magnetization (TRM) by the oxidized multiphase samples, the direction of magnetization was found in some cases to be opposite to that of the applied magnetic field. This self-reversal is thought to be due to a negative magnetostatic interaction between the two titanomagnetites. The interaction field was estimated as being of the order of a few hundred Amps per metre and is thought to be magnetostatic in origin. The region of self-reversal was confined to a narrow oxidation range although partial self-reversal was observed over a much wider range.

Magnetic behavior of magnetite and rocks stressed to failure — Relation to earthquake prediction

Polycrystalline magnetite and rock samples have been subjected to uniaxial isothermal compression to failure in an attempt to see whether any magnetic expression of dilatancy could be observed. The change in magnetic susceptibility, remanent moment and total moment were recorded continuously as a function of increasing compression. Changes in the slopes of the magnetization-stress curves were observed for stresses corresponding to the onset of dilatancy, and anomalous effects were seen at the approach to failure. Marked directional changes of the magnetization vector were frequently present. Samples carrying weak field thermo-remanent magnetization stressed in zero field showed anomalous increases in intensity of magnetization. In contrast samples carrying saturation isothermal remanent magnetization were demagnetized on the application of stress in zero field. Single-domain material showed virtually no change due to compression. For multidomain material, magnetic observations permitted prediction of failure in the laboratory. This study suggests the eventual possibility of earthquake prediction using magnetic precursors, particularly with three-component measurements, and it indicates that multidomain materials are better transducers of the seismomagnetic effect than single-domain materials.

Thermoremanent magnetization in submicroscopic magnetite

Induction and demagnetization characteristics of thermoremanent magnetization (TRM) have been studied for four synthetic magnetites with mean particle sizes of 0.04–0.22 μm. The experiments are intended to simulate the acquisition and laboratory demagnetization of natural remanent magnetization carried by submicroscopic but non-single-domain titanomagnetite particles in igneous rocks. Although hysteresis properties indicate that the magnetites are above single-domain (SD) size, their weak-field TRM are SD-like. TRM intensity, for a 1-oe inducing field, ranges from 6 to 12 emu/cm3. Median alternating demagnetization fields are between 275 and 400 oe, while blocking temperatures are generally within 50°C of the Curie point. However, neither Neel's SD theory nor Stacey's pseudo-SD theory can account for the observed dependence of TRM on inducing field, since theoretically TRM should saturate in rather low fields. Neel's two-domain theory fits the strong-field data very well but is inadequate in the weak-field region. These failures indicate that theoretically assumed two- and four-domain structures are not appropriate in particles just above SD size. Blocking temperature data are consistent with well-developed multidomain (MD) structure (i.e., thermal activation volume associated with a domain wall notably less than particle volume) in 0.22-μm particles but a wavelike spin structure without recognizable domains in particles ≤0.1 μm in size. The dependence of weak-field TRM on grain size d also supports this interpretation: TRM varies approximately as d−1 between 0.1 μm and the pseudo-SD threshold at 15–20 μm, but below 0.1 μm there is no clear size dependence. Particles with wavelike spin structure have not been recognized previously but may well explain the MD-like hysteresis and SD-like TRM of many igneous rocks.

The effect of internal stress on the thermoremanence of nickel

In order to assess the influence of internal stress on magnetic properties of interest in paleomagnetic research, experiments were made on six strained samples from a nickel single crystal. The samples were (100) disks 7.5 mm in diameter and 2.5 mm thick that were cold rolled to produce dislocation densities between 108 and 1011cm2. The intensities of weak field thermoremanent magnetization (TRM) produced in the samples were proportional to their dislocation densities. Direct control of magnetization by stress fields near dislocations is shown to be energetically unfavorable, and a model is developed to explain relationships between remanence and dislocation density based on the pinning of domain walls. Some of the remanence remains after alternating field demagnetization in 400 oe, but possible higher stabilities are masked by a significant viscous component.