Studies of partial rotational remanent magnetization and rotational remanent magnetization at slow speeds of rotation

Summary. Rotational remanent magnetizations and partial rotational remanent magnetizations have been induced in four specimens using alternating magnetic fields of 55 mT maximum peak strength and 128 Hz, and speeds of rotation between 0.0016 and 0.4 rev s⁻¹. Each partial rotational remanent magnetization ( PRRM ), was produced by rotating the specimen only at the maximum setting of the alternating field. The variation of PRRM with (a) speed of rotation, ω, and (b) total angle of rotation, θ, was investigated. In (a), PRRM fell slowly but steadily as ω increased; for (b) it rose sharply as θ increased up to 60° and reached a maximum for θ between 90° and 120°. Alternating field demagnetizations of PRRMs were performed with the specimen (a) at rest, and (b) rotating about an axis perpendicular to the field. Rotation significantly enhanced the demagnetization process. Variation of the time T , taken to remove the inducing alternating field produced no detectable effect in the case of PRRM , but affected the value of ω at which a given feature of the RRM —ω curve appeared, and the product θ_F(=ω T ) appears to be more important than either ω or T separately. Current theories on RRM can be used to explain some of the new experimental data on PRRM .     

Directions of anhysteretic and rotational remanent magnetizations in rotating samples

Summary. These experiments support Stephenson's predictions that partial anhysteretic remanent magnetizations produced in rotating samples deviate from the steady field direction, which is collinear with the rotation axis, by amounts depending on the angle, between the alternating magnetic field axis and the rotation axis. A similar effect was observed for partial rotational remanent magnetizations. Possible differences between the two remanence types were noted.     

The origin of rotational remanent magnetization

Summary. Rotational remanent magnetization, RRM, is the magnetization acquired when a sample is rotated during alternating field demagnetization. Although the existence of RRM has been well documented in different laboratories, until now no physical mechanism explaining its origin has been given. We propose that the RRM originates from thermal fluctuations biased by a precessional torque associated with the alternating field. Our theory is consistent with the observation that no directional preference exists in the experimental situation until the sample is rotated relative to the alternating field. Moreover, our theory predicts that the combined sample rotation and precession will produce a RRM that switches direction when the frequency of sample rotation increases from any value below the frequency of the alternating field to any value above that frequency as observed in experiments. Although no precise theory is given for the intensity of RRM, the model presented here can qualitatively explain previous intensity observations.     

The angular dependence of rotational and anhysteretic remanent magnetizations in rotating rock samples

Summary. Recent experimental work by Edwards has demonstrated that rotational remanent magnetization (RRM) is not a maximum when the alternating field is normal to the rotation axis of the sample (a rock) but is greatest when the angle is about 75°. Experiments involving the production of ARM during sample rotation gave a similar result with a maximum at about 60°. These results are explained here in terms of the response of an isotropic assembly of identical single-domain particles to a strong alternating magnetic field.     

An experiment relating to rotational remanent magnetization and frequency of demagnetizing field

Summary. Several tests have been carried out to investigate how the generation of rotational remanent magnetization depended on the frequency of the applied demagnetizing field. The equipment used is described. The investigation was carried out using two specimens, one being a synthetic specimen of magnetite, and the other a cylindrical rock sample. These specimens gave virtually identical behaviour with varying frequency, unlike the differing behaviours reported previously by Wilson & Lomax. For each of the separate alternating field frequencies used (ranging from 50 to 1210 Hz), as the rotational speed of each specimen was reduced from 0.1 cycle s⁻¹, the corresponding rotational remanent magnetization increased to a maximum value when the rotational speed was in each case just a little greater than 0.01 cycle s⁻¹, after which the rotational remanence decreases as the rotational speed decreases.     

A magnetization process of sediments: laboratory experiments on post-depositional remanent magnetization

Summary. Palaeomagnetic studies require a theory of magnetization mechanism of sediments and a method of estimating magnetic field intensity from their remanences. This paper establishes a physical basis for the generation of the remanence in deep-sea and lake sediments experimentally.
Redeposition experiments have been carried out under centrifugal force in weak magnetic fields. The centrifuging method produces post-depositional remanent magnetization (post-DRM) in the compacted sediment, and its remanence and susceptibility are compatible with those of natural sediments and reconstituted materials of other redepositional experiments. Three properties of the post-DRM have been deduced from the experiments: (1) the efficiency of acquisition of post-DRM decreases with increase in density during the compaction process, (2) the total post-DRM is equal to the sum of the partial post-DRM (addition law), and (3) time is not a substantial factor for alignment of the magnetic particles. These results lead to the conclusions that the magnetic particles do not rotate steadily but in a series of steps, and that the density change is the crucial factor giving rise to the post-DRM.
A mathematical formula representing the remanence record in sediments is proposed on the basis of the experimental results and the model. The principal equation is expressed as an integral of the product of three parameters over time when sediments have been compacted; the field intensity variation, characteristic function of the sediment and the time derivative of the density change.     

The additivity of partial thermal remanent magnetization in magnetite

Summary. Experiments were done to test the additivity of partial thermal remanent magnetizations (PTRMs) for prepared samples containing magnetite particles whose sizes range from SD (single domain) to MD (multidomain). The experiments compare the sum of two PTRMs with total-TRM, all produced by the same external field of 0.47 oe. The most significant conclusion of this paper is that, to first order, the additivity of PTRMs is obeyed for the magnetites of this study regardless of particle size. However, small, higher order deviations from additivity occur such that ΣPTRM > TRM by an average of about 1 per cent. Though small, these departures from additivity are significant at the 99 per cent confidence level, and they can be understood in terms of Néel's theory for SD particles. The small departures from additivity are intrinsic to the experimental procedure in which some particles acquire remanence twice, in each of the two PTRM steps. In the limit of small inducing fields additivity should be obeyed exactly for the magnetites of this study and for samples of interest in palaeomagnetism. The deviations from additivity should have no effect on palaeointensity determinations by the Thelliers' version of the Thellier palaeointensity method. For palaeointensity determinations by Coe's version of the Thellier method the effects of deviations from additivity would be very small, less than 4 per cent on the average for a worst-case experimental configuration, and these effects can be minimized by producing PTRMs parallel to the original NRM and by using weak laboratory fields.     

An investigation of the scattered remanent magnetization of the Dunnet Head sandstone

Summary. The directions of remanent magnetism of samples of the Dunnet Head sandstone from Scotland are very scattered on a scale down to a few millimetres, although an overall mean direction is reasonably well defined. The scattered directions show considerable stability against thermal demagnetization and there is evidence that haematite pigment is an important carrier of the remanence. It is concluded that the origin of the inhomogeneous magnetization is a disturbed ambient field during acquisition of chemical remanence by the pigment.