Remanent Magnetization Vector Research Articles

Petromagnetism and paleomagnetism of kimberlite pipesof the Verkhnemunskoe deposit (Yakutsk diamondiferous province)

The purpose of the studies of kimberlite pipes of the Verkhnemunskoe diamond field is to calculate the Middle Paleozoic paleomagnetic pole to clarify the trajectory of the apparent migration of the pole and reconstructions of the paleogeographic position of the Siberian platform at the time of the manifestation of active tectono-magmatic processes. The Verkhnemunskoye deposit is located within the Verkhnemunskoye kimberlite field of the Yakutsk diamondiferous province and includes five kimberlite pipes (Deimos, Zapolyarnaya, Komsomolskaya-Magnitnaya, Novinka and Poiskovaya), the age of which according to geological and isotopic data is estimated as Late Devonian-Early Carboniferous (372–347 Ma). For the first time scalar and vector physical parameters of kimberlites and captured xenoliths from different structural-material complexes of the Earth’s crust, as well as the host terrigenous sedimentary rocks of the Early Paleozoic were obtained, which are necessary for the development of physical-geological models of the Verkhnemunskoe field deposits. A relatively deep level of erosional shearing of the field has been established. The primary (synchronous with the formation of the field) natural residual magnetization was preserved in the kimberlite cohesive mass. The main carrier minerals of natural remanent magnetization vectors of kimberlites are unaltered magnesioferrite and magnetite, which indicates their thermo-sufficient nature. The natural remanent magnetization vectors of captured xenoliths indicate that the influence of hypergenic processes did not strongly affect the NRM vectors of kimberlites. Firing test is positive. The paleomagnetic pole with coordinates Φ = 26.5°N, Λ = 142.2°E, dp/dm = 6.2/7.8° was calculated from the obtained clusters of N = 10 vectors of the primary natural remanent magnetization of kimberlite pipes. On its basis, we reconstructed the paleogeographic position of the Siberian Platform, which at the time of the kimberlite intrusion was located in the middle latitudes of the northern hemisphere and was facing north with its southern edge.

Dynamic physical-geological model of complex folding according to paleomagnetic data

The purpose of the study is to evaluate the possibility of using the mathematical apparatus of spatial rotations to solve the determination issues of the nature and age of natural remanent magnetization vectors under complex tectonic dislocations of rocks manifested in the areas of modern folded structures of platform framing. Dynamic physical-geological models are shown to be useful when forming complex tectonic-magmatic systems. Mathematical (computer) modeling in comparison with other methods of studying geological processes features higher accuracy, cost-effectiveness and unambiguity of data interpretation in achieving the set goal. On the basis of dynamic physical-geological model of complex folding, an algorithm is developed and the results of mathematical modeling of natural remanent magnetization vectors are given for solving direct and inverse problems on correct application of the fold test under complex rock deformations. The dynamic physical-geological model of complex folded structure formation shows that the characteristic natural remanent magnetization vectors identified in laboratory experiments on demagnetization can be used to determine their age relative to the folding stages, and, depending on this, fully or partially restore the number, sequence and direction of tectonic dislocations. This will enable us to solve the mineralogenic and geodynamic problems of folded region development more effectively on the basis of paleomagnetic data.

2.5D magnetization vector inversion of vector magnetic data

The remanent magnetization vector records the earth’s magnetic field at the time of the formation process of magnetized geologic units. Recovering the magnetization vector from magnetic data can provide extra information regarding the source properties to differentiate geologic units and reveal thermal evolution or tectonic history. To provide this information, the magnetization vector inversion (MVI) method has been used to invert for the magnetization vector. The MVI usually works with the total-field magnetic anomaly magnitude ([Formula: see text]). The magnitude is an approximation of the projection of the magnetic anomaly vector onto the normal geomagnetic field. For highly magnetic sources, the approximation error of [Formula: see text] cannot be ignored. However, this approximation error can be avoided by using measured vector magnetic data. To reduce the severe ambiguity of 3D MVI, specific constraints based on extra prior information are usually used. For 2D or 2.5D cases, MVI only inverts for magnitude and inclination of the magnetization, which encounters less ambiguity than that of the 3D case. We have developed a 2.5D MVI approach using vector magnetic data. To reduce the divergent and smooth trends in magnetization vectors recovered by MVI in the Cartesian framework, we make use of a focusing constraint method to improve the imaging. The inversion results of synthetic data indicate that the method is able to recover magnetization magnitude and inclinations close to the true values and is fairly robust to inappropriate choice of the profile location and heading. Finally, the method is applied to the measured airborne vector magnetic data from the Qixin area of the East Tianshan Mountains in China. The distributions and directions of magnetization of the mafic-ultramafic rocks and linear structures are revealed, which provides the information for distinguishing geologic units and geologic differentiation.

Interpretation for magnetic data at low latitude areas using continuous wavelet transform and marquardt algorithm

As analyzing geomagnetic data at low latitude areas for instance the Mekong Delta (latitudes 11,07o), significant problem is that both of the magnetization and ambient field are not vertical totally, making magnetic anomalies antisymmetrical and often skewed to the location of the sources. In this paper, two-dimensional continuous wavelet transform (2-D CWT), using Farshad-Sailhac complex wavelet function is studied and applied for reducing the magnetic anomaly to a symmetrical one - this located on the source of the anomaly, and then determining the position of the center of the object causing anomalies by wavelet transform modulus maxima (WTMM) method. Next, magnetic data is extracted in two perpendicular directions passing through the center of the source to perform one-dimensional continuous wavelet transform (1-D CWT) to estimate the shape, depth and size of the source. Then, using the Marquardt algorithm to solve the inverse problem by least-squares method to further identify other characteristic parameters of the source such as: vertical size, remanent magnetization vector. The reliability of the proposed method is verified through theoretical models before application for analyzing the geomagnetic data in the Mekong Delta. The results are consistency with deep hole data, having small root mean square error, contribute to a better interpretation of the geological nature of the magnetic anomaly sources in the study area.

Separation of magnetic anomalies into induced and remanent magnetization contributions

ABSTRACTInversion of magnetic data is complicated by the presence of remanent magnetization, and it provides limited information about the magnetic source because of the insufficiency of data and constraint information. We propose a Fourier domain transformation allowing the separation of magnetic anomalies into the components caused by induced and remanent magnetizations. The approach is based on the hypothesis that each isolated source is homogeneous with a uniform and specific Koenigsberger ratio. The distributions of susceptibility and remanent magnetization are subsequently recovered from the separated anomalies. Anomaly components, susceptibility distribution and distribution of the remanent and total magnetization vectors (direction and intensity) can be achieved through the processing of the anomaly components. The proposed method therefore provides a procedure to test the hypotheses about target source and magnetic field, by verifying these models based on available information or a priori information from geology. We test our methods using synthetic and real data acquired over the Zhangfushan iron‐ore deposit and the Yeshan polymetallic deposit in eastern China. All the tests yield favourable results and the obtained models are helpful for the geological interpretation.

Geochemistry and Magnetism of Peat Deposits in the Khor–Kiya Interfluve, Sikhote–Alin (Preliminary Results)

This paper presents geochemical, microprobe, paleomagnetic, and rock magnetic data from peat accumulated over the past 10 000 years in the area between the Khor and Kiya rivers (Khabarovsk krai). A horizon ranging in thickness between 75 and 100 cm with elevated concentrations of Ni, Zn, Bi, and Se is identified as a result of comprehensive studies. Magnetite and carbon spherules, as well as native iron found at the bottom of the horizon suggest an impact event that occurred about 5500 years ago. For the first time, peat deposits are a target of paleomagnetic research. Distinct inclination variations of the remanent magnetization vector are established in the horizon at a depth of 50 to 75 cm, which corresponds in time to the “Etrussia” geomagnetic excursion (2500 years ago).

Susceptibility and remanent magnetization inversion of magnetic data with a priori information of the Köenigsberger ratio

SUMMARY Magnetic susceptibility and natural remanent magnetization of rocks are useful parameters to study geological structures and geodynamic processes. Traditional widely used algorithms for the inversion of magnetic data can recover the distribution of the apparent susceptibility or total magnetization intensity, but do not provide information on the remanent magnetization. In this paper, we propose a framework to directly invert for the magnetic susceptibility and the natural remanent magnetization vector using surface or airborne magnetic data, assuming that the Köenigsberger ratio of the rock is known or approximately deducible. The susceptibility and remanence are computed using two different approaches: (1) the susceptibility, intensity, and direction of the remanent magnetization are continuously recovered for each discretized cell and (2) the remanence direction is assumed to be uniform in each subzone and is iteratively computed as discrete values. Both processes are implemented using the preconditioned conjugate gradient algorithm. The method is tested on three synthetic models and one field data set from the Zaohuohexi iron-ore deposit, Qinghai Province, northwest China. The results of the continuous inversion show the trend of the remanent magnetization directions, while the discrete inversion yields more specific values. This inversion framework can determine the source bodies’ geometry and position, and also provide superposed and comprehensive information on the natural remanent magnetization, which may be useful to investigate geological bodies bearing stable primary remanent magnetization.

Artificial intelligence techniques to the interpretation of geophysical measurements

SummaryIntegration of geology and geophysics thinking requires a common earth model, that accommodates, with errors, all the features from the geophysics interpretation products, topological rules from geology, field mapping and drill logs, and the link between the two via physical rock property estimates. Automation of intelligent search, inference engines to this problem involves 200+ individual processes, to yield a family of plausible models. The true limiting factor is managing technical complexity and communicating to the geoscience team, not compute power.All models are wrong and are destined to be replaced as further data or insights are gleaned. So, taking too much time trying to create the ultimate “correct” interpretation is wasteful, hence the need for automation.An example of a simple pattern recognition technique to locate kimberlite pipes that have a magnetic and/or gravity response is given. This uses a range of pipe diameters, depths, topography, density and susceptibility, and variable directions of remanent magnetization vectors, in an automated manner.

Remanent magnetization mapping: A tool for greenfields magmatic Ni-Cu-PGE exploration undercover: Part 1

There are many challenges for greenfields exploration, particularly in vast, sparsely populated, partially covered terrains, e.g., a paucity of prior exploration data, and difficulties in gaining access to land. However, in many cases good aeromagnetic coverage is freely available, and for some styles of mineralization (e.g., magmatic Ni-Cu-PGE), aeromagnetic data alone can provide critical insights into the age, size and depth of multiple targets, across large regions, early in the exploration life cycle. This study investigates whether an understanding of remanent magnetization from three suites of mafic intrusions exposed in the Huckitta area, NT, Australia, can be utilized to provide a first pass exploration tool for exploration under shallow cover to the east. The study demonstrates that although the measured remanent magnetization vectors present were complex in some cases, the majority are sub-parallel (oriented steep down to the south), which implies a common mode of acquisition. It was found that the magnetizations were acquired during the Alice Springs Orogeny (ca 450–300 Ma), with the dominant magnetization occurring at ca 330 Ma. Therefore the magnetizations present do not relate to cooling of the parent magmas, but rather to cooling of lower crustal rocks from temperatures above the Curie point (>580 °C) during exhumation of the host terrain. Whilst this knowledge does not allow discrimination of the three suites remotely based purely on their remanent magnetization vectors, the recognition of a common magnetization vector present in all mafic rocks does allow us to better map mafic rocks under cover, and better constrain magnetic modelling and exploration targeting, as discussed in the second paper.

Paleomagnetic data on the sedimentation rate near the Mendeleev Rise (Arctic Ocean)

Seven cores of bottom sediments were collected during the cruise of R/V Akademik Fedorov in 2000 along the latitudinal profile crossing the Mendeleev Rise near 82° N, and then studied. The length of cores varies from 240 to 334 cm. In all cores, the boundary of the changing remanent magnetization vector corresponding to the Brunhes-Matuyama boundary was established reliably. In five cores, this boundary is at a depth of 86–102 cm; in two cores, collected to the west of the Mendeleev Rise, at depth levels of 188 and 208 cm. The correlation of paleomagnetic and lithological data demonstrates that the upper boundary of a bench of speckled pelites, overlapped by a bench of variegated pelites, is confined to the Olduvai episode or horizons lying slightly below. The average sedimentation rate of sediments from the Mendeleev Rise during the last 2–2.5 Ma have not exceeded 1–1.3 mm/ka, increasing towards the shelf seas of northeast Russia.

The remote determination of magnetic remanence

SummaryThe remote determination of magnetic remanence in rocks is a method that has largely been ignored because of the ambiguity associated with the estimation of both the Koenigsberger ratio and remanent magnetization direction. Our research shows that the resultant magnetization direction can be derived directly through inversion of magnetic data for an isolated magnetic anomaly. The resultant magnetization direction is a property of the target magnetic rocks and a robust inversion parameter. The departure angle of the resultant magnetization vector from that of the inducing magnetic field is an important indicator of the existence of remanent magnetization and the inversion process can detect departures that are not easily detected by visual inspection. This departure angle is called the Apparent Resultant Rotation Angle or ARRA.The induced field vector, remanent magnetization vector and resultant magnetization vector lie on a great circle. We find the intersection of the polar wander vector trace with the great circle to obtain one or more possible solutions for the remanent magnetization direction. Geological deduction will normally allow us to reduce the ambiguity for multiple solutions to obtain the most likely remanent magnetization direction. Once the remanent magnetization direction is established, it is then possible to determine the Koenigsberger ratio and magnetic susceptibility for the target.We illustrate the methodology with some synthetic models and targets from Australian magnetic surveys. Magnetic remanence is a physical property of the rock that is distinct from susceptibility and this methodology provides a new tool to help with the categorization and prioritization of exploration targets.

Iron deficiency in pyrrhotite of suevites from the Chesapeake Bay impact crater, USA—A consequence of shock metamorphism?

Abstract– Pyrrhotite from suevite of the 35 Ma Chesapeake Bay impact structure (CBIS) shows a shock metamorphism and we report on several mineralogical and magnetic features. Pyrrhotite shows strong brittle deformation with a high density of stacking faults, twinning parallel to the hexagonal (001) planes and average fault distances in the order of 10 nm. Although the determination of a superstructure was not possible due to the lattice defects, the reflections of the NiAs subcell, which is typical of all pyrrhotite modifications, were clearly detected. This phase is ferrimagnetic with a Curie temperature (TC) between 350 and 365 °C, and suevite with this phase does not show the 34 K transition. The most peculiar feature is the low metal/sulfur ratio of 0.81, which indicates a distinctly higher vacancy concentration than for 4C pyrrhotite and a composition close to smythite (Fe9S11). This phase carries a stable natural remanent magnetization and is relatively hard magnetic. Steep inclinations of the natural remanent magnetization vector, however, suggest that this phase has been remagnetized by the drilling process. A possible explanation is the magnetic domain size of faultless areas of about 10 nm in diameter, which is at the lower limit of the single domain size near the threshold, below which superparamagnetic behavior occurs. The low thermal stability of this phase excludes postshock heating above 300 °C for the suevite of the CBIS. Our results imply that the iron‐deficient pyrrhotite is produced by shock metamorphism, although an iron loss due to shock has never been reported before for pyrrhotite.

Remanence anisotropy effect on the palaeointensity results obtained from various archaeological materials, excluding pottery

Abstract The effect of magnetic anisotropy on the palaeointensity results has been evaluated in different materials, including samples from archaeological structures of various ages, such as baked clay from prehistoric domestic ovens or pottery kilns, burnt soil from ancient fires, and bricks and bricks or tiles used in the kiln’s construction. The remanence anisotropy was estimated by the thermoremanent (TRM) anisotropy tensor and isothermal remanence (IRM) tensor methods. The small anisotropy effect (less than 5%) observed in the palaeointensity results of baked clay from the relatively thin prehistoric oven’s floors estimated previously through IRM anisotropy was confirmed by TRM anisotropy of this material. The new results demonstrate the possibility of using IRM anisotropy evaluation to correct baked clay palaeointensity data instead of the more difficult to determine TRM anisotropy ellipsoid. This is not always the case for the palaeointensity results from bricks and tiles. The anisotropy correction to palaeointensity results seems negligible for materials other than pottery. It would therefore appear that the palaeointensity determination is more sensitive to the degree of remanence anisotropy P and the angle between the natural remanent magnetization (NRM) vector and the laboratory field direction, than to the angle between the NRM and the maximum axis of the remanence anisotropy ellipsoid (K max).

Re-computing palaeopoles for the effects of tectonic finite strain

The pre-Messinian limestone cover (∼ 58–8 Ma) to the Troodos ophiolite (∼ 88 Ma) of southern Cyprus is penetratively strained as shown by ubiquitous magnetic fabrics and, in many sites, stylolitic cleavage. These define a gently N-dipping foliation and an N-plunging extension. South-vergent folding and thrusting is well known in very localized whereas the bulk of the strained limestone cover dips gently south, disturbed by faulting. The magnetic fabrics and stylolitic cleavage define the axes of finite strain in all sites studied, and the calcite matrix was suitably ductile to permit the original palaeomagnetic directions to be de-strained assuming continuum behaviour. The optimum de-straining (30–40% shortening in a flattening strain) is compatible with the stylolitic cleavage development, restores bedding to the near-horizontal, and restores the characteristic remanent magnetization vectors (ChRMs) to concentrated, symmetrical Fisherian distributions. The strain-corrected ChRMs yield more reasonable palaeopole locations for the Lefkara and Pakhna Limestone and more uniform micro-plate rotation rates. Corrected palaeopoles reveal a relatively uniform anticlockwise rotation of the Troodos plate since the creation of the late Cretaceous (∼ 88 Ma) ocean lithosphere. It did not accelerate during the deposition of the limestone cover as required by palaeopoles calculated from data not corrected for finite strain but turned at ∼ 1.5° Ma − 1 since ∼ 58 Ma.

Reevaluation of magnetic chrons in the North Atlantic between 35°N and 47°N: Implications for the formation of the Azores Triple Junction and associated plateau

In this paper we present a new magnetic compilation for an area of the North Atlantic located between 35°N and 47°N and up to anomaly 33r. We also present a strategy to pick magnetic isochrones and compute finite rotation poles. This technique is based on a continuous reduction to the pole technique and some basic assumptions regarding the direction of the remanent magnetization vector. A cost function that measures the misfit between interpreted and rotated isochrones and the systematic exploitation of the parameter space is used to compute the best set of finite Eulerian rotations for the chrons 5, 6, 6C, 11–12, 13, 18, 20, 21, 22, 23, 24, 25, 26, 28, 29, 30, 32, 33, and 33r. This set of chrons and poles is used to discuss the evolution of the North Atlantic close to Iberia and, in particular, the onset and early development of the Azores Triple Junction area. We show that the relative motion between the Eurasian and the African plates can be coherently described in terms of rigid plate kinematics, respecting both the anomalies shapes and the precise location of the main structural elements of the area: the Pico Fracture Zone, the East Azores Fracture Zone, and the Gloria Fault. We distinguish from the magnetic point of view two different areas of the Azores plateau: the South Azores domain where almost undisturbed NNW magnetic lineations can be found and the Azores domain close to the topographic highs and with no systematic magnetic stripping with the exception of a few recent lineations, probably Matuyama and Brunhes. We present an approximate reconstruction of the plate configuration after chron 18 to conclude that the attachment of Iberia to Eurasia was younger than previously thought (lower Miocene), triggering the formation of the Azores domain, in which stretching took place essentially in the last 20 Ma at an average rate of ∼3.8 mm/a, and progressively attaching the South Azores domain to the African plate by a northward progression of the triple junction.

Multivectorial paleointensity determination by the Thellier method

Theoretical and experimental study of shape of the Arai‐Nagata diagrams for the natural remanent magnetization (NRM) consisting of two low‐ and high‐temperature partial thermal remanent magnetization vectors directed at angle θ to each other is considered. Assuming that the NRM is carried by single‐domain and/or small pseudosingle‐domain grains, that is, the Thellier laws of additivity and independence are valid, a theoretical consideration showed that the low‐temperature portion of the diagram has a hyperbolic shape. A striking feature of the diagram is that at obtuse angle θ and strong enough low‐temperature component it exhibits a minimum. The theoretical conclusions were examined experimentally; the results of the experiments are in a good agreement with the theoretical predictions. The shape of the low‐temperature portion is predetermined by the angle θ, the relative intensity of low‐temperature component, and the field h in which this component was imparted. These three parameters can be found from the results of Thellier experiments by the least squares method. It opens a way to determine the paleofield h and the angle θ which are characteristics of the low‐temperature (presumably secondary) remanent magnetization. A few examples of such the determinations on samples from Proterozoic collections are reported.

Crystallographic-magnetic correlations in single-crystal haemo-ilmenite: new evidence for lamellar magnetism

SUMMARY 17 single crystals were identified by electron backscatter diffraction (EBSD) and isolated from coarse massive haemo-ilmenite ore from South Rogaland, Norway. These were studied using the EBSD results, natural remanent magnetization (NRM), and anisotropy of magnetic susceptibility (AMS), to gain a better understanding of angular relationships between crystallographic axes and magnetic properties of haemo-ilmenite in relation to lamellar magnetism. Electron microprobe analyses gave the following average end-member compositions for ilmenite host: 21.1 per cent MgTiO3, 73.7 FeTiO3, 0.5 MnTiO3, 4.3 Fe2O3, 0.2 Cr2O3 and 0.3 V2O3; and for the coarsest (∼3 μm) haematite exsolution lamellae: 3.5 MgTiO3, 22.4 FeTiO3, 71.4 Fe2O3, 1.6 Cr2O3, 1.0 V2O3 and 0.1 Al2O3, making this sample the most Mg- and Cr-rich haemoilmenite studied in the province, but with similar element fractionations between the coexisting phases. TEM work on similar material suggests the presence of much thinner exsolution down to 1–2 nm. The EBSD, NRM and AMS results from 12 out of 17 crystals indicate a good agreement between the orientation of crystallographic axes, NRM direction and principal axes of the magnetic susceptibility ellipsoid, with the NRM located in the (0001) basal plane [NRM ∧ (0001) < 6.5 ◦ ] and the crystallographic c axis quasi-parallel to the minimum axis of the susceptibility ellipsoid [c ∧ k3 < 13.5 ◦ ]. In addition, in 10 of these 12 crystals, the remanent magnetization vector is parallel or nearly parallel to the positive direction of a crystallographic a axis [NRM ∧ a < 20 ◦ ], hence parallel to a principal magnetic moment direction in haematite as determined by Besser et al., and not parallel to the spin-canted direction of end-member haematite. This is consistent with a basic property of lamellar magnetism, where the magnetic moment is parallel to the principal moments (sublattice magnetization directions) in haematite. Relationships in three additional crystals with NRM ∧ a = 22 ◦ –33 ◦ , only two with good agreement, can be interpreted as consistent with having a magnetic vector quasi-parallel to the spin-canted direction of haematite.

Tectonic rotations during the Chile Ridge collision and obduction of the Taitao ophiolite (southern Chile)

Abstract The &lt; 6 Ma young Taitao ophiolite, exposed at the westernmost promontory of the Taitao Peninsula, is located approximately 40 km southeast of the Chile triple junction and consists of a complete sequence of oceanic lithosphere. Systematic sampling for paleomagnetic study was performed to understand the complex obduction processes of the ophiolite onto the forearc of the South American Plate. Two representative demagnetization paths of remanent magnetization vectors were observed. One is characterized by stable univectorial demagnetization paths and was observed in volcaniclastic rocks and dyke complexes. Orientations of their remanent magnetization vectors indicate various degrees of counterclockwise rotations. The other is characterized by multivectorial demagnetization paths and was observed in the plutonic units (gabbros and ultramafic rocks). From these, two distinct stable remanent magnetization vectors were isolated; one has high coercivity and the other has low coercivity along the demagnetization paths with little influence of viscous magnetizations. This suggests that the complex deformation history involved at least two rotational events. The clockwise rotation, inferred from high coercivity remanent magnetization vectors, was attributed to a ridge collision event and the counterclockwise rotation, inferred from the low coercivity remanent magnetization vectors, was attributed to an accommodation phase into the South American forearc during obduction and final emplacement of the ophiolite. Folds developed during this period. Paleomagnetic restorations of the internal structures of the plutonic units and dyke complexes suggest that they probably originated in a mid‐oceanic ridge environment near a transform fault. The counterclockwise rotation of the plutonic and dyke complex units during the obduction generated tectonic gaps between these and the basement. The volcaniclastic rocks must have been deposited at nearly their present location, filling the tectonic gaps, as less effect of tectonic rotation was identified on these rocks.

Measurement of vector minor loops

An experimental vector analysis of the rotational magnetization, remanent magnetization and reversible magnetization properties of longitudinal recording samples is presented. It is found that for large fields the locus of the magnetization vector, the remanent magnetization vector and the reversible magnetization are all ellipses whose aspect ratios are fundamentally different. For smaller rotating fields, the trajectory of the magnetization vector and the remanent magnetization vector are spirals whose limit cycle is an ellipse; however, the reversible magnetization vector is an ellipse. These experimental results are In agreement with a recently proposed vector hysteresis model for recording media.

Flux line orientation in YBa2Cu3O7−δ crystals in low magnetic fields

The isothermal remanent magnetization was measured on a twinned YBa2Cu3O7−δ crystal as a function of magnetic field, temperature and angle between the applied magnetic field and the c axis of the crystal. The angular dependence of the field of first vortex penetration as well as the orientation of the isothermal remanent magnetization vector, determined from the data, are different at different temperatures. Our results point to a crossover between two different pinning mechanisms at a certain temperature.