Magnetic Reversal Frequency Research Articles

Palaeomagnetism of the Vendian traps of Volyn, southwestern margin of the East European Platform, P. 2: magnetostratigraphy

We present new results of palaeomagnetic studies of the Vendian (Ediacaran) rocks of the Volyn Basalt Province revealed by six boreholes on the NW Ukraine. This is a continuation of previous studies of upper part of the Volyn series described by Bakhmutov et al. [2021]. In the recent works [Shcherbakova et al., 2020; Thallner et al., 2022], the results of palaeointensity determinations of the Volyn series basalts showed the ultra-low dipole moment of Earth’s magnetic field which coincident with other palaeomagnetic data for Ediacaran indicating an extremely weak geomagnetic field. Clear stratification and correlation of the basalt and tuff layers by magnetic parameters allow us to determine the magnetic field reversals throughout the stratigraphic succession of the Volyn series of about 400 m thick. In basalts, tuffs and baked contact rocks, a high-temperature characteristic component of remanent magnetization (ChRM) with all signs of primary magnetization have been isolated. For the directions of inclination of ChRM-components at least six magnetic polarity reversals were revealed. The geochronological ages of rocks is in the range of 580—545 Ma, but the errors in the age estimation are too large for separation of individual formations within the pulses of activity or differentiation of individual eruptions. Therefore the formation time of the entire stratum remains uncertain, and the key issue for interpretation of the magnetic polarity reversal frequency is the duration of accumulation of traps. Two possible interpretations of palaeomagnetic results are considered. In the first, which takes into account different stages of magmatic activity, the time interval of accumulation can be about 10 million years. Then the average frequency of geomagnetic inversions is close to mean for the Phanerozoic. We prefer the other interpretation when the formations of the Large Igneous Provinces have been occurred over a short time interval (e.g. 0.5 Myr). Taking into account the results of palaeointensity estimations for the same samples, which shown the extremely weak geomagnetic field, the hypothesis of the «hyperactivity» of the field with a frequency of at least 12 reversals per one million years on the end of the Ediacaran gain the additional confirmation.

Determination of Eddy-Current and Hysteresis Losses in the Magnetic Circuits of Electrical Machines

We consider the problem of separation of the total losses in the electric steel of a magnetic circuit into two components: the losses caused by hysteresis and the losses caused by eddy currents. The solution of this technical problem would enable us to guarantee the possibility of efficient design and construction electric machines with magnetic circuits characterized by low magnetic losses. We deduce computational formulas for the hysteresis and eddy-current components of losses, which contain the levels of total losses measured in the open-circuit tests for two different frequencies of magnetization reversal and the ratio of these frequencies. It is shown that the optimal ratio of the frequencies of magnetization reversal is equal to 1.2. We also present the plots of dependences and the expressions aimed at finding the exponent of dependence of the frequency of magnetization reversal on the measured open-circuit losses and the values of frequency.

Dual-shot dynamics and ultimate frequency of all-optical magnetic recording on GdFeCo

Although photonics presents the fastest and most energy-efficient method of data transfer, magnetism still offers the cheapest and most natural way to store data. The ultrafast and energy-efficient optical control of magnetism is presently a missing technological link that prevents us from reaching the next evolution in information processing. The discovery of all-optical magnetization reversal in GdFeCo with the help of 100 fs laser pulses has further aroused intense interest in this compelling problem. Although the applicability of this approach to high-speed data processing depends vitally on the maximum repetition rate of the switching, the latter remains virtually unknown. Here we experimentally unveil the ultimate frequency of repetitive all-optical magnetization reversal through time-resolved studies of the dual-shot magnetization dynamics in Gd27Fe63.87Co9.13. Varying the intensities of the shots and the shot-to-shot separation, we reveal the conditions for ultrafast writing and the fastest possible restoration of magnetic bits. It is shown that although magnetic writing launched by the first shot is completed after 100 ps, a reliable rewriting of the bit by the second shot requires separating the shots by at least 300 ps. Using two shots partially overlapping in space and minimally separated by 300 ps, we demonstrate an approach for GHz magnetic writing that can be scaled down to sizes below the diffraction limit.

Solution of Topical Issues in Theory of Transformers

In the article, the authors summarized their experience in solving some controversial issues in the theory of transformers, and briefly outlined the essence of their developments. For power transformers of the TM series, the presence of an excess of the active resistance of the primary winding relative to the reduced active re-sistance of the secondary winding was established. The obtained correction factors, taking into account the electrical skin effect in the windings and their heating, can be used to calculate short-circuit currents and heat distribution in other types of transformers. An expression is given that makes it possible to correct the transfor-mation ratio of one of the transformers connected to parallel operation in such a way that it becomes possible to fully load both devices, despite their different short-circuit voltages. Expressions are presented that make it possible to reveal the ratio of hysteresis and eddy-current losses in the magnetic circuit of a particular trans-former, as well as the exponent with which the losses in the magnetic circuit depend on the frequency of mag-netization reversal. A method is proposed for determining the three components of losses in a magnetic circuit, based on an idle experiment at three frequencies. A method is described for determining the thickness of sheets of a magnetic circuit, at which the total losses in it are minimal, and a method for determining the heating time constant of dry transformers is presented, which makes it possible to accelerate their thermal tests.

Investigation of the frequency characteristics of composite iron powders with insulating oxide coatings

This article discusses methods for studying the frequency characteristics of composite iron powders with insulating oxide coatings. The main dependences of hysteresis losses on induction for a full cycle at a frequency of magnetization reversal for various cross-sections of iron powders, sharply differing in the shape of the hysteresis curve and the values of the main magnetic characteristics, are studied, and also frequency dependences in the range of 50 Hz - 1 MHz of the figure of merit and inductance for samples based on composites are considered from powder Hoganes ASC100.29 after annealing in air, in water vapor and hydrogen at a temperature of 400 ° С for two hours. Based on the analysis of the dependencies, it can be explained by the fact that a material with a thin layer has a lower electrical resistivity, which means that the total electromagnetic losses will also be greater due to an increase in the contribution of losses to eddy currents. At the same time, the magnetic permeability at a frequency of 1 kHz differs by 30 units, and with an increase in the frequency to 1 MHz, it decreases by 10-15% for both materials. This makes it possible to use iron-based soft magnetic materials for electrical engineering.

Determination of eddy current losses and hysteresis losses in magnetic circuits of electrical machines

The division of the total core losses in the electrical steel of the magnetic circuit into two components – losses dueto hysteresis and eddy currents – is a serious technical problem, the solution of which will effectively design and construct electrical machines with magnetic circuits having low magnetic losses. In this regard, an important parameter is the exponent α, with which the frequency of magnetization reversal is included in the total losses in steel. Theoretically, this indicator can take values from 1 to 2. Most authors take α equal to 1.3, which corresponds to the special case when the eddy current losses are three times higher than the hysteresis losses. In fact, for modern electrical steels, the opposite is true. To refine the index α, an attempt was made to separate the total core losses on the basis that the hysteresis component is proportional to the first degree of the magnetization reversal frequency, and the eddy current component is proportional to the second degree. In the article, the calculation formulas of these components are obtained, containing the values of the total losses measured in idling experiments at two different frequencies, and the ratio of these frequencies. It is shown that the rational frequency ratio is within 1.2. Presented the graphs and expressions to determine the exponent α depending on the measured no-load losses and the frequency of magnetization reversal.

Magnetic reversal frequency in the Lower Cambrian Niutitang Formation, Hunan Province, South China

Magnetic reversal frequency in the Lower Cambrian Niutitang Formation, Hunan Province, South China

18-Pulse Transformer Rectifier Unit with an Amorphous Magnetic Core for Aircraft

This paper deals with a transformer-rectifier unit with a magnetic core made of amorphous steel. Transformers with various types of amorphous alloys have been compared. The most optimal amorphous alloy to use in a transformer is determined. The simulation of the electromagnetic and thermal processes occurring in transformers with a magnetic core made of amorphous steel was carried out, the amplitude and shape of the voltage and current were determined and the maximum temperature in nominal operating mode was defined. The dependencies of the specific losses in the studied samples of amorphous steel on the frequency of magnetization reversal, magnetic flux density and temperature were obtained. An experimental layout of a transformer rectifier unit with a magnetic core made of amorphous steel, loaded onto an 18-pulse rectifier with a power of 10.5 kVA and operating at a frequency of 400-800 Hz with a voltage of 115/27 V was designed and manufactured. An experimental study was carried out on the developed layout to verify the obtained data. The experimental studies confirmed the correctness of the chosen design methodology and of the design scheme of the transformer. It is important to note that the thermal loads detected on the transformer during its testing are within the permissible values.

The interrelationships of magnetic and magneto acoustic-emission characteristics of heat-treated steels of various chemical composition

The relationship between the amplitude of magnetoacoustic emission and the residual magnetic induction of the materials is shown. The dependence of magnetoacoustic emission (MAE) on the frequency of magnetization reversal of a group of steels of different chemical composition was investigated. In all the investigated cases, the MAE amplitude maximum corresponds to a field frequency of 3-5 Hz. The magnetic properties of two groups of steels subjected to various thermal treatments have been studied. It is shown that the residual magnetic induction of steels is a sufficiently universal parameter for controlling softening thermal treatments and that the amplitude of magnetoacoustic emission can be used as a testing parameter in scanning systems of structurescopy of ferromagnetic steels.

Three distinct reversing modes in the geodynamo

The data that describe the long-term reversing behavior of the geodynamo show strong and sudden changes in magnetic reversal frequency. This concerns both the onset and the end of superchrons and most probably the occurrence of episodes characterized by extreme geomagnetic reversal frequency (>10–15 rev./Myr). To account for the complexity observed in geomagnetic reversal frequency evolution, we propose a simple scenario in which the geodynamo operates in three distinct reversing modes: i—a “normal” reversing mode generating geomagnetic polarity reversals according to a stationary random process, with on average a reversal rate of ∼3 rev./Myr; ii—a non-reversing “superchron” mode characterizing long time intervals without reversal; iii—a hyper-active reversing mode characterized by an extreme geomagnetic reversal frequency. The transitions between the different reversing modes would be sudden, i.e., on the Myr time scale. Following previous studies, we suggest that in the past, the occurrence of these transitions has been modulated by thermal conditions at the core-mantle boundary governed by mantle dynamics. It might also be possible that they were more frequent during the Precambrian, before the nucleation of the inner core, because of a stronger influence on geodynamo activity of the thermal conditions at the core-mantle boundary.

Magnetic reversal frequency scaling in dynamos with thermochemical convection

Scaling relationships are derived for the frequency of magnetic polarity reversals in numerical dynamos powered by thermochemical convection. We show that the average number of reversals per unit of time scales with the local Rossby number Roℓ of the convection. With uniform core–mantle boundary (CMB) heat flux, polarity reversals are absent below a critical value Roℓcrit≃0.05, beyond which reversal frequency increases approximately linearly with Roℓ. The relative standard deviation of the dipole intensity fluctuations increases with reversal frequency and Roℓ. With heterogeneous CMB heat flux that models the large-scale seismic heterogeneity in Earth’s lower mantle, reversal frequency also exhibits linear dependence on Roℓ, and increases approximately as the square root of the amplitude of the CMB heterogeneity. Applied to the history of the geodynamo, these results imply lower CMB heat flux with Roℓ⩽Roℓcrit during magnetic superchrons and higher, more heterogeneous CMB heat flux with Roℓ>Roℓcrit when geomagnetic reversals were frequent. They also suggest that polarity reversals may have been commonplace in the early history of other terrestrial planets. We find that zonal heterogeneity in CMB heat flux produces special effects. Close to Roℓcrit enhanced equatorial cooling at the CMB increases reversal frequency by concentrating magnetic flux at low latitudes, whereas far beyond Roℓcrit enhanced polar cooling at the CMB increases reversal frequency by amplifying outer core convection.

The frequency dependence of demagnetizing factor of a Fe-based amorphous ribbon

The frequency dependence of total and internal demagnetizing factors has been investigated. A minimum of the demagnetizing factor at optimal frequency of magnetization reversal is revealed. The influence of the demagnetizing factor on the frequency dependence of magnetic properties of amorphous ribbon is studied.

Distribution of the induction of a quasi-stationary magnetic field created in a ferromagnet by an attachable electromagnet

As a result of numerical calculations, the spatial and time distributions of the induction of a quasi-stationary magnetic field, which was created in a studied ferromagnet by U-shaped and solenoidal attachable electromagnets (AEMs) at frequencies of 1 and 10 Hz, were obtained. The presence of a phase shift between the amplitude values of the magnetic-field induction at different depths in the specimen, which increases with the frequency of the exciting current, was revealed. The dependences of the magnetic-field penetration depth into the ferromagnet on the magnetic reversal frequency were studied. It was found that for the frequency range under study the magnetic-field penetration depth for both types of AEM is smaller than that for the corresponding stationary case.

Dynamic magnetization properties of a superferromagnetic metal-insulator multilayer observed by magneto-optic Kerr microscopy

Longitudinal magneto-optical Kerr effect (MOKE) magnetometry and microscopy have been performed on a superferromagnetic (SFM) metal-insulator multilayer of [Co80Fe20(tn=1.3 nm)/Al2O3(3 nm)]10 to understand the dynamic behavior of the SFM domains. Frequency dependent hysteresis loops measured by MOKE show that the coercive field Hc increases as the rate (frequency) of magnetization reversal is increased similar to dynamic hysteresis loops observed on conventional ferromagnetic ultrathin films. This dynamic hysteresis behavior arises due to SFM friction-controlled domain wall motion which is observed by MOKE microscopy. We see that the nucleation field for SFM domains increases for hysteresis loops with higher frequency as expected for a ferroic system.

Magnetic reversal frequency and apparent polar wander of the Siberian platform in the earliest Palaeozoic, inferred from the Khorbusuonka river section (northeastern Siberia)

SUMMARY Marly limestones that crop out along the Khorbusuonka river (northeastern Siberian platform) define a well-dated Lower and Middle Cambrian sequence which was sampled for magnetostratigraphy. In the upper part of the section, from the uppermost Lower Cambrian (Toyonian stage) to the Middle Cambrian (Amgan and Mayan stages), thermal demagnetization isolates two magnetic components carried either by magnetite, haematite or both. The first is parallel to the recent geomagnetic field at the site and is removed in the low to middle temperature range. The second component is isolated at higher temperatures and has dual magnetic polarities. This component yields a positive (but partial since we consider only two sites) fold test with other Middle Cambrian data previously obtained from the distant Kulumbe river section, and was probably acquired during sedimentation or at an early stage of diagenesis. It records a very high magnetic reversal frequency, of at least six to eight reversals per Myr, during the entire Middle Cambrian (assuming a ∼10 Myr duration for this epoch). A much more complex palaeomagnetic behaviour is observed within the upper Tommotian‐lower Atdabanian (Lower Cambrian) part of the Khorbusuonka section. Demagnetization diagrams show large overlaps between magnetic components, which prevents the isolation of a clear magnetic component at high temperatures. Analysis of these overlaps is, however, consistent with the existence of an ill-determined primary magnetic direction for the Lower Cambrian, which is very different from that isolated in the Middle Cambrian. Although still in need of further confirmation, the possibility that a large drift in Siberian apparent polar wander occurred over a rather short time during the Lower Cambrian appears to be supported by our data.

Middle Cambrian high magnetic reversal frequency (Kulumbe River section, northwestern Siberia) and reversal behaviour during the Early Palaeozoic

We present a magnetostratigraphic study of Middle Cambrian sediments along the Kulumbe River (northwestern Siberia). The deposits are ∼850 m thick and lie within the Mayan stage as constrained by trilobite fossils. Palaeomagnetic analyses reveal two magnetic components. The first component, parallel to the direction of the present day field at the site, is isolated in the low to middle temperature range. A high temperature component (HTC) is then isolated up to 580 or 680°C, carried either by magnetite and/or by hematite. The HTC defines a sequence of 28 magnetic polarity intervals, some of them being defined by only one sample. Considering that the studied section has a duration of ∼5 Myr, we propose that the magnetic reversal frequency was high (∼4–6 reversals per Myr) during part of the Middle Cambrian, among the highest values known within the Phanerozoic. The reversal frequency may have been roughly similar during the Lower Cambrian. We further suggest a drastic decrease of the magnetic reversal frequency between the Lower–Middle Cambrian and the end of the Tremadoc (Ordovician) when a superchron probably occurred. However, this behaviour is still challenged by other scenarios which depend on the chosen Early Palaeozoic time scale and on the reliability of some magnetostratigraphic data.

Upper Cambrian to Middle Ordovician magnetostratigraphy from the Kulumbe river section (northwestern Siberia)

We present the magnetostratigraphy of an Upper Cambrian to Middle Ordovician sedimentary section sampled along the Kulumbe river in northwestern Siberia. Thermal demagnetization yields the determination of two magnetic components. A component of steep inclination is interpreted as a remagnetization acquired during the emplacement of the Siberian traps at the Permian–Triassic boundary. Another component with a moderate inclination, essentially carried by hematite and sometimes by magnetite, likely represents a syndepositional or nearly syndepositional magnetization of the studied deposits. A sequence of 17 magnetic intervals is observed between the upper part of the Upper Cambrian and the Tremadoc, three of them being poorly defined by single samples. We also find that the Llanvirn magnetostratigraphic sequence contains only one reversed polarity interval. Together with magnetostratigraphic results previously obtained from the Moyero river section from Siberia, our data suggest the occurrence of a 25–30 Myr-long period during the Lower and the Middle Ordovician characterized by a very low magnetic reversal frequency (possibly containing a superchron), and dominated by reversed polarity. This geomagnetic feature would be compatible with a time constant of about 150 Myr in the reversal process through the whole Phanerozoic.

Upper Anisian to Lower Carnian magnetostratigraphy from the Northern Calcareous Alps (Austria)

We present a magnetostratigraphic study of three pelagic limestone sections (Gamsstein, Mendlingbach and Mayerling) from the eastern part of the Northern Calcareous Alps. Together these sections, which contain a rich conodont fauna, yield a sedimentary record encompassing the uppermost Anisian, the Ladinian and the Lower Carnian. Thermal demagnetization and isothermal rémanent magnetization experiments indicate that the magnetization is essentially carried by a mineral of the magnetite family. The high unblocking temperature components isolated from the three sections provide clear magnetic polarity zonations. Correlations between these results, based on the biostratigraphic data and tephrochronology, allow the construction of a composite magnetic polarity sequence from the Illyrian substage (Upper Anisian) to the Julian 2 zone (Lower Carnian). This sequence contains 17 well‐defined polarity reversals, and eight more poorly defined intervals. Correlations can be suggested between the new data and other magnetostratigraphic results previously obtained from marine sections. We estimate that the magnetic reversal frequency was 2.5 to 3.6 reversals per million years during the Ladinian.

Hettangian and Sinemurian magnetostratigraphy from Paris Basin

A long core (1110 m) drilled at Montcornet (northeastern Paris Basin) provides early Jurassic magnetostratigraphic data coupled with biochronological control. About 600 paleomagnetic samples were obtained from a 148‐m‐thick series of Hettangian and Sinemurian rocks. A composite demagnetization using thermal (up to 300°C) followed by alternating field technique (up to 100 mT) is used to separate the magnetic components. A low unblocking temperature component (<250°C) with an inclination of about 65° is interpreted as a present‐day field overprint. The characteristic remanent component with both normal and reversed antipodal directions is then isolated between 5 and 70 mT. Eighty‐one polarity intervals are recognized in this study. The higher reversal frequency of the late Hettangian/early Sinemurian time interval contrasts with a lower reversal rate observed in the rest of the early Liassic. A rough mean estimate of about 5 reversals/m.y. can be proposed for the earliest Jurassic. These results represent a significant contribution to the magnetic polarity reversal timescale for a time interval hitherto poorly known and add to the magnetic reversal frequency curve of the last 350 m.y.

Magnetostratigraphy of the Moyero River Section (North-Western Siberia): Constraints On Geomagnetic Reversal Frequency During the Early Palaeozoic

SUMMARY We present the magnetostratigraphy of the early Palaeozoic Moyero river section in north-western Siberia. the section investigated, which consists mainly of reddish limestones, marls and greyish limestones, extends from the upper Cambrian to the lower Silurian. Palaeomagnetic analyses reveal two magnetic components. One is interpreted as a remagnetization acquired during Siberian trap emplacement; the other is believed to be early diagenetic, consistent with the established lower Palaeozoic apparent polar wander path for Siberia. Reversed polarity dominates from the upper Cambrian to the middle Ordovician, while normal polarity dominates from the upper Ordovician to the lower Silurian. Only a few reversals, which are poorly defined, are observed through the section, in particular during the Llandeilo. These data are in agreement with previous magnetostratigraphic data from the Baltic platform and Siberia and indicate that magnetic reversal frequency was low during the Ordovician. Because of the time-delayed process of acquisition of the magnetization carried by haematite, we cannot clearly ascertain if a reversed superchron occurred during the lower to middle Ordovician. However, our results suggest that a 15 Myr-long reversed interval existed during the Arenig.