Stable Magnetization Research Articles

Условия возникновения спонтанной осциллирующей магнитной релаксации в синтетических ферримагнетиках Pt/Co/Ir/Co/Pt

We had established that the non-monotonic magnetic relaxation appeared, when two or three magnetic fields, switching between stable magnetization states, coincide with each other, in Pt/Co/Ir/Co/Pt synthetic ferrimagnets with perpendicular magnetic anisotropy. Switching fields between stable magnetization states depend on sum of exchange interlayer energy, Zeeman energy and magnetization reversal energy barrier of each layer. This total energy depends on layers thickness and temperature. We had shown that with choose of temperature and free layer thickness we can achieve double or triple points on energy states diagram, which is reason of nonmonotonic magnetization relaxation.

Constraining the Intracontinental Tectonics of the SW Central Asian Orogenic Belt by the Early Permian Paleomagnetic Pole for the Turfan‐Hami Block

AbstractIn order to better understand the kinematics and geodynamics of the late Paleozoic to early Mesozoic transition from accretionary to intracontinental tectonics in the southwest Central Asian Orogenic Belt, we performed a paleomagnetic study on Lower Permian strata in the Turpan‐Hami Basin, NW China. Magnetite and hematite are shown to be the principal magnetic remanence carriers. A typical sedimentary fabric that has not been modified by postdepositional deformation is recognized via magnetic fabric study. Stable characteristic remanent magnetizations, all of reverse magnetic polarity, are revealed by step demagnetizations. Remanence age is determined to be of Early Permian affinity, based on a fold test, remanence behavior of minerals, and geochronologic data. With inclination shallowing corrections, the first Early Permian paleomagnetic pole for the Turpan‐Hami Block is acquired as λ = 75.7°N, φ = 276.3°E, A95 = 5.7°, and N = 10 sites. Comparisons of this new pole with published ones from neighboring cratonic blocks provide kinematic constraints on the tectonic transition and subsequent intracontinental tectonics of this region. The Turpan‐Hami and Yili blocks have kept their relative positions since the Early Permian. Significant latitudinal movement of 14.7° ± 7.2° occurred between the Turpan‐Hami and South Junggar blocks and could be mainly accommodated by late Paleozoic crustal shortening of at least ~530 km across an intra‐arc basin in the Bogda belt. The Turpan‐Hami Block has experienced large‐magnitude relative rotations of −41.1° ± 7.1° and 21.2° ± 9.4° with respect to Tarim and South Junggar since the Early Permian, respectively, corresponding to dominant intracontinental tectonics characterized by large‐magnitude strike‐slip displacements along megashear zones.

Nanogoethite as a Potential Indicator of Remagnetization in Red Beds

AbstractRed beds are well‐known for recording stable natural remanent magnetization (NRM). However, discriminating primary NRM from secondary remanence in red beds is difficult. The Paleogene Nangqian red beds in eastern Tibetan Plateau variably record an overprint related to nearby magmatism and thus provide a great opportunity to characterize remagnetization in red beds. Through comprehensive rock magnetic, Mössbauer spectroscopic, and petrographic analyses, we find that remagnetization was controlled by temperature. Remagnetized red beds contain abundant authigenic hematite and goethite, with some larger grains unblocking at the Néel temperatures and the remainder (nanoparticles) unblocking at lower temperatures. In contrast, red beds retaining primary NRM are characterized by dominance of detrital hematite and magnetite, presence of fine‐grained authigenic hematite, and absence of authigenic goethite and magnetite. High temperature behaviors of NRM/susceptibility are indicative of remagnetization, but the presence of goethite appears to be a more sensitive criterion for diagnosing remagnetization in red beds.

Quantum state manipulation of single atom magnets using the hyperfine interaction

The magnetic quantum states of holmium single atom magnets on MgO(100) have proven extremely robust when exposed to high magnetic fields and temperatures up to 35 K. Here we address the stability of Ho at small magnetic fields, where the hyperfine interaction creates several avoided level crossings. Using spin-polarized scanning tunneling microscopy, we demonstrate quantum state control via Landau-Zener tunneling and stable magnetization at zero field. Our observations indicate a total spin ground state of ${J}_{z}=\ifmmode\pm\else\textpm\fi{}8$. Combined quantum and classical control render Ho a promising qubit candidate.

Triassic (Anisian and Rhaetian) palaeomagnetic poles from the Germanic Basin (Winterswijk, the Netherlands)

In this paper, we provide two new Triassic palaeomagnetic poles from Winterswijk, the Netherlands, in the stable interior of the Eurasian plate. They were respectively collected from the Anisian (~ 247–242 Ma) red marly limestones of the sedimentary transition of the Buntsandstein Formation to the dark grey limestones of the basal Muschelkalk Formation, and from the Rhaetian (~ 208–201 Ma) shallow marine claystones that unconformably overlie the Muschelkalk Formation. The magnetization is carried by hematite or magnetite in the Anisian limestones, and iron sulfides and magnetite in the Rhaetian sedimentary rocks, revealing for both a large normal polarity overprint with a recent (geocentric axial dipole field) direction at the present latitude of the locality. Alternating field and thermal demagnetization occasionally reveal a stable magnetization decaying towards the origin, interpreted as the Characteristic Remanent Magnetization. Where we find a pervasive (normal polarity) overprint, we can often still determine well-defined great-circle solutions. Our interpreted palaeomagnetic poles include the great-circle solutions. The Anisian magnetic pole has declination D ± ∆Dx = 210.8 ± 3.0°, inclination I ± ∆Ix = − 26.7 ± 4.9°, with a latitude, longitude of 45.0°, 142.0° respectively, K = 43.9, A95 = 2.9°, N = 56. The Rhaetian magnetic pole has declination D ± ∆Dx = 32.0 ± 8.7°, inclination I ± ∆Ix = 50.9 ± 8.1°, with a latitude, longitude of 60.6°, 123.9° respectively, K = 19.3, A95 = 7.4°, N = 21. The poles plot close to the predicted location of global apparent polar wander paths (GAPWaPs) in Eurasian coordinates and are feasible for future apparent polar wander path construction. They confirm that the intracontinental, shallow-marine Germanic Basin, in which the Muschelkalk Formation was deposited, existed at a palaeolatitude of 14.1° [11.3, 17.1] N, in a palaeo-environment reminding of the Persian Gulf today. In Rhaetian times, palaeolatitudes of 31.6° [24.8, 39.8] N were reached, on its way to the modern latitude of 52°N.

Diagenesis of magnetic minerals at the Southwest Pacific DSDP Site 277

ABSTRACTWe present a paleomagnetic and rock magnetic record from a 465.2 m long drill core recovered at DSDP Site 277 in the Southwest Pacific. Our study sought to construct a magnetostratigraphy and thus enable further paleoenvironmental work on this sedimentary succession. Magnetic remanence measurements revealed a stable magnetisation in the upper 34.8 m and lower 58.7 m of the core. However, between 34.8 and 397.5 m sediments are very weakly magnetised, do not carry a stable magnetisation, and rock magnetic analyses revealed a lack of remanence carrying minerals in this interval. We suggest that during the period spanning Paleocene to Oligocene sufficiently high organic matter (reagents) accumulation resulted in complete reduction and dissolution of ferri/ferromagnetic grains.

Spin–Orbit Torque and Spin Hall Effect-Based Cellular Level Therapeutic Spintronic Neuromodulator: A Simulation Study

Artificial modulation of a neuronal subset through ion channels activation can initiate firing patterns of an entire neural circuit in vivo. As nanovalves in the cell membrane, voltage-gated ion channels can be artificially controlled by the electric field gradient that caused by externally applied time varying magnetic fields. Herein, we theoretically investigate the feasibility of modulating neural activities by using magnetic spintronic nanostructures. An antiferromagnet/ferromagnet (AFM/FM) structure is explored as neuromodulator. For FM layer with perpendicular magnetization, stable bidirectional magnetization switching can be achieved by applying in-plane currents through AFM layer to induce the spin-orbit torque (SOT) due to the spin Hall effect (SHE). This Spin-orbit Torque Neurostimulator (SOTNS) utilizes in-plane charge current pulses to switch the magnetization in FM layer. The time changing magnetic stray field induces electric field that modulates the surrounding neurons. The Object Oriented Micromagnetic Framework (OOMMF) is used to calculate space and time dependent magnetic dynamics of SOTNS structure. The current driven magnetization dynamics in SOTNS has no mechanically moving parts. Furthermore, the size of SOTNS can be down to tens of nanometers, thus, arrays of SOTNSs could be fabricated, integrated together and patterned on a flexible substrate, which gives us much more flexible control of the neuromodulation with cellular resolution.

Paleomagnetism of the Mn wad deposit at Niimi hot springs, Hokkaido, Japan

The Niimi hot springs is located at the foot of Mt. Shyakunage in the active Niseko volcanic area, Hokkaido, Japan. The manganese (Mn) wad deposit was precipitated from the hot spring water by biomineralization processes. However, no precise direct age dates for the diagenesis of such Mn oxides have been reported. Here is reported the first paleomagnetic application to date Mn oxides on 144 disseminated Mn wad and 42 pyroclastic clay specimens. The Mn wad specimens were collected at 14 levels between ∼5.5 and ∼62 cm in thickness on a three-dimensional grid pattern, and the clay specimens were collected at seven levels between 85 and 118 cm in thickness. Alternating field step demagnetization isolates either one or two stable characteristic remanent magnetization (ChRM) components in the Mn wad specimens. The lower coercivity component resides in pseudosingle domain titanomagnetite and shows clustered ChRM directions at each sampling level. Conversely, the higher coercivity component resides in single domain titanomagnetite and shows more random directions. The higher coercivity component is likely carried by pyroclastic fragments that landed on the site. The remanent directions of the clay specimens reside in multidomain magnetite, are less stable, and show a scattered distribution. When the observed ChRM directions of the Mn wad specimens are compared with the paleosecular variation record from Lake Biwa in Japan, the Mn wad deposition records the secular variation and appears to have lasted at least ∼1600 years from ∼3.5 to ∼1.9 kyr BP.

Paleomagnetism of the native copper mineralization, Keweenaw Peninsula, Michigan

The age and genesis of Michigan’s world-class native copper deposits are poorly constrained. The copper is hosted by basaltic flow tops and conglomeratic interbeds of the 1095 ± 2 Ma Keweenawan Portage Lake Formation. Progressive thermal demagnetization isolates stable hematite remanent magnetization components at 28 paleomagnetic sites. Paleomagnetic tilt tests show that magnetite in massive flow interiors is primary (1095 ± 2 Ma) and that hematite throughout the formation is syntectonic. The altered cupriferous deposits contain primary ∼1095 Ma and secondary ∼1053 Ma hematite in various proportions. The Caledonia Mine’s basaltic mineralization carries the ∼1053 Ma hematite dominantly whereas the Delaware Mine’s conglomeratic interbed mineralization carries the ∼1095 Ma hematite dominantly. The ∼1095 Ma hematite is attributed mostly to magnetite exsolution during flow extrusion and to weathering oxidation between extrusion events. An infusion of epigenetic hydrothermal fluids emplaced the native copper with additional hematite and polarity self-reversing titanohematite at 1053 ± 7 Ma. Importantly, paleomagnetic evidence supports a 1053 ± 7 Ma age also for the White Pine stratiform sedimentary copper mineralization, for the oxidation of the Oronto Group clastic rocks to red beds, and for the time limits of major tectonic uplift and deformation on the Keweenaw Peninsula.

Preferred orientation of ferromagnetic phases in rock-forming minerals: insights from magnetic anisotropy of single crystals

In the early days of paleomagnetism, David Strangway was interested in understanding why igneous rocks are faithful recorders of the Earth’s magnetic field. He recognized that ferromagnetic (s.l.) grains that could be discerned by optical microscopy were too large to carry a stable remanent magnetization, and speculated whether fine-grained, ferromagnetic (s.l.) inclusions or exsolutions in silicate minerals are responsible. When these inclusions or exsolutions are randomly oriented, or the silicate hosts are randomly oriented in a rock, they can be a good recorder of the field. If these minerals, however, show an alignment within the silicate host, and the host is preferentially aligned due to flow structures or deformation, then the paleomagnetic direction and paleointensity could be biased. We examine the magnetic anisotropy arising from the ferromagnetic (s.l.) phases in silicate-host minerals. Single crystals of phyllosilicate, clinopyroxene, and calcite show most consistent ferrimagnetic fabric with relation to the minerals’ crystallographic axes, whereas olivine and feldspar display only a weak relationship. No discernable relationship is found between the ferrimagnetic anisotropy and crystallographic axes for amphibole minerals. Our results have implications when single crystals are being used for either studies of field direction or paleointensity or in cases where silicate minerals have a preferential orientation. Phyllosilicate minerals and pyroxene should be screened for significant magnetic anisotropy.

Storage or cooking pots? Inferring pottery use through archaeomagnetic assessment of palaeotemperatures

In this paper we report the results of an archaeomagnetic and rock-magnetic study performed on a set of Neolithic and Chalcolithic potsherds from El Portalón de Cueva Mayor site (Sierra de Atapuerca, Burgos, Spain) to investigate their thermal history and obtain information about their function during ancient times. Two types of palaeomagnetic behaviour have been observed. One yields stable and univectorial magnetization diagrams (Type 1) while the other yields multivectorial diagrams with an intermedium component interpreted as a partial thermoremanence (p-TRM) (Type 2), acquired during a partial reheating of the pottery during their use. To investigate the connection of these two types with the ancient function of the potsherds, a set of modern pots was experimentally fired and then partially reheated al known temperatures. The results obtained reproduced patterns similar to those observed in the archaeological samples. Type 1 (univectorial) samples that reached only a high heating temperature (>600 °C) during their manufacture were most probably used as storage vessels. By contrast, Type 2 (multivectorial) samples probably recorded a p-TRM acquired during their last use (reheating) related to cooking activities (<450 °C). Additionally, a detailed microprofile performed on an experimental pot heated twice at different temperatures allows interesting inferences to be drawn on the magnetic record in archaeological pottery and its implications. This study shows that the magnetic measurements can be a useful tool to infer the last re-heating temperatures in prehistoric ceramics and to relate them to the use of those pieces in the past.

Last 3000 years of geomagnetic field intensity from India: New reference palaeointensity data from two east coast archaeological sites and archaeomagnetic dating insights

A comprehensive rock magnetic and absolute archaeointensity (AI) investigation was carried out on baked bricks from the east coast of India with an aim to obtain a set of well-defined palaeointensity (PI) data. The new PI data from India comprise a significant first step to improve the regional intensity reference curve, which can be used as reliable dating tool mainly for archaeomagnetic material. Standard Thellier-Coe experiments were applied on 12 independent brick samples collected from a Buddhist monument at Adurru in the Godavari delta and a Medieval Fort at Kolleti Kota site situated on an island in the Kolleru Lake. The samples thus collected represent two different archaeological periods, one around the 3rd century BCE and the other between the 4th and 7th centuries CE respectively. The data obtained from thermal demagnetization of Saturation Isothermal Remanent Magnetization (SIRM), temperature dependent magnetic susceptibility (χ–T), Isothermal Remanent Magnetization (IRM) acquisition curves, and hysteresis measurements showed that the primary carriers of Natural Remanent Magnetization (NRM) are fine-grained Pseudo-Single Domain (PSD) (Stable Single Domain (SSD) + Superparamagnetic (SP)) titanomagnetite with various titanium contents. This implies that the samples had significant stable Thermoremanent Magnetization (TRM), and, therefore, good candidates for PI analysis. Strict selection and quality-control criteria for detecting magnetomineralogical alteration and intensity data corrected for anisotropy and the cooling rate effects on TRM acquisition, offer high quality PI determinations with respect to the previous available data that missed such corrections.A total 33 specimen level PI determinations that correspond to linear NRM–TRM plots, were made using the Thellier GUI software. The corrected data yielded mean PI values of 44.6 ± 1.4 μT for the Buddhist Monument, and 36.6 ± 2.9 μT for the Fort site. By comparison and evaluation of new PI data with Global Geomagnetic Field Model predictions, the sampled bricks were archaeomagnetically dated using the Matlab tool developed by Pavón-Carrasco et al. (2011). Both ARCH3K.1 and SHA.DIF.14 K global models were used to evaluate the age estimates. The SHA.DIF.14 K model provided the most accepted age estimates that varied between 305 and 182 BCE for the early historic Buddhist Structure at Adurru, and 516–889 CE for the Medieval Fort at Kolleti Kota. These new reference archaeointensity data highlight the potential of archaeomagnetic dating to improve the archaeologically determined time periods of bricks, and also show the need for new high quality reference data to improve the geomagnetic field models from India.

Palaeomagnetism for chronologies of recent alpine lake sediments: successes and limits

Chronologies of lake-sediment records covering the last centuries to millennia are usually based on both short-lived radionuclides and radiocarbon dating. However, beyond the range of short-lived radionuclides, age model accuracy often suffers from large radiocarbon uncertainties. For high-altitude records, this issue is even more prominent as terrestrial plant fragments for radiocarbon dating are often lacking due to the sparse vegetation in such environments. In this study, we evaluate the potential of the geomagnetic field secular variations as a complementary tool to establish more robust age–depth relationships. Our palaeomagnetic study, applied to five high-altitude lakes from the western European Alps, first shows that recent unconsolidated sediments can carry stable remanent magnetization. The analysis of the magnetic parameters indicates that low-coercivity pseudo-single domain magnetite grains carry the natural magnetization. Nevertheless, the quality of palaeomagnetic secular variation records varies from one lake to another. This quality can be illustrated through the calculation of the declination/inclination maximum angular variations and their comparison to the expected value. Compared with available models, the declination variations are usually too large and the inclination too high. We discuss the validity of palaeosecular variation (PSV) of the Earth’s magnetic field regarding rock magnetism, magnetization processes and possible deformation during coring. From a magnetic point of view, the quality of data is variable, but the characteristic remanent magnetization direction is consistent at site level between neighbouring lakes and with the reference curve, suggesting that geomagnetic field secular variations are approximately recorded. Finally, we attempt to correlate the declination/inclination variations of the characteristic remanent magnetization measured in the five records to the reference geomagnetic model to provide additional chronological markers for age–depth modelling. These stratigraphic chrono-markers appear in systematic agreement with our previous chronological data and enable a reduction of dating uncertainties up to 30% when including these chrono-markers in the age–depth modelling. This agreement supports the interpretation that PSV may have been recorded more or less accurately depending on the studied lake. Therefore, coupled with a comprehensive understanding through other analysis (sedimentology, dating, geochemistry), PSV can be used to improve the age models in the more favourable cases.

Non-volatility using materials with only volatile properties: Vertically integrated magnetoelectric heterostructures and their potential for multi-level-cell devices

Different (1–3) heterostructures, such as BiFeO3-CoFe2O4 and BiFeO3-CuFe2O4 on Pb(Mg1/3Nb2/3)0.74Ti0.26O3 (PMN-26PT), were selected for study as possible materials for magnetoelectric (ME) random access memory. The (1–3) heterostructures were deposited, and multimagnetic states were found under different E-field (E) conditions. Upon removal of E, two possible remnant magnetization states remained stable. If an H-field (H) was also applied, two additional stable remnant magnetization states were found. Our investigations demonstrate (1–3) heterostructures with nonvolatility even though the individual phases/substrates had only volatile properties. This simplifies materials selection for multistate systems based on these heterostructures, averting difficulties with compositional nonuniformity and property repeatability, in particular, with regard to PMN-xPT crystal substrates. With such N≥4 magnetic state systems, a multilevel-cell memory device could readily be built with high ME coupling and numerous accessible magnetic states.

A palaeomagnetic study of the Middle Permian and Middle Triassic limestones from Shan State, Myanmar: Implications for collision of the Sibumasu Terrane and Indochina Terrane

We report herein a new palaeomagnetic study on the Middle Permian and Middle Triassic limestones from Shan State of eastern Myanmar, aiming to resolve the spatial location of the Sibumasu Terrane during the collision of tectonic blocks in Southeast Asia and to constrain the timing of convergence between the Sibumasu Terrane and Indochina Terrane. Based on the rock magnetic and palaeomagnetic experiments, three stable characteristic remnant magnetizations are recognized from limestone specimens of Middle Permian and Middle Triassic age. The characteristic Component C of the Middle Permian limestone passed the fold test at 95% confidence level, indicative of a pre‐folding origin. The mean direction of this component is D = 10.7°, I = 17.9° (κ = 130.3, α95 = 10.8°) before and D = 13.7°, I = −25.3° (κ = 1894.9, α95 = 2.8°) after tilt correction, corresponding to a palaeomagnetic pole at 254.1°E, 53.2°N (A95 = 1.7°). This suggests that the Sibumasu Terrane was placed at a palaeolatitude of ~13.0°S during the Middle Permian. The Component C separated from the Middle Triassic samples also passed the fold test with a mean direction of D = 207.3°, I = −8.3° (κ = 12.9, α95 = 6.2°) before tilt correction and D = 205.6°, I = −33.5° (κ = 27.1, α95 = 4.2°) after tilt correction, corresponding to a palaeomagnetic pole at 187.3°E, 65.8°N (dp/dm = 2.7°/4.8°). This implies that the Sibumasu Terrane was located at a palaeolatitude of ~18.3°N during the Middle Triassic. Based upon the detailed palaeomagnetic analysis, this study supports the view that the collision of the Sibumasu Terrane and Indochina Terrance occurred during or before the Middle–Late Triassic times.

Rock-magnetic and paleomagnetic study on a 27-m-long core from Lake Chapala, western Mexico: Paleoenvironmental implications for the last 10 ka

We report detailed rock-magnetic and paleomagnetic results from a 27-m-long sedimentary core collected at depocenter of Lake Chapala, western Trans-Mexican Volcanic Belt. Rock-magnetic parameters point to PSD grain-size titanomagnetites as the dominant magnetic mineralogy. Observed variations, in both concentration and grain-size dependent magnetic parameters, reflect changes in the sediment supply through the studied depth, but mainly during the last half century. Stable characteristic remanent magnetization, representative of secular variation of the geomagnetic field, was retrieved after alternating field demagnetization of the natural remanent magnetization (NRM). The comparison of the obtained magnetic inclination and declination logs against the corresponding curves predicted by the Global Geomagnetic Field Model SHA.DIF.14k enables to establish an alternative sedimentation-rate independent age-depth model for the whole core, in reasonably good agreement with the one calculated using conventional radiocarbon dating. This approach works not only for Lake Chapala, but also for Lake Chalco. The correlation established between rock-magnetic parameters and paleoenvironments, based on diatom identification of representative samples within specific zones, can be considered as a novel proxy methodology. This approach opens a new perspective for new, more detailed rock-magnetic, paleomagnetic and paleoenvironmental investigations for better paleoclimatic reconstructions.

Transient Chaos in Nanomagnets Subject to Elliptically Polarized AC Applied Fields

Magnetization dynamics in single-domain particles driven by elliptically polarized ac external fields is studied. It is shown that such dynamics exhibits chaotic and fractal nature at a moderate power level. This is due to the phenomenon of heteroclinic tangle produced by the combined action of ac excitations and saddle-type dynamics. By using the special perturbation technique based on the Melnikov function, analytical formulas for the threshold ac excitation amplitudes necessary to create the heteroclinic tangle are derived as a function of frequency and general elliptic polarization of the applied ac field. Numerical simulations confirm that the heteroclinic tangle implies the erosion of the safe basin around stable magnetization states.

First report of coupled Early Permian paleomagnetic and geochronologic data from the Dunhuang block (NW China), and implications for the tectonic evolution of the Paleo-Asian ocean

In order to better understand the tectonic relationship between the Dunhuang block (DHB) and its adjacent blocks, and to constrain the timing of the closure of the Paleo-Asian ocean, a combined geochronologic and paleomagnetic investigation has been undertaken on Early Permian tuff, basalt flows and sandstones from the Shuangbaotang Formation in the northwestern part of the DHB. U-Pb zircon dating indicates that the age of the strata is between 280.6 ± 2.9 Ma and 291.4 ± 2.6 Ma. Thermal demagnetization of a three-component isothermal remanent magnetization (IRM), and Curie point experiment suggest that magnetite dominates in the rock samples analyzed. In addition, there is a minor amount of hematite in some sandstones. Stepwise thermal demagnetization successfully isolated stable characteristic remanent magnetization (ChRM) from 11 tuff layers, two lava flows and nine sandstone beds. Two components were isolated from all samples: a lower temperature component (LTC) and a higher temperature component (HTC). The LTC is near the direction of the present-day geomagnetic field and produced a negative fold test, indicating it is a viscous remanent magnetization in the present-day geomagnetic field. Most of the HTC are reverse polarity (Normal = 4, reverse = 158), which is in accordance with the Kiaman Reversed Superchron that spans the Late Carboniferous-Permian interval. In addition, the HTC of all studied sites passed the fold and reverse tests, suggesting that they likely represent primary remanent magnetization. The tilt-corrected mean direction from all sites (tuff, basalts and sandstones) is Ds = 1.7°, Is = 43.1°, ks = 403, α95 = 1.5°, N = 22. The mean paleopole of the site-mean direction-corresponded VGPs lies at 74.5°N, 268.5°E with A95 = 1.6°. Considering the consistent inclination values recorded among the studied tuff, basalts and sandstones, and the low degrees of anisotropy within all samples, we suggest that there is no significant inclination shallowing caused by depositional compaction in the sedimentary layers of the studied section. Taking into account the results from this study as well as previous Late Paleozoic paleomagentic studies from adjacent tectonic blocks, we conclude that the DHB formed part of the amalgamated Dunhuang-North China-Alxa-Qaidam mega-block during the Early Permian, but was separated from the Tarim block by a small ocean (here named the Qiemo-Xingxingxia paleo-ocean) at this time. A comparison of the Early Permian paleolatitudes of these and other adjacent blocks suggest that the Paleo-Asian ocean (sensu lato) was still open at this time. Combined with other geological evidence, a paleogeographic reconstruction of the Paleo-Asian ocean has been reconstructed for the Early Permian.

Temperature invariable magnetization in Co-Al-Fe alloys by a martensitic transformation

Recently, it was reported that in a dual-phase ferromagnetic Fe-Ga alloy, highly thermally stable magnetization is achieved by a diffusional transformation between the two phases with different magnetizations, which compensates for the inevitable magnetization reduction. Contrasting with the compensation mechanism through a diffusional transformation, here we report that a diffusionless martensitic transition can also be used as a compensation mechanism for achieving nearly temperature invariable magnetization. We found that ferromagnetic Co-Al-Fe alloys with a non-thermoelastic martensitic transformation exhibit almost unchanged magnetization up to 830 K (about 81% TC). In-situ heating transmission electron microscopy reveals that the gradual transformation from a hexagonal close-packed (hcp) martensite phase with lower magnetization into a face-centered-cubic (fcc) parent phase with higher magnetization occurs over a wide temperature range, giving rise to such temperature invariable magnetization. The electrical resistivity measurement reveals that the fcc → hcp transformation can be repeated during heating-cooling cycles, which can be used to design temperature invariable magnets with a long cycling life.

Palaeo- and rock magnetic investigations of Late Quaternary sediments from the Upper Congo deep-sea fan: on the difficulty in obtaining palaeomagnetic secular variation records from low latitudes

We report here on results of palaeo- and rock magnetic investigations of two sediment cores from the Upper Congo deep-sea fan. The sediments have a high organic content and contain a heterogeneous Fe-mineral assemblage with biogenic magnetite and detrital (Ti-)magnetite as the main magnetic carrier minerals. Pyrite, hematite, and Fe-oxyhydroxides were identified by comparing high-temperature magnetic susceptibility curves with those from Fe-minerals of known composition. According to AMS 14C dates, the 6.8 m-long profile spans the last 37 kyr. Sediments older than 20 ka are affected by reductive diagenesis that has led to a loss of the fine-grained magnetic mineral fraction. Sediments younger than 20 ka have stable magnetizations. Characteristic remanent magnetization records of inclination and declination were obtained for each core. There is a little agreement between these records, modelled curves, and other sediment records from Equatorial Africa, so no composite record could be established. The cores are not ideal relative palaeointensity recorders and estimates using different normalizers did not yield consistent signals from both cores. Normalization methods used for relative palaeointensity estimation were not developed for sediments that contain large amounts of ultra-fine-grained biogenic magnetite; therefore, the relative palaeointensity estimates should be considered with caution. However, in view of the incoherent picture given by the scarce available palaeointensity information from the region off South-West Africa, the GeoB6517-2 record may provide a tentative relative palaeointensity record for comparison, at least for the past 10 kyr.