Isothermal Remanent Magnetization Curves Research Articles

A Depth‐Transect of Ocean Deoxygenation During the Paleocene‐Eocene Thermal Maximum: Magnetofossils in Sediment Cores From the Southeast Atlantic

AbstractThe Paleocene‐Eocene Thermal Maximum (PETM, ∼56 Ma) presents a past analog for future global warming. Previous studies provided evidence for major loss of dissolved oxygen during the PETM, although understanding the degree and distribution of oxygen loss poses challenges. Magnetofossils produced by magnetotactic bacteria are sensitive to redox conditions in sediments and water columns, and have been used to reconstruct paleoredox conditions over a range of geological settings. Here, we present records of well‐preserved magnetofossils from cores along Walvis Ridge in the Southeast Atlantic that span the PETM across a depth transect (∼1,500–3,600 m paleodepth). Hysteresis, isothermal remanent magnetization curves, first‐order reversal curve diagrams, and low‐temperature magnetic measurements document large variations in magnetic properties of magnetofossils, which relate to time and water depth. Abundant magnetofossil grains are present within the studied sediments, and their morphologies change with paleodepth, as shown by transmission electron microscope observations. Magnetofossils from samples within the PETM onset at the deeper sites (∼2,600–3,600 m paleodepth) have lower coercivity values, a higher oxidation degree, and smaller grain sizes than those from shallower sites (∼1,500–1,800 m paleodepth), likely reflecting changes in paleoredox conditions at different paleodepths. We use the magnetofossil records to reconstruct relative changes in dissolved oxygen content at different water depths through the PETM, and suggest that ocean deoxygenation likely expanded downwards in the early stages of the PETM. We thus demonstrate the value of magnetofossil records for paleoenvironmental reconstructions over time and space, particularly for sediments that lack carbonate fossils.

Assessing the Magnetic Mineralogy of the Pre-Variscan Manteigas Granodiorite: An Unexpected Case of a Magnetite-Series Granitoid in Portugal

The Early Ordovician Manteigas granodiorite has been characterized as having a magnetic susceptibility with high heterogeneity and values compatible with the presence of ferromagnetic phases, such as magnetite. However, granitoids with magnetite are rare in the Iberian Massif. Therefore, an in-depth study of mineralogical composition was carried out on the Manteigas granodiorite by evaluating its magnetic susceptibility, analyzing the isothermal remanent magnetization (IRM) and the IRM curve, identifying the magnetic coercivity components, evaluating the magnetic grain size and performing a petrographic study. Low concentrations of very small grains of magnetite and/or titanomagnetite have been found in areas with low magnetic susceptibility and with values not compatible with ferromagnetic phases. The petrographic study has explained this phenomenon as a result of the transformation of magnetite into hematite (martitization), indicating a redox reaction. A high concentration of magnetite and/or titanomagnetite, occurring as multidomain crystals, has been found in areas with high magnetic susceptibility. This study indicates that the Manteigas granodiorite has a deep magma origin, and formed under oxidizing conditions since it has magnetite instead of ilmenite, which is an uncommon phenomenon in the Iberian massif.

ПЕРІОДИЧНІСТЬ ЗЕМЛЕТРУСІВ У КАСПІЙСЬКОМУ МОРІ

Magnetic minerals form or alter in the presence of hydrocarbons, making them a potential magnetic proxy for identifying hydrocarbon migration pathways. We concentrate in the precision magnetic mineralogical analyzes to understand what magnetic minerals in soil from the hydrocarbon fields are responsible for the magnetic signal. To illustrate the results of our studying, we consider the study area of the hydrocarbon deposits near the village Balabanivka, Bogodukhiv district, Kharkiv region. A collection of soil samples is represented by deep medium-humus chernozems. The thermomagnetic analyzes, hysteresis parameters, and isothermal remanent magnetization curves (IRM) were studied for the samples collected near the well (point PR 28) and at the area located out of the hydrocarbon influence zone (point PR 0403). To study the thermomagnetic parameters, different types of the magnetization and magnetic susceptibility (MS, χ), and hysteresis loops we used the Variable Field Translation Balance (VFTB) instrument. The results are confirmed by the highest values of the gas geochemical parameters. The MS variations at temperatures ranging from 200 to 400 ˚C may reflect the presence of either iron sulphides or maghemite. The magnetite occurs for all soils. The wasp-waisted hysteresis loops identified either a mixture of two magnetic minerals (magnetite and hematite) and the admixture of the superparamagnetic and larger grains of the same minerals.

Micromagnetic Calculations of the Effect of Magnetostatic Interactions on Isothermal Remanent Magnetization Curves: Implications for Magnetic Mineral Identification

AbstractIsothermal remanent magnetization (IRM) curves are used widely in rock magnetic and environmental magnetic studies to characterize magnetic properties of natural samples. Previous theoretical and experimental investigations indicate that magnetostatic interactions can strongly affect the IRM behavior. In order to quantitatively investigate this effect, we modeled IRM curves and hysteresis properties for magnetite assemblages with variable morphologies (grain size, elongation, and shape) using the finite‐element micromagnetic modeling code MERRILL. We also conducted an analytical study of IRM behavior for noninteracting single domain grains utilizing the Stoner–Wohlfarth model. It is found that multiple Gaussian components do not necessarily reflect multiple magnetic mineral populations. For example, magnetostatic interactions can produce coercivity distributions that resemble results of some samples containing a detrital and extracellular (D + EX) and a biogenic soft or hard (BS/BH) coercivity components. Simulated IRM curves for different morphologies of interacting flattened grains are similar, indicating that the effect of interactions can overshadow the effect of grain morphologies. Moreover, our micromagnetic and analytical calculations consistently indicate that the IRM curves for noninteracting fine‐grained assemblages are mostly determined by grain shape (elongated or flattened) and elongation distributions of the magnetic mineral assemblages. Our simulations present possible outcome of IRM curves for magnetic mineral assemblages with controlled grain size, shape and microstructures, which provides important theoretical constraints on characterizing complex magnetic mineral components in natural samples through IRM curves and hysteresis.

Assessment of Magnetic Techniques for Understanding Complex Mixtures of Magnetite and Hematite: The Inuyama Red Chert

AbstractMagnetite and hematite mixtures occur widely in nature. Magnetic unmixing of the signals recorded by these minerals can be important for assessing the origin of their respective paleomagnetic remanences and for extracting geological and paleoenvironmental information. However, unmixing magnetic signals from complex magnetite and hematite mixtures is difficult because of the weak magnetization and high coercivity of hematite. We assess here the relative effectiveness of first‐order reversal curve (FORC) and extended FORC‐type diagrams, FORC‐principal component analysis (PCA), isothermal remanent magnetization (IRM) curve decomposition, and PCA of remanent hysteretic curves for unmixing magnetic components in samples from the magnetically complex Inuyama red chert, Japan. We also further characterize the domain state and coercivity distributions of both magnetite and hematite with FORC‐PCA and IRM acquisition analysis in the red chert. We show that IRM curve decomposition can provide valuable component‐specific information linked to coercivity, while FORC‐PCA enables effective magnetic domain state identification. PCA of remanent hysteretic curves provides useful information about the most significant factors influencing remanence variations and subtle coercivity changes. To identify components in complex magnetite and hematite mixtures, we recommend PCA analysis of remanent hysteretic curves combined with FORC analysis of representative samples to identify domain states and coercivity distributions.

Interaction field in nanocrystalline exchange-coupled Sm-Y-Co alloys

Nanocrystalline Sm0.5Y0.5Co5 alloys were prepared by mechanical milling, and their magnetic properties were investigated using a physical property measurement system (PPMS) to observe the presence of magnetic interactions and determine the dipolar interaction field. Isothermal remanent magnetization curves (mr) and the normalized direct current demagnetization (md) were measured. The dipolar interaction field the for nanocrystalline Sm0.5Y0.5Co5 alloys was determined, and was representative of the magnetic interactions.

Remanence magnetization of iron oxide particles with different size distribution

Remanent magnetization curves for two samples of iron oxide particles with different size distributions were investigated. It was observed that the isothermal remanent magnetization curve and the DC demagnetization remanence curves are strongly depended on fine particle size. Moreover, it was figured out that remanent curves are quite complicated for the assembly of larger particles. In terms of remanence dependence analysis, it was shown that favourable interaction mechanism for the assembly of iron oxide particles strongly depends on particle size. The isothermal remanent magnetization curves were analysed in terms of the Stoner-Wohlfarth model. It leads to determine particle anisotropy field. It was found that the anisotropy field depends on particle size. It was assumed that observed differences related to different magnetic structure of particles.

Paleomagnetic results from Lower Devonian sandstones of the Niqiuhe Formation in the Duobaoshan area and its constraints on paleoposition of the Xing’an block

Two common but contradictory views toward the timing of the final closure of the eastern Paleo-Asian Ocean (PAO) are either the Pre-Late Devonian or the Late Permian to Early Triassic. Previous paleomagnetic studies support a consolidation of the unified North China and Mongolia by the Late Permian. However, Pre-Late Permian paleogeography of the eastern Central Asian Orogenic Belt (CAOB) is not well understood due to the scarcity of reliable paleomagnetic records. The Xing’an block plays a key role in the evolution of the CAOB, however, no paleomagnetic results are available so far to provide constraints on its Late Paleozoic paleo-position. We performed a systematic paleomagnetic study on a purple siltstone section of the Lower Devonian Niqiuhe Formation near the Duobaoshan County, Heilongjiang Province. Representative fresh end samples were subjected to rock magnetic analysis to identify their dominant magnetic carriers and particle size distribution. Rock magnetic experiments include measurements of magnetic susceptibility variation dependence of temperature ( κ - T ) curves, hysteresis loops, acquisition of isothermal remanent magnetization (IRM) and back-field demagnetization of saturated IRM (SIRM) curves. The results indicate that the magnetic remanence carriers are dominant by hematite with minor contributions of magnetite. 112 cylindrical specimens were thermally demagnetized using an ASC TD-48S furnace and measured in a 2G-755-4K SQUID system in the Magnetotectonics Lab of Peking University. A total of 11 sites revealed identical bi-components character. The low temperature component resembles the present geomagnetic field in geographic coordinates, while the high temperature characteristic remanence (ChRM) component was reversed polarity both before and after tilt correction. Optimal concentration is achieved at 35.7% unfolding percentage, which suggests the magnetization was acquired during the late stage of a folding process. A previous structural study indicated that stress field of the research area switched from NW-SE to N-S orientation during the Late Permian, which is supported by the predominant NE-SW oriented Ordovician to Devonian strata as well as the angular unconformity between the Devonian and Upper Permian strata. The folding time of the Niqiuhe Formation is likely Middle Devonian to Middle Permian. Supported by evidence from a suite of granites near our sampling section (less than 3 km) dating at 309−299 Ma along with the universal reversed polarity of all 11 sites, we argue that the ChRM component was acquired during the Permo-Carboniferous Reversed Superchron (~318−262 Ma), when the study area was affected by a granitic intrusion thermal event (309−299 Ma). The ChRM component ( D / I =28.6°/−33.2°, k =18.6, α 95=10.9°) corresponds to a paleomagnetic pole of λ p/ φ p=17.3°S/97.1°E ( dp / dm =7.0°/12.4°). For a common reference site (43°N/114°E), our results suggest the Xing’an block was situated at 27.7°±12.4°N during the Late Carboniferous. Coeval results obtained from rocks of different lithology suggest 28.5°±9.7°N and 19.9°±14.8°N for the Uliastai continental margin and the Central Mongolia-Euruna block. After applying a blanket flattening factor f =0.6 for results obtained from clastic sedimentary rocks, the paleolatitudinal difference between the Xing’an block, Uliastai continental margin, and the Central Mongolia-Erguna block is less than 3.2°±12.8°. Late Carboniferous paleomagnetic poles from those three blocks are distributed along a small circle. Significant post-Permian local rotations are likely attributed to extensive Mesozoic strike-slip faults among the eastern CAOB. Those three blocks were probably consolidated into a unified block, situated at ~28°−30°N during the Late Carboniferous and dominant by the cold water Borel realm fauna. By contrast, the North China and Songliao-Xilinhot blocks was situated at an equatorial region (4.3°S−12°N) and dominant by the warm water Tethyan realm fauna, while Siberia was located at ~45°−50°N. The Paleo-Asian Ocean therefore remained open and the main basin was the Hegenshan ocean rather than the Solonker ocean.

Magnetic properties of surface sediments as proxies of recent anthropogenic pollution in the Anllóns riverbed (NW Spain)

A study of surface sediments from the Anllons riverbed (northwestern Spain) was conducted to assess the use of their magnetic properties as proxies of recent trace element contamination (i.e., As, Zn, Pb, Cr, Ni and Cu). Thermomagnetic curves, hysteresis parameters, specific magnetic susceptibility χ, frequency-dependent magnetic susceptibility κ FD%, magnetic remanence and saturation isothermal remanent magnetization ratios (i.e., S and SIRM/χ) reveal the presence of mostly fine-grained magnetite and maghemite in these samples. The Tomlinson Pollution Load Index (PLI), as well as the magnetic parameters χ, κ FD%, SIRM/χ and S-ratios, vary more or less in agreement to each other when they are sequentially arranged throughout a 30-km-long transect that goes from the town of Carballo to the river mouth in Ponteceso. From that profile, it is possible to identify an anomaly of χ that lies between two small peaks of the PLI and seems to define a contaminated area probably linked to a diffuse low-impact source of pollution. There is also a conspicuous peak of the PLI that is likely associated to a nearby gold mine. The variations of the κ FD%’s along the transect are the ones that best reflect the changes in the PLI values, regardless of the level of contamination. This result suggests that adsorption is the main mechanism of trace elements removal by Fe oxides in these samples. Direct signal analyses, used to decompose the isothermal remanent magnetization curves, reveal the presence of four magnetic fractions, with varying relative concentrations all over the profile, namely single-domain magnetite, close to the superparamagnetic threshold (SD/SP) and/or maghemite ( $$\log B_{1/2}$$ ca 1.2), single-domain magnetite ( $$\log B_{1/2}$$ ca 2), hematite ( $$\log B_{1/2}$$ ca 2.5) and goethite ( $$\log B_{1/2}$$ ca 3.5). Integrating all the available information, it was sketched out a likely sequence of events that included magnetic minerals neoformation in parent soils, their subsequent chemical transformations and the contamination episodes that affected the region.

Anisotropy evolution of nanoparticles under annealing: Benefits of isothermal remanent magnetization simulation

Isothermal remanent magnetization (IRM) combined with Direct current demagnetization (DcD) are powerful tools to qualitatively study the interactions (through the Δm parameter) between magnetic particles in a granular media. For magnetic nanoparticles diluted in a matrix, it is possible to reach a regime where Δm is equal to zero, i.e. where interparticle interactions are negligible: one can then infer the intrinsic properties of nanoparticles through measurements on an assembly, which are analyzed by a combined fit procedure (based on the Stoner–Wohlfarth and Néel models). Here we illustrate the benefits of a quantitative analysis of IRM curves, for Co nanoparticles embedded in amorphous carbon (before and after annealing): while a large anisotropy increase may have been deduced from the other measurements, IRM curves provide an improved characterization of the nanomagnets intrinsic properties, revealing that it is in fact not the case. This shows that IRM curves, which only probe the irreversible switching of nanomagnets, are complementary to widely used low field susceptibility curves.

A rock magnetic profile through the ejecta flap of the Lockne impact crater (central Sweden) and implications for the impact excavation process

The well-documented, well-preserved, and well-exposed Lockne crater is a reference crater for marine-target impacts on Earth. The large amount of data allows detailed analysis of the cratering and modification processes. A unique feature of Lockne as compared with other similar craters is its pristine ejecta layer. Here, we provide the first complete lithological description coupled with an analysis of the rock magnetic properties of the Lockne-9 core drilled through the ejecta flap. Low-field bulk magnetic susceptibility, magnetic hysteresis, isothermal remanent magnetization curves (IRM), and the corresponding model of the coercivity spectra, backfield IRM, and thermomagnetic curves are used to fully characterize the magnetic mineralogy (i.e., pseudo-single domain (PSD) magnetite and pyrite). Variation of the magnetic properties with depth reveals a characteristic maximum in the magnetic susceptibility and magnetization within the crystalline ejecta. The magnetic properties of rocks affected by the impact show a slight weakening in the coercivity of magnetic minerals in comparison with rocks not affected by the impact. Altogether, this suggests to us that the high magnetization zone already existed before the impact event took place. Therefore, it can be inferred that during the cratering process, the Lockne ejecta was repositioned en masse from the central part of the crater in the form of an ejecta flap. This stands in contrast to the standard ballistic emplacement model wherein individual particles move in an ejecta curtain.

Ficus benjamina leaves as indicator of atmospheric pollution: a reconaissance study

We present a diagnostic study to evaluate the suitability of Ficus benjamina tree leaves as a captor of heavy metal particles from atmospheric dusts in urban areas. Leaf samples were taken at 16 localities within three areas in Morelia, state of Michoacan (Mexico’s medium size city, 830000 inhabitants). Measurements of magnetic susceptibility were conducted to determine the magnetic enhancement using samples from green, relatively unpolluted area, as a reference. The samples collected at areas with heavy traffic (main avenues) yielded values almost ten times higher than the values obtained for the unpolluted reference. Isothermal Remanent Magnetization curves are proportional to the degree of pollution. Associations of almost pure magnetite with heavy metals were revealed by scanning electron microscope.

Climatic control of magnetic granulometry in the Mircea Vodă loess/paleosol sequence (Dobrogea, Romania)

A detailed rock-magnetic investigation of loess/palaeosol samples from the section at Mircea Vodă (Dobrogea Plateau, Romania) is presented. Investigation of climatic control on magnetic granulometry used several grain-size and concentration-dependent proxies (magnetic susceptibility and its frequency dependence, anhysteretic remanent magnetization, isothermal remanent magnetization, hysteresis properties and FORC distributions) and an unmixing model for isothermal remanent magnetization curves. The paleosol layers in the Mircea Vodă sequence experienced significant pedogenic alteration, resulting in high amounts of ultrafine grained SP, grains at SP/SD threshold, stable SD grains and PSD magnetite/maghemite grains. The loess layers in the Mircea Vodă sequence are dominated by MD and/or PSD oxidized magnetite probably of aeolian origin. The unmixing model for IRM curves shows the presence of three components with different coercivities. The soft (∼21 mT) and medium (∼42 mT) coercivity components are produced by pedogenesis during interglacial and interstadial periods. The hard coercivity component (∼60 mT) is of aeolian origin, being dominant in loess layers.

Rock magnetic characterization of early and late diagenesis in a stratigraphic well from the Llanos foreland basin (Eastern Colombia)

AbstractWe have carried out rock magnetic characterizations of different lithofacies along the stratigraphic well Saltarín 1A, in order to learn about the various diagenetic events that could have affected the Miocene sequence of the Llanos foreland basin (Colombia). Thermomagnetic and low-temperature susceptibility measurements performed on some selected samples were complemented with analyses of Scanning Electron Microscopy (SEM), Energy Dispersive X-ray (EDX) and isothermal remanent magnetization (IRM) acquisition curves. The identification of the magnetic mineral assemblages at each depth level analysed, as well as their relative concentrations, were determined from a direct signal analysis (DSA) of the IRM curves. Samples from the top of the Guayabo formation reveal the presence of hydrocarbons-related microscopic framboids of pyrite with partial replacement of magnetite. The bottom of the Guayabo formation shows hematite and goethite and appears to record a thoroughly documented Middle Miocene global regression. In samples from the León formation, pyrrhotite could have resulted from an early diagenesis that took place in a lacustrine environment via sulphate reduction. Traces of crude oil in samples from the Carbonera formation, and the additional occurrence of hematite and magnetite, suggest that a hydrocarbons-mediated late diagenesis could also have affected the lowermost levels of Saltarín 1A.

End-member modelling of isothermal remanent magnetization (IRM) acquisition curves: a novel approach to diagnose remagnetization

SUMMARY To identify remagnetization is essential for palaeomagnetic studies and their geodynamic implications. The traditional approach is often based on directional analysis of palaeomagnetic data and field tests, which may be inconclusive if the apparent polar wander path (APWP) is poorly constrained or if the remagnetization predates folding. In several cases, rock magnetic work, particularly, the measurement of hysteresis loops allows identification of the so-called ‘remagnetized’ and ‘non-remagnetized’ trends. However, for weakly magnetic samples, this approach can be equivocal. Here, to improve the diagnosis of remagnetization, we investigated 192 isothermal remanent magnetization (IRM) acquisition curves (up to 700 mT) of remagnetized and non-remagnetized limestones from the Organy` a Basin, northern Spain. Also, 96 IRM acquisition curves from non-remagnetized marls were studied as a cross-check for the non-remagnetized limestones. A non-parametric end-member modelling approach is used to analyse the IRM acquisition curve data sets. First, remagnetized and non-remagnetized groups were treated separately. Two or three end-members were found to adequately describe the data variability: one end-member represents the high-coercivity contribution, whereas the low-coercivity part can be described by either one end-member or two reasonably similar endmembers. In the remagnetized limestones, the low-coercivity end-members tend to saturate at higher field values than in the non-remagnetized limestones. When the entire data set was processed together, a three-end-member model was judged optimal. This model consists of a high-coercivity end-member, a low-coercivity end-member that saturates at ∼300–400 mT and a low-coercivity end-member that approximately saturates at 700 mT. Higher contributions of the latter end-member appear to occur dominantly in the remagnetized limestones, whereas the reverse is true for the non-remagnetized limestones, so they plot in clearly distinguishable areas. Meanwhile, the IRM curves from non-remagnetized marls show a behaviour similar to that of the non-remagnetized end-member in the limestones. Therefore, this new approach can be a very useful tool to diagnose remagnetization in weakly magnetic limestones and marls. We recommend applying it to other areas of potentially remagnetized low-intensity sediments.

Henkel analysis of Fe/Er multilayers prepared by electron-beam evaporation

Fe/Er multilayers were prepared by high-vacuum electron-beam evaporation system and their microstructures have been investigated. Henkel analysis was performed via isothermal remanent magnetization curve and direct current demagnetization curve. Magnetic measurement indicates that Fe/Er multilayers have an easy axis in film plane and a small value of positive magnetoresistance. It is showed that the Er layer will change to an amorphous structure when its thickness is below 3.5 nm. Hence, Δm plots were established according to Henkel analysis data. The results show that Δm plot has a strong dependence on the thickness of Fe and Er layers. When the thickness of Fe layer remained unchanged, Δm plot indicated that the magnetic coupling was oscillating with the layer thickness; and when the thickness of Er layer was 8.8 nm, Δm plot showed a positive-negative overturn of magnetic interaction, which revealed the interplay between the demagnetization and exchange coupling effect in multilayer. The explanation of Δm plot was also discussed via a simplified model, i.e., a competition between demagnetizing interaction and magnetizing interaction.

Using time- and temperature-dependent Preisach models to investigate the limitations of modelling isothermal remanent magnetization acquisition curves with cumulative log Gaussian functions

SUMMARY A number of recent studies have applied the cumulative log Gaussian (CLG) modelling tech- nique to bulk isothermal remanent magnetization (IRM) curves to investigate the coercivity contributions of different minerals contained within a natural sample. Here, we present a series of Preisach-Neel models used to investigate how robust the assumption of fitting lognormal coercivity distributions is in the presence of local interaction fields and thermal relaxation. Our models indicate that the starting state of the magnetic system, magnetic interaction and thermal relaxation have a strong influence on the form of IRM curve, meaning that in a sub- stantial number of cases the lognormal assumption fails and the CLG procedure can produce misleading interpretations. In some cases the failure of the CLG assumptions produces models which introduce spurious additional coercivity components into the fitted curve.

Compensation for the demagnetizing field in remanence measurements of perpendicular magnetic recording films

It is necessary to compensate for the demagnetizing field in remanence measurements of perpendicular magnetic recording films.In this paper,a new method of compensating for the demagnetizing field is proposed and an effective reverse field Heff is used to evaluate the dc demagnetization remanence curves,isothermal remanent magnetization curves,and the ΔM plots for perpendicular magnetic recording films.The demagnetization factor N is important for this method,and three methods are used to estimate the value of N. The results of the ΔM plots indicate that the intergranular interactions in Ba ferrite films seem to be of exchange coupling,while those in Co-Cr-Ta films are of magnetostatic interactions.

An attempt to determine the microwave paleointensity on historic Paricutín volcano lava flows, Central Mexico

Reportamos un estudio preliminar de magnetismo de rocas y paleointensidad con la técnica de microondas de los flujos de lava históricos entre 1943 y 1948 del volcán Paricutín. Esos flujos de lava muestran afloramientos frescos y bien preservados. La mayoría de las muestras estudiadas se caracterizan por diagramas ortogonales simples. Las curvas de magnetización remanente isotermal muestran saturación en campos bajos a moderados sugiriendo la serie de titanomagnetitas. De los experimentos de histéresis se determinó que los portadores magnéticos son probablemente titanomagnetitas ricas en hierro con un comportamiento de dominio sencillo a pseudosencillo. La técnica de paleointensidad por microondas se aplicó a tres muestras seleccionadas usando el método de Kono y Ueno (1977); i.e., la dirección del campo aplicado en laboratorio fue perpendicular a la dirección de la magnetización remanente. Los resultados de paleointensidad fueron de 11.38, 26.37 y 51.6 microTeslas, que son significantemente diferentes de los valores esperados. La dispersión observada puede deberse a la pequeña fracción de la magnetización remanente natural usada para la determinación de la paleointensidad, o de no haber usado una corrección por la razón de enfriamiento para muestras naturales.

Magnetic characterization of perpendicular recording media

In this paper, we describe techniques for the magnetic characterization of perpendicular recording media. Such measurements made using traditional techniques, such as the vibrating sample magnetometry (VSM) and alternating gradient force magnetometer (AGFM), have to be corrected for the sample shape demagnetizing factor, which is often found not to be equal to -4/spl pi/. For measurements other than the simple hysteresis loop, such as remanence curves, this correction must be carried out in real time and we describe the method by which this can be achieved and the process for achieving the correct demagnetization of perpendicular films prior to measurements of the isothermal remanent magnetization curve. A further complication is that real perpendicular media have a soft underlayer beneath the recording layer, which swamps and confuses signals from instruments such as VSM or AGFM. Hence, we describe the construction and use of a magnetooptical Kerr effect magnetometer, which does not penetrate significantly into the soft layer and enables the perpendicular layer to be measured independently. We describe the properties of a traditional alloy perpendicular medium and a Co-Pd multilayer system, which in the latter case exhibits multiple switching behavior. We also address the issue of the effect of the soft underlayer on the coupling in similar longitudinal films and find that the presence of the underlayer induces significant additional coupling effects that may well give rise to an increase in noise in recorded signals.