Magnetic hysteresis in natural materials

Magnetic hysteresis loops and the derived hysteresis ratios R_H and R_I are used to classify the various natural dilute magnetic materials. R_I is the ratio of saturation isothermal remanence (I_R) to saturation (I_S) magnetization, and R_H is the ratio of remanent coercive force (H_R) to coercive force (H_C). The R_H and R_I values depend on grain size, the characteristics of separate size modes in mixtures of grains of high and low coercivity, and the packing characteristics. Both R_H and R_I are affected by thermochemical alterations of the ferromagnetic fraction. Hysteresis loop constriction is observed in lunar samples, chondrite meteorites and thermochemically-altered basaltic rocks, and is due to mixtures of components of high and low coercivity. Discrete ranges of R_H and R_I for terrestrial and lunar samples and for chondrite meteorites provide for a classification of these natural materials based on their hysteresis properties.     

A preliminary magnetic study of soil samples from west-central Minnesota

Various rock magnetic techniques were applied to characterize magnetically the samples of a soil profile taken from west-central Minnesota. There is a marked change in magnetic properties as a function of depth in the core. X-ray analysis and Curie temperature measurements carried out on the magnetic fractions indicate that magnetite is the dominant iron oxide in both the top soil and the subsoil. The intensity of anhysteretic remanent magnetization (ARM) decreases sharply as the depth increases. In contrast, the stability of ARM was found to be higher for the subsoil. The surface soil sample was capable of acquiring a significant amount of viscous remanent magnetization (VRM). The VRM acquisition coefficient (S_a) of the subsoil (S_{a= 3.18 × 10^?6emu g^?1, 3.18 × 10^?6A m² kg^?1}) was about ten times weaker than that of the top soil sample (S_a = 3.868 × 10^{?7emu g?1, 3.868 × 10?7A m2 kg?1}). The magnetic domain state indicator, the ratio of coercivity of remanence to coercive force, H_cr/H_c, was 1.5 and 3.85 for the top soil and subsoil, respectively. It appears that the observed variations in magnetic properties down the present soil core is due only to a difference in grain size. We conclude that the magnetic grains in surface soil samples were more single-domain (SD) like whereas the magnetite grains in the subsoil samples were more likely in pseudo-single-domain (PSD) or small multidomain (MD) range. The observed lower stability for the surface soil samples is attributed to the presence of superparamagnetic grains whose presence was confirmed by transmission electron micrographs.     

A magnetic study of single-crystal titanomagnetite (Fe2.4Ti0.6O4)

Single crystals of approximate composition Fe_2.4Ti_0.6O₄ were prepared from which spherical samples of diameters 1–2 mm were obtained. The measured values of the Königsberger ratio, the ratio of saturation remanence to saturation magnetization and a Lowrie-Fuller test showed that they were multidomain in character. The temperature variation of the coercive force and saturation magnetization was measured between room temperature and the Curie point. The field dependence of intensity of acquired thermoremanent magnetization (TRM) was determined. The predictions of some of the theoretical models for multidomain TRM, which, of necessity, apply to simplifications of real materials (either natural or synthetic), compare favourably with the results of the present study. The validity of the assumptions made in this comparison is discussed.     

Rock magnetic properties and ore microscopy of the iron ore deposit of Las Truchas, Michoacan, Mexico

Iron ore and host rocks have been sampled (90 oriented samples from 19 sites) from the Las Truchas mine, western Mexico. A broad range of magnetic parameters have been studied to characterize the samples: saturation magnetization, Curie temperature, density, susceptibility, remanence intensity, Koenigsberger ratio, and hysteresis parameters. Magnetic properties are controlled by variations in titanomagnetite content, deuteric oxidation, and hydrothermal alteration. Las Truchas deposit formed by contact metasomatism in a Mesozoic volcano-sedimentary sequence intruded by a batholith, and titanomagnetites underwent intermediate degrees of deuteric oxidation. Post-mineralization hydrothermal alteration, evidenced by pyrite, epidote, sericite, and kaolin, seems to be the major event that affected the minerals and magnetic properties. Magnetite grain sizes in iron ores range from 5 to >200 μm, which suggest dominance of multidomain (MD) states. Curie temperatures are 580±5°C, characteristic of magnetite. Hysteresis parameters indicate that most samples have MD magnetite, some samples pseudo-single domain (PSD), and just a few single domain (SD) particles. AF demagnetization and IRM acquisition indicate that NRM and laboratory remanences are carried by MD magnetite in iron ores and PSD–SD magnetite in host rocks. The Koenigsberger ratio falls in a narrow range between 0.1 and 10, indicating the significance of MD and PSD magnetites.     

Magnetic hysteresis as a function of low temperature for deep-sea basalts containing large titanomagnetite grains-inference of domain state and controls on coercivity

Hysteresis loops to 1200 oersteds (9.55×10⁴ A m^?1) are measured between 295 K and 105 K for two deep-sea basalts (DSDP, Leg 34 and 37) containing large (～200 μm) unexsolved titanomagnetite grains. The Curie points, electron microprobe analyses and saturation magnetizations of the magnetic grains are the same as for unoxidized synthetic titanomagnetite (xFe₂TiO₄·(l ? x)Fe₃O₄) with x=0.6.As temperature is lowered from 295 to 190 K, coercive force H_c slowly rises from ～40 Oe to ～95 Oe approximately in proportion to the rise in the magnetostriction constant λ. Presumably, H_c is controlled by λ through internal stresses impeding domain wall motion. As expected of multidomain grains, the ratio of saturation remanence to saturation magnetization (in 1200 oersted cycles) j_R/j_S rises approximately in proportion to H_c, with a constant of proportionality consistent with titanomagnetite (x=0.6).As temperature is lowered from 190 to 120 K, H_c rises rapidly to ～400 Oe as a roughly linear function of the magnetocrystalline anisotropy constant K₁. Perhaps H_c is now controlled by K₁ through non-magnetic inclusions impeding domain wall motion.As temperature is lowered from 120 to 105 K, H_c rises even more rapidly to ～600 Oe. The control over H_c seems to have changed again, though most of the titanomagnetite is in grains large enough to still contain a few domains. The ratio j_R/j_S reaches 0.7 by 105 K and appears to be saturating towards the theoretical limit of 0.83.     

Study of magnetic contrasts applied to hydrocarbon exploration in the Maturín Sub-Basin (eastern Venezuela)

We have studied near-surface magnetic contrasts in nine oil wells from an oil prospective area in eastern Venezuela (compressive deformation front of the northeastern Maturín Sub-Basin). Samples are drill cuttings taken at intervals of about 15 m within approximately the first 1200 m of most of these wells. Rock magnetic experiments, electron paramagnetic resonance (EPR), extractable organic matter (EOM), X-ray diffraction and scanning electron microscopy (SEM) are used to discriminate between two anomalies in magnetic susceptibility (MS), related to either a reducing environment caused by the underlying reservoir (Type A), or to primary lithological contrasts (Type B). Contour maps of MS, S-ratio and organic-matter-free radical concentrations (OMFRC), for the A-like depth levels, show a major central zone of anomalous high values. This area is probably associated with either maximum accumulation of hydrocarbons in the reservoir or with their southerly migration from the northern petroleum source towards the deformation front. We argue that such a result could be used, for future exploration and production ventures in the region, as a preliminary characterization of the reservoir. We also show in a Day plot, that saturation remanence — saturation magnetization ratio (M_rs/M_s) and coercivity of remanence — coercive force ratio (H_cr/H_c) stand as additional criteria to discriminate between these two types of magnetic contrasts. In fact, for the area of study, hysteresis ratios show three distinct trends corresponding to samples from A-like, B-like and non-anomalous depth levels.     

Low temperature magnetic properties of basalts containing near ~TM30 titanomagnetite

The paper is devoted to studying the mineral composition and magnetic properties, mainly at the cryogenic temperatures, of the Middle–Late Devonian basalts from North Timan. The magnetic minerals in these basalts are dominated by intermediate-composition titanomagnetites (TM25–TM30) which demonstrate unusual magnetic properties in a wide temperature range. At room temperature, a low coercive force coexists with relatively high M_rs/M_s ratios. At cryogenic temperatures, the dependences of magnetic susceptibility on the temperature and frequency of the applied field are characteristic of this titanomagnetite composition, whereas the remanent saturation magnetization acquired at 2 K is destroyed at significantly lower temperatures compared to the synthetic analogs. The obtained results again highlight the necessity of studying the low-temperature properties of titanomagnetite samples with a controlled composition and grain size.     

On the possibility of obtaining relative paleointensities from lake sediments

Relative paleointensities are obtained from a 6-m sediment core from Lake St. Croix, Minnesota, spanning the time range from 445 to 1740 years B.P. To normalize the natural remanent magnetization (NRM) for variations in the magnetic content, a laboratory-induced remanence is chosen, whose alternating field (AF) demagnetization curves most closely resemble the NRM demagnetization curves. By plotting the ratio of the NRM to the normalizing remanence versus AF demagnetizing field, H_AF, for samples of the same sediment horizon, as well as for samples from different horizons, estimates are obtained for expected uncertainties in the relative paleointensities. For the Lake St. Croix sediments the anhysteretic remanence (ARM) demagnetization curves are very similar to those of the NRM's, and ARM is therefore used as the normalization parameter. Because the sediment exhibits homogeneous remanence properties throughout, and H_AF = 100Oe is the optimum “cleaning” field for the entire core, NRM₁₀₀/ARM₁₀₀ is evaluated to represent the fluctuations of the relative paleointensity. Our relative paleointensity data exhibit the same general features as obtained from archeomagnetic studies. The intensity increases as one goes back in time with a peak near 800 years B.P., representing an increase in the intensity of up to 60%. Apparent periodicities in the intensity of 300–400 years are observed.     

TRM in low magnetic fields: a minimum field that can be recorded by large multidomain grains

Thermally acquired remanent magnetization is important for the estimation of the past magnetic field present at the time of cooling. Rocks that cool slowly commonly contain magnetic grains of millimeter scale. This study investigated 1-mm-sized magnetic minerals of iron, iron–nickel, magnetite, and hematite and concluded that the thermoremanent magnetization (TRM) acquired by these grains did not accurately record the ambient magnetic fields less than 1 μT. Instead, the TRM of these grains fluctuated around a constant value. Consequently, the magnetic grain ability to record the ambient field accurately is reduced. Above the critical field, TRM acquisition is governed by an empirical law and is proportional to saturation magnetization (M_s). The efficiency of TRM is inversely proportional to the mineral's saturation magnetization M_s and is related to the number of domains in the magnetic grains. The absolute field for which we have an onset of TRM sensitivity is inversely proportional to the size of the magnetic grain. These results have implications for previous reports of random directions in meteorites during alternating field demagnetization, or thermal demagnetization of TRM. Extraterrestrial magnetic fields in our solar system are weaker than the geomagnetic field by several orders of magnitude. Extraterrestrial rocks commonly contain large iron-based magnetic minerals as a common part of their composition, and therefore ignoring this behavior of multidomain grains can result in erroneous paleofield estimates.     

Variation of the magnetic properties in a basaltic dyke with concentric cooling zones

The effects of the variation of magnetic grain size on the magnetic properties of rocks have been studied throughout a reversely magnetized basaltic dyke with concentric cooling zones.Except in a few tachylites in which the magnetic mineral is a Ti-rich titanomagnetite, in the bulk of the dyke the magnetization is carried by almost pure magnetite grains. Although the percentage p of these magnetic oxides varies slightly, the large changes in the various magnetic parameters observed across the dyke are essentially attributable to large variations in the grain size of the magnetic particles.From the outer scoria region, where the magnetic grains are a mixture of single-domain (SD) and superparamagnetic (SP) grains, to the tachylite zone with finely crystallized basaltic glass containing interacting elongated SD particles, one observes an increase of both the ratio of the saturation remanent magnetization and the saturation induced magnetization J_rs/J_is, the bulk coercive force H_c, the median destructive field MDF, the intensity of the remanent magnetization J_r, and the Koenigsberger ratio Q. In the tachylites these parameters reach unusually high values, for subaerial basalts:

$\text{〈}\text{J}{}_{\mathrm{rs}}\text{J}{}_{\mathrm{is}}\text{〉 = 0.3, 〈H}{}_{\mathrm{c}}\text{〉 = 460 O}{}_{\mathrm{e}}\text{, 〈MDF〉 = 620 O}{}_{\mathrm{e}}\text{r.m.s., 〈J}{}_{\mathrm{r}}\text{〉 = 2.7 · 10}{}^{\mathrm{?2}}\text{e.m.u. cm}{}^{\mathrm{?3}}\text{〈Q〉 = 24}$

These parameters decrease in the basalt toward the centre of the dyke where pseudo-single-domain (pseudo-SD) particles coexist together with multidomain (MD) grains. The susceptibility remains approximately constant from the inner basalt to the tachylite, but increases in the scoria up to values 10 times higher owing to the presence of SP particles. The magnetic viscosity increases also drastically toward the margin of the dyke due to an increase of the fraction of the SD particles just above the superparamagnetic threshold.     

On the magnetic and mineralogical properties of basalts

Summary Systematic and extensive investigations on the magnetic and mineralogical properties of samples from basaltic lavas and doleritic dykes have been made. The results obtained lead to several interesting conclusions. Rocks giving Curie temperatures of around 250°C exhibit a high degree of magnetic stability, yield largeQn values (>40) and often contain ore minerals possessing skeletal crystal structure. Also, such rocks can be saturated in low fields of the order of 1000 Oe, and give a value of around 0.5 for the ratioR of saturation remanence to the saturation intensity of magnetization. In contrast to this, rocks giving Curie temperatures of about 580°C are comparatively less stable, give smaller values ofQn, require higher fields for saturation and yield values ofR less than 0.2. Rocks with Curie temperatures ranging from 250°C to 550°C show general magnetic properties intermediate to those of the above two categories.     

Magnetic hysteresis in the F.M.R. spectra of fine-grained spherical iron: Possible evidence for a new carrier of hard remanence in lunar soils and rocks

Magnetic hysteresis effects have been observed in ferromagnetic resonance (FMR) spectra obtained at 9 and 16 GHz for certain simulated lunar glasses which were reduced by H₂ in the melt and rapidly quenched. Transmission electron microscopy has revealed that these samples contained spherical particles in the size range ～0.01–0.5 μm. FMR spectra obtained at 35 GHz (applied field ～ 12.5 kOe) exhibited a line shape characteristic of spherical, single-domain (SD) iron particles with no hysteresis. Computer simulations of the latter spectra confirmed that the average deviation from sphericity must be ?3% and that (2K₁/M_s) ≈ + 600 Oe for the precipitated magnetic phases. The principal features of the spectra obtained at all three frequencies have been explained on the basis of a simple theoretical treatment for spherical iron particles which have 2 domains in applied fields ?7 kOe, but become saturated at higher fields. Isothermal remanent magnetization (IRM) of these samples has been studied by both FMR and standard static techniques; the mean coercive force measured by the former (～4 kOe) contrasts with the mean value determined by the latter (～550 Oe). Apparently, FMR singles out and even amplifies the contributions of two-domain particles (which are magnetically hard), while the static measurement is more sensitive to the average of all particles present. The intensity of the FMR hysteresis of typical lunar soils is found to be ～1% of the total FMR intensity. In spite of this seemingly small value, two-domain iron particles may be important carriers of natural remanent magnetization (NMR) in certain lunar rocks.     

New paleomagnetic constraints on the late Cretaceous and early Cenozoic tectonic history of the Eastern Pontides

The Pontides are characterized by a series of Mesozoic-Cenozoic fold belts comprising a N-vergent foreland fold and thrust belt in the Western Pontides and a concave, upward-shaped fold belt in the Eastern Pontides. The curvature of the fold belt follows the Caucasus which may imply a phase of oroclinal bending. In order to test whether the fold curvature represents a phase of oroclinal bending, a paleomagnetic study has been carried out in the Eastern Pontides on late Cretaceous and middle Eocene volcanic and sedimentary rocks from 29 sites. Rock magnetic studies reveal medium-temperature components with an unblocking temperature of 400–580 °C, indicating pseudo-single domain titanomagnetite as the most abundant carrier of magnetic remanence in the middle Eocene rocks studied here. In the upper Cretaceous rocks, a high-temperature component with an unblocking range of 580–650 °C was isolated. Stepwise thermal and alternating field demagnetization isolated two components of remanent magnetization in middle Eocene rocks comprising a low unblocking temperature/coercivity component near the present field direction and a characteristic remanent magnetization (ChRM) component of D_s = 332.3°, I_s = 49.9° (k = 33.3, α₉₅ = 9.2°, N = 15 sites). A positive fold test at a 95% confidence level and a reversal test indicate a primary magnetization. Component analysis of the upper Cretaceous rocks identifies a stable ChRM D_s = 160.3°, I_s = −45.0°, (k =  85.6, α₉₅ = 6.0°, N =  8 sites) following removal of secondary remanence. Their ChRM direction passes fold and reversal tests at a 95% confidence level. Both the upper Cretaceous and middle Eocene paleomagnetic data from the Eastern Pontides and the Lesser Caucasus clearly demonstrate evidence of oroclinal bending that occurred contemporaneouslywith the convergence between Arabia and Eurasia in the Paleocene.     

Magnetic petrology of eastern North America diabases, I. Olivine-normative dikes from western South Carolina

The oxide mineralogy and magnetic properties were examined in a suite of fifteen olivine-normative diabase dike samples from western South Carolina in an attempt to elucidate their magnetic petrology. Titanomagnetite (1–2 vol.%) is the dominant Fe-Ti oxide mineral. Ilmenite and secondary magnetite are generally present in very minor amounts. Chromite constitutes up to 0.5 vol.%; its abundance and composition correlate with bulk rock Cr. Various types of fine-scale microstructure are evident in titanomagnetite crystals. The most important are patterned anisotropism and the development of trellis-type ilmenite lamellae. Microprobe analyses indicate: (1) titanomagnetite compositions, x, are mostly between 0.4 and 0.55, and (2) low analytical totals are characteristic of most titanomagnetites. Curie temperatures of the diabases are 500–540°C, which are several hundred degrees higher than predicted from the observed titanomagnetite x's (150–300°C). We attribute these higher Curie temperatures to oxidation of the titanomagnetites, which has produced “titanomaghemites” having visible microstructure and yielding low analyses (because they are cation deficient). Natural remanence magnetization and REM (ratio of natural remanence to saturation remanence) vary between 4 and 100 × 10⁻⁴ A m² kg⁻¹ and 0.0019 and 0.032, respectively. These properties inversely correlate with Cr content and demonstrably contrast Cr-rich and Cr-poor samples. Initial susceptibility, saturation magnetization and coercivity values show a two- to three-fold range. Variations in initial susceptibility and coercivities appear to be largely related to the type and extent of oxidation-induced microstructure in the titanomagnetites.     

Thermoremanent and chemical magnetization of exsolution products of nanosized titanomagnetites

Based on the theory of two-phase interacting nanoparticles, the formation of thermoremanent and chemical remanent magnetization in nanosized titanomagnetites is modeled. It is shown that the value of thermoremanent magnetization barely depends on the degree of titanomagnetite exsolution whereas, chemical remanent magnetization which emerges during the exsolution increases up to at most the value of thermoremanent magnetization. The values of the ratio of thermoremanent to ideal magnetization, R _t , fall within the limits 0.8 ≤ R _t ≤ 1. The analogous ratio of chemical remanent magnetization to the ideal R _c are below R _t at all stages of the exsolution. Besides, the magnetic interaction between the nanoparticles reduces the values of thermoremanent and chemical magnetization but barely affects the ratio.     

Stable Eocene Magnetization Carried by Magnetite and Iron Sulphides in Marine Marls (Pamplona-Arguis Formation,Southern Pyrenees,Northern Spain)

In order to establish the magnetic carriers and assess the reliability of previous paleomagnetic results obtained for Eocene marine marls from the south Pyrenean basin, we carried out a combined paleo- and rock-magnetic study of the Pamplona-Arguis Formation, which crops out in the western sector of the southern Pyrenees (N Spain). The unblocking temperatures suggest that the characteristic remanent magnetization (ChRM) is carried by magnetite and iron sulphides. The ChRM has both normal and reversed polarities regardless of whether it resides in magnetite or iron sulphides, and represents a primary Eocene magnetization acquired before folding. Rock magnetic results confirm the presence of magnetite and smaller amounts of magnetic iron sulphides, most likely pyrrhotite, in all the studied samples. Framboidal pyrite is ubiquitous in the marls and suggests that iron sulphides formed during early diagenesis under sulphate-reducing conditions. ChRM directions carried by magnetic iron sulphides are consistent with those recorded by magnetite. These observations suggest that magnetic iron sulphides carry a chemical remanent magnetization that coexists with a remanence residing in detrital magnetite. We suggest that the south Pyrenean Eocene marls are suitable for magnetostratigraphic and tectonic purposes but not for studies of polarity transitions, secular variations and geomagnetic excursions, because it is difficult to test for short time differences in remanence lock-in time for the two minerals. The presence of iron sulphide minerals contributing to the primary magnetization in Eocene marine marls reinforces the idea that these minerals can persist over long periods of time in the geological record.     

Textural remanence: A new model of lunar rock magnetism

In reexamining the accumulated magnetic data on lunar rocks, several common patterns of magnetic behavior are recognized. Their joint occurrence strongly suggests a new model of lunar rock magnetism, which appeals only to partial preferred textural alignment of the spontaneous moments of magnetic grains, without requiring the existence of ancient lunar magnetic fields. This magnetic fabric, mimetic to locally oriented petrofabric, gives rise to an apparent “textural remanent magnetization” (TXRM). In order to account for the observed intensity of “stable remanence” in lunar rocks, only a minute fraction (10^?3 to 10^?5) of the single-domain iron grains present need be preferentially aligned. Several mechanisms operating on the lunar surface, including shock and diurnal thermal cycling, appear adequate for producing the required type and degree of magnetic alignment in all lunar rock classes. The model is supported by a wide variety of direct and indirect evidence and its predictions (e.g. regarding anisotropic susceptibility and remanence acquisition) can be experimentally tested.     

On the grain size dependence of the behavicur of fine magnetic particles in rocks

The grain size dependence of the ratio of saturation remanent magnetization to saturation magnetization (J _R :J _S ), weak field susceptibility (X ₀), thermoremanent magnetization (TRM) and its stability against AF demagnetization are interpreted in terms of nucleation theory. It is concluded that each of these parameters exhibits grain size dependence due to two effects. The first is the increasing difficulty with which domain walls are neucleated as grain size decreases. The second is an intrinsic grain size dependence of the parameters in multidomain particles.     

A note on magnetic modelling with remanence

The general problem of magnetic modelling involves accounting for the effect of both remanent magnetization and the application of an external magnetic field. However, as far as the disturbing field of a magnetic body in a magnetic environment is concerned, there is an equivalence between the effects of these two causations that allows the remanence to be represented in terms of an equivalent primary magnetic H field. Moreover, due to the linearity of the magnetic field in terms of its causations, the general modelling problem involving an applied magnetic field in the presence of remanence can be simply and more efficiently dealt with in terms of an equivalent primary field acting in the absence of any remanent magnetization.