Superparamagnetic Grains Research Articles

Magnetic hysteresis properties of greigite (Fe 3S 4) and a new occurrence in Holocene sediments from Swedish Lappland

A new occurrence of ferrimagnetic greigite (Fe 3S 4) is reported in freshwater Holocene sediments from Gaskkamus Gorsajávri in Swedish Lappland. The presence of greigite was inferred from the mineral magnetic analysis of sediment cores. The greigite-bearing sediments possess a high saturation isothermal remanent magnetisation to susceptibility (SIRM/χ) ratio, a characteristic shared by other natural sediments known to contain greigite. The hysteresis properties (measured using a vibrating sample magnetometer) of natural greigite extracted from lake sediments show much higher saturation coercivity (( B 0) C) and M RS M S ratio values (approximately 50 mT and 0.5 respectively) than synthetic samples of greigite (approximately 12 mT and 0.2). The results suggest that synthetic greigite samples contain crystals ranging from small non-remanence carrying superparamagnetic grains to large multi-domain grains. In contrast, the natural samples probably consist predominantly of pseudo-single-domain and single-domain crystals capable of carrying a chemical remanent magnetisation. However, the unstable nature of greigite under laboratory conditions creates difficulty in characterising its physical attributes.

Rock magnetism of the Early Pliocene Trubi Formation at Eraclea Minoa (Sicily)

The most important magnetic mineral in the finegrained marls of the Early Pliocene Trubi formation near Eraclea Minoa is magnetite. Some goethite is also present. The magnetite is of high purity and of primary origin.Hysteresis and low‐temperature measurements show that most magnetite grains are single‐domain. The presence of superpara‐magnetic grains is suspected. Multi‐domain grains are rare. It is stewn that AF demagnetization does not yield correct NRM directions.

Magnetic grain size and viscous remanent magnetization of pelagic clay

Unfossiliferous pelagic clay (red clay) cores from the Pacific generally have unstable remanent magnetization except for tens of centimeters to several meters below the surface. The origin of this instability has been considered to be viscous remanent magnetization (VRM). We conducted a rock‐magnetic study of a pelagic clay core from the South Pacific, which has the unstable‐to‐stable transition at about 1.5 m below the surface. Using the suspension method of Yoshida and Katsura (1985), we determined downcore variation of complete alignment magnetization (CAM) and magnetic moment distribution (geometric mean moment, mG, and log standard deviation, log a) assuming a lognormal distribution. Mean magnetic grain diameter was calculated from the mG using the saturation magnetization of magnetite, 4.8×105 A M−1, on the assumption that all the magnetic grains are single‐domain magnetites of spherical shape. This assumption is supported by thermomagnetic analyses, isothermal remanent magnetization (IRM) acquisition experiments, and the ratio of CAM to saturation IRM The grain diameter ranges from about 0.02 to 0.15 μm and decreases with depth below about 1.5 m. The derived grain size holds even when the magnetites in the sediments suffered oxidation because the saturation magnetization of maghemite is close to that of magnetite. The frequency dependence of magnetic susceptibility showed a downward increase in the amount of superparamagnetic grains. On the other hand, the magnetic viscosity acquisition coefficient normalized by the CAM increases with depth. We conclude that magnetic grain size can control the magnitude of the secondary magnetization, VRM, of pelagic clay. It has been proposed that the major source of pelagic clay is atmospherically transported dust. Intensified global atmospheric circulation by the change of climate would raise the grain size of the magnetic minerals of the eolian component, which can cause the unstable‐to‐stable transition of the remanent magnetization.

Magnetic and Structural Properties of Granular Iron-Silicon Dioxide Thin Films

ABSTRACTFe-SiO2 granular films were produced by co-sputtering from separate Fe and SiO2 targets. Films deposited on silicon nitride membranes were thin enough to observe directly by transmission electron microscopy (TEM). The coercive force (Hc) was strongly dependent on temperature and volume fraction of Fe as observed in previous investigations, but with lower Hc. Two features of the films with low Fe concentrations were remarkable. The hysteresis loops show high remanence, and the room temperature TEM images show stable domain structure and ripple patterns. This behavior is inconsistent with a model of non-percolating, non-interacting superparamagnetic Fe grains.

The alignment of interstellar grains

The magnetic-field alignment of interstellar silicate grains containing superparamagnetic (SPM) inclusions is investigated theoretically. It is shown that the observed dependence of polarization on wavelength can be fully explained in terms of the alignment mechanisms and the size distribution of the SPM grains, at least for interstellar polarizations peaking at wavelength 550 nm or higher. The implications of this finding for models of grain coagulation and shattering are considered.

Magnetic studies on Shergotty and other SNC meteorites

Hysteresis measurements on three shergottite and two nakhlite meteorites indicate single domain grain size behavior for the highly shocked Shergotty, Zagami, and EETA 79001 meteorites, with more multidomain-like behavior for the unshocked Nakhla and Governador Valadares meteorites. High viscosity and initial susceptibility for Antarctic shergottite ALHA 77005 indicate the presence of superparamagnetic grains in this specimen. Thermomagnetic curves likewise reveal a range of oxidation states for the high ulvospinel titanomagnetite grains which dominate the magnetic properties of these first five meteorites. Thermomagnetic analysis indicate Shergotty and Zagami as the least initially oxidized, while EETA 79001 appears to be the most oxidized. Cooling of the meteorite samples from high temperature in air results in a substantial increase in magnetization due to the production of magnetite through oxidation exsolution of titanomagnetite. However, vacuum heating substantially suppresses this process, and in the case of EETA 79001 and Nakhla, results in a rehomogenization of the titanomagnetite grains.Remanence measurements on several subsamples of Shergotty and Zagami meteorites reveal a large variation in intensity that does not seem related to the abundance of remanence carriers. The other meteorites carry only weak remanence, suggesting weak magnetizing fields as the source of their magnetic signal. A paleointensity experiment on a weakly magnetized subsample of Shergotty reveals a low temperature component of magnetization acquired in a field of 2,000 gammas. Also present was a high temperature component reflecting a paleo-field strength of between 250 and 1,000 gammas, depending on the nature and degree of alteration that the sample may have undergone with heating. This is consistent with an earlier paleointensity estimate of 1,000 gammas for ALHA 77005. The weak field environment that these meteorites seem to reflect is consistent with either a Martian or asteroidal body origin, but inconsistent with a terrestrial origin.

FMR study of coherent fine magnesioferrite particles in MgO-line shape behavior

Ferromagnetic resonance (FMR) in fine coherent precipitates (10 nm 3 < v <10 3 nm 3) of magnesium ferrite embedded in MgO matrix has been studied. Room temperature results show that with decreasing particle size the resonance line width increases but at very small particle sizes ( v<100 nm 3) shows an anomalous decrease. Asymmetric and distorted resonance line shape become more regular in consequence of motional averaging of crystalline anisotropy at higher temperatures and for smaller particle sizes. Simple model of independent superparamagnetic grains explains the influence of particle size distribution on the FMR line shape.

Relations between magnetism and superconductivity in amorphous Zr1−xFex alloys

In the amorphous sputtered Zr1−xFex alloy with x = 24% we observe the separation of two amorphous phases upon annealing or aging. One is Fe-rich and behaves like an assembly of superparamagnetic grains. The other is superconducting.

Theoretical analysis of thermomagnetic properties, low-temperature hysteresis and domain structure of titanomagnetites

Magnetic hysteresis of coarse-grained titanomagnetites at room temperature is characterised by low coercive force, low relative remanence, and a high ratio of coercivity of remanence to coercive force. These properties are generally interpreted in terms of multidomain structure. At low temperatures, however, ulvöspinel-rich compositions exhibit hysteresis properties similar to those of single-domain assemblages, and on this basis Radhakrishnamurty and Deutsch have proposed an alternative interpretation of the domain structure of titanomagnetites having x ⩾ 0.3 in terms of a mixture of single-domain and superparamagnetic particles. Low apparent Curie temperatures are attributed to the effects of thermal agitation above the blocking temperature. We have examined theoretically the effects of thermal agitation on the low- and high-field thermomagnetic curves and find that observed Curie temperatures in general represent an intrinsic property of the magnetic mineral present, rather than reflecting thermal agitation. The high coercive force and relative remanence at low temperatures for titanomagnetites having x > 0.5 can be explained on the basis of the interaction of domain walls with crystal defects when the large increases in magnetocrystalline anisotropy and magnetostriction with decreasing temperature are taken into account. We discuss the evidence for the existence of domain walls in coarse-grained ulvöspinel-rich titanomagnetites and conclude that multidomain structure is well established. It is also shown that fine titanomagnetite grains may have more than one blocking temperature. In any temperature interval for which superparamagnetic grains are present they will disproportionately influence susceptibility and low-field hysteresis.

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 m 2 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 m 2 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.

Some observations on short-term magnetic viscosity behaviour at room temperature

Experimental results on short-term magnetic viscosity behaviour at room temperature are described. A simple method, using a Digico spinner magnetometer, is used to measure short-term magnetic moment decays. These decays follow linear relationships with the logarithm of time showing relaxation times of less than 100–1000 s, i.e. superparamagnetic grain assemblages at room temperature. These effects are found to be very common in a wide variety of rock types, and may affect routine measurements of remanent magnetization.

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: 〈 J rs J is 〉 = 0.3, 〈H c〉 = 460 O e, 〈MDF〉 = 620 O e r.m.s., 〈J r〉 = 2.7 · 10 −2 e.m.u. cm −3 〈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.

Nature of magnetic grains and their effect on the remanent magnetization of basalts

Abstract The nature of magnetic grains in basalts obtained from different parts of the world has been investigated. Results indicate that magnetic behaviour attributable to cation-deficient magnetite is common in basalts younger than Cretaceous, while that due to multidomain magnetite is widespread in much older rocks. Superparamagnetic grains occur in basalts more abundantly than originally presumed, which seem to be mainly responsible for the viscous remanent magnetization of such samples. Basalts which are inferred to contain predominantly optimum single-domain grains are found to be most suitable for palaeomagnetic work. However, many samples generally contain a wide range of grain sizes and this can account for the observed variation in their magnetic stability. One possible mechanism for the formation of such magnetic grains in basalts and its implications to palaeomagnetism is presented. Basalts whose magnetic behaviour is completely reversible on heating and cooling are very rare and because of this fact the reliability of palaeointensity determinations, involving heating of the samples even for one time, may be reduced to a considerable extent.

The frequency dependence of the viscous component of the magnetic susceptibility of lunar rock and soil samples

The susceptibility of two lunar samples (a soil and a low metamorphic grade breccia) has been measured in a weak field — 0.001 Oe — and as a function of frequency from 0.032 to 1.0 Hz. The measurements were made using a superconducting magnetometer. The results show that the susceptibility decreases linearly with the log of frequency. This observation is in agreement with a theoretical model for viscous decay based on the Néel theory of single-domain and superparamagnetic grains. The relation derived agrees with a model in which there is a uniform distribution of relaxation times.

Effects of vapor-phase deposition processes on the optical, chemical, and magnetic properties OE the lunar regolith

The processes of solar wind sputtering and meteoritic impact vaporization have created materials in the lunar regolith which were deposited from a vapor phase. Although the quantity of such exotic condensed substances should theoretically be comparable with that of materials which have been melted by impacts, their existence in the fines has not been generally recognized. We have investigated the physical and chemical properties of materials deposited from vapors generated by hydrogen-ion sputtering and thermal evaporation of lunar and artificial ferrosilicates. Both processes are highly reducing. The deposits are enriched in Fe, have large, nonselective, optical absorptivities, and contain abundant sub-microscopic, superparamagnetic grains of metallic Fe which exhibit the characteristicg=2.1 ESR resonance. The sputter-deposited films are enriched in heavy elements. Thus the hypothesis that the lunar fines contain several percent of materials deposited from the vapor phase accounts in a natural manner for many of the unusual optical, physical and chemical properties of lunar soils. The vapor-deposits are probably concentrated in the agglutinate particles of the regolith.

Preisach Diagrams and Magnetic Viscosity Phenomena for Soils and Synthetic Assemblies of Iron Oxide Grains

Preisach diagrams for soils derived from sedimentary geological strata and for synthetic samples containing dispersed magnetite grains are presented and are used as the basis for distinguishing between superparamagnetic, stable single domain and multidomain grains. In particular it is established that between 10 and 20 percent of the total magnetic constituents in a typical soil are well-dispersed stable single domain grains with diameters of about 250A, the majority of the remainder being superparamagnetic grains with slightly smaller diameters.The Preisach-Neel theory for interacting single domain grains is extended so that the associated magnetic viscosity phenomena can be represented on the Preisach diagram in terms of a fluctuating viscosity field and experimental results justifying the introduction of this concept are included. This presentation provides a clear indication of those areas of the Preisach diagram, and hence the components of the overall magnetic grain distribution in the sample, that are associated with any particular viscosity process. It is thus shown that the value for the coefficients for the decay of viscous remanent magnetism can be used for comparing the magnetic grain size distributions in different samples.

Critical Phenomena in Very Small Spin Systems

AbstractThe discontinuities observable at second order phase transitions (e.g. the divergence of the specific heat) can occur only if the size of the considered system is very large (strictly speaking, infinite). Therefore some pronounced modifications of critical phenomena have to be expected in very small systems. We investigate these modifications by numerical calculations (using mainly the Monte Carlo method) for the case of both Ising S = 1/2 and classical Heisenberg spin systems (the total number of spins in the considered systems being in the range from 10 to 103). Assuming systems with free surfaces one finds also a pronounced dependence on the shape of the system. The shape dependence of the energy can be related to the reduction of the ground state energy of the system. Considering the behaviour of the energy, specific heat, and magnetization, one observes both a rounding of the critical anomalies and a shift to lower temperatures. This behaviour is in good agreement with the conclusions which can be drawn from the wellknown behaviour of the exactly soluble two‐dimensional Ising S = 1/2 model. The results for the three‐dimensional systems presented here give information concerning the rounding phenomena which should be observable at the “phase transition” of superparamagnetic grains or similar systems.

Response of an isolated magnetic grain suspended in a liquid to a rotating field

We have studied the response of an isolated uniaxial magnetic grain suspended in a liquid to an applied fieldh rotating with frequencyω. In the presence of an applied static field (H≫h), at low frequencies (i.e. for fast relaxation), the easy axis followsh, while at high frequencies the behavior is similar to that of a bulk sample. In zero static field, the response of a ferromagnetic grain is more complicated; there exists a critical frequencyωe below which a steady state is reached, with the easy axis followingh. Forω>ωe the mechanical behavior depends crucially on the initial conditions. Finally, a superparamagnetic grain has a (different) critical frequencyωe, below which it reacts similarly to the ferromagnetic particle, while forω>ωe it does not follow steadily the rotating field, but can only oscillate about its initial position.