Özden Özdemir

A comparison of different magnetic methods for determining the relative grain size of magnetite in natural materials: Some results from lake sediments

Abstract A new rapid method for identifying relative grain size variations in magnetic involves the parameter anhysteretic susceptibility (χARM, i.e. specific ARM obtained in a 1 Oe steady field), which is particularly sensitive to the single domain (SD) and small pseudo-single domain (PSD) grains of the finer magnetite fraction. A second parameter, low-field susceptibility (χ), is relatively more sensitive to the coarser magnetite fraction (larger PSD and smaller multidomain (MD) grains). We can then obtain a measure of the ratio of coarse- to fine-grain magnetite for large numbers of samples by plotting χARversusχ. A simple idealized model based on sized magnetite samples is proposed to explain the use of the χARMversusχ plot for detecting relative grain-size changes in the magnetic content of natural materials. The sediments of three lakes that contain magnetite or a similar magnetic carrier and have a wide range of values of χARM and χ are used to test the model.The model is used to interpret the magnetic variations observed, and the interpretations are supported by high-field hysteresis measurements of the same sediments. The combination of the high-field hysteresis method of Day et al. [1] and the χARM vs. χ method is a powerful technique allowing the rapid identification of both the relative grain size and domain state for large numbers of samples containing magnetite. The χARMvs.χ method should be used as an intial means of identifying distinct groups of samples.The high-field hysteresis method should then be applied to a few representative samples from each group to confirm the initial interpretation.

The effect of oxidation on the Verwey transition in magnetite

At the Verwey transition (Tv≈110–120 K), magnetite transforms from monoclinic to cubic spinel structure. It has long been believed that magnetic remanence and susceptibility would change markedly at Tv in the case of coarse grains but only slightly or inappreciably in the case of fine (<1 µm) grains. We find on the contrary that remanence changes at Tv by 50–80% in both large and small crystals, if they are stoichiometric. However, minor surface oxidation suppresses the transition, and the fact that fine grains oxidize more readily leads to an apparent size dependence. Our experiments used submicron magnetite cubes with mean sizes of 0.037, 0.076, 0.10 and 0.22 µm which were initially non-stoichiometric (oxidation parameter z from 0.2–0.7). A saturation isothermal remanent magnetization (SIRM) given in a 2.5 T field at 5 K decreased steadily during zero-field warming to 300 K with little or no indication of the Verwey transition. After the oxidized surface of each crystal was reduced to stoichiometric magnetite, the SIRM decreased sharply during warming by 50–80% around 110 K. The change in SIRM for the 0.22 µm grains was almost identical to that measured for a 1.5 mm natural magnetite crystal. Thus a 1012 change in particle volume does not materially affect the remanence transition at Tv but oxidation to z=0.3 essentially suppresses the transition. The effect of the degree of oxidation on Tv provides a sensitive test for maghemitization in soils, sediments and rocks.

Changes in remanence, coercivity and domain state at low temperature in magnetite

Abstract Submicron magnetite crystals with mean sizes of 0.037, 0.10 and 0.22 μm undergo major changes in hysteresis properties and domain states in crossing the Verwey transition ( T V ≈120 K). The 0.037 μm crystals are single-domain (SD) both in the cubic phase at room temperature T 0 and in the monoclinic phase below T V . The 0.10 and 0.22 μm crystals have a mixture of SD and two-domain (2D) states at room temperature T 0 , but mainly SD structures below T V , in agreement with micromagnetic calculations. Coercive force H c increases on cooling through T V , by a factor 3–5 in the submicron magnetites and 40 in a 1.3 mm single crystal, because of the high crystalline anisotropy and magnetostriction of monoclinic magnetite. As a result, domain walls and SD moments are so effectively pinned below T V that all remanence variations in warming or cooling are reversible. However, between ≈100 K and T 0 , remanence behavior is variable. Saturation remanence (SIRM) produced in monoclinic magnetite at 5 K drops by 70–100% in warming across T V , with minor recovery in cooling back through T V (ultimate levels at 5 K of 23–37% for the submicron crystals and 3% for the 1.3 mm crystal). In contrast, SIRM produced in the cubic phase at 300 K decreases 5–35% (submicron) or >95% (1.3 mm) during cooling from 300 to 120 K due to continuous re-equilibration of domain walls, but there is little further change in cooling through T V itself. However, the submicron magnetites lose a further 5–15% of their remanence when reheated through T V . These irreversible changes in cycling across T V , and the amounts of the changes, have potential value in determining submicron magnetite grain sizes. The irreversibility is mainly caused by 2D→SD transformations on cooling through T V , which preserve or enhance remanence, while SD→2D transformations on warming through T V cause remanence to demagnetize.

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

Abstract 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 −6 emu g −1 , 3.18 × 10 −6 A 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 −7 A 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.

Inversion of titanomaghemites

Metastable titanomaghemite inverts to a multiphase intergrowth when heated above 250–300°C. In the present study, pure and aluminum-substituted titanomaghemites were heated to 600°C to produce inversion products. Saturation hysteresis properties-saturation magnetization (JS), saturation remanence (JRS), and coercive force (HC-were measured at intervals during heating and cooling between 20 and 600°C. X-ray cell edge and magnetic viscosity were measured at room temperature before and after heating. As a result of inversion, the magnetic and other physical properties changed considerably. JS of the final inversion product was always higher than JS of the starting titanomaghemite. Decreases in the ratio JRSJS and in HC and an increase in magnetic viscosity after inversion reflect the subdivision of originally single-domain (SD) homogeneous titanomaghemite grains into superparamagnetic (SP) or nearly SP-size subgrains. The phase assemblage in the final inversion product depends on the degree of low-temperature oxidation of the starting titanomaghemite. The spinel phase in the intergrowth was near-stoichiometric titanomagnetite or magnetite, and the other phases included ilmenite, haematite, anatase, and/or pseudobrookite. Aluminum had no stabilizing effect on the titanomaghemite structure.

High-temperature hysteresis and thermoremanence of single-domain maghemite

Abstract Magnetic properties have been measured for acicular maghemite crystals of single-domain (SD) size between room temperature and the Curie temperature T C of 645°C. The transformation of maghemite to hematite takes place at 752°C, far above T C of the present γ-Fe 2 O 3 . The observed high-temperature stability of this material made it possible to study the thermoremanent magnetization (TRM) and high-temperature hysteresis properties of γ-Fe 2 O 3 . The temperature dependence of coercive force shows that the main contribution to the coercivity is from shape anisotropy. Magnetization reversal is dominated by incoherent rotation. The experimental results are consistent with the fanning mode of reversal in a chain of spheres. The weak-field TRM was linear with applied field, as predicted from Neel SD theory.

Thermoremanence and Néel temperature of goethite

We have measured thermoremanence (TRM) and the temperature dependence of high-field susceptibility χ both parallel and perpendicular to the crystallographic c-axis, for a sample of well crystallized natural goethite (αFeOOH). Susceptibility χ⟂ measured perpendicular to the c-axis was almost temperature independent between 50 and 300 K, while χ∥ measured parallel to the c-axis increased almost linearly with temperature over the same range. These are the dependences expected for an antiferromagnetic (AFM) substance with sublattice magnetizations along the c-axis. Extrapolation of the χ⟂ and χ∥ data trends to their point of intersection gives an estimate for the AFM Neel temperature TN of (120±2)°C. TRMs produced by cooling in a weak field applied either parallel or perpendicular to the c-axis had intensities of 2.4 × 10−4 Am²/kg and 1.2 × 10−5 Am²/kg, respectively. Since (MTRM)⟂ is only 5% of (MTRM)∥, the weak ferromagnetism of goethite must be parallel to the AFM spin axis, not perpendicular to it as in the case of hematite. The ferromagnetism is very hard: TRM was unaffected by AF demagnetization to 100 mT and by thermal demagnetization to 90°C. Above 90°C, TRM decreased sharply, reaching zero at (120±2)°C. Thus the ferromagnetic Curie point TC coincides with TN, as in hematite. However, the weak ferromagnetism cannot be due to spin canting, as it is in hematite, because canting of the sublattices would produce a net moment perpendicular to the c-axis, rather than parallel to the c-axis as observed.

Effect of grain size and domain state on thermal demagnetization tails

Thermal demagnetization of viscous remanence (VRM) and partial thermoremanence (pTRM) of 20- and 135-µm natural magnetites reveals a broad spectrum f(TUB) of unblocking temperatures TUB, both > and < TB, the blocking temperature. In contrast, 0.04-µm single-domain (SD) grains demagnetize sharply at TUB ≈ TB. High- and low-TUB tails of f(TUB) were wider for 135-µm multidomain (MD) grains than for 20-µm PSD grains. 10-mT alternating-field demagnetization rendered the VRM of the 20-µm magnetite more SD-like in subsequent thermal cleaning, selectively erasing both low- and high-TUB tails of f(TUB). Primary and secondary remanences are then cleanly separated, and non-linear Thellier paleointensity determination becomes linear. Our results agree well with (TB, TUB) data for 105-year thermoviscous overprints in the Milton Monzonite over the range of log(time) common to the two studies. Because (TUB)av ≈ TB for PSD and MD as well as SD grains, the Pullaiah et al. paleothermometry method will work if (TUB)av values are used instead of (TUB)max values.

Low-temperature properties of a single crystal of magnetite oriented along principal magnetic axes

Abstract We have measured saturation induced and remanent magnetizations and induced magnetization as a function of field at low temperatures, between 300 K and 10 K, on an oriented 1.5-mm single crystal of magnetite. The induced magnetization curves along the cubic [001], [1 1 0] , and [110] axes at 10 K have very different approaches to saturation. The crystal is easy to magnetize along [001] but difficult along [1 1 0] and [110], the hard directions of magnetization for monoclinic magnetite. The temperature dependence of saturation magnetization between the Verwey transition temperature, Tv = 119 K, and 10 K is also different along the three axes, indicating that below Tv the crystal has uniaxial symmetry. The room-temperature saturation remanence (SIRM) produced along [001] decreases continuously in the course of zero-field cooling, levelling out at the isotropic temperature, Ti = 130 K, where the first magnetocrystalline anisotropy constant becomes zero. At Ti, 86% of the initial SIRM was demagnetized. The domain wall pinning responsible for this soft remanence fraction must be magnetocrystalline controlled. The remaining 14% of the SIRM is temperature independent between Ti and Tv and must be magnetoelastically pinned. This surviving hard remanence is the core of the stable magnetic memory. The Verwey transition at 119 K, where the crystal structure changes from cubic to monoclinic, is marked by a discontinuous increase in remanence, indicating that the cubic [001] direction suddenly becomes an easy direction of magnetization. The formation of monoclinic twins may also affect the intensity of remanence below Tv. Reheating from 10 K retraces the cooling curve, with a decrease at Tv back to the original remanence level, which is maintained to 300 K. When SIRM is not along [001], the initial SIRM is larger but the reversible changes across the Verwey transition are much smaller. The SIRM produced at 20 K is an order of magnitude larger than the 300 K SIRM, but the only change during warming is a discontinuous and irreversible drop to zero at Tv.

Intermediate magnetite formation during dehydration of goethite

Abstract The dehydration of goethite has been studied by low-temperature induced magnetization (LTIM) and X-ray diffraction on well-characterized acicular crystals. Fresh samples were heated in air to temperatures between 155°C and 610°C. Goethite and hematite were the magnetically dominant phases after all runs except 500°C and 610°C, for which only hematite was found. However, partially dehydrated goethites after the 238–402°C runs had broad peaks or inflections in the LTIM curves around 120 K, suggesting the formation of an intermediate spinel phase. These samples were next given a saturation remanence in a field of 2 T at 10 K and the remanence was measured continuously during zero-field warming to 300 K. There was a decrease in remanence at the Verwey transition (120 K), diagnostic of magnetite. The possible formation of a small amount of magnetite is of serious concern in studies of goethite-bearing sediments and rocks. Chemical remanent magnetization (CRM) of this strongly magnetic spinel phase could significantly modify the direction as well as the intensity of the original goethite CRM. As well, it would be a new source of paleomagnetic noise as far as primary remanence carried by other mineral phases is concerned.