Low temperature magnetic properties of titanomagnetites

New measurements of high field magnetisation (I_s), remanence (I_s), and coercive force (H_c) are presented between 4 and 300 K for x = 0.4, 0.5, 0.6 and 0.8 (Fe_3−xTi_xO₄). For x = 0.4 a pronounced minimum is found at T100 K and for X = 0.5 and 0.6 broad minima occur around T200 K, apparently coinciding with the temperature for K₁0. The magnetic properties below T60 K ar complex and were found to be significantly changed by cooling in the presence of a high magnetic field. With no applied field during cooling, a distinct decrease in I_s is observed for T60 K, at which temperature there is a peak in the value of I_rs. The effe cooling is to eliminate the sharp decrease in I_s, reduce H_c and to increase I_rs below 60 K to a value the peak value, giving essentially a square hysteresis loop. The results are interpreted in terms of a form of crystallographic phase transition coupled to the magnetisation direction, possibly by the magnetostriction. Square hysteresis loops in ferrites have been explained by the presence of Jahn-Teller ions and, in the present case, the low temperature of the observed effect may be a consequence of the weak Fe²⁺ Jahn-Teller ion coupled to other effects such as spin-lattice coupling.

Details of this work can be found in Schmidbauer, E. and Readman, P.W., 1982. Low temperature magnetic properties of Ti-rich Fe---Ti spinels. J. Magn. Magn. Mat., 27: 114–118. A paper reporting further work on Fe_2.4Ti0.6O0.4 is in preparation.     

The laboratory simulation of deuteric oxidation of titanomagnetites: effect on magnetic properties and stability of thermoremanence

Synthetic single crystals of titanomagnetite of nominal composition Fe_2.4Ti_0.6O₄ have been oxidized at 1275°C in controlled gas atmospheres, producing multiphase intergrowths to simulate the natural process of deuteric oxidation. The evolution of the intergrowths was monitored using the conventional techniques of petrology: optical and electron microscopy and X-ray and electron microprobe analyses. In addition, the measured magnetic properties — particularly the temperature-dependence of hysteresis properties — provided further information about the composition and concentrations of magnetic phases, and their domain state, as oxidation proceeded. The evolution of a trellis pattern of ilmenite lamellae, characteristic of the “exsolution” stages of deuteric oxidation, was observed in the oxidized crystals. The interlamellar spinel region consisted of two iron-enriched titanomagnetites, one thought to occur along the lamellar boundaries. The magnetic hardness of both phases was found to be greater than the original homogeneous multidomain titanomagnetite crystals, although neither phase achieved monodomain characteristics, and the stability of thermoremanence (TRM) remained quite low (median destructive fields (MDF) of the order of a few thousend A m^?1). The lamellae made little contribution to the total remanence. A sharp rise in magnetic hardness, observed during the post-exsolution stages of oxidation, was due to the presence of fine-grain monodomain magnetite, thought to be distributed within the haemoilmenite matrix, but probably not having been formed by lamellar subdivision. The crystals could now carry an intense and stable TRM with MDFs of many tens of thousands of A m^?1.     

High-temperature hysteresis and other magnetic properties of synthetic monodomain titanomagnetites

A suite of synthetic titanomagnetites of composition Fe_2.4?δAl_δTi_0.6O₄ and Fe_2.6?δAl_δTi_0.4O₄ (δ = 0, 0.1 and 0.2 in both cases) have been prepared by a method of partial self-buffering and pulverized in a ball mill to particle size of about 200–500 Å. Magnetic hysteresis parameters-saturation and remanent magnetizations and coercive force were measured between room temperature and the Curie temperatures and other parameters-X-ray cell edge, initial susceptibility and coercive force of remanence were determined at room temperature. The intrinsic magnetic “hardness” increases with increasing content of Al³⁺ and Ti⁴⁺, both probably corresponding to an increase in the concentration of Fe²⁺ ions on the tetrahedral sites of the spinel structure. The room-temperature hysteresis properties were compared with those resulting from monodomain models for the work done to magnetically saturate an assemblage of grains and the approach to saturation, and the separate contributions from coexisting anisotropies of cubic and uniaxial symmetries (assumed present) inferred. The cubic anisotropy energy constants so derived are larger than those determined from multidomain single crystals. The derived cubic constants are also larger than the derived uniaxial anisotropy constants. The latter, however, dominate the behaviour (e.g., coercive force) because of the lower symmetry. The materials appear to be entirely in the stable monodomain state at room temperature.     

The magnetic anisotropy of basalts and its variations under oxidation of titanomagnetites

Summary Using four samples of basaltic rocks from the Bohemian Massif, Nos 201, 202, 206 and 218, the curves of rotational moments in a magnetic field of 4 × 10⁵ A/m were studied as a function of the degree of oxidation of titanomagnetites. The amplitudes of the individual harmonics were determined by harmonic analysis for samples oxidized to various degrees. It was found that the proportion of the harmonic A ₁ withsin increases with the degree of oxidation. Simultaneously, the anisotropy of the magnetic susceptibility changes in a field of 60 A/m and the structure changes from linearly parallel to plane parallel.     

Characteristic magnetic properties of titanomagnetites in continental igneous rocks

Curie temperatures, hysteresis, alternating field properties and anhysteretic and ordinary susceptibilities have been used to characterize the titanomagnetites in a large collection of continental granites, diorites, syenites, anorthosites, gabbros, diabases and basalts. Low-Curie-point titanomagnetites or titanomaghemites were found only in basalts. In all shallow and deep-seated intrusive rocks, the predominant magnetic phase was nearly-titanium-free titanomagnetite with a Curie point of 520–580°C. Most felsic plutonic rocks owed their magnetic properties to coarse, discrete titanomagnetites with truly multidomain properties. Many mafic plutonic rocks (anorthosites, gabbros, norites) displayed bimodal magnetic properties, strong-field properties being due to the discrete titanomagnetites and weak-field properties being due to fine magnetite inclusions in deuterically altered silicates. The Lowrie-Fuller test and the anhysteretic induction curve were the most diagnostic tests of this bimodal behaviour. Grain-size variation within a single diabase dike or sill had a strong expression in all magnetic properties, except H_R/H_c and the Lowrie-Fuller test. On the other hand, the Lowrie-Fuller test was a sensitive indicator of changes in “effective” grain size in basalts due to the subdivision of grains by ilmenite lamellae.     

Changes of the magnetic susceptibility and its anisotropy of basalts by oxidation of titanomagnetites

Summary The changes of the anisotropy of the magnetic susceptibility of alkaline Tertiary basalts under titanomagnetite oxidation are investigated. The oxidation of the titanomagnetites was achieved by the following thermal processing of rock samples: 60 mins at a temperature of 400°C, 300 mins at T=400°C and 60 mins at T=500°C. It is shown that considerable changes of the values of the magnetic susceptibility, as well as of the degree of the preferred orientation of the titanomagnetites and of the ellipsoid of anisotropy in the sample occur as a result of the oxidation of titanomagnetites, contained in the samples, due to the said thermal processing. Moreover, the micro-structure of the grains, created by the oxidation of the titanomagnetites, causes the mostly linearly parallel configuration to change to a planar-parallel configuration.     

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.     

Magnetic properties of synthetic titanomagnetites

Magnetic properties and crystal structure parameters of synthetic solid solutions Fe₃O₄Fe₃TiO₄, Fe₂O₄MgFe₂O₄ and Fe₃O₄Mg₂TiO₄ have been studied. Basic regularities in the behaviour of saturation magnetisation (I_s), Curie temperature (T_C) and cubic lattice parameter a during the substitution of Ti and Mg ions for Fe ions have been found. As the concentration of Ti ions increases, I_s reduces from 70 Gs·cm³ g^?1 to 0, T_C changes from 580 to 130°C and a from 8.391 to 8.520 Å. Growth of the Mg concentration leads to changes in I_s to 19.8 Gs·cm³, g^?1, T_C, to 440°C and a, to 8.360 Å. The full Fe ions substitution gives $\text{“a”=8.440}\text{A}\text{?}$.Chemical compositions of the samples, in which the valency variation of Fe ions at oxidation leads to an increase in susceptibility and T_C, have been determined.     

On the complex magnetic behaviour of titanomagnetites

The bulk magnetic properties, including Curie temperature, susceptibility and hysteresis at various temperatures, and the Mössbauer spectra of coarse synthetic members of the titanomagnetite series are compared as a function of titanium content, using some previously reported and some new results. It has been noted previously that the titanium-rich members fail to show the expected “true multidomain” behaviour observed in pure magnetite. One possible explanation is that the behaviour observed may be due to a tendency to inhibit domain wall formation in some titanomagnetites. In this paper we discuss some other possible mechanisms to account for the magnetic properties of such titanomagnetites observable even when domain-wall-related structures do form in them. These mechanisms suggest that magnetocrystalline anisotropy and its temperature dependence in titanomagnetites may be related to local magnetic inhomogeneities, and do not depend only on crystal structure as in the case of classical ferromagnetic materials.     

Low-temperature oxidation of synthetic Al- and Mg-doped titanomagnetites

Al- and Mg-doped titanomagnetites were synthesized at 1300°C using the gas-mixing technique. A composition, representative of average natural titanomagnetites in ocean floor basalts, was sought.

The samples were ball-milled in acetone to average grain sizes of 0.5 μm and 5 μm and the material was then oxidised, in air, at temperatures below 300°C. This procedure formed titanomaghemite, a cation-deficient titanomagnetite.

Low-temperature oxidation is described as the diffusion of Fe-ions out of the spinel lattice and the process is observed to be distinctly dependent upon grain size.     

Exsolution microstructures in titanomagnetites and their magnetic significance

A survey, carried out using transmission electron microscopy, of exsolution-derived microstructures developed in titanomagnetites is presented. Microstructures, probably produced by spinodal decomposition, consist of a three-dimensional lamellar framework of ulvospinel, separating magnetite-enriched blocks. In coarser textures the magnetite-enriched regions have a plate morphology, and the ulvospinel-enriched lamellae develop secondary exsolution textures. The implications of exsolution for the magnetic properties of titanomagnetites are discussed. The coarsening of exsolution textures will cause the blocking temperature of the magnetite-enriched regions to increase with time. The development of magnetite-enriched plates may alter the magnetic properties of the titanomagnetite, as may the stress associated with some of the small scale, coherent microstructures.     

The microstructure and domain structure of multiphase oxidized titanomagnetites

An estimation of the domain state of 15 natural and synthetic samples containing both homogeneous and multiphase oxidized titanomagnetites was made by means of J_rs/J_s and H_cr/H_c ratios, the Lowrie-Fuller criterion, the thermomagnetic criterion, F criterion and the Preisach diagram. The J_rs/J_s and H_cr/H_c ratios and the Lowrie-Fuller criterion are shown to be not sufficiently informative for a determination of the domain state. In the case where the lamellae thickness became thinner than 0.1 μm, titanomagnetite grains demonstrate multidomain behaviour independent of the size of the interlamellar regions (cells). If the lamellae become thicker than 0.1 μm the domain state depends on the size of the cells. Single-domain behaviour is obtained for a cell size less than 1 × 1 μm; in agreement with the results of others, larger cells have multidomain properties.     

An investigation of ulvospinel composition and cation migration during maghemitization in DSDP,Leg 61, titanomagnetites

Titanomagnetites of basalts and diorites from an off-axis volcanic episode in the Pacific Ocean (Leg 61, DSDP) were studied for their ulvospinel composition and for changes in the iron-titanium ratio with progressive low-temperature oxidation. Thermomagnetic analysis and microscopy indicate that these Fe/Ti spinels show relatively little low-temperature oxidation, despite their Cretaceous age. Microprobe investigation shows that the Fe/Ti spinels have a wide range of ulvospinel content directly reflecting the range of parent rock chemistries. This is an important demonstration that ulvospinel contents are not constant but vary within a restricted range of compositions, namely at least x =0.5 to x=0.75 (Fe_2?xTi_xO₄). Recognition of this fact is important in attempting to elucidate changes in cation ratio as a result of progressive low-temperature oxidation.Examination of samples of different oxidation states for cation ratio changes as a result of low-temperature oxidation revealed lower titanium in the more oxidized samples and Ti enrichment in the less oxidized ones. This is probably a reflection of changing x values with progressive crystallization of individual sills, and thus unrelated to cation ratio changes with oxidation.In more than 100 grains analyzed, no evidence was found for a more oxidized shell on the exteriors of grains, suggesting that the oxidation process did not proceed uniformly with respect to grain geometry.     

Hysteresis properties of titanomagnetites: Grain-size and compositional dependence

Sized fractions of x = 0.6, 0.4, 0.2 and 0.0 titanomagnetites were studied with a vibration magnetometer. In the course particles (d > 150 μm), no compositional dependence of hysteresis parameters was found. H_C was less than 50 Oe, H_R/H_C > 4 and J_R/J_S < 10^?2, reflecting multi-domain behaviour. In contrast, fine particles ($\text{d ? 0.1 μ}\text{m}$) revealed systematic grain-size dependence of parameters with coercive force as high as 2,000 Oe in x = 0.6 titanomagnetite. Grain-size dependence studies revealed broad transition sizes for the onset of true multi-domain behaviour depending upon which factor is chosen. In magnetite it varies from 10 to 20 μm. The experimental critical size for single-domain behaviour for magnetite is about 0.1 μm and for x = 0.6 titanomagnetite 1–2 μm.     

Elastic properties,chemical composition,and crystal structure of minerals

The observed systematic relations between velocities, density, and crystal structure in minerals are viewed as part of the general problem of the interactions of ions and electrons in solids. A complete explanation of these systematics would result from an adequate physical model of interactions between atoms in a crystal. For ionic models, understanding such interactions mainly requires knowledge of inter-ionic repulsive forces, which can be calculated from solutions for the electronic energy levels. The main forms of systematics developed in the last few years can be classed either as bulk modulus-molar volume (K-V _M ) or, equivalently, velocity-density (v-p) relations. Isostructural and isochemical (having constant mean atomic weight ) trends exist, and for both classes of systematics it is possible to produce semi-empirical universal expressions with the ability to predict elastic properties to a few percent. The best estimates of composition of the Earth's interior come from comparisons of mineral elastic properties with seismic data. On the basis of current velocity profiles of the Earth, the mantle appears to be chemically homogeneous.     

The domain structure of synthetic stoichiometric TM10–TM75 and Al-, Mg-, Mn- and V-doped TM62 titanomagnetites

The existence of domain structure has been questioned for titanomagnetites of typical oceanic basalt composition owing to the unusual temperature dependence of their susceptibility, resembling that of spin glasses. In order to make a direct test of domain structure, a series of stoichiometric titanomagnetites between magnetite (TM0) and 75% ulvöspinel content (TM75) as well as a titanomagnetite of typical oceanic basalt composition have been synthesised using the double-sintering technique at 1300°C, in controlled atmospheres. The purity, stoichiometry and homogeneity of these materials were tested by optical, X-ray and microprobe studies as well as by magnetic measurements.Domain structures were observed using the Bitter-pattern technique after ionic polishing to produce stress-free surface of the bulk material. The optimum time required for ionic polishing was found to increase with the ulvöspinel content and to be correlated with the magnetostrictive constant θ. Magnetite showed a domain configuration which is also typical for nickel (mostly lamella-shaped domains, pine-tree-shaped closure domains, high domain wall mobility in small external fields, straight domain walls). The tendency to form lamella-shaped domains is present up to TM75 (which has a Curie temperature of only 40°C), but with an increasing tendency to form curved domain walls and to have fewer and also differently shaped closure domains. This is demonstrated in a series of photographs. The results constitute unequivocal evidence for the existence of a domain structure in the classical sense in a broad range of stoichiometric pure and doped (Al, Mg, Mn, V) titanomagnetites.     

MHD nature of the origination,dynamics, geoeffectiveness,and disappearance of the four-sector structure of the solar magnetic field during the cycle 23 decline phase

The MHD nature of the origination, dynamics, geoeffectiveness, and disappearance of the four-sector structure of the solar magnetic field during the cycle 23 decline phase has been established. A prolonged ordered MHD process including the chain of the interrelated phenomena (unknown before this study), which begin and end in one of the main zones of active longitudes and are responsible for the above nature of the four-sector structure, has been detected as a result of the simulation of the large-scale open solar magnetic field and an analysis of the dynamics of this field fluxes. These phenomena are as follows: the extreme concentration of the photospheric field sources of the same sign in the zone of active longitudes; blocking of regular differential rotation by these sources; origination of a nonstationary MHD disturbance in the form of a four-sector structure, traveling in the direction of solar rotation at a nearly Alfvén velocity; upset of blocking, displacement of blocking sources from the zone, and their shearing motion relative to a traveling MHD disturbance; deceleration and dissipation of a four-sector MHD disturbance; and reconstruction of a bisector structure. The interactions during this process, which lasted from May 2004 to December 2005, were accompanied by the generation of an ordered succession of heliospheric and solar-terrestrial disturbances including the series of nine extrastorms that were observed from July 2004 to September 2005 and were the last storms in the finished cycle 23 of solar activity.     

Elasticity,crystal structure and phase transitions

It is demonstrated that in many cases the ratio of elastic velocities of polymorphs of a material depends primarily on the crystal structures involved, and only secondarily on the composition. Crucial to this demonstration is the use of estimates, from the Hashin-Shtrikman bounds, of the elastic properties of mixtures of simple oxides isochemical to particular compounds. Normalization to the oxide mixture properties displays the interrelationships between the densities and velocities of different crystal structures. A wide range of velocity-density trends through possible phase transitions is found. Transitions involving increases of Si coordination from 4 to 6 tend to have lower slopes. Velocity-density correlations are reasonably consistent with the average trends through phase transitions.Comparison of models of the transition zone with the characteristics of the olivine-spinel-oxides transitions indicates that in some studies the lower mantle density may have been underestimated or the mean atomic weight of the lower mantle overestimated. The transition zone of Model B1 of Jordan and Anderson, which was derived entirely from seismic data, is consistent, overall, with the olivine-oxides transition and uniform composition, but the individual discontinuities are not consistent with the olivine-spinel and spinel-oxides transitions.