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 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.     

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.     

Change of structure,phase composition and magnetic properties with the oxidation of titanomagnetites

Summary Phase, structural and magnetic changes, occurring under oxidation and at increased temperatures, are studied on four samples of magnetic fractions. The samples of magnetic fractions, containing titanomagnetites at different oxidation levels, were oxidized at a temperature of 400°C for 1, 60 and 300 mins. With the aid of X-rays and Mössbauer's spectrometry it has been proved that under oxidation non-stoichiometric titanomagnetites and titanomagnetites plus ilmenite and pseudobrookite are formed.     

The effects of heating on low-temperature susceptibility and hysteresis properties of basalts

The variation of the low-field susceptibility of basalts down to liquid-nitrogen temperature always falls into one of three types that depend on the composition and grain size of the titanomagnetite grains present. Group 1 basalts contain predominantly unoxidised, multidomain homogeneous titanomagnetites having x 0.3. Group 2 basalts contain predominantly titanomagnetite grains with many exsolved ilmenite lamallae that subdivide the grains so that they act similarly to single domains. Group 3 basalts contain predominantly multidomain magnetite or magnetite-rich titanomagnetite having x 0.15. After repeated heating to 615°C, the group 1 basalts gradually oxidise above 300°C to produce the characteristics of group 2 basalts, owing to the exsolution of ilmenite. On the other hand, both group 2 and 3 basalts are stable to oxidation until at least 500°C. They are therefore the most useful material for palaeointensity studies.     

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.     

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.     

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.     

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.     

一个变泥质岩包体样品的系统热磁试验及其矿物学意义

利用一件采自河北汉诺坝周坝地区变泥质岩包体样品,结合系统的低温和高温磁性测量结果,探讨了应用热磁实验鉴别样品中所含原生磁性矿物的多解性问题. 结果表明, 饱和等温剩余磁化强度（SIRM）在室温～250℃以及280℃～380℃的降低分别由高钛钛磁铁矿的剩磁解阻过程（一种物理过程）以及由磁赤铁矿转换成赤铁矿（一种矿物相变）引起.样品在500℃以后磁化率的升高则是由磁铁矿从钛磁赤铁矿中出溶所致.因此,κ-T（即磁化率随温度变化）曲线中呈现约580℃居里点是由加热过程中次生的磁铁矿引起,而并非代表原始（即加热前）样品中的磁铁矿成分.     

Theoretical analysis of telluric anomaly of a basin structure

The solution of a boundary value problem modelling a two-dimensional basin structure has been obtained by using the Schwartz-Christoffel conformal transformation technique and numerical methods of solving non-linear differential equations. Utilizing this solution, the telluric field and its first horizontal derivative have been theoretically computed for field directions perpendicular to the strike of the structure. On the basis of systematic analysis of a large number of such anomaly curves, two nomograms have been prepared to be used in the quantitative interpretation of telluric data. An interpretation procedure to evaluate the geometric parameters of the basin from the observed telluric data is outlined. This procedure is demonstrated on an actual field example.     

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.     

Low-temperature magnetic hysteresis properties of multidomain single-crystal titanomagnetite

The low-temperature magnetic hysteresis characteristics of multidomain titanomagnetite were fitted by a mixed coercivity model in which magnetocrystalline-controlled and magnetostriction-controlled domain-wall pinning play an important part. The effective contribution of the former was found to decrease systematically with increasing titanium content for compositions in the range Fe_2.48Ti_0.52O₄Fe_2.35Ti_0.65O₄. In agreement with the model, the observed magnetic remanence transitions and the characteristic peaks on the susceptibility vs. temperature curve became less pronounced as the titanium content increased. The problems in using these features as a means of identifying the carriers of natural remanent magnetization are discussed.     

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.     

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.     

The low-temperature electrical properties of carbonaceous meteorites

Electrical conductivities and dielectric constants have been measured over the temperature range 90–300°K on several carbonaceous chondrites and some terrestrial analogues. The conductivities of meteorites of different petrologic subtypes range over many orders of magnitude and the low-temperature activation energies are typically much smaller than those observed in terrestrial materials at higher temperatures. The electrical properties of carbonaceous chondrites vary systematically with chemical-mineralogical characteristics in that: (1) activation energy at low temperature is greater in the more volatile-rich meteorites containing hydrated silicates, and (2) conductivity is greater in the more reduced meteorites of higher petrologic subtype. These new data on the electrical properties of chondrites hold important implications for both the thermal and magnetic histories of small bodies in the early solar system.     

Results of optical, thermomagnetic and electron microprobe analyses of titanomagnetite grains

This paper presents results of a study of titanomagnetite from specimens of ten different rock units. The rock units comprise trachytes, basalts, ignimbrites, nephelinites, olivine melanephelinites and welded tuffs from Kenya, with ages ranging from recent to Precambrian. The correlations of the stability index with opaque petrology and with thermomagnetic analyses of the titanomagnetite and titanomaghemite bearing specimens appear to be significant. Electron microprobe analyses of the titanomagnetite grains indicate a high percentage of “impurities” in the low stability index specimens and elevation of the Curie point in the high stability index specimens.     

钢骨混凝土框架滞回分析研究

根据非线性有限元原理，编制了基于恢复力模型的钢骨混凝土框架滞回全过程分析程序。程序设计中考虑了材料非线性、几何非线性、预应力作用、轴力二次矩等因素的影响，并考虑了钢骨的特点。通过与已有钢骨混凝土框架试验结果的对比，验证了该程序的合理性。在此基础上，应用该程序对八榀钢骨混凝土框架进行了滞回分析，分析表明钢骨混凝土框架具有良好的抗震性能。     

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.