Karl Fabian

Magnetic exchange bias of more than 1 Tesla in a natural mineral intergrowth

Magnetic exchange bias is a phenomenon whereby the hysteresis loop of a soft magnetic phase is shifted by an amount H(E) along the applied field axis owing to its interaction with a hard magnetic phase. Since the discovery of exchange bias fifty years ago, the development of a general theory has been hampered by the uncertain nature of the interfaces between the hard and soft phases, commonly between an antiferromagnetic phase and a ferro- or ferrimagnetic phase. Exchange bias continues to be the subject of investigation because of its technological applications and because it is now possible to manipulate magnetic materials at the nanoscale. Here we present the first documented example of exchange bias of significant magnitude (>1 T) in a natural mineral. We demonstrate that exchange bias in this system is due to the interaction between coherently intergrown magnetic phases formed through a natural process of phase separation during slow cooling over millions of years. Transmission electron microscopy studies show that these intergrowths have a known crystallographic orientation with a known crystallographic structure and that the interfaces are coherent.

Northern Hemisphere Glaciation during the Globally Warm Early Late Pliocene

The early Late Pliocene (3.6 to ∼3.0 million years ago) is the last extended interval in Earths history when atmospheric CO2 concentrations were comparable to todays and global climate was warmer. Yet a severe global glaciation during marine isotope stage (MIS) M2 interrupted this phase of global warmth ∼3.30 million years ago, and is seen as a premature attempt of the climate system to establish an ice-age world. Here we propose a conceptual model for the glaciation and deglaciation of MIS M2 based on geochemical and palynological records from five marine sediment cores along a Caribbean to eastern North Atlantic transect. Our records show that increased Pacific-to-Atlantic flow via the Central American Seaway weakened the North Atlantic Current and attendant northward heat transport prior to MIS M2. The consequent cooling of the northern high latitude oceans permitted expansion of the continental ice sheets during MIS M2, despite near-modern atmospheric CO2 concentrations. Sea level drop during this glaciation halted the inflow of Pacific water to the Atlantic via the Central American Seaway, allowing the build-up of a Caribbean Warm Pool. Once this warm pool was large enough, the Gulf Stream–North Atlantic Current system was reinvigorated, leading to significant northward heat transport that terminated the glaciation. Before and after MIS M2, heat transport via the North Atlantic Current was crucial in maintaining warm climates comparable to those predicted for the end of this century.

A low-temperature phase diagram for ilmenite-rich compositions in the system Fe2O3-FeTiO3

Abstract An approximate low-temperature, metastable phase diagram is drawn for the system (1 - X) Fe2O3-(X)FeTiO3. It is based on published and new magnetic data from nine synthetic samples with bulk compositions in the range 0.6 < X < 1.0. Fields are plotted for (1) the paramagnetic phase (PM); the Fe2O3-rich ferrimagnetic phase (FM); (2) the FeTiO3-rich antiferromagnetic phase (AF); and (3) a re-entrant spin-glass phase (RSG). In addition, two subfields are plotted: (1) FM′, a subfield of the FM-phase, which occurs below a characteristic temperature TK, at which the magnetic susceptibility drops sharply on cooling, and (2) PM′, a subfield of the PM-phase (traditionally called superparamagnetic) forms below a sharp rise in susceptibility at TS, and exhibits measurable dispersion in the magnetic susceptibility at T < TS. The diagram is drawn with a bicritical point, Tλλ′, at X ≈ 0.87, T ≈ 39 K, which is the intersection of second-order magnetic phase boundaries for the paramagnetic → ferrimagnetic [PM(PM′) → FM] transition, TC(X), and the PM(PM′) → AF transition, TN(X). In addition, the RSG phase is plotted as one of four stable phases at Tλλ′, a construction that is not required by the phase rule, but is strongly favored by the physics of competition between the incompatible magnetically ordered structures of the FM- and AF-phases. These phase relations are at such low temperature as to be of little consequence for terrestrial magnetism, however, they may well be essential for interpreting the magnetism of the Moon, Mars, and other cold planets. These phase relations are also essential for the characterization of fine natural and synthetic intergrowths, and for understanding magnetic materials for low-temperature technological applications.

Magnetic force microscopy reveals meta-stable magnetic domain states that prevent reliable absolute palaeointensity experiments

Obtaining reliable estimates of the absolute palaeointensity of the Earths magnetic field is notoriously difficult. The heating of samples in most methods induces magnetic alteration--a process that is still poorly understood, but prevents obtaining correct field values. Here we show induced changes in magnetic domain state directly by imaging the domain configurations of titanomagnetite particles in samples that systematically fail to produce truthful estimates. Magnetic force microscope images were taken before and after a heating step typically used in absolute palaeointensity experiments. For a critical temperature (250u2009°C), we observe major changes: distinct, blocky domains before heating change into curvier, wavy domains thereafter. These structures appeared unstable over time: after 1-year of storage in a magnetic-field-free environment, the domain states evolved into a viscous remanent magnetization state. Our observations qualitatively explain reported underestimates from otherwise (technically) successful experiments and therefore have major implications for all palaeointensity methods involving heating.

Automated paleomagnetic and rock magnetic data acquisition with an in‐line horizontal “2G” system

Todays paleomagnetic and magnetic proxy studies involve processing of large sample collections while simultaneously demanding high quality data and high reproducibility. Here we describe a fully automated interface based on a commercial horizontal pass-through “2G” DC-SQUID magnetometer. This system is operational at the universities of Bremen (Germany) and Utrecht (Netherlands) since 1998 and 2006, respectively, while a system is currently being built at NGU Trondheim (Norway). The magnetometers are equipped with “in-line” alternating field (AF) demagnetization, a direct-current bias field coil along the coaxial AF demagnetization coil for the acquisition of anhysteretic remanent magnetization (ARM) and a long pulse-field coil for the acquisition of isothermal remanent magnetization (IRM). Samples are contained in dedicated low magnetization perspex holders that are manipulated by a pneumatic pick-and-place-unit. Upon desire samples can be measured in several positions considerably enhancing data quality in particular for magnetically weak samples. In the Bremen system, the peak of the IRM pulse fields is actively measured which reduces the discrepancy between the set field and the field that is actually applied. Techniques for quantifying and removing gyroremanent overprints and for measuring the viscosity of IRM further extend the range of applications of the system. Typically c. 300 paleomagnetic samples can be AF demagnetized per week (15 levels) in the three-position protocol. The versatility of the system is illustrated by several examples of paleomagnetic and rock magnetic data processing.

Stability of equidimensional pseudo–single-domain magnetite over billion-year timescales

Significance When magnetic crystals form in rocks and meteorites, they can record the ancient magnetic field and retain this information over geological timescales. Scientists use these magnetic recordings to study the evolution of the Earth and the Solar System. Previous theories for the recording mechanisms of the magnetic crystals in rocks and meteorites are based on the idea that the magnetic structures within crystals are near uniform. However, from numerous studies we know this not to be true. The crystals are too large in size and display complex nonuniform “vortex” structures. In this study we have shown that vortex structures are capable of recording and retaining magnetic signals over billions of years. Interpretations of paleomagnetic observations assume that naturally occurring magnetic particles can retain their primary magnetic recording over billions of years. The ability to retain a magnetic recording is inferred from laboratory measurements, where heating causes demagnetization on the order of seconds. The theoretical basis for this inference comes from previous models that assume only the existence of small, uniformly magnetized particles, whereas the carriers of paleomagnetic signals in rocks are usually larger, nonuniformly magnetized particles, for which there is no empirically complete, thermally activated model. This study has developed a thermally activated numerical micromagnetic model that can quantitatively determine the energy barriers between stable states in nonuniform magnetic particles on geological timescales. We examine in detail the thermal stability characteristics of equidimensional cuboctahedral magnetite and find that, contrary to previously published theories, such nonuniformly magnetized particles provide greater magnetic stability than their uniformly magnetized counterparts. Hence, nonuniformly magnetized grains, which are commonly the main remanence carrier in meteorites and rocks, can record and retain high-fidelity magnetic recordings over billions of years.

Nonlinear Preisach maps: Detecting and characterizing separate remanent magnetic fractions in complex natural samples

Natural remanent magnetization carriers in rocks can contain mixtures of magnetic minerals that interact in complex ways and are challenging to characterize by current measurement techniques. Here a non-linear mapping scheme is described that efficiently enhances sensitivity and the resolution power of remanent Preisach maps. Using this scheme a large dynamic range of magnetic moments and coercivities can be reliably resolved. The method is applied to synthetic and natural standard samples containing magnetite and hematite, as well as to natural samples from remanent magnetic anomalies where complex microstructures are observed. It is shown that certain offset high coercivity patterns in remanent Preisach maps may serve as fingerprints for exsolution structures of ilmenite in hematite or hematite in ilmenite, and that in some magnetite-bearing remanent anomalies the magnetite coercivity is increased beyond its intrinsic coercivity range. Experimental results and theoretical considerations indicate a minimal coercivity of about 10xa0mT for SD magnetite, such that observation of lower coercivities implies PSD or MD grain sizes. A diagnostic hematite pattern with a peak downwards offset of 17 ± 2% of the intrinsic coercivity is found that is stable over a large range of intrinsic coercivities, and may be related to shielding of internal defect or lamellar moments by a spin canting response to the internal field.

Experimental study of the magnetic signature of basal-plane anisotropy in hematite

The crystal symmetry of hematite in the basal plane predicts three easy magnetization axes for the sublattice spin orientation above the Morin transition. Spin canting then leads to three preferred magnetization axes perpendicular to these easy axes. By combining detailed crystallographic orientation by EBSD measurements with dense magnetic hysteresis and remanence curves as a function of rotation angle, the relation between crystallography and magnetic properties has been experimentally verified for the basal plane of a natural hematite crystal. The measurements lead to a better understanding of the interplay between spin canting, remanence and magnetic susceptibility at different field strengths. The measurement results coincide qualitatively with theoretical predictions, and provide experimental evidence for quantitative evaluation by more complex micromagnetic modeling.

Energy barriers in three-dimensional micromagnetic models and the physics of thermoviscous magnetization

A first principle micromagnetic and statistical calculation of viscous remanent magnetization (VRM) in an ensemble of cubic magnetite pseudo single-domain particles is presented. This is achieved by developing a fast relaxation algorithm for finding optimal transition paths between micromagnetic local energy minima. It combines a nudged elastic band technique with action minimization. Initial paths are obtained by repetitive minimizations of modified energy functions. For a cubic pseudo-single domain particle, 60 different local energy minima are identified and all optimal energy barriers between them are numerically calculated for zero external field. The results allow to estimate also the energy barriers in in weak external fields which are necessary to construct the time dependent transition matrices which describe the continuous homogeneous Markov processes of VRM acquisition and decay. By spherical averaging the remanence acquisition in an isotropic PSD ensemble was calculated over all time scales. The modelled particle ensemble shows a physically meaningful overshooting during VRM acquisition. The results also explain why VRM acquisition in PSD particles can occur much faster than VRM decay and therefore can explain for findings of extremely stable VRM in some paleomagnetic studies.