Michael Winklhofer

Detection of noninteracting single domain particles using first‐order reversal curve diagrams

We present a highly sensitive and accurate method for quantitative detection and characterization of noninteracting or weakly interacting uniaxial single domain particles (UNISD) in rocks and sediments. The method is based on high-resolution measurements of first-order reversal curves (FORCs). UNISD particles have a unique FORC signature that can be used to isolate their contribution among other magnetic components. This signature has a narrow ridge along the Hc axis of the FORC diagram, called the central ridge, which is proportional to the switching field distribution of the particles. Therefore, the central ridge is directly comparable with other magnetic measurements, such as remanent magnetization curves, with the advantage of being fully selective to SD particles, rather than other magnetic components. This selectivity is unmatched by other magnetic unmixing methods, and offers useful applications ranging from characterization of SD particles for paleointensity studies to detecting magnetofossils and ultrafine authigenically precipitated minerals in sediments.

Ultrastructural analysis of a putative magnetoreceptor in the beak of homing pigeons

With the use of different light and electron microscopic methods, we investigated the subcellular organization of afferent trigeminal terminals in the upper beak of the homing pigeon, Columba livia, which are about 5 μm in diameter and contain superparamagnetic magnetite (SPM) crystals. The SPM nanocrystals are assembled in clusters (diameter, ∼1–2 μm). About 10 to 15 of these clusters occur inside one nerve terminal, arranged along the cell membrane. Each SPM cluster is embedded in a solid fibrous cup, open towards the cell surface, to which the cluster adheres by delicate fiber strands. In addition to the SPM clusters, a second inorganic iron compound has been identified: noncrystalline platelets of iron phosphate (about 500 nm wide and long and maximally 100 nm thick) that occur along a fibrous core of the terminal. The anatomic features suggested that these nerve endings could detect small intensity changes of the geomagnetic field. Such stimuli can induce deformations of the SPM clusters, which could be transduced into primary receptor potentials by mechanosensitive membrane receptor channels. The subepidermal fat cells surrounding the nerve endings prevent the inside from external mechanical stimuli. These structural findings corresponded to conclusions inferred from rock magnetic measurements, theoretical calculations, model experiments, and behavioral data, which also matched previous electrophysiologic recordings from migratory birds. J. Comp. Neurol. 458:350–360, 2003.

Extracting the intrinsic switching field distribution in perpendicular media: A comparative analysis

We introduce a method based on the first-order-reversal-curve (FORC) diagram to extract the intrinsic (microscopic) switching-field distribution (SFD) of perpendicular recording media (PRM). To demonstrate the viability of the method, we micromagnetically simulated FORCs for PRM with known SFD. The extracted SFD is compared with the SFD obtained by means of two different methods that are based on recoil loops, too, which, however, rely on mean-field approximations and assumptions on the shape of the SFD. The FORC method turns out to be the most accurate algorithm over the technologically relevant range of magnetic quality factors Q, where the other methods overestimate the width of the SFD.

Magnetic blocking temperatures of magnetite calculated with a three‐dimensional micromagnetic model

We present an analysis of thermal stability of magnetic remanence in fine grains of magnetite (grain size d = 15–120 nm). In order to model incoherent transitions between single-domain (SD) and pseudo-single-domain (PSD) magnetization configurations, we employ a three-dimensional constrained minimization method proposed by Enkin and Williams [1994]. Using this approach, one can track in detail the transition from one local energy minimum state into another by constraining the magnetization vectors of appropriate cells in a discrete model. For each particle, we obtain the energy barriers EB(T) from 25° to 578°C. Magnetic blocking temperatures (TB) are calculated by integrating EB(T) for two extreme cooling schedules representing laboratory and geological timescales. The computed blocking temperatures for laboratory timescales are in excellent agreement with the experimentally determined blocking temperatures for magnetite by Dunlop [1973b]. The results of our computations are summarized as relaxation time versus blocking temperature curves, which deviate from the curves of Pullaiah et al. [1975] for particles with grain sizes in the SD-PSD transition region. A consequence of the dependence of TB on timescale is that some PSD size particles are blocked in vortex states on geologic timescales but are blocked in the SD state on laboratory timescales. Paleointensity determinations with the Thellier method on such samples can therefore underestimate the paleofield. The superparamagnetic to SD threshold size dS is determined as 50 nm for cubic grains, whereas a small aspect ratio of q=1.1 is sufficient to depress dS to 27 nm. SD particles of magnetite with small shape anisotropy and cubic grains with 58 nm≤d≤72 nm are reliable carriers of paleomagnetic information.

Chains of magnetite crystals in the meteorite ALH84001: Evidence of biological origin

The presence of magnetite crystal chains, considered missing evidence for the biological origin of magnetite in ALH84001 [Thomas-Keprta, K. L., Bazylinski, D. A., Kirschvink, J. L., Clemett, S. J., McKay, D. S., Wentworth, S. J., Vali, H., Gibson, E. K., Jr., & Romanek, C. S. (2000) Geochim. Cosmochim. Acta 64, 4049–4081], is demonstrated by high-power stereo backscattered scanning electron microscopy. Five characteristics of such chains (uniform crystal size and shape within chains, gaps between crystals, orientation of elongated crystals along the chain axis, flexibility of chains, and a halo that is a possible remnant of a membrane around chains), observed or inferred to be present in magnetotactic bacteria but incompatible with a nonbiological origin, are shown to be present. Although it is unlikely that magnetotactic bacteria were ever alive in ALH84001, decomposed remains of such organisms could have been deposited in cracks in the rock while it was still on the surface on Mars.

A quantitative assessment of torque-transducer models for magnetoreception

Although ferrimagnetic material appears suitable as a basis of magnetic field perception in animals, it is not known by which mechanism magnetic particles may transduce the magnetic field into a nerve signal. Provided that magnetic particles have remanence or anisotropic magnetic susceptibility, an external magnetic field will exert a torque and may physically twist them. Several models of such biological magnetic-torque transducers on the basis of magnetite have been proposed in the literature. We analyse from first principles the conditions under which they are viable. Models based on biogenic single-domain magnetite prove both effective and efficient, irrespective of whether the magnetic structure is coupled to mechanosensitive ion channels or to an indirect transduction pathway that exploits the strayfield produced by the magnetic structure at different field orientations. On the other hand, torque-detector models that are based on magnetic multi-domain particles in the vestibular organs turn out to be ineffective. Also, we provide a generic classification scheme of torque transducers in terms of axial or polar output, within which we discuss the results from behavioural experiments conducted under altered field conditions or with pulsed fields. We find that the common assertion that a magnetoreceptor based on single-domain magnetite could not form the basis for an inclination compass does not always hold.

A new model for a magnetoreceptor in homing pigeons based on interacting clusters of superparamagnetic magnetite

We present a new model of magnetic-field reception in magnetite-containing nerve terminals, which have recently been identified in the upper-beak skin of homing pigeons. The potentially magnetoreceptive nerve cells comprise chain-like aggregates with up to 20 closely spaced clusters of superparamagnetic (SP) magnetite. We designed experiments on superparamagnetic model systems to simulate the behaviour of the aggregates in varying magnetic fields. Magnetic-field induced interactions between the clusters in an aggregate gives rise to attractive and repulsive forces between the clusters. The resulting stress on the surrounding cellular structures varies with field direction and intensity. Our model is able to explain the principal features of the magnetic sense in homing pigeons as derived from behavioural experiments.

Clusters of superparamagnetic magnetite particles in the upper-beak skin of homing pigeons evidence of a magnetoreceptor?

Previous electrophysiological studies on bobolinks, an American migratory songbird, and on homing pigeons suggested that the skin of the upper beak may be involved in magnetic-field perception, which makes this tissue likely to contain a magnetic-field receptor. In the upper-beak skin of homing pigeons, we localised high concentrations of Fe3+, which form distinct coherent elongated structures extending up to 200 μ in length. Rather than being randomly distributed over the tissue, these structures always occur in the same skin layer, the stratum laxum of the subcutis. Using transmission electron microscopy (TEM), we identified the material as aggregates of magnetite (Fe3O4) nanocrystals, in the grain-size range of superparamagnetism at ambient temperatures. The nanocrystals (with grain-sizes between 2 and 5 nm) form densely packed, encapsulated clusters of 1 to 3 μ in diameter. It is demonstrated that such a cluster undergoes shape changes as the magnetic field changes, and thus could represent the core of a magnetic-field receptor. This interpretation is supported by the fact that the found structures are adjacent to nervous material. This paper was presented at the “Biogenic Iron Minerals” symposium held in Tihany, Hungary (May 2000)

Magnetic characterization of isolated candidate vertebrate magnetoreceptor cells

Over the past 50 y, behavioral experiments have produced a large body of evidence for the existence of a magnetic sense in a wide range of animals. However, the underlying sensory physiology remains poorly understood due to the elusiveness of the magnetosensory structures. Here we present an effective method for isolating and characterizing potential magnetite-based magnetoreceptor cells. In essence, a rotating magnetic field is employed to visually identify, within a dissociated tissue preparation, cells that contain magnetic material by their rotational behavior. As a tissue of choice, we selected trout olfactory epithelium that has been previously suggested to host candidate magnetoreceptor cells. We were able to reproducibly detect magnetic cells and to determine their magnetic dipole moment. The obtained values (4 to 100 fAm2) greatly exceed previous estimates (0.5 fAm2). The magnetism of the cells is due to a μm-sized intracellular structure of iron-rich crystals, most likely single-domain magnetite. In confocal reflectance imaging, these produce bright reflective spots close to the cell membrane. The magnetic inclusions are found to be firmly coupled to the cell membrane, enabling a direct transduction of mechanical stress produced by magnetic torque acting on the cellular dipole in situ. Our results show that the magnetically identified cells clearly meet the physical requirements for a magnetoreceptor capable of rapidly detecting small changes in the external magnetic field. This would also explain interference of ac powerline magnetic fields with magnetoreception, as reported in cattle.

The osmotic magnetometer: a new model for magnetite-based magnetoreceptors in animals

Abstract Homing pigeons and migratory birds are well known examples for animals that use the geomagnetic field for their orientation. Yet, neither the underlying receptor mechanism nor the magnetoreceptor itself is known. Recently, an innervated structure containing clusters of magnetite nanocrystals was identified in the upper beak skin of the homing pigeon. Here we show theoretically that such a cluster has a magnetic-field-dependent shape, even in fields as weak as the Earths magnetic field; by converting magnetic stimuli into mechanical strain, the clusters can be assumed as primary units of magnetoperception in homing pigeons. Since the orientation of the strain ellipsoid indicates the direction of the external magnetic field, a cluster of magnetite nanocrystals also has the potential to serve as the basis of the so-called inclination compass of migratory birds. It is quantitatively demonstrated that the magnetic-field-induced shape change of a cluster can be amplified as well as counterbalanced by means of osmotic pressure regulation, which offers an elegant possibility to determine the magnetic field strength just by measuring changes in concentration.