Biomedical subjects

A Scholl

Publications and source records attributed to A Scholl.

At least 19 recordsLinked to original sources

PubMed2006

Electrical control of antiferromagnetic domains in multiferroic BiFeO3 films at room temperature.

Multiferroic materials, which offer the possibility of manipulating the magnetic state by an electric field or vice versa, are of great current interest. In this work, we demonstrate the first observation of electrical control of antiferromagnetic domain structure in a single-phase multiferroic material at room temperature. High-resolution images of both antiferromagnetic and ferroelectric domain structures of (001)-oriented multiferroic BiFeO3 films revealed a clear domain correlation, indicating a strong coupling between the two types of order. The ferroelectric structure was measured using piezo force microscopy, whereas X-ray photoemission electron microscopy as well as its temperature dependence was used to detect the antiferromagnetic configuration. Antiferromagnetic domain switching induced by ferroelectric polarization switching was observed, in agreement with theoretical predictions.

Journal Article↗

PubMed2006

Interface coupling transition in a thin epitaxial antiferromagnetic film interacting with a ferromagnetic substrate.

We report experimental evidence for a transition in the interface coupling between an antiferromagnetic film and a ferromagnetic substrate. The transition is observed in a thin epitaxial NiO film grown on top of Fe(001) as the film thickness is increased. Photoemission electron microscopy excited with linearly polarized x rays shows that the NiO film is antiferromagnetic at room temperature with in-plane uniaxial magnetic anisotropy. The anisotropy axis is perpendicular to the Fe substrate magnetization when the NiO thickness is less than about 15 A, but rapidly becomes parallel to the Fe magnetization for a NiO coverage higher than 25 A.

Journal Article↗

PubMed2005

ACE-inhibitor treatment attenuates atrial structural remodeling in patients with lone chronic atrial fibrillation.

OBJECTIVE: Chronic atrial fibrillation (AF) is characterized by a remodeling process which involves the development of fibrosis. Since angiotensin II has been suspected to be involved in this process, the aim of our study was to investigate a possible influence of an ACE-I therapy in patients with chronic AF regarding the occurrence of left atrial structural remodeling. METHODS: Atrial tissue samples were obtained from patients with lone chronic AF or sinus rhythm (SR). Collagen I, vascular endothelial growth factor (VEGF) and basic fibroblast growth factor (bFGF) protein expression were measured by quantitative Western Blotting techniques and calculated as mean +/- SEM. Histological tissue samples were used for calculating microvessel density (microvessel/mm(2) +/- SEM). RESULTS: In AF, the collagen amount was higher (1.78 +/- 0.21; p = 0.01) vs. SR (0.37 +/- 0.07) accompanied by declining microcapillary density (AF: 145 +/- 13 vs. SR: 202 +/- 9; p = 0.01). Additionally, a negative correlation (p = 0.01) between collagen content and microcapillary density was observed. To investigate the influence of an ACE-I therapy on this remodeling process, patient groups were divided into AF and SR both with or without ACE-I. Interestingly, there was a significantly lower expression of collagen I in AF with ACE-I (1.04 +/- 0.26) vs. AF without ACE-I treatment (2.07 +/- 0.24, p = 0.02). The microcapillaries were not diminished in AF with ACE-I (180 +/- 15) vs. SR with ACE-I (196 +/- 9), but there was a significant rarification in AF without ACE-I (123 +/- 18; p = 0.03). The expression of VEGF and bFGF did not reveal any significant differences. CONCLUSION: In patients undergoing ACE-I treatment: atrial structural remodeling was attenuated and the loss of atrial microcapillaries was prevented.

Angiotensin-Converting Enzyme Inhibitors↗

PubMed2005

X-ray absorption spectromicroscopy studies for the development of lithography with a monomolecular resist.

Soft X-ray absorption microscopy was applied to image and characterize molecular patterns produced by electron irradiation of aliphatic and aromatic thiol-derived self-assembled monolayers (SAMs) on Au substrates. The measurements were performed at all relevant absorption edges. The fabricated patterns could be clearly imaged with a lateral resolution better than 150 nm, which, for example, allowed us to distinguish a fine structure of 1 microm features. The X-ray absorption microspot spectra derived from different areas of the SAM patterns provided specific chemical information on pristine and irradiated areas and unexpected features in these patterns. The quality of the microspot spectra is comparable with that of the analogous X-ray absorption spectra acquired with standard equipment from homogeneous SAMs. In particular, a chemical transformation of the functional tail groups within the irradiated areas of the patterned aromatic SAMs could be directly monitored.

Spectrum Analysis↗

PubMed2004

Magnetic stripe domains in coupled magnetic sandwiches.

Magnetic stripe domains in the spin reorientation transition region are investigated in (Fe/Ni)/Cu(001) and Co/Cu/(Fe/Ni)/Cu(001) using photoemission electron microscopy. For (Fe/Ni)/Cu(001), the stripe domain width decreases exponentially as the Fe/Ni film approaches the spin reorientation transition point. For Co/Cu/(Fe/Ni)/Cu(001), the Fe/Ni stripe orientation is aligned with the Co in-plane magnetization, and the stripe domain width decreases exponentially with increasing the interlayer coupling between the Fe/Ni and Co films. By considering magnetic stripes within an in-plane magnetic field, we reveal a universal dependence of the stripe domain width on the magnetic anisotropy and on the interlayer coupling.

Journal Article↗

PubMed2004

Creation of an antiferromagnetic exchange spring.

We present evidence for the creation of an exchange spring in an antiferromagnet due to exchange coupling to a ferromagnet. X-ray magnetic linear dichroism spectroscopy on single crystal Co/NiO(001) shows that a partial domain wall is wound up at the surface of the antiferromagnet when the adjacent ferromagnet is rotated by a magnetic field. We determine the interface exchange stiffness and the antiferromagnetic domain wall energy from the field dependence of the direction of the antiferromagnetic axis, the antiferromagnetic pendant to a ferromagnetic hysteresis loop. The existence of a planar antiferromagnetic domain wall, proven by our measurement, is a key assumption of most exchange bias models.

Journal Article↗

PubMed2004

Vortex core-driven magnetization dynamics.

Time-resolved x-ray imaging shows that the magnetization dynamics of a micron-sized pattern containing a ferromagnetic vortex is determined by its handedness, or chirality. The out-of-plane magnetization in the nanometer-scale vortex core induces a three-dimensional handedness in the planar magnetic structure, leading to a precessional motion of the core parallel to a subnanosecond field pulse. The core velocity was an order of magnitude higher than expected from the static susceptibility. These results demonstrate that handedness, already well known to be important in biological systems, plays an important role in the dynamics of microscopic magnets.

Journal Article↗

PubMed2003

Magnetic phase transition in Co/Cu/Ni/Cu(100) and Co/Fe/Ni/Cu(100).

Magnetic phase transitions in coupled magnetic sandwiches of Cu/Co/Cu/Ni/Cu(100) and Cu/Co/Fe/Ni/Cu(100) are investigated by photoemission electron microscopy. Element-specific magnetic domains are taken at room temperature to reveal the critical thickness at which the magnetic phase transition occurs. The results show that a coupled magnetic sandwich undergoes three types of magnetic phase transitions depending on the two ferromagnetic films' thickness. A phase diagram is constructed and explained in the process of constructing Monte Carlo simulations, which corroborate the experimental results.

Journal Article↗

PubMed2003

Correlation between exchange bias and pinned interfacial spins.

Using x-ray magnetic circular dichroism, we have detected the very interfacial spins that are responsible for the horizontal loop shift in three different exchange bias sandwiches, chosen because of their potential for device applications. The "pinned" uncompensated interfacial spins constitute only a fraction of a monolayer and do not rotate in an external magnetic field since they are tightly locked to the antiferromagnetic lattice. A simple extension of the Meiklejohn and Bean model is proposed to account quantitatively for the exchange bias fields in the three studied systems from the experimentally determined number of pinned moments and their sizes.

Journal Article↗

PubMed2002

Lateral versus perpendicular segregation in mixed polymer brushes.

Grafting of incompatible polymers on a substrate prevents macrophase separation and the chains self-assemble laterally. Mixed brushes are exposed to different solvents and the morphology is observed via atomic force and x-ray photoemission microscopy. In a nonselective solvent the different species segregate into parallel cylinders ("ripple structure"). Upon exposure to a selective solvent, we encounter a transition to a "dimple" structure, in which the unfavored component forms clusters. Simultaneously, we observe an enhanced perpendicular segregation. The experimental observations are compared to self-consistent field calculations, where qualitative agreement is found.

Journal Article↗

PubMed2001

Spectroscopic identification and direct imaging of interfacial magnetic spins.

Using x-ray absorption spectromicroscopy we have imaged the uncompensated spins induced at the surface of antiferromagnetic (AFM) NiO(100) by deposition of ferromagnetic (FM) Co. These spins align parallel to the AFM spins in NiO(100) and align the FM spins in Co. The uncompensated interfacial spins arise from an ultrathin CoNiOx layer that is formed upon Co deposition through reduction of the NiO surface. The interfacial Ni spins are discussed in terms of the "uncompensated spins" at AFM/FM interfaces long held responsible for coercivity increases and exchange bias. We find a direct correlation between their number and the size of the coercivity.

Journal Article↗

PubMed2001

Spin reorientation at the antiferromagnetic NiO(001) surface in response to an adjacent ferromagnet.

Polarization dependent x-ray photoemission electron microscopy was used to investigate the spin structure near the surface of an antiferromagnetic NiO(001) single crystal in response to the deposition of a thin ferromagnetic Co film. For the cleaved NiO surface we observe only a subset of bulklike antiferromagnetic domains which is attributed to minimization of dipolar energies. Upon Co deposition a spin reorientation near the NiO interface occurs, with the antiferromagnetic spins rotating in plane, parallel to the spins of the Co layer. Our results demonstrate that the spin configuration in an antiferromagnet near its interface with a ferromagnet may significantly deviate from that in the bulk antiferromagnet.

Journal Article↗

PubMed2001

Studies of the magnetic structure at the ferromagnet-antiferromagnet interface.

Antiferromagnetic layers are a scientifically challenging component in magnetoelectronic devices, such as magnetic sensors in hard-disk heads, or magnetic random-access memory (RAM) elements. In this paper, it is shown that photoelectron emission microscopy (PEEM) is capable of determining the magnetic structure at the interface of ferromagnets and antiferromagnets with high spatial resolution (down to 20 nm). Dichroism effects at the L edges of the magnetic 3d transition metals, using circularly or linearly polarized soft X-rays from a synchrotron source, give rise to a magnetic image contrast. Images, acquired with the PEEM2 experiment at the Advanced Light Source, show magnetic contrast for antiferromagnetic LaFeO3, microscopically resolving the magnetic domain structure in an antiferromagnetically ordered thin film for the first time. Magnetic coupling between LaFeO3 and an adjacent Co layer results in a complete correlation of their magnetic domain structures. From field-dependent measurements, a unidirectional anisotropy resulting in a local exchange bias of up to 30 Oe in single domains could be deduced. The elemental specificity and the quantitative magnetic sensitivity render PEEM a perfect tool to study magnetic coupling effects in multilayered thin-film samples.

Journal Article↗

PubMed2000

Direct observation of the alignment of ferromagnetic spins by antiferromagnetic spins

The arrangement of spins at interfaces in a layered magnetic material often has an important effect on the properties of the material. One example of this is the directional coupling between the spins in an antiferromagnet and those in an adjacent ferromagnet, an effect first discovered in 1956 and referred to as exchange bias. Because of its technological importance for the development of advanced devices such as magnetic read heads and magnetic memory cells, this phenomenon has received much attention. Despite extensive studies, however, exchange bias is still poorly understood, largely due to the lack of techniques capable of providing detailed information about the arrangement of magnetic moments near interfaces. Here we present polarization-dependent X-ray magnetic dichroism spectro-microscopy that reveals the micromagnetic structure on both sides of a ferromagnetic-antiferromagnetic interface. Images of thin ferromagnetic Co films grown on antiferromagnetic LaFeO3 show a direct link between the arrangement of spins in each material. Remanent hysteresis loops, recorded for individual ferromagnetic domains, show a local exchange bias. Our results imply that the alignment of the ferromagnetic spins is determined, domain by domain, by the spin directions in the underlying antiferromagnetic layer.

Journal Article↗

PubMed2000

Oscillatory decay of magnetization induced by domain-wall stray fields

The demagnetization of a hard ferromagnetic layer via the fringing fields of domain walls created by reversing the moment of a neighboring soft ferromagnetic layer is explored experimentally. An unusual oscillatory decay of the magnetic moment of the hard layer is observed using structures in which the demagnetization occurs after a few hundred cycles. This surprising observation is confirmed on a microscopic scale by detailed imaging of the magnetization of the hard layer using high resolution photoemission electron microscopy and by micromagnetic simulations.

Journal Article↗

PubMed2000

Observation of antiferromagnetic domains in epitaxial thin films

Antiferromagnetic domains in an epitaxial thin film, LaFeO(3) on SrTiO(3)(100), were observed using a high-spatial-resolution photoelectron emission microscope with contrast generated by the large x-ray magnetic linear dichroism effect at the multiplet-split L edge of Fe. The antiferromagnetic domains are linked to 90 degrees twinned crystallographic regions in the film. The Neel temperature of the thin film is reduced by 70 kelvin relative to the bulk material, and this reduction is attributed to epitaxial strain. These studies open the door for a microscopic understanding of the magnetic coupling across antiferromagnetic-ferromagnetic interfaces.

Journal Article↗

PubMed2000

Assessment of natural and artificial propagation of the white-clawed crayfish (Austropotamobius pallipes species complex) in the Alpine region with nuclear and mitochondrial markers.

In a joint analysis of nuclear (allozyme) and mitochondrial markers (sequence and restriction fragment length polymorphism (RFLP) analysis of the 12S and 16S genes), five genetically distinct groups of the Austropotamobius pallipes Lereboullet species complex were detected in the Alpine region. The geographical distribution of these evolutionary lineages coincided largely with several taxa (A.p. pallipes, A. berndhauseri, A.(p.) italicus) formerly defined on the basis of morphological characters. A low level of genetic variability was found within these lineages. For some populations, the combination of the two markers gave additional information about the likelihood of their natural or artificial origin. A hybrid zone was detected between A.p. pallipes and A. berndhauseri in the Lake Geneva area. The mosaic distribution of the five evolutionary lineages indicated that conservation efforts should be aimed at the level of local populations.

Alleles↗

PubMed1996

Genetic differentiation of continental and island populations of Bombus terrestris (Hymenoptera: Apidae) in Europe.

Ten microsatellite loci and a partial sequence of the COII mitochondrial gene were used to investigate genetic differentiation in B. terrestris, a bumble bee of interest for its high-value crop pollination. The analysis included eight populations from the European continent, five from Mediterranean islands (six subspecies altogether) and one from Tenerife (initially described as a colour form of B. terrestris but recently considered as a separate species, B. canariensis). Eight of the 10 microsatellite loci displayed high levels of polymorphism in most populations. In B. terrestris populations, the total number of alleles detected per polymorphic locus ranged from 3 to 16, with observed allelic diversity from 3.8 +/- 0.5 to 6.5 +/- 1.4 and average calculated heterozygosities from 0.41 +/- 0.09 to 0.65 +/- 0.07. B. canariensis showed a significantly lower average calculated heterozygosity (0.12 +/- 0.08) and observed allelic diversity (1.5 +/- 0.04) as compared to both continental and island populations of B. terrestris. No significant differentiation was found among populations of B. terrestris from the European continent. In contrast, island populations were all significantly and most of them strongly differentiated from continental populations. B. terrestris mitochondrial DNA is characterized by a low nucleotide diversity: 0.18% +/- 0.07%, 0.20% +/- 0.04% and 0.27% +/- 0.04% for the continental populations, the island populations and all populations together, respectively. The only haplotype found in the Tenerife population differs by a single nucleotide substitution from the most common continental haplotype of B. terrestris. This situation, identical to that of Tyrrhenian islands populations and quite different from that of B. lucorum (15 substitutions between terrestris and lucorum mtDNA) casts doubts on the species status of B. canariensis. The large genetic distance between the Tenerife and B. terrestris populations estimated from microsatellite data result, most probably, from a severe bottleneck in the Canary island population. Microsatellite and mitochondrial DNA data call for the protection of the island populations of B. terrestris against importation of bumble bees of foreign origin which are used as crop pollinators.

Alleles↗