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Biomedical subjects

J Stöhr

Publications and source records attributed to J Stöhr.

At least 19 recordsLinked to original sources
PubMed2005

Dissipation of spin angular momentum in magnetic switching.

Applying one ultrashort magnetic field pulse, we observe up to 10 precessional switches of the magnetization direction in single crystalline Fe films of 10 and 15 atomic layers. We find that the rate at which angular momentum is dissipated in uniform large angle spin precession increases with time and film thickness, surpassing the intrinsic ferromagnetic resonance spin lattice relaxation of Fe by nearly an order of magnitude.

Journal Article↗
PubMed2004

Lensless imaging of magnetic nanostructures by X-ray spectro-holography.

Our knowledge of the structure of matter is largely based on X-ray diffraction studies of periodic structures and the successful transformation (inversion) of the diffraction patterns into real-space atomic maps. But the determination of non-periodic nanoscale structures by X-rays is much more difficult. Inversion of the measured diffuse X-ray intensity patterns suffers from the intrinsic loss of phase information, and direct imaging methods are limited in resolution by the available X-ray optics. Here we demonstrate a versatile technique for imaging nanostructures, based on the use of resonantly tuned soft X-rays for scattering contrast and the direct Fourier inversion of a holographically formed interference pattern. Our implementation places the sample behind a lithographically manufactured mask with a micrometre-sized sample aperture and a nanometre-sized hole that defines a reference beam. As an example, we have used the resonant X-ray magnetic circular dichroism effect to image the random magnetic domain structure in a Co/Pt multilayer film with a spatial resolution of 50 nm. Our technique, which is a form of Fourier transform holography, is transferable to a wide variety of specimens, appears scalable to diffraction-limited resolution, and is well suited for ultrafast single-shot imaging with coherent X-ray free-electron laser sources.

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

The ultimate speed of magnetic switching in granular recording media.

In magnetic memory devices, logical bits are recorded by selectively setting the magnetization vector of individual magnetic domains either 'up' or 'down'. In such devices, the fastest and most efficient recording method involves precessional switching: when a magnetic field B(p) is applied as a write pulse over a period tau, the magnetization vector precesses about the field until B(p)tau reaches the threshold value at which switching occurs. Increasing the amplitude of the write pulse B(p) might therefore substantially shorten the required switching time tau and allow for faster magnetic recording. Here we use very short pulses of a very high magnetic field to show that under these extreme conditions, precessional switching in magnetic media supporting high bit densities no longer takes place at well-defined field strengths; instead, switching occurs randomly within a wide range of magnetic fields. We attribute this behaviour to a momentary collapse of the ferromagnetic order of the spins under the load of the short and high-field pulse, thus establishing an ultimate limit to the speed of deterministic switching and magnetic recording.

Journal Article↗
PubMed2004

Future possibilities of the Linac Coherent Light Source.

A study of the potential for the development of the Linac Coherent Light Source (LCLS) beyond the specifications of the baseline design is presented. These future developments include delivery of X-ray pulses in the 1 fs regime, extension of the spectral range, increase of the FEL power, exploitation of the spontaneous emission, and a more flexible time structure. As this potential is exploited, the LCLS can maintain its role as a world-leading instrument for many years beyond its commissioning in 2008 and initial operation as the world's first X-ray free-electron laser.

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↗
PubMed2004

Biochemistry of transmembrane signaling mediated by trimeric G proteins.

Many extracellular signals are at the cell surface received by specific receptors, which upon activation transduce information to the appropriate cellular effector molecules via trimeric G proteins. The G protein-mediated cascades ultimately lead to the highly refined regulation of systems such as sensory perception, cell growth, and hormonal regulation. Transmembrane signaling may be seriously deranged in various pathophysiological conditions. Over the last two decades the major experimental effort of our group has been devoted to better understanding the molecular mechanisms underlying transmembrane signaling regulated by G proteins and to the closely related process of desensitization of hormone response. This review provides general information about the basic principles of G protein-regulated transmembrane signaling as well as about our contribution to the current progress in the field.

Adipose Tissue↗
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↗
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

Atomic-beam alignment of inorganic materials for liquid-crystal displays.

The technique used to align liquid crystals-rubbing the surface of a substrate on which a liquid crystal is subsequently deposited-has been perfected by the multibillion-dollar liquid-crystal display industry. However, it is widely recognized that a non-contact alignment technique would be highly desirable for future generations of large, high-resolution liquid-crystal displays. A number of alternative alignment techniques have been reported, but none of these have so far been implemented in large-scale manufacturing. Here, we report a non-contact alignment process, which uses low-energy ion beams impinging at a glancing angle on amorphous inorganic films, such as diamond-like carbon. Using this approach, we have produced both laptop and desktop displays in pilot-line manufacturing, and found that displays of higher quality and reliability could be made at a lower cost than the rubbing technique. The mechanism of alignment is explained by adopting a random network model of atomic arrangement in the inorganic films. Order is induced by exposure to an ion beam because unfavourably oriented rings of atoms are selectively destroyed. The planes of the remaining rings are predominantly parallel to the direction of the ion beam.

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↗
PubMed2001

Differences in the behaviour of Sprague--Dawley and Lewis rats during repeated passive avoidance procedure: effect of amphetamine.

The present paper investigated the differences in passive avoidance learning between Sprague--Dawley and Lewis rats. After initial habituation (experimental Part 1), measured as latencies to enter the dark, preferable compartment, the effect of treatment with amphetamine (8 mg kg(-1)b.w.), the retention performance compared with controls (saline) was tested in both rat strains in Parts 2--4. The intervals between Parts 2--4 were 24 or 49 days. Each experimental part consisted of testing lasting 6 days. On the 7th day the rats received drug treatment 1 h before the application of foot shock. The differences between rat strains were already detectable at the beginning of the study. During the repeated exposures of rats in Part 1, only Lewis rats, in contrast to Sprague--Dawley rats, exhibited the habituation. The repeated testing of rats in Parts 2--4, due to previous experience with an aversive stimulus, was considered as the retention test. In Parts 2--3 we observed only minor differences in the responses of both rat strains tested. Also no significant differences were observed between rat strains after amphetamine treatment that induced an amnesia-like effect in all retention trials. However, data shown in Part 4 revealed the largest differences between both strains. Control Lewis rats exhibited significantly higher retention responses than Sprague--Dawley rats. In the latter strain we observed no differences in avoidance latencies between controls and amphetamine treated rats. In Lewis rats the difference in avoidance performance between controls and amphetamine treated animals was highly significant due to their enhanced retention performance. In conclusion, the results presented in this study extend the known behavioural differences in tested rat strains to the passive avoidance procedure that, in addition, was performed for a total period of 4 months. Due to a known deficiency of hypothalamo-pituitary-adrenal axis activity in Lewis rats it can be hypothesized that the behavioural dissociation of this strain from Sprague--Dawley rats could be related to the different activity of this regulatory axis in the rat strains tested.

Amphetamine↗
PubMed1999

A more mature phenotype of blood mononuclear phagocytes is induced by fluvastatin treatment in hypercholesterolemic patients with coronary heart disease.

Monocytes are recruited as the principal inflammatory cells into the atherosclerotic lesion. In a previous study we demonstrated that a low HDL-cholesterol and the apo E4 allele are associated with an increased proportion of blood monocytes that are characterized by a high expression of Fcgamma-RIIIa (CD16), a dim expression of the lipopolysaccharide (LPS) receptor (CD14) and a high expression of beta1- and beta2-integrins (Rothe et al. Arterioscler Thromb Vasc Cell Biol 1996;16:1437-1447). In this study, 79 hypercholesterolemic patients were treated either with the HMG CoA reductase inhibitor fluvastatin in combination with diet or with placebo and diet in a double-blind and randomized multicenter study, and monitored for the potential effects on the phenotype of peripheral blood monocytes. At baseline, in the whole group of hypercholesterolemic patients the population size of these more mature monocytes (CD14dimCD16+) was positively correlated to triglyceride (P = 0.003) and total serum cholesterol levels (P = 0.012) confirming our previous study. Fluvastatin treatment for 52 weeks was associated with a 24.2% reduction in LDL-cholesterol (P < 0.001) as well as a 40.7% decrease in the expression density of CD14 on all monocytes (P = 0.027). A 24.5% decrease (P < 0.001) of the population of less differentiated CD14brightCD16- monocytes and an 83.1% increase (P = 0.029) of the population of more differentiated CD14dimCD16+ monocytes further confirmed this modification of the phenotype of peripheral blood monocytes. The positive pre-study correlation of the CD14dimCD16+ monocyte subset to the serum cholesterol concentration, but inverse changes of both parameters under fluvastatin therapy, in conclusion indicate that fluvastatin exerts an as yet uncharacterized immunomodulatory effect on either monocyte maturation and differentiation, or extravasation which may also depend on the endothelial phenotype that is independent of the change in serum lipids.

Adult↗
PubMed1999

Effects of immobilization stress combined with water immersion and chronic amphetamine treatment on the adenylyl cyclase activity in rat neurohypophysis.

Several papers have indicated the participation of cyclic AMP as a second messenger for the release of neurohypophysial hormones. Since very little is known about the effects of stress and drugs of abuse on this process, we studied the activity of adenylyl cyclase in the neurohypophyses after immobilization stress and chronic amphetamine treatment. Our findings indicate the involvement of cyclic AMP in the regulation of neurohypophysis as well as the increase in total adenylyl cyclase both after application of immobilization stress combined with water immersion and after chronic amphetamine treatment.

Adenylyl Cyclases↗
PubMed1998

X-PEEM Study on Surface Orientation of Stylized and Rubbed Polyimides.

X-ray photoelectron emission microscopy (X-PEEM) was used to study the surface orientation of stylized and rubbed polyimide thin films. Using soft X-rays produced by a synchrotron light source, this technique combines high spatial resolution imaging with near-edge X-ray absorption fine structure (NEXAFS) spectroscopy to yield information on the surface orientation of the films. Stylizing is an ideal model of the rubbing process since the local stress acting on the polyimide to orient the molecules can be calculated. The minimum normal stress necessary to orient the surface of BPDA-PDA films was found to be 45 MPa much lower than the bulk yield stress of 200-300 MPa. Studies of the polyimide films oriented by the conventional rubbing method showed lateral inhomogeneities in the orientation of the polymer at the surface.

Journal Article↗
PubMed1998

Enhanced upregulation of the Fc gamma receptor IIIa (CD16a) during in vitro differentiation of ApoE4/4 monocytes.

We recently reported a positive correlation of the pool size of lipopolysaccharide receptor (CD14)dim and Fc gamma receptor IIIa (CD16a)+ monocytes in peripheral blood to the apolipoprotein E4 (apoE4) phenotype and a negative correlation to high density lipoprotein (HDL) cholesterol levels (Arterioscler Thromb Vasc Biol. 1996;16:1437-1447). In this study, the in vitro differentiation of mononuclear phagocytes derived from healthy blood donors homozygous for the E3/3 or the E4/4 phenotype was analyzed during 7 days of culture in serum-free medium supplemented with macrophage colony-stimulating factor (M-CSF). The CD16a expression, which indicates Fc receptor-dependent phagocytic activity, increased to a significantly higher level in apoE4/4 monocytes than in apoE3/3 cells. The costimulatory molecule CD40, which indicates antigen-presenting capacity, was upregulated more strongly in apoE3/3 monocytes compared with E4/4 cells, but the difference did not reach a significant level. The expression of differentiation-associated surface proteins (CD14, CD33, CD45) and adhesion molecules (CD11a, CD11b, CD11c, CD49d) was not significantly different between apoE3/3 and apoE4/4 monocytes. However, a significantly decreased intracellular apoE concentration and a reduced amount of secreted apoE were found in apoE4/4 monocytes during in vitro differentiation. No differences were found in the surface expression of the low density lipoprotein receptor-related protein (CD91) and the uptake of fluorescence labeled low density lipoprotein between apoE3/3 and apoE4/4 monocytes. These data indicate that the apoE4/4 phenotype significantly influences the M-CSF-dependent differentiation of monocytes toward a more CD16a-positive phagocytic phenotype.

Adult↗