Coupled atomistic spin-lattice simulations of ultrafast demagnetization in 3d ferromagnets.

Despite decades of research, the role of the lattice and its coupling to the magnetisation during ultrafast demagnetisation processes is still not fully understood. Here we report on studies of both explicit and implicit lattice effects on laser induced ultrafast demagnetisation of bcc Fe and fcc Co. We do this using atomistic spin- and lattice dynamics simulations following a heat-conserving three-temperature model. We show that this type of Langevin-based simulation is able to reproduce observed trends of the ultrafast magnetization dynamics of fcc Co and bcc Fe. The parameters used in our models are all obtained from electronic structure theory, with the exception of the lattice dynamics damping term, where a range of parameters were investigated. It was found that while the explicit spin-lattice coupling in the studied systems does not impact the demagnetisation process notably, the lattice damping has a large influence on the details of the magnetization dynamics. The dynamics of Fe and Co following the absorption of a femtosecond laser pulse are compared with previous results for Ni and similarities and differences in the materials' behavior are analysed. For all elements investigated so far with this model, we obtain a linear relationship between the value of the maximally demagnetized state and the fluence of the laser pulse , which is in agreement with experiments. Moreover, we demonstrate that the demagnetization amplitude is largest for Ni and smallest for Co. This holds over a wide range of the reported electron-phonon couplings, and this demagnetization trend is in agreement with recent experiments.

Author Correction: Enhanced skyrmion stability due to exchange frustration.

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Enhanced skyrmion stability due to exchange frustration.

Skyrmions are localized, topologically non-trivial spin structures which have raised high hopes for future spintronic applications. A key issue is skyrmion stability with respect to annihilation into the ferromagnetic state. Energy barriers for this collapse have been calculated taking only nearest neighbor exchange interactions into account. Here, we demonstrate that exchange frustration can greatly enhance skyrmion stability. We focus on the prototypical film system Pd/Fe/Ir(111) and use an atomistic spin model parametrized from first-principles calculations. We show that energy barriers and critical fields of skyrmion collapse as well as skyrmion lifetimes are drastically enhanced due to frustrated exchange and that antiskyrmions are metastable. In contrast an effective nearest-neighbor exchange model can only account for equilibrium properties of skyrmions such as their magnetic field dependent profile or the zero temperature phase diagram. Our work shows that frustration of long range exchange interactions - a typical feature in itinerant electron magnets - is a route towards enhanced skyrmion stability even in systems with a ferromagnetic ground state.

Electromechanical piezoresistive sensing in suspended graphene membranes.

Monolayer graphene exhibits exceptional electronic and mechanical properties, making it a very promising material for nanoelectromechanical devices. Here, we conclusively demonstrate the piezoresistive effect in graphene in a nanoelectromechanical membrane configuration that provides direct electrical readout of pressure to strain transduction. This makes it highly relevant for an important class of nanoelectromechanical system (NEMS) transducers. This demonstration is consistent with our simulations and previously reported gauge factors and simulation values. The membrane in our experiment acts as a strain gauge independent of crystallographic orientation and allows for aggressive size scalability. When compared with conventional pressure sensors, the sensors have orders of magnitude higher sensitivity per unit area.

Colossal magnetic anisotropy of monatomic free and deposited platinum nanowires.

Whenever a nanosystem such as an adatom, a cluster or a nanowire spontaneously magnetizes, a crucial parameter is its magnetic anisotropy, the intrinsic preference of magnetization to lie along an easy axis. Anisotropy is important in nanosystems because it helps reduce the magnitude of thermal (superparamagnetic) fluctuations, it can modify the flow of current, and it can induce new phenomena, such as the quantum tunnelling of magnetization. We discuss here, on the basis of density functional calculations, the novel and unconventional feature of colossal magnetic anisotropy--the strict impossibility of magnetization to rotate from the parallel to the orthogonal direction--which, owing to a quantum mechanical selection rule, the recently predicted Pt nanowire magnetism should exhibit. Model calculations suggest that the colossal magnetic anisotropy of a Pt chain should persist after weak adsorption on an inert substrate or surface step.

Magnetism in atomic-size palladium contacts and nanowires.

We have investigated Pd nanowires theoretically, and found that, unlike either metallic or free atomic Pd, they exhibit Hund's rule magnetism. In long, monostrand nanowires, we find a spin moment of 0.7 mu(B) per atom, whereas for short, monostrand nanowires between bulk leads, the predicted moment is about 0.3 mu(B) per nanowire atom. In contrast, a coaxial (6,1) nanowire was found to be nonmagnetic. The origin of the nanowire magnetism is analyzed.

Geometry of the valence transition induced surface reconstruction of Sm(0001).

We present a structural determination of the surface reconstruction of the Sm(0001) surface using surface x-ray diffraction, scanning tunneling microscopy, and ab initio calculations. The reconstruction is associated with a large (22%) expansion of the atomic radius for the top monolayer surface Sm atoms. The mechanism driving the surface reconstruction in Sm is unique among all elements and is connected to the strong correlations of the 4f electrons in Sm and the intermediate valence observed in certain Sm compounds. The atoms constituting the top monolayer of Sm(0001) have vastly different chemical properties compared to the layer underneath and behave as if they were an adsorbate of a different chemical species.

Vascular tumours in occlusive disease of the iliac-femoral vessels.

Two cases of intraluminal tumours of the iliac-femoral vessels are presented. One was a benign haemangioendothelioma of the common femoral vein and the other a leiomyosarcoma of the iliac artery. Their clinical features were similar to those of patients having venous thrombosis and atherosclerotic occlusive disease respectively. Consequently every physician specialized in vascular disease should be aware of intraluminal tumours as an uncommon explanation of these common clinical pictures.

Brain cortical amino acids measured by intracerebral dialysis in portacaval shunted rats.

The extracellular amino acid content was measured in the parietal cortex in portacaval and sham operated rats, using the brain dialysis technique. The amino acid content of the perfusate was determined for 10 min before and during stimulation with potassium chloride. Basal levels of aspartate, glutamine, glycine, methionie, valine, phenylalanine and leucine were 2-to 6-fold higher in the PC-shunted as compared to the sham operated rats. For glutamate, taurine, and GABA no differences were observed between the two groups. After KCl stimulation the release of glutamate and GABA increased significantly in both groups. For GABA this rise was approximately twice as high in the PC-shunted rats (+300%, P less than 0.01) as in the sham operated rats (+150%, P less than 0.01 as compared to basal). In the sham operated, but not in the PC-shunted rats, methionine and valine levels rose significantly (+200%, P less than 0.05) and glutamine release decreased (-50%, P less than 0.05). These findings suggest that the brain metabolism of amino acids is altered after a portacaval shunt. This could in turn alter the neurotransmission and partly explain the low spontaneous motor activity seen in these animals.

Growth rate of abdominal aortic aneurysms as measured by computed tomography.

The risk of rupture of an abdominal aortic aneurysm increases with size. It has thus been recommended that small aneurysms be continuously followed with some type of imaging technique to detect when aneurysm size constitutes an indication for surgery. The present study focuses on the growth rate of abdominal aortic aneurysms in 35 patients who were subjected to repeated computerized tomographic examinations of their abdominal aortic aneurysms. Several aneurysms were measured more than twice resulting in 57 different examinations. The mean growth rate of the transverse diameter was 0.52 cm/year. The individual growth rates were, however, variable. Aneurysms with an initial transverse diameter exceeding 6 cm showed a slightly but not significantly faster increase in size compared with smaller aneurysms. No correlation between initial size and growth rate could be established. Six patients died during the study period, two from myocardial infarction, three after elective aneurysm operations and one, refused for elective operation, died after rupture. It is concluded that the growth rate measured with computed tomography agrees well with previously reported estimates obtained with ultrasonography. It is recommended that small aneurysms particularly in patients with relative contraindications to surgery be followed with repeated examinations of size.

Brain amino acids measured by intracerebral dialysis in portacaval shunted rats.

Changes in brain amino acid uptake and metabolism have been proposed as a possible etiological factor in hepatic encephalopathy. By use of a brain dialysis technique (a thin tube implanted in the brain of the living animal), the extracellular amino acid concentrations in the striatum of portacaval (PC)-shunted and sham-operated rats were measured. Leucine, phenylalanine, methionine, and glutamine were increased two- to sixfold in the PC-shunted rats, whilst no changes were seen for GABA, valine, glutamate, or isoleucine, confirming previous reports. Aspartate levels were 350% higher in the PC-shunted rats, and this rise, as well as that of phenylalanine, was significantly correlated with the lower motor activity observed in the PC-shunted rats, suggesting a possible importance of these amino acids in the etiology of hepatic encephalopathy. The amino acid concentrations measured in whole blood demonstrated the well-known pattern of low levels of branched-chain amino acids and increased concentrations of phenylalanine, glutamine, and histidine.

Thrombectomy and arteriovenous fistula for iliofemoral venous thrombosis in fertile women.

Thirteen women of fertile age with left-sided iliofemoral venous thrombosis were treated with thrombectomy and a temporary arteriovenous fistula in the groin. In nine patients, the appearance of thrombosis could be related to pregnancy; in three of these, obstetric complications occurred. Diagnosis was obtained by antegrade and retrograde phlebography. After three months, the arteriovenous fistula was closed. Anticoagulant treatment was given from the time of diagnosis and for a total of six months. Patency of the iliac vein was determined by phlebography or venous occlusion plethysmography, or both. Based upon these findings, a short term patency rate of 85 per cent was achieved. Analysis of the material indicates a multifactorial pathogenic mechanism in the development of iliofemoral venous thrombosis. Important factors are pregnancy, obstetric complications, mechanical factors causing flow resistance in the iliac vein, hormonal factors and defective fibrinolysis.