Coherent Excitation of Heterosymmetric Spin Waves with Ultrashort Wavelengths.

In the emerging field of magnonics, spin waves are foreseen as signal carriers for future spintronic information processing and communication devices, owing to both the very low power losses and a high device miniaturization potential predicted for short-wavelength spin waves. Yet, the efficient excitation and controlled propagation of nanoscale spin waves remains a severe challenge. Here, we report the observation of high-amplitude, ultrashort dipole-exchange spin waves (down to 80 nm wavelength at 10 GHz frequency) in a ferromagnetic single layer system, coherently excited by the driven dynamics of a spin vortex core. We used time-resolved x-ray microscopy to directly image such propagating spin waves and their excitation over a wide range of frequencies. By further analysis, we found that these waves exhibit a heterosymmetric mode profile, involving regions with anti-Larmor precession sense and purely linear magnetic oscillation. In particular, this mode profile consists of dynamic vortices with laterally alternating helicity, leading to a partial magnetic flux closure over the film thickness, which is explained by a strong and unexpected mode hybridization. This spin-wave phenomenon observed is a general effect inherent to the dynamics of sufficiently thick ferromagnetic single layer films, independent of the specific excitation method employed.

Magnetic vortex core reversal by excitation with short bursts of an alternating field.

The vortex state, characterized by a curling magnetization, is one of the equilibrium configurations of soft magnetic materials and occurs in thin ferromagnetic square and disk-shaped elements of micrometre size and below. The interplay between the magnetostatic and the exchange energy favours an in-plane, closed flux domain structure. This curling magnetization turns out of the plane at the centre of the vortex structure, in an area with a radius of about 10 nanometres--the vortex core. The vortex state has a specific excitation mode: the in-plane gyration of the vortex structure about its equilibrium position. The sense of gyration is determined by the vortex core polarization. Here we report on the controlled manipulation of the vortex core polarization by excitation with small bursts of an alternating magnetic field. The vortex motion was imaged by time-resolved scanning transmission X-ray microscopy. We demonstrate that the sense of gyration of the vortex structure can be reversed by applying short bursts of the sinusoidal excitation field with amplitude of about 1.5 mT. This reversal unambiguously indicates a switching of the out-of-plane core polarization. The observed switching mechanism, which can be understood in the framework of micromagnetic theory, gives insights into basic magnetization dynamics and their possible application in data storage.

Explaining the paradoxical diversity of ultrafast laser-induced demagnetization.

Pulsed-laser-induced quenching of ferromagnetic order has intrigued researchers since pioneering works in the 1990s. It was reported that demagnetization in gadolinium proceeds within 100 ps, but three orders of magnitude faster in ferromagnetic transition metals such as nickel. Here we show that a model based on electron-phonon-mediated spin-flip scattering explains both timescales on equal footing. Our interpretation is supported by ab initio estimates of the spin-flip scattering probability, and experimental fluence dependencies are shown to agree perfectly with predictions. A phase diagram is constructed in which two classes of laser-induced magnetization dynamics can be distinguished, where the ratio of the Curie temperature to the atomic magnetic moment turns out to have a crucial role. We conclude that the ultrafast magnetization dynamics can be well described disregarding highly excited electronic states, merely considering the thermalized electron system.

Spin Interactions in bcc and fcc Fe beyond the Heisenberg Model.

Usually, the adiabatic magnetic exchange-energy hypersurface is parametrized in terms of the bilinear Heisenberg interactions in pairs of atoms. For general magnetic configurations, this model is not complete even if it includes pairs with up to infinite interatomic distances. In contrast, the modeling by an in principle infinite spin-cluster expansion is complete for all conceivable magnetic configurations. In the present Letter, it is shown for bcc and fcc iron that a very accurate representation can be reached with a finite expansion with 20 terms which include biquadratic or multispin non-Heisenberg interactions.

Speeding up the extended Kalman filter approach for denoising XMCD movies of fast magnetization dynamics.

The Kalman filter is a well-established approach to get information on the time-dependent state of a system from noisy observations. In former times it has been used for systems with only a few degrees of freedom (typically about 10). The fast magnetization dynamics is often investigated by x-ray magnetic circular dichroism movies (XMCD movies) where the number of components of the state vector is very large (typically about 105). For such systems the Jacobian matrix which is required in the extended Kalman filter approach cannot be calculated numerically (as it is done in former papers) by use of the Landau-Lifschitz-Gilbert equation of motion in a finite time because of the many degrees of freedom of the state vector. In the present paper it is shown that the calculation of the Jacobian matrix can be much speeded up by using good analytical approximations, which are derived by a tensorial Green's function method to solve the linearized Landau-Lifschitz-Gilbert equation of motion. This makes it possible to investigate the dynamics of magnetic vortices and spin waves in circular discs of Permalloy, which are initiated by time-dependent external magnetic fields.

Towards denoising XMCD movies of fast magnetization dynamics using extended Kalman filter.

The Kalman filter is a well-established approach to get information on the time-dependent state of a system from noisy observations. It was developed in the context of the Apollo project to see the deviation of the true trajectory of a rocket from the desired trajectory. Afterwards it was applied to many different systems with small numbers of components of the respective state vector (typically about 10). In all cases the equation of motion for the state vector was known exactly. The fast dissipative magnetization dynamics is often investigated by x-ray magnetic circular dichroism movies (XMCD movies), which are often very noisy. In this situation the number of components of the state vector is extremely large (about 10(5)), and the equation of motion for the dissipative magnetization dynamics (especially the values of the material parameters of this equation) is not well known. In the present paper it is shown by theoretical considerations that - nevertheless - there is no principle problem for the use of the Kalman filter to denoise XMCD movies of fast dissipative magnetization dynamics.

Effects of hypertonic-hyperoncotic infusion on the human atrial natriuretic factor in a standardized clinical trial.

Hypertonic-hyperoncotic solutions (HHT) are presently being utilized to resuscitate patients in shock. However, so far, the effects of HHT on human atrial natriuretic factor (hANF) have not yet been investigated in a clinical trial. The effects of HHT on hANF were studied in a standardized preoperative and clinical setting in patients undergoing aneurysmectomy. Twenty-three conscious patients were included in two groups: 11 HHT and 12 HES (HHT: 7.5% NaCl/10% HES 200; HES: .9% NaCl/10% HES 200). Stepwise infusion of 50 mL was titrated preoperatively according to individual Frank-Starling relationships. Central venous pressure (CVP) and pulmonary capillary wedge pressure (PCWP) were determined before, during and after volume application. hANF, cGMP were also measured before and 1, 10, 30, 60, and 120 min after administration. The volumes necessary to produce the same volume status were: 213.6 +/- 63.6 mL of HHT, 409.9 +/- 136.2 mL of HES (p < .001). The sodium load was 273.9 +/- 81.5 mmol of [HHT], 63.1 +/- 21.0 mmol of [HES] (p < .001).(ABSTRACT TRUNCATED AT 250 WORDS)

Optimal preoperative titrated dosage of hypertonic-hyperoncotic solutions in cardiac risk patients.

Hypertonic-iso/hyperoncotic solutions have been the subject of numerous studies, mostly used in a fixed dosage (4 mL/kg bw or 250 mL). Nearly no study exists to prove whether this is the appropriate dosage especially in cardiac risk patients with accompanying diseases. We have compared preoperative volume loading with either 10% hydroxyethyl-starch/7.5% NaCl (HHT-HES) or 10% hydroxyethyl-starch/.9% NaCl (HES) in 50 mL bolus infusions. Volume loading was done with either HES or HHT-HES in 2 x 20 patients before aortic aneurysmectomy. The endpoint of stepwise infusion represented the highest cardiac index (CI) at the lowest possible wedge pressure (PCWP) (turning point of each individual Frank Starling relation). 167.5 mL (+/- 45.5 mL = 2.41 mL/kg bw) of HHT-HES and 440 mL (+/- 26.15 mL = 6.33 mL/kg bw) of HES were necessary. We observed a significant higher increase of the CI in the HHT-HES group. Significant increases of PCWP, pulmonary artery pressure, and central venous pressure occurred within the groups without any significant differences between the groups (p < .05). Results of the study showed: 1) The commonly used fixed dosage of 4 mL/kg bw of HHT-HES is too high in cardiac risk patients with slight hypovolemia. 2) HHT-HES should be given in an individual titration. 3) In the HHT-HES group we observed a positive inotropic effect (higher CI). 4) With the individual titration of HHT-HES no negative side effects occurred (especially no hypotension).

Electron theory of fast and ultrafast dissipative magnetization dynamics.

For metallic magnets we review the experimental and electron-theoretical investigations of fast magnetization dynamics (on a timescale of ns to 100 ps) and of laser-pulse-induced ultrafast dynamics (few hundred fs). It is argued that for both situations the dominant contributions to the dissipative part of the dynamics arise from the excitation of electron-hole pairs and from the subsequent relaxation of these pairs by spin-dependent scattering processes, which transfer angular momentum to the lattice. By effective field theories (generalized breathing and bubbling Fermi-surface models) it is shown that the Gilbert equation of motion, which is often used to describe the fast dissipative magnetization dynamics, must be extended in several aspects. The basic assumptions of the Elliott-Yafet theory, which is often used to describe the ultrafast spin relaxation after laser-pulse irradiation, are discussed very critically. However, it is shown that for Ni this theory probably yields a value for the spin-relaxation time T(1) in good agreement with the experimental value. A relation between the quantity α characterizing the damping of the fast dynamics in simple situations and the time T(1) is derived.