Showing 1–7 of 7 results for author: Bauer, H

Magnetism of monomer MnO and heterodimer FePt@MnO nanoparticles

Authors: X. Sun, A. Klapper, Y. Su, K. Nemkovski, A. Wildes, H. Bauer, O. Köhler, A. Schilmann, W. Tremel, O. Petracic, Th. Brückel

Abstract: We report about the magnetic properties of antiferromagnetic (AF) MnO nanoparticles (NPs) with different sizes (6-19nm). Using a combination of polarized neutron scattering and magnetometry we were able to resolve previously observed peculiarities. Magnetometry, on the one hand, reveals a peak in the zero field cooled (ZFC) magnetization curves at low temperatures (~25K) but no feature around the… ▽ More We report about the magnetic properties of antiferromagnetic (AF) MnO nanoparticles (NPs) with different sizes (6-19nm). Using a combination of polarized neutron scattering and magnetometry we were able to resolve previously observed peculiarities. Magnetometry, on the one hand, reveals a peak in the zero field cooled (ZFC) magnetization curves at low temperatures (~25K) but no feature around the Néel temperature at 118K. On the other hand, polarized neutron scattering shows the expected behavior of the AF order parameter vanishing around 118K. Moreover, hysteresis curves measured at various temperatures reveal an exchange bias effect indicating a coupling of an AF core to a ferromagnetic (FM)-like shell. ZFC data measured at various fields exclude a purely superparamagnetic (SPM) scenario. We conclude that the magnetic behavior of MnO particles can be explained by a superposition of SPM-like thermal fluctuations of the AF-Néel vector inside the AF core and a strong magnetic coupling to a ferrimagnetic Mn$_2$O$_3$ or Mn$_3$O$_4$ shell. In addition, we have studied heterodimer ('Janus') particles, where a FM FePt particle is attached to the AF MnO particle. Via the exchange bias effect, the magnetic moment of the FePt subunit is stabilized by the MnO. △ Less

Submitted 14 February, 2017; originally announced February 2017.

Journal ref: Phys. Rev. B 95, 134427 (2017)

Non-linear spin-wave excitation at low bias fields

Authors: Hans G. Bauer, Peter Majchrak, Torsten Kachel, Christian H. Back, Georg Woltersdorf

Abstract: Non-linear magnetization dynamics is essential for the operation of many spintronics devices. For microwave assisted switching of magnetic elements the low field regime is of particular interest. In addition a large number of experiments uses high amplitude FMR in order to generate d.c. currents via spin pumping mechanism. Here we use time resolved X-ray magnetic circular dichroism experiments to… ▽ More Non-linear magnetization dynamics is essential for the operation of many spintronics devices. For microwave assisted switching of magnetic elements the low field regime is of particular interest. In addition a large number of experiments uses high amplitude FMR in order to generate d.c. currents via spin pumping mechanism. Here we use time resolved X-ray magnetic circular dichroism experiments to determine the number density of excited magnons in magnetically soft Ni_80Fe_20 thin films at small bias fields and large rf-excitation amplitudes. Our data shows that the common model of non-linear ferromagnetic resonance is not suitable to describe the low bias field limit. Here we derive a new model of parametric spin-wave excitation which correctly predicts threshold amplitudes and decay rates also at low bias fields. In fact a new series of critical modes with amplitude phase oscillations is found, generalizing the theory of parametric spin-wave excitation. △ Less

Submitted 24 October, 2014; originally announced October 2014.

Resonant dynamics of chromium condensates

Authors: Tomasz Świsłocki, Jarosław H. Bauer, Mariusz Gajda, Mirosław Brewczyk

Abstract: We numerically study the dynamics of a spinor chromium condensate in low magnetic fields. We show that the condensate evolution has a resonant character revealing rich structure of resonances similar to that already discussed in the case of alkali-atoms condensates. This indicates that dipolar resonances occur commonly in the systems of cold atoms. In fact, they have been already observed experime… ▽ More We numerically study the dynamics of a spinor chromium condensate in low magnetic fields. We show that the condensate evolution has a resonant character revealing rich structure of resonances similar to that already discussed in the case of alkali-atoms condensates. This indicates that dipolar resonances occur commonly in the systems of cold atoms. In fact, they have been already observed experimentally. We further simulate two recent experiments with chromium condensates, in which the threshold in spin relaxation and the spontaneous demagnetization phenomena were observed. We demonstrate that both these effects originate in resonant dynamics of chromium condensate. △ Less

Submitted 12 October, 2013; originally announced October 2013.

Comments: 8 pages, 9 figures

Journal ref: Phys. Rev. A 89, 023622 (2014)

Low-amplitude magnetic vortex core reversal by non-linear interference between azimuthal spin waves and the vortex gyromode

Authors: Markus Sproll, Matthias Noske, Hans Bauer, Matthias Kammerer, Ajay Gangwar, Georg Dieterle, Markus Weigand, Hermann Stoll, Christian H. Back, Gisela Schütz

Abstract: We demonstrate a non-linear interference due to an active 'dual frequency' excitation of both, the sub-GHz vortex gyromode and multi-GHz magneto-static spin waves in ferromagnetic micrometer sized platelets in the vortex state. When the sub-GHz vortex gyromode is excited simultaneously a significant broadband reduction of the switching threshold for spin wave mediated vortex core reversal is obser… ▽ More We demonstrate a non-linear interference due to an active 'dual frequency' excitation of both, the sub-GHz vortex gyromode and multi-GHz magneto-static spin waves in ferromagnetic micrometer sized platelets in the vortex state. When the sub-GHz vortex gyromode is excited simultaneously a significant broadband reduction of the switching threshold for spin wave mediated vortex core reversal is observed in both, experiments and micromagnetic simulations. Consequently, the magnetic field amplitudes required for vortex core reversal can be lowered by nearly one order of magnitude. Moreover, additional spin wave resonance frequencies are found which emerge only if the vortex gyromode is actively excited simultaneously which can be explained by frequency doubling and by the broken symmetry of the vortex state. △ Less

Submitted 17 July, 2013; originally announced July 2013.

Comments: 5 pages, 4 figures

Microwave assisted magnetization reversal in single domain nanoelements

Authors: H. T. Nembach, H. Bauer, J. M. Shaw, M. L. Schneider, T. J. Silva

Abstract: We studied the microwave assisted magnetic reversal of 65 nm by 71 nm elliptical Ni80Fe20 nanomagnets. Hysteresis curves were measured by magneto-optical Kerr effect for a range of microwave frequencies and amplitudes. The coercive field Hc was reduced by 26% for an rf field of 0.08Hc when the microwave frequency coincided with the minimum of the experimentally determined ferromagnetic resonance… ▽ More We studied the microwave assisted magnetic reversal of 65 nm by 71 nm elliptical Ni80Fe20 nanomagnets. Hysteresis curves were measured by magneto-optical Kerr effect for a range of microwave frequencies and amplitudes. The coercive field Hc was reduced by 26% for an rf field of 0.08Hc when the microwave frequency coincided with the minimum of the experimentally determined ferromagnetic resonance frequency with varying dc field. The experimental results for the fractional reduction in Hc with rf field amplitude are in good agreement with numerical simulations for an array of interacting macrospins with a physically realistic shape anisotropy distribution △ Less

Submitted 2 June, 2009; originally announced June 2009.

Path integral Monte Carlo simulation of helium at negative pressures

Authors: G. H. Bauer, D. M. Ceperley, Nigel Goldenfeld

Abstract: Path integral Monte Carlo (PIMC) simulations of liquid helium at negative pressure have been carried out for a temperature range from the critical temperature to below the superfluid transition. We have calculated the temperature dependence of the spinodal line as well as the pressure dependence of the isothermal sound velocity in the region of the spinodal. We discuss the slope of the superflui… ▽ More Path integral Monte Carlo (PIMC) simulations of liquid helium at negative pressure have been carried out for a temperature range from the critical temperature to below the superfluid transition. We have calculated the temperature dependence of the spinodal line as well as the pressure dependence of the isothermal sound velocity in the region of the spinodal. We discuss the slope of the superfluid transition line and the shape of the dispersion curve at negative pressures. △ Less

Submitted 4 November, 1999; v1 submitted 11 September, 1999; originally announced September 1999.

Comments: 6 pages, 7 figures, submitted to Physical Review B Revised: new reference, replaced figures

The metal-insulator transition in amorphous Si_{1-x}Ni_x: Evidence for Mott's minimum metallic conductivity

Authors: A. Mobius, C. Frenzel, R. Thielsch, R. Rosenbaum, C. J. Adkins, M. Schreiber, H. -D. Bauer, R. Grotzschel, V. Hoffmann, T. Krieg, N. Matz, H. Vinzelberg, M. Witcomb

Abstract: We study the metal-insulator transition in two sets of amorphous Si_{1-x}Ni_x films. The sets were prepared by different, electron-beam-evaporation-based technologies: evaporation of the alloy, and gradient deposition from separate Ni and Si crucibles. The characterization included electron and scanning tunneling microscopy, glow discharge optical emission spectroscopy, and Rutherford back scatt… ▽ More We study the metal-insulator transition in two sets of amorphous Si_{1-x}Ni_x films. The sets were prepared by different, electron-beam-evaporation-based technologies: evaporation of the alloy, and gradient deposition from separate Ni and Si crucibles. The characterization included electron and scanning tunneling microscopy, glow discharge optical emission spectroscopy, and Rutherford back scattering. Investigating the logarithmic temperature derivative of the conductivity, w = d ln sigma / d ln T, we observe that, for insulating samples, w(T) shows a minimum increasing at both low and high T. Both the minimum value of w and the corresponding temperature seem to tend to zero as the transition is approached. The analysis of this feature of w(T,x) leads to the conclusion that the transition in Si_{1-x}Ni_x is very likely discontinuous at zero temperature in agreement with Mott's original views. △ Less

Submitted 29 October, 2001; v1 submitted 7 December, 1998; originally announced December 1998.

Comments: RevTex-file: 25 pages, 17 Postscript figures, 1 GIF figure (original Ps file can be obtained on request). Published version. Changes of v2: Sec. VII completed by two paragraphs on nature of transition, several refs. added

Journal ref: Phys. Rev. B 60, 14209 (1999)