Invited Paper MI+AS+NS+SP-TuA9
Imaging Magnetization Dynamics on its Genuine Time Scale

Tuesday, October 29, 2013, 4:40 pm, Room 202 A

A variety of excellent microscopies that provide magnetic contrast on the nanoscale matured to powerful tools. Today's scanning-probe techniques feature ultimate spin resolution, namely imaging of the magnetization of single adatoms [1]. The temporal resolution of optical and x-ray methods reaches down to femtoseconds. It is intriguing to have spatial and time resolution simultaneously. The relevant frequency scale for ferromagnets is given by the ferromagnetic resonance which lies in the GHz range. Thus the required time resolution is in the sub-nanosecond regime. Magnetic microscopies available at synchrotron sources enable real-time imaging and provide lateral resolution down to the nanometer scale [2,3].

We investigate the switching criteria of nanometer-scaled magnetic vortices in micron-sized Permalloy squares. The vortices are excited by high frequency magnetic fields. Continuous core reversal is demonstrated for a wide range of frequencies and amplitudes of excitation by ferromagnetic absorption spectroscopy and for selected frequencies and amplitudes with time-resolved scanning x-ray microscopy. The boundary of this switching regime is derived from the Thiele equation when a critical velocity of v_crit ≈ 250 m/s is considered [4].

Complexity created by periodic arrangement of well-understood building blocks plays an important role in biochemistry, photonics, and nanoelectronics. The periodic arrangement of atoms or molecules as basis determines the physical and even the chemical properties of crystals. With the flexibility of nanometer-precise electron-beam lithography we engineer magnetic interactions yielding two-dimensional magnonic crystals that benefit from the magnetic vortex core as crystal basis. Using scanning transmission x-ray microscopy at the MAXYMUS beamline at BESSY II in Berlin, Germany we image the magnonic crystal dynamics. We observe self-organized vortex core state formation by adiabatic reduction of high frequency magnetic field excitation [5]. The experimental results are described analytically by coupled Thiele equations of motion and are compared to micromagnetic simulations.

Financial support of the Deutsche Forschungsgemeinschaft via Sonderforschungsbereich 668 and Graduiertenkolleg 1286 is gratefully acknowledged. This work has been supported by the excellence cluster “The Hamburg Centre for Ultrafast Imaging” of the Deutsche Forschungsgemeinschaft.

[1] A. Khajetoorians et al., Science 332, 1062 (2011)

[2] P. Fischer and C. Fadley, Nanotechnol. Rev. 1, 5 (2012)

[3] A. Vogel et al., Phys. Rev. Lett. 106, 137201 (2011)

[4] M. Martens et al., Phys. Rev. B 87, 054426 (2013)

[5] C. Adolff et al., submitted