The working group “Magnetization Dynamics” investigates dynamic processes in ferromagnetic thin-film systems. Structures with layer thicknesses in the range of 5 to 50 nanometers are studied. Lateral dimensions range from 50 nanometers up to few micrometers, covering the length scales of patterned magnetic storage elements. Soft magnetic materials, like iron, cobalt and nickel, as well as their alloys, exhibit fundamental magnetic lengths (so-called exchange lengths) on this length scale. Switching times of current magnetic applications approach the sub-nanosecond regime. This intrinsic time scale of the magnetization dynamics is defined by the gyroscopic precession of individual magnetic moments.
Both, the static equilibrium configuration and the dynamic behavior of magnetic microstructures depend essentially on their shape and size. In response to the excitation with fast varying magnetic fields, fundamental magneto-dynamic phenomena appear, like fast movement of magnetic domain walls, the dynamics of the magnetic vortices as well as fast switching of the magnetization. In the case of magnetic multilayers, the dynamical coupling of different magnetic layers is observed. Furthermore, magnetic relaxation processes after the excitation are of great interest. Fundamental questions currently being intensively investigated are, for example, the source and the role of damping in magnetization dynamics.
Spatial extent of the smallest micromagnetic configurations, i.e. the magnetic domain walls and the magnetic vortex-cores, is determined by the characteristic exchange length of the material. The exchange length of technologically highly important soft magnetic nickel-iron-alloy (Ni 80%, Fe 20%, Permalloy) amounts to less than 6 nanometers. Analytical tools with high temporal and spatial resolution are necessary in order to investigate the static and the dynamic behavior of such structures in detail. We are using time-resolved magnetic transmission x-ray microscopy for the visualization of magnetization dynamics in small magnetic structures.
Transmission x-ray miscroscopy
– imaging of the magnetization dynamics -
Fresnel zone plates serve as focusing and imaging elements in a transmission x-ray microscope. They consist of concentric rings, which serve as circular diffraction grids, and constructively focus the soft x-rays to a focal-point. State of the art zone plates are manufactured with modern lithographic procedures and can achieve very high lateral resolutions down to 20 nanometers.
There are two different types of transmission x-ray microscopes. The imaging transmission x-ray microscope (ITXM) illuminates the whole sample and the magnified picture is recorded in transmission by means of a CCD-Camera. In the case of scanning transmission x-ray microscope (STXM) the sample is scanned with a small spot of focussed x-rays and the transmitted intensity is measured by a small photodetector.
Soft x-rays are produced by a synchrontron radiation source. Our experiments are being performed by the „Advanced Light Source“ in Berkeley, USA http://www-als.lbl.govIn an electron storage ring, the so-called synchrotron, bunches of electrons are accelerated to very high velocities close to the speed of light. At certain places along the storage ring, the electrons are deflected by strong magnets and emit very intensive, bundled and polarized x-rays. The polarization-property of synchrontron radiation is the essential requirement for the magnetic x-ray microscopy. As a magnetic contrast mechanism, the x-ray magnetic circular dichroism (XMCD) is used. Due to this effect, the absorption of circularly polarized x-rays depends on the orientation of the magnetization relative to the propagation direction of the x-rays. Magnetic regions oriented in the direction of propagating x-rays will appear bright; the regions oriented against the propagation direction of x-rays will appear dark. In this way the XMCD effect leads to a light-dark contrast of the transmitted intensity. The x-ray circular dichroism appears, when the energy of x-rays corresponds to a certain absorption edge of a magnetic atom. Local magnetization distribution and the spin dynamics in magnetic multilayers, can be resolved layer-by-layer and element-selectively by adjusting the energy of the x-rays to the appropriate element-specific absorption edge. The inherently pulsed temporal structure of synchrotron radiation is used for the visualization of dynamic processes. Temporal resolution is given by short x-ray flashes with duration of less than 100 picoseconds. A common method for the imaging of magnetization dynamics is a pump-probe“ experiment. The magnetic equilibrium state is excited by periodic magnetic pump-pulses and afterwards stroboscopically probed with x-ray flashes.
In addition to the experiments, micromagnetic simulations of the spin dynamics are also being performed by our working group. They permit a detailed insight into fast switching processes. Investigation of the switching behavior of thin Permalloy platelets, excited by external magnetic field, is of special interest.
New concepts for magnetic storage are being investigated in this context. Square-shaped platelets exhibit as equilibrium state the so-called Landau pattern with a vortex in the central area of the element. In the vortex core the magnetization rotates in an area of approximately 20 nanometers perpendicular to the plane. The magnetization of the vortex core can be switched by suitable external magnetic fields from the orientation “up“ to “down“ and vice versa. Thus the polarity of the tiny vortex cores could be used as a memory cell for an information bit.