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Research in the AG Kuch

Spectroscopy and spectromicroscopy of new magnetic materials

Magnetic read headA read head of a magnetic hard disk contains a trilayered magnetic structure as sensor.

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To explore the fundamental effects that determine the behavior of such trilayered structures is one of the objectives of the research group.

General

The main object of research in the AG Kuch are nanoscopic magnetic systems such as molecules adsorbed on solid surfaces or ultrathin films including multilayers, surfaces, and patterned structures. We aim at the fundamental investigation of new functional properties that are important, or may become important, for applications in magnetic data storage technology, magnetic sensors, or magnetoelectronic devices.

Our research employs a variety of different measurement techniques that are either implemented in our labs at the physics department of the Freie Universität in Berlin-Dahlem, or at BESSY in Berlin-Adlershof, where we make use of synchrotron radiation.

Current research topics

Molecules at surfaces

The rapidly progressing miniaturization in microelectronics leads to the vision of using molecules as functional parts of a future molecular nanotechnology. Within the DFG-funded research network Sonderforschungsbereich 658, "Elementary Processes in Molecular Switches at Surfaces", we explore the magnetic properties of molecules and metal-organic surface assemblies on solid surfaces and how they can be reversibly manipulated by external parameters. We use mainly spectroscopic techniques to extract element-resolved information.

In recent research we tackled magnetic coupling phenomena in such systems, and the reversible switching of molecules at surfaces, for example in adsorbed spin-crossover (SCO) molecules, which can be reversibly switched between a low-spin state and a high-spin state. Techniques we use include x-ray absorption spectroscopy (XAS) and photoelectron spectroscopy (XPS).

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Ultrathin antiferromagnetic films

Ferromagnetic materials that are brought in contact with antiferromagnets usually exhibit a strongly enhanced coercivity and a shift of the magnetization loop along the field axis. The latter is termed "exchange bias", and is frequently applied in magnetoresistive devices to manipulate or pin the magnetization direction of ferromagnetic layers in a multilayered stack.

In antiferromagnetic materials the direction of atomic magnetic moments systematically varies from one atom to the next. Thus a structural characterization of the interface on the scale of atomic distances is crucial. We tackle this issue by growing single-crystalline ultrathin antiferromagnetic films, like FeMn, NiMn, or CoO, on suitable single-crystal substrates, where we can characterize structure and morphology by LEED and STM. The magnetic interaction with an adjacent ferromagnetic film is studied by MOKE, XMCD, MLD, and PEEM.

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Coupling mechanisms in magnetic multilayered systems

Ferromagnetic layers in a multilayered stack that are separated by non-magnetic spacer layers interact through different coupling mechanisms such as oscillatory interlayer exchange coupling by confined electronic states in the spacer layer, and the so called Nèel “orange peel” coupling by magnetostatic interaction emerging from conformal interface roughness.

In addition to these effects we address also coupling mechanisms that become important if the specimen is either morphologically or magnetically non-uniform on the nanoscale. The former can be realized by patterning into small elements. Magnetic non-uniformity already arises if magnetic domains are present. An example is the interaction between ferromagnetic layers by local magnetic stray fields.

A suitable method to address such local coupling phenomena is the element-resolved imaging of magnetic domains using photoelectron emission microscopy (PEEM) with X-ray magnetic circular dichroism (XMCD) providing magnetic contrast. The element-sensitivity of XMCD can be turned to different magnetic layers, even to buried layers. It is such possible to study the local interaction between ferromagnetic layers containing different elements.

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Magnetization dynamics

Accelerating the operating speed of devices based on magnetoresistive effects implies control of the magnetic damping in ultrathin films and multilayers. It is determined by the time it takes to reverse the magnetization direction of one or more ferromagnetic layers. A thorough understanding and control of the magnetic damping is inevitable for establishing faster switching schemes that take advantage of the magnetization precession ("precessional switching").

Using ferromagnetic resonance (FMR) in ultra-high vacuum we try to identify the relevant mechanisms for the damping of the magnetization precession in such technologically relevant systems.

In contrast to FMR, where of the magnetization precession is probed in the frequency domain, we also investigate magnetization dynamics in “real time”. In this case, the sample is excited by laser pulses or short magnetic field pulses. The response of the sample is stroboscopically investigated by magnetic microscopy using photoelectron emission microscopy (PEEM) with X-ray magnetic circular dichroism (XMCD), providing magnetic contrast. Element-resolved microscopic magnetic domain images reveal the influence of, for example, coupling effects in magnetic multilayer systems or the fast magnetization reversal of one of the layers.

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The methods we use include:

Activity reports