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Molecular Magnets


Molecular magnetism deals with the magnetic properties of both isolated and assemblies of molecules. The research interest in molecular magnetism is motivated by the need for a better understanding of the fundamental principles that govern magnetic behavior, as well as the need for designing new magnetic materials in bottom-up routines.
Some molecular magnets possess a highly symmetrical frustrated spin structure and provide us with an ideal test-bed for the theory on frustrated quantum spin systems. By means of both polarized neutron scattering and inelastic neutron scattering we are able to study the unusual spin-spin interactions and magnetic excitations in geometrically frustrated molecular magnetic systems, e.g. Mo72M30 (M=Fe, Cr, V). In a further step, molecular magnets will be assembled on substrates. Such systems of interacting molecular magnets could serve as both possible model systems for magnetic data storage and also as candidates for future quantum computing.

Ulrich Rücker

Magnetic Nanoparticles

Nanoparticles can be considered as multi-functional 'building blocks' for novel materials with tunable physical and chemical properties. In terms of future information technology and energy research, nanoparticles with magnetic, optic and electronic properties are of particular interest. Magnetic nanoparticles can be used for novel data storage media, spintronic devices or permanent magnets with improved properties. Plasmonic nanoparticles could be used for telecommunication or solar cells and the use of semiconducting nanoparticles in electronics and quantum information systems is currently being discussed .  

When different functionalities are combined into one 'nanoparticle composite', it becomes possible to produce multi-functional materials with completely novel properties, e.g. magneto-plasmonic or magneto-electric properties.


Nano particles JCNS

The figure shows polarized neutron diffraction data on MnO nanoparticles (A) and FePt-MnO nanoparticle dimers (B) obtained from the DNS instrument (MLZ) at the JCNS outstation in Garching (in collaboration with the group of Prof. W. Tremel, University of Mainz). With this technique it is possible to separate the 'nuclear' (structural) and magnetic contribution to the scattering. Consequently we can systematically study the magnetic ordering inside the particles.

Further projects with nanoparticles comprise the study of self-organized, regular arrangements of nanoparticles, i.e. so-called 'supracrystals'. Here we focus on the collective magnetic properties of such supracrystals, how these properties can be controlled, which inter-particle interactions are relevant and how we can systematically tune these interactions.  The magnetic inter-particle correlations can be studied using polarized neutron scattering at small angles (GISANS), depicted in (C). The detector image is a real experimental result obtained  using the instrument MARIA (MLZ) at the JCNS outstation in Garching (in collaboration with Dr. S. Disch, ILL, and the group of Prof. L. Bergström, University Stockholm, Sweden).

Oleg Petracic

Ordering phenomena in transition metal oxide heterostructures


With the ongoing miniaturization of electronic devices, interface effects become more and more important. Moreover, interfaces can induce material properties which are absent in bulk materials. Transition metal oxides are the most suitable candidates for new interface effects, because they come with a broad variety of properties themselves. These properties, originating from the highly correlated electronic system, range from magnetism to piezoelectricity. With these prerequisites we can deliberately change the charge and orbital order at the layer boundaries and thus manipulate magnetic and electric properties, for example. We produce multilayers of manganites with molecular beam epitaxy and high pressure sputtering. We are able to grow films to an accuracy of one unit cell by monitoring the intensity of the RHEED pattern during growth. Additionally, polarized neutron reflectometry is the method of choice to determine the magnetic depth profile of multilayers. For structural characterization we use X-ray reflectometry and X-Ray diffractometry.

The figure shows the angle-dependent reflectivity for X-rays of a 20-fold bilayer of SrMnO3/LaMnO3. In the background, a RHEED image is shown, used during the growth to determine the layer boundaries. In the lower left corner, a LEED image shows how the spacing of the surface atoms can be deduced. The illustration in the upper right shows a heterostructure of different Perovskites. On top of that, you see an AFM picture of the surface, where the atomic steps can be distinguished.

In future, we plan to increase the structuring of heterostructures in order to pave the way for new spintronic devices.

Anirban Sarkar

Layer-by-layer Magnetometry on Neutron-Polarizing Supermirrors


Polarizing supermirrors are commonly used for the polarization of cold neutron beams. They consist of a series of ferromagnetic and non-magnetic layers with a gradient in the layer thicknesses. In the figure, reflectivity and off-specular scattering of polarized neutrons with polarization analysis is shown, recorded during the re-magnetization process of a remanent supermirror. From these measurements, we can deduce that the bottommost magnetizations flip first along the applied magnetic field and that magnetic fluctuations exist in the layers whose magnetizations have not yet flipped. Those affirmations are confirmed by simulations of the measured intensities within the framework of the Distorted Wave Born Approximation (DWBA), whereupon the dynamic effects of perturbation theory are taken into account.

Emmanuel Kentzinger

Interface Characterization of GMR Systems


The magnetoelectronics properties (interlayer coupling, magnetoresistance) of thin film devices depend strongly on their interface morphologies. A non-destructive characterization can be performed by x-ray scattering under grazing incidence. The form factor contrast between the layers can be significantly enhanced by anomalous scattering at a synchrotron radiation source. The figure shows off-specular scattering from an Fe/Cr/Fe trilayer at two x-ray energies, once for vanishing contrast and once for maximal contrast between Fe and Cr, leading to a very different modulation of the measured intensities.

Emmanuel Kentzinger

Magnetic Nanostructures


Here we investigate structural and magnetic correlations in thin film systems relevant to magneto-electronics. These correlations are studied on a length scale close to the spin diffusion length of the electrons. They can be used to observe interfacial roughness, magnetic domains or magnetic roughness.

Emmanuel Kentzinger

Rare Earth Superlattices


Here we investigate structural and magnetic correlations in thin film systems relevant to magneto-electronics. These correlations are studied on a length scale close to the spin diffusion length of the electrons. They can be used to observe interfacial roughness, magnetic domains or magnetic roughness.

Jörg Voigt

Laterally-structured Layered Magnetic Systems


The magnetization behavior and magnetic domain formation in magnetic layered structures are mainly governed by the crystalline anisotropy of the ferromagnetic layers, the interlayer coupling, the shape anisotropy of the thin films (dipolar forces) and the external magnetic field.
The structuring of selected layers with electron or laser interference lithography (e.g. into stripes of 500 nm width) induces additional shape anisotropy in the structured layers and provides a means of influencing the domain state. We use off-specular polarized neutron scattering under grazing incidence to investigate on the one hand, the interaction between the stripes due to the magnetic coupling of the unstructured layers and on the other, dipolar forces, to obtain information on the magnetic domain formation in the individual layers of the nano-structured sample.

Ulrich Rücker