Dr. Axel Enders - Research Nuggets

Stripes are Stars! Pt helps Fe to stay magnetized!

The magnetic anisotropy energy is among the most important functional properties of magnetic elements. It determines the orientation and stability of the magnetization as well as the mechanisms and the dynamics of the magnetization reversal. New materials with extremely large anisotropy values are currently sought for the development of ultrahigh density magnetic recording media. As one example, FePt or CoPt alloys exhibiting L10 structure are currently in the focus of intensive research, as they were found to exhibit unprecedented anisotropy values. It was recently discovered that also atomically small nanostructures of Fe and Pt can, under certain conditions, exhibit magnetic anisotropy values similar to those of their L10 bulk counterparts. This discovery was possible after advances in nanostructure synthesis have been made to embed Fe atoms as monatomic Fe stripes or in other configurations into the surface of Pt single crystals. It was found that the Pt plays a critical role in the magnetism of these structures, as it dictates the structure's magnetic anisotropy and thus helps the Fe to stay magnetized. Intriguingly, in the system studied, the interatomic exchange, magneto-crystalline anisotropy and Dzyaloshinski-Moriya interactions are all of the same order of magnitude, which leads to an interesting nanoscale interplay between ordinary magnetization states and noncollinear spin structures and hence to rather complex magnetic properties of the FePt nanostructures.

Interactions at the organics/metal interface

Designing molecular architectures in reduced dimensions is a potentially elegant and versatile approach towards the nanoscale control of complex matter. It allows scientists to engineer, atom-by-atom, molecule-by-molecule, highly organized systems with desired properties ranging from hydrogen storage and molecular switching to magnetic ordering. Porphyrin molecules are of particular interest because many different metal atoms can be inserted into the porphyrin ring, offering a wealth of possibilities to control, characterize and optimize properties. In our research, we create and analyze two-dimensionally ordered networks of porphyrin with various species of central metal atoms. The two-dimensional character of the structures allows us to probe the electronic and magnetic properties of individual elements within the networks. Our objective is to determine the interactions between the central metal atoms, such as Ni or Co, and the substrate (Ag or Pt), which can potentially result in magnetic ordering or useful catalytic properties.

Animation of the formation of the pillow effect of surface charges as a benzene approaches a Cu(111) surface. Calculations: E. Zurek and S. Simpson, SUNY Buffalo.

Self-Assembled Clusters and Cluster/Substrate Interactions

The creation of nanostructures with well-defined geometry, chemistry, and long-range periodicity is a key challenge in nanomagnetism. This includes but is not limited to information technology, where progress will be based on nanoscale monodisperse and aligned magnetic units, densely packed into an ordered monolayer and with stable remanent magnetization at room temperature, accessible switching fields, and negligible interactions. In this project, we synthesize clusters of Co on mechanically stable, periodically corrugated boron-nitride layers by repeated cycles of buffer-layer-assisted growth. Alternatively, we employed reconstructed surfaces, such as carbureated W(110) surfaces, to achieve templating of deposited clusters. The approach combines the advantages of well-established preparation methods for surface-supported clusters, namely the versatility of cluster deposition from the gas phase and the positional accuracy of the directed growth on template surfaces. Our work showed that the approach to full coverage is critically slowed down by attractive inter-particle interaction, which results in the coalescence and growth of some of the clusters. Ordered cluster layers exhibiting 80 terra clusters per square inch, as those in this work, would be suitable for application as patterned media for ultrahigh density magnetic data storage if the Co particles could be replaced by aligned FePt particles.

Visualization of Many-Body Wave Functions for Nanomagnetic Structures

Quantum mechanical calculations of nanoscale objects such as molecules and clusters help greatly in advancing our current understanding of materials and properties. They require, however, the consideration of the atomistic basis of such structures and usually result in challenging computations of many-body wave functions. We have combined state-of-the-art computational approaches with experiments to visualize quantum phenomena. We studied model systems such as tetraphenylporphyrin molecules designed to accommodate magnetic atoms. The high spatial precision of our scanning tunneling microscope (STM) is exploited to image selected molecular orbitals, thereby giving us a picture of many-body wave functions. The theoretical modeling of the STM images of these structures requires the consideration of a huge number of atoms. By comparing theory with experiment we are able to test important quantum-mechanical concepts, including the local partition-function theorem by Kohn and Yaniv, which shows that physical properties are largely determined on a local scale, even if the wave functions extend to infinity. Ultimately, this will help us understand how macroscopic properties emerge from quantum mechanics. This research is supported by the National Science Foundation, Division of Materials Research, Materials Research Sciences and Engineering Program, Grant 0820521.

Functional Magnetic Structures in Contact with Oxide Surfaces

Thin oxide films are studied with regards to their ferroelectric and pyroelectric properties. But we are also interested how the electric polarization of those films changes the properties of deposited adsorbates, such as ferromagnetic nanostructures. One example of this work are deposited Fe nanoclusters clusters on ultrathin films of BaTiO3. Our goal is to establish a model system in which a recently proposed, new type of magneto-electric effect can be studied experimentally. This new effect, proposed by Tsymbal et al. arises at the Fe-BaTiO3 interface and is due to changes in the Fe-Ti bond length at the interface during reversal of the electric polarization of the oxide. This effect is thus distinctively different from commonly observed magneto-elastic coupling that is responsible for the majority of experimentally observed magnetoelectric effects. An experimental challenge is the growth of near-perfect metal-oxide interface, due to the tendency of the metal to oxidize at the interface. In our new approach we employ atomically smooth BaTiO3 thin films and deposit Fe clusters using buffer layer assisted growth, thereby reducing the chemical reactivity during deposition and improving the interface quality. First X-ray magnetic circular dichroism studies have shown that the magnetic moment of smallest clusters of Fe is not suppressed, which indicates that no oxide is formed. A spin reorientation transition from in-plane to out-of-plane is observed for a Fe coverage of 0.5 atomic layers. The high structural quality achieved is of great promise for the study of the atomistic origin of magneto-electric interface effects. Other projects concentrate on imaging the magnetism in ultrathin films of Cr2O3, or chromia. We employ spin-polarized scanning tunneling microscopy to make the layer-wise antiferromagnetism in epitaxial chromia films visible. Ultimately, the magnetism in those films is switchable with strong electric fields and we will perform such studies using the strong E-field produced by the tip of our STM. The ultimate goal of these projects is to explore avenues to magnetic switching with applied electrical fields.