T. Butz

Induced Magnetic Ordering by Proton Irradiation in Graphite

We provide evidence that proton irradiation of energy 2.25 MeV on highly oriented pyrolytic graphite samples triggers ferro- or ferrimagnetism. Measurements performed with a superconducting quantum interferometer device and magnetic force microscopy reveal that the magnetic ordering is stable at room temperature.

pi-electron ferromagnetism in metal-free carbon probed by soft x-ray dichroism.

Elemental carbon represents a fundamental building block of matter and the possibility of ferromagnetic order in carbon has attracted widespread attention. However, the origin of magnetic order in such a light element is only poorly understood and has puzzled researchers. We present a spectromicroscopy study at room temperature of proton irradiated metal-free carbon using the elemental and chemical specificity of x-ray magnetic circular dichroism. We demonstrate that the magnetic order in the investigated system originates only from the carbon pi-electron system.

The role of hydrogen in room-temperature ferromagnetism at graphite surfaces

We present a x-ray dichroism study of graphite surfaces that addresses the origin and magnitude of ferromagnetism in metal-free carbon. We find that, in addition to carbon {pi} states, also hydrogen-mediated electronic states exhibit a net spin polarization with significant magnetic remanence at room temperature. The observed magnetism is restricted to the top {approx}10 nm of the irradiated sample where the actual magnetization reaches {approx_equal} 15 emu/g at room temperature. We prove that the ferromagnetism found in metal-free untreated graphite is intrinsic and has a similar origin as the one found in proton bombarded graphite.

Experimental evidence for two-dimensional magnetic order in proton bombarded graphite

We have prepared magnetic graphite samples bombarded by protons at low temperatures and low fluences to attenuate the large thermal annealing produced during irradiation. An overall optimization of sample handling allowed us to find Curie temperatures

The novel ultrastable HVEE 3.5 MV Singletron™ accelerator for nanoprobe applications

T_c \gtrsim 350

Magnetic order in graphite: Experimental evidence, intrinsic and extrinsic difficulties

K at the used fluences. The magnetization versus temperature shows unequivocally a linear dependence, which can be interpreted as due to excitations of spin waves in a two dimensional Heisenberg model with a weak uniaxial anisotropy.

Quantitative microanalysis of perineuronal nets in brain tissue

Abstract Recently, HVEE has completed a novel 3.5 MV single ended accelerator (Singletron™) for the University of Leipzig, Germany. For one of the main applications, the system will be connected to a nanobeamline to achieve submicron resolution. Because the energy stability and ripple of the beam, and beam brightness are of vital importance for the performance of a nanoprobe, special care has been taken in optimizing these parameters. The system consists of an RF source which is directly mounted on the accelerator tube, a switching magnet to bend the beam into a chamber for standard analysis purposes and an analysis magnet that directs the beam into the nanoprobe. The stability of the beam energy was measured at a terminal voltage of 1.881 MV. These measurements were taken during factory acceptance with large production equipment operational, which negatively influenced the stability of the mains. The measured stability was found to be approx. ±50 eV over 5 h, but it is anticipated that this figure will be as good as ±20 eV (i.e. ∼ 10−5) under normal laboratory conditions. The terminal voltage ripple was measured at 2.25 MV to be 25 Vpp (i.e. ∼ 1.1 × 10−5). Finally, the beam brightness of a 2.25 MeV hydrogen beam was measured by the use of two micrometer slit systems. A brightness of approx. 18 Amps · rad−2 m−2 eV−1 was obtained. In this article we will describe the considerations which have led to the layout of the present system.

From micro- to nanoprobes: auspices and horizons

We discuss recently obtained data using different experimental methods including magnetoresistance measurements that indicate the existence of metal-free high-temperature magnetic order in graphite. Intrinsic as well as extrinsic difficulties to trigger magnetic order by irradiation of graphite are discussed in view of recently published theoretical work.

Active compensation of stray magnetic fields at LIPSION

Abstract The relevance of the perineuronal nets (specialised extracellular matrix surrounding a part of the neurons in brain tissue) as a possible protection of neurons against oxidative stress induced by metal ions (e.g. Al, Fe, Cu and Zn) is an actually discussed hypothesis. It is assumed that the perineuronal nets are able to bind metal ions and thus reduce the oxidative stress to neurons. Therefore, we used nuclear microscopy (μPIXE) in order to investigate the concentration and distribution of iron in rat brain loaded with colloidal iron with special emphasis to the perineuronal nets in the extracellular matrix. The elemental microanalysis was performed on 6 μm thin resin embedded sections. The perineuronal nets accumulated more Fe than other extracelullar matrix components leading to well defined, neuron-related structures in the Fe maps. In order to quantify the affinity, the iron accumulations in the perineuronal nets were analysed for different Fe loadings. We present first results that support the hypothesis that perineuronal nets act as chemical protection mechanism against oxidative stress due to their ability to bind metal ions.

Identification of air pollution sources by single aerosol particle fingerprints - micro-PIXE spectra

High energy ion microprobes with submicrometre resolution are expected to develop into true nanoprobes provided the resolution limiting problems can be solved. Thus far, a drastic improvement of the ion source brightness appears to be the most promising route. A few examples of feasible, but rather demanding applications of nanoprobes are given.