A. Setzer

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.

Room temperature ferromagnetism in ZnO films due to defects

ZnO films were prepared by pulsed laser deposition on a-plane sapphire substrates under N2 atmosphere. Ferromagnetic loops were obtained with the superconducting quantum interference device at room temperature, which indicate a Curie temperature much above room temperature. No clear ferromagnetism was observed in intentionally Cu-doped ZnO films. This excludes that Cu doping into ZnO plays a key role in tuning the ferromagnetism in ZnO. 8.8% negative magnetoresistance probed at 5K at 60kOe on ferromagnetic ZnO proves the lack of s-d exchange interaction. Anomalous Hall effect (AHE) was observed in ferromagnetic ZnO as well as in nonferromagnetic Cu-doped ZnO films, indicating that AHE does not uniquely prove ferromagnetism. The observed ferromagnetism in ZnO is attributed to intrinsic defects.

Ferromagnetism in oriented graphite samples

We have studied the magnetization of various well characterized samples of highly oriented pyrolitic graphite (HOPG), Kish graphite, and natural graphite to investigate the recently reported ferromagneticlike signal and its possible relation to ferromagnetic impurities. The magnetization results obtained for HOPG samples for applied fields parallel to the graphene layers---to minimize the diamagnetic background---show no correlation with the magnetic impurity concentration. Our overall results suggest an intrinsic origin for the ferromagnetism found in graphite. We discuss possible origins of the ferromagnetic signal.

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.

Room temperature ferromagnetism in Mn-doped ZnO films mediated by acceptor defects

Mn-doped ZnO films with preferred c-axis growth orientation were prepared by pulsed laser deposition under N2 atmosphere on a-plane sapphire substrates. Large positive magnetoresistance amounting to 60% was observed at 5K. Clear anomalous Hall effect was observed at 20K. Ferromagnetism with Curie temperature higher than 290K has been observed, and a deep acceptor trap due to Zn vacancies with a thermal activation energy amounting to 0.815eV has been detected by deep-level transient spectroscopy. For comparison, only paramagnetism was observed in Mn-doped ZnO films with donor traps prepared under O2 atmosphere. Their results clearly demonstrate that the ferromagnetism in Mn-doped ZnO originates from the parallel alignment of magnetic moments mediated by acceptor defects.

Magnetic-field-driven superconductor–insulator-type transition in graphite

Abstract A magnetic-field-driven transition from metallic- to semiconducting-type behavior in the basal-plane resistance takes place in highly oriented pyrolytic graphite at a field H c ∼1 kOe applied along the hexagonal c-axis. The analysis of the data reveals a striking similarity between this transition and that measured in thin-film superconductors and Si MOSFETs. However, in contrast to those materials, the transition in graphite is observable at almost two orders of magnitude higher temperatures.

Observation of intrinsic magnetic domains in C60 polymer

A C60 polymer has been characterized for the first time with respect to impurity content and ferromagnetic properties by laterally resolved particle induced X-ray emission (PIXE), superconducting quantum interference device (SQUID) and magnetic force microscopy (MFM) in order to detect intrinsic ferromagnetic domains. In parts of the pure regions (concentration of magnetic impurities <1 μg/g), we found stripe-domain magnetic images with different orientations of domain magnetization. The size of the regions where magnetic domains were observed is ∼30% of the pure region. All these results reveal that the polymerized C60 sample is a mixture of magnetic and non-magnetic parts and only a fraction of the sample contributes to the ferromagnetism.

Can Doping Graphite Trigger Room Temperature Superconductivity? Evidence for Granular High‐Temperature Superconductivity in Water‐Treated Graphite Powder

Granular superconductivity in powders of small graphite grains (several tens of micrometers) is demonstrated after treatment with pure water. The temperature, magnetic field and time dependence of the magnetic moment of the treated graphite powder provides evidence for the existence of superconducting vortices with some similarities to high-temperature granular superconducting oxides but even at temperatures above 300 K. Room temperature superconductivity in doped graphite or at its interfaces appears to be possible.

Defect-Induced Magnetism in Solids

In recent years, the number of nominally nonmagnetic solids showing magnetic order induced by some kind of defect has increased continuously. From the single element material graphite to several covalently bonded nonmagnetic compounds, the influence of defects like vacancies and/or nonmagnetic ad-atoms on triggering magnetic order has attracted the interest of experimentalists and theoreticians. We review and discuss the main theoretical approach as well as recently obtained experimental evidence based on different experimental methods that support the existence of defect-induced magnetism in nonmagnetic as well as in magnetic materials.

Interfacial strain effects in epitaxial multiferroic heterostructures of PbZrxTi1−xO3∕La0.7Sr0.3MnO3 grown by pulsed-laser deposition

Ferroelectric PbZrxTi1−xO3 and ferromagnetic La0.7Sr0.3MnO3 films were grown on SrTiO3(100) substrates in order to fabricate multiferroic epitaxial heterostructures. Multilayers of PbZr0.2Ti0.8O3∕La0.7Sr0.3MnO3 with 5nm thin individual layers preserve good magnetic properties and have a transition temperature of ∼320K. The variation of the magnetic coercive field of thin La0.7Sr0.3MnO3 films, sandwiched between PbZrxTi1−xO3 films of increasing Zr content in the same epitaxial heterostructure, demonstrates the influence of the interfacial biaxial strain.