R. Höhne

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.

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.

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.

Can carbon be ferromagnetic

In pure carbon materials, such as fullerenes and highly oriented pyrolytic graphite (HOPG), novel magnetic and electrical properties have recently been discovered. In particular, the discovery of weak ferromagnetism in rhombohedral C 6 0 polymers (RhC 6 0 ) has attracted the attention of the scientific community and the press. In this paper, the ferromagnetic correlations found in HOPG and Rh-C 6 0 are commendted upon.

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.

Retraction: Magnetic carbon.

This corrects the article DOI: 10.1038/35099527

Magnetism in carbon structures

We discuss different magnetic phenomena observed in carbon-based structures, in particular the diamagnetism, paramagnetism and ferromagnetism observed in graphite, disordered carbon, fullerenes and irradiated carbon structures.

A comparison of the magnetic properties of proton- and iron-implanted graphite

Abstract.In this work we have investigated the changes of the magnetic properties of highly oriented pyrolytic graphite samples after irradiation either with ~3×1014 protons or 3.5×1013 ... 3.5×1014 iron ions with energies in the MeV range. Our results show that iron and proton irradiations can produce similar paramagnetic contributions depending on the implantation temperature. However, only protons induce a ferromagnetic effect.

Exchange anisotropy in epitaxial Fe3O4/CoO and Fe3O4/CoxFe3−xO4 bilayers grown by pulsed laser deposition

Magnetic exchange anisotropy has been studied in epitaxial Fe3O4 based bilayers grown by pulsed laser deposition on MgO substrates. Measurements of the exchange biasing of the Fe3O4 magnetization curves were performed on (100)- and (111)-oriented Fe3O4 (20 nm)/CoO (20 nm) bilayers and (100)-oriented Co0.16Fe2.84O4 (190 nm)/Fe3O4 (40–370 nm) bilayers. The effective exchange bias field Heb and coupling energy were found to be 10–102 kA/m and 1–2 mJ/m2, respectively. The dependence of Heb on the thickness d of the soft magnetic component (magnetite) of the cobalt ferrite/magnetite bilayer Heb∼1/d is found to be in agreement with the predictions of a Stoner–Wohlfarth-type model. The order of magnitude and the temperature dependence of Heb for the Fe3O4/CoO bilayers are similar to those of bilayers prepared by molecular beam epitaxy.Magnetic exchange anisotropy has been studied in epitaxial Fe3O4 based bilayers grown by pulsed laser deposition on MgO substrates. Measurements of the exchange biasing of the Fe3O4 magnetization curves were performed on (100)- and (111)-oriented Fe3O4 (20 nm)/CoO (20 nm) bilayers and (100)-oriented Co0.16Fe2.84O4 (190 nm)/Fe3O4 (40–370 nm) bilayers. The effective exchange bias field Heb and coupling energy were found to be 10–102 kA/m and 1–2 mJ/m2, respectively. The dependence of Heb on the thickness d of the soft magnetic component (magnetite) of the cobalt ferrite/magnetite bilayer Heb∼1/d is found to be in agreement with the predictions of a Stoner–Wohlfarth-type model. The order of magnitude and the temperature dependence of Heb for the Fe3O4/CoO bilayers are similar to those of bilayers prepared by molecular beam epitaxy.