1,408 research outputs found
Trace element content and magnetic properties of commercial HOPG samples studied by ion beam microscopy and SQUID magnetometry
In this study, the impurity concentration and magnetic response of nine
highly oriented pyrolytic graphite (HOPG) samples with different grades and
from different providers were determined using ion beam microscopy and SQUID
magnetometry. Apart from sideface contaminations in the as-received state, bulk
contamination of the samples in most cases consists of disk-shaped micron-sized
particles made of Ti and V with an additional Fe contamination around the grain
perimeter. The saturation magnetization typically increases with Fe
concentration, however, there is no simple correlation between Fe content and
magnetic moment. The saturation magnetization of one, respectively six, out of
nine samples clearly exceeds the maximum contribution from pure Fe or Fe3C. For
most samples the temperature dependence of the remanence decreases linearly
with T - a dependence found previously for defect-induced magnetism (DIM) in
HOPG. We conclude that apart from magnetic impurities, additional contribution
to the ferromagnetic magnetization exists in pristine HOPG in agreement with
previous studies. A comparative study between the results of ion beam
microscopy and the commonly used EDX analysis shows clearly that EDX is not a
reliable method for quantitative trace elemental analysis in graphite,
clarifying weaknesses and discrepancies in the element concentrations given in
the recent literature.Comment: submitted to Carbo
Comment on: "Revealing common artifacts due to ferromagnetic inclusions in highly oriented pyrolytic graphite", by M. Sepioni, R.R. Nair, I.-Ling Tsai, A.K. Geim and I.V. Grigorieva, EPL 97 (2012) 47001
This comment addresses several issues in the paper by Sepioni et al., where
it is stated that the ferromagnetism in pristine highly oriented pyrolytic
graphite (HOPG) reported by several groups in the previous years is most likely
due to impurity contamination. In this comment, clear arguments are given why
this statement is not justified. Furthermore, it is pointed out, that there are
already measurements using element-sensitive microscopic techniques, e.g. X-ray
Magnetic Circular Dichroism (XMCD) that directly proved the intrinsic origin of
the ferromagnetism in graphite, also in pristine HOPG.Comment: 1, 0 figures, 9 reference
-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 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 (XMCD). We
demonstrate that the magnetic order in the investigated system originates only
from the carbon -electron system.Comment: 10 pages 3 color figure
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 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 10 nm of
the irradiated sample where the actual magnetization reaches 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.Comment: 10 pages, 5 figures, 1 table, submitted to New Journal of Physic
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 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.Comment: 4 pages, 3 figure
3D-Hydrogen Analysis of Ferromagnetic Microstructures in Proton Irradiated Graphite
Recently, magnetic order in highly oriented pyrolytic graphite (HOPG) induced
by proton broad- and microbeam irradiation was discovered. Theoretical models
propose that hydrogen could play a major role in the magnetism mechanism. We
analysed the hydrogen distribution of pristine as well as irradiated HOPG
samples, which were implanted to micrometer-sized spots as well as extended
areas with various doses of 2.25 MeV protons at the Leipzig microprobe LIPSION.
For this we used the sensitive 3D hydrogen microscopy system at the Munich
microprobe SNAKE. The background hydrogen level in pristine HOPG is determined
to be less than 0.3 at-ppm. About 4.8e15 H-atoms/cm^2 are observed in the
near-surface region (4 um depth resolution). The depth profiles of the implants
show hydrogen located within a confined peak at the end of range, in agreement
with SRIM Monte Carlo simulations, and no evidence of diffusion broadening
along the c-axis. At sample with microspots, up to 40 at-% of the implanted
hydrogen is not detected, providing support for lateral hydrogen diffusion.Comment: accepted for publication in Nucl. Instr. and Met
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