Trace element content and magnetic properties of commercial HOPG samples studied by ion beam microscopy and SQUID magnetometry

Abstract

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 Fe 3 C. For most samples the temperature dependence of the remanence decreases linearly with T -a dependence found previously for defectinduced 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.

Defect-induced magnetism in SiC: Interplay between ferromagnetism and paramagnetism

“…The broad bump is due to the secondary electron Bremsstrahlung background. For commercially available, purest graphite, there is detectable transition metal contamination (mainly Ti, V, Fe, Ni) as revealed by PIXE [39]. In sharp contrast, the transition metal impurity in the SiC we used for this study, if there is, is below the detection limit of around 1 ppm (Fig.…”

“…in our SiC substrates, we have performed PIXE using 2 MeV protons with a broad beam of around 1 mm 2 , as shown in Figure 1. PIXE is a sensitive method to detect trace impurities in bulk volume without significant structural destruction [35,39]. In the spectrum, the sharp peak is the Si K-line X-ray emission.…”

“…On the one hand, measurement artifacts in SQUID magnetometry may occur: improper mounting of samples and wrong use of sample holders can easily generate ferromagnetic like signal [32][33][34]. On the other hand, the debate over the purity of graphite and oxide substrates continues in parallel [35][36][37][38][39]. Pristine graphite and oxides substrates are often ferromagnetic due to different contaminations or due to intrinsic defects [36,[40][41][42], which hamper the interpretation of the observed ferromagnetism.…”

Growth of Pt-Ni Nanoparticles of Different Composition using Electrodeposition and Characterization of Their Magnetic Properties

“…This was expected since bulk Pt 3 Ni alloys have no net magnetic moment in comparison to PtNi 3 [5]. It should be noted that a small difference between ZFC and FC data measured on HOPG (and Pt 3 Ni nanoparticles on HOPG) is an evidence of already well-known defectinduced ferromagnetism observed in several carbon-based materials [6]. Although, only about 3 µg of the alloy (as estimated from the particle size distribution and density of deposited nanoparticles) is contributing to the total magnetic moment, a clear evidence of dominant magnetic moment of nanoparticles was obtained for PtNi 3 sample.…”

Evidence for room temperature superconductivity at graphite interfaces

“…For comparison, we measured also: the untreated graphite powder (RWA-virgin), a pellet obtained from a high-purity graphite cylinder used for spectroscopy calibration (SK-AS01) and a graphite bulk sample (HOPG-AC-ZYA) from Advanced Ceramics, which impurity concentration and magnetization behavior was throughly characterized in [13]. This last bulk sample shows a ferromagnetic response for fields parallel to the graphene planes, originated by defects and/or hydrogen with a saturation field of µ 0 H sat 0.2 T [13].…”