Revealing common artifacts due to ferromagnetic inclusions in highly oriented pyrolytic graphite

Abstract

We report on an extensive investigation to figure out the origin of room-temperature ferromagnetism that is commonly observed by SQUID magnetometry in highly-oriented pyrolytic graphite (HOPG). Electron backscattering and X-ray microanalysis revealed the presence of micron-size magnetic clusters (predominantly Fe) emu/g, i.e., 5 orders of magnitude less than the saturation magnetization of Fe, and the whole subject remains controversial, especially concerning the role of possible contamination, as well as the mechanism responsible for the strong interaction required to lead to ferromagnetic ordering at room temperature.Trying to clarify the situation, we have carried out extensive studies of magnetic behaviour of HOPG crystals obtained from different manufacturers (ZYA-, ZYB-, and ZYH-grade from NT-MDT and SPI-2 and SPI-3 from SPI Supplies). These crystals are commonly used for studies of magnetism in graphite; e.g., ZYA-grade crystals were used in refs. [1,[3][4][5][6][7] and ZYH-grade in ref.[2]. We have also observed weak ferromagnetism, independent of temperature between 300K and 2K and similar in value to the one reported previously for pristine (non-irradiated) HOPG. Below, we show that the observed ferromagnetism in ZYA-, ZYB-, and ZYH-grade crystals is due to micron-sized magnetic inclusions (containing mostly Fe), which can easily be visualized by scanning electron microscopy (SEM) in the backscattering mode. Without the intentional use of this technique, the inclusions are easy to overlook. No such inclusions were found in SPI crystals and, accordingly, in our experiments these crystals were purely diamagnetic at all temperatures (no ferromagnetic signals at a level of 10 -5 emu/g).

Disentangling defect-induced ferromagnetism in SiC

“…The impurity concentration of Fe or Ni is of the range of 10 14 /cm 3 , which corresponds to 0.015 µg/g 37 . This impurity level is around one order of magnitude lower than that of the purest graphite samples 38 , for which the origin of the defect induced magnetism is still under intensive discussion [39][40][41] . Therefore, SiC presents a suitable model system for studying defect-induced ferromagnetism in addition to carbon based materials.…”

Comment on “Revealing common artifacts due to ferromagnetic inclusions in highly oriented pyrolytic graphite” by Sepioni M. et al .

“…Furthermore, it is important to emphasize that whatever the ultimate origin of the ferromagnetic signals found by SQUID magnetometry, also in pristine, unirradiated ZY graphite samples, one can clearly rule out that "...the contamination could be the reason for the reported ferromagnetism", as the authors claimed in [1]. In most of the alluded works [3,6,9,7], and in some other publications, the net magnetization difference between the same sample after and before ion irradiation is plotted, or else both measurements are shown together.…”

“…Apparently the authors in [1] are not aware of the proper use of the Particle Induced X-ray Emission (PIXE) technique and its ability for elemental imaging with lateral resolution of ≈ 1 µm. As it is stated in previous publications [2,3] PIXE is typically carried out using 2000 keV protons (not 200 keV as stated in Ref.…”

“…As it is stated in previous publications [2,3] PIXE is typically carried out using 2000 keV protons (not 200 keV as stated in Ref. [1]), which penetrate 38 µm deep into graphite and excite X-rays efficiently up to a depth of ≈ 20 µm. The absorption of the K-X-rays of e.g.…”

“…the authors in [1] to read the nowadays broad literature on the defect-induced magnetism (DIM) in pure carbonbased and other materials more carefully.…”