Magnetic Susceptibility of Carbons and Polycrystalline Graphites. II

Abstract

The change in magnetic susceptibility of carbons of varying crystallite sizes resulting from a shift of the Fermi level has been studied by investigating residue compounds of carbons with bisulfate ions. The susceptibility decreases monotonically with the concentration of the ions and the relative decrease is progressively larger for larger crystallites. As a result, instead of the plateau in susceptibility in the case of untreated carbons extending from diameters of 150 A to sizes greater than 1500 A, one finds a maximum in susceptibility at about 200 A diameter for carbons with a finite (not too small) ion concentration. The temperature dependence of susceptibility for both untreated carbons and residue compounds has also been studied over the temperature range from 80\ifmmode^\circ\else\textdegree\fi{}K to 1300\ifmmode^\circ\else\textdegree\fi{}K. The susceptibilities of the untreated carbons all decrease at higher temperatures and tend towards a temperature-independent limit at low temperatures. For small crystallites, the rapid decrease begins at higher ambient temperature, and the low-temperature limit is smaller. The plateau in susceptibility as a function of crystallite size is present for all ambient temperatures, but the beginning of the plateau shifts somewhat to smaller crystallite size for higher ambient temperatures. A qualitative discussion is given which leads to the conclusion that the Landau-Peierls theory is inadequate to explain all the experimental findings if the simplified band model deduced from other electronic properties of carbons is used.