Local superconductivity and ferromagnetism interplay in graphite-sulfur composites

Abstract

The superconductivity of graphite-sulfur composites is highly anisotropic and associated with the graphite planes. The superconducting state coexists with the ferromagnetism of pure graphite, and a continuous crossover from superconducting to ferromagnetic-like behavior could be achieved by increasing the magnetic field or the temperature. The angular dependence of the magnetic moment m(α) provides evidence for an interaction between the ferromagnetic and the superconducting order parameters.An interplay between ferromagnetic (FM) and superconducting (SC) behavior of graphite and related systems has recently been observed [1,2] and theoretically analyzed [3,4]. In particular, it has been demonstrated that the SC of graphite-sulfur (C-S) composites occurring within some sort of "grains" or domains [5,2] is highly anisotropic and associated with the graphite planes [2]. The SC domains coexist with the FM of pure graphite [1], and a continuous crossover from SC to FM-like behavior could be achieved increasing either the applied magnetic field H or the temperature T.Here we focus our attention on the highly anisotropic nature of the SC state of the C-S composites, which was explored by means of the angular dependence of the sample magnetic moment m(α, T, H), where α is the angle between the applied magnetic field H and the largest sample surface. The main conclusion of this work is that SC and FM order parameters interact in such a way that the FM component of m(α, T, H) is rotated by 90 • below the SC transition temperature T c (H).The here studied graphite-sulfur sample was thoroughly characterized in Ref. [2]. In summary, the C-S sample was prepared using graphite rods from Carbon of America Ultra Carbon, AGKSP grade, ultra "F" purity (99.9995%) (AlfaAesar, # 40766) and sulfur chunks from American Smelting and Refining Co. that are spectrographically pure (99.999+ %). A pressed pellet (φ = 6 mm, ∼7000 lb) of graphite was prepared by pressing graphite powder, the graphite powder was produced by cutting and grinding the graphite rod on the edge and side area of a new and clean circular diamond saw blade. The graphite pellet was encapsulated with sulfur chunks (mass ratio ∼ 1:1) in quartz tube under 1/2 atmosphere of argon and heat treated in a tube furnace at 400 • C for one hour and then slowly cooled (4 • C/h) to room temperature. X-ray diffraction measurements (θ-2θ geometry and rocking curves) of the reacted sample yielded a spectrum with only the superposition of the (00 ) diffraction peaks of graphite with the orthorhombic peaks of sulfur with no extra peak due to a compound, second phase or impurity. The c-axis lattice parameter (c = 6.72Å) of the sample is equal to the pristine graphite powder pellet, which testifies against sulfur intercalation. The diffraction pattern also shows a strong (00 ) preferred orientation, which was confirmed by rocking curve scans that give a ∆θ = 6 • (FWHM) for the (002) peak, due to the highly anisotropic (plate-like) shape of the graphite grains. The sample (∼ 5 × 2.5 × 1.7m...

Interaction between superconducting and ferromagnetic order parameters in graphite-sulfur composites

“…4(a), it can be seen that m(α) ~ sin α for T = 12 K and m(α) ~ sin (α + 90°) = cos α for T = 5 K. These oscillatory behaviors imply that the magnetic response of the sample can vary from paramagnetic (or ferromagnetic) to diamagnetic depending simply on the sample/field configuration used during the measurement. The m(α, T, H) behavior of this graphite-sulfur sample in the normal state, T = 12 K, is essentially the same as found in pure HOPG graphite samples, which will be reported elsewhere [19]. The m(α) dependence for T = 5 K of Fig.…”

“…should describe all types of m(α, H, T) behavior. The continuous lines through the experimental data of figures 4(a-f) are fits with this function (1), which also describes well the m(α, H, T) dependences of pure HOPG samples [19].…”

The Magnetization of a Composite Based on Reduced Graphene Oxide and Polystyrene

“…One paper [ 6 ] demonstrated that the superconductivity of graphite–sulfur composites occurs within some sort of “grains” or domains. In particular, magnetization measured at T = 7 K showed a characteristic type II superconductor hysteresis loop.…”