Coexistence of superconductivity and ferromagnetism in the graphite-sulphur system

Abstract

Superconducting characteristics such as the Meissner-Ochsenfeld state, screening supercurrents and hysteresis loops of type-II superconductors were observed from the temperature and magnetic field dependences of the magnetic moment, m(T, H), in graphite powders reacted with sulfur for temperatures below 9.0 K. The temperature dependence of the lower critical field H c1 (T) was determined and the zero-temperature penetration depth, λ(0), was estimated (λ(0) = 2270 Å). The superconductivity was observed to be highly anisotropic and to coexist with a ferromagnetic state that has a Curie temperature well above room temperature. A continuous transition from the superconducting state to the ferromagnetic state could be achieved by simply increasing the applied magnetic field.

Local superconductivity and ferromagnetism interplay in graphite-sulfur composites

“…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.…”

“…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].…”

“…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].…”

“…Fig. 2 shows the hysteresis loops m(H) -m o (H) measured with the ZFC procedure (H c) for T = 7, 8, 9 and 10 K, after subtraction of the diamagnetic background signal m o = χ H, where χ = −7.12 × 10 −6 emu g −1 Oe −1 for all these measured temperatures (for more details see [2]). Fig.…”

“…[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+ %).…”