In addition to the double phase transition (with the Curie temperatures TC=300K and TCt=144K), a low-temperature anomaly in the dependence of the magnetization is observed in the bulk magnetic graphite (MG) (with an average granular size of L10nm), which is attributed to the manifestation of the size effects below the quantum temperature TL2/L2 and is well fitted by the periodic function ML(T)sin[M(T)Λ(T)/L] with M(T) being the bulk magnetization and Λ(T)/T the thermal de Broglie wavelength. The best fits of the high-temperature data (using the mean-field Curie–Weiss and Bloch expressions) produced reasonable estimates for the model parameters, such as defects mediated effective spin exchange energy J12meV (which defines the intragranular Curie temperature TC) and proximity mediated interactions between neighboring grains (through potential barriers U created by thin layers of nonMG) with energy Jt=exp(d/ξ)J5.8meV (which defines the intergranular Curie temperature TCt) with d1.5nm and ξ/U2nm being the intergranular distance and characteristic length, respectively.

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