Manifestation of finite temperature size effects in nanogranular magnetic graphite

S. Sergeenkov, N. S. Souza, C. Speglich, V. A.G. Rivera, C. A. Cardoso, H. Pardo, A. W. Mombrú, F. M. Araújo-Moreira

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In addition to the double phase transition (with the Curie temperatures TC =300 K and TCt =144 K), a low-temperature anomaly in the dependence of the magnetization is observed in the bulk magnetic graphite (MG) (with an average granular size of L≃10 nm), which is attributed to the manifestation of the size effects below the quantum temperature T L∞h2/L2 and is well fitted by the periodic function ML (T) sin [M (T) (T) /L] with M (T) being the bulk magnetization and (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 J≃12 meV (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/) J≃5.8 meV (which defines the intergranular Curie temperature TCt) with d≃1.5 nm and /U≃2 nm being the intergranular distance and characteristic length, respectively.

Original languageEnglish
Article number116101
JournalJournal of Applied Physics
Issue number11
StatePublished - 2009
Externally publishedYes

Bibliographical note

Funding Information:
This work has been financially supported by the Brazilian agencies CNPq, CAPES, and FAPESP.


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