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arXiv · 0912.5347

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. Mombru·F. M. Araujo-Moreira
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Abstract

In addition to the double phase transition (with the Curie temperatures T_C=300K and T_{Ct}=144K), a low-temperature anomaly in the dependence of the magnetization is observed in the bulk magnetic graphite (with an average granular size of L=10nm), which is attributed to manifestation of the size effects below the quantum temperature. 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=12meV (which defines the intragranular Curie temperature T_C) and proximity mediated interactions between neighboring grains (through potential barriers created by thin layers of non-magnetic graphite) with energy J_t=exp(-d/s)J=5.8meV (which defines the intergranular Curie temperature T_{Ct}) with d=1.5nm and s=2nm being the intergranular distance and characteristic length, respectively.

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S. Sergeenkov, N. S. Souza, C. Speglich, V. A. G. Rivera, C. A. Cardoso, H. Pardo, A. W. Mombru, F. M. Araujo-Moreira. 2009-12-29. Manifestation of finite temperature size effects in nanogranular magnetic graphite. https://doi.org/10.1063/1.3264571

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