Manifestation of finite temperature size effects in nanogranular magnetic graphite

作者：Sergeenkov S^*; Souza N S; Speglich C; Rivera V A G; Cardoso C A; Pardo H; Mombru A W; Araujo Moreira F M

来源：Journal of Applied Physics, 2009, 106(11): 116101.

DOI：10.1063/1.3264571

• Curie temperature
• grain size
• graphite
• magnetisation
• nanostructured materials
• size effect

In addition to the double phase transition (with the Curie temperatures T-C=300 K and T-Ct=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 similar or equal to 10 nm), which is attributed to the manifestation of the size effects below the quantum temperature T-L proportional to h(2)/L-2 and is well fitted by the periodic function M-L(T)proportional to sin[M(T)Lambda(T)/L] with M(T) being the bulk magnetization and Lambda(T)proportional to h/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 similar or equal to 12 meV (which defines the intragranular Curie temperature T-C) and proximity mediated interactions between neighboring grains (through potential barriers U created by thin layers of nonMG) with energy J(t)=exp(-d/xi)J similar or equal to 5.8 meV (which defines the intergranular Curie temperature T-Ct) with d similar or equal to 1.5 nm and xi proportional to h/U>similar or equal to 2 nm being the intergranular distance and characteristic length, respectively.

• 出版日期2009-12-1