Manifestation of finite temperature size effects in nanogranular magnetic graphite
Abstract
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 (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 TC) and proximity mediated interactions between neighboring grains (through potential barriers created by thin layers of non-magnetic graphite) with energy Jt=exp(-d/s)J=5.8meV (which defines the intergranular Curie temperature TCt) with d=1.5nm and s=2nm being the intergranular distance and characteristic length, respectively.