Manifestation of finite temperature size effects in nanogranular magnetic graphite

TL;DR

This paper investigates finite temperature size effects in nanogranular magnetic graphite, revealing double phase transitions and low-temperature anomalies linked to grain size and quantum effects.

Contribution

It provides experimental evidence and theoretical analysis of size effects and intergranular interactions in magnetic graphite with 10nm grains, including model parameter estimates.

Findings

Identification of double phase transition at 300K and 144K

Observation of low-temperature anomaly related to size effects

Estimation of exchange energies J and J_t from data

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.