Thermal stability of the magnetic moment in amorphous carbon thin film - An experimental and ab-initio study

Abstract

The effect of vacuum annealing on the microstructure and its consequence on the thermal stability of magnetic moment in amorphous carbon (a-C) thin film is studied. The magnetic a-C thin film of thickness 8 nm is vacuum annealed for 2 h at 523 K and 923 K. Microstructural and chemical characterization using Raman spectroscopy and x-ray photoelectron spectroscopy, respectively, reveals that with increasing annealing temperature, the sp2 fraction in the film increases. The magnetic measurement carried out at room temperature (RT) using SQUID-VSM reveals a weak ferromagnetic signal in the film due to the interaction between the unpaired sp2 sites, which prevails till 923 K. In the as-deposited sample, an isolated sp2 atom surrounded by sp3 atom results in the paramagnetic contribution, and an sp2-sp2 atom with twisted π-orbitals results in weak ferromagnetic coupling. Compared with the as-deposited case, initially, the saturation magnetization increases when the sample is annealed at 523 K and later decreases when annealed at 923 K. Subsequently, the ab-initio molecular dynamics calculation predicts that due to the increase in sp2 fraction, the net magnetization of the system increases from 300 K to 523 K. π-Orbital Axis Vector (POAV) analysis shows that further annealing to 923 K enhances π-orbital overlapping, which leads to the annihilation of moments sites. The present study differentiates between the two structures that contribute to the magnetism in a-C in terms of their thermal stability. Our simulation indicates that the twisted π-orbitals structure is thermodynamically more stable than the structure where sp3 surrounds sp2 hybridized atom.