Temperature driven magnetic transitions in FePd ₃ filled monolayer carbon foam and Fe ₃ C/α-Fe filled carbon nanotubes

An unusual observation of magnetic transition and Pd/C rehybridization effects is reported in novel monolayer carbon foam materials filled with doubled FePd ₃ crystal lattices. We reveal the presence of not previously observed sharp Pd/C interfaces, as confirmed by X-ray photoelectron spectroscopy, which give rise to localized paramagnetic features in the ESR spectra for a g value of 2.05 at 130 K. Also, as revealed by vibrating sample magnetometry and electron spin resonance measurements, an increase in the magnetization values with the decrease of the temperature is found. This result is compatible with a structural rearrangement from a face centered cubic metastable paramagnetic FePd ₃ phase into a primitive ferromagnetic cubic Fe ₃ Pd phase. Comparative studies performed in Fe ₃ C/α-Fe filled thin walled carbon nanotubes also give further confirmations of these unusual types of transitions by revealing a bcc to fcc phase shift with the decrease of the temperature as confirmed by zero field cooled measurements of the magnetization and ESR. In addition, a temperature dependent variation in the intensity and position of the π-electron's differential absorption feature is observed and implies the possible existence of antiferromagnetic correlations in the carbon-nanotubes structure.