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An unusual observation of magnetic transition and Pd/C rehybridization effects is reported in novel monolayer carbon foam materials filled with doubled FePd3 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 FePd3 phase into a primitive ferromagnetic cubic Fe3Pd phase. Comparative studies performed in Fe3C/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.

Ferromagnetically-filled carbon nanotubes have been recently considered important candidates for application into data recording quantum disk devices. Achievement of high filling rates of the ferromagnetic materials is particularly desirable for applications. Here we report the novel observation of carbon nanotubes continuously filled along the capillary with unusual μm-long faceted Fe3C nanowires. Anomalous magnetic features possibly due to strain effects of the crystal facets are reported. Magnetization measurements revealed unusual stepped magnetic hysteresis-loops at 300 K and at 2 K together with an anomalous decrease in the coercivity at low temperature. The observed unusual shape of the hysteresis is ascribed to the existence of an antiferromagnetic transition within or at the boundary of the ferromagnetic facets. The collapse in the coercivity value as the temperature decreases and the characteristic width-enhancement of the hysteresis with the field increasing appear to indicate the existence of layered antiferromagnetic phases, possibly in the strain-rich regions of the nanowire facets. Zero field cooled (ZFC) and field cooled (FC) magnetic curves evidenced presence of magnetic irreversibilities, an indicator of a possible spin-glass-like behavior induced by competing antiferromagnetic and ferromagnetic interactions. Characterization performed with low temperature XRD measurements, further revealed a slight variation in the average Fe3C unit cell parameters, suggesting the absence of additional unit-cell volume induced ferromagnetic transitions at low temperature.

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