Possible exchange-bias in Fe-filled multiwall carbon nanotubes in presence of coexisting α-Fe and Fe3C phases

Abstract

Ferromagnetic Fe₃C and α-Fe nanowires are low dimensional nanoscale systems with tuneable saturation magnetization and coercivities. These materials have been frequently identified as technological targets for applications in spintronics, spin-wave transport and exchange-bias when confined inside carbon nanotubes (CNTs). Here we investigate the presence of exchange-bias in films of Fe-filled CNTs fabricated by pyrolysis of ferrocene/dichlorobenzene mixtures. By employing a combination of characterization techniques, namely X-ray diffraction (XRD), transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) we identify the presence of three main Fe-phases, namely Fe₃C, α-Fe and γ-Fe. Control of the nanowire phase-composition was obtained by employing a custom designed cooling approach consisting on the combination of slow and rapid cooling methods. By tuning the duration of the slow-cooling method in the range from 0 to 40 min, we were able to reproducibly control the structural relaxation of Fe₃C into α-Fe and γ-Fe. Magnetization vs Field signals acquired at T ~ 2 K revealed a shift of ~105 Oe in the position of the ferromagnetic hysteresis (under field cooling) acquired from those CNTs exhibiting coexisting α-Fe and Fe₃C ferromagnetic phases. The hysteresis-shift was found to deplete and almost-vanish (~31 Oe) when converting the Fe₃C phase into α-Fe and γ-Fe. Further characterization by atomic and magnetic force microscopy (AFM/MFM) revealed two main categories of contrast, namely one deriving from spontaneous magnetization with a direction perpendicular to the long axis of the CNT and another indicative of spontaneous magnetization along the nanowire-axis direction. Coexisting single and alternate types of contrast were interestingly identified and can be interpreted as the result of magnetic contributions arising from coexisting Fe₃C and α-Fe phases.