We have employed magnetization measurements, Mössbauer and ESR spectroscopic techniques, in order to study the ferromagnetic insulating (FMI) compound La1-xCaxMnO3 (x=0.175) doped with 1% 57Fe. We have used two samples; one prepared in air which has cation vacancies and a second in inert atmosphere, which is stoichiometric. An abrupt change of the experimental results is obtained, by all techniques, in the ferromagnetic insulating regime, in the temperature region of T O/O//≈60 K, where an orbital rearrangement is suggested to occur. An analysis of these findings points to an orbital rearrangement transformation. Ferromagnetic resonance reveals considerable differences between stoichiometric and cation deficient samples, indicating anisotropy of the exchange interactions in the former sample with significant temperature dependence, most pronounced in the vicinity of TO/O//. © EDP Sciences, Società Italiana di Fisica, Springer-Verlag 2005.

The electronic properties of boron-doped multiwall carbon nanotubes (MWNTs) have been studied using static magnetization and electron spin resonance. A relatively strong ferromagnetic signal has been identified in the dc magnetization response that shows the occurrence of ferromagnetism, coexisting with the orbital and spin magnetism of the conduction electrons. The small diamagnetic susceptibility and the weak temperature variation of the g factor, which is observed in the narrow conduction-electron spin resonance (CESR), indicate a Fermi level shift of ∼0.2eV, according to the quasi-two-dimensional graphite band model. The temperature dependence of the spin susceptibility reveals considerable enhancement compared to undoped MWNTs and the presence of thermally activated behavior, complying with an increased density of states and the formation of localized impurity states close to the Fermi level. Line-shape analysis of the CESR spectra in terms of two separate spin systems implies enhanced doping inhomogeneity. © 2005 The American Physical Society.

Electron spin resonance (ESR) has been applied to investigate the magnetic heterogeneity in electron doped La1-xCaxMnO3 (0.80≤x≤0.95). A low field ferromagnetic resonance (FMR) mode is observed for lightly doped compounds (x = 0.90,0.95), signifying the formation of ferromagnetic (FM) spin clusters within the antiferromagnetic G-type AFM phase. The anomalous temperature variations of the resonance field, linewidth and FMR intensity, as well as the observation of thermal cycling effects below TC, emphasize the non-trivial dynamics of the FM phase, which is attributed to the temperature dependent size evolution of the underlying spin clusters towards canted AFM and FM domains. For heavier electron doping (x = 0.80,0.85), distinct AFM behaviour is evinced in the vicinity of TN in the monoclinic C-type AFM phase, characterized by the absence of critical relaxation. Additional weak FMR lines are observed for x = 0.80 and 0.85, whereas a narrow superparamagnetic-like signal is detected for x = 0.95. © 2005 IOP Publishing Ltd.

Ferromagnetic resonance (FMR) and ac conductivity have been applied to study a polymer composite containing as filler a binary mixture of magnetite (Fe3 O4) and cementite (Fe3 C) nanoparticles (30-50 nm) dispersed in a diamagnetic carbon matrix, which was synthesized by the carburization of nanocrystalline iron. Ac conductivity measurements showed thermally activated behavior involving a range of activation energies and power law frequency dependence at high frequencies similar to conducting polymer composites randomly filled with metal particles. Ferromagnetic resonance measurements revealed a relatively narrow FMR line at high temperatures indicating the presence of ferromagnetic nanoparticles, where thermal fluctuations and interparticle interactions determine the FMR temperature variation. An abrupt change of the FMR spectra was observed at T<81 K (ΔT≤1 K) coinciding with a sharp anomaly resolved in the temperature derivative of the ac conductivity. This behavior is attributed to the Verwey transition of Fe3 O4 nanoparticles, where the concurrent skin depth variation unveils the FMR of large magnetite conglomerates and thus allows discriminating their contribution from relatively isolated nanoparticles. © 2005 American Institute of Physics.