The magnetic properties of the Mg2FeV3O11-x ternary vanadate, characterized by disorder between diamagnetic Mg2+ and high-spin Fe3+ ions, are studied using dc magnetization and electron paramagnetic resonance (EPR). The dc susceptibility shows antiferromagnetic interactions between Fe3+ spins with a Curie-Weiss temperature of Θ = -50(1) K, followed by spin-glass-like freezing at Tf ≈ 2.8 K, suggesting significant spin frustration. Temperature-dependent EPR measurements confirm the antiferromagnetic coupling of Fe3+ spins at high temperatures, while a distinct divergence is observed at T ≈ 50 K. This behavior is associated with the formation of spin clusters providing two different energy scales for the magnetic interactions. The magnetic response of Mg2FeV3O11-x is similar to that of the Zn-analogue compound, though the observed differences of the implicated energy scales indicate that magnetic inhomogeneity depends on the extent of cation disorder.

A new poly(ester-b-ether) copolymer filled with magnetic γ-Fe2O3 nanoparticles of low concentration (0.1 and 0.3 wt%) has been synthesized, with poly(ethylene terephthalate) (PET) as a rigid segment and poly(tetraoxymethylene) (PTMO) as the flexible segment at the ratio of 50 wt%. The filler was used in the form of suspension or solid powder. The dielectric spectroscopy results for these materials, in a wide range of frequencies and temperatures, are presented and compared with similar results for a pure polymeric matrix. Two relaxation processes, α and β, have been observed clearly in all samples at about 253 K and 163 K, respectively. The α process was shifted towards the lower temperatures as the nanoparticle content increased, while the β-relaxation was unaffected by the doping. The interfacial polarization effect was observed in the samples with nanoparticles, less pronounced for the larger nanoparticles. © 2006 Elsevier B.V. All rights reserved.

Ferromagnetic resonance (FMR) has been applied to study the variation of magnetic anisotropy and phase separation tendencies in the ferromagnetic insulating state of stoichiometric hole-doped La1-x Cax Mn O3 (0.125≤x≤0.19). A strongly anisotropic ferromagnetic phase is identified in the ferromagnetic insulating regime. The magnetic anisotropy evolves from positive uniaxial at x=0.125 and 0.15 to negative cubiclike at x=0.175 with increasing magnitude, whereas it decreases appreciably at x=0.19, which is close to the ferromagnetic metallic phase boundary. Minor contributions from ferromagnetic inhomogeneities characterized by weaker magnetic anisotropy are traced at low temperatures for x=0.125 and 0.15, while their temperature dependence implies coupling with the major anisotropic phase. A growing tendency towards the formation of spatially separate ferromagnetic regions in the paramagnetic regime is observed at x≥0.175, as the ferromagnetic metallic phase is approached. Persistent anomalies in the temperature dependence of the FMR parameters are traced concurrently with the structural transformation at 60-70 K. © 2006 The American Physical Society.

The magnetic properties of γ- Fe2 O3 ferrimagnetic nanoparticles embedded in a multiblock poly(ether-ester) copolymer have been investigated by static magnetization and ferromagnetic resonance (FMR) measurements at two different dispersion states. Significant variation of the magnetic response is identified below T≈120 K, most pronounced in the marked resonance field shift of the FMR spectra, independently of the dispersion state of the nanocomposites. This behavior correlates favorably with the dynamic relaxation of the copolymer, indicating a matrix freezing effect that is attributed to the magnetoelastic coupling of the oxide nanoparticles with the surrounding polymer. At low temperatures, the dc magnetization and FMR measurements vary considerably for the two nanocomposites, indicating essential differences in their ground state, related to the different morphology of the samples and the concomitant variation of interparticle interactions. © 2006 American Institute of Physics.

Two samples of the Zn2FeV3O11 compound were synthesized using different preparation routes: a solid state reaction of the ZnO-Fe2O3-V2O5 oxides and a direct reaction of the Zn2V2O7 and FeVO4 vanadates. In both cases the temperature dependences of the EPR spectra revealed a dominant contribution of antiferromagnetically coupled Fe3+ ions in the high-spin state. Specific differences in the EPR spectra of the two samples were recorded at all the investigated temperature intervals. A short range, or less likely, partial magnetic order involving a fraction of the Fe3+ distributed on the two octahedral crystallographic sites appeared to take place below 50 K. The differences observed between the EPR spectra of the two Zn2FeV3O11 samples might suggest the existence of small variations in the cation disorder phenomena of Fe3+ and Zn2+ ions or the presence of an oxygen deficiency process.