S. Glenis

Matrix effects on the magnetic properties of γ-Fe2O3 nanoparticles dispersed in a multiblock copolymer

The magnetic properties of γ-Fe2O3 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≈120K, 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.

Magnetic frustration in the site ordered Mg3Fe4(VO4)6 vanadate

The magnetic properties of the site ordered multicomponent vanadate Mg3Fe4(VO4)6 are studied using dc magnetization and electron paramagnetic resonance (EPR) measurements. The static susceptibility shows antiferromagnetic interactions between Fe3+ spins with a Curie-Weiss temperature Θ=−111(1)K, while a transition to a spin-glass-like state is observed at T≈8.5K, indicating appreciable spin frustration. EPR measurements corroborate the presence of antiferromagnetically coupled Fe3+ spins from high temperatures, while a distinct change in the temperature variation of the EPR parameters is observed at T<80K, complying with the mean-field energy scale provided by the Curie-Weiss temperature of the system. The resulting magnetic inhomogeneity that persists despite the absence of cation disorder between magnetic and diamagnetic metal ions is attributed to the presence of small amounts of oxygen deficiency that modifies the connectivity and superexchange coupling between Fe3+ spins and leads to a disordered mag...

Spectroscopic and magnetic properties of two di-copper(II) complexes with macrocyclic schiff bases

Abstract Two binuclear copper(II) complexes with macrocyclic Schiff bases Cu2LI(CH3COO)2·5H2O (complex I) and Cu2LII(CH3COO)2·2H2O (complex II) were synthesized and then characterized by IR, UV, and thermogravimetric analysis (TGA) measurements. TGA was used to investigate the desolvation of lattice water molecules. IR spectra demonstrated the formation of the cyclic compound and together with chemical elemental analysis were used to propose the structure of the complexes. The UV spectra of both complexes are typical for binuclear copper(II) complexes with Robson-type ligands. Variable-temperature magnetic susceptibility measurements corroborated by EPR and low-temperature isothermal magnetization data confirmed the formation of copper dimers with antiferromagnetic exchange coupling constants of −400 and −1250 cm−1 for complexes I and II, respectively, residing outside the usual range for the phenoxide bridged Cu(II) complexes. This implies the possibility that additional superexchange paths through the macrocyclic ligand may affect the intradimer exchange interaction as well as the phenoxide oxygen bridges.

Magnetism in pristine and chemically reduced graphene oxide

The evolution of magnetism for graphene oxide (GO) before and after chemical reduction was investigated by means of static magnetization and electron spin resonance (ESR) spectroscopy. Strong paramagnetism with a saturation magnetization of ∼1.2 emu/g and weak antiferromagnetic interactions were identified in pristine GO. Apart from spin-half defect centers, ESR spectroscopy indicated the excitation of high spin states, consistently with the high spin (S = 2) magnetic moments derived from the magnetization analysis, corroborating the formation of spatially “isolated” magnetic clusters in GO. A marked reduction of GOs magnetization (∼0.17 emu/g) along with an appreciable rise of diamagnetism (−2.4 × 10−6 emu/g Oe) was detected after chemical reduction by sodium borohydride, reflecting the drastic removal of paramagnetic defects and the concomitant growth of sp2 domains in reduced graphene oxide (rGO). ESR revealed a large drop of the spin susceptibility for rGO, which, in addition to the main paramagnetic...

The influence of small doses of fast neutron irradiation on the critical temperature and EPR spectra of YBa2Cu3O7-δ high temperature superconductors

Abstract The effect of the increase of the critical temperature T c by the irradiation of the Y123 samples with small doses of fast neutrons has been investigated by the temperature dependence of resistance and EPR studies. The results indicated that T c and EPR signals of all specimens are increased with increasing irradiation fluences. These results are explained in terms of redistribution of oxygen, which promotes charge ordering in the superconducting CuO 2 planes.

Influence of water molecule coordination on the magnetic properties of polyamine copper dinitrate complexes

The magnetic properties of two biogenic polyamine copper complexes were investigated by dc magnetization and electron paramagnetic resonance (EPR) measurements. The variation in temperature of the low field magnetization reveals that the absence or presence of water molecules in the copper coordination sphere results, respectively, in the enhancement or suppression of antiferromagnetic interactions between Cu2+ ions. Analysis of the EPR spectra shows considerable temperature dependence of both the g values and EPR linewidths that persist in the paramagnetic regime for both complexes. Such variation of EPR parameters is attributed to the interplay of demagnetizing field effects, pertinent to the purely paramagnetic compound, and to the presence of short-range magnetic order that applies to the antiferromagnetic polyamine complex.

Low temperature magnetic phase transition and interlayer coupling in double-wall carbon nanotubes

The magnetic properties of double wall carbon nanotubes (DWCNTs) were investigated using electron spin resonance (ESR) spectroscopy. An asymmetric resonance line of low intensity was identified and analyzed by the superimposition of a narrow and a broad metallic lineshape, attributed to the distinct contributions of defect spins located on the inner and outer DWCNTs shells. The spin susceptibilities of both ESR components revealed a ferromagnetic phase transition at low temperatures (T < 10 K) with small variation in the corresponding Curie-Weiss temperatures, approaching closely that of metallic single wall carbon nanotubes. Interlayer coupling between the DWCNT layers is suggested to effectively reduce the difference between the transition temperatures for the inner and outer shells and enhance spin-spin interactions between defect spins via the RKKY-type interaction of localized spins with conduction electrons.

Comparing the effects of oxygen content on the superconducting properties of YBa2Cu3Ox and RBa2Cu3Ox with R=Nd, Eu and Gd

Abstract Previous studies have shown that a Josephson weak link transition occurs in some YBa 2 Cu 3 O x samples at very low fields, H H C1 . This transition is distinct from the vortex glass–fluid transition which of course occurs for H > H C1 . The Josephson weak link transition for YBa 2 Cu 3 O x was observed only in samples annealed in oxygen. Moreover, the transition temperature T c of YBa 2 Cu 3 O x is also sensitive to oxygen content. T c is 93 K for air-sintered and vacuum-annealed samples but only 90 K for oxygen-annealed samples. We have extended these studies to RBa 2 Cu 3 O x samples with R=Nd, Eu and Gd. Like the Y-based systems, all three show the Josephson weak link transition but only for oxygen-annealed samples. Unlike the Y-based system, the values of T c for RBa 2 Cu 3 O x are not sensitive to oxygen content with T c being 95 K for all samples. Moreover, the Y- and R-based systems show different specific heat behavior. C / T vs. T for both oxygen and vacuum-annealed RBa 2 Cu 3 O x samples show a single peak at 95 K. Vacuum-annealed YBa 2 Cu 3 O x samples also show a single C / T peak at 93 K, but oxygen-annealed YBa 2 Cu 3 O x samples show two peaks in C / T at 90 and 93 K.

Magnetic Properties of n CoO/(1- n )ZnO Nanocomposites Obtained by Calcination

Magnetic properties of the nCoO/(1-n)ZnO (n = 0.4, 0.50, 0.60 and 0.70) nanocomposites obtained by using traditional wet chemical synthesis method followed by calcination at 600∘C were investigated by dc magnetometry and magnetic resonance spectroscopy. XRD measurements revealed the presence of only two phases: ZnO (hexagonal nanocrystals with sizes in 64–300 nm range) and spinel phase Co3O4 (spheroidal nanocrystals with sizes in 14–21 nm range). Magnetic dc susceptibility measurements in 2–300 K range revealed dominating paramagnetic behavior in the whole temperature range and the presence of a strong temperature-independent component. With exception of n = 0.70 sample, no behavior connected with the expected phase transition to antiferromagnetic phase in Co3O4 and superparamagnetism was registered. Experimental results could be consistently explained by assuming that the most of high-spin Co2+ ions are involved in formation of antiferromagnetic pairs or clusters. Low intensity electron paramagnetic resonance spectra registered at RT were attributed to two different magnetic components – one involving paramagnetic Co2+ ions at Zn2+ sites in ZnO and the other due to Co2+ in Co3O4 phase or more probably the superparamagnetic resonance of Co3O4 nanoparticles. The former component dominates in nanocomposites with small concentration of cobalt, the latter in highly Co concentrated samples.

Electronic properties of M2InV3O11 (M(II) = Zn(II) and Co(II)) compounds

The electronic properties of multicomponent vanadate oxides M2InV3O11 (M(II) = Zn(II) and Co(II)) were investigated by electrical resistivity and electron paramagnetic resonance (EPR) measurements. Replacement of non-magnetic Zn(II) cations with magnetic Co(II) ions resulted in a significant drop in the electrical conductivity and an increase in the activation energy. The EPR spectroscopy revealed the presence of VO2+ vanadyl ions in both compounds, while the presence of divalent cobalt ions was identified in the Co2InV3O11 oxide at low temperatures. The concentration of VO2+ vanadyl ions was found to be about one order higher for the vanadate oxide without magnetic ions. It is suggested that the increased concentration of VO2+ ions could be responsible for the enhanced conductivity of Zn2InV3O11.