J. Typek

Competing Magnetic Interactions in Zn_3Fe_4V_6O_{24} Studied by Electron Paramagnetic Resonance

A multicomponent vanadate M3Fe4V6O24 sample with non-magnetic M = Zn(II) ions was synthesized by the solid state reaction method using stoichiometric mixtures of the 80mol% FeVO4 and 20mol% Zn3(VO4)2. The temperature dependence of the EPR spectra was performed in the 90–280 K temperature range. The resonance field and the integrated intensity of the EPR line showed minimum value of both parameters at ≈ 200 K. It is suggested that a part of the sample is displaying tendency to form an antiferromagnetic ordered state (or the magnetic clusters) above this temperature while below the ferromagnetic interaction of the main part of material is dominating. This behaviour is attributed to the inherent magnetic inhomogeneity of the system due to the presence of the ferromagnetic or antiferromagnetic spin clusters.

Magnetic properties of ZnFe2O4 nanoparticles

Fine particles of ZnFe2O4 were synthesized by a wet chemical method in the (80 wt.% Fe2O3 + 20 wt.% ZnO) system. The morphological and structural properties of the mixed system were investigated by scanning electron microscopy, X-ray diffraction, inductively coupled plasma atomic emission, and X-ray photoelectron spectroscopy. The major phase was determined to be the ZnFe2O4 spinel with particle size of 11 nm. The magnetic properties of the material were investigated by ferromagnetic resonance (FMR) in the temperature range from liquid helium to room temperature. A very intense, asymmetric FMR signal from ZnFe2O4 nanoparticles was recorded, which has been analyzed in terms of two Callen-lineshape lines. Temperature dependence of the FMR parameters was obtained from fitting the experimental lines with two component lines. Analysis of the FMR spectra in terms of two separate components indicates the presence of strongly anisotropic magnetic interactions.

Study of magnetic inhomogeneity in β-Cu3Fe4V6O24

The temperature dependence of dc magnetization and electron paramagnetic resonance (EPR) spectra of the β-Cu3Fe4V6O24 multicomponent vanadate were investigated. Dc magnetic measurements showed the presence of strong antiferromagnetic interactions (Curie-Weiss temperature, Θ ∼ 80 K) at high temperatures, while zero-field-cooled (ZFC) magnetization revealed a cusp-like maximum in low fields at Tf1 = 4.4 K, which coincides with the splitting of the ZFC and FC curves. Another maximum was registered at Tf2 = 3.0 K. These two temperatures (Tf1 and Tf2) could be regarded as freezing temperatures in the spin glass state of two magnetic sublattices of Fe1 and Fe2 ions. The EPR spectrum of β-Cu3Fe4V6O24 is dominated by a nearly symmetrical, very intense and broad resonance line centered at geff ∼ 2.0 that could be attributed to iron ions. Below 10 K, an additional EPR spectrum with g1 = 2.018(1) and g2 = 2.175(1) appears, as well as a very weak line at geff = 1.99(1). The former spectrum is probably is due to divalent copper ions, and the latter line due to vanadium V4+ complexes. The temperature dependence of EPR parameters (g-factor, linewidth, integrated intensity) was determined in the range of 3–300 K. Two low-temperature maxima in the temperature dependence of the integrated intensity (at 40 and 6 K) were fitted with a function suitable for pairs of exchange-coupled Fe3+ ions. A comparison of dc magnetic susceptibility and EPR integrated intensity indicates the presence of spin clusters, which play an important role in determining the low-temperature magnetic response of β-Cu3Fe4V6O24.

Study of magnetic properties of two samples from FeVO4-Co3V2O8 system

Two samples containing phases formed in the FeVO4-Co3V2O8 system were prepared by a conventional sintering method. The sample designated as H5 was one-phase with the howardevansite-type structure, while the sample designated as HL7 contained a mixture of H-type and lyonsite-type structures. The temperature dependence of the electron paramagnetic resonance (EPR) spectra and static magnetic susceptibility χ was investigated in the temperature range from liquid helium to room temperature. Both the EPR spectra and the dc magnetic susceptibility showed anomalous behavior indicating that the magnetic competition process may be responsible. A comparison of the obtained results with previous studies on related compounds with the same structure, i.e. M3Fe4V6O24 (M = Mg(II), Zn(II), and Cu(II)) revealed that the observed anomaly shifted to lower temperatures on replacing the non-magnetic ions by magnetic Co(II) ions. The temperature dependence of the inverse susceptibility χ−1 indicates the existence of antiferromagnetic interactions between Fe(III) and Co(II) spins in sample H5. The obtained values of the Curie-Weiss temperatures are lower than for the Mn3Fe4V6O24 compound and comparable to compounds from M3Fe4V6O24 systems with M diamagnetic cations. The introduction of cobalt cations intensifies the magnetic frustration what is reflected in the temperature dependence of the magnetic susceptibility at low temperatures.

Temperature Dependence of the FMR Spectra of Polymer Composites with Nanocrystalline α-Fe/C Filler

Two different concentrations of nanocrystalline material: α-Fe/C were prepared by the carburization of nanocrystalline iron and characterized by XRD and SEM. The nanoparticles were next used as fillers in polymer nanocomposites using the in situ polycondensation reaction in a poly(ether-ester) matrix with two concentrations: 0.1 wt. % and 0.3 wt. %. The temperature dependence of the ferromagnetic resonance (FMR) spectra was investigated to study magnetic interactions in the compounds. The introduced FMR parameters (intensity and position of the right peak) describe well the temperature dependence of FMR spectra of strongly interacting magnetic nanoparticles. The FMR spectra depend strongly on the concentration of magnetic nanoparticles, which influence the magnetic interactions between them. Two main critical points of the matrix (the glass state and the freezing of benzene rings) influence the behaviour of the FMR spectra.

Magnetic resonance study of annealed and rinsed N-doped TiO2 nanoparticles

AbstractNanoparticles of nitrogen-modified TiO2 (N-doped TiO2) calcined at 300°C and 350°C, have been prepared with and without water rinsing. Samples were characterized by x-ray diffractrometry (XRD) and optical spectroscopy. The electron paramagnetic resonance (EPR) spectra from centers involving oxygen vacancies were recorded for all samples. These could be attributed to paramagnetic surface centers of the hole type, for example to paramagnetic oxygen radicals O−, O2−etc. The concentration of these centers increased after water rising and it further increased for samples annealed at higher temperature. Additionally, for samples calcined at 300°C, and calcined at 350°C and rinsed, the EPR spectra evidenced the presence of magnetic clusters of Ti3+ ions. The photocatalytic activity of samples was studied towards phenol decomposition under unltraviolet-visible (UV-Vis) irradiation. It was found that, in comparison to the starting materials, the rinsed materials showed increased photocatalytic activity towards phenol oxidation. The light absorption (UV-Vis/DRS) as well as surface Fourier transform infrared/diffuse reflectance spectroscopy (FTIR/DR) studies confirmed a significantly enhanced light absorption and the presence of nitrogen groups on the photocatalysts surfaces, respectively. A significant increase of concentration of paramagnetic centers connected with oxygen vacancies after water rising has had an essential influence on increasing their photocatalytic activity.

Magnetic resonance study of co-modified (Co,N)-TiO2 nanocomposites

Abstract Three nCo,N-TiO2 nanocomposites (where cobalt concentration index n = 1, 5 and 10 wt %) were prepared and investigated by magnetic resonance spectroscopy at room temperature. Ferromagnetic resonance (FMR) lines of magnetic cobalt agglomerated nanoparticle were dominant in all registered spectra. The relaxation processes and magnetic anisotropy of the investigated spin system essentially depended on the concentration of cobalt ions. It is suggested that the samples contained two magnetic types of sublattices forming a strongly correlated spin system. It is suggested that the existence of strongly correlated magnetic system has an essential influence of the photocatalytic properties of the studied nanocomposites.

Magnetic Resonance Study of Nickel and Nitrogen Co-modified Titanium Dioxide Nanocomposites

Nickel and nitrogen co-modified TiO2, nNi,N-TiO2 (n = 1, 5 and 10 wt% of Ni) nanocomposites were prepared by impregnation of amorphous titanium dioxide with Ni(NO\(_{3})_{2}\cdot \) 5H2O followed by high temperature calcination at 800 ∘C in ammonia atmosphere. Temperature dependence of the FMR/EPR spectra of nNi,N-TiO2 samples in 4–290 K range has been investigated. The FMR spectra of nickel nanoparticle agglomerates were studied by decomposition into three components. Temperature dependence of FMR parameters (resonance field, two types of linewidth, integrated intensity) of components were analyzed to determine their origin. In addition, the EPR spectra of trivalent titanium ions were recorded in the low temperature range. The connection between photocatalytic activity of the investigated nanocomposites and their magnetic properties was discussed.

Magnetic properties of co-modified Fe,N-TiO 2 nanocomposites

Abstract Iron and nitrogen co-modified titanium dioxide nanocomposites, nFe,N-TiO2 (where n = 1, 5 and 10 wt% of Fe), were investigated by detailed dc susceptibility and magnetization measurements. Different kinds of magnetic interactions were evidenced depending essentially on iron loading of TiO2. The coexistence of superparamagnetic, paramagnetic and ferromagnetic phases was identified at high temperatures. Strong antiferromagnetic interactions were observed below 50 K, where some part of the nanocomposite entered into a long range antiferromagnetic ordering. Antiferromagnetic interactions were attributed to the magnetic agglomerates of iron-based and trivalent iron ions in FeTiO3 phase,whereas ferromagnetic interactions stemmed from the F-center mediated bound magnetic polarons.

FMR and Magnetization Study of ZnFe 2 O 4 Nanoparticles in 0.40Fe 2 O 3 /0.60ZnO Nanocomposite

Zinc oxide (ZnO) nanocrystals containing Fe2O3 have been synthesized by the calcination method. Ferromagnetic resonance (FMR) and dc magnetization measurements of 0.40(Fe2O3)/0.60(ZnO) nanocomposite have been carried out in the 4-300 K range to study the magnetic properties of agglomerated magnetic zinc ferrite ZnFe2O4 (ZFO) nanoparticles with an average crystallite size of 12 nm. Temperature dependence of the resonance field, linewidth, and the integrated intensity calculated from FMR spectra have been determined to obtain the value of the uniaxial anisotropy field and to establish the ranges of different relaxation types. Magnetization measurements in ZFC and FC modes as well as the study of hysteresis loops allowed calculating different magnetic characteristics - blocking/freezing temperature, magnetic moment, anisotropy constant, and anisotropy field. The observed magnetic properties of 0.40(Fe2O3)/0.60(ZnO) nanocomposite were explained based on the core-shell model of ZFO nanoparticles. From comparison of FMR and dc magnetization measurements, the temperature ranges of magnetic phases existing in ZFO nanoparticles in 0.40Fe2O3/0.60ZnO nanocomposite are proposed.