Janis Kliava

Diluted and non-diluted ferric ions in borate glasses studied by electron paramagnetic resonance

Abstract Electron paramagnetic resonance (EPR) spectra of lithium borate glass (1 - x )(0.63B 2 O 3 · 0.37Li 2 O) · x Fe 2 O 3 , with x varying from 0.001 to 0.1, were measured at different microwave frequencies and temperatures. For low Fe 3+ concentrations (Fe 2 O 3 molar contents from 0.001 to 0.01), X-band EPR spectra, consisting of a g ef = 4.3 peak accompanied by a shoulder continuing down to g ef = 9.7 , are computer simulated on the basis of a ‘rhombic’ spin-Hamiltonian with Zeeman and fine-structure terms. A good fit to the experimental spectra for various Fe 2 O 3 contents is observed with the same values of the spin-Hamiltonian parameters and assuming a Lorentzian lineshape and a linewidth increasing linealry with the concentration of Fe 3+ ions. It is concluded that the spectrum is due to diluted Fe 3+ ions in a relatively strong crystal field of orthorhombic symmetry, with largely distributed fine-structure parameters. From the concentration dependence of the line width, by extending to glasses a theoretical EPR linewidth expression derived for polycrystalline systems, the minimum distance between diluted Fe 3+ ions is estimated as 4.9 A. A diluted state of Fe 3+ ions in the glass network in this range is also confirmed by the temperature dependence of the g ef = 4.3 resonance which follows a Curie law. For intermediate concentrations of Fe 3+ ions (Fe 2 O 3 molar contents from 0.01 to 0.1), the width of the g ef = 4.3 line is proportional to the square root of concentration, still indicating dipolar interactions. On the other hand, the microwave frequency dependence of a broad g ef ≈ 2 line, which coexists at these concentrations with the g ef = 4.3 line, shows that the former line is due to pairs or small clusters of exchange-coupled Fe 3+ ions. The temperature dependence of the g ef ≈ 2 line intensity in 0.1 mol Fe 2 O 3 glass is consistent with a more antiferromagnetic character by comparison with the 0.05 mol Fe 2 O 3 glass, which is attributed to an appearance, at higher Fe 2 O 3 contents, of iron-containing microclusters not incorporated in the random glass network, with smaller distances between the paramagnetic ions. These microcluster are probably the origin of a new narrow line superposed with the broad g ef ≈ 2 line in the low-temperature EPR spectra.

Magnetic resonance of superparamagnetic iron-containing nanoparticles in annealed glass

In this work, we study borate glasses doped with a low concentration of iron oxide by X band (9.5 GHz) electron magnetic resonance. These glasses (composition: 0.63B2O3–0.37Li2O–0.75×10−3 Fe2O3 in mole %) were annealed at increasing temperatures Ta, starting at the glass transition temperature. A new composite resonance at gef≈2.0 arises in the spectra measured at room temperature (300 K). The narrow component of this resonance is predominant in glasses annealed at lower Ta while the broad component increases in intensity as Ta increases. This resonance is ascribed to an assembly of superparamagnetic nanoparticles of a crystalline iron-containing compound. Numerical simulations assuming a lognormal particle volume distribution show that the mean particle diameter increases from 5.3 to 8.5 nm as Ta increases from 748 to 823 K. The integrated spectra intensity shows that the total number of spins in the nanoparticles increases rapidly with Ta. At lower anneal temperatures Ta, a striking increase occurs in t...

Mo5+ ions as EPR structural probes in molybdenum phosphate glasses

Abstract Glasses of P 2 O 5 -Li 2 MoO 4 -Li 2 O system with constant Li 2 MoO 4 molar contents (10%), containing different amounts of the alkaline oxide (Li 2 O) ranging from 0 to 30 mol%, have been studied by electron paramagnetic resonance (EPR) spectroscopy. It is found that less than 1% of molybdenum ions are reduced to Mo 5+ , this percentage decreasing with the Li 2 O content. The components of g - and A -tensors are determined by computer simulations and related to structural properties using molecular orbital method. The distorted octahedral environment of Mo 5+ sites is confirmed and minimal Mo-Mo distance is evaluated from the EPR linewidth as ≈0.38 nm. At Li 2 O contents ≥20 mol%, the coordination polyhedra of Mo 5+ become better ordered though more distorted. The Mo-O bond covalency increases with increasing Li 2 O content.

Lineshapes in magnetic resonance spectra

In magnetic resonance, and in particular, in superparamagnetic resonance studies at variable temperatures, a correlation between the apparent resonance magnetic field and the apparent linewidth is often observed. In order to account for this correlation, we consider the resonance lineshapes resulting from different phenomenological equations of damped motion of the magnetic moments in the cases of a linear paramagnet and of a perfect soft ferromagnet. The Bloch-Bloembergen, modified Bloch, Gilbert, Landau-Lifshitz and Callen equations are analysed. In most cases we obtain analytical expressions for the apparent resonance-field shift. Finally, we report an experimental variable-temperature study of the superparamagnetic resonance of ultrafine Fe2O3 particles in sol-gel glass. Computer simulations using the Landau-Lifshitz lineshape provide good fits of the resonance spectra at different temperatures for the same magnetic and morphological parameters of the particles.

Superparamagnetic resonance of annealed iron-containing borate glass

A lithium borate glass containing a small amount of iron oxide is studied by electron magnetic resonance at room temperature after repeated annealing steps between 460 and . As the anneal temperature increases, the sharp line characteristic of isolated iron ions decreases in intensity and finally disappears. Simultaneously, a narrow line emerges at , superposed with a broader one, the narrow and the broader components predominating respectively after annealing at lower and at higher temperatures. Computer simulations of spectra have been carried out, based on a model of resonance of ferromagnetic single-domain nanoparticles randomly dispersed in the devitrified glass (superparamagnetic resonance). As the anneal temperature increases, the most probable particle diameter obtained assuming a log-normal distribution of diameters increases from 2.9 to 4.7 nm showing a saturation at higher anneal temperatures, whereas the relative number of larger particles grows continuously.

SPIN TRANSITION WITH A VERY LARGE THERMAL HYSTERESIS IN A MOLECULAR CRYSTAL : AN EPR STUDY OF FE(PM-PEA)2(NCS)2

Abstract Magnetic susceptibility and X- and Q-band EPR studies are reported for the Fe 2+ spin transition compound (Fe( PM-PEA ) 2 (NCS) 2 ) in powder form. An exceptionally large thermal hysteresis loop, about 60 K, has been found by both techniques. The EPR studies have been carried out on compound doped with Mn 2+ (0.1‰ of Mn/Fe). An original EPR spectra simulation program has been elaborated in order to yield data on the structural modifications accompanying the spin transition. The fine structure parameters obtained by simulations in the high-spin state of Fe 2+ , D =460, E =25, and in the low-spin state of this ion, D =235, E =20 (in 10 −4 cm −1 ) confirm that the spin transition is accompanied by a crystalline phase transformation. The Mn 2+ EPR spectra in the spin transition region can be convincingly fitted by linear combinations of the spectra observed in the high-spin and low-spin states of Fe 2+ , indicating that the spin transition occurs in domains of like-spin ions.

Magnetic and optical properties and electron paramagnetic resonance of gadolinium-containing oxide glasses

Magnetic susceptibility, electron paramagnetic resonance (EPR) and optical absorption have been studied in a glass system 20La2O3–22Al2O3–23B2O3– 35(SiO2+GeO2) with a part of La2O3 substituted by Gd2O3 in different concentrations. Positive Weiss constants have been found in more heavily doped glasses and ascribed to clustering of Gd3+ ions. Computer simulations of the EPR spectra show that the short-range ordering in the environment of the Gd3+ ions is well preserved. The relative distribution widths of the ligand coordinates are less than 2%. In the more heavily doped glasses the EPR spectra are superpositions of signals arising from isolated ions and ferromagnetic clusters. The increase of Gd3+ concentration is shown to change substantially the strong optical absorption edge while only small changes of f–f absorption band characteristics are observed. This difference is associated with the different effect of the Gd ion clustering on the mechanisms of the strong absorption in the ultraviolet region and the f–f absorption.

Electron paramagnetic resonance and Mössbauer effect studies in iron-doped 57Fe isotope enriched phosphate glasses

Abstract Electron paramagnetic resonance and Mossbauer spectroscopy measurements of phosphate glasses doped with 57Fe isotope enriched Fe2O3 have been carried out. The electron paramagnetic resonance spectra have been computer simulated using an approach based on the eigenfield method applied to the ‘rhombic’ spin Hamiltonian, which contains only the Zeeman and quadrupole fine structure terms. In order to account for the structural disorder in glass, different distribution densities of fine structure parameters D and E have been tried: a two-dimensional Gaussian function of D and λ = | E D | and ‘Czjzeks and related functions’. An agreement between the experimental and computer simulated spectra found with the Gaussian distribution density suggests the presence of a large fraction of Fe3+ sites with axial or feebly rhombic distortions (λ ≤ 0.08). The mean value of the axial fine structure parameter D is consistent with a highly distorted environment of Fe3+ ions in the phosphate glasses (but somewhat less in comparison with the borate glasses). It is shown that the gef = 2.0 absorption in the phosphate glasses consists of two different features: a narrow one results from Fe3+ ions isolated in the glass matrix, while the broad one is due to crystallite inclusions.

Metacenter and ship stability

We address the location of the metacenter M of a floating body such as a ship. Previous studies of M in relation to the stability of a ship have mainly used geometrical approaches and were limited to near equilibrium. We develop a quantitative approach to the location of M for a general shape of the cross-section of a floating body in a rolling/pitching motion and for an arbitrary heel angle. We show that different definitions of M refer to one and the same special point of the floating body. We discuss the relation between the height of M with respect to the line of flotation and the contribution of the buoyancy force to ship stability. We provide expressions and graphs of the buoyancy, flotation, and metacentric curves for some simple shapes of floating bodies.

Formation and evolution of magnetic nanoparticles in borate glass simultaneously doped with Fe and Mn oxides

Evolution of the phase state of paramagnetic additions at various stages of synthesis and subsequent thermal treatment of glasses of the system Al2O3–K2O–B2O3 simultaneously doped with Fe2O3 and MnO is studied by means of a combination of experimental techniques: Faraday rotation (FR), electron magnetic resonance (EMR), transmission electron microscopy (TEM), Mossbauer spectroscopy, and magnetic measurements. Both FR and EMR show that magnetically ordered clusters occur already at the first stage of the glass preparation. In particular, for the ratio of the Fe and Mn oxides in the charge close to 3:2, fine magnetic nanoparticles are formed with characteristics similar to those of manganese ferrite. By computer simulating the EMR spectra at variable temperatures, a superparamagnetic nature of these nanoparticles is confirmed and their mean diameter is estimated as approximately 3.2 nm. In the thermally treated glasses larger magnetic nanoparticles are formed, giving rise to FR spectra, characteristic of ma...