Ronaldo Sergio De Biasi

Anisotropy field of small magnetic particles as measured by resonance

The anisotropy field as measured by resonance is calculated for a coherent assembly of small magnetic particles possessing both crystal and shape anisotropy. The theoretical results are compared with experimental data on precipitated cobalt and magnesioferrite particles.

Effect of gamma irradiation on the ductile-to-brittle transition in ultra-high molecular weight polyethylene

Ultra-high molecular weight polyethylene (UHMWPE) is used in the manufacture of medical applications due to its excellent mechanical properties, abrasion resistance and biocompatibility. The products are usually sterilized by gamma radiation. It is well known that gamma irradiation introduces structural changes that may be detrimental to the integrity and performance of the polymer, limiting its useful life. Therefore, it is imperative to have a fundamental understanding of the fracture behaviour of the material. A commercial general grade ultra-high molecular weight polyethylene (UHMWPE) was irradiated with several total doses and the effects of gamma radiation on the mechanical behaviour of the polymer and its morphology were evaluated by tensile tests, electron spin resonance spectroscopy (ESR) and scanning electron microscopy (SEM). The experimental data show that the gamma irradiation changes the molecular structure of UHMWPE, degrades its tensile properties and produces a gradual ductile-to-brittle transition.

Determination of particle size distribution by FMR measurements

Knowledge of particle size distribution is very important for the study of magnetic fluids, magnetic powders and other magnetic systems. In this work, we describe a simple method for its determination from FMR measurements. The method was applied to the case of MgFe2O4 precipitates in (Mg,Fe)O.

Martensitic transformation of austenitic stainless steel orthodontic wires during intraoral exposure.

INTRODUCTION Our purpose was to study the mechanical properties and phase transformations of orthodontic wires submitted to in-vivo exposure in the mouth for different periods of time. METHODS Stainless steel wires were tied to fixed orthodontic appliances of 30 patients from the orthodontics clinic of Universidade Federal do Rio de Janeiro School of Dentistry in Brazil. According to the duration of the clinical treatment, the patients were divided into 3 groups. After in-vivo exposure, the samples were studied by mechanical testing (torsion) and ferromagnetic resonance. Statistical analyses were carried out to evaluate the correlation between time of exposure, mechanical properties, and austenite-to-martensite transformation among the groups. RESULTS The results were compared with as-received control samples. The torque values increased as time in the mouth increased. The increase in torque resistance showed high correlations with time of exposure (P = 0.005) and austenite-martensite phase transformation. CONCLUSIONS The resistance of stainless steel orthodontic wires increases as the time in the mouth increases; this effect is attributed to the austenite-to-martensite transformation.

Influence of vanadium concentration on the ESR linewidth of V4+ in rutile

Abstract Electron spin resonance spectra of vanadium-doped rutile have been studied at 77 K for V concentrations between 0.01 and 1.00 mol%. The results suggest that the range of the exchange interaction between V 4+ ions is about 0.81 nm.

Influence of chromium concentration on the electron magnetic resonance linewidth of Cr3+ in SrTiO3

Electron magnetic resonance (EMR) spectra of Cr3+ ions in samples of chromium-doped strontium titanate (SrTiO3) have been studied at room temperature for chromium concentrations between 0.20 and 1.00 mol%. According to previous studies, chromium substitutes Ti4+ sites in the lattice and its preferred valence state is Cr4+, which is EMR silent in the X-band, but the trivalent state can be produced by illumination or codoping with Nb. In the present work, the codoping method was used; the results show the electron magnetic resonance linewidth of the Cr3+ spectrum increases with increasing chromium concentration and that the range of the exchange interaction between Cr3+ ions is about 0.96 nm.

Powder ESR spectra of paramagnetic impurities in axial symmetry sites

Abstract The resonance fields and amplitudes of the powder ESR spectra of paramagnetic impurities in axial symmetry sites have been computed as functions of the zero-field splitting parameter D for an effective spin S in the range 1⩽S⩽ 5 2 . The results, which are presented in graphical form, may be used for the interpretation of powder spectra.

Influence of the S3 term on the EPR spectrum of Cr3+ in cubic symmetry sites in MgO

Abstract The EPR spectrum of Cr3+ in cubic symmetry sites in MgO was investigated in the temperature range 100 to 300 K. The orientation dependence of the resonance field for the 1 2 → − 1 2 transition is found to be consistent with a spin Hamiltonian of the form H = gβH·S + uβ{ S x 3 H x + S y 3 H y + S z 3 H z − ( 1 5 ) (S·H)[3S(S + 1) − 1]}. The spin-Hamiltonian parameters are: at 300 K, g = 1.97965, u = + 10.0 × 10−5; at 100 K, g = 1.97971, u = +6.1 x 10−5; The temperature dependence of both parameters is approximately linear in the range 100 to 300 K.

Electron Magnetic Resonance of Diluted Solid Solutions of Gd3+ in BaTiO3

Electron magnetic resonance (EMR) spectra of gadolinium-doped barium titanate (BaTiO3) have been studied at room temperature for gadolinium concentrations between 0.20 and 2.00 mol%. The results suggest that the Gd3+ ions occupy substitutional sites, replacing the Ba2+ ion, that the electron magnetic resonance linewidth increases with increasing gadolinium concentration, and that the range of the exchange interaction between Gd3+ ions is about 0.98 nm, of the same order as that of the same ion in other host lattices, such as strontium titanate (SrTiO3), strontia (SrO), quicklime (CaO), magnesia (MgO) and zircon (ZrSiO4). The fact that the electron magnetic resonance linewidth of the Gd3+ ion increases, regularly and predictably, with Gd concentration, shows that the Gd3+ ion can be used as a probe to study, rapidly and non-destructively, the crystallinity and degradation of BaTiO3.

Spin-lattice relaxation of Fe3+ and Mn2+ in magnesium oxide

Abstract The spin-lattice relaxation time of Fe3+ and Mn2+ in MgO has been measured in the range 100–340 K. In the case of Fe3+, a T-4.80 behavior was observed; in contrast, the relaxation time for Mn2+ was found to be proportional to T-1.05.