K. Skeff Neto

Particle sizing of magnetite-based magnetic fluid using atomic force microscopy: A comparative study with electron microscopy and birefringence

Atomic force microscopy (AFM), transmission electron microscopy (TEM), and static magnetic birefringence (SMB) were used to unfold the particle size polydispersity profile of a magnetite-based magnetic fluid sample. The data obtained from different techniques were curve fitted using the lognormal distribution function, from which the mean particle diameter (Dm) and the standard deviation (σ) were obtained. In comparison to the TEM data, the AFM data show a reduction of Dm (about 20%) and an increase of σ (about 15%). In contrast, close agreement between the TEM and SMB data was found.

Magneto-optical properties of a highly transparent cadmium ferrite-based magnetic fluid

The magneto-optical properties of a highly stable ionic magnetic fluid sample containing CdFe2O4 nanoparticles were investigated using static magnetic birefringence, zero-field optical transmissivity, and transmission electron microscopy. From our measurements we found that the transmittivity and the birefringence of the CdFe2O4-based sample is several times greater than a typical magnetic fluid sample (γ-Fe2O3 based), giving this magnetic material great potential for magneto-optics applications. We also found that the birefringence can be increased by several orders of magnitude, allowing full manipulation of the observed negative differential transmitted optical intensity feature.

The influence of aggregates and relative permeability on the magnetic birefringence in ionic magnetic fluids

The static magnetic birefringence (SMB) of nickel-ferrite ionic magnetic fluid was investigated within the oscillating dipole-interaction anisotropy concept proposed by Xu and Ridler [J. Appl. Phys. 82, 326 (1997)]. The model was extended to include the magnetic texture of particle agglomerates and the field dependence of the magnetic permeability. The SMB data of samples subjected or not to a magnetic aging process and presenting particle concentration in the range of 2×1016 to 8×1016 particle/cm3 were successfully described. The particle size distribution obtained from the fit of the SMB data was discussed in comparison with the data obtained from transmission electron microscopy.

Electron spin resonance in superparamagnetic particles dispersed in a non-magnetic matrix

Abstract The temperature dependence of the electron magnetic resonance line broadening (ΔH pp) of superparamagnetic particles immersed in an inert matrix is discussed on the basis of the method of moments.

Mössbauer thermal decomposition studies of Fe(II) sulphate

Abstract Investigations of thermal decomposition of FeSO 4 .H 2 O by Mossbauer resonance, thermal differential analysis (DTA), thermal gravimetric analysis (TGA), infrared (i.r.), magnetic susceptibility, X-rays and chemical analyses are being reported. An unidentified thermal product A (Q.S. = 1·45 mm/sec) is magnetically ordered at 77K and transition to paramagnetic state takes place at 123·5 K. In aqueous vapours compound A changes reversibly to a non-magnetically ordered compound B (probably Buterlite). The empirical formulae have been given.

Detection of ferromagnetic resonance by photoacoustic effect

By exploring the simple periodic heat‐flux principle of the photoacoustic cell we propose and demonstrate experimentally the usefulness of the photoacoustic cell for studying the ferromagnetic resonance. An interesting feature of this technique is that it offers special advantages over conventional spectroscopic methods used in ferromagnetic resonance because of the simplicity of measurements. An application is made for the case of an iron thin foil.

Irreversibility of zero-field birefringence in ferrofluids upon temperature reversal

The temperature dependence of the zero-field birefringence was investigated using acid and basic MnFe2O4 ionic magnetic fluids, in the range of 290–350 K. Approaching a characteristic temperature (Tc) from below, which depends upon the sample characteristics, the zero-field birefringence goes critically down to zero. Furthermore, the birefringence shows an irreversible path upon heating and cooling the samples above Tc. The experimental data are successfully explained as long as dimers are included in the model calculation and the thermal disruption of them follows a critical behavior.

Magnetic resonance of zinc- and copper-ferrite ionic magnetic fluids: temperature effects

Abstract Magnetic resonance was used to investigate ionic magnetic fluids based on ZnFe 2 O 4 and CuFe 2 O 4 nanoparticles. Temperature and angular variation measurements were performed with field-frozen samples, using an X-band spectrometer. The resonance line shape analysis indicates the presence of four components, irrespective of the temperature and sample orientation. Intermediate values of the anisotropy field were associated to dimers, while low and high values of the anisotropy field were associated to large- and small-sized monomers, respectively. The temperature dependence of the resonance field associated to each resonance line allowed quantitative investigation of the surface anisotropy component. Positive as well as negative surface anisotropy components, as obtained from the temperature dependence of the resonance field, are reported.

Spin - freezing phenomenon in pseudobrookite

Abstract The temperature dependence of EPR linewidth broadening in polycrystalline Fe 2 TiO 5 is discussed. From the comparison between our EPR data and those reported in the literature we can conclude that our sample exhibits a true thermodynamic spin-glass transition at T f =53.4±0.5 k. We present an expression to explain the behavior of the EPR linewidth broadening as a function of temperature.

Spectroscopic characterization of superparamagnetic particles of thermolytic products of ferric sulphate hydrates

Abstract Superparamagnetic particles of chemically pure samples, in the system Fe(OH)SO 4 /Fe(OH)SO 4 ·(H 2 O), are produced by thermal decomposition of ferric sulphate hydrates. The control of particle size distribution is achieved by successive hydration and dehydration processes monitored by X-ray diffraction, electron microscopy, Mossbauer and IR spectroscopy. The particle size modification is related for the particle growth and two mechanisms are suggested thereon.