Review of Irradiation Effects on Ferrites : Results in the World from 1970 to 1995

In this paper, we present the main irradiation effects observed on ferrites in the world between 1970 and 1995. The crystallographic and magnetic changes induced by irradiation are shown for various kinds of particles : fast neutrons, low energy ions and high energy heavy ions and for different ferrite structures : spinels, garnets and hexaferrites.The observed effects depend both on the mechanism of particle crystal interaction and on the structure of the ferrite. From crystalline point of view, several effects can be induced : increase of lattice parameter, change of site and valence for cations and anions, amorphisation, creation of tracks showing either elongated extended defects or continuous amorphous cylinders. From magnetic point of view, the following effects can be induced : - the magnetization and the Curie temperature can either increase, or decrease, or be constant, - the anisotropy can change and even cancel. These effects are reproducible and stable at 300K. Recently, it was found that Fe3O4 is the most irradiation resistant ferrite

Volume anomaly in ferrimagnetism

The volume anomaly ΔV/V due to the magnetic energy corresponding to the exchange interactions is experimentally determined for YIG. The experimental values (from 77 K to Tc) agree with the values deduced from the theoretical expression based on the Néel's theories of volume anomaly and of ferrimagnetism. These results are compared with those obtained by other authors on ferromagnetic and antiferromagnetic materials with localized magnetic moments : a reduced curve is obtained.L'anomalie de volume ΔV/V due à l'énergie magnétique associée aux interactions d'échange est déterminée expérimentalement pour Y3Fe5O 12. Les valeurs mesurées de 77 K à Tc coincident avec les valeurs obtenues à partir de l'expression basée sur l'approximation du champ moléculaire et sur les théories de Néel de l'anomalie de dilatation et du ferrimagnétisme. Ces résultats sont comparés à ceux obtenus par d'autres auteurs pour des matériaux ferromagnétique et antiferromagnétique à moments localisés : une courbe réduite est obtenue

Experimental Study of Microwave Susceptibility in Ferrite Composite Materials

The microwave susceptibility of NiFe2O4 and Co2Z was measured between 100 and 10000 MHz. The samples were either samples with different porosities or composites with different volume fractions, in the case of NiFe2O4, or composites with different granulometry, in the case of Co2Z. The measured susceptibility undergoes substantial changes at the crossover from a ceramic state to a composite of isolated particles and for different granulometries of Co2Z in the range 10 to 600 µm. We show that a model in which the susceptibility is uniform inside the particles cannot account for these results. We suggest that in order to compute the susceptibility of composites, a microscopic approach is required

Study of the Gyromagnetic Resonance Damping in Relaxing Rare Earth Substituted YGd2Fe5O12 Garnets

The influence of magnetic state (saturated or unsaturated states) on the gyromagnetic resonance damping in relaxing rare earth substituted Y1-xDyxGd2Fe5O12 garnets (with x=0;0.15;0.3;0.6) has been studied. The damping parameter α, as defined by Landau-Lifshitz equation, has been determined either by FMR at 10GHz (saturated state) or by measurement of the frequency dependence of the complex initial permeability in the 0.1GHz-10GHz band (unsaturated state). On the one hand, the damping parameter αsat has been calculated by fitting the FMR spectra of single crystals with the theoretical lorentzian shape. On the other hand, the damping parameter αunsat of polycrystals has been calculated using models which take into account the interaction between magnetic domains. In the fully magnetized state the introduction of relaxing rare earth (Dy) is found to increase significantly αsat values (from 0.02 to 0.3 when x varies from 0 to 0.6). On the contrary, the introduction of Dy hardly increases αunsat values. It appears that the difference between αsat and αunsat values stems not only from the interaction between magnetic domains but also from the modification of the damping in each magnetic domain owing to the presence of magnetic domain walls

Characterization of carbon and iron nanostructures synthesized by the DC arc discharge method: influence of the location in the reactor and of the pressure

International audienceX-ray diffraction, Mössbauer spectroscopy, Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) techniques were used to characterize the Fe-C phases in the soots synthesized by the DC arc discharge method. Various equilibrium and non-equilibrium Fe-C compounds were identified, with fractions depending on both the location in the reactor and the helium gas pressure. The soots obtained are composed of the same five phases (C-graphite, α-Fe, γ-Fe, Fe3C and Fe5C2) whatever the helium gas pressure and wherever they are situated in the reactor. However, the location in the reactor has a considerable influence on the size of the particles in the nanostructure. The Fe-C compounds in the Pyrex vessel samples (CL) seem only to be present in the form of nanoparticles embedded in an amorphous gangue, while the water-cooled copper cylinder samples (RS) contain, in addition to these nanoparticles, large composite crystalline particles