Basic concepts for high permeability in soft ferrites

The main concepts used to obtain high values of initial magnetic permeability in ferrimagnetic toroidal samples, arc presented. Generally the main contribution to the permeability is due to the mechanism of domain wall bulging which is proportional to (Ms2 / K) x D, where Ms is the saturation magnetization, K the magnetic anisotropy constant and D the domain wall size (length) fixed either by the mean grain size, or by the mean distance between two wall pinning centers. Then, three concepts can be applied to increase the permeability. The first is based on the cancellation of anisotropy K, by using the phenomenon of compensation of the negative anisotropy of Fe3+ ions by ions with positive anisotropy, as for example for MnZn ferrite or TbSc(YIG). The second concept is to use the relative variation of magnetization Ms2(T) and anisotropy K(T) : for some specific temperatures, Ms2(T) /K(T) can have a maximum as for example near the Curie temperature "the Hopkinson peak" for all ferrites. The third concept is to increase the distance D, for example by increasing the mean grain size during the synthesis of polycrystalline samples of good crystalline quality. A striking linear law between permeability and grain size is observed in many materials : spinels, garnets, hexaferrites and even metals. So, the appropriate choice of adjustable parameters related, either to the ionic structure or to the granular structure, allows to obtain high permeability values

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

MATÉRIAU FERROMAGNÉTIQUE FEUILLETÉ POUR HYPERFRÉQUENCES

Dès 1949 Sakurai [1] envisageait l'emploi de matériaux électromagnétiques " artificiels ". Clogston et d'autres [2, 3, 4] cherchèrent à réduire les pertes joule des câbles coaxiaux ; ils proposèrent un fin feuilletage en lame alternativement conductrice et isolante pour supprimer l'effet de peau. Le développement de la microélectronique et celui des films minces reposent aujourd'hui le problème. Nous calculons la constante de propagation d'un milieu finement stratifié. En plus des pertes électromagnétiques des deux composants on distingue trois nouvelles sources de pertes, celles dues à la résistivité du métal, celles dues à l'épaisseur du film métallique, et celles dues à une propagation non parallèle aux couches. Les expériences confirment la théorie. A titre d'exemple nous présentons une ligne à sens unique en microstrip, où le ferrite est remplacé par un permalloy feuilleté, la non-réciprocité atteint 10 dB.The use of " artificial " electromagnetic material was first envisaged in 1949 [1]. Clogston and others [2, 3, 4] seek to reduce joule loss in coaxial cables ; they proposed the alternate stacking of thin conducting and insulating lamina to minimize the skin effect. Due to the development of microelectronics and thin films to-date, this problem is again of interest. In this paper the propagation constant in a stratified medium is calculated. In addition to the two ordinary components of dielectric and magnetic losses in insulating media, we found an additional conductivity-dependent loss which can be further subdivided into three components : that due to the finite resistivity of the metal, that due to the finite thickness of the metallic film, and that due to wave propagation which is not parallel to the film plane. The results of our experiments are in accord with our theory. As an example, we present here the attenuation data on a microstrip unidirectional transmission line, where the ferrite is replaced by a laminated permalloy-dielectric medium ; the isolation to insertion loss ratio for this case is 10 dB

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

DISTANCE BETWEEN MAGNETIC IONS AND FUNDAMENTAL PROPERTIES IN FERRITES

A partir du ferrite spinelle de base NiFe2O4, on a introduit des ions de rayon ionique de plus en plus grand, Zn2+ et Cd2+, avec une concentration x de plus en plus forte. Ces matériaux de composition Ni1-xZnxFe2O4 et Ni1-xCdxFe2O4 ont été préparés en utilisant une technologie spéciale. Le pas du réseau croît linéairement avec le rayon moyen d'ion en site tétraédrique. La variation de température de Curie due à une variation de distance entre les ions magnétiques est évaluée. La contribution à la température de Curie due à la distribution des porteurs magnétiques est mise en évidence par une courbe unique hypothétique intercomposition. De la même façon, l'influence de la variation de distance entre les ions magnétiques sur l'aimantation Ms a été déterminée. Enfin les résultats obtenus sur les spinelles sont comparés avec ceux déduits de la littérature pour les grenats.In the basic spinel NiFe2O4, ions of larger and larger ionic radius, Zn2+ and Cd2+, have been introduced with higher and higher concentrations. The corresponding materials of composition Ni1-xZnxFe2O4 and Ni1-xCdxFe2O4 have been prepared by using a special technologic process. The lattice constant increases linearly with the mean ionic radius in tetrahedral site. The variation of the Curie temperature due to a variation of the distance between magnetic ions is evaluated. The contribution to the Curie temperature due to the magnetic carriers distribution is shown by plotting a single intercomposition hypothetic curve. In the same way the influence upon the magnetization value of the variation of the distance between magnetic ions has been evaluated. Results obtained on spinels have been compared with those deduced from literature data in the case of garnets

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