R h r n 6 .-A partir du ferrite spinelle de base NiFe204, 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 materiaux de composition Ni1-~Zn~Fe204 et Nil-zCdzFe204 ont ete preparbs en utilisant une technologic speciale. Le pas du rtseau croit linkairement avec le rayon moyen d'ion en site tetrakdrique. La variation de temperature de Curie due a une variation de distance entre les ions magnktiques est Cvaluie. La contribution la temperature de Curie due a la distribution des porteurs magnttiques est mise en Cvidence par une courbe unique hypothktique intercomposition. De la m&me facon, I'influence de la variation de distance entre les ions magnktiques sur I'aimantation Ms a kt6 determinee. Enfin les rbultats obtenus sur les spinelles sont compares avec ceux deduits de la litterature pour les grenats.

on the occasion of his 80th birthday AC permeability measurements of the reentrant spin glass system CdC:r2,In2-,S4 (0.85 < x 1) are performed on toroidal shaped samples by using the transformer technique since the demagnetizing factor is minimized. The complex permeability observed is very close to the true initial permeability. The abrupt decrease of p' a t low temperatures is an intrinsic effect, probably related to the transition to the reentrant spin glass state. The out-of-phase permeability presents a maximum a t the transition temperature TRSG. The frequency dependence of 2'1~s~ is studied (130 Hz 5 5 v 5 100 kHz). Its variation corresponds with the explanation of reentrant properties based on the existence of clusters of which the freezing destroys the ferromagnetic order.En utilisant la mkthode du transformateur, des mesures de permkabilitk alternative ont Cte effectukes sur des bchantillons en forme de tore du systhme rbentrant verre de spin CdCr2,Inz -,S, (0,85 < x 1). Le facteur dkmagnktisant &ant minimisb, la permkabilitb complexe observbe est trhs proche de la permkabilitb initiale intrinshque. A basse tempbrature, la dkcroissance rapide de p' est un effet intrinshque, probablement relib A la transition vers un &tat spin glass rbentrant.La partie complexe de la permkabiliti: prksente un maximum B la tempkrature de transition TRSG.La variation de TRSG avec la frkquence a Otk btudibe (130 Hz 5 v 5 100 kHz); elle est en accord avec l'interprbtation des propribtbs rbentrantes basCe sur l'existence de clusters dont le gel dbtruit l'ordre ferromagnbtique.

The domain wall mobility of ferrimagnets is determined by using an original permeability spectrometer, which operates over a very broad frequency range (1 Hz to 1 GHz) from 77 to 600 K. Using such spectrometer permits to separate the diffusion relaxation and the domain wall relaxation. Three series of samples are investigated (YIG and Ni‐Zn ferrites with various contents of diffusants and Gd‐Y mixed garnets) in order to test the present theories of the domain wall damping: the diffusion damping theory and the spin damping theory. In fact, neither the diffusion damping nor the spin damping can account for the experimental mobility values.

When cycling a hysteresis loop, bursts of high frequency ultrasonic waves are emitted. This general phenomenon, which must not be confused with the classical magnetostrictive effect at twice the cycling frequency, is called the magnetoacoustic emission (MAE). By using literature and authors' results, qualitative and quantitative, the authors review the past and present knowledge of the MAE phenomenon to stress its fundamental origin: the domain wall (DW) creation/annihilation processes. On a qualitative point of view, the shape demagnetizing effect is shown to drastically change the MAE profiles, which allow us to clearly distinguish the MAE from the classical magnetic Barkhausen noise. Quantitatively and in agreement with their interpretation, the authors have established that the total MAE activity along an hysteresis loop is proportional to the hysteresis losses during the same loop; this law has been confirmed on several series of polycrystalline ferrimagnets and is valid whatever the composition, the grain size, or the measuring temperature is. The noncorrelation between the MAE and the magnetostriction is also demonstrated, contrary to some previously reported results and interpretations. Related to the DW creation/annihilation processes, the MAE might be the missing link in the process of the conversion of the magnetic energy into heat during a hysteresis loop.

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