Néel's theory of magnetostatic coupling between two magnetic layers with inplane magnetization separated by a non-magnetic spacer has been extended to the case of multilayers with perpendicular anisotropy. It is shown that the presence of a correlated roughness between the successive interfaces induces an interlayer coupling through the spacer analogous to the well-known orange peel coupling. However, depending on the parameters describing the interfacial roughness, the magnetic anisotropy and the exchange stiffness constant, this coupling can favor either parallel or an antiparallel alignment of the magnetization in the two ferromagnetic layers. This model was used to quantitatively interpret the variation of interlayer coupling vs. thickness of Pt spacer layer in out-of-plane magnetized exchange-biased spin-valves comprising (Pt/Co) multilayers as free and pinned layers. It is shown that the net coupling can be interpreted by the coexistence of perpendicular orange peel and oscillatory RKKY couplings. Interestingly, since these two couplings have different thickness dependence, in certain range of Pt thickness, the coupling changes sign during growth, being antiferromagnetic at the early stage of the growth of the top (Co/Pt) multilayer but ferromagnetic once the growth is completed.

We investigate the potential of the extraordinary Hall effect (EHE) in magnetic thin films with out-of-plane anisotropy for sensors, memories or logic applications. The scalability of EHE at decreasing lateral dimension has been first explored. In order for EHE to provide output voltage compatible with CMOS technology, it is shown that the longitudinal resistivity of the magnetic material must be considerably increased at decreasing size while keeping a large Hall angle. Then the EHE properties of various classes of materials with out-of-plane anisotropy ((Co/Pt) multilayers, FePt ordered alloys, rare-earth/transition metal alloys, CoSiOx and CoPtSiOx heterogeneous composites) are measured and compared in order to evaluate their potential for the envisioned applications. It is concluded that while EHE can readily be used for large devices (size > micrometres), no materials are yet available which offer suitable scalability towards the 22 nm microelectronic node.

Perpendicular exchange bias has been observed for IrMn/ ͓Co/ Pt͔ n and ͓Pt/ Co͔ n / IrMn multilayers in the as-deposited state. The exchange bias field is largest when the IrMn film is grown on magnetically saturated Co/ Pt multilayers ͑8.12 mT for n =3͒, whereas it is considerably smaller when domain formation in the IrMn film occurs before Co/ Pt deposition ͑3.30 mT for n =3͒. After annealing at 220°C in an out-of-plane magnetic field the perpendicular exchange bias field and magnetic anisotropy are considerably larger for the Co/ Pt multilayers with an IrMn film at the bottom. The apparent correlation between bias and anisotropy is explained by the dependence of the perpendicular exchange bias field on the orientation of the Co spins near the Co/ IrMn interface.

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We review models for the nucleation of magnetisation reversal, i.e. the formation of a region of reversed magnetisation in an initially magnetically saturated system. For small particles models for collective reversal, either uniform (Stoner-Wohlfarth model) or non-uniform like curling, provide good agreement between theory and experiment. For microscopic objects and thin films, we consider two models, uniform (Stoner-Wohlfarth) reversal inside a nucleation volume and a droplet model, where the free energy of an inverse bubble is calculated taking into account volume energy (Zeeman energy) and surface tension (domain wall energy). In macroscopic systems, inhomogeneities in magnetic properties cause a distribution of energy barriers for nucleation, which strongly influences effects of temperature and applied field on magnetisation reversal. For these systems, macroscopic material parameters like exchange interaction, spontaneous magnetisation and magnetic anisotropy can give an indication of the magnetic coercivity, but exact values for nucleation fields are in general hard to predict.To cite this article: J. Vogel, J. Moritz, O. Fruchart, C. R. Physique 7, 977 (2006). RésuméNucléation du renversement de l'aimantation, de nanoparticules aux systèmes macroscopiques. Nous passons en revue différents modèles traitant de la nucléation dans des systèmes magnétiques. La nucléation est l'étape qui initie le renversement de l'aimantation dans des objets magnétiques préalablement saturés en champ. Pour des particules d'extension spatiale réduite -typiquement inférieureà la largeur de paroi de domaines magnétiques-les modèles de renversement collectif, homogène (modèle de Stoner-Wohlfarth) ou par exemple de type 'curling' donnent un bon accord avec l'expérience. Pour des objets microscopiques et des couches minces, nous discutons deux approches, l'une supposant le renversement homogène de l'aimantation dans un volume d'activation de taille réduite, l'autre fondée sur l'énergie de formation d'une gouttelette. Cette dernière est calculée en tenant compte de l'énergie de volume (terme Zeeman) et de surface (énergie de paroi). Les deux approches sont utilisées pour déterminer des volumes d'activation ou ajuster des courbes expérimentales. Il semblerait que le modèle de gouttelette soit plus pertinent pour expliquer les variations thermiques des champs de renversement ou leur comportement dynamique. Dans les systèmes macroscopiques, les inhomogénéités du matériau donnent lieuà des distributions d'énergie de barrière qu'il est nécessaire d'intégrer dans les calculs. Ainsi les paramètres comme l'anisotropie ou l'échange peuvent donner une indication de la coercitivité, mais il est en général difficile de prévoir sa valeur exacte. Pour citer cet article : J. Vogel, J. Moritz, O. Fruchart, C. R. Physique 7, 977 (2006).

Langmuir probe measurements in front of high power ion cyclotron resonant frequency antennas are not possible or simply too noisy to be analyzed properly. A linear experiment is a radio frequency (RF) magnetized plasma discharge reactor designed to probe the rectified potential in front of such antennas but at low power level (1 kW) to next improve antenna design and mitigate sheath effects. The maximum magnetic field is 0.1 T, and the RF amplifier can work between 10 kHz and 250 MHz allowing ion cyclotron resonances for argon or helium. The first measurements with no magnetic field are presented here, especially 2D potential maps extracted from the RF compensated probe measurements yield ni ≈ 10(15) m(-3) and Te ≈ 2 eV for RF power lower than 100 W. Series resonances in the chamber are highlighted and allow to deduce the plasma parameters from a simple equivalent impedance model of the plasma in helium gas. Next studies will be focused on magnetized plasmas and especially magnetized RF sheaths.

Contact between a Co/ Pt multilayer and an IrMn film leads to perpendicular exchange bias. The exchange bias field does not depend on the degree of ͑111͒ film texture and for Co/ Pt multilayers with IrMn at the bottom it can be enhanced by magnetic field annealing. The perpendicular exchange bias of the Co/ Pt-IrMn system is limited by a misalignment between the Co spins and the film normal, which is due to a negative magnetic anisotropy contribution from the Co/ IrMn interface ͑K S Co/IrMn = −0.09 mJ/ m 2 ͒. The insertion of a 3 Å thick Pt layer at the Co/ IrMn interface maximizes the perpendicular exchange-bias field. © 2005 American Institute of Physics. ͓DOI: 10.1063/1.1855699͔ Exchange-biased ferromagnetic ͑FM͒ films with in-plane magnetization are utilized in applications such as read heads, magnetic sensors, and magnetic random access memory. Exchange bias in these structures is established by contacting the FM film with an antiferromagnetic ͑AFM͒ layer and the effect manifests itself by a shift in the hysteresis loop and enhanced coercivity. 1 For practical applications, IrMn is an interesting AFM material since it combines good corrosion resistance with relatively high interfacial exchange energy, high blocking temperature ͑T B Ϸ 250°C͒, and small critical thickness. [2][3][4][5][6][7] Although it has been intensively studied for systems with in-plane magnetization, it has hardly been used in recent studies on perpendicular exchange bias. [8][9][10][11][12][13][14][15][16][17][18] In a recent paper, Sort et al. showed that Co/ Pt multilayers with out-ofplane magnetization can be biased by growing an IrMn layer on top. 19 In this paper we present an overview on the exchange-biasing properties of IrMn in systems with perpendicular magnetic anisotropy. We show that Co/ Pt multilayers with an IrMn film on top or at the bottom exhibit a considerable perpendicular exchange bias in the as-deposited state. Magnetic field annealing ͑bottom-pinned structures͒ and/or the insertion of a thin Pt layer at the Co/ IrMn interface further enhances the exchange-bias field.The Co/ Pt multilayers were deposited by dc magnetron sputtering onto thermally oxidized Si substrates. A metal shadow mask was used to define Hall bars with a linewidth of 200 m. The relatively large extraordinary Hall effect ͑EHE͒ of the Co/ Pt-IrMn structures, which depends linearly on the out-of-plane moment, was utilized to measure the exchange bias field, the coercivity, and the perpendicular magnetic anisotropy. The crystalline structure of the multilayers was characterized with x-ray diffraction ͑XRD͒. For the field annealing experiments we used a vacuum furnace ͑p Ͻ 1 ϫ 10 −6 mbar͒ and a superconducting magnet. Figure 1 clearly shows that Co/ Pt multilayers with outof-plane magnetization can exhibit perpendicular exchange bias when covered with or grown on top of an IrMn layer. In the as-deposited state the exchange-bias field is largest for Co/ Pt multilayers with IrMn on top. In this case, domain formation in the antiferromagnetic IrMn film i...