A phenomenological approach for polycrystalline exchange-bias bilayers is proposed which explains the coercivity enhancement as well as its temperature and coupling strength dependences. In the model, it is assumed that uncompensated interfacial antiferromagnetic grains can switch their magnetizations irreversibly, producing a rotatable anisotropy. A preferential distribution of the antiferromagnetic easy axes is also considered. An inhomogeneous ferromagnetic magnetization reversal is allowed, assuming that the ferromagnet is divided into domains, each coupled to a stable antiferromagnetic grain only. The antiferromagnetic anisotropy distribution affects the angular dependence of the coercivity, reducing its value in the vicinity of the exchangebias direction, also smoothing the loop shift variations, more notably for small ferromagnetic uniaxial anisotropy. The inclusion of the rotatable anisotropy changes the shape of the magnetization curves and their characteristics. The larger the relative contribution of the rotatable anisotropy to the effective uniaxial anisotropy, the closer the loop shift angular variation gets to a pure cosine behavior, and no significant effect on the coercivity for strong coupling is detected. The frequently observed peak in the temperature variation of the coercivity is also explained considering the variation of the rotatable anisotropy, which is directly connected to the temperature dependence of the unstable antiferromagnetic grains' magnetization.

This work investigates the structure and interface perpendicular magnetic anisotropy ͑PMA͒ of electrodeposited Cu/Co/Au͑111͒ sandwiches with variable Co thickness ͓2-20 monolayers ͑ML's͔͒. In optimum deposition conditions, polar magneto-optical Kerr effect measurements show that the axis of easy magnetization is perpendicular to the layers for thicknesses below ca. 7.2 ML's. This value is among the best ever reported for the Cu/Co/Au͑111͒ structure. While extended x-ray-absorption fine structure indicates that layers are hcp, in situ STM imaging suggests that magnetoelastic effects contribute significantly to PMA. The correlation observed between the strength of PMA and film structure is discussed in details.

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Ferromagnetic resonance ͑FMR͒ and magnetization ͑MAG͒ measurements were used to study the exchange interaction between the antiferromagnetic and ferromagnetic layers in an IrMn/ Cu/ Co system as a function of the Cu spacer thickness. Although the experimental angular variations of the exchange-bias fields H eb FMR and H eb MAG coincide, the coupling strengths J and the Co layers' anisotropy fields H U , obtained via numerical simulations, are different. For all Cu thicknesses J FMR Ͼ J MAG and H U FMR Ͻ H U MAG. The exchange coupling decreases exponentially with the spacer thickness and is a short-range interaction. These characteristics were explained in the framework of a model considering polycrystalline magnetic layers with independent easy axis distributions, taking into account the rotatable anisotropy. The role of antiferromagnetic grains at the interface with different sizes and different magnetic stabilities is essential for understanding the behavior of this exchange-biased system.

Granular Co 10 Cu 90 alloys displaying giant magnetoresistance have been obtained by melt spinning followed by an appropriate heat treatment in the range 0-700°C. Their structural and magnetic properties have been studied on a microscopic scale using 59 Co NMR technique and thermoremanent magnetization measurements. The study reveals that in the as-quenched samples Co is involved in two main structural components: small, irregular, strained Co particles ͑60% of the entire Co population͒ and a composition modulated CoCu alloy. A high modulation amplitude of the concentration profile in the alloy subdivides the latter in two parts with distinctly different properties. One part consists of ferromagnetic alloy ͑average Cu concentration of about 20%͒ with a blocking temperature of about 35 K ͑involving 6% of the entire Co population in a sample͒. The other part, containing the remaining 34% of the entire Co population, is a paramagnetic alloy with a blocking temperature below 4.2 K. The ferromagnetic alloy is magnetically soft-its transverse susceptibility is lower by a factor of 7 than the transverse susceptibility of the quenched-in Co particles. The latter population has a blocking temperature of about 150-200 K. During the heat treatment, each of the two main structural components undergoes respective decomposition processes: both of them display two temperature regimes. One process consists in dissolving the quenched-in Co particles after annealing at around 400°C, followed at higher temperatures by a nucleation and growth of the more regular in shape Co particles. The other process resembles a spinodal decomposition of the quenched-in CoCu alloy, resulting in sharpening of the concentration profile and eventually leading to Co cluster formation in samples annealed above 450°C. Both processes end at about T an ϭ700°C, in formation of large, pure Co clusters that are ferromagnetic at least up to 400 K.

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Short-time dynamics of correlated magnetic moments in superparamagnetic Cu-Co melt spun alloys exhibiting giant magnetoresistance