Exchange bias, referred to the interaction between a ferromagnet ͑FM͒ and an antiferromagnet ͑AFM͒, is a fundamental interfacial magnetic phenomenon, which is key to current and future applications. The effect was discovered half a century ago, and it is well established that the spin structures at the FM/AFM interface play an essential role. However, currently, ad hoc phenomenological anisotropies are often postulated without microscopic justification or sufficient experimental evidence to address magnetization-reversal behavior in exchange-bias systems. We advance toward a detailed microscopic understanding of the magnetic anisotropies in exchange-bias FM/AFM systems by showing that symmetry-breaking anisotropies leave a distinct fingerprint in the asymmetry of the magnetization reversal and we demonstrate how these emerging anisotropies are correlated with the intrinsic anisotropy. Angular and vectorial resolved Kerr hysteresis loops from FM/AFM bilayers with varying degree of ferromagnetic anisotropy reveal a noncollinear anisotropy, which becomes important for ferromagnets with vanishing intrinsic anisotropy. Numerical simulations show that this anisotropy naturally arises from the inevitable spin frustration at an atomically rough FM/AFM interface. As a consequence, we show in detail how the differences observed for different materials during magnetization reversal can be understood in general terms as originating from the interplay between interfacial frustration and intrinsic anisotropies. This understanding will certainly open additional avenues to tailor future advanced magnetic materials.

A detailed study of the angular dependence of the magnetization reversal in polycrystalline ferromagnetic ͑FM͒/antiferromagnetic Co/IrMn bilayers with noncollinear FM and unidirectional anisotropies shows a peculiar asymmetric magnetic behavior. The anisotropy configuration is set via a field cooling ͑FC͒ procedure with the magnetic field misaligned with respect to the easy magnetization direction of the FM layer. Different magnetization reversal modes are observed for either positive or negative angles with respect to the FC direction. The angular dependence of both coercivity and exchange bias also clearly displays the broken symmetry of the induced noncollinearity. Our findings are reproduced with a modified Stoner-Wohlfarth model including the induced anisotropy configuration. Our results highlight the importance of the relative angle between anisotropies in exchange bias systems, opening a new path for the tailoring of their magnetic properties. © 2009 American Institute of Physics. ͓doi:10.1063/1.3236768͔ Ferromagnetic/antiferromagnetic ͑FM/AFM͒ structures 1 are at the heart of today's spintronic devices, stabilizing the direction of FM reference layers, while taking advantage of the interfacial exchange interaction effects. 2 The most notable consequences of the FM/AFM exchange coupling are a shift in the hysteresis loop of the FM layer, called exchange bias 0 H E , an enhanced coercivity 0 H C , and an asymmetry in the magnetization reversal process. Experiments have shown that pinned ͑unpinned͒ uncompensated AFM spins at the interface are correlated with 0 H E ͑Ref. 3͒ ͑coercivity enhancement 4 ͒, and that the competition between anisotropies determines the asymmetric behavior of the magnetization reversal. 5,6 Different intrinsic parameters ͑e.g., materials, anisotropies, thicknesses, and shapes͒ 2 as well as extrinsic ones ͓e.g., field cooling ͑FC͒ procedures 7-10 ͔ have been explored to understand the exchange coupling phenomena in FM/AFM systems, aiming at improving the performance of magnetic devices. In general, the interfacial exchange coupling effects depend on the strength of the anisotropies 5 as well as their relative orientation, 6 exhibiting a complex phase diagram of different reversal modes. 5,6,[11][12][13][14][15] In fact, the relative orientation between the intrinsic FM anisotropy and the induced interfacial unidirectional anisotropy can be controlled by different FC procedures, varying both strength, 7,8 FC angle, 6,9,10 and/or interfacial magnetic frustration. 14,15 In this letter we present a detailed study on the magnetization reversal of FM/AFM systems with a noncollinear uniaxial, K U , and unidirectional, K E , anisotropy configuration. Our work reveals the importance of taking into account the misalignment between the K U direction and the direction of the applied field during the FC procedure in order to properly account for the asymmetry of the magnetization reversal and the angular dependences of 0 H C and 0 H E .The collinear and noncollinear relative orientation between t...

A vectorial magneto-optic Kerr effect (v-MOKE) setup with simultaneous and quantitative determination of the two in-plane magnetization components is described. The setup provides both polarization rotations and reflectivity changes at the same time for a given sample orientation with respect to a variable external magnetic field, as well as allowing full angular studies. A classical description based on the Jones formalism is used to calculate the setup's properties. The use of different incoming light polarizations and/or MOKE geometries, as well as the errors due to misalignment and solutions are discussed. To illustrate the capabilities of the setup a detailed study of a model four-fold anisotropy system is presented. Among others, the setup allows to study the angular dependence of the hysteresis phenomena, remanences, critical fields, and magnetization reversal processes, as well as the accurate determination of the easy and hard magnetization directions, domain wall orientations, and magnetic anisotropies.

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We present a detailed study on the magnetic properties, including anisotropy, reversal fields, and magnetization reversal processes, of well characterized half-metallic epitaxial La 0.7 Sr 0.3 MnO 3 (LSMO) thin films grown onto SrTiO 3 (STO) substrates with three different surface orientations, i.e., (001), (110), and (118). The latter shows step edges oriented parallel to the [110] (in-plane) crystallographic direction. Room temperature high resolution vectorial Kerr magnetometry measurements have been performed at different applied magnetic field directions in the whole angular range. In general, the magnetic properties of the LSMO films can be interpreted with just the uniaxial term, with the anisotropy axis given by the film morphology, whereas the strength of this anisotropy depends on both structure and film thickness. In particular, LSMO films grown on nominally flat (110)-oriented STO substrates presents a well defined uniaxial anisotropy originated from the existence of elongated in-plane [001]-oriented structures, whereas LSMO films grown on nominally flat (001)-oriented STO substrates show a weak uniaxial magnetic anisotropy, with the easy axis direction aligned parallel to residual substrate step edges. Elongated structures are also found for LSMO films grown on vicinal STO(001) substrates. These films present a well-defined uniaxial magnetic anisotropy, with the easy axis lying along the step edges, and its strength increases with the LSMO thickness. It is remarkable that this step-induced uniaxial anisotropy has been found for LSMO films up to 120 nm thickness. Our results are promising for engineering novel half-metallic magnetic devices that exploit tailored magnetic anisotropy.