Magneto-Resistance and Induced Domain Structure in Tunnel Junctions
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MAGNETO-RESISTANCE AND INDUCED DOMAIN STRUCTURE IN TUNNEL JUNCTIONS M. Hehn, O. Lenoble, D. Lacour and A. Schuhl Laboratoire de Physique des Matériaux, UMR CNRS 7556, B.P. 239, 54506 VANDOEUVRE LES NANCY Cedex, France. C. Tiusan, D. Hrabovsky* and J.F. Bobo* IPCMS, 23 rue du Loess, 67037 STRASBOURG Cedex, France. * LPMC, 135 Av. de Rangueil, 31077 Toulouse Cedex, France ABSTRACT Magnetization reversals in sputtered Co electrodes of a magnetic tunnel junction are studied using transport measurements, magneto-optic Kerr magnetometry and microscopy. Using the tunnel magneto-resistive effect as a probe for micromagnetic studies, we first evidence the existence of an unexpected domain structure in the soft Co layer. This domain structure originates from the duplication of the domain structure of the hard Co layer template into the soft layer via ferromagnetic inter-electrode coupling. A detailed analysis of the kerr microscopy images shows that all features appearing in the variation of tunnel resistance as a function of the applied field are associated to the domain phase evolution of each electrode. By tailoring the magnetic properties of the hard Co layer, we have demonstrated that the appearance of the domain duplication is driven by the magnetic anisotropy of the hard layer. Finally, a brief theoretical description of the domain duplication process allows us to extract the main parameters governing the effect.
INTRODUCTION The increasing interest in magnetic tunnel junctions (MTJs) for spin electronic devices requires the understanding and the control of the magnetic properties of their ferromagnetic (FM) electrodes. A coupling between the two electrodes of the MTJ takes usually place due to their proximity. These interactions between a magnetically hard (reference) and a soft (detection) layer are of particular importance as they influence the reversal characteristics of the FM layers, and thus, the magneto-resistive behavior of the tunnel device. We illustrate explicitly the effect of the magneto-static coupling on the transport and magnetic properties of those systems.
EXPERIMENTAL DETAILS For this purpose, we prepared Glass/Co(10nm)/Al(eAl nm, tox mn)/Co(eCo nm)/ Co (20nm) cross like tunnel junctions by using sputtering and ex situ changed shadow masks. Details on the junction fabrication (oxidation process to make the AlOx tunnel barrier, geometry for CCP measurements...) can be found elsewhere [1,2]. When Co is deposited at low Ar pressure (5×10-3 mbar), the grain diameter is less than 2nm and the magnetization reversal of a Co(10nm) layer is sharp with nucleation and propagation of domain walls [3]. In addition, this low pressure process leads to the appearance of an anisotropy axis due to the geometry of the deposition. When the Ar pressure increases up to an optimum, equal to 1.5×10-2 mbar, the grain size and the coercive field increase up to a maximum. In this case, *
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the magnetization behavior is consistent with a reversal occurring through the appearance of ripples domain structures [4]. More
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