Interlayer Exchange Coupling in Single Crystal Magnetic Tunnel Junctions Studied by Electron Holography
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1026-C22-05
Interlayer Exchange Coupling in Single Crystal Magnetic Tunnel Junctions Studied by Electron Holography Etienne Snoeck1, Coriolan Tiusan2, Fanny Greullet2, Gerard Benassayag1, Pierre Baules1, and Alain Schuhl3 1 CEMES, CNRS, 29 rue J. Marvig, Toulouse, 31400, France 2 Laboratoire de Physique des Matériaux, CNRS - Université de Nancy, Bd. des Aiguillettes, Vandoeuvre-lès-Nancy, 54506, France 3 SPINTEC, CEA - CNRS, Av. des Martyrs, Grenoble, 38054, France ABSTRACT Ion irradiation has been performed on epitaxial Fe/MgO/Fe magnetic tunnel junctions exhibiting antiferromagnetic exchange coupling. Electron holography and VSM experiments show that the amplitude and the nature of the coupling are modified by the irradiation. We demonstrate that the roughness induced by the irradiation at the Fe/MgO interfaces destroys the AF coupling. The biquadratic coupling experimentally observed in the irradiated samples is the result of the competition between local antiferromagnetic and ferromagnetic coupling. For the largest irradiation doses, the net coupling becomes ferromagnetic. INTRODUCTION An antiferromagnetic (AF) coupling between two ferromagnetic (F) layers separated by an thin insulating barrier has been evidenced by Faure-Vincent et al [1] in Fe/MgO/Fe/MgO(001) epitaxial magnetic tunnel junctions (MTJs). The authors have shown that the AF coupling takes place for a MgO spacer thickness less than one nanometer. The experimental evidence for an AF coupling supports the theoretical model proposed by Slonczewski [2] of an interlayer exchange coupling (IEC) due to spin-polarized quantum tunneling of electrons between the two Fe layers. This coupling is due to the competition between IEC which favors anti-parallel alignment of the two ferromagnetic layers at very small MgO thickness and roughness-induced magnetostatic orange-peel coupling favoring parallel alignment. The most critical parameters for the coupling strength in a Fe/MgO/Fe system are the MgO barrier thickness and the quality of the top and bottom Fe/MgO interfaces. In order to study in more detail the influence of the interfacial microstructure on the IEC, we irradiated epitaxial Au/Pd/Fe/MgO/Fe/MgO(001) MTJs with N+ ions at 150 keV. The modification of the MTJ interfacial structure/morphology as a function of the irradiation dose has been investigated by x-ray reflectivity (XRR) and transmission electron microscopy (TEM), and their magnetic behavior has been studied macroscopically by Vibrating Sample Magnetometer (VSM) and locally by Electron Holography (EH). EXPERIMENT The MTJs were grown in a molecular beam epitaxy (MBE) system on MgO(100) substrates in a UHV chamber whose base pressure is 4×10-11 Torr. A 10 nm thick MgO was deposited on the substrate at 450°C. The bottom 30 nm thick Fe electrode was deposited from a Knudsen cell at room temperature then annealed at 450°C for 20 minutes. A 0.6 nm thick MgO insulating barrier
was evaporated at 100°C on the Fe surface using an electron gun. The two-dimensional layer-bylayer growth was asserted by RH
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