Evaluation of AlO x in Co/AlO x /Co spin tunneling junctions by XPS

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Evaluation of AlOx in Co/AlOx/Co spin tunneling junctions by XPS Yuki Otaka, Hideo Kaiju, Mao Nishiyama, Naoki Sakaguchi and Kazuo Shiiki Department of Applied Physics and Physico-Informatics, Keio University, Yokohama, Kanagawa, 223-8522, Japan. ABSTRACT Oxidation states of AlOx in Co/AlOx/Co spin tunneling junctions were studied using X-ray photoelectron spectroscopy. Co(10nm)/AlOx/Co(50nm) spin tunneling junctions were fabricated on glass substrates by ion-beam mask sputtering. The Al layers were deposited in various thicknesses and were oxidized in pure O2 gas for 24-48 hours at room temperature. Co/AlOx bilayers simultaneously fabricated were analyzed by XPS. The thicker the Al layer is deposited, the more the unoxidized Al remains. MR ratio of the junctions increased as the unoxidized Al decreased, and this result is qualitatively in agreement with the LMTO calculation result. The O/Al ratio of AlOx layer is about 1.9-2.0, which means existence of the higher order oxides than Al2O3. XPS depth profiles showed that the unoxidized Al remains not only near the interface with Co but also near the surface when the deposited Al layer thickness is thick. INTRODUCTION In recent years, spin tunneling junctions have attracted much interest for applications either in magnetic random access memories or in magnetic field sensors for magnetic disk storage because of their large magnetoresistance ratio at room temperature[1,2]. Spin tunneling junctions consist of two ferromagnetic metal electrodes separated by a very thin insulating layer. Aluminum oxide (AlOx) is often used as the insulator in spin tunneling junctions. However, it is very difficult to form ideal aluminum oxide Al2O3. It could be conjectured that the formation of the nonideal aluminum oxide significantly influences the junction property. In the present study, oxidation states of AlOx were investigated using X-ray photoelectron spectroscopy (XPS). Moreover, the influence of oxidation states to the junction property is analyzed. EXPERIMENTS Co(10nm)/AlOx/Co(50nm) spin tunneling junctions were fabricated on glass substrates by ion-beam mask sputtering. The pressure during sputtering was approximately 10-4 Torr. The base pressure was less than 3.0×10-9 Torr. The Al layers were deposited in various thicknesses and were oxidized in pure O2 gas for 24-48 hours at room temperature. The junction area was about 0.1mm × 0.1mm, which was evaluated by the laser scanning microscope. The magnetic anisotropy of each Co layer was orthogonalized by sputtering under a magnetic bias field. The external magnetic field was applied in the direction of the magnetic easy axis for the upper Co (50nm) thin film layer and in the direction of magnetic hard axis for the lower Co (10nm) thin film layer. MR ratio of the junctions was measured by dc 4 probe method. Co/AlOx bilayers simultaneously fabricated were analyzed by XPS. The MgKα X-ray source was operated at 10kV and 10mA. Photoelectron spectra were obtained with analyzer pass energy of 20eV. The pressure during the analysis