IEEE TRANSACTIONS ON MAGNETICS, VOL. 44, NO. 11, NOVEMBER 2008 2577 Interfacial Oxidation Enhanced Perpendicular Magnetic Anisotropy in Low Resistance Magnetic Tunnel Junctions Composed of Co/Pt Multilayer Electrodes Jeong-Heon Park , Chando Park , and Jian-Gang Zhu , Fellow, IEEE Data Storage Systems Center, Carnegie Mellon University, Pittsburgh, PA 15213 USA Western Digital Corporation, Fremont, CA 94539 USA Low-resistance magnetic tunnel junctions utilizing perpendicular magnetic anisotropy of Co/Pt multilayer electrodes have been in- vestigated. In these junctions, AlO tunnel barrier has been prepared by repeated natural oxidation processes. Each natural oxidation process has been optimized by varying the thickness of pre-oxidized Al layer prior to oxidation. The perpendicular magnetic anisotropy of Co/Pt multilayer has been found to be dependent on the degree of oxidation at the interface of tunnel barrier and Co adjacent layer. It has been observed that magnetization of Co/Pt multilayer is tilted when Co layer adjacent to tunnel barrier is relatively thick, which is attributed to the lack of interfacial anisotropy contribution near the interface of Co adjacent layer and AlO barrier. Interfacial oxi- dation treated on the monolayer thick Co adjacent layer has enhanced perpendicular magnetic anisotropy of the entire Co/Pt multilayer electrodes, enabling sharp transition between well-defined parallel and anti-parallel states at micro-fabricated perpendicular magnetic tunnel junctions of m resistance-area product. Index Terms—Magnetoresistance, magnetic tunnel junction, natural oxidation, perpendicular magnetic anisotropy. I. INTRODUCTION M AGNETIC TUNNEL JUNCTIONS (MTJs) using perpendicular magnetic anisotropy (PMA) have been suggested as a promising candidate to overcome the density limits of the present in-plane-magnetized junctions for scalable magnetic memory application [1]. This is mainly because perpendicular magnetic tunnel junctions (pMTJs) have low saturation magnetization with negligible shape dependence, which is greatly favorable in terms of switching field fluctua- tion predicted as the dimension of devices shrinks. There have been continuing efforts to develop pMTJs with high tunneling magnetoresistance (TMR) by utilizing perpendicularly magne- tized magnetic materials including rare earth-transition metal alloys or exchange-coupled multilayers such as Co/Pt or Co/Pd [1]–[3]. In the previous study, authors experimentally demon- strated pMTJs composed of Co/Pt multilayers with PMA, which yielded robust tunneling magnetoresistance (TMR) of % TMR ratio at room temperature [4]. However, previous researches on pMTJs have dealt with junctions of extremely high resistance, over 100 k m resistance-area (RA) product, which should be inapplicable to high density memory devices. In this paper, low resistance pMTJs with thin AlOx tunnel barrier by natural oxidation have been investigated. On optimizing the tunnel barrier, the authors focus on verifying the effect of oxidizing Co monolayer adjacent to AlOx barrier. This has been designed in order to enhance the PMA of Co/Pt multilayer by producing rich Co-O bonding at the interface [5]. In this paper, modification of magnetization switching behavior in pMTJs with interfacial oxidation treatment has Digital Object Identifier 10.1109/TMAG.2008.2003071 been addressed. The effect of interfacial oxidation has been in- vestigated through magnetization-magnetic field measurement and TMR transfer curves at the micron-size fabricated pMTJ devices of m RA product. II. EXPERIMENTAL SETUP All film stacks including magnetic layers, leads and tunnel barrier were prepared by rf/dc sputtering below torr base pressure. Co/Pt multilayers have been optimized to have perpendicular anisotropy by changing thickness of each layer and repeat of bilayer. The entire MTJ film stack is composed of two different Co/Pt multilayers, AlOx tunnel barrier, and Co adjacent layers which bridge electrodes and the tunnel barrier. AlO tunnel barrier has been prepared by repeated processes of thin Al layer sputtering followed by natural oxidation under 100 mT oxygen pressure for 5 min. We split tunnel barrier ox- idation process by the treatment of so called interfacial oxida- tion; one group of pMTJ samples has been treated and the other has not. The typical interfacial oxidation treatment on AlO bar- rier and Co adjacent layers is depicted in Fig. 1. In this figure, each natural oxidation process has been optimized for the full oxidation of Al layer. For enriching Co-O bonding at the bottom interface, 3 thick Al is deposited on top of the bottom Co adjacent layer prior to the first natural oxidation. For the top interface oxidation, monolayer thick Co is deposited on top of 3 Al layer with both layers being oxidized by the third nat- ural oxidation step. For non-treated samples, 5 Al layer was deposited without Co adjacent layer before each natural oxida- tion step, and these procedure was repeated two or three times. After film deposition, magnetization switching has been inves- tigated with film level perpendicular hysteresis by alternating gradient magnetometer (AGM) measurement. In order to obtain TMR transfer curves, micron-size MTJ test patterns were fab- ricated using conventional optical lithography and ion milling processes. 0018-9464/$25.00 © 2008 IEEE