Thermodynamic Evaluation of the Interface Stability Between Selected Metal Oxides and Co
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Peter F. Ladwig Materials Science Program, University of Wisconsin, Madison, Wisconsin, 53706
Y. Austin Chang Materials Science and Engineering Department and Materials Science Program, University of Wisconsin, Madison, Wisconsin 53706
Feng Liu, Bharat. B. Pant, and Allan E. Schultz Recording Head Operations, Seagate Technology, Bloomington, Minnesota 55435 (Received 24 June 2003; accepted 7 January 2004)
For an interface to be considered thermodynamically stable, the phases in contact must be in equilibrium with each other (connected by a stable tie-line) and have negligible mutual solubility on the phase diagram. The stability of Co based magnetic tunnel junctions (MTJs), with Co/MxO1−x/Co structures (M ⳱ Al, Gd, Hf, La, Mg, Si, Ti, Ta, Y and Zr), were evaluated with regard to these two conditions. Specifically, low temperature ternary isothermal phase diagrams were calculated and evaluated for the Co–M–O systems. All of these systems have at least one oxide in equilibrium with Co and thus have at least one thermodynamically stable tunnel barrier candidate for use in Co based MTJs. In light of the assumptions made in this analysis, along with the uncertainty in applying bulk enthalpy data to thin films, the current evaluation of interfacial stability serves as a first step in identifying suitable stable tunneling barrier materials in MTJs for detailed study.
I. INTRODUCTION
Magnetic tunnel junctions (MTJs) consist of two ferromagnetic metallic electrodes separated by a insulating (tunnel barrier) layer and are studied for their applications in magnetic random access memories and tunneling magnetoresistive (TMR) read heads. To ensure successful device operation, several characteristics of MTJs are critical, including the stoichiometry and uniformity of the insulator (tunnel barrier) layer, thermodynamic stability, and the smoothness and abruptness of the ferromagnetinsulator interface.1 The most successful tunnel barrier materials to date are Al2O3, AlN and MgO,2 but other materials such as HfO2,3 ZrO2,4 Ta2O5,5 and Gd2O36 also show potential to be good tunnel barriers in MTJs. In the present study, we estimate the potential of particular oxide insulators with regard to a key characteristic of successful operation, thermodynamic stability. We chose to investigate the stability of Co in contact with the tunnel barriers of choice based on the widespread use of Co and Co–Fe alloys (with Fe concentrations near 10 at.%) a)
Address all correspondence to this author. e-mail:[email protected] DOI: 10:1557/JMR.2004.0153 J. Mater. Res., Vol. 19, No. 4, Apr 2004
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in TMR devices. Specifically, we chose to investigate the following systems: Co–Al–O, Co–Ti–O, Co–Mg–O, Co– Gd–O, Co–La–O, Co–Si–O, Co–Ta–O, Co–Hf–O, Co– Zr–O, and Co–Y–O based on two principles. The first principle is that the metal should be a good oxide former. In other words, the metal oxide should have a large negative enthalpy of formation. The second principle is that the insulator barrier should
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