Abstract
Perpendicular CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJs) prepared from (Co25Fe75)1−xBx alloys are found to have better annealing stability when made with 30at% boron relative to a more typical 20at% boron. A comparison of film-level properties shows that perpendicular magnetic anisotropy (PMA) increases significantly for 30at%B, while the range of electrode thicknesses that maintain a perpendicular easy axis also increases. Because capping layer interdiffusion has been previously suggested to play a role in the breakdown of PMA with annealing temperature, we have isolated its effect by studying the annealing process of thin Ta/CoFeB(2nm)/Ta trilayers. Through analysis of the decrease in Curie temperature during annealing, we can infer that higher boron content indeed suppresses growth of the intermixed CoFeB-Ta dead layer. For device structures and processing conditions where interdiffusion is a limiting factor, increasing boron content is shown to result in substantially improved tunneling magnetoresistance (TMR).
Highlights
As a primary candidate for use in spintronic applications, the CoFeB/MgO material system has been studied extensively since its introduction.[1,2] CoFeB alloys were broadly chosen based on a boron concentration expected to minimize the thermodynamic driving force for crystallization, which occurs near 20at%B.3 Alloys at this concentration have been found to optimize the tunneling magnetoresistance (TMR) vs.annealing temperature behavior of in-plane magnetic tunnel junctions (MTJs) where CoFeB electrodes are typically in the range of 3-5nm thick.[4]
With the widespread transition towards perpendicular MTJs where much thinner layers are required,[5] annealing stability has become a critical challenge and necessitates that alloy selection be revisited in order to leverage secondary effects beyond amorphization during growth
Despite the fact that perpendicular magnetic anisotropy (PMA) originates at the CoFeB/MgO interface,[7] the capping layer has a critical impact on the observed anisotropy[8,9,10] that is not well understood
Summary
As a primary candidate for use in spintronic applications, the CoFeB/MgO material system has been studied extensively since its introduction.[1,2] CoFeB alloys were broadly chosen based on a boron concentration expected to minimize the thermodynamic driving force for crystallization, which occurs near 20at%B.3 Alloys at this concentration have been found to optimize the TMR vs.annealing temperature behavior of in-plane MTJs where CoFeB electrodes are typically in the range of 3-5nm thick.[4]. Annealing temperature behavior of in-plane MTJs where CoFeB electrodes are typically in the range of 3-5nm thick.[4] With the widespread transition towards perpendicular MTJs where much thinner layers are required,[5] annealing stability has become a critical challenge and necessitates that alloy selection be revisited in order to leverage secondary effects beyond amorphization during growth.
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