Full length article Nano-analysis of Ta/FeCoB/MgO tunnel magneto resistance structures H. Bouchikhaoui a , P. Stender a , Z. Balogh a , D. Baither a , A. Hütten b , K. Hono c , G. Schmitz a, * a Institute of Materials Science, University of Stuttgart, Germany b Department of Physics, University of Bielefeld, Germany c National Institute for Materials Science, 1-2-1 Sengen, Tsukuba 305-0047, Japan article info Article history: Received 17 July 2015 Received in revised form 4 June 2016 Accepted 22 June 2016 Keywords: Tunnel magnetoresistance Magnetic tunnel junction CoFeB MgO Atom probe tomography abstract The partitioning and segregation of B during the crystallization of amorphous FeCoB in Ta/FeCoB/MgO layered structures is investigated by atom probe tomography to obtain a better understanding of the beneficial impact of Ta capping layers on FeCoB/MgO/FeCoB magnetic tunneling junctions. Boron, initially uniformly dissolved in the amorphous FeCoB layer, is rejected from FeCo grains on crystallization and first segregates at the interface to the Ta layer where it prevents nucleation of crystalline FeCo. Only later, it is fully absorbed by the Ta layer. In the studied thin film structures, B neither segregates at nor dissolves into the MgO barrier even after prolonged heat treatment. Important kinetic parameters are derived from detailed isochronal annealing series. They allow the quantitative modelling of the observed process. The combination of high affinity to B but low diffusivity makes Ta the unique capping material to achieve an optimum performance. © 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. 1. Introduction The theoretical prediction of over 1000% tunneling magneto- resistance (TMR) by the preferential spin-dependent tunneling of certain wave functions in a coherent Fe(001)/MgO(001)/Fe(001) magnetic tunneling junction (MTJ) [1,2] led to the experimental demonstration of giant tunneling magnetoresistance (TMR) above 150% at room temperature in MgO based MTJs [3,4]. Subsequently, Djayaprawara et al. [5] reported that a heat-treated CoFeB/ MgO(001)/CoFeB MTJs prepared by magnetron sputtering exhibits a much larger TMR of 230% at room temperature. The high TMR obtained in these MTJs was attributed to the growth of an (001) oriented MgO barrier on an amorphous CoFeB layer, and subse- quent templated crystallization of bcc CoFe with grain-to-grain epitaxy between CoFe and the MgO barrier [6,7]. The TMR ratio of CoFeB/MgO/CoFeB MTJ was found to be further improved by deposition of additional capping layers [8e11]. In particular, Ta capping shows a remarkably positive impact on the TMR ratio of the CoFeB/MgO/CoFeB based MTJ, which is currently used in read head sensors in hard disk drives. Since the TMR values are signifi- cantly affected by the CoFe/MgO interface and the B segregation after the crystallization of amorphous CoFeB, several attempts have been made to quantify B during the recrystallization process. Miyagima et al. [8] and Karthik et al. [9] investigated the boron distribution in CoFeB/MgO/CoFeB pseudo spin valves by high res- olution transmission electron microscopy (HRTEM) and electron energy loss spectroscopy (EELS) with various capping layers. Kozina et al. [12] studied these structures using hard X-ray photoemission HAXPES. They reported a strong dependence of the crystallization process of the CoFeB layer on annealing temperature and capping layer. They stated to see a tendency of boron to diffuse preferen- tially to the Ta capping layer and rarely to the MgO barrier. Recently, Greer et al. [13] used standing-wave hard X-ray photoemission spectroscopy (SW-HXPS). They deduced a concentration of 19.5% of boron uniformly distributed within the MgO barrier and less amount of boron, up to about 2.5 at%, segregated at the interface to the Ta capping. Although these methods give important insight, the conclusions have been deduced through complex data processing. In general, it is difficult to access the segregation of light elements at buried interfaces without real 3D information in atomic resolu- tion. Therefore, we apply in this work laser-assisted atom probe tomography (LA-APT). Atom Probe Tomography (APT) is well known as an outstanding analysis technique on the sub-nanometer scale. It delivers three dimensional reconstructions of the atomic arrangement with single atom sensitivity. The recent extension of the atom probe technique * Corresponding author. E-mail address: guido.schmitz@imw.uni-stuttgart.de (G. Schmitz). Contents lists available at ScienceDirect Acta Materialia journal homepage: www.elsevier.com/locate/actamat http://dx.doi.org/10.1016/j.actamat.2016.06.045 1359-6454/© 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Acta Materialia 116 (2016) 298e307