PHYSICAL REVIEW B 91, 134413 (2015) Effect of CoO/Ni orthogonal exchange coupling on perpendicular anisotropy of Ni films on Pd(001) P. Ku´ swik, 1, 2 P. L. Gastelois, 1, 3 M. M. Soares, 4 H. C. N. Tolentino, 5, 6 M. De Santis, 5, 6 A. Y. Ramos, 5, 6 A. D. Lamirand, 5, 6 M. Przybylski, 1, 7, * and J. Kirschner 1, 8 1 Max-Planck-Institut f¨ ur Mikrostrukturphysik, 06120 Halle, Germany 2 Institute of Molecular Physics, Polish Academy of Sciences, 60-179 Pozna´ n, Poland 3 Centro de Desenvolvimento da Tecnologia Nuclear, 31270-901 Belo Horizonte, MG, Brazil 4 European Synchrotron Radiation Facility, 38043 Grenoble, France 5 University Grenoble Alpes, Institut N´ eel, 38042 Grenoble, France 6 CNRS, Institut N´ eel, 38042 Grenoble, France 7 Faculty of Physics and Applied Computer Science, and Academic Centre for Materials and Nanotechnology, AGH University of Science and Technology, 30-059 Krak´ ow, Poland 8 Naturwissenschaftliche Fakult¨ at II, Martin-Luther-Universit¨ at Halle-Wittenberg, 06120 Halle, Germany (Received 21 April 2014; revised manuscript received 10 February 2015; published 10 April 2015) The effect of orthogonal exchange coupling between antiferromagnetic CoO and ferromagnetic Ni/Pd(001) on the perpendicular anisotropy of Ni films is studied. The thickness range in which Ni films show a perpendicular easy magnetization axis is extended by growing CoO on top of them, however, only at temperatures below T N of CoO. The perpendicular orientation of Ni spins and the in-plane orientation of CoO spins are confirmed by the magneto-optic Kerr effect/x-ray magnetic circular dichroism and x-ray magnetic linear dichroism, respectively. Additionally, a perpendicular exchange bias shows up at low temperature. DOI: 10.1103/PhysRevB.91.134413 PACS number(s): 75.70.Cn, 75.30.Gw, 75.50.Ee, 75.70.Ak I. INTRODUCTION The interface exchange coupling mechanism in the antifer- romagnet (AFM)/ferromagnet (FM) system has been widely investigated in the last few decades. A lot of attention has been given to distinguish the coupling direction between the FM and AFM spins in such systems [1–3]. It is of particular interest to exploit the FM/AFM coupling which modifies magnetic anisotropy and switches the easy magnetization axis from in plane to out of plane. This is very important for many novel applications where perpendicular magnetization is strongly required, in particular, for tunneling magnetoresistance (TMR) based reading heads and for magnetic random access memory (MRAM) [4,5]. Very recently, it was shown that the magnetic easy axis of FM layers can be changed from in plane to out of plane due to the collinear coupling between the perpendicularly oriented unpinned magnetic moments of the Mn AFM layer in the Mn/Fe system [6]. Existing models explain the exchange bias effect for systems with an uncompensated AFM surface (i.e., all spins are oriented in the same direction) and with collinear (i.e., parallel or antiparallel) FM/AFM exchange coupling at the in- terface [7]. However, theoretical calculations for compensated interfaces (i.e., neighboring spins of the interfacial AFM plane are oriented antiparallel) show that the AFM and FM spins may be coupled orthogonally due to the “spin-flop” mechanism [8]. In the last decade, the compensated antiferromagnet (AFM) surface, e.g., CoO(001) in contact with a ferromagnetic layer, has been deeply investigated [9–11]. A lot of attention has been paid to the system with a CoO layer, because as an AFM layer, it has strong magnetic anisotropy [12,13] and an ordering temperature close to room temperature (RT) [13,14]. Moreover, recently, it was experimentally and theoretically * marprzyb@agh.edu.pl shown that the orientation of the magnetic moments of CoO can be controlled via the strain applied to that layer [15,16], which can open new opportunities to investigate the coupling direction in CoO/FM systems. Up to now, one of the main interests was focused on the CoO/Fe system to distinguish the direction of the coupling between CoO and Fe spins [14,17,18]. It was found that this interfacial coupling favors perpendicular alignment [17], however, due to the substrate influence, collinear coupling was also observed [18]. It should be noted that the quality of the CoO/Fe interface plays an important role because the large number of uncompensated spins in the nominally compensated surface of CoO can change the coupling direction from orthogonal to collinear [14]. Theoretically, it is well known that such an orthogonal coupling can generate an effective uniaxial anisotropy [19]. This has been observed experimentally for many FM systems with in-plane anisotropy, for instance, NiO/Fe or CoO/Fe bilayers grown on a Ag(001)-stepped surface [18,20,21]. However, there are only a few reports regarding orthogonal coupling in the layered system with perpendicular mag- netic anisotropy (PMA) [22–24]. In magnetron sputtered CoO/[Co/Pt] multilayers, it was observed that the in-plane spin orientation of the CoO layer supplies the additional in-plane contribution to the anisotropy of the Co/Pt, thereby reducing the PMA of the Co/Pt multilayer [22]. On the other hand, in a well-crystallized CoO/FePt(L1 0 ) bilayer, where the FM layer shows perpendicular anisotropy, it was reported that the Co spins were aligned in the sample plane [23,24]. However, such a robust orthogonal coupling has never been shown as a source of additional uniaxial anisotropy favoring perpendicular magnetization. In this paper, we show both that orthogonal FM/AFM coupling may occur in a plane perpendicular to the sample surface and that the easy magnetization axis of the FM layer can be reoriented perpendicular to the sample plane due to this coupling. We have previously demonstrated that 1098-0121/2015/91(13)/134413(5) 134413-1 ©2015 American Physical Society