Tunable magnetic anisotropy of antiferromagnetic NiO in (Fe)/NiO/MgO/Cr/MgO(001) epitaxial multilayers

www.nature.com/scientificreports OPEN Tunable magnetic anisotropy of antiferromagnetic NiO in (Fe)/ NiO/MgO/Cr/MgO(001) epitaxial multilayers W. Janus 1*, T. Ślęzak 1, M. Ślęzak 1, M. Szpytma 1, P. Dróżdż 1, H. Nayyef 1, A. Mandziak 3, D. Wilgocka‑Ślęzak 2, M. Zając 3, M. Jugovac 4, T. O. Menteş 4, A. Locatelli 4 & A. Kozioł‑Rachwał 1 We report on the magnetic properties of antiferromagnetic NiO(001) thin films in epitaxially grown NiO/MgO(dMgO)/Cr/MgO(001) system for different thicknesses of MgO, dMgO. Results of X-ray Magnetic Linear Dichroism show that together with an increase of dMgO, rotation of NiO spins from in-plane towards out-of-plane direction occurs. Furthermore, we investigated how the proximity of Fe modifies the magnetic state of NiO in Fe/NiO/MgO(dMgO)/Cr/MgO(001). We proved the existence of a multidomain state in NiO as a result of competition between the ferromagnet/antiferromagnet exchange coupling and strain exerted on the NiO by the MgO buffer layer. Conventional spintronic devices utilize electron’s spin degree of freedom to store information in the ferromagnetic layer1. However, recent demonstrations of magnetotransport effects in antiferromagnets (AFMs) make them excellent candidates for usage as active elements in future spintronic devices2. AFMs offer several advantages over ferromagnetic counterparts, such as higher packing density due to the absence of stray magnetic fields, robustness against external magnetic field and a potentially higher operation speed as a result of terahertz spin d ynamics3–5. Different methods were utilized to modify the spin structure in AFMs, i.e. magnetic, optical, electrical and strain manipulation6. Magnetic methods include the application of relatively large external magnetic fields to align magnetic moments of AFMs7,8 and indirect control of the AFM spin structure via interfacial exchange coupling with a ferromagnet (FM) in FM/AFM bilayer systems, where a relatively small magnetic field is n eeded9–13. The FM/AFM bilayers containing antiferromagnetic NiO are considered as model systems to study ferromagnet-antiferromagnet coupling14. Previous studies do not provide a conclusive view on the relative orientation between magnetic moments of FM and AFM layers in FM/NiO structure. While for the ideal FM/AFM interface a perpendicular alignment of AFM and FM magnetic moments across the interface is expected14, experimental observations of collinear and non-colinear coupling have been reported for FM/NiO15–19. Recently, NiO has been proposed as an active element of spintronics devices. Demonstration of spin hall m agnetoresistance20–22, current-induced switching23–25 and optical generation of ultrafast spin current in NiO/Pt bilayers26 opened a new path toward the realization of NiO-based spintronic devices. NiO is a transition metal oxide with a rock-salt crystal structure. In the bulk, below its Néel temperature of TN = 523 K, the magnetic moments of N i2+ cations are ferromagnetically aligned within each (111) plane and antiferromagnetically aligned between neighboring (111) planes27. Previous works demonstrated that appropriate strain engineering and finite size effects could modify the magneto-crystalline anisotropy and influence the direction of the magnetic moments in ultrathin NiO layers28. It was found that compressive strain leads to an inplane NiO spin alignment, while tensile strain preferentially stabilizes an out-of-plane orientation of AFM spins. Lattice distortion in NiO can be induced either by the growth of AFM on f erroelectric29,30 or lattice miss-matched substrates31–34. Representatives of the latter approach are epitaxial NiO/Ag(001) and NiO/MgO(001) systems. It was shown that compressive strain in a thin NiO layer grown on Ag(001) stabilizes in-plane AFM domains31,35,36 while out-of-plane direction of AFM spins is preferred in NiO grown on MgO(001) substrate (aAg = 4.086 Å < 1 Faculty of Physics and Applied Computer Science, AGH University of Science and Technology, Kraków, Poland. 2Jerzy Haber Institute of Catalysis and Surface Chemistry Polish Academy of Sciences, Krakow, Poland. 3SOLARIS National Synchrotron Radiation Centre, Jagiellonian University, Krakow, Poland. 4Elettra-Sincrotrone Trieste S.C.P.A., Basovizza, Trieste, Italy. *email: Scientific Reports | (2023) 13:4824 | https://doi.org/10.1038/s41598-023-31930-z 1 Vol.:(0123456789) www.nature.com/scientificreports/ aNiO = 4.176 Å < aMgO = 4.21 Å)32,37. Additionally, some previous studies have reported that in thin NiO layers grown on MgO(001) substrate direction of AFM spins can substantially deviate from its bulk counterpart25,38. In our recent work, we performed systematic studies on the magnetic properties of NiO in Fe/NiO/Cr/MgO multilayers33. We proved that NiO grown on wedge-shaped Cr buffer undergoes continuous strain-induced spin reorientation transition (SRT) from nearly out-of-plane to in-plane direction as the strains change from tensile to compressive(aCr = 4.07 Å < aNiO = 4.176 Å < aMgO = 4.21 Å). Furthermore, for ultrathin NiO layers, we demonstrated an orthogonal (spin-flop) coupling between Fe and NiO spins. In the present approach, we combine strain and ferromagnetic proximity to tune the magnetic spin structure of NiO. While strain induced by the substrate allows tuning magnetic anisotropy of the AFM from its bottom interface, interaction with the ferromagnetic cover layer enables to influence the magnetic state of the AFM from its top interface. Our results show that the insertion of an MgO layer between NiO and Cr buffer in a NiO/MgO/ Cr stack strongly influences the spin structure in the AFM layer. Together with an increase of MgO interlayer thickness, we noted rotation of the NiO spins towards the out-of-plane direction. Furthermore, we investigated the magnetic response of NiO spins to the FM capping layer. Comparison of measured and simulated angular dependencies of the X-ray Magnetic Linear Dichroism (XMLD) spectra indicates that a domain structure of NiO is driven by a change of Fe and MgO thickness in Fe/NiO/MgO/Cr. Experimental Samples were grown in an ultrahigh vacuum (UHV) chamber using molecular beam epitaxy on polished MgO(001) single crystals. The MgO substrates were annealed for 1 h at 753 K to obtain a clean surface before the deposition of thin films. A 50 Å MgO homoepitaxial layer was evaporated at 723 K using electron beam evaporation. Next, a Cr buffer layer was deposited at 473 K and annealed at 753 K to improve its surface quality. The Cr buffer thickness dCr was chosen to be 200 Å to ensure a fully relaxed s urface33. Following Cr deposition, the MgO wedge-shaped layer with a thickness dMgO in the range of (0–100) Å was grown at room temperature using a movable shutter and subsequently covered by a homogenous 20 Å-thick NiO layer. The NiO film was grown at room temperature (RT) by reactive deposition of Ni under the oxygen partial pressure of 1 × 10−6 mbar. At this stage, one- (...truncated)