key: cord-0912798-6bgkbkmm
authors: Lin, Weinan; Yang, Baishun; Chen, Andy Paul; Wu, Xiaohan; Guo, Rui; Chen, Shaohai; Liu, Liang; Xie, Qidong; Shu, Xinyu; Hui, Yajuan; Chow, Gan Moog; Feng, Yuanping; Carlotti, Giovanni; Tacchi, Silvia; Yang, Hongxin; Chen, Jingsheng
title: Perpendicular magnetic anisotropy and Dzyaloshinskii-Moriya interaction at an oxide/ferromagnetic metal interface
date: 2020-06-25
journal: Physical review letters
DOI: 10.1103/physrevlett.124.217202
sha: 5ec332e620ee515ea15351b7a51f921e506bb714
doc_id: 912798
cord_uid: 6bgkbkmm

We report on the study of both perpendicular magnetic anisotropy (PMA) and Dzyaloshinskii-Moriya interaction (DMI) at an oxide/ferromagnetic metal (FM) interface, i.e. BaTiO3 (BTO)/CoFeB. Thanks to the functional properties of the BTO film and the capability to precisely control its growth, we are able to distinguish the dominant role of the oxide termination (TiO2 vs BaO), from the moderate effect of ferroelectric polarization in the BTO film, on the PMA and DMI at the oxide/FM interface. We find that the interfacial magnetic anisotropy energy of the BaO-BTO/CoFeB structure is two times larger than that of the TiO2-BTO/CoFeB, while the DMI of the TiO2-BTO/CoFeB interface is larger. We explain the observed phenomena by first-principles calculations, which ascribe them to the different electronic states around the Fermi level at the oxide/ferromagnetic metal interfaces and the different spin-flip processes. This study paves the way for further investigation of the PMA and DMI at various oxide/FM structures and thus their applications in the promising field of energy-efficient devices.

(TiO2 vs BaO), from the moderate effect of ferroelectric polarization in the BTO film, on the PMA and DMI at the oxide/FM interface. We find that the interfacial magnetic anisotropy energy of the BaO-BTO/CoFeB structure is two times larger than that of the TiO2-BTO/CoFeB, while the DMI of the TiO2-BTO/CoFeB interface is larger. We explain the observed phenomena by first principles calculations, which ascribe them to the different electronic states around the Fermi level at the oxide/ferromagnetic metal interfaces and the different spin-flip process. This study paves the way for further investigation of the PMA and DMI at various oxide/FM structures and thus their applications in the promising field of energy-efficient devices.

Keywords: Dzyaloshinskii-Moriya interaction, perpendicular magnetic anisotropy, oxide/ferromagnetic metal interface Perpendicular magnetic anisotropy (PMA) and Dzyaloshinskii-Moriya interaction (DMI) in conventional ferromagnetic metals (FM) are attracting great interest as they are proposed as key components to design and realize energy-efficient spintronic devices, especially in the recent developed spin-orbit based devices [1, 2] . One strategy to enhance the PMA of a conventional FM is to introduce a heavy metal neighbour layer with large spin orbit coupling (SOC) strength at the beginning [3, 4] , and later evolve to bring in an oxide layer next to it [5, 6] , which can result in similar strength of PMA. For example, the MgO/CoFeB used in perpendicular magnetic tunnel junctions is promising for realizing the next-generation high-density nonvolatile memory and logic chip [5] . On the other hand, the DMI, described by the Dzyaloshinskii-Moriya vector D, is the antisymmetric exchange interaction that promotes canted spin configuration, instead of the parallel or antiparallel spin alignments obtained by usual Heisenberg exchange interaction [7] [8] [9] . Though the concept has been introduced several decades ago, only recently it is recognized that DMI can play an important role in electrically manipulation of the magnetization in various materials for achieving potential energy-efficient devices [2, [10] [11] [12] , such as fast domain wall motion [12] and skyrmion lattice formation [13, 14] . Similar to PMA, DMI requires the presence of a sizeable SOC, as well as of broken inversion symmetry that is naturally present at interfaces. Therefore, heavy metals, such as Pt and Ir, are usually introduced to engineer the interfacial DMI [13, 15] even if it has been shown more recently that an oxide layer can be also exploited to the same aim. This possibility has been analysed from the theoretical point of view in different studies [16, 17] even if experimental data are still lacking in the literature. To this respect, a large variation in the interfacial DMI under the application of an electric field has been observed very recently in the Ta/CoFeB/TaOx system [18] . Oxide materials are versatile, featuring peculiar degrees of freedom, such as the terminations in a complex oxide and the polarization in a ferroelectric oxide, which can be exploited to manipulate the DMI strength. However, a detailed analysis of these effects has been not performed so far.

In this work, we make use of the ferroelectric BaTiO3 (BTO) as an oxide layer to investigate both PMA and DMI at the oxide/FM interface. The precise control of the termination and the polarization of the BTO film helps us to distinguish the role of the termination and polarization in influencing the strength of the PMA and DMI at the oxide/FM interface. With the help of first principles calculations, we ascribe the modulation of the PMA and DMI at the interface to the dominant role of the oxide termination and thus to the different electronic states and spin flip possibilities around the Fermi level.

The studied structures were synthesised in a pulsed laser deposition (PLD)-Sputter combined chamber. The terminations of the BTO film (BaO and TiO2) are realized by controlling the terminations of the SrTiO3 (STO) substrate and the layer-by-layer growth of the BTO film (monitored by the high energy electron diffraction (RHEED) system), i.e. TiO2 terminated STO results in TiO2 terminated BTO, while SrO terminated STO results in BaO terminated BTO. The TiO2 terminated STO is obtained via the conventional buffer-HF solution treatment [19] , while the SrO termination is obtained via the deposition of a SrRuO3

(SRO) single layer on top of the treated STO [20] . In this study, 15 unit cell (uc) of BTO are grown on STO substrates of both terminations, as shown in Fig. S1 of the Supplementary Materials. Next, the film is transferred in the sputter chamber for in situ growth of the CoFeB film with various thicknesses and then the heavy metal capping layer (Ta or Pt), followed by an annealing procedure (300 °C for 1h). As shown in inset of Fig. 1(a) and (b), the terraced morphology preserved in structures of both terminations. According to our previous works [21, 22] , besides the different termination of the BTO layers, the different terminated STO substrates will lead to different as-grown polarizations of the BTO layer, i.e. TiO2 (SrO) termination results in the down (up) ferroelectric polarization in the BTO film. In the following, we use the BaO-BTO and TiO2-BTO to indicate these two types of BTO structures, which also hold the up and down ferroelectric polarizations, respectively. we also prepared a controlled structure with 2uc SRO inserted between a 1.33 nm CoFeB and polarized BTO to screen the influence of the ferroelectric polarization [25] . The calculated Keffteff of the controlled sample is 0.228 mJ/m 2 , which is close to the value for the CoFeB grown on the BaO-BTO film, 0.222 mJ/m 2 . Therefore, one concludes that the dominant factor for the substantial difference of the interfacial magnetic anisotropy energy is the oxide termination of the BTO/CoFeB interface.

The interfacial DMI at the oxide/FM interface is investigated by Brillouin light scattering (BLS) [26] [27] [28] [29] .

In ultrathin films, the presence of i-DMI causes a frequency asymmetry between Damon-Eshbach (DE) modes propagating in opposite in-plane directions, perpendicular to the sample magnetization, following the relation:

where is the effective DMI constant, k is the spin waves (SWs) wave vector, is gyromagnetic ratio. The An in-plane magnetic field H=3.5 kOe, sufficiently large to saturate the magnetization in the film plane, was applied along the z axis. Meanwhile, the in-plane k was swept along the perpendicular direction (x axis), corresponding to the DE geometry. Due to the conservation of momentum in the light scattering process, the magnitude of k is connected to the incidence angle of light θ, by the relation k = 4π sin θ/λ. In order to estimate the effective DMI constant D, the SW dispersion (frequency vs wave vector k) is measured, changing k from 0 to 2.07 ×10 7 rad/m. In order to clarify the underlying mechanism of the different PMA and DMI for the two terminated BTO/CoFeB heterostructures, we performed first principles calculations using the Vienna ab-initio simulation package (VASP) [30] [31] [32] [33] [34] . The atomic structures of TiO2-termination, with down polarization, and BaO-termination, with up polarization, used in the calculations are shown in Fig. 3(a) and (b Because the DMI mainly comes from the interfaces at FM/HM and FM/Oxide [17, 35] , we separated the trilayers structure into two parts, BTO/Fe and Fe/Pt, for the calculation of the DMI, whose schematics are shown in Fig. 4 . From the obtained values reported in [37] . Concerning the difference of DMI in the two terminated surfaces, one should consider, that besides the spin orbital coupling in inversion symmetry broken system, the bandfilling of the 3d atom plays an important role in determining the DMI strength in conventional 3d/5d

interfaces [38] . Though without heavy metals in these two terminated interfaces, the band-filling of the Fe layer may still dominate the differences of their DMI strengths [39] . As seen in Fig. 3 Finally, in order to gain a deeper insight into the role of ferroelectric polarizations on the observed differences of the PMA and DMI, we have performed first principles calculations for the TiO2-BTO/Fe structure with up polarization. The calculated MAE increases by 6% with respect to the down polarization, but remains far smaller than that of the BaO terminated structure. Moreover, the calculated DMI of TiO2-BTO/Fe with up polarization turns out to reduce its magnitude by about one third, but it is still of the same order of magnitude of TiO2-BTO/Fe with down polarization and is much larger than that of the BaO-BTO/Fe structure. The potential atomic relaxation effect on the PMA and DMI has been considered [34] , which is moderate compared to the effect from the terminations. Therefore, one concludes that the termination of the BTO layer, rather than the ferroelectric polarization, plays a dominant role in the modulation of the PMA and DMI in the investigated BTO/CoFeB structures.

In summary, the effects of changing oxide termination and ferroelectric polarization on PMA and DMI at the BTO/CoFeB interface have been investigated. We found that the choice of the termination strongly affects both PMA and DMI strength. In particular, a larger PMA has been observed for the CoFeB films grown on a BaO-BTO substrate, while a higher value of the DMI constant has been found for a TiO2-BTO substrate. First principle calculations show that this behaviour can be ascribed to the different electronic states around the Fermi level at the oxide/FM interfaces. This provides another degree of freedom to manipulate the PMA and DMI in a FM layer. These results may inspire further studies of the interface characteristics in various oxide/FM systems, paving the way to the design of layered structures with tailored DMI to be exploited in forthcoming energy-efficient devices.

See Supplementary Materials for the detail of the first principles calculations, the calculated density of states of the interfacial BaTiO3, Ti and O, the analyses of the different orbital-resolved contributions to MAE in the two terminated structures and the atomic relaxation effect