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Giant interfacial induced Dzyaloshinskii-Moriya interaction in polycrystalline Co/Pt multilayers

Multilayers of heavy metals and ferromagnets are the basis for the formation of various structures with inhomogeneous chiral magnetic distributions. The most informative method for studying magnetic states in such structures with a high spatial resolution is Lorentz transmission electron microscopy (L-TEM). Here, we report on the observation of chiral N\'eel domain walls by means of L-TEM in Co/Pt multilayers with perpendicular magnetic anisotropy. We explain the existence of chiral N\'eel internal structure due to the overcritical, giant interfacial induced Dzyaloshinskii-Moriya interaction. Our results are confirmed by micromagnetic modeling, which allows taking into account the effect of local inhomogeneity of the magnetic anisotropy of polycrystalline films on formation of spin textures.


I. INTRODUCTION
asymmetry into the structure of surfaces "FM/HM" and "HM/FM" of the multilayer, and therefore, the iDMI value in multilayers can differ at the "top" and "bottom" interfaces of the ferromagnetic film 31 . Thus, the presence of iDMI due to the structural asymmetric interfaces can be expected in magnetron-deposited polycrystalline multilayers, which consist of only two metals, for example, cobalt and platinum.
Another way to get iDMI in the multilayer films system is an interlayered mechanism due to spin-orbit coupling not on the interface but on interlayers of heavy metal 32,33 . Recently this mechanism of iDMI has been demonstrated in cobalt-palladium multilayer between platinum top and bottom layers 26 . The most fascinating question wether one can get large enough iDMI in more homogeneous films, e.g. cobalt-platinum multilayer instead of cobalt-palladium.
There are several experimental methods which can be applied to study the distribution of magnetization in nanostructures and can provide information on the presence of the iDMI in thin film systems. The Brillouin light scattering 34 and neutron scattering technique 35,36 are used to study the Dzyaloshinskii-Moria interaction. However, these methods have a low spatial resolution, which does not allow to obtain local magnetization distributions arising in structures with iDMI. The most qualitative and quantitative method for studying inhomogeneous states in thin films with a high spatial resolution is Lorentz transmission electron microscopy (L-TEM). In this paper, we present the experimental results using the L-TEM method, which demonstrate the existence of iDMI in multilayer Co/Pt films. The micromagnetic modeling was used to show the effect of nanocrystalline structure of Co and Pt layers on magnetization distributions in multilayer films, which allowed us to explain the features of the experimentally observed Fresnel contrast.

II. SAMPLES PREPARATION AND METHODS
The magnetic multilayer films with Ta(3.2)/Pt(5.5)/[Co(0.5)/Pt(1.0)] ×5 /Pt(3.8) structure (where 5 denotes the layer repetition numbers, thickness in nm) are grown by DC magnetron sputtering on 50-nm-thick Si 3 N 4 membrane windows. The films are fabricated in a high vacuum magnetron AJA 2200 system at the basic argon pressure of 4 × 10 −3 Torr. The preliminary pumping of the working chamber is carried out to a vacuum of 10 −5 Torr. The sputtering is carried out from separate Ta, Co, and Pt targets at room temperature. The deposition rates of the materials are 0.1 nm/s for cobalt films and 0.3 nm/s for platinum. The thicknesses of the films are determined by the deposition time after preliminary calibration of the thicknesses of individual layers, measured by of an electron microscope and defocus value ∆z where ρ is a vector in the detector plane and r is a vector in the plane of the film. In general case the sample is tilted and the film's and detector's plane do not coincides. The total value of the intensity (taking into account the material contrast I 0 ( ρ) is an inverse Fourier transform (IFFT) of the convolution of the Fourier image of (1) with the phase transfer function 45 where λ is electron wavelength, θ is beam divergence and q ⊥ is a reciprocal lattice vector in the detector's plane. The Fresnel contrast simulations are done taking to account TEM transfer function for 80 keV electron energy, defocus 2 mm and beam divergence θ = 0.09 mrad. Then these calculated images are compared with the LTEM images obtained experimentally.

III. RESULTS AND DISCUSSIONS
TEM images are obtained for the overall characterization of the microstructures of Co/Pt multilayers. The high-resolution TEM image (HRTEM) of the Co/Pt cross-section clearly shows the crystal lattice of cobalt and platinum layers with an interplanar spacing of 0.225 nm (Fig. 1a)), which can best correspond to the (111) plane of the Co 1+x Pt 3−x solid solution with Pm3m space group of symmetry lattice. However, the Bright Field (BF) image of a cross-section of a multilayer film shows the contrast between the Co and Pt layers (Fig. 1b) which indicates the layered structure The following experimental observations may indicate the presence of a nonzero iDMI. The first one is existence of striped-type contrast on L-TEM images with the Fresnel contrast (Figs. 4b) in a some range of external fields (excitations of the objective lens). The micromagnetic nature of stripes is the following. Domain walls in magnetic film with perpendicular magnetic anisotropy are Bloch-type (6a) in the absence of iDMI. Such distribution is favorable due to minimal magnetostatic energy. A weak iDMI tends to tilt the domain wall (6b) as iDMI minimizes the energy for magnetic distribution with nonzero magnetostatic charges ∇ · M = 0. When the iDMI interaction is larger then a critical value 48 , the domain wall becomes pure Néel-type (6c). This value for our magnetic parameters is |D cr | = 0.44µ 0 M 2 s t/π = 52.8 µJ/m 2 , where t is the film thickness. Sharp steps on hysteresis curve (point "2" at Fig. 3) as between 20-30 mT appears only for |D| D cr . The magnetization distribution at this point is a topological 360 degrees domain walls (Fig. 6d), which look like black-white stripes in Fresnel mode (Fig. 5b&d). The interplay of magnetostatic and iDMI leads to the situation when two 180 degrees Néel domain walls are too close but they can't annihilate (on the contrary to Bloch walls). The iDMI value for micromagnetic simulation can be selected easily from experimental data. On the one hand it should be larger than critical value D cr , on the other hand a large iDMI (about 65 − 75µJ/m 2 ) leads to formation of isolated Néel skyrmions instead of 360 degrees walls.
We also have observed weak Fresnel contrast at zero tilting angles. The explanation is the fol- lowing. In case of pure Néel walls in homogeneous film there should be zero magnetic contrast at zero tilt 13 . The analysis of Fresnel contrast with simulation of polycrystalline nature of magnetron sputtered films shows nonzero contrast for pure Néel walls at zero tilt conditions (Fig. 7).
The small contrast variations arise due to the magnetization tends to lie along wandering axis of the uniaxial magnetic anisotropy of nanocrystalline film structure. The tilting angle is 30 degrees along diagonal of the image. The origin of contrast on Fig. 7a&b is following. Wandering anisotropy axes leads to a weak canting of spins in domain wall from pure Néel configuration. That leads to a nonzero values for magnetization rotor and gives rise to local Fresnel contrast. Due to nanocrystalline origin of this contrast it is of the same order as the contrast from small magnetization cants of "homogeneously" magnetized areas. Thus the nanocrystalline structure of magnetron sputtered films can be the origin of weak magnetic contrasts in polycrystalline films 22,27 .

IV. CONCLUSION
Summing up, we experimentally have investigated the crystal and magnetic structure of the Co/Pt multilayer films with perpendicular magnetic anisotropy. Basing upon some previous re-search 26 we see the significant role of interlayers in formation of homochiral magnetic textures.
In this paper we have shown the existense of interlayer contribution in iDMI even in case of "symmetrical" material structure of multilayer. It is surprising fact that the magnetron sputtered