Ta Multilayer Film with

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Surfaces, Interfaces, and Applications

Construction of a Room-temperature Pt/Co/Ta Multilayer Film with Ultrahigh-density Skyrmions for Memory Application Lei Wang, Chen Liu, Nasir Mehmood, Gang Han, Yadong Wang, Xiulan Xu, Chun Feng, Zhipeng Hou, Yong Peng, Xingsen Gao, and Guanghua Yu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b00155 • Publication Date (Web): 28 Feb 2019 Downloaded from http://pubs.acs.org on March 1, 2019

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Construction of a Room-temperature Pt/Co/Ta Multilayer Film with Ultrahigh-density Skyrmions for Memory Application Lei Wang1, Chen Liu2, Nasir Mehmood3, Gang Han1, Yadong Wang3, Xiulan Xu1, Chun Feng1, *, Zhipeng Hou3, *, Yong Peng2, *, Xingsen Gao3, and Guanghua Yu1 1

Department of Materials Physics and Chemistry, University of Science and Technology Beijing, Beijing 100083, China; Beijing Laboratory of Metallic Materials and Processing for Modern Transportation, University of Science and Technology Beijing, Beijing 100083, China

2

Key Laboratory for Magnetism and Magnetic Materials of Ministry of Education, Lanzhou University, Lanzhou

730000, People’s Republic of China 3

Guangdong Provincial Key Laboratory of Quantum Engineering and Quantum Materials and Institute for Advanced Materials, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou 510006, China.

ABSTRACT: Magnetic skyrmions are chiral quasiparticles that show promise for storage of information. At present, the achievable skyrmion density (ηSk) is generally low (10-20 μm-2) due to the lack of effective manipulation. Here, both the magnetic anisotropy (Keff) and interfacial Dzyaloshinskii−Moriya interaction (DMI) of [Pt/Co/Ta]n multilayer films are elaborately modulated by changing the Co thickness (tCo) to study the ηSk dependence of intrinsic properties of the films systematically. The experimental and simulated results confirm that both the DMI and Keff have significant modifications on the ηSk, and their respective contributions vary with the tCo. Only when the magnetic anisotropy transits from out-of-plane to in-plane at an appropriate tCo range (1.8-2.1 nm), Keff decreases and DMI increases with the tCo. Both of the factors are favorable to the skyrmion formation and increase the density synergistically, toggling a maximal ηSk value of 45 μm-2. These findings provide a criterion for designing the high ηSk magnetic film 1


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which may advance the development of high-density skyrmion-based magnetic memorizer. KEYWORDS:

Magnetic

skyrmion,

magnetic

memory

devices,

magnetic

anisotropy,

Dzyaloshinskii−Moriya interaction, micromagnetic simulation

2



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1. INTRODUCTION The low-dimensional topological spin textures in magnetic materials are fundamentally important and technologically attractive in terms of high information-storage density, low threshold of controllability and exotic physical behaviors. One example is the magnetic skyrmion, a topologically protected swirling spin configuration with the typical size ranging from a few nanometers to several micrometers. Unlike the conventional magnetic domain walls, only an ultra-low current density is needed to drive skyrmions to move due to their special spin arrangement and flexibility in shape-deformation. This fascinating topological property, together with their nanoscale size and stable particle-like features make magnetic skyrmions promising candidate for carrying magnetic information in further high-density and low-power consumption spintronic devices such as skyrmion-based racetrack memories, logic computing gates, and oscillators 1-4. Magnetic skyrmions are first experimentally identified in a small-angle neutron diffraction study on chiral B20-type magnet MnSi

5

and subsequently are observed in a number of chiral

non-centrosymmetric magnets or ferromagnetic multilayers

6-11,

where the symmetry breaking

results in a strong Dzyaloshinskii-Moriya interaction (DMI). Among the numerous material systems, the ferromagnetic multilayers, including [Pt/Co/Ta]n, [Pt/Co/MgO]n, [Ir/Co/Pt]n, and [Ir/Fe/Co/Pt]n 12-16, are of particular appeal for practical applications because they are allowed for straightforward integrating into modern electronic technology. Recently, most investigations on the ferromagnetic multilayers are focus on the current-driven dynamics of skyrmions

17-22.

However, the investigations on how to enhance the skyrmion density (ηSk), which is of great significance for achieving ultrahigh-density information storage, are scarcely reported. Presently, 3


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the achievable ηSk in magnetic films is generally lower than 20 μm-2 due to the absence of effective tunability

12, 23-25,

which greatly restricts the storage density in the future. As we know,

ηSk is affected by intrinsic properties of material systems, including Keff and DMI, and some extrinsic parameters, such as boundary effect and size of sample. Now the research interest is whether we can obtain dense skyrmions by an intrinsic tunability. Recently, a high density of 60 μm-2 was achieved in a quaternary complex [Ir/Fe/Co/Pt]n

16

film system by designing the Ir/Fe

and Co/Pt interfaces with opposite chirality to increase the DMI value. However, the in-depth correlation between the intrinsic properties with ηSk was not studied systematically. Therefore, it is desirable to study the influence of intrinsic properties on ηSk and enhance ηSk in a simple and controllable material. [Pt/Co/Ta]n multilayer film is a typical skyrmion material system due to a remarkable DMI enhancement by the bottom Pt and top Ta. 26 In this work, [Pt/Co/Ta]n multilayer films were selected to tune the Keff and interfacial DMI elaborately and study the corresponding ηSk evolution by adjusting the ferromagnetic layer thickness. The achievable ηSk is significantly enhanced to 45 μm-2. Based on the experimental results and micromagnetic simulations, we find the high density of skyrmions results from a synergistic effect of magnetic anisotropy and DMI on ηSk.

2. EXPERIMENTAL SECTION Sample Preparation: The Ta (5 nm)/[Pt (3 nm)/Co (tCo)/Ta (1.9 nm)]12 multilayer films were deposited by magnetron sputtering at room temperature under the vacuum better than 5×10-5 Pa and the working pressure of 0.45 Pa. The thickness of Co layer (tCo) was adjusted from 0.9-2.7 nm. A thin Ta (5nm) seed layer was deposited to provide a flat surface for growing high-quality Pt/Co/Ta multilayers. 4



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Sample Characterization: All the characterizations were conducted at 300 K. Magnetic properties of the multilayer films were measured using a physical property measurement system (PPMS, Quantum Design) with in-plane fields up to 20 kOe and out-of-plane fields up to 10 kOe. The high-angle annular dark field (HAADF) images and energy dispersive X-ray spectroscopy (EDS) mapping were obtained by FEI Tecnai G2 F20 at 200 kV. The magnetic domain structure of the films was characterized by a transmission electron microscopy (Tecnai-F30) using a Lorentz mode at room temperature. During the measurements, the sample stage was tilted by 30º away from the horizontal direction and the under-focus amount was set 55 μm. The magnetic field ranging from 0-1 kOe was applied parallel to the electron beam. Micromagnetic Simulation: The domain structure evolution in the films was simulated by using the Mumax3 software. In the simulation, 2×2 μm2 square system of 12 repetitions of a trilayer stack consisting of Pt (3nm), Co (1.8nm), and Ta (1.9nm) with applying the 2D periodic boundary conditions and the mesh size of 4×4 nm2 was used. The saturation magnetization (MS), the exchange constant (A), and the temperature (T) were chosen to be 1.003×106 A/m, 1.0×10-11 J/m, and 300 K, respectively. The initial magnetization state was set to random magnetization. In order to match the simulation conditions with a polycrystalline sample, the disordered film was considered by using the grain size of 30 nm and the anisotropy constant and magnetic anisotropy axis variations of 10%. The exchange coupling between grains was set to be 90%.

3. RESULTS AND DISCUSSION In this work, Ta (5 nm)/[Pt (3 nm)/Co (tCo)/Ta (1.9 nm)]12 multilayer films were deposited on Si substrates by magnetron sputtering. The detailed structure is schematically illustrated in Fig. 1a. The HAADF image in Fig. 1c shows the flat and distinct interfaces in the as-deposited sample. 5


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Moreover, the line scanning EDS in Fig. 1d and elemental mapping in Figs. 1e-i well confirms the periodical arrangement of Pt, Co, and Ta layers without obvious atomic diffusion at the interface. The real average thickness of the Pt, Co, and Ta layers are 2.94 nm, 2.14 nm, and 1.89 nm, respectively, which are consistent with the designed thicknesses. These results demonstrate the good interfacial quality in the as-prepared multilayers, which is critical to skyrmion formation. Next, we will turn to study the magnetic domain structure evolution. In the Pt/Co/Ta multilayer film, interfacial DMI exists between S1 and S2 of two adjacent Co atoms close to heavy metals (Ta or Pt) with a strong spin-orbit coupling. The Hamiltonian can be expressed as HDMI = D12·(S1×S2) 27, where D12 is the DMI vector as shown in Fig. 1b. The competition among the DMI, Heisenberg interaction, dipolar interactions and Zeeman energy results in different spin states and domain structures, which evolves from the labyrinth domain at zero field to the chiral skyrmion under appropriate fields, and finally to the shrinkage and disappearance of the skyrmion as shown in Fig. 2c. These domain structure evolutions are consistent with the results reported in other literatures

12, 28-31.

The density and size of skyrmion as a function of the measured field are

summarized in Figs. 2a and 2b. Here, the skyrmion density was calculated by counting the numbers of skyrmion in a large region of 5×5 μm2 and then dividing the number to the area. Noticeably, the maximum ηSk in the sample is tunable with adjusting the Co layer thickness. Figure 3 depicts the room-temperature Lorentz TEM images corresponding to the maximum ηSk for each sample, which clearly demonstrate the evolution of skyrmion density with tCo. The quantitative relationship between the maximum ηSk and tCo is shown in Fig. 4a. When tCo 1.9×106 erg/cm3), meanwhile, ηSk keeps at a low level and is not variable with the tCo. During the transition from PMA to IMA at tCo=1.8-2.1 nm, the film demonstrates appropriate magnetic anisotropy (|Keff| 1.5×106 erg/cm3) and ηSk decreases rapidly with thickening tCo. These results suggest that there is a harsh anisotropy condition for the skyrmion formation, where an appropriate IMA is beneficial for obtaining a high-ηSk Pt/Co/Ta multilayer film. On the other hands, variation of the Co layer thickness also affects interfacial DMI. The DMI constant D can be calculated by the equation 𝜎𝐷𝑊 = 4 𝐴𝐾𝑢,𝑒𝑓𝑓 - 𝜋|𝐷|

32, 33,

where 𝜎𝐷𝑊 is the

domain wall energy, A is the exchange stiffness and 𝐾𝑢,𝑒𝑓𝑓 is the effective uniaxial anisotropy constant. 𝜎𝐷𝑊 can be obtained by measuring magnetic domain width

34,

where the detailed

process is shown in Fig. S4 and related discussion in the supporting information. Then, D can be estimated by using the experimental Keff and assuming A=1×10-11 J/m 13. The variations of D and ηSk with tCo are shown in Fig. 5b. The measured D values are equivalent to the reported values in [Pt/Co/Ta]n multilayer system

13,23,35.

The D value decreases rapidly with tCo lower than 1.7 nm,

accompanying with a low ηSk value in the film. However, when tCo is thickened to 1.8-2.1 nm, both the D and ηSk values increase rapidly, implying that the increment of D favors for the skyrmion formation and the enhancement of ηSk. With further increasing tCo over 2.1 nm, the ηSk value decreases quickly although the D value keeps the increasing trend. In order to clarify the relationship among Keff, DMI, and ηSk, a micromagnetic simulation was carried out by the Mumax3 software package 36,37 based on the Landau–Lifshitz–Gilbert equation 38,39.

For a continuous interface in the multilayer, the interfacial electronic structure is not affected 8



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much with thickening tCo. Thus, the tCo-dependent Keff variation is mainly caused by the evolution of uniaxial anisotropy (Ku). Therefore, we used the Ku as a variate in the simulation. The simulated domain structure evolution is shown in Fig. 6. Moreover, the typical ηSk variations with Ku and D are summarized in Fig. 7. The simulation results are consistent with the experimental results. As we can see, ηSk always displays an increasing trend with the increment of D. That is because DMI tends to twist the spin structure to the non-collinear direction, which helps for the formation of skyrmion 40. On the other hand, the moderate decrement of Ku is advantageous to enhance the skyrmion density. In fact, the domain wall energy is one of important factors to affect the nucleation energy barrier for skyrmions 41. The Ku decrement may decrease the nucleation energy for skyrmions by tuning the domain wall energy. This toggles the enhancement of ηSk in the film. However, when Ku is too low to overcome the demagnetization energy, only in-plane vortex domain can be formed, resulting in the vanishing of skyrmion. Based on the aforementioned results, the tunability behaviors of the Keff, DMI on ηSk in the Fig. 5 can be well understood. When tCo

Ta Multilayer Film with

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