Tunable Giant Anomalous Hall Angle in Perpendicular Multilayers by

Research Article Cite This: ACS Appl. Mater. Interfaces 2019, 11, 24751−24756

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Tunable Giant Anomalous Hall Angle in Perpendicular Multilayers by Interfacial Orbital Hybridization Jingyan Zhang,† Wenlin Peng,† Guanghua Yu,*,† Zhanbing He,‡ Feng Yang,§ Wei Ji,§ Chen Hu,∥ and Shouguo Wang*,†,‡ Beijing Advanced Innovation Center for Materials Genome Engineering, Department of Materials Physics & Chemistry, and ‡State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 100083, China § Department of Physics, Renmin University of China, Beijing 100872, China ∥ Department of Physics, McGill University, Montreal H3A2T8, Canada Downloaded via GUILFORD COLG on July 17, 2019 at 05:33:34 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

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S Supporting Information *

ABSTRACT: A spintronic device based on the spin-dependent Hall effect has attracted great interest because of its great potential applications in the multivalue storage and logic gate, which is a promising candidate to break the bottleneck of the information industry in the big data period. It is a technological challenge to implant spintronic devices into semiconductor integrated circuits. The anomalous Hall angle (θ), defined as the deviation of the electron flow from the current direction, is the key parameter to evaluate the capacity of Hall device compatibility. However, the bottleneck for the device is low θ (less than 5%) at room temperature (RT), making it difficult to directly complement with the semiconductor circuit which limits its potential application. Here, we report a simple perpendicular multilayered structure with θ up to 5.1% at RT. Wide working temperature (250−350 K) across RT for our samples will accelerate the potential applications in spintronic memory. A giant Hall angle at RT originates from the enhanced side-jump scattering at the atomic-scale-modified interfacial structure. The high θ at RT together with wide working temperature is practically significant and may provide the way for further 3D spintronic devices based on the spin-dependent Hall effect with ultrahigh storage density and ultralow power consumption. KEYWORDS: anomalous Hall angle, perpendicular magnetic anisotropy, interfacial structure, side-jump scattering, spin-orbit coupling

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INTRODUCTION

a high output voltage, which is directly compatible with CMOS technology. Output voltage in the spintronic device based on the spindependent Hall effect is determined by the anomalous Hall angle (θ). In general, θ is expressed as θ= (ρxy/ρxx) × 100%, where ρxy and ρxx is anomalous Hall resistivity and longitudinal resistivity, respectively. For the nanoscale Hall storage cell, it was pointed that θ in perpendicular magnetic materials should be larger than 5% at room temperature (RT), so that the device can be directly implanted into the large-scale integrated circuit compatible with CMOS.19 Previously, some values about θ were reported in ferromagnetic materials at low temperatures and high magnetic fields.20−26 For example, the Hall angle for the GdPtBi ternary alloy was up to 15% at 9.2 K with 4 T.27 Ando et al. reported the value of θ up to 5.7% at RT in the perpendicular MnGa epitaxial film.28 However, low working temperature and the high-quality epitaxial film greatly limit their applications in devices.27−30 Pt-based films have strong spin−orbit coupling (SOC), in which the Hall angle is

Spin manipulation has provided an open way for the information storage/communication and logic operation in past decades, where the semiconductor industry is under the limit of Moor’s law.1−4 The composite materials with the spindependent effect are explosively launched with the development of nanotechnology, but more attention has been paid to manipulate the spin without external magnetic fields (e.g., spin polarized current and electric field) to achieve the high-quality spintronic device.5−8 Recently, spin−orbit torque (SOT) induced by the spin Hall effect has been demonstrated theoretically and experimentally to tune anomalous Hall resistance.9−13 Therefore, the memory device based on the spin-dependent Hall effect with the imbalanced transverse spin polarization charge is considered as a promising candidate for storage and logic with potential ultrahigh density, ultralow power consumption, and ultrahigh speed.14−17 Therefore, more focus has been put on intrinsic properties of the device based on the spin-dependent Hall effect. Unfortunately, few work was reported on the hybrid spin device/complementary metal/oxide-semiconductor (CMOS) transistor circuit.18 The most challengeable point is that the spin device should provide © 2019 American Chemical Society

Received: April 16, 2019 Accepted: June 19, 2019 Published: June 19, 2019 24751

DOI: 10.1021/acsami.9b06204 ACS Appl. Mater. Interfaces 2019, 11, 24751−24756

Research Article

ACS Applied Materials & Interfaces far below 5% at RT.19 However, these films have thus far not been able to simultaneously satisfy the requirements of RT operation, large enough θ, stable storage configuration (perpendicular magnetic anisotropyPMA), and simple structure. Therefore, it is a challenging goal to achieve the large θ in perpendicular magnetic multilayers at RT. In this work, the value of θ up to 5.1% at RT was obtained in perpendicular Co/Pt multilayers sandwiched by CoO functional layers. Furthermore, a wide working temperature (250− 350 K) was achieved in this multilayered structure. Our results about spin-transport measurements and first-principles calculation show that giant θ originates from the enhanced sidejump scattering at interfaces. This work may provide a feasible approach for the further application of high-performance spin devices compatible with CMOS technology.

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Pt(0.6)]n/MgO(5) (blue hexagon), and CoO(5)/[Co(0.6)/ Pt(0.6)]n/CoO(5) (red circle), respectively. With increasing n, the value of θ for three samples shows a monotonic increase. It is worthy to point that the θ value up to 5.1% is obtained in CoO(5)/[Co(0.6)/Pt(0.6)]n/CoO(5) (in nm) with n = 7. Compared with pure Co/Pt multilayers and the multilayers sandwiched by MgO, the value of θ for the sample sandwiched by CoO is always larger. With increasing n from 2 to 10, θ increases, but it is below 5% both for pure Co/Pt multilayers and the multilayers sandwiched by MgO layers. The corresponding hysteresis loops were shown in the Supporting Information. The pure [Co/Pt] and CoO/[Co/Pt]n/CoO (n = 2−10) multilayers exhibit in-plane magnetic anisotropy and PMA, respectively, as shown in Figure S1. For example, the Hall loop in the inset of Figure 1c shows two stable Hall resistance at zero fields, indicating PMA for the CoO/[Co/ Pt]7/CoO sample. The PMA for multilayers with CoO comes from the exchange coupling between Co/Pt multilayers and CoO, in good agreement with the previous study.31 For MgO(5)/[Co(0.6)/Pt(0.6)]n/MgO(5) samples, the easy axis is in plane when n ≤ 7, but it transforms to be perpendicular when n > 7. According to above results, θ beyond 5.0% can be achieved in CoO/[Co/Pt]n/CoO for n ≥ 7 with PMA at RT. Therefore, it will be useful to the hybrid AHE-based memory cell made of our multilayers into the CMOS circuit without any amplifier. For the CoO/[Co/Pt]7/CoO sample, the microstructural characterization was carried out by transmission electron microscopy (TEM). Figure 2a shows the low-magnification

EXPERIMENTAL METHOD

The core stack structures of [Co(0.6)/Pt(0.6)]n, CoO(5)/[Co(0.6)/ Pt(0.6)] n/CoO(5), and MgO(5)/[Co(0.6)/Pt(0.6)]n/MgO(5) (thickness in nm) with several period numbers (n = 2−10) were fabricated on thermally oxidized Si wafers at RT by the magnetron sputter system (AJA ATC-1800F). The base pressure was better than 2.0 × 10−7 Torr, and the Ar pressure was 2.0 × 10−3 Torr during sputtering. The multilayered sample was patterned into Hall bar (shown in Figure 1a) by lithography with ion milling for transport

Figure 1. (a) Schematic diagram of the anomalous Hall measurement. (b) Scanning electron micrograph for the Hall bar with the width of 30 μm. (c) Anomalous Hall angle as a function of the period number (n = 2−10) for the sample with the structure of [Co/Pt]n (black squares), MgO/[Co/Pt]n/MgO (blue hexagons), and CoO/[Co/ Pt]n/CoO (red circles) at 300 K, respectively. Inset: Hall loop for sample CoO/[Co/Pt]n/CoO with n = 7.

Figure 2. (a) Low power transmission electron micrograph and (b) high-resolution section transmission electron micrograph for sample CoO/[Co/Pt]n/CoO with n = 7. (c) Scanning transmission electron microscopy image for sample CoO/[Co/Pt]7/CoO. (d) Diffraction pattern of I and II regions selected in (b).

measurements in a physical property measurement system by the standard four-probe technique. The transport measurements were carried out with a current of 1 mA. Microstructures were investigated by a Tecnai F30 and a JEM-ARM200F transmission electron microscope equipped with a Cs corrector. The SOC energy for the multilayers was obtained by first-principles calculation using Nanodcal software.

cross-sectional transmission electron micrograph image, where a continuous flat Co/Pt multilayered structure with sharp metal/oxide interfaces was clearly observed. High-resolution transmission electron microscopy (HRTEM) from the red square in Figure 2a was shown in Figure 2b, demonstrating the good crystalline nature of the Co/Pt multilayers. The core stack-[Co/Pt]7 sandwiched by CoO layers was shown by the dark region. Two regions called I and II were further examined, as shown in Figure 2d. Diffraction reflections with an interplanar spacing of approximately 0.22 nm correspond to the fcc CoPt(111) plane, indicating the good crystalline structure of the core stack. It is well known that (111) CoPt

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RESULTS AND DISCUSSION The image of the Hall bar by scanning electron microscopy was shown in Figure 1b, where the width of the Hall bar was about 30 μm. Figure 1c shows the θ value as a function of the period number (n) for the samples with structures of [Co(0.6)/Pt(0.6)]n (black square), MgO(5)/[Co(0.6)/ 24752


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ACS Applied Materials & Interfaces

It is necessary to investigate the temperature sensitivity of AHE for industrial applications in the interested temperature region. Figure 4a presents Hall loops for sample CoO/[Co/

crystalline texture is critical to obtain the multilayers with PMA. Some details can be found in HRTEM as shown in Figure 2b, where significantly crystalline nature of CoO was observed near the CoPt/CoO interface. Meanwhile, the local epitaxial relationship between CoPt and CoO can be established in the HRTEM image as shown in Figure S2. The red square region in Figure 2a was further examined, and the high-angle annular dark-field scanning transmission electron microscopy (STEM) image was presented in Figure 2c. The continuous and alternative light and dark stripes were observed in the STEM image, corresponding to Pt layers and Co layers, respectively. More details can be found in Figure S3. It strongly indicates that there are continuous layered structures in Co/Pt multilayers in spite of the Co (Pt) layer with only 0.6 nm thickness. The sharp interfaces in our Co/Pt multilayers will be greatly useful to enhance spin-dependent electron scattering, leading to larger AHE. Figure 3a presents the value of θ as a function of Co thickness for the sample CoO(5)/[Co(tCo)/Pt(0.6)]7/CoO-

Figure 4. (a) Hall loops for sample CoO/[Co/Pt]7/CoO at 250, 300, and 350 K. (b) Saturation anomalous Hall resistivity and anomalous Hall angle (θ) as a function of test temperature T, respectively.

Pt]7/CoO at 250, 300, and 350 K, respectively. The good squareness of Hall loops at 250−350 K can be clearly seen, suggesting that the sample of CoO/[Co/Pt]7/CoO exhibits PMA at various temperatures. In this case, the remanence at zero fields provides two stable states to achieve information storage and logic operation. The coercivity (Hc) of the Hall storage cell shown in Figure S5 decreases 60% (420−260 Oe) as increasing temperature from 250 to 400 K. Figure 4b presents the saturation Hall resistivity and the θ value as a function of test temperature (T). Saturation Hall resistivity for CoO/[Co/Pt]7/CoO has a slight change, keeping 3.6 μΩ·cm at all temperatures investigated here. The θ value for the Hall storage cell shows a nonlinear variation as increasing T. The anomalous Hall angle is up to 5.0% at 250 K and 4.9% at 400 K, respectively. It is worthy to point that the anomalous Hall angle is beyond 5.0% during the temperature region (250−350 K), indicating that the Hall storage cell has the capacity of being compatible directly with CMOS technology at the wide working temperature region. Generally, AHE in ferromagnets has both the Berry phase mechanism (intrinsic mechanism) and extrinsic scattering mechanism, where the Berry phase mechanism comes from the SOC in materials, and the extrinsic scattering mechanism is related to the multilayered structure, respectively. To determine the intrinsic AHE, the first-principles calculation about SOC strength of the multilayered structure with Nanodcal software was performed. The core stack [Co/Pt]n with n = 2 was selected as the basic configuration. Based on this, different metal/oxide interfaces were introduced to investigate SOC energy in the multilayered structure. More details can be found in the Supporting Information. To investigate the SOC energy at oxide/metal interfaces, the interfacial oxygen environment should be taken into consideration. It was demonstrated that the different metalterminated interface has significant role on spin-dependent

Figure 3. (a) Anomalous Hall angle as a function of Co thickness (tCo) for sample CoO(5)/[Co(tCo)/Pt]7/CoO(5) (in nm). (b) Anomalous Hall angle as a function of the thickness of the oxide layer (t) for MgO(t)/[Co/Pt]7/MgO(t) and CoO(t)/[Co/Pt]7/ CoO(t) (in nm), respectively.

(5) (in nm). With Co thickness tCo increasing, θ shows a nonlinear change, and the maximum value reaches 5.2% when tCo = 0.7 nm, indicating that the giant Hall angle is due to the interfacial effect, not the Co layer. Figure 3b shows the θ value as a function of the oxide layer thickness for two samples. It is grateful to find that the θ value for multilayers sandwiched by CoO can be kept constant as high as 5.1% as tCoO increasing. However, for the samples with MgO, a slight decrease of the θ value was observed with tMgO increasing. Moreover, the effect of the bottom oxide/[Co/Pt]n interface and the top [Co/Pt]n/ oxide interface on the θ value can be found in the Supporting Information. It indicates that different ferromagnetic/oxide interfaces play an important role on the magnetic transport properties. Therefore, it is necessary to investigate the influence of different metal/oxide interfaces on AHE behavior. 24753


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Figure 5. (a) Four metal/oxide interface configurations (A1, A2, B1, and B2) of the first-principles theoretical calculations. (b) SOC energy for the atomic layer for four oxide/metal interfaces. (c) Variation of SOC energy at each atomic layer for four configurations. (d) Variation of SOC energy as a function of the period number for the four configurations with CoO or MgO.

value of ΔEsoc/Esoc decreases with n increasing for A2 and B2. For example, when n = 2, the ΔEsoc/Esoc value for A2 and B2 is −2.40 and −2.47%, respectively. When n is increased to 7, the ΔEsoc/Esoc value for A2 and B2 decreases to −0.69 and −0.72%, respectively. However, the ΔEsoc/Esoc value remains constant (almost zero) for A1 and B1. According to the firstprinciples calculation, SOC energy in the multilayered structure has a slight change when metal/oxide interfaces (A2, and B2) are introduced into the multilayers. The change (ΔEsoc/Esoc) will turn smaller as n increases. Moreover, different oxides (MgO or CoO) have a similar role on the variation of SOC energy (ΔEsoc) when n = 2−10. Therefore, the effect of different metal/oxide interfaces on SOC is not large enough to result in different AHE behaviors. Above results shown in Figure 5 indicate that the variation of SOC energy for MgO/[Co/Pt]n/MgO and CoO/[Co/Pt]n/ CoO with n = 7 is almost equal. It is reasonable to conclude that the intrinsic AHE induced by the Berry phase mechanism for two samples is nearly equal. Therefore, the different AHE behaviors for two samples should originate from the extrinsic scattering mechanism. In our [Co/Pt]n multilayers, the skew scattering and side-jump scattering have a considerably important role on AHE behavior due to the polycrystalline nature and complex interfacial structure. To check the extrinsic mechanism, the Hall transport measurements were carried out for the two samples with several period numbers at different temperatures. The AHE in our multilayers can be expressed as ρxy = a × ρxx + b × (ρxx)2, where ρxy and ρxx is anomalous Hall resistivity and longitudinal resistivity, respectively. The linear fitting ρxy/ρxx ≈ ρxx for the [Co/Pt]n multilayers (n = 2−10) sandwiched by CoO was plotted in Figure 6a, where the intercept and the slope correspond to parameter a (skewscattering mechanism, SS) and b (side-jump scattering mechanism, SJ), respectively. Figure 6d presents the values of a and b as a function of n for the CoO/[Co/Pt]n/CoO sample. The value of a shows a monotonic and significant decrease with n increasing. The value of a for CoO/[Co/Pt]n/

tunneling behavior.32,33 In this study, the interfacial SOC energy is greatly influenced by oxygen-terminated or metalterminated interface. For example, the MgO/Co interface has two cases: the Mg-terminated interface (Mg−Co) and oxygenterminated interface ((Mg)O−Co) correspond to oxygen-poor and oxygen-rich states, respectively. During the magnetron sputtering process, the oxidation layer is usually the oxygenpoor state, so four metal/oxide interface configurations (A1: Mg−Co, A2: Mg−Pt, B1: Co−Co, and B2: Co−Pt) are shown in Figure 5a. The SOC energy at each atomic layer (Co or Pt) with and without different metal/oxide interfaces (A1, A2, B1, B2, and pure [Co/Pt]2) was presented in Figure 5b. The SOC energy mainly comes from heavy metallic Pt layers, whereas the SOC energy at Co layers is inappreciable. When four metal/oxide interfaces were introduced in the core stack ([Co/ Pt]n multilayers), the SOC energy at the Pt atomic layer near the metal/oxide interface (A2 and B2) has a significant change. However, the SOC energy at each atomic layer inside the multilayers and Co atomic layer near metal/oxide interfaces (A1 and B1) has little change. For the Pt/MgO interface (A2) and Pt/CoO interface (B2) as shown in Figure 5c, the variation of SOC energy has a fast decrease as the Pt atomic layer increases (ΔEsoc at Pt-1 > ΔEsoc at Pt-2 > ΔEsoc at Pt-3), suggesting the strong interfacial dependence of ΔEsoc. The maximum variation of SOC energy ΔEsoc is only 0.1 eV at the Pt-1 adjacent metal/oxide interface, which is far less than SOC energy of the Pt atomic layer (Esoc ≈ 0.8 eV). However, the ΔEsoc value at Pt-3 is almost zero (0.02 eV). For the MgO/Co interface (A1) and CoO/Co interface (B1), the value of ΔEsoc is almost the same at each Co atomic layer near the metal/ oxide interface (Co-4, Co-5, and Co-6) compared with the pure Co/Pt multilayered structure. Therefore, our focus will be given on two A2 and B2 configurations. The total ΔEsoc value for the [Co/Pt]2 multilayers with A2 is −0.13 eV, and the value of total ΔEsoc for the multilayers with B2 is −0.14 eV. Figure 5d displays ΔEsoc/Esoc as a function of the period number (n) for four configurations. The results show that the 24754


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should be cooperative, whereas, the opposite sign of a and b values was observed in the multilayered structure with MgO layers. The sign and value changing of parameter b can be observed for the samples. The side-jump scattering coming from the Co/Pt interface and metal/oxide interface will lead to the change of parameter b. When the period number is increased, the sign of parameter b has a change. Moreover, the b value is 108.7, −982.6, and −165.7 S/cm for the CoO/[Co/ Pt]4/CoO, MgO/[Co/Pt]4/MgO, and pure [Co/Pt]4, respectively. It should be mentioned that the different metal/oxide interfaces can also change the sign of the parameter b. It is reasonable to conclude that the increment of AHE due to side jump scattering leads to a giant Hall angle in our samples, which comes mainly from metal/CoO interfaces.

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CONCLUSIONS In this paper, the anomalous Hall angle up to 5.1% at RT, together with the wide working temperature region, was experimentally achieved in the multilayered structure with PMA. Such giant Hall angle originates from the improvement of AHE induced by side-jump scattering due to continuous Co/Pt interfaces and local metal/CoO epitaxial interfaces. The anomalous Hall angle beyond 5.0% is greatly useful to implant multivalue Hall storage (such as Hall balance) into the CMOS circuit without any amplifier, which can improve the storage density and response speed. This work may provide a promising approach to promote the application of the highperformance SOT-assistant Hall storage device.

Figure 6. (a) ρxy/ρxx ≈ ρxx curves for the sample CoO/[Co/Pt]n/ CoO (n = 2−10). ρxy/ρxx ≈ ρxx curve for (b) sample CoO/[Co/Pt]7/ CoO and (c) MgO/[Co/Pt]7/MgO, respectively. (d) Parameter a and b values as a function of the period number (n) for sample CoO/ [Co/Pt]n/CoO (n = 2−10), respectively.

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CoO reaches 29.0 × 10−2 as n = 2, and it greatly decreases to 2.3 × 10−2 as n = 10. However, the value of b shows a nonlinear change with n increasing. It is well worthy to point out that the sign of b also changes with n increasing. b is negative as n ≤ 3, while it becomes positive when n ≥ 4. It suggests that the relationship between skew scattering and side-jump scattering mechanisms change from competitive to cooperative. It is reasonable to conclude that the enhanced scattering mechanism (cooperative relationship between SS and SJ) should be due to the continuous Co/Pt interfaces in the multilayered structure, leading to the enhancement of the anomalous Hall angle. To further investigate the effect of the different oxide/metal interfaces on AHE behavior, the samples sandwiched by CoO or MgO with n = 7 were selected. Figure 6b,c shows the ρxy/ρxx ≈ ρxx curve for two samples, respectively. The parameter a and b values for CoO/[Co/ Pt]7/CoO are 3.1 × 10−2 and 298.2 S/cm, respectively. However, the values of a and b for [Co/Pt]7 multilayers sandwiched by the MgO layer are 4.1 × 10−2 and −101.2 S/ cm, respectively. The value of a for the sample with MgO is 30% larger than that for the Co/Pt multilayers with CoO, suggesting that AHE-induced skew scattering in the multilayers with MgO is larger than that in CoO/[Co/Pt]7/CoO. Generally, skew scattering comes mainly from the impurity and interfaces. The rough metal/MgO interfaces will cause more electron scattering. It is worthy to emphasize that a large enhancement of parameter b can be observed in the multilayers with CoO. The value of b in the sample sandwiched by CoO is 195% larger than that in MgO/[Co/Pt]7/MgO, indicating that side-jump scattering at the metal/CoO interface is stronger than that at the metal/MgO interface. Moreover, the sign of a and b for CoO/[Co/Pt]7/CoO is the same (positive), suggesting that the skew scattering and side-jump scattering

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b06204.

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Magnetic properties of the multilayers, Hall measurements for the multilayers with different metal/oxide interfaces, microstructure characterization, first-principles calculation for the multilayers, and scattering mechanism analysis (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (G.Y.). *E-mail: [email protected] (S.W.). ORCID

Jingyan Zhang: 0000-0003-0012-6191 Wenlin Peng: 0000-0001-9334-837X Zhanbing He: 0000-0002-3964-2815 Wei Ji: 0000-0001-5249-6624 Chen Hu: 0000-0003-2333-2182 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was supported by the Natural Science Foundation of China (grant nos. 11874082, 51431009, and 51625101), the National Key Basic Research Program of China (grant no. 2015CB921401), the Fundamental Research Funds for the Central Universities Grant FRF-TP-16-001C2, and the Beijing Laboratory of Metallic Materials and Processing for Modern Transportation. 24755


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Tunable Giant Anomalous Hall Angle in Perpendicular Multilayers by

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