Interface-Induced Anomalous Hall Conductivity in a Confined Metal

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Surfaces, Interfaces, and Applications

Interface-Induced Anomalous Hall Conductivity in a Confined Metal Oleg E. Parfenov, Dmitry V. Averyanov, Andrey M. Tokmachev, Igor A. Karateev, Alexander N. Taldenkov, Oleg Kondratev, and Vyacheslav G. Storchak ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b10962 • Publication Date (Web): 24 Sep 2018 Downloaded from http://pubs.acs.org on September 25, 2018

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ACS Applied Materials & Interfaces

Interface-Induced Anomalous Hall Conductivity in a Confined Metal Oleg E. Parfenov,† Dmitry V. Averyanov,† Andrey M. Tokmachev,† Igor A. Karateev,† Alexander N. Taldenkov,† Oleg A. Kondratev,† and Vyacheslav G. Storchak†* †

National Research Center “Kurchatov Institute”, Kurchatov Sq. 1, Moscow 123182, Russia

KEYWORDS: spintronics, anomalous Hall effect, Hanle effect, EuSi2, silicon, antiferromagnetism

ABSTRACT – The mature silicon technological platform is actively explored for spintronic applications. Metal silicides are an integral part of the Si technology used as interconnects, gate electrodes, diffusion barriers; their epitaxial integration with Si results in premier contacts. Recent studies highlight the exceptional role of electronic discontinuities at interfaces in the spin-dependent transport properties. Here we report a new type of Hall conductivity driven by sharp interfaces of Eu silicide, an antiferromagnetic metal, sandwiched between two insulators – Si and SiOx. Quasi-ballistic transport probes spin-orbit coupling at the interfaces, in particular charge-spin interconversion. Transverse magnetic field results in anomalous Hall effect signals of unusual line shape. The interplay between opposite-sign signals from the two interfaces allows for efficient control over the magnitude and sign of the overall effect. Selective engineering of

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interfaces singles out a particular spin signal. The two-channel nature of the effect and its high tunability offer new functional possibilities for future spintronic devices.

INTRODUCTION A fundamental property of materials with large spin-orbit coupling (SOC) is interconversion between macroscopic charge and spin degrees of freedom. The spin Hall effect (generation of a spin current)1-3 and Rashba-Edelstein effect (current-induced spin polarization)4,5 are probably the most prominent mechanisms of charge-spin interconversion. The interface emerges as a fount of spin-based phenomena,6,7 especially due to the Rashba SOC;8 the SOC strength at interfaces is comparable to other relevant energy scales.7 Highly efficient spin-to-charge conversion through the Rashba coupling manifests itself at interfaces in the 2D electron system LaAlO3/SrTiO3,9,10 metallic bilayers,11,12 metal/insulator hybrid devices,13,14 as well as surfaces of topological insulators.15-17 The prospect of chemical engineering is a strong advantage of interfacial spin effects18. Theoretical analysis shows that spin Hall effect currents in thin films are dominated by the interface.19 Simple models describing a metal confined between two insulators20,21 result in a giant spin Hall conductivity due to interfacial scattering. So far, the majority of related works consider systems with a diffusive spin transport. However, (quasi)-ballistic regime offers a unique probe of the interface in the absence of extrinsic bulk scattering;22 in metals, it is predicted to induce various spin transport phenomena.20,23 Particularly interesting is the case of the spin relaxation time sufficiently large for the Larmor frequency Ω = /ħ ~ to be attained in modest magnetic fields . It sets conditions for clear observation of the Hanle effect, an effect extensively used to probe and manipulate spin polarization in various materials with large ,24-26 which can be readily


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combined with charge-spin interconversion mechanisms. Such study does not necessarily involve spin injection or detection – the spin variable can serve as an intermediate link between charge currents, as in a variety of magnetoresistance (MR) effects combining charge-to-spin and spin-to-charge processes.27-30 However, observation of the corresponding effects in longitudinal MR can be obscured by other contributions such as geometrical MR. In principle, it should be much easier to identify effects in the Hall effect measurements – they reveal themselves as a non-linear (anomalous) part of the Hall conductivity as soon as the other, traditional mechanisms for the anomalous Hall effect (AHE)31 are ruled out by the choice of material. The change of the current direction (required for the Hall effect contribution) can be effectuated via spin dephasing in a magnetic field which not only modulates the spin polarization in its initial direction but also creates a transverse spin polarization.32-35 Therefore, one can imagine an effect in the Hall conductivity emerging from the following combination of three processes (Scheme 1): i) efficient charge-to-spin conversion (such as provided by interfaces) resulting in in-plane spin polarization transverse to the current; ii) rotation of the non-equilibrium spin polarization by an out-of-plane magnetic field away from its original direction; iii) spin-to-charge conversion producing a transverse current. The proposed probe of the interfacial spin transport would ideally require an epitaxial film of a centrosymmetric compound formed by a heavy element (for large interfacial SOC), exhibiting quasi-ballistic transport, enhanced structural coherency and sharp interfaces. Recently, we developed a robust way to grow films of tetragonal europium silicide EuSi2 (Supporting Information Figure S1) on Si001 .36 The metallic films demonstrate superb structural quality of EuSi2 and atomically abrupt interfaces. EuSi2 is an antiferromagnetic (AFM) metal with the Néel temperature ≈ 40 K. Thus, it can bridge the gap between the advanced Si technological


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platform and AFM spintronics.37,38 Once a spin gap opens up below the spin relaxation is strongly suppressed. At low temperature, the electron mobility in EuSi2 is rather high for an AFM material36 – thus, the local AFM moments do not contribute to spin scattering. The promising combination of properties of EuSi2/Si001 suggests the use of such system as a playground material for studies of spin-related phenomena, in particular those at the interfaces. Here, we produce SiOx/EuSi2/Si001 structures using molecular beam epitaxy (MBE), study their structural, magnetic and transport properties. In particular, we establish that the electron transport is quasi-ballistic and probes interfacial spin effects. As a result, a new type of anomalous Hall conductivity is observed and ascribed to metal/insulator interfaces. We demonstrate the dependence of the effect on materials, synthesis conditions, temperature, and scaling of the device. The effect is taken under control by engineering EuSi2 films exhibiting the AHE from each interface separately or from both interfaces together. RESULTS AND DISCUSSION Synthesis and Characterization. Tetragonal europium silicide, EuSi2, is synthesized the same way as many other silicides – by direct reaction of Si and metal: Eu + 2 Si = EuSi2. Although small EuSi2 crystallites of various orientations are commonly produced when Eu is deposited on Si001 ,39,40 the synthesis under controlled MBE conditions leads to epitaxial EuSi2/Si001 films.36 Tetragonal EuSi2 is the most stable polymorph; the recently discovered trigonal EuSi2 could be more preferable due to a higher Néel temperature but its synthesis is rather tricky, the films are pseudo-morphic (unstable when their thickness exceeds 7 nm)41 and


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exhibit thickness-dependent 2D ferromagnetism.42 To probe interfacial spin effects we fabricated a number of epitaxial films of EuSi2 on Si001 differing by their thickness (see Methods). The procedure is carried out by directing a molecular beam of Eu atoms at a heated Si001 substrate in an MBE system. According to RHEED, the initial stages of the growth involve 3 distinct surface reconstructions of Eu on Si001 (Supporting Information Figure S2). Further growth of EuSi2 requires a steady supply of Si to the surface, which is likely to proceed via the well-known vacancy mechanism. All the films are covered with SiOx which makes the top interface with EuSi2 and also protects the film from the ambient atmosphere. The epitaxial quality of the films is confirmed by RHEED images taken in situ (Supporting Information Figure S2) and ex situ θ-2θ X-ray diffraction (XRD) scans showing only 0 0 4 peaks from EuSi2 and those from the substrate (Supporting Information Figure S3). Electron microscopy images of the films reveal that EuSi2 is grown epitaxially and evenly (Figure 1a). A close look (Figures 1b and 1c) attests the sharpness of both SiOx/EuSi2 and EuSi2/Si001 interfaces – the major origin of the thickness fringes detected by XRD (Supporting Information Figure S4). The temperature dependence of magnetization of the SiOx/EuSi2/Si001 structures certifies that the Eu moments support the AFM order below 40 K with the magnetic easy axis normal to the surface (Supporting Information Figure S5). Magnetic properties of the films do not depend on , suggesting that the films reach the bulk limit. The resistivity measurements are also consistent with a spin gap opening below 40 K (Figure 1d). At low temperature, the - curves (Figure 1e) exhibit a linear trend thus reporting the absence of any FM ordering.


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The electron transport in the films is quasi-ballistic at low temperature. The linear dependence of the conductivity on the film thickness (Figure 1f) is a fingerprint of the mean free path exceeding . We performed simple calculations within the model of free electrons employing the carrier concentration and mobility estimated from resistance and Hall effect measurements at = 2 K: = is a product of the Fermi velocity and relaxation time , determined from = ⁄3

!

and = #⁄$∗ assuming that the effective electron mass

$∗ = $& . In the EuSi2 films, is several times larger than : for example, = 90 nm for = 29 nm and = 480 nm for = 132 nm. Interfacial Anomalous Hall Effect. As stated above, we suggest the Hall effect measurements to observe anomalous (non-linear in ) conductivity arising from interfaces. Both interfaces of the structure SiOx/EuSi2/Si001 – SiOx/EuSi2 and EuSi2/Si – are of superb quality and can be a source of spin-related phenomena. The resistances of SiOx and Si are much higher than that of EuSi2: SiOx is an insulator; silicon is a semiconductor at high temperature, however, in the temperature range of our transport experiments thermal excitations are frozen out. Therefore, the system can be described as a metal confined between two insulators. In the absence of bulk scattering the two metal/insulator interfaces take central stage. The electric fields at the interfaces lead to the interfacial SOC. A potential mechanism for the interfacial Hall conductivity (Scheme 1) would require charge-spin interconversion which comes from spin-momentum coupling, as in Rashba-type Hamiltonian = '() *+ − + *) -. We are aiming at creation of transverse (Hall) resistance without any spin injection. Thus, it is ultimately a charge-to-charge conversion where the spin variable is linking the charge variables. The spins polarized in-plane at the interface can be manipulated with an out-of-plane magnetic field . The rotation of spins (the Hanle effect) results in a specific functional form (with respect to ) for the arising transverse spin


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polarization which is directly transferred to the overall effect. In the classical Hanle effect, transverse spin polarization arises from convolution of spin depolarization and spin precession, i.e. by integration of the product of exp− 1⁄ and sinΩ1 over time variable. It acquires the form of an odd-Lorentzian with respect to Ω 32-35 and, therefore, an odd-Lorentzian with respect to :

6789: 5)+ ~

Ω ~ 1 ! 1 + ⁄ < ! 1 + Ω

6789: where < is the magnetic field at the maximum of 5)+ or, alternatively, the half width at the

half maximum for the corresponding Lorentzian. Similar dependence on is derived for spin current swapping in the predicted Hanle spin Hall effect.43 Thus, one would expect an oddLorentzian term in 5)+ for each interface. Measurements at 2 K of the SiOx/EuSi2/Si001 structure ( = 57 nm) employing macroscopic Hall bars with a lateral size of 5 mm show that 5)+ is essentially non-linear (Figure 2a) suggesting the AHE. The non-linear part of 5)+ (Figure 2b) is nontrivial, indicating the presence of more than one AHE contribution. The experimental curves are fit according to

5)+ =

5= ∙ + ?@ + ? @@ , 2 @ ! 1 + ⁄ < 1 + ⁄

Interface-Induced Anomalous Hall Conductivity in a Confined Metal

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