Magnetically intercalated multilayer silicene

Silicene, a Si-based analogue of graphene, is predicted to exhibit topological electronic phases with exotic properties capable to revolutionize electronics. In particular, the silicene structure is highly advantageous for spintronics. However, lack of synthetic routes to free-standing and magnetically functionalized silicene compounds prevents experimental corroboration of the predictions. Here we synthesize EuSi2, multilayer silicene intercalated with inherently magnetic Eu atoms, on SrSi2/Si(001) templates. The resulting films are formed by crystallites of two mutually orthogonal orientations. The structure is firmly established with electron diffraction, X-ray diffraction and electron microscopy. The compound EuSi2 exhibits non-trivial magnetic and transport properties. The data are compared with those for EuSi2 films grown on SrSi2/Si(111) templates.


Introduction
The exceptional rise of graphene to prominence in condensed-matter physics has driven a surge in studies of two-dimensional (2D) materials.Buckled 2D Xene sheets formed by group IVA atoms arranged in a honeycomb lattice are of particular interest to nanotechnology due to diversity and high tunability of their electronic states [1].The most important member of the family of 2D Xenes is silicene, a Si-based analogue of graphene [2,3].Similar to graphene, physical properties of silicene are largely determined by massless Dirac fermions.The difference is that the band gap in silicene is easily tuned by chemical functionalization and electric fields.Despite technological difficulties, silicene is demonstrated as a promising material for field-effect transistors [4].Electric and exchange fields induce different topological states in silicene [3,5] leading to unconventional properties including quantum anomalous Hall effect, quantum spin Hall effect and valley polarized quantum Hall effect.
Silicene is thought to be especially valuable for application in spintronics [3].Unlike graphene, silicene benefits from relatively strong spin-orbit interaction, large spin-coherence length and spin-diffusion time.Especially useful are valley-spin coupling and accessible topological states with non-trivial spin transport.However, to make use of these functionalities magnetic states in silicene should be induced, for example by chemical modification.In this respect, adsorption or intercalation of metal atoms are considered as a prospective way to bring diverse magnetic properties into the silicene physics [3].
A lot of predictions are made about unique and exotic properties of silicene compounds but their experimental realization is well behind the theory and electronic structure calculations.The problem is in synthesis of silicene and its derivatives.Synthesis of 2D materials is usually tricky [6] but in the case of silicene it is vastly aggravated by its high chemical reactivity caused by significant buckling of silicene layers.Free-standing silicene is yet to be produced.2D silicene structures are formed by adsorption of Si on metal substrates; however, a strong hybridization of silicene with the substrate destroys the Dirac states [2,3,7].
It is suggested that intercalation of silicene with active atoms restores Dirac cones [7].This hypothesis is experimentally corroborated in multilayer silicene intercalated with Ca, a polymorph of CaSi 2 [8].Recently we succeeded in synthesis of Sr-intercalated multilayer silicene, a new polymorph of SrSi 2 [9].It is engineered by epitaxial stabilization on Si(111) and Si(001).In contrast to CaSi 2 , layered SrSi 2 enjoys a regular stacking of silicene layers, thus avoiding the momentum shift of Dirac cones from high-symmetry points [9].
Both CaSi 2 and SrSi 2 are nonmagnetic.To induce magnetic properties into silicene lattice we proposed a synthesis of multilayer silicene intercalated with inherently magnetic atoms [10].We chose Eu because it is the most active among magnetic metals, the half-filled f-shell of Eu 2+ provides strong magnetic properties, and the isomorphism of Eu(II) and Sr compounds provides an opportunity for successful synthesis.In Ref. [10] we thoroughly explored synthesis and properties of EuSi2 grown on SrSi 2 /Si(111) with silicene layers parallel to the surface.However, many unique properties of silicene lattice come from electric fields acting normal to silicene surface [3,5].Here, we explore transport across magnetic silicene layers by studying in detail EuSi 2 synthesized on Si(001) with a SrSi 2 template layer.

Methods
The films of silicides are grown by molecular beam epitaxy in the Riber Compact 12 system.The UHV system consists of Gamma Vacuum Titan Ion Pump, Cryo-Torr 8 cryopump, cryopanels and a titanium sublimation pump.The base pressure in the growth chamber is less than 10 -10 Torr.The metals (Sr and Eu) of 4N quality are effused from Knudsen cell sources.The intensity of molecular beams is measured with a Bayard-Alpert ionization gauge.The substrate temperature is determined with a PhotriX ML-AAPX/090 infrared pyrometer.The surface is controlled in situ with a RHEED diffractometer.
X-ray diffraction (XRD) study of the films employs Rigaku SmartLab diffractometer with a 9 kW copper anode.Electron microscopy studies are carried out for cross-sections prepared in Helios scanning electron microscope/focused ion beam (FIB) dual system.The surface of the samples is covered by a layer of Pt.Ga + ions are used for FIB milling, thinning and cleaning cross-sections.The specimens are protected from oxidation by carbon deposition.The structure of the films is studied with a Titan 80-300 microscope.
Magnetic properties of EuSi 2 are studied with MPMS XL-7 Superconducting Quantum Interference Device employing the reciprocating sample option.Transport properties of the films are determined in the van der Pauw configuration with a Lake Shore 9709A measurement system.Ohmic contacts to metallic films are produced by placing a Ga-Sn-Ag alloy.

Results
Multilayer silicene intercalated with Eu atoms (trigonal EuSi 2 ) is less stable than the tetragonal EuSi 2 .The latter is formed in the reaction of Eu atoms with Si surface, irrespective of its orientation [11].We employ epitaxial stabilization to produce trigonal EuSi 2 .The first step is to prepare a template of layered SrSi 2 on a clean Si(001) surface following the recipe proposed in Ref. [9].The synthesis goes through a number of surface reconstructions of Sr on Si(001) [12].Due to natural incommensurability of the hexagonal lattice of SrSi 2 and the cubic lattice of Si(001), the resulting film is formed by crystallites of two orientations differing by lateral rotations by 90°.Silicene layers are orthogonal to the Si(001) surface for both orientations.
EuSi 2 is then grown by directing a beam of Eu atoms (10 -8 Torr) on top of the SrSi 2 /Si(001) template kept at 450 °C.Si atoms go through SrSi 2 via the well-known vacancy mechanism.The structure is capped with SiO x to prevent its degradation by atmosphere.RHEED images (Fig. 1a) show that the film is formed by two types of EuSi 2 crystallites with orthogonal orientations repeating the morphology of the underlying SrSi 2 .Regretfully, the epitaxial stabilization is short-range: the film adopts the most stable form of EuSi 2 [11] when the thickness of EuSi 2 exceeds 7 nm.It becomes evident in RHEED images (Fig. 1b) revealing appearance of the pattern corresponding to tetragonal EuSi 2 .The structure of the films is further characterized ex situ.XRD -2 scan for the SiO x /EuSi 2 /SrSi 2 /Si(001) film is shown in Fig. 2.Although peaks from SrSi 2 and EuSi 2 are not resolved due to isomorphism of the compounds and closeness of the corresponding lattice constants, the scan is consistent with the proposed orientation of crystallites and the absence of any alien phases.The atomic structure of the films is revealed by electron microscopy.Fig. 3 shows HAADF-STEM image for one of the crystallites with silicene layers running normal to the cross-section.The image confirms the orientational relation between SrSi 2 and EuSi 2 .It also reveals the presence of edge dislocations: extra halfplanes of atoms are introduced at the EuSi 2 /SrSi 2 boundary to account for some mismatch between the lattices.Fig. 4 shows a crystallite of the other orientation, with silicene layers parallel to the crosssection.We employ the bright-field mode with reduced Z-contrast to show the honeycomb structure of silicene.The boundary between EuSi 2 and SrSi 2 is formed by a single layer of mixed Eu x Sr 1-x Si 2 silicide marked as red rectangle.The aim of making trigonal EuSi 2 is to get a magnetic (spin) response from a silicene-based system.Fig. 5 shows temperature dependence of the magnetic moment measured in magnetic field of 1 T directed along 3 axes.The film demonstrates antiferromagnetic (AFM) response with moments oriented normal to the plane and parallel to silicene layers.Thus, we succeeded in inducing magnetism into a system based on silicene layers.It is reasonable to expect that the proximity of magnetic Eu 2+ ions can result in non-trivial transport properties of the films, especially for the transport across a stack of silicene layers.Temperature dependence of resistivity   and its temperature derivative for EuSi 2 grown on Si(001) and Si(111) are shown in Fig. 6.In the latter case the transport is carried out along silicene layers, while in the former case carriers move partly along the layered structure and partly across it.Naturally, SrSi 2 also contributes into the film conductivity but this transport channel shows no anomalies [9].The resistivity of a magnetic metal is dominated by local spin fluctuations; the temperature dependence of    ⁄ should follow that of the magnetic specific heat.The opposite sign of derivative changes at the AFM transition for the EuSi 2 /SrSi 2 /Si(001) and EuSi 2 /SrSi 2 /Si(111) structures is a consequence of different direction of magnetic moment with respect to the current in the two films.In Ref. [10] we found an unconventional magnetoresistance and its temperature dependence in transport across silicene layers.Here, we study Hall resistance   for the films.In the case of the EuSi 2 /SrSi 2 /Si(111) structure, Hall response is unexceptional -dependence of   on B is linear at all T; the sign of the derivative corresponds to electron carriers.The situation is quite different for the EuSi 2 /SrSi 2 /Si(001) structure, as shown in Fig. 7.The behaviour of the Hall resistance at low temperatures (including those above T N ) is quite similar to that of EuSi 2 on Si(111).However, the sign of the Hall effect changes from electron-like to hole-like as T approaches room temperature.More importantly, this change in the electronic structure is accompanied by anomalous contribution to the Hall resistivity, which is highly unusual for a paramagnet.This behaviour may be caused by interplay of more than one band.

Conclusions
Silicene is one of the most promising materials for nanoelectronics.Its structure ensures efficient transport properties which can be manipulated by external stimuli.A major promise of silicene is for spin-dependent phenomena and transport.However, the bottleneck of silicene studies -synthesis -precludes the theoretical predictions from experimental verification.Here, we attempted to bridge the gap between the theory and experiment by studies of stacks of silicene layers intercalated with magnetic Eu atoms.We described the complex synthesis procedure and verified the atomic structure of the resulting layered polymorph of EuSi 2 grown on Si(001) with a SrSi 2 template.The film exhibits a complex magnetic structure dominated by antiferromagnetism.This conclusion is confirmed by studies of transport across silicene layers.The electronic structure of the films changes as temperature goes up from 2 K to 300 K, signified by changes in the carrier type and unexpected room-temperature anomaly in Hall measurements.The results call for additional studies, both experimental and theoretical.This work is partially supported by NRC "Kurchatov Institute", the Russian Foundation for Basic Research (grants 16-07-00204, 16-29-03027, and 17-07-00170), and the Russian Science Foundation (grant 14-19-00662).The measurements are partially carried out using the equipment of the resource centres of electrophysical, laboratory X-ray, and electron microscopy techniques of NRC "Kurchatov Institute".

Fig. 4 .
Fig. 4. Cross-sectional BF-STEM image of the SiO x /EuSi 2 /SrSi 2 /Si(001) film.It shows the boundary between EuSi 2 and SrSi 2 phases as well as hexagons of Eu (red), Sr (blue) and Si (yellow).The aim of making trigonal EuSi 2 is to get a magnetic (spin) response from a silicene-based system.Fig.5shows temperature dependence of the magnetic moment measured in magnetic field of 1 T directed along 3 axes.The film demonstrates antiferromagnetic (AFM) response with moments oriented normal to the plane and parallel to silicene layers.

Fig. 6 .
Fig. 6.Temperature dependence of resistivity   (blue) and its temperature derivative    ⁄ (red) for thin films: a) SiO x /EuSi 2 /SrSi 2 /Si(001) and b) SiO x /EuSi 2 /SrSi 2 /Si(111).Thus, we succeeded in inducing magnetism into a system based on silicene layers.It is reasonable to expect that the proximity of magnetic Eu 2+ ions can result in non-trivial transport properties of the films, especially for the transport across a stack of silicene layers.Temperature dependence of resistivity   and its