SrTiO3 Oxide Interfaces - ACS

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Tunable magnetic phases at FeO/SrTiO oxide interfaces Mai Hussein Hamed, Ronja Anika Hinz, Patrick Lömker, Marek Wilhelm, Andrei Gloskovskii, Peter Bencok, Carolin Schmitz-Antoniak, Hebatalla Elnaggar, Claus M. Schneider, and Martina Müller ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b20625 • Publication Date (Web): 23 Jan 2019 Downloaded from http://pubs.acs.org on January 24, 2019

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ACS Applied Materials & Interfaces

Tunable magnetic phases at Fe3O4/SrTiO3 oxide interfaces Mai Hussein Hamed,†,‡ Ronja Anika Hinz,† Patrick Lömker,† Marek Wilhelm,† Andrei Gloskovskii,¶ Peter Bencok,§ Carolin Schmitz-Antoniak,† Hebatalla Elnaggar,k Claus M. Schneider,†,⊥ and Martina Müller∗,†,# †Peter-Grünberg-Institut (PGI-6), Forschungszentrum Jülich GmbH, 52425 Jülich, Germany ‡Faculty of Science, Helwan University, 11795 Cairo, Egypt ¶Photon Science, Deutsches Elektronen-Synchrotron DESY, 22607 Hamburg, Germany §Diamond Light Source, OX11 0DE Didcot, UK kDebye Institute for Nanomaterials Science, 3584 CG Utrecht, The Netherlands ⊥Fakultät für Physik and Center for Nanointegration Duisburg-Essen (CeNIDE), 47048 Duisburg, Germany #Experimentelle Physik I, Technische Universität Dortmund, 44227 Dortmund, Germany E-mail: [email protected]

Abstract

Keywords

We demonstrate the emergence and control of magnetic phases between magnetite (Fe3 O4 ), a ferrimagnetic halfmetal, and SrTiO3 , a transparent non-magnetic insulator considered the bedrock of oxide-based electronics. The Verwey transition (TV ) was detected to persist from bulk-like down to ultrathin Fe3 O4 films, decreasing from 117±4 K (38 nm) to 25±4 K (2 nm), respectively. Element-selective electronic and magnetic properties of the ultrathin films and buried interfaces are studied by angledependent HAXPES and XMCD techniques. We observe a reduction of Fe2+ ions with decreasing film thickness, accompanied by an increase of Fe3+ ions in both tetrahedral and octahedral sites, and conclude on the formation of a magnetically active ferrimagnetic 2 u.c. γFe2 O3 intralayer. In order to manipulate the interfacial magnetic phase, a postannealing process causes the controlled reduction of the γFe2 O3 that finally leads to stoichiometric and ferrimagnetic Fe3 O4 /SrTiO3 (001) heterointerfaces.

Verwey transition, Magnetite, Fe3-δ O4 , spin moment, spintronics, heterointerfaces

1

Introduction

Oxide heterostructures offer a multitude of functionalities for the exploration of novel physics and next generation of electronics applications. In particular, the interfaces of oxide heterostructures reveal exciting phenomena, for example the formation of a conducting two dimensional electron system (2DES) at the interface of insulating heteroepitaxial oxides 1,2 , functional metal oxides via interfacial redox reaction 3,4 , large Rashba spin-orbit interaction at oxide heterointerfaces 5 and magnetic interfaces between non-magnetic oxides 6 . Also the enhancement of the saturation magnetization of ultrathin magnetic insulators due to auxetic unit cell reduction was observed 7,8 . Magnetite Fe3 O4 is a cornerstone material for spin-based oxide electronics applications, due

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and formation of deficient FeO 22 and the formation of polycrystalline films 23 are possible. Some studies tried to understand the properties of Fe3 O4 /SrTiO3 heterointerfaces, where they claim the formation of the antiferromagnetic FeO phase in the interface because of an exchange bias effect 24 . In this study, we emphasize the influence of reduced film thicknesses and interfaces on the magnetic and electronic properties of ultrathin heterostructures. In order to give a precise understanding of the interfacial properties, bulk and surface sensitive elemental X-ray techniques have been used. By understanding the heterointerfaces properties and the mechanisms for their formation, we can control the oxide interfaces for tailored applications.

to its half metallicity and up to 100 % negative spin polarization at the Fermi level 9,10 . If interfaced with an oxide electrode, exciting phenomena arise from its magnetic functionality, such as spin generation and spin-to-charge conversion, spin-dependent transport 11 (spin filter, spin injection, spin valve and magnetic tunnel junctions 12–15 ), spin Seebeck effect 16 and biomedical applications 17 . Fe3 O4 crystallizes in an inverse spinel structure where Fe3+ cations occupy the tetrahedral sites and a mixture of Fe3+ and Fe2+ occupy the octahedral sites as it is shown in Figure 1. The ferrimagnetic orders with a Curie temperature of 860K and the net magnetic moment of 4 µB /f.u. is a consequence of the antiferromagnetic coupling between tetrahedral and octahedral sites and of the ferromagnetic coupling between the octahedral cations. Another well know property for magnetite is the metalinsulator transition (Verwey transition) which occurs at a temperature TV =120 K 18 .

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Experimental details

Magnetite films with thicknesses between 2 and 38 nm have been grown on conductive 0.1 % Nb-doped SrTiO3 (001) TiO2 -terminated substrates by pulsed laser deposition (PLD). The substrate temperature and oxygen partial pressure were kept at 400 ◦C and 2 × 10−6 mbar, respectively. The laser fluence was set to 1.5 J/cm2 with a repetition rate of 5 Hz producing ionized particles from a Fe2 O3 (purity 99.9 % - Praxair) rotating target. The substrate is mounted in a distance of 50 mm away from the target. The film thicknesses and interface- and surface roughness were examined by X-ray reflectivity (XRR) using a Philips XPert MRD with CuKα -radiation. Atomic force microscopy (AFM) using an Asylum Research Cypher AFM in tapping mode was employed to assess the films surface roughness that were found to be 0.2-0.4 nm. Bulk magnetic properties were investigated by a vibrating sample magnetometer (VSM) using a Quantum Design Dynacool physical properties measurement system (PPMS). Magnetic moment versus temperature M (T ) was measured in zero field cooling (ZFC) mode with 500 Oe applied field. Hysteresis loops were recorded at T = 50 K and a magnetic field

Figure 1: Schematics of the Fe3 O4 /SrTiO3 heterostructure with focus on the inverse spinel Fe3 O4 unit cell, where Fe3+ cations (green) equally occupy tetra- (Td ) and octahedral (Oh ) sites, while Fe2+ cations (blue) occupy Oh sites. The ferromagnetic (red arrow) and antiferromagnetic (blue arrow) coupling between the Oh sites, and Td and Oh sites, respectively, account for the net magnetic moment of 4 µB /f.u.. Growing Fe3 O4 on SrTiO3 , a mainstay electrode material for oxide electronics, is a challenging task because of a −7.5 % compressive lattice mismatch, which may lead to misfit dislocations, twins and formation of antiphase boundaries (APBs) 19 . In addition the high surface roughness 20,21 , cationic inversion


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H=±5 T parallel to the in-plane [100]-axis. Element-selective and depth-dependent chemical information of the films and buried interfaces were analyzed by Hard X-ray Photoelectron Spectroscopy (HAXPES) at PETRA III (DESY, Hamburg). The information depth is depending on the photon energy and the inelastic mean free path λ; for Fe3 O4 using 6 keV the information depth is approximately equal to 25 nm. This is allowing us to probe the electronic structure of the films surfaces, interfaces and substrates. Element- and magnetic site-selective X-ray Absorption Spectroscopy (XAS) and X-ray Magnetic Circular Dichroism (XMCD) were performed at Diamond Light Source (Didcot, UK) at beamline I10 (BLADE) with the photon energy ranging from 0.4 to 2 keV. A near-grazing emission geometry θ=80° has been used, with 1 T magnetic field and 10K temperature. XAS spectra were recorded in total electron yield mode (TEY) by measuring the sample drain current. Four absorption spectra were taken for each sample using left and righthanded circular polarization. The quantitative cation sites occupancy was determined by comparing experimental XMCD spectra with model spectra calculated by the CTM4XAS 5.5 software 25 , using the parameters given in Ref. 26 . The inverse spinel structure can be written in the form of 3+ •• •• Fe3+ [Fe2+ 1 – 3δ Fe1+2δ VOδ ]O4 , where VO indicates the vacancies in Oh sites, for magnetite δ=0 and maghemite δ= 0.33, respectively 27 . The thickness of Fe3 O4 and γ-Fe2 O3 can be calculated using the cation ratio obtained by XMCD 1 − 3δ F e2+ = . 3+ Fe 2 + 2δ

ments employing the sum rules 28,29 mSpin + 7 < Tz >= −

3p − 2q Nh , r

(3)

2q Nh , (4) 3r where Nh =13.5 is the number of Fe empty orbitals per formula unit 30 , Tz indicates the magnetic dipole moment operator that can be neglected to the first order for cubic systems 31 , and p and q are Z Z + − p= (σ − σ )dω q = (σ + − σ − )dω, mOrb = −

L3

L3,2

(5) as determined from the XMCD integration, where r denotes as Z 1 r= (σ + + σ − )dω. (6) 2 L3,2

3 3.1

Results Verwey Transition

The critical temperature TV is considered as an indicator for the Fe3 O4 stoichiometry as it is strongly influenced by the oxygen content 32,33 . Moreover, the Fe3 O4 magnetization decreases at TV because of a lowering of the symmetry to a monoclinic structure and an increase of the anisotropy 34 . For nonstoichiometric Fe3-δ O4 single crystals, it is reported that the Verwey temperature decreases with increasing δ. However, if δ > 0.045, no Verwey transition could be found 33 . Generally, TV is influenced by the growth conditions 35–38 , substrate choice 35,39,40 and film thicknesses 35,36,41,42 . For Fe3 O4 /MgO heterostructures with 0.32 % lattice mismatch, many studies report that TV decreases with decreasing film thickness, however, it was hardly detected for magnetite films thinner than 10 nm 35,36,41,42 . To date, studies on Fe3 O4 /STO (−7.5 % lattice mismatch) did not report a Verwey transition in films thinner than 17 nm 24,35,40 . In order to determine the Verwey transition TV in Fe3 O4 /SrTiO3 , zero field cooling (ZFC) magnetization versus temperature mea-

(1)

If fractions (x) of Fe3 O4 and (1 − x) of γ-Fe2 O3 are present, they can be related as (F e3 O4 )x + (γ − F e2 O3 )(1−x) = F e3−δ O4 (2) By determining the XMCD intensities, we calculate the Fe spin and orbital magnetic mo-


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In order to investigate the impact of the STO heterointerface and reduced dimensionality on the Fe3 O4 chemical and magnetic properties, depth- and element-selective-magnetic studies will be discussed in the following.

3.2 (b)

Buried Fe3 O4 /SrTiO3 Interface

In order to identify the chemical structure of the ultrathin Fe3 O4 films and the interface to the SrTiO3 substrates, we used depth-dependent HAXPES. Figure 3 (a) depicts Fe 2p core level spectra for 2, 4, 8 and 38 nm thick films. To identify their chemical properties, the experimental spectra were compared to reference data of FeO 45 , Fe2 O3 and Fe3 O4 44 . A wellknown feature to identify the particular Fe oxides is the satellite peak as shown in Figure 3 (a). For FeII O and FeIII 2 O3 , the satellite binding energies are 715.5 eV and 719 eV, respectively, which can be easily distinguished by both XPS (the resolution ∆E=0.8 eV) and HAXPES (∆E=0.2 eV). For Fe3 O4 , which has mixed oxidation state of Fe2+ and Fe3+ , the satellites from both states merge, see Figure 3 (a). For 38 nm thick Fe3 O4 , we find indeed that the two satellites are merged. Moreover, the Fe 2p3/2 core level depicted in Figure 3 (b) is composed of the Fe3+ peak and a shoulder that refers to Fe2+ . Consequently, we observe a bulk-like stoichiometry of 38 nm thick Fe3 O4 films. To investigate the buried Fe3 O4 /SrTiO3 interfaces, HAXPES was conducted for dF e3 O4 =2, 4 and 8 nm, shown in Figure 3 (a). We observe that the Fe3+ satellite peak appears for the 8 nm film and its intensity enhances with decreasing Fe3 O4 thickness. This enhancement is accompanied with a decrease of the Fe 2p3/2 peak shoulder, which is indicative for Fe2+ . Both observations suggest the formation of the Fe2 O3 phase in addition to Fe3 O4 . The fact that we do not observe this feature for dF e3 O4 =38 nm suggests that the modification of the chemical properties from Fe3 O4 to Fe2 O3 occurs at the Fe3 O4 /SrTiO3 interface . In order to clarify this hypothesis, angle-dependent HAXPES is performed.

Figure 2: Magnetic detection of Verwey transition in Fe3 O4 on SrTiO3 (001): (a) M (T ) of 38 nm thick film, the first and second derivatives for the magnetization curve to detect TV and ∆TV . (b) Verwey transition temperature TV and transition width ∆TV as a function of film thicknesses. surements (M (T )) were performed for Fe3 O4 thicknesses between 2-38 nm. In Figure 2 (a), the M (T ) curve of a 38 nm thick film is shown exemplarily. The magnetization drops about 14 % below TV , whereby TV was determined as 117±4 K by the maximum of ∂M/∂T . The transition width ∆TV was calculated from ∂ 2 M/∂T 2 as 12±2 K. This value is very close to the bulk values of 120 K 18 and suggests a bulklike stoichiometry of the 38 nm thick Fe3 O4 film. For the thickness series d=2-38 nm, TV and ∆T (d) are depicted in Figure 2 (b). TV decreases with film thickness as observed for Fe3 O4 /MgO 35,36,41,42 , yet the Verwey transition is still detectable in our ultrathin Fe3 O4 films of 2 and 4 nm thickness. TV decreases slowly for thickness above 8 nm to nearly 100 K, but for thickness less than 8 nm, the curve exponentially decreases to 25 K for a 2 nm film. Moreover, the transition width ∆TV shows an inflection point at 8 nm Fe3 O4 film thickness. The decrease of TV and broadening of transition is due to epitaxial strain and growth defects 43 , as well as reduced domain sizes 35 .


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(c)

(b)

(d)

Figure 3: (a) Fe 2p core level HAXPES spectra of Fe3 O4 /SrTiO3 (001) for varying thickness recorded at hν = 6 keV and reference spectra 44,45 . (b) Fe 2p3/2 core level. (c) Fe 2p core level of 8 nm thick Fe3 O4 , recorded at θ=5° and 60° emission at hν = 4 keV. (d) Fe 2p3/2 core level. Figure 3 (c) and (d) illustrate the angledependent HAXPES experiment on 8 nm Fe3 O4 using both a normal emission θ=5° (interface sensitive) and grazing emission θ=60° (surface sensitive) geometry. The Fe3+ satellite peak intensity reduces for 60° measurement, whereas, the Fe2+ shoulder of Fe 2p3/2 core level peak is enhanced. In contrast to previous studies 22,24 , which claimed the formation of interfacial FeO layer for the heteroxides Fe3 O4 /SrTiO3 system, our findings point toward the formation of an interfacial Fe2 O3 phase. In particular, α-Fe2 O3 crystallizes in a corundum structure with antiferromagnetic properties, whereas γ-Fe2 O3 is a defected ferrimagnetic spinel with a saturation magnetization of Ms =2.5 µB /f.u.. In order to reveal, which interfacial Fe2 O3 phase is likely formed, we apply XMCD to distinguish between ferrimagnetic and antiferromagnetic properties.

3.3

for α-Fe2 O3 and γ-Fe2 O3 46 . XAS spectra of Fe L2,3 -edge are shown in Figure 4 (a), (b) and (c). The pre-edge peaks A and D are mainly contributed to Fe2+ Oh and its intensities reduce with decreasing thickness. The peaks B, C, E and F originated from mixed contributions from 3+ 3+ Fe2+ Oh , FeTd and FeOh sites. We observe both altered intensity ratios and different energy positions, which indicates the changing of the Fe2+ and Fe3+ ratios with film thickness. The observed alteration of their ratios confirms a redistribution of cations. We conclude from these findings, that the absorption fine structure indicates the presence of γ-Fe2 O3 at the interface. Another and more stronger proof for the formation of interfacial γ-Fe2 O3 is the XMCD fine structure. Figure 4 (d), (e) and (f) illustrate the Fe L2,3 edge XMCD spectra. For L3 edge, the two preedge peaks a and b are mainly contributed to Fe2+ Oh , and their peak intensities decrease with decreasing thickness. The peaks c, d and e are well-known fingerprints for distinguishing Fe3 O4 from γ-Fe2 O3 27,47 . The Fe2+ Oh (c peaks) 3+ and FeOh (e peaks) are ferromagnetically cou-

Interface Magnetic Properties

Next, we determine the element-specific magnetic properties of the Fe3 O4 /SrTiO3 interface. The XAS fine structure thereby strongly differs


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(d)

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(b)

(c)

(e)

(f)

Figure 4: (a)Fe L2,3 -edge XAS spectra of Fe3 O4 films with varying thickness, (b) L3 edge XAS fine structure, (c) L2 edge XAS fine structure, (d) Fe L2,3 -edge XMCD spectra of Fe3 O4 films with varying thickness (solid lines) in comparison with the model spectra (doted lines) calculated for Fe2+ and Fe3+ in both Td and Oh sites, (e) L3 edge XMCD fine structure and (f) L2 edge XMCD fine structure. pled (negative XMCD L3 edge) and antiferromagnetically coupled to Fe3+ Td (d peaks) (positive XMCD L3 edge). The intensity ratio between the c and e peaks Ic /Ie is equal to 1.5 and 0.4 for Fe3 O4 and γ-Fe2 O3 , respectively 47 . For the 38 nm thick film, Ic /Ie nearly equals 1.5. The ratio is decreasing continuously with decreasing film thickness to Ic /Ie =0.6 for a 2 nm thick film. This is once more indicating the formation of an interfacial Fe2 O3 intralayer. We find a magnetic contribution from the γ-Fe2 O3 (ferrimagnetic), but no α-Fe2 O3 (antiferromagnetic). Furthermore, the L2 edge XMCD (Figure 4 (f)) reflects the mixed state of Fe3 O4 and γ-Fe2 O3 . The peak f corresponds to Fe2+ Oh and decreases with decreasing thickness. The peak 3+ 3+ g corresponds to both Fe2+ Oh , FeTd and FeOh and splits into two small peaks for the 2 nm thick film. The h peak corresponds to Fe3+ Td , and its intensity increases with decreasing thickness. The XMCD fine structure clearly reflects the formation of γ-Fe2 O3 at the Fe3 O4 /SrTiO3 interface. In order to identify the magnetic thicknesses of γ-Fe2 O3 and Fe3 O4 , XMCD calculations are

(a)

(b)

Figure 5: (a) Calculated Fe cation occupancy in Td and Oh lattice sites for various Fe3 O4 film thicknesses. (b) Calculated magnetic thicknesses of Fe3 O4 and the interfacial γ-Fe2 O3 intralayer.


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To study the magnetic properties of the interface, the sum rules are used 28,29 . Figure 6 (a) gives XAS and XMCD spectra exemplarily for 38 nm Fe3 O4 , and their integration to determine the values of p, q and r according to Eq.(5) and (6). Spin and orbital magnetic moments with respect to films thickness are given in Figure 6 (b). For the 38 nm thick sample, the saturation magnetization MS measured by VSM is of 3.3±0.2 µB /f.u., which is reduced from the theoretical bulk value 4 µB /f.u. 48 . The macroscopic magnetization is affected by structural dislocations and formation of antiferromagnetic anti-phase boundaries, which is a well known phenomenon for Fe3 O4 49,50 . However, this cannot explain the reduction of spin magnetic moment measured by XMCD technique to 2.6±0.5 µB /f.u., which is 80 % of MS determined by VSM. We notice that theoretical studies have shown that sum rules on 3ds systems can show up to 30 % error of ground state properties, which could explain the difference observed 51 In recent studies on Fe3 O4 films and nanoparticles 52,53 , mS measured with an applied magnetic field of 1 T equals 75-80 % of the magnetic moment measured at higher applied field. As shown in Figure 6 (b), both mS and MS are decreasing with film thickness to 1.2±0.3 µB /f.u. for 2 nm film as expected because γ-Fe2 O3 has a lower ferrimagnetic magnetic moment MS =2.5 µB /f.u. than Fe3 O4 (MS =4 µB /f.u.).

performed. We calculate the occupancy of Fe3+ and Fe2+ in the different lattice sites Td and Oh shown in Figure 5 (a). Starting from the 38 nm 3+ 2+ film, the ratios for Fe3+ Td , FeOh and FeOh equal 33.3 %, in agreement with stoichiometric Fe3 O4 2+ 3+ which has 1:1:1 ratio for Fe3+ Td :FeOh :FeOh . With decreasing the Fe3 O4 thickness, the cation ratios change. The Fe2+ Oh ratio decreases to 6 % for a 2 nm thick film, accompanied by an in3+ crease of the Fe3+ Td and FeOh ratios to 42 % and 52 %, respectively. By using Eqs. 1 and 2, we calculate the magnetic thickness dm of the interfacial γ-Fe2 O3 layer and Fe3 O4 , as compiled in Figure 5 (b). We determine a γ-Fe2 O3 intralayer of 1.3± 0.3 nm which is independent of the overlaying Fe3 O4 thickness. (a)

(b)

3.4

Substrate-induced Oxidation

Interface

The previously discussed characterization of the magnetic and electronic structure indicates the formation of γ-Fe2 O3 in the interface. Figure 7 sketches a crystalline model of the resulting heterosystem. However, the mechanism for the formation of the interfacial γ-Fe2 O3 phase between Fe3 O4 films and STO substrates remains to be discussed. Taking into account the thermodynamic properties of all constituents, SrTiO3 has lower Gibbs free energy of formation ∆GoF =−1588.413 76 kJ/mol than Fe3 O4 (∆GoF =−1015.4568 kJ/mol) 54 , therefore STO is not expected to oxidize Fe3 O4 . However, oxy-

Figure 6: (a) XAS and XMCD and their integration for 38 nm thick Fe3 O4 . (b) Spin and orbital magnetic moments calculated from XMCD spectrum compared to MS measured by VSM for varying film thickness.


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(a)

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(b)

TiO2-

Figure 7: Schematics of the interfacial oxidation process via oxygen diffusion and the counteracting reduction process via annealing.

Figure 8: (a) Magnetization versus magnetic field for 4 nm Fe3 O4 as grown and post annealed at different temperatures. (b) HAXPES Fe 2p core level spectra of the resulting samples.

gen vacancies perturb the formation energy 55,56 for TiO2 -terminated STO to 461.53 kJ/mol 57 . Consequently, the formation of an oxidized γFe2 O3 intralayer may be explained by the oxygen diffusion from the STO substrate, due to the reduced surface formation energy by oxygen vacancies of TiO2 -terminated STO surfaces, which enables the migration of oxygen from STO into Fe3 O4 . We note, that this mechanism may simultaneously lead to the formation of reduced Ti3+ at the STO interface, which is the key prerequisite for the formation of a redox 2DES 3 .

3.5

films of d=4 nm, as we expect that they are composed of 50 % γ-Fe2 O3 and 50 % Fe3 O4 . Two films were prepared in the same environment. Subsequently, the first film was annealed at T =400 ◦C for 90 min in PO2 =2 × 10−6 mbar, whereas the second sample was annealed at T =700 ◦C in PO2 =2 × 10−6 mbar. Figure 8 depicts the magnetic and chemical properties of the as grown sample and the two samples annealed at 400 and 700 ◦C. Starting with the magnetic properties of the films shown in Figure 8 (a), the saturation magnetization MS for the film annealed at 400 ◦C is 1.4±0.2 µB /f.u., which nearly equals an as-grown 4 nm film without annealing (MS =1.5±0.2 µB /f.u.). This suggests that a γFe2 O3 phase formed in the interface as expected from our thermodynamics analysis. However, for the film annealed at 700 ◦C, MS enhances to 3.1±0.3 µB /f.u., which is near to values measured for as grown 38 nm films. We therefore may have succeeded to obtain Fe3 O4 by postannealing at T =700 ◦C. HAXPES experiments

Towards Stable Fe3 O4 /SrTiO3 Interfaces

Finally, we investigate how to grow Fe3 O4 ultrathin films free from γ-Fe2 O3 on STO substrates. One route is to reduce α-Fe2 O3 to Fe3 O4 by annealing in low oxygen pressure < 10−7 mbar and high temperature >800 ◦C, which has been shown for Fe3 O4 films grown on Al2 O3 58 . However in our case with the active STO substrate this conditions may be not suitable. We chose


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were conducted on both films as shown in Figure 8 (b). The appearance of satellite peak A for sample annealed at 400 ◦C refers to the formation of a Fe2 O3 phase, in accordance to the magnetization measurements. Moreover, for the sample annealed at 700 ◦C, the satellite peak A disappears, whereas the shoulder of Fe 2p3/2 enhances and indicates the presence of Fe3 O4 . While the surface roughness for the sample annealed at 700 ◦C increases to 1.7±0.2 nm compared to 0.6±0.1 nm for the sample annealed at 400 ◦C, the crystalline quality of the post-annealed sample (700 ◦C) increases remarkably, as indicated by the XRD and rocking curve of Fe3 O4 (004) reflection (not shown here). However, no Verwey transition was observed. We hereby demonstrated the tuning between two ferrimagnetic phases of iron oxide by taking control of the redox chemical processes at the Fe3 O4 /SrTiO3 interface.

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of iron oxide devices in which the functionalities of the magnetic phases, and their coupling, can be obtained from controlled physicochemical reactions and engineering of their interfaces. Acknowledgement M. H. Hamed acknowledges R. Dittmann for providing the PLD setup as well as O. Petracic for providing measurement time at VSM at FZJ. This work has been funded by the Helmholtz Association under Grant HGF-NG-811. M. H. Hamed thanks the Egyptian Ministry of Higher Education for providing a PhD scholarship.

References (1) Ohtomo, A.; Hwang, H. Y. A High-Mobility Electron Gas at the LaAlO3 /SrTiO3 Heterointerface. Nature 2004, 427, 423–426. (2) Thiel, S.; Hammerl, G.; Schmehl, A.; Schneider, C. W.; Mannhart, J. Tunable Quasi-Two-Dimensional Electron Gases in Oxide Heterostructures. Science 2006, 313, 1942.

Conclusion

In summary, we grow Fe3 O4 on SrTiO3 with thicknesses d=2-38 nm and investigated the interfacial chemical and magnetic properties. The Fe3 O4 films stoichiometriy are assured by mapping the Verwey transition down to the ultrathin 2 nm film. We performed angledependent HAXPES and XMCD, which prove the formation of an interfacial ferrimagnetic γFe2 O3 layer of 2 u.c. in the Fe3 O4 /SrTiO3 heterosystem. The formation of an oxidized intralayer is due to the diffusion of oxygen across the TiO2 -terminated and oxygendeficient SrTiO3 surface, which is lowered in surface Gibbs formation energy. We succeed to reduce the oxidized interfacial phase γ-Fe2 O3 by post-annealing at high temperatures. At elevated temperatures, the mobility of surface oxygen ions increases and causes the reduction of γ-Fe2 O3 and the formation of Fe3 O4 in the interface. Our results open the exciting perspective of enabling the continuous tuning, depending on Fe3 O4 film thickness and redox process parameters, of the spin-dependent properties of Fe3 O4 -based heterointerfaces. More generally, our results allow a simple and versatile design

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SrTiO3 Oxide Interfaces - ACS

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