Incommensurate Growth of Thin and Ultrathin Films of Single-Phase

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Incommensurate Growth of Thin and Ultrathin Films of Single-Phase Fe3O4(001) on SrTiO3(001) Juan Rubio-Zuazo,*,†,‡ Laura Onandia,†,‡ Eduardo Salas-Colera,†,‡ Alvaro Muñoz-Noval,†,‡ and German R. Castro†,‡ †

SpLine Spanish CRG Beamline at the ESRF, 71 Avenue de Martyrs, F-38043 Grenoble, France Instituto de Ciencia de Materiales de Madrid-ICMM/CSIC, Cantoblanco Madrid E-28049, Spain

‡

ABSTRACT: We present structural, electronic, and morphological characterization of thin (7.5 nm, 9 unit cells) and ultrathin (1.7 nm, 2 unit cells) Fe3O4 layers grown on SrTiO3(001) by oxygen assisted molecular beam epitaxy (MBE) and pulsed laser deposition (PLD) methods. The analysis show single-phase single oriented (001) layers that grow forming a coincidence lattice of the order 13. Such an incommensurate growth is present for all the layers, independent of the evaporation method and layer thickness. The magnetite layers, which are strain-free, provide a sharp interface with the substrate. The films grown by the MBE method present negligible interaction with the substrate and smooth surfaces. The films grown by the PLD evaporation method present an expansion of the last atomic layer of the substrate and rough layer surface. However, the obtained structural domain size of 10 nm is identical for the layers grown by both evaporation techniques corresponding to the size of the coincidence lattice. We determine that the magnetic and transport properties inherent to the Fe 3O4/STO heterostructure are not related to the lattice strain but to the formation of antiphase boundary defects.

typically an insulating oxide ultrathin film, on the magnetite film usually requires high sample temperature under high oxygen pressures. As a consequence, it is of extreme importance for the spintronic technology to grow single-phase magnetite thin films on oxide substrates different than MgO. Fe3O4 has been successfully grown on SrTiO3 and SrTiO3:Nb substrates by the pulsed laser deposition and molecular beam epitaxy techniques. Recently, it has been demonstrated the presence of in-plane magnetization at room temperature with a well-defined easy axis that can be tuned with the modification of the growth parameters.7 The strong magnetic anisotropy on epitaxial thin films has been assigned to the substrate induced strain due to the large lattice mismatch (∼7.5%) between the Fe3O4 and the SrTiO3 lattices.8,9 However, although epitaxial growth on Fe3O4/SrTiO3 heterostructures has been claimed in several works,9−11 convincing experimental demonstration is still missing. Epitaxial growth has been claimed from reflection high energy electron diffraction (RHEED) in-plane structural

1. INTRODUCTION Magnetite (Fe3O4) has attracted much attention because it presents some exotic behavior as half-metal character, metal-toinsulator transition (Verwey transition) at 120 K, ferrimagnetic nature with a 850 K Curie temperature, and multiferroicity at low temperatures. These properties make magnetite an ideal candidate for many technological applications, for instance as an electrode in tunnel magneto-resistance (TMR) memories based on high spin-polarized currents1 or in photovoltaic devices.2 The response of these devices depends drastically on the quality of the magnetite layer and the nature of the buried interface with the substrate. Sharp interfaces, strain-free and dislocation-free layers are mandatory to maintain the inherent properties of magnetite.3 Recently, magnetic behavior in ultrathin films of magnetite with thicknesses down to 1 nm4 has been obtained, evidencing its potentiality. Thin films of Fe3O4 have been mostly grown on MgO substrates due to the perfect match between both lattices. However, the MgO/Fe3O4 stacking cannot be heated above 670 K without Mg diffusion to the magnetite layer5,6 and with the consequent inhibition of its intrinsic properties. Such a fact imposes a severe limitation for electrodes in TMR devices as the growth of a tunnel barrier, © 2014 American Chemical Society

Received: October 22, 2014 Revised: December 19, 2014 Published: December 23, 2014 1108

DOI: 10.1021/jp510615j J. Phys. Chem. C 2015, 119, 1108−1112

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The Journal of Physical Chemistry C characterization11 or elucidated from out-of-plane X-ray diffraction (XRD) measurements in a θ−2θ configuration.2 An issue that remains is a detailed investigation of the growth mechanism of Fe3O4 on SrTiO3 being of crucial importance to corroborate the argued relationship between the misfit strain and the magnetic properties.

2. EXPERIMENTAL METHOD Thin (7.5 nm) and ultrathin (1.7 nm) films of Fe3O4 were grown on SrTiO3 (001) substrates by the pulsed laser deposition (PLD) and molecular beam epitaxy (MBE) methods. The PLD deposition was performed using a Nd:YAG laser with 355 nm wavelength, 10 Hz and 1 J/cm2 irradiance power, and using a polycrystalline Fe3O4 target. A metal iron rod was used for the MBE deposition. For both evaporation techniques, the films were deposited in an oxygen atmosphere of 2 × 10−6 mbar keeping the substrate at 350 °C. The evaporation rate was maintained very low (0.25 nm/min for the PLD growth and 1 nm/h for the MBE growth) in order to ensure good oxidation of the iron and good crystallization of the layer. Faster deposition rates revealed the coexistence of Fe3O4 and FeO phases, especially at the interface with the substrate. Before evaporation, the SrTiO3 (STO) substrates were etched using the “Arkansas” method to achieve single TiO2 termination.12 Then, the samples were heated to 500 °C under UHV conditions during 5 min. The crystallinity of the surface was checked with RHEED using a primary electron beam of 29 keV, and the absence of oxygen vacancies was checked by X-ray photoelectron spectroscopy (XPS) on the Ti 2p level. After the evaporation, the samples were transferred in vacuum to the analysis chamber of the synchrotron beamline BM25-SpLine at the European Synchrotron Radiation Facility devoted to the simultaneous combination of grazing incidence X-ray diffraction/X-ray reflectivity (GIXRD/XRR) and hard Xray photoelectron spectroscopy (HAXPES). It should be mentioned that, although for the present case, the samples were preserved from contact with air, the layers remain very stable under atmospheric pressure (even the ultrathin films). The atomic and electronic structure of the layers was followed simultaneously by GIXRD and XPS/HAXPES. For the ultrathin films (1.7 nm thick layers), a standard X-ray tube was used (Mg Kα radiation) for the XPS analysis while synchrotron hard X-rays (HAXPES), photon energy of 12 keV, were used to create electrons with high enough kinetic energy to probe the whole depth from the thick layers. XRR was also used for an accurate determination of the layer thicknesses. After the electronic and structural analysis, ex situ morphological characterization was performed by scanning electron microscopy (SEM) using a LEO 1530 microscope. A Schottkytype field-emission electron source was used, operated at a fixed energy of 20 keV.

Figure 1. Low-angle reflectivity for a 2-unit-cell thick layer grown by (a) MBE and (b) PLD, and for a 9-unit-cell thick layer grown by (c) MBE and (d) PLD. Dots represent the experimental data while the corresponding fit is represented by a solid line. The photon energy used was 12 keV.

and 0.6 nm and a surface roughness of 0.4 and 0.8 nm were obtained for the layers grown by MBE and PLD, respectively. Bulk Fe3O4 has a spinel structure with space group Fd3m and a lattice parameter of a = 0.8397 nm. The mismatch with the SrTiO3 lattice (a = 0.390 nm) is of ∼7.5%. As the Fe3O4 inplane lattice parameter is a1 = a2 ∼ 2aSTO, the magnetite grows with its crystallographic axis along the (10) and (01) directions of the STO substrate; i.e., both lattices are parallel, in order to minimize the lattice mismatch between both materials. The Fe3O4 out-of-plane lattice parameter is a3 ∼ 2aSTO. The Miller indices of the magnetite layer are approximated (2h,2k,2l), where h, k, and l are the indices of the cubic STO. It is expected that the STO only contributes to the diffraction signal for the reflections with integer h, k, and l values. However, due to the double lattice parameter of the Fe3O4 compared to STO, it is expected that the magnetite films present extra peaks at approximated half-integer h, k, and l values. Reciprocal space maps are shown for the (111) (Figure 2a,c) and (1/2 1/2 1/2)

3. EXPERIMENTAL RESULTS AND DISCUSSION Figure 1 shows synchrotron-based low-angle X-ray reflectivity and the corresponding fits for samples analyzed in this work. All samples present a sharp buried interface and smooth surface giving rise to intense Kiessig fringes, although for the similarity of densities between STO and Fe3O4. Layer thicknesses t of 1.7 nm (2 unit cells) and 7.5 nm (9 unit cells) were obtained for both the MBE and the PLD evaporation techniques. From the fitting of the experimental data, an interface roughness of 0.4

Figure 2. HK reciprocal space maps for 7.5 nm (a, b) and 1.7 nm (c, d) thick films. The magnetite diffraction peak does not coincide with the STO diffraction peak, evidencing an incommensurate growth. 1109

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The Journal of Physical Chemistry C (Figure 2b,d) reflections for a 7.5 nm (Figure 2a,b) and 1.7 nm (Figure 2c,d) thick films. It can be clearly seen that the layers present an incommensurate growth; i.e., they do not adapt their lattice parameters to those of the substrate. The magnetite grows on the STO maintaining its bulk (completely strain-free) lattice parameter as obtained from the high-resolution scans presented in Figure 3. It should be stressed that the same

Figure 4. (a) (1 1)-CTR and (b) (0.465 0.465)-ROD scans for the four layers studied in this work. The layers grown by PLD show higher interface roughness and expansion of the last atomic plane of the STO substrate as compared to the layers grown by MBE. The four layers show the same atomic structure based on the relative intensity between magnetite Bragg peaks.

Figure 5 shows the Fe 2p XPS and HAXPES spectra for the 1.7 and 7.5 nm layers, respectively. It should be stressed that

Figure 3. In-plane and out-of-plane high-resolution GIXRD scans for 7.5 nm (a, b) and 1.7 nm (c, d) thick films. The obtained in-plane and out-of-plane lattice parameters for the magnetite correspond to its bulk value. A completely strain-free film is then obtained.

behavior is present for the layers evaporated using the PLD and MBE techniques. The in-plane structural domain size is found to be 11 ± 1 nm, as obtained from Figure 3, independent of the layer thickness and evaporation technique. We explain the incommensurate growth by the presence of a coincidence lattice of the order 13 (13*aFe3O4 = 28*aSTO). It should be mentioned that we do not observe the diffraction peaks associated with the coincidence lattice because the structural domain is not large enough to provide constructive interference. In fact, the in-plane structural domain size is of the order of the size of the commensurate lattice. Figure 4 shows out-of-plane scans (l scans) along the (1 1 l) and (0.465 0.465 l) directions for 7.5 and 1.7 nm thick layers grown by PLD and MBE. Because of the incommensurate growth, only the STO contributes to the (1 1 l)-CTR scan. The intensity between Bragg peaks at the crystal truncation rods (CTRs) can be then directly associated with the buried interface between Fe3O4 and STO. By comparison of the CTRs from the four layers studied in this work with the CTR from a clean substrate, it can be inferred (see Figure 4a) that the layers grown by PLD present a relaxation of the last atomic plane of the STO based on the asymmetry of the surface diffraction signal at both sides of the L = 1 Bragg peak.13,14 The GIXRD spectra from the MBE layers shows a tiny asymmetry, evidencing a lower modification of the STO surface than in the case of the PLD layers. The layers grown by PLD also show a larger decrease of the surface diffraction signal at the anti-Bragg position, evidencing higher interface roughness than for the MBE layers. It should be also noted that the relative intensities between magnetite Bragg peaks (Figure 4b) are identical for all the layers, indicating the presence of the same atomic structure independent of the layer thickness and deposition technique.

Figure 5. (From top to bottom) Fe 2p XPS - Mg Kα (hν = 1253.6 eV) for a 1.7 nm layer grown by MBE and 1.7 nm layer grown by PLD; Fe 2p HAXPES (synchrotron beam - hν = 12 keV) for a 7.5 nm layer grown by MBE and 7.5 nm layer grown by PLD.

the Fe 2p electron kinetic energy corresponds to ca. 500 and 11300 eV for photon energies of 1253.6 eV and 12 keV, respectively. The information depth is of the order of 3 nm for XPS15 and 50 nm for HAXPES,16 being hence much larger than the thickness of the corresponding layers. In this way, the photoemission spectra is representative of the whole layer. It is well-known the presence of satellite structures on the iron oxide Fe 2p XPS spectra due to charge transfer screening effects,17,18 which provide detailed features allowing the identification of the iron +2 and +3 oxidation states. The Fe3O4 is characterized by the absence of such a satellite structure, due to the presence of intercalated Fe2+ and Fe3+ ions in the lattice. The 1.7 and 7.5 nm layers, grown by PLD and MBE, show the characteristic spectra of Fe3O4, in accordance with the GIXRD patterns. It should be noted that the information obtained from the photoemission spectra is not only representative from the crystalline part of the sample but also from possible noncrystalline (amorphous) parts. Hence, we can elucidate, from the combination of GIXRD and XPS/HAXPES, the 1110

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Figure 6. RHEED pattern along the ⟨110⟩ and SEM image for a 1.7 nm layer grown (a) by MBE and (b) by PLD and for a 7.5 nm layer grown (c) by MBE and (d) by PLD. The layer grown by MBE revealed a 2D growth, whereas the layer grown by PLD revealed an island growth. As a reference, the RHEED pattern along the ⟨110⟩ direction for a clean substrate is shown in the center of the figure.

patterns obtained by Hamie et al.11 and Wei et al.,2 demonstrate that Fe3O4 grows without accumulating strain, indicating that the exotic magnetic behavior reported on Fe3O4/STO is due to the formation of antiphase boundary defects caused by the incommensurate growth. The strain-free growth of the magnetite also implies a negligible modification of the layer−substrate interface leading to very sharp interfaces.

growth of single-phase single oriented Fe3O4 (001) with the absence of crystalline and noncrystalline contributions of other iron oxide phases. In situ RHEED patterns along the ⟨110⟩ direction and ex situ SEM images were obtained for the four samples. Figure 6 shows the corresponding patterns for the 1.7 and 7.5 nm layers grown by MBE (Figure 6a,c) and PLD (Figure 6b,d). The RHEED pattern along the ⟨110⟩ direction from the clean substrate is shown at the center of the figure. Smooth diffraction stripes were obtained for the layers grown by MBE, being representative of a smooth and homogeneous layer, in accordance with the SEM images. However, sharp diffraction spots were obtained for the layers grown by PLD, being representative of an island-type growth as also revealed by the SEM images. The mean dimension of the islands, as obtained from the SEM images, is 10.5 nm, in accordance with the structural domain size obtained from the GIXRD patterns. The island growth obtained by the PLD method, as compared to the 2D growth obtained by the MBE method, is also reflected in the XRR spectra shown in Figure 1, where the measured signal vanishes more rapidly for the samples grown by PLD than by MBE, indicating the presence of a more rough surface for the PLD layers than for the MBE layers. It should be stressed that, although the layers grown by MBE present a 2D growth, the structural domain size is identical for all the layers, independent of the layer thickness and evaporation method. The structural domains sizes are then limited by dislocations and defects, as previously observed by TEM by Hamie et al.11 However, they associate the dislocations and defects to the strain accumulated by the Fe3O4 layer due to the modification of the lattice in order to match the STO. The high-resolution in-plane and out-of-plane GIXRD scans presented in this work, in contrast to the poor resolution of the RHEED and θ−2θ

4. CONCLUSIONS Incommensurate growth of Fe3O4 (001) thin and ultrathin layers on SrTiO3 (001) has been revealed by high-resolution synchrotron based GIXRD. Such a growth, which is present for all layers independent of the evaporation technique and layer thickness, is based on a coincidence lattice of the order 13. Antiphase boundary defects are present at the border of the coincidence lattice, playing an important role on the magnetic and transport properties of Fe3O4/SrTiO3 heterostructures. Morphological characterization by RHEED and SEM revealed smooth surfaces for the layers grown by MBE and the presence of islands at the surface of the layers grown by PLD. The mean size of such islands matches the size of the coincidence lattice and the size of the structural domains. The electronic characterization by XPS and HAXPES showed single-phase character for all the layers.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (J.R-Z.). Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript 1111

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The Journal of Physical Chemistry C Notes

(18) Krasnikov, S. A.; Vinogradov, A. S.; Hallmeier, K. H.; Höhne, R.; Ziese, M.; Esquinazi, P.; Chassé, T.; Szargan, R. Oxidation Effects in Epitaxial Fe3O4 Layers on MgO and MgAl2O4 Substrates Studied by X-ray Absorption, Fluorescence and Photoemission. Mater. Sci. Eng., B 2004, 109, 207−212.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to the SpLine staff for their valuable help and for the financial support from the Spanish MINECO and Consejo Superior de Investigaciones Cientificas under Grant Nos. MAT2011-23785 and 20106OE013.

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REFERENCES

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DOI: 10.1021/jp510615j J. Phys. Chem. C 2015, 119, 1108−1112