Epitaxial Stabilization of Ε-fe 2 O 3 " 00l… Thin Films on Srtio 3 " 111…

Thin films of the metastable and elusive ␧-Fe 2 O 3 have been epitaxially stabilized on SrTiO 3 ͑111͒ substrates. The ␧-Fe 2 O 3 films present a ͑001͒ orientation perpendicular to the substrate and three in-plane domains measuring a few nanometers and showing atomically sharp interfaces. We argue that this domain structure, rather than the epitaxial-strain, plays an essential role in stabilizing the ␧-Fe 2 O 3 by minimizing the energy of ͑100͒ surfaces. The ␧-Fe 2 O 3 films show a large in-plane coercivity ϳ8 kOe which combined with the magnetoelectric character claimed for this oxide may lead to novel applications in spintronics. Progress on magnetoelectric devices, such as tunnel junctions 1 or the magnetoelectric random access memories 2 is fundamentally hampered by the scarcity of suitable materials displaying substantial room-temperature magnetoelec-tric coupling. Within the oxide realm, antiferromagnetic Cr 2 O 3 had been explored 3 although most work is limited to single crystal-based studies. In particular, GaFeO 3 , a polar ferrimagnet, was among the first materials for which a large linear magnetoelectric effect was reported 4 and has been the object of a renewed interest because it displays magnetically induced second harmonic generation 5 and optical magneto-electric effects. 6 Only very recently, the growth of GaFeO 3 films has also been reported. 7–10 ␧-Fe 2 O 3 is a metastable structural intermediate phase of maghemite ͑␥-Fe 2 O 3 ͒ and hematite ͑␣-Fe 2 O 3 ͒ which interestingly , is isostructural to the polar GaFeO 3. At room-temperature, it presents a noncentrosymmetric structure ͑space group Pna2 1 ͒ where four Fe 3+ ions are antiferromag-netically coupled along a, resulting in a ferrimagnetic ordering. 11,12 ␧-Fe 2 O 3 has a large room-temperature coerciv-ity of 20 kOe 13 and enlarges the family of magnetoelectric oxides since it presents a coupling of its magnetic and dielec-tric properties. 14 It has been recently established that the large magnetic anisotropy in ␧-Fe 2 O 3 is provoked by distortions of the Fe 3+ coordination polyhedra which are responsible for a substantial spin-orbit coupling. 15 Thus, a priori, ␧-Fe 2 O 3 should display superior properties than other known room-temperature magnetoelectric oxides owing it to ͑i͒ its ferrimagnetic character and in contrast to the antiferromag-netism displayed by Cr 2 O 3 , and to ͑ii͒ its Curie temperature, which is much higher ͑510 K͒ than that of GaFeO 3 ͑200 …

Progress on magnetoelectric devices, such as tunnel junctions 1 or the magnetoelectric random access memories 2 is fundamentally hampered by the scarcity of suitable materials displaying substantial room-temperature magnetoelectric coupling.Within the oxide realm, antiferromagnetic Cr 2 O 3 had been explored 3 although most work is limited to single crystal-based studies.In particular, GaFeO 3 , a polar ferrimagnet, was among the first materials for which a large linear magnetoelectric effect was reported 4 and has been the object of a renewed interest because it displays magnetically induced second harmonic generation 5 and optical magnetoelectric effects. 6[9][10] -Fe 2 O 3 is a metastable structural intermediate phase of maghemite ͑␥-Fe 2 O 3 ͒ and hematite ͑␣-Fe 2 O 3 ͒ which interestingly, is isostructural to the polar GaFeO 3 .At roomtemperature, it presents a noncentrosymmetric structure ͑space group Pna2 1 ͒ where four Fe 3+ ions are antiferromagnetically coupled along a, resulting in a ferrimagnetic ordering. 11,12-Fe 2 O 3 has a large room-temperature coercivity of 20 kOe 13 and enlarges the family of magnetoelectric oxides since it presents a coupling of its magnetic and dielectric properties. 14It has been recently established that the large magnetic anisotropy in -Fe 2 O 3 is provoked by distortions of the Fe 3+ coordination polyhedra which are responsible for a substantial spin-orbit coupling. 15Thus, a priori, -Fe 2 O 3 should display superior properties than other known room-temperature magnetoelectric oxides owing it to ͑i͒ its ferrimagnetic character and in contrast to the antiferromagnetism displayed by Cr 2 O 3 , and to ͑ii͒ its Curie temperature, which is much higher ͑510 K͒ than that of GaFeO 3 ͑200 K͒.Moreover, the fact that this is a single-metal, single-valent oxide offers an advantage over other multimetallic and/or mixed valence systems in which a control of site occupancies is required for optimum response.So far, -Fe 2 O 3 has only been grown as nanoparticles in confined nanoreactors by sol-gel based methods 12,13,16 or as nanowires. 17,18Therefore, in spite of its obvious interest, implementation of -Fe 2 O 3 in practical devices was challenged by the absence of thin films.
In the present letter, we show that using appropriate substrates, epitaxial stabilization of -Fe 2 O 3 thin films has been achieved.We claim that twinning and interface energies, rather than the common substrate-induced epitaxial strain, play crucial roles on the stabilization of this phase.
Iron oxide films of about 100 nm thickness were grown on several substrates as follows: Si ͑100͒, MgO ͑100͒, yttriastabilized zirconia ͑111͒, and STO ͑111͒.A Kr excimer laser ͑248 nm wavelength, 34 ns pulse duration͒ was used at a 5 Hz repetition rate, focused at ϳ1.5 J / cm 2 on a ␣-Fe 2 O 3 target placed at a distance of 5 cm.The target was prepared by sintering ␣-Fe 2 O 3 powders ͑Aldrich 99.9% purity͒.The growth conditions were chosen following the conditions that yielded high density 1D nanowire growth in Ref. 17.The substrate temperature was 800 °C.The oxygen pressure was maintained at 9.86ϫ 10 −5 atm during the sample growth and increased to 1 atm once the substrate temperature reached 500 °C on cooling.The crystal structure of the films was analyzed by x-ray diffraction ͑XRD͒ using Cu K␣ radiation in a Bruker-AXS D8 Advance diffractometer and by transmission electron microscopy ͑TEM͒ and scanning TEM ͑STEM͒.Conventional TEM and selected-area electron diffraction ͑SAED͒ were carried out on a FEI CM-200 transmission electron microscope operating at 200 kV.Atomic resolution Z-contrast images were taken at 300 kV in a FEI Titan 80-300 S scanning transmission electron microscope and in two dedicated STEMs VG HB603U at 300 kV and NION UltraSTEM at 100 kV.Specimens for TEM and STEM were prepared by ion milling to electron transparency ͑ϳ100 nm͒.A Quantum Design superconducting quantum interference device magnetometer was used for the magnetic characterization.
We performed XRD analysis of the films grown on the different substrates.Textured growth has only been observed on YSZ͑111͒, showing ␥-Fe 2 O 3 ͑0kl͒ reflections ͑not shown͒, and on STO͑111͒ where -Fe 2 O 3 has been stabilized.The -2 XRD scan of the latter ͓Fig.1͑a͔͒ only shows reflections corresponding to -Fe 2 O 3 .The peaks observed at 2 = 18.94°, 38.17°, 58.68°, and 81.42°can be, respectively, ascribed to the ͑002͒, ͑004͒, ͑006͒, and ͑008͒ reflections, indicating an out-of plane ͕001͖ ʈ ͕111͖ STO orientation relationship of the -Fe 2 O 3 film.We find that the c cell parameter extracted by the Nelson Riley extrapolation method 19 to = 90°is 9.467͑5͒ Å which is essentially the same reported for nanoparticles at room temperature, 20 indicating that the film is strain-relaxed.In order to get an average picture of the in-plane texture of the -Fe 2 O 3 film, two pole figures were recorded for 2 = 30.06°and 2 = 32.4°whichcorrespond to the Bragg condition of the ͑013͒ and ͑110͒ reflections of -Fe 2 O 3 and STO, respectively ͓see Fig. 1͑b͔͒.The pole figure exhibits the three equivalent ͕110͖ STO poles at a tilt angle ϳ 35.3°, providing the in-plane orientation of the substrate for reference.Decreasing the tilt to the substrate normal one finds six poles rotated at 60°one to each other in azimuth ͑͒ at ϳ 20°which correspond to the ͕013͖ planes of the ͑00l͒-oriented -Fe 2 O 3 .In order to understand the inplane orientation which is derived from this pole arrangement, one must consider that the projections of the ͕013͖ and ͕110͖ STO poles on the ͑001͒ and ͑111͒ STO planes are parallel to ͓010͔ and ͓112 ¯͔STO directions, respectively.The angle between the ͓112 ¯͔STO and ͓101 ¯͔STO , contained in the ͑111͒ STO plane is 30°, and since the ͕013͖ pole only consists of the ͑013͒ and ͑01 ¯3͒ planes, with poles 180°apart in , it can be deduced from the pole figure that the -Fe 2 O 3 film presents three in-plane domains rotated 120°to each other, reflecting the threefold symmetry of the STO͑111͒ planes.Thus, the in-plane orientation of the first domain, D1, is ͓010͔ ʈ ͓101 ¯͔STO ͑i.e., ͓100͔ ʈ ͓12 ¯1͔ STO ͒ and domains D2 and D3 have ͓100͔ ʈ ͓112 ¯͔STO and ͓100͔ ʈ ͓2 ¯11͔ STO orientations.This in-plane domain structure is represented in Fig. 1͑c͒.
Figure 2͑a͒ shows a cross-sectional TEM image of the -Fe 2 O 3 layer grown on STO ͑111͒ substrate.The film presents a uniform thickness ͑of about 110 nm͒ and a uniform grain structure.The coalesced grains have a rectangular shape and a flat surface.Some grains also present irregularities at the interface in the form of voids.Nevertheless, the corresponding SAED pattern, Fig. 2͑b͒, shows an epitaxial growth of the -Fe 2 O 3 film on STO, in good agreement with the XRD observations.The diffraction spots of the SAED pattern can be indexed by considering the superposition of the diffraction patterns obtained along the zone axes ͓100͔ and ͓1 ¯1 ¯0͔ of -Fe 2 O 3 and ͓12 ¯1͔ of STO indicating the presence of the domains D1 and D2 with the c-axis normal to the substrate surface.The SAED pattern also confirms the relaxation of the film, showing the splitting of the film and substrate diffraction spots ͓see arrows in Fig. 2͑b͔͒.
Figure 2͑c͒ shows a Z-contrast STEM image of the -Fe 2 O 3 / STO interface along the ͓12 ¯1͔ STO zone axis.The -Fe 2 O 3 / STO interface is sharp and presents periodic strain contrast due to misfit dislocations.Figure 2͑c͒ also shows domains D1 and D2 which are ϳ5 nm across, extending the full thickness of the film.Figure 2͑d͒, a higher resolution Z-contrast image, reveals an atomically sharp interface between the two -Fe 2 O 3 domains.
Even though previous studies of GaFeO 3 grown on STO͑111͒ reported the same ͑001͒ texture 10 as observed in this study for -Fe 2 O 3 , the conditions of -Fe 2 O 3 stabilization are more critical than for GaFeO 3 .For instance, while GaFeO 3 ͑110͒ grows epitaxially on MgO ͑100͒, 9 Fe 2 O 3 de- posited at essentially the same conditions does not.This may be related to the fact that -Fe 2 O 3 nanoparticles present a marked tendency to grow anisotropically along the ͓100͔ direction, 21,22 likely because the energy of the ͑100͒ surfaces is particularly high.Indeed, any ͓100͔ out of plane texture would be clearly unfavorable.However, the domain structure that we have observed ͑i.e. domains ϳ5 nm at 120°to each other͒ could contribute to reduce the energy of the -Fe 2 O 3 ͑100͒ surfaces through the domain boundaries.It is interesting to note that the growth of -Fe 2 O 3 with ͓001͔ in-plane is not observed, even with the c lattice parameter of -Fe 2 O 3 ͑c = 9.468 Å͒ presenting a low mismatch along ͓12 ¯1͔ STO ͓i.e., ͑ͱ6a STO − c ͒ / c ϳ 1%͔.This means that either ͓100͔ is out of plane implying the existence of high energy ͑100͒ surfaces, or both a and c in-plane with ͓001͔ ʈ ͓12 ¯1͔ STO and ͓100͔ ʈ ͓303 ¯͔STO which has a high lattice mismatch ͓͑ͱ2a STO − a ͒ / a ϳ 8.5%͔.Hysteresis loops M͑H͒ were measured, at room temperature, with the magnetic field applied parallel and perpendicular to the film plane ͑Fig.3͒.The shape of the loop measured with in-plane field is consistent with the domain structure described above and with the data reported for magneticfield oriented nanorods. 20In the films, the crystallographic a-axis, which corresponds to the easy magnetization axis, forms a network of domains oriented at 120°from each other.Therefore, assuming that the magnetization process takes place via coherent rotation of the magnetization vector, the resulting H C should be intermediate between the value measured in a in-plane field applied along the easy a-axis and perpendicular to it and accordingly, the shape of the loop will be less rectangular than the loop measured along a. Indeed, this is what we observed experimentally ͑H C ϳ 8 kOe͒.Finally, one notices a magnetization step in the low-field region of the return branch of the M͑H͒ curve.This shoulder could be attributed to the presence of a small fraction of superparamagnetic ␥-Fe 2 O 3 . 23Such claim is further confirmed by the shape of the magnetization loop measured with the field perpendicular to the film plane, and to the easy axis ͑Fig.3͒.
In conclusion, we have shown that it is possible to prepare epitaxial thin films of the metastable -Fe 2 O 3 with good quality on STO ͑111͒ substrates using pulsed laser deposition.The coercivity of the films measured from hysteresis loops is ϳ8 kOe, which is well below the ϳ20 kOe measured in nanoparticles, but consistent with the domain structure observed in this study.The -Fe 2 O 3 films present a ͑001͒ orientation perpendicular to the substrate and three types of in-plane domains rotated 120°to each other with ͓100͔ ʈ ͓12 ¯1͔ STO .The domains show atomically sharp interfaces, with sizes between 5-10 nm.We argue that this domain structure can play an essential role in the film stabilization by minimizing the energy of the -Fe 2 O 3 ͑100͒ surfaces.The high coercivity displayed by the -Fe 2 O 3 films and the magnetoelectric character of -Fe 2 O 3 can be appealing for its application in spintronics and its future integration into devices.
This research was partially supported by the Ministerio de Ciencia e Innovacion ͑Grant Nos.MAT2009-08024 and CSD2007-00041͒, the Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, U.S. Department of Energy ͑S.J.P and M.V.͒, and the SHaRE User Facility ͑J.G. and J.C.I.͒.

FIG. 1 .FIG. 2 .
FIG. 1. ͑Color online͒ ͑a͒ XRD -2 scan of -Fe 2 O 3 film grown on ͑111͒ STO.͑b͒ Pole figures of the ͕013͖ and ͕110͖ STO poles.The divisions of the azimuth and polar coordinates are of 10°.͑c͒ Schematic drawing of the domain structure.The short arrows indicate the unit cell of STO, and the long arrows refer to the in-plane directions on ͑111͒ STO.

FIG. 3 .
FIG.3.͑Color online͒ Hysteresis loops at 300 K measured with the magnetic field applied parallel ͑full circles͒ and perpendicular ͑empty squares͒ to the substrate.Data has been corrected for the diamagnetic contribution of the substrate.