Structural and magnetic properties of epitaxial delafossite CuFeO2 thin films grown by pulsed laser deposition

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(1)Structural and magnetic properties of epitaxial delafossite CuFeO2 thin films grown by pulsed laser deposition Toyanath Joshi, Tess R. Senty, Robbyn Trappen, Jinling Zhou, Song Chen, Piero Ferrari, Pavel Borisov, Xueyan Song, Mikel B. Holcomb, Alan D. Bristow, Alejandro L. Cabrera, and David Lederman Citation: Journal of Applied Physics 117, 013908 (2015); doi: 10.1063/1.4905424 View online: http://dx.doi.org/10.1063/1.4905424 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/117/1?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Magnetic and structural properties of BiFeO3 thin films grown epitaxially on SrTiO3/Si substrates J. Appl. Phys. 113, 17D919 (2013); 10.1063/1.4796150 Structural and magnetic properties of epitaxial Fe 3 O 4 / ZnO and ZnO / Fe 3 O 4 bilayers grown on c-Al 2 O 3 substrate J. Appl. Phys. 108, 103909 (2010); 10.1063/1.3511348 Optical and magnetic properties of CuMnO 2 epitaxial thin films with a delafossite-derivative structure Appl. Phys. Lett. 95, 032109 (2009); 10.1063/1.3186790 Epitaxial growth of one-dimensional Ca 3 Co 2 O 6 thin films prepared by pulsed laser deposition Appl. Phys. Lett. 91, 172517 (2007); 10.1063/1.2802731 Preparation and characterization of La Mn O 3 thin films grown by pulsed laser deposition J. Appl. Phys. 100, 023910 (2006); 10.1063/1.2217983. Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Download to IP: 146.155.28.40 On: Mon, 16 May 2016 20:18:15.

(2) JOURNAL OF APPLIED PHYSICS 117, 013908 (2015). Structural and magnetic properties of epitaxial delafossite CuFeO2 thin films grown by pulsed laser deposition Toyanath Joshi,1 Tess R. Senty,1 Robbyn Trappen,1 Jinling Zhou,1 Song Chen,2 Piero Ferrari,3 Pavel Borisov,1 Xueyan Song,2 Mikel B. Holcomb,1 Alan D. Bristow,1 Alejandro L. Cabrera,3 and David Lederman1. 1 Department of Physics and Astronomy, West Virginia University, Morgantown, West Virginia 26506-6315, USA 2 Department of Mechanical and Aerospace Engineering, West Virginia University, Morgantown, West Virginia 26506-6070, USA 3 Pontificia Universidad Catolica, Instituto de Fısica, Santiago, Chile. (Received 4 September 2014; accepted 21 December 2014; published online 6 January 2015) Growth of pure phase delafossite CuFeO2 thin films on Al2O3 (00.1) substrates by pulsed laser deposition was systematically investigated as a function of growth temperature and oxygen pressure. X-ray diffraction, transmission electron microscopy, Raman scattering, and x-ray absorption spectroscopy confirmed the existence of the delafossite phase. Infrared reflectivity spectra determined a band edge at 1.15 eV, in agreement with the bulk delafossite data. Magnetization measurements on CuFeO2 films demonstrated a phase transition at TC  15 6 1 K, which agrees with the first antiferromagnetic transition at 14 K in the bulk CuFeO2. Low temperature magnetic phase is best described by commensurate, weak ferromagnetic spin ordering along the C 2015 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4905424] c-axis. V. I. INTRODUCTION. CuFeO2 has the delafossite structure with rhombohedral space group R-3m and lattice constants a ¼ b ¼ 0.309 nm and c ¼ 1.709 nm.1 The crystal structure consists of a triangular lattice of magnetic Feþ3 layers separated by nonmagnetic layers of CuþO2, stacked along the c axis of a hexagonal unit cell. Among other magnetic delafossites, CuFeO2 has been studied as an archetypical triangular lattice antiferromagnet.2,3 Despite the obvious spin frustration promoted by the triangular configuration, a collinear spin order is adopted as the magnetic ground-state below TN2  11 K, with antiferromagnetically aligned spins parallel to the c-axis within the four sub-lattices. A second transition to a sinusoidal amplitude-modulated collinear-incommensurate spin structure takes place at T > TN2 with the subsequent transition to the paramagnetic state above TN1  14 K.2,3 Magnetic fields (7 < B ⱗ13 T) applied along the c-axis at temperatures as below TN2 induce a transformation to a noncollinear-incommensurate structure,4 which is at the origin of a ferroelectric polarization due to the magnetoelectric effect.5 Materials demonstrating magnetoelectric effect are promising in terms of designing new devices for data storage and sensor applications. Delafossite CuFeO2 therefore represents an extremely interesting example of a material where noncollinear spin structure results in a breaking of spatial inversion symmetry. An open question remains, however, regarding which sort of transformations would happen to the spin order in thin films of CuFeO2. Previous studies on other non-collinear magnets, for example, TbMnO3 (Ref. 6) and BaFe10.2Sc1.8O19,7 have shown that growth-induced strain, twinned growth domains, and other structural effects can affect the spin order in thin films. Obtaining pure phase CuFeO2 thin films is non-trivial because it is necessary to maintain the lowest possible Cu 0021-8979/2015/117(1)/013908/8/$30.00. valency (þ1) in order to avoid forming the comparably stable spinel compound CuFe2O4, which is a high Tc (455 K) ferrimagnet with the bulk magnetization values 1.3–2.3 lB per formula unit.8 CuFeO2 thin films have been grown using several techniques, such as rf magnetron sputtering9,10 and chemical solution deposition,11 all of which require an additional annealing step in a reducing argon or nitrogen atmosphere in order to achieve an acceptable crystalline film quality. Pulsed laser deposition (PLD) has been shown to produce epitaxial, mostly single phase films of CuFeO2.12,13 In Ref. 12, thin films of CuFeO2 were grown on amorphous glass substrates. Growth pressures between 0.1 and 5 mTorr oxygen and the same substrate temperature of 750  C were tested, and 1 mTorr was found as the best growth pressure, when out-of-plane oriented, in-plane textured (0001) CuFeO2 films were formed. Ref. 13 reported the growth on (0001) Al2O3 substrates. Growth O2 pressures between 0.3 and 225 mTorr were tested, and the pressure of 75 mTorr was reported to yield epitaxial quality CuFeO2 films (as compared to the sample grown at 225 mTorr), while the substrate temperature was kept the same at 500  C. Here, we report on a careful optimization of both the substrate temperature and the O2 growth pressure and find that the best growth parameters for Al2O3 c-cut substrates are 550–600  C substrate temperature and 0.1 mTorr O2 growth pressure. Magnetization measurements on CuFeO2 films in Ref. 13 were explained as demonstrating spin glass behavior at low temperatures below 60 to 80 K, but additionally weak ferromagnetism was also claimed to be present at 5 K due to the growth strain. No magnetic transitions around 11–14 K were observed. On the other hand, magnetic measurements on our best CuFeO2 films show a phase transition at 15 6 1 K, in agreement with the bulk transition at 14 K. Furthermore, we. 117, 013908-1. C 2015 AIP Publishing LLC V. Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Download to IP: 146.155.28.40 On: Mon, 16 May 2016 20:18:15.

(3) 013908-2. Joshi et al.. describe the low temperature magnetic phase in our films as commensurate, weak ferromagnetic ordering along the c-axis with the saturation moment of about 0.2–0.3 lB per unit cell. II. MATERIALS AND METHODS A. Sample preparation. CuFeO2 films were deposited onto Al2O3 (00.1) substrates from a stoichiometric polycrystalline CuFeO2 target (Shanghai Optics) using a Neocera PLD system with a KrF excimer laser (248 nm) from Coherent, Inc. The base pressure in the vacuum chamber was approximately 1  108 Torr. O2 gas was introduced into the chamber during the growth and controlled by a mass flow controller. The distance between target and substrate was kept at 7.3 cm and the energy density at the target was approximately 2 J/cm2. The pulse repetition rate was set to 2 Hz. The surface quality of the substrates and films was monitored using an in situ Reflection High-Energy Electron Diffraction (RHEED) system from STAIB Instruments. Prior to each sample growth, the Al2O3 substrates were pre-cleaned and annealed at 1200  C for 2 h in air in order to obtain step-and-terrace surface quality, as verified by atomic force microscopy (AFM) using a Veeco Multimode microscope.14 B. Sample characterization. A four-axis Rigaku x-ray diffraction (XRD) system with a Cu Ka rotating anode and Huber goniometers was used for structural characterization of the deposited films. CuFeO2 and CuFe2O4 phases were identified using powder diffraction files # 01–075–2146 and #00–025-0283, respectively. Information about the surface oxidation states was obtained from x-ray photoelectron spectroscopy (XPS) measurements using a PHI 5000 VersaProbe x-ray photoelectron spectrometer. The in-plane epitaxial relationship between the film and the substrate was established by x-ray /-angle scans. Film thickness and surface roughness analysis were performed by x-ray reflectivity (XRR) and by AFM. The cross-sectional samples for transmission electron microscopy (TEM) measurements were prepared by mechanical polishing and ion-milling. The samples were cooled using liquid nitrogen during the ion-milling process. Diffraction contrast imaging, electron diffraction, and microchemical analysis were performed in a JEOL 2100 high resolution TEM (HRTEM) equipped with a LaB6 electron source and an x-ray energy dispersive spectrometer (EDS). Reflectance measurements were conducted using Fourier Transform Infrared (FTIR) spectroscopy with a Nicolet Nexus 870 ESP FTIR spectrometer with a halogen light source and a liquid nitrogen cooled HgCdTe detector. Spectra were obtained for photon energies between 0.347 eV and 1.451 eV with 0.001 eV resolution. Raman spectra were obtained with a LabRam010 system from Instruments, S.A. (Horiba) using a 5.5 mW He–Ne laser (632.8 nm wavelength). This instrument uses an Olympus confocal optical microscope with a light spectrometer in a back-scattering geometry, where the incident beam is linearly polarized and. J. Appl. Phys. 117, 013908 (2015). spectral detection is unpolarized. The spectra were taken at room temperature using a 100 objective (10 lm spot size) and with an energy resolution of approximately 1 cm1. X-ray absorption near-edge spectroscopy (XANES) measurements were performed at beamline 6.3.1 at the Advanced Light Source at Lawrence Berkeley National Laboratory. The Cu L-edge spectra were measured at 80 K in total electron yield mode with the energy resolution about 0.2 eV and at normal incidence. The spectra have been normalized to the incident x-ray flux. The incident photon energy was set according to the Ti L3,2 edge from a reference BaTiO3 sample. Both the Ti edge and the Cu L3,2 as shown in the CuO reference exhibited the same energy offsets. A Superconducting Quantum Interference Device (SQUID) magnetometer MPMS from Quantum Design was used to measure magnetization of the thin films as a function of temperature and magnetic field. III. RESULTS AND DISCUSSION A. RHEED and X-ray diffraction studies. Figure 1 shows typical RHEED patterns recorded on a blank Al2O3 (00.1) substrate for [21.0] and [10.0] azimuths [Figs. 1(a) and 1(b)] and on a CuFeO2 thin film along the same azimuthal directions [Figs. 1(c) and 1(d)]. Streaky patterns with six-fold rotation symmetry indicate twodimensional growth and smooth surface of our films. The crystal quality of the delafossite CuFeO2 thin films has a significant dependence on oxygen pressure and growth temperature. Two sets of experiments were performed to optimize the growth conditions. First, film quality was investigated as a function of the oxygen growth pressure for films grown at a fixed substrate temperature of 550  C [Fig. 2(a)]. Subsequently, samples were grown with varying substrate temperatures while keeping the oxygen pressure fixed at 0.1 mTorr [Fig. 2(b)]. X-ray reflectivity analysis performed on these samples (not shown here) demonstrated that the average film thickness for all samples was approximately 17 nm. X-ray diffraction measurements confirmed that all samples were composed of [00.1]-oriented CuFeO2 films.. FIG. 1. RHEED images from Al2O3 substrate (a) and (b), and from as grown CuFeO2 film (c) and (d) grown in 0.1 mTorr oxygen pressure and at 600  C substrate temperature.. Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Download to IP: 146.155.28.40 On: Mon, 16 May 2016 20:18:15.

(4) 013908-3. Joshi et al.. J. Appl. Phys. 117, 013908 (2015). FIG. 3. c-lattice constants calculated for samples grown at 500  C as a function of pressure (red triangles) and in 0.1 mTorr pressure as a function of growth temperature (black circles).. FIG. 2. X-ray diffraction spectra for the samples grown at 550  C as a function of O2 pressure (a) and in 0.1 mTorr of O2 pressure as a function of growth temperature (b).. Previously, the major problem with the growth of singlephase delafossite films has been the presence of an additional impurity in the form of [111]-oriented spinel CuFe2O4, which has a better lattice match (7% mismatch) with the (00.1) Al2O3 substrates with respect to CuFeO2/Al2O3 (11% mismatch).11 The growth in O2-pressures above 0.1 mTorr resulted in an increase in height of the CuFe2O4 impurity diffraction peaks, as shown in Fig. 2(a). This can be explained by a higher yield of Cu2þ oxidation states with increasing oxygen growth pressures. In this case, Cu ions are overoxidized for the delafossite structure and thus contribute to a spinel structure instead. On the other hand, oxygen growth pressures or substrate temperatures that are too low [Fig. 2(b)] impede the oxidation of iron to Fe3þ ions. Based on the results shown in Fig. 2(a), we estimated the optimal O2 growth pressure to be 0.1 mTorr. XRD spectra from the samples grown at 0.1 mTorr of oxygen pressure and different temperatures [Fig. 2(b)] showed that the pure phase could be obtained at 550–600  C. At higher temperatures, additional spinel impurities appear in the XRD spectra, probably for the same reasons as in the case of the higher O2 pressures, that is, an increased oxidation of Cu ion to the 2þ state and the better lattice match with the substrate, as a result of the larger thermal energies of the ablated species when they land on the substrate’s surface. The c-lattice constants of CuFeO2 films calculated from XRD spectra as a function of growth conditions are shown in Fig. 3. Decreasing the growth temperature or the O2 growth pressure led to a monotonic increase in the c-constant with respect to the bulk value, 1.709 nm. This effect is a combined. action of the compressive elastic strain from the substrate, because of the 11% lattice mismatch between substrate and film (the substrate structure is represented in the form of oxygen sublattice, see below in text), and the formation of oxygen vacancies due the reduced iron valency, that is, Fe2þ vs. Fe3þ. Increasing the growth temperature favors the formation of crystallographic lattice defects, thus helping to release the elastic stress in the film lattice, and also prevents the Fe2þ ion formation. The in-plane orientation of the film with respect to the substrate was determined from XRD /-scans around (10.4) peaks of the substrate and (10.2) peaks of the film grown in 0.1 mTorr oxygen pressure at 600  C [Fig. 4(a)]. Six-fold rotational symmetry of the film was observed, thus confirming in-plane epitaxial growth of CuFeO2 on sapphire substrates. The six-fold symmetry of the film suggests the presence of in-plane twin crystallographic structures, in which two variants are rotated by 180 relative to each other. Those peaks are rotated by 30 with respect to the three-fold peaks from the substrate. This can be explained by the film growth starting from the oxygen sub-lattice in the sapphire, which is rotated 30 with respect to the [10.0] Al2O3 direction, in a similar manner as thin films of CuGaO2 grown on Al2O3 substrates.15 XRD rocking curves and the fringes around Bragg diffraction peaks of the film [Fig. 2(b)] as a function of growth temperature reveal an improving film structural quality (smaller full width at half maximum values, for example) with increasing growth temperature. However, the formation of the spinel impurity at higher growth temperatures sets an upper bound. Rocking curves for the samples grown at 600  C and 0.1 mTorr O2 pressure with different total thicknesses are shown in Fig. 4(b). Two peaks at the same orientation contributing to each of the rocking curves were identified by fitting the experimental data to the sum of two Lorentzian peak functions both centered around Dx ¼ 0. One peak is sharp (full width at half maximum, FWHM ¼ 0.16 –0.19 for all thicknesses) and the other is broad (FWHM ¼ 2.90 , 2.00 , and 1.29 for 9 nm, 16 nm, and 41 nm thick films, respectively), most likely indicating. Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Download to IP: 146.155.28.40 On: Mon, 16 May 2016 20:18:15.

(5) 013908-4. Joshi et al.. FIG. 4. (a) Phi scan of (10.2) and (10.4) peaks for the CuFeO2 film grown at 600  C and in 0.1 mTorr of O2 pressure (red curve) and for Al2O3 substrate (black curve), respectively. (b) Rocking curves around the (006) CuFeO2 peak for the samples grown under the same conditions as in (a) with thicknesses of 9 nm, 16 nm, and 41 nm. Solid lines represent fits to two Lorentzian peak functions.. J. Appl. Phys. 117, 013908 (2015). (commensurate lattice mismatch) is expected to be similar to other 11% mismatch systems, such as thin AlN films on (0001) Al2O3. Note that ZnO grown on Al2O3 demonstrates a 6/7 ratio. Edge-type dislocations are supposed to be then incorporated into the strained interface layer of CuFeO2 after each 8 unit cells grown along the h1–1.0i directions of Al2O3. Ideally, if it is possible to form a fully relaxed film of CuFeO2 after the growth of the first few monolayers on top of Al2O3 substrate, then the film growth can be continued in a layer-by-layer mode up to the much larger thicknesses. Contrary to this scenario, we observed the broader peak starting to appear in the rocking curves at film thicknesses above 9 nm (11 ML), which leads us to a conclusion that some residual strain is still accumulated during the initial growth of CuFeO2, likely due to deviations in the 8/9 ratio during the interface layer formation. The final result of this failure to relax initial growth of CuFeO2 would be then screw-type dislocations forming at thicknesses above 9 nm. An alternative scenario involves a switching of the growth regime from 2D into the 3D mode, when the film is formed of coalescent columns and remains of epitaxial quality as verified by our RHEED and XRD scans (Figs. 1 and 4). Based on further reports of successful growth of fully relaxed ZnO films on Al2O3 substrates,20 further optimization of the growth procedure in case of CuFeO2 films might also allow similar improvement of the film quality. B. XPS measurements. the presence of two layered film phases with different concentration of crystalline defects. The relative height of the broad peaks was also greater in thicker films in comparison to the thinner ones. Further understanding of this behavior can be provided by comparison to the epitaxy results in other thin film systems with relatively large mismatch, in particular, grown on the same kind of substrates: ZnO on (0001) Al2O3 (mismatch 14%)16 and AlN on (0001) Al2O3 (mismatch 11%).17 For the case of ZnO films grown on Al2O3 substrates, similar rocking curve line-shapes have been reported, surprisingly with features similar to our case, with the same thickness limit (9 nm) above which the broader peak in the rocking curves became visible, and with the peak relative intensity increasing and its FWHM decreasing with increasing film thickness.18 This behavior is explained in Ref. 18 by the sharp part of the rocking curve corresponding to the portion of the film being the closest to the substrate interface, which contains relatively few structural defects, while the broad peak component corresponds to a defect-rich film layer on top of the defect-poor one. The conventional picture of the highly strained film below the critical thicknesses cannot be applied to highly mismatched systems, as the strain relaxation would have to take place within a single monolayer. Instead, the concept of the so-called domain matching epitaxy19 or extended atomic distance mismatch17 has been developed. Following these ideas, in our case we can expect almost perfect matching (0.1%) between 8 unit cell spacings in (0001) CuFeO2 (inplane lattice parameter 3.09 Å) and 9 unit cell spacings in the oxygen sublattice in (0001) Al2O3 (in-plane lattice parameter 2.75 Å). That is, the 8/9 in-plane domain alternation ratio. The presence of spinel impurities in the films grown in oxygen pressures above 0.1 mTorr was also verified by XPS measurements similar to Ref. 13. Figure 5(a) compares Cu XPS spectra obtained from samples grown at 550  C in 0.1 mTorr (red, solid circles) and 10 mTorr (black, open circles) of O2 pressure. In agreement with the XRD data in Fig. 2(a), the two samples had different Cu valency states but the same Fe valency states (Fe3þ) [Fig. 5(b)]. The film grown in 10 mTorr oxygen pressure had two main Cu peaks at binding energies that match the values of the Cu2þ spectrum of CuO,21,22 indicating the presence of the CuFe2O4 phase [Fig. 5(a)]. Furthermore, the presence of satellite peaks and comparatively lower peak intensity indicates the presence of open d-orbitals, also consistent with Cu2þ. This is in contrast to the two Cuþ valency-related peaks measured on the film grown in 0.1 mTorr oxygen pressure, which confirms the presence of a single CuFeO2 phase. In this case, the positions of the binding energy peaks match with peak values of Cuþ in Cu2O21,22 and the absence of satellite peak indicates completely filled d-orbitals. C. TEM and epitaxial relationship. A film grown under the above mentioned optimum growth conditions (600  C at 0.1 mTorr of O2), but slightly thinner than the others discussed above, was examined using HRTEM and EDS to verify the epitaxial relationship between film and substrate and the chemical composition. Film thickness, using XRR [Fig. 6], was determined to be 8.9 nm with a surface roughness of 0.58 nm, while the AFM surface scan for the same sample [Fig. 7(f)] shows the. Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Download to IP: 146.155.28.40 On: Mon, 16 May 2016 20:18:15.

(6) 013908-5. Joshi et al.. J. Appl. Phys. 117, 013908 (2015). FIG. 6. X-ray reflectivity data (open circles) with the corresponding fit (line) using GenX software. Film thickness found from the fit was 8.9 nm with roughness 0.58 nm.. FIG. 5. (a) and (b) XPS spectra for Cu and Fe, respectively, for the samples grown at 550  C and in 0.1 mTorr (red, solid circles) and 10 mTorr (black, open circles) O2 pressure.. surface root-mean square roughness was approximately 0.31 nm. The HRTEM image in Fig. 7(a), obtained from the interface between the Al2O3 substrate and the CuFeO2 film, clearly indicates that the film had a uniform lateral thickness with very few dislocations visible in cross-section extended film planes throughout the film. This is in agreement with the rocking curve results shown in Fig. 4(b) for the 9 nm film discussed above. On the other hand, the interface layer between the substrate (area A) and the film (area C) does show some disorder, as expected for the initial growth taking place on a high mismatch substrate. The Fourier transforms from the HRTEM images of the substrate [Fig. 7(b)], the interface between the substrate and. the film [Fig. 7(c)], and the film itself [Fig. 7(d)] result from the epitaxial relationship of the film’s lattice structure to that of the substrate, corresponding to [10.0] Al2O3//[12.0] CuFeO2 and [00.1] Al2O3//[00.1] CuFeO2. The crystal orientation relationship between the film and substrate was also confirmed by electron diffraction patterns obtained from a larger lateral length scale of approximately 200 nm [Fig. 7(e)]. This epitaxial relationship is consistent with the x-ray diffraction data discussed above. The EDS data indicate an atomic ratio of Fe:Cu ¼ 1:1, which is also consistent with CuFeO2. D. Raman spectroscopy and optical band gap measurement. Fig. 8(a) shows a typical Raman spectrum of a pure phase CuFeO2 film with 21 nm thickness. Three Raman peaks at 350, 511, and 689 cm1 were observed whose positions and relative magnitudes agreed well with the literature data for bulk single crystal23 and polycrystalline24 CuFeO2. Modes at 350 and 689 cm1 are assigned to Eg and A1g vibrational modes, respectively, typical for delafossites, while the broad feature at 511 cm1 is generally attributed to non-zero wavevector phonons and explained by the presence of crystalline defects.23 IR reflectance data obtained from a. FIG. 7. (a) HRTEM image taken from the interface between the substrate and the thin film grown at 600  C and 0.1 mTorr O2 pressure. (b)–(d) The Fourier transforms of the data in framed areas A, B, and C in (a), respectively. (e) Selected area electron diffraction patterns and the indexing confirm the crystal orientation relationship between the film and the substrate. (f) AFM topography image of the same CuFeO2 film.. Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Download to IP: 146.155.28.40 On: Mon, 16 May 2016 20:18:15.

(7) 013908-6. Joshi et al.. J. Appl. Phys. 117, 013908 (2015). FIG. 9. The Cu L-edge XANES spectrum taken on the CuFeO2 thin film. Blue solid and green dotted lines signify experimental data and the corresponding best fit based on a linear combination of normalized XANES profiles of 10% CuO (black solid line) and 90% Cu2O (red solid line) extracted from the study of Grioni et al. on the corresponding bulk samples.28. FIG. 8. (a) Raman shift and (b) optical reflectivity measurement of sample grown at 600  C and in 0.1 mTorr pressure. Inset graph in (b) shows the derivative of the reflectivity data. The red lines in the inset are linear extensions from the low and high energy regions to determine the band gap energy (1.15 eV). The peak near 1.5 eV is an artifact due to the upper energy limit of the photodetector used.. pure phase sample grown under optimal conditions are shown in Fig. 8(b). The reflectance exhibited a slow increase with increasing photon energy. The first derivative of the reflectance data indicates that there is an abrupt increase in slope at approximately 1.15 eV (Fig. 8 inset), which is due to the presence of the indirect band gap of CuFeO2.25,26 The observed slope below this photon energy may be the signature of an Urbach tail, due to the presence of defects27 and is in agreement with the assertion that the rocking curves have a disorder-induced wider peak for films thicker than 9 nm. E. Cu L-edge XANES. In order to investigate the ratio between Cuþ/Cu2þ ions in our samples which have pure delafossite phase according to the XRD data, we performed synchrotron XANES measurements on one of the CuFeO2 thin films (thickness 17 nm) using photon energies around the Cu L3,2 edge (Fig. 9). Pure phase CuFeO2 was verified by XRD and XPS prior to the measurements. A comparison with the spectra from the literature taken on Cu2O (¼“Cuþ”) and CuO (¼“Cu2þ”)28,29 allows identification of features in the total spectrum resulting from the different Cu valencies. For the purpose of a quantitative analysis of the Cu valencies, the experimental data have been vertically offset to obtain zero absorption for the Cu pre-edge and then re-scaled to yield a best fit to a. linear combination of XANES reference spectra taken on Cu2O and CuO bulk samples. The reference data have been extracted from Ref. 28 and normalized at a photon energy of 1000 eV in accordance with Ref. 28. Although the fit reproduces qualitatively all of the main features of the experimental spectrum, it does not describe the experimental data perfectly. One of the major reasons could be the difference between the investigated thin film geometry and the reference bulk samples. Therefore, the fit results are to be understood as an indication of the order of magnitude of the Cuþ/ Cu2þ ionic ratio, rather than an exact measurement. We obtained a 90%/10% ratio between Cuþ and Cu2þ cations in the studied CuFeO2 film. Note that the measurements were performed in total electron yield mode with a typical escape depth of 5 nm. This small (<10%) amount of Cu2þ could be formed due to a post-oxidation of the top Cuþ layer (<1 nm) to Cu(OH)2 after the film has been exposed to air. F. Magnetic properties. For the purpose of magnetic characterization, two different thin film samples were chosen. Sample A has been grown at 600  C substrate temperature and in 0.1 mTorr O2 oxygen pressure with a film thickness of 40 nm. Our structural analysis by XRD and XPS (Figs. 5 and 6) indicated that this sample was composed of pure phase delafossite CuFeO2 with the stacking c-axis oriented out-of-plane. Magnetic measurements were performed with the magnetic field applied in (perpendicular to the c-axis) or out of the film plane (parallel to the c-axis). Sample B was grown at 550  C substrate temperature in 10 mTorr O2 growth pressure with a thickness of 25 nm. XRD patterns showed epitaxial (111) growth of the cubic spinel phase CuFe2O4 without any delafossite phaserelated peaks. Temperature dependences of the magnetizations measured during warming in field H ¼ 100 Oe after zero-field cooled (ZFC) and measured during field cooling (FC) in the same field are shown in Figs. 10(a), 10(b), and 10(c). The spinel film (sample B, Fig. 10(c)) exhibits a magnetic response that is one to two orders of magnitude larger than. Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Download to IP: 146.155.28.40 On: Mon, 16 May 2016 20:18:15.

(8) 013908-7. Joshi et al.. FIG. 10. (a)–(c) ZFC and FC magnetization vs. temperature measurements on (0001) CuFeO2 [(a) and (b)] and on (111) CuFe2O4 (c) thin films grown using the same CuFeO2 target. Magnetic field H ¼ 100 Oe was applied inplane [(a) and (c)] or out-of-plane (b) with respect to the substrate plane. Green arrows indicate magnetic phase transition at 15 6 1 K in CuFeO2. (d) Magnetic hysteresis loops measured at 5 K on the sample shown in (a) and (b) with different field orientations. (e) Magnetic hysteresis loop measured at 5 K on the sample shown in (c), in-plane orientation. Magnetic response from the Al2O3 (0001) substrates has been subtracted for all measurements.. that of the delafossite film (sample A, Figs. 10(a) and 10(b)). The magnetic transition temperature in the spinel sample B is also higher than the upper limit of the temperature range used in the measurements (320 K). This agrees well with the spinel bulk properties. On the other hand, ZFC and FC curves for the delafossite film (Figs. 10(a) and 10(b)) start to deviate from each other below 60 K, with the ZFC curve demonstrating a broad peak in the range of 35 K to 45 K. This is a clear sign of uncompensated remanent magnetization established in this low temperature region. This is somewhat similar to the weak ferromagnetic response found in thin films of other materials with bulk non-collinear spin ordering, for example, 30–50 nm films of TbMnO3 or YMnO3 grown on SrTiO3 substrates.6,30 It has been suggested that growth strain is responsible for disturbing the balance between different sub-lattice magnetic interactions, hence resulting in a weak ferromagnetism. Nevertheless, a step-like feature can be clearly seen in the FC curves for both field orientations (Figs. 10(a) and 10(b)) at TC 15 6 1 K, which is in an excellent agreement with the TN1 ¼ 14 K in the bulk CuFeO2.31 No indication of the second transition below TN1 was observed, which was likely to be due to the same strain-induced effects mentioned above. In the bulk CuFeO2, a step-like magnetization decrease is expected at TN2  11 K, but not at TN1  14 K, with the onset of the collinear-commensurate structure only.27 Hence, the observed step at 15 6 1 K could be a sign of an extended region of the commensurate for T < TN1. The magnetic response from the CuFeO2 film is stronger for the in-plane than for the out-of-plane field orientation, which is in agreement with the bulk CuFeO2 where the magnetic response with the field applied perpendicular to c-axis is stronger than the response along the c-axis.31 The magnetic hysteresis loops measured on CuFeO2 film (Fig. 10(d)) at T ¼ 5 K with different magnetic field. J. Appl. Phys. 117, 013908 (2015). orientations also show an in-plane magnetic anisotropy. Both field orientations exhibit hysteresis loops with relatively low coercivity (max. 100–200 Oe) and saturation moment values (0.3 lB and 0.2 lB per CuFeO2 for in-plane and out-of-plane field orientation, respectively). Such behavior is well described by antiferromagnetic spins aligned along the caxis, in agreement with the bulk spin order below TN2, with the additional canted moment in the film plane, however. Magnetization hysteresis loops measured in high fields up to 70 kOe for both field orientations and in the temperature range of 5 K to 10 K did not show any step-like behavior normally related to the collinear-noncollinear spin order transformations, as is normally observed in the bulk CuFeO2.4 The magnetic hysteresis loop measured on the spinel sample CuFe2O4 (Fig. 10(e)) at T ¼ 5 K shows a saturation moment that is an order of magnitude stronger than the one in the CuFeO2 sample. Its absolute value (5 lB per CuFe2O4 formula unit) is somewhat higher than in the bulk CuFe2O4, where typical values of 1.3 lB or 2.2 lB are reported for the tetragonal or cubic phases, respectively.8 Following the simple spin distribution model from Ref. 8, 32, and 33, a general cation distribution between tetrahedral (¼“tet”) and octahedral (¼“oct”) sites in the spinel can be described by formula (Cu1-dFed)tet(CudFe2-d)octO4, where d signifies the Cu cation inversion. Assuming 1lB and 5lB as magnetic spin moments for Cu2þ and Fe3þ ions, respectively (no Fe2þ were identified by the XPS measurements shown in Fig. 5(b)), the saturation magnetization per formula unit is then calculated as l ¼ lB(9–8d). In this model, the magnetizations in the bulk tetragonal and cubic phases are described by d ¼ 100% or d ¼ 85%, respectively. In our CuFe2O4 samples, the observed saturation magnetization can then be explained by the reduced Cu inversion d ¼ 50%, which has been reported for CuFe2O4 thin films.32 Note that in order to satisfy the nominal stoichiometry of the CuFeO2 target used in the PLD growth process, the spinel film should contain additional amount of Cu or CuOx. While no unreacted Cu was identified from the XPS data, no additional XRD peaks from possible Cu or CuOx phases were observed, which suggests an amorphous or polycrystalline structural disorder in these impurities. The only likely signature of CuO could be small steps (Fig. 10(c), blue arrows) observed in both ZFC and FC data at 215 K and 220 K, respectively, which is relatively close to the bulk antiferromagnetic transitions in CuO at 213 K and 230 K.34 Formation of CuO is also more likely than Cu2O, since the CuFe2O4 formation does favor Cu2þ over Cuþ. We expect a negligible contribution to the saturation magnetization of the CuFe2O4 sample (Fig. 10(e)) by CuO impurities, since even in case of CuO nanoparticles known to exhibit magnetization higher than the bulk one, the estimated values are 2–3 orders of magnitude below our experimental data.35 An alternative scenario, in which CuO does not form at all, involves a nonstoichiometric spinel (Cu0.75Fe0.25)tet(Cu0.75Fe1.25)octO3.75 with 1:1 ratio of Cu2þ/Fe3þ and with 5lB per formula unit saturation magnetization. To the best of our knowledge, such a compound has never been reported, so we can only speculate about its existence.. 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(9) 013908-8. Joshi et al.. IV. CONCLUSIONS. We have successfully deposited (00.1) pure phase epitaxial delafossite CuFeO2 thin films using PLD on Al2O3 (00.1) single crystal substrates. The impurity spinel phase CuFe2O4 tended to form for growth conditions resulting in over-oxidation of Cu to Cuþ2 ions, and was also favored by the slightly better lattice match of the spinel phase on the substrate. The spinel phase was not found in CuFeO2 films grown at optimal growth conditions, 600  C substrate temperature in a 0.1 mTorr O2 pressure. X-ray diffraction, RHEED, and TEM measurements showed that the films grown in such conditions are highly epitaxial to the substrate. The chemical purity of the film was verified by XPS and XANES and confirmed using HRTEM and Raman scattering. The indirect bandgap of the pure-phase delafossite CuFeO2 film was measured as 1.15 eV, using IR reflectivity. Magnetization measurements on CuFeO2 films showed weak ferromagnetic behavior at low temperatures with the spins aligned along the c-axis. An antiferromagnetic transition has been identified at 15 6 1 K, and referred to the extended region of the commensurate spin order existing in these films. The lack of additional transitions in the CuFeO2 film, which are observed in the bulk as functions of temperature and magnetic field, is explained to be a result of induced strain in the sample. Further growth experiments performed on substrates with different lattice mismatch, or piezoelectric substrates, might provide additional information on how strain affects the magnetic order in CuFeO2. ACKNOWLEDGMENTS. We thank J. P. Lewis and his group at WVU for useful discussions. This work at WVU was supported in part by the WV Higher Education Policy Commission (grant HEPC.dsr.12.29) and by FAME, one of six centers of STARnet, a Semiconductor Research Corporation program sponsored by MARCO and DARPA (contract # 2013-MA2382). Work at PUC was supported by FONDECyT (Grant No. 1130372). 1. A. Pabst, Am. Mineral. 31, 539 (1946). M. Mekata, N. Yaguchi, T. Takagi, T. Sugino, S. Mitsuda, H. Yoshizawa, N. Hosoito, and T. Shinjo, J. Phys. Soc. Jpn. 62, 4474 (1993). 3 S. Mitsuda, N. Kasahara, T. Uno, and M. Mase, J. Phys. Soc. Jpn. 67, 4026 (1998). 4 S. Mitsuda, M. Mase, K. Prokes, H. Kitazawa, and H. Katori, J. Phys. Soc. Jpn. 69, 3513 (2000). 2. J. Appl. Phys. 117, 013908 (2015) 5. T. Kimura, J. C. Lashley, and A. P. Ramirez, Phys. Rev. B 73, 220401 (2006). 6 X. Marti, V. Skumryev, C. Ferrater, M. Garcia-Cuenca, M. Varela, F. Sanchez, and J. Fontcuberta, Appl. Phys. Lett. 96, 222505 (2010). 7 P. Borisov, J. Alaria, T. Yang, S. McMitchell, and M. Rosseinsky, Appl. Phys. Lett. 102, 032902 (2013). 8 J. Smit and H. P. Wijn, Ferrites (Wiley, New York, 1959). 9 A. Barnabe, E. Mugnier, L. Presmanes, and Ph. Tailhades, Mater. Lett. 60, 3468 (2006). 10 E. Mugnier, A. Barnade, L. Presmanes, and Ph. Tailhades, Thin Solid Films 516, 1453 (2008). 11 L. Zhang, P. Li, K. Huang, Z. Tang, G. Liu, and Y. Li, Mater. Lett. 65, 3289 (2011). 12 D. H. Choi, S. J. Moon, J. S. Hong, S. Y. An, L. B. Shim, and C. S. Kin, Thin Solid Films 517, 3987 (2009). 13 S. Z. Li, J. Liu, X. Z. Wang, B. W. Yan, H. Li, and J. M. Liu, Physica B 407, 2412 (2012). 14 K. Simeonov and D. Lederman, Surf. Sci. 603, 232 (2009). 15 K. Ueda, T. Hase, H. Yanagi, H. Kawazoe, H. Hosono, H. Ohta, M. Orita, and M. Hirano, J. Appl. Phys. 89, 1790 (2001). 16 J. Narayan, K. Dovidenko, A. K. Sharma, and S. Oktyabrsky, J. Appl. Phys. 84, 2597 (1998). 17 C. J. Sun, P. Kung, A. Saxler, H. Ohsato, K. Haritos, and M. Razeghi, J. Appl. Phys. 75, 3964 (1994). 18 T.-B. Hur, Y.-H. Hwang, H.-K. Kim, and H.-L. Park, J. Appl. Phys. 96, 1740 (2004). 19 J. Narayan and B. C. Larson, J. Appl. Phys. 93, 278 (2003). 20 P. Pant, J. D. Budai, R. Aggarwal, R. J. Narayan, and J. Narayan, J. Phys. D: Appl. Phys. 42, 105409 (2009). 21 J. Ghijsen, L. H. Tjeng, J. van Elp, H. Eskes, J. Westerink, G. A. Sawatzky, and M. T. Czyzyk, Phys. Rev. B 38, 11322 (1988). 22 S. Poulston, P. M. Parlett, P. Stone, and M. Bowker, Surf. Interface Anal. 24, 811 (1996). 23 O. Aktas, K. D. Truong, T. Otani, G. Balakrishnan, M. J. Clouter, T. Kimura, and G. Quirion, J. Phys.: Condens. Matter 24, 036003 (2012). 24 S. P. Pavunny, A. Kumar, and R. S. Katiyar, J. Appl. Phys. 107, 013522 (2010). 25 F. A. Benko and F. P. Koffyberg, J. Phys. Chem. Solids 48, 431–434 (1987). 26 J. W. Lekse, M. K. Underwood, J. P. Lewis, and C. Matranga, J. Phys. Chem. C 116, 1865 (2012). 27 F. Urbach, Phys. Rev. 92, 1324 (1953). 28 M. Grioni, J. van Acker, M. Czyzyk, and J. Fuggle, Phys. Rev. B 45, 3309 (1992). 29 P. Jiang, D. Prendergast, F. Borondics, S. Porsgaard, L. Giovanetti, E. Pach, J. Newberg, H. Bluhm, F. Besenbacher, and M. Salmeron, J. Chem. Phys. 138, 024704 (2013). 30 X. Marti, V. Skumryev, A. Cattoni, R. Bertacoo, V. Laukhin, C. Ferrater, M. Garcia-Cuenca, M. Varela, F. Sanchez, and J. Fontcuberta, J. Magn. Magn. Mater. 321, 1719 (2009). 31 T. Lummen, C. Strohm, H. Rakoto, and P. van Loosdrecht, Phys. Rev. B 81, 224420 (2010). 32 A. Yang, Z. Chen, A. Zuo, A. Arena, J. Kirkland, C. Vittoria, and V. Harris, Appl. Phys. Lett. 86, 252510 (2005). 33 D. Thapa, N. Kulkarni, S. Mishra, P. Paulose, and P. Ayybu, J. Phys. D: Appl. Phys. 43, 195004 (2010). 34 R. Villarreal, G. Quirion, M. Plumer, M. Poirier, T. Usui, and T. Kimura, Phys. Rev. Lett. 109, 167206 (2012). 35 A. Punnoose, H. Magnone, M. S. Seehra, and J. Bonevich, Phys. Rev. B 64, 174420 (2001).. Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. 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Figure

Figure 1 shows typical RHEED patterns recorded on a blank Al 2 O 3 (00.1) substrate for [21.0] and [10.0] azimuths [Figs

Figure 1

shows typical RHEED patterns recorded on a blank Al 2 O 3 (00.1) substrate for [21.0] and [10.0] azimuths [Figs p.3
FIG. 3. c-lattice constants calculated for samples grown at 500  C as a func- func-tion of pressure (red triangles) and in 0.1 mTorr pressure as a funcfunc-tion of growth temperature (black circles).
FIG. 3. c-lattice constants calculated for samples grown at 500  C as a func- func-tion of pressure (red triangles) and in 0.1 mTorr pressure as a funcfunc-tion of growth temperature (black circles). p.4
FIG. 2. X-ray diffraction spectra for the samples grown at 550  C as a func- func-tion of O 2 pressure (a) and in 0.1 mTorr of O 2 pressure as a function of growth temperature (b).
FIG. 2. X-ray diffraction spectra for the samples grown at 550  C as a func- func-tion of O 2 pressure (a) and in 0.1 mTorr of O 2 pressure as a function of growth temperature (b). p.4
FIG. 4. (a) Phi scan of (10.2) and (10.4) peaks for the CuFeO 2 film grown at 600  C and in 0.1 mTorr of O 2 pressure (red curve) and for Al 2 O 3 substrate (black curve), respectively
FIG. 4. (a) Phi scan of (10.2) and (10.4) peaks for the CuFeO 2 film grown at 600  C and in 0.1 mTorr of O 2 pressure (red curve) and for Al 2 O 3 substrate (black curve), respectively p.5
FIG. 6. X-ray reflectivity data (open circles) with the corresponding fit (line) using GenX software
FIG. 6. X-ray reflectivity data (open circles) with the corresponding fit (line) using GenX software p.6
FIG. 7. (a) HRTEM image taken from the interface between the substrate and the thin film grown at 600  C and 0.1 mTorr O 2 pressure
FIG. 7. (a) HRTEM image taken from the interface between the substrate and the thin film grown at 600  C and 0.1 mTorr O 2 pressure p.6
FIG. 5. (a) and (b) XPS spectra for Cu and Fe, respectively, for the samples grown at 550  C and in 0.1 mTorr (red, solid circles) and 10 mTorr (black, open circles) O 2 pressure.
FIG. 5. (a) and (b) XPS spectra for Cu and Fe, respectively, for the samples grown at 550  C and in 0.1 mTorr (red, solid circles) and 10 mTorr (black, open circles) O 2 pressure. p.6
FIG. 9. The Cu L-edge XANES spectrum taken on the CuFeO 2 thin film.
FIG. 9. The Cu L-edge XANES spectrum taken on the CuFeO 2 thin film. p.7
FIG. 8. (a) Raman shift and (b) optical reflectivity measurement of sample grown at 600  C and in 0.1 mTorr pressure
FIG. 8. (a) Raman shift and (b) optical reflectivity measurement of sample grown at 600  C and in 0.1 mTorr pressure p.7
FIG. 10. (a)–(c) ZFC and FC magnetization vs. temperature measurements on (0001) CuFeO 2 [(a) and (b)] and on (111) CuFe 2 O 4 (c) thin films grown using the same CuFeO 2 target
FIG. 10. (a)–(c) ZFC and FC magnetization vs. temperature measurements on (0001) CuFeO 2 [(a) and (b)] and on (111) CuFe 2 O 4 (c) thin films grown using the same CuFeO 2 target p.8

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