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High Quality Magnetic Oxide Thin Films Prepared via Aqueous Solution Processing Peter Richter,† Paul N. Plassmeyer,‡ Julia Harzdorf,§ Tobias Rüffer,§ Heinrich Lang,§ Jana Kalbacova,† Nathanael Jöhrmann,# Steffen Schulze,# Michael Hietschold,# Sri Sai Phani Kanth Arekapudi,∥,⊥ Manfred Albrecht,∥,⊥ Dietrich R.T. Zahn,† Catherine J. Page,*,‡ and Georgeta Salvan*,† †

Semiconductor Physics, Technische Universität Chemnitz, 09107 Chemnitz, Germany Department of Chemistry, University of Oregon, Eugene, Oregon 97403, United States § Inorganic Chemistry, Technische Universität Chemnitz, 09107 Chemnitz, Germany # Solid Surfaces Analysis, Technische Universität Chemnitz, 09107 Chemnitz, Germany ∥ Surface and Interface Physics, Technische Universität Chemnitz, 09107 Chemnitz, Germany ⊥ Institute of Physics, University of Augsburg, 86159 Augsburg, Germany ‡

S Supporting Information *

ABSTRACT: Recent progress in multiferroic materials and spintronic devices has renewed interest in metal oxide ferromagnetic and ferrimagnetic materials. Here, we report the preparation of thin films of nanocrystalline ferrimagnetic CoFe2O4 (CFO) using an environmentally benign aqueous solution processing route. The evolution of the structural, optical, and magnetic properties as a function of postdeposition annealing temperature is reported. For the highest annealing temperature (800 °C), the remanence, coercivity, and resistivity values are comparable with those of films fabricated by epitaxial growth methods and exceed the quality of CFO nanocrystals prepared by other wet-chemical syntheses. In addition to the ability to make high quality ferrite films, the aqueous solution processing strategy offers great flexibility for tuning film properties by incorporating or substituting additional transition metal ions.

1. INTRODUCTION Thin film metal oxides are of interest as gate dielectrics,1 semiconductors,2 transparent conductors,3 ionic conductors,4 superconductors,5 and in photovoltaics.6 Transition metal oxides are particularly interesting because partially filled d shells impart a range of magnetic and electronic properties. In this latter class of compounds, ferro- or ferrimagnetic metal oxides have been explored for a host of magnetic applications. Spinel structure metal oxides (e.g., ferrites like CoFe2O4) are attractive for magneto-electric devices7 as they often show multiferroic effects.8−11 They can be used in microwave12,13 and power storage14,15 applications or can serve as building blocks of novel high-density magnetic recording media.16 Finally, ferrites may show intrinsic spintronic functionality17,18 and can potentially serve as electrode materials in organic spintronic heterojunctions, as has been shown using perovskite structure manganites.19,20 A wide variety of techniques are employed to prepare thin films of magnetic oxides. Physical vapor phase deposition techniques such as pulsed laser deposition (PLD),21−23 molecular beam epitaxy (MBE),24,25 and sputtering26,27 generally yield high quality oxide films with good crystallinity and reproducibly tunable electronic and magnetic properties. However, these techniques require sophisticated specialized © 2016 American Chemical Society

instrumentation, are restricted to the deposition of small sample areas, and are not easily amenable to doping with foreign ions. As an alternative, there are several chemical synthesis routes for the preparation of ferrite nanoparticles and powders such as sol−gel,28,29 microemulsion,30−33 combustion,34,35 chemical coprecipitation,36−38 hydrothermal synthesis,39−41 and forced hydrolysis.42,43 In general, the particle sizes and the resulting material properties can be tuned depending on the processing parameters, but the integration of ferrite nanoparticles and powders into functional devices remains difficult. Reports of wet-chemical preparation routes to thin ferrite films are relatively scarce. There are a few studies utilizing sol−gel and other solution deposition strategies,44−48 while others focus on the modification and/or optimization of the spin coating method49,50 in order to obtain thin films of spinel type oxides. In addition, a spin coating strategy has also been employed to fabricate multiferroic spinel/perovskite layer stacks.51,52 Apart from the fact that these fabrication procedures generally involve the use of environmentally problematic solvents, the reported films often show poor magnetic and Received: March 9, 2016 Revised: June 28, 2016 Published: June 28, 2016 4917

DOI: 10.1021/acs.chemmater.6b01001 Chem. Mater. 2016, 28, 4917−4927

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Chemistry of Materials

Figure 1. Illustration of the suggested PIC procedure to fabricate magnetic spinel oxides by spin coating of an aqueous precursor solution with subsequent annealing. 500 rpm/s, a final rotation speed of 3000 rpm, and a total rotation time of 30 s. The wet films were immediately transferred to a hot plate set at 200 °C, and the temperature was increased to 400 °C at an approximate rate of 2 K/s and kept at this temperature for 10 min. During this soft annealing procedure, most of the remaining water, hydroxides, and nitrates are expelled from the film, as indicated by thermogravimetric analyses of the metal nitrates and the dried precursor solution (not shown here). In a last heating step, the samples were annealed for 1 h in air at 600 or 800 °C. The use of a gradual temperature increase and decrease (15 K/min) prevents rapid phase changes and allows smooth crystallite growth in the material. 2.3. Film Characterization. Transmission IR measurements were performed on a Nicolet 6700 FT-IR spectrometer using films prepared on double-side polished silicon substrates (2000 Ω·cm) with processing conditions matching those described above. A bare silicon substrate subjected to the same thermal treatment at each annealing temperature was used for the background correction. Spectroscopic ellipsometry data was recorded with a J. A. Woollam Co., Inc. M-2000 ellipsometer in a spectral range from 0.75 to 5.0 eV at incidence angles of 45°, 55°, 65°, and 75°. In order to determine the diagonal elements of the dielectric tensor (and the complex refractive indices) of the CFO layers and their thicknesses, a transfer matrix based optical layer modeling procedure was applied to the experimentally obtained ellipsometric parameters Ψ and Δ using the software WVASE32. The CFO layers are treated as being isotropic and thus εxx = εyy = εzz. For the Kramers−Kronig consistent modeling of the ellipsometry data obtained from the different ferrite layers, it has proven to be adequate to employ a set of 8 Gaussian oscillators in order to represent the feature rich dispersion of the dielectric functions and account for a possible transparent region in the spectra. Magneto-optical Kerr effect (MOKE) spectroscopy was used to measure the wavelength dependent polarization change of a light beam as it is reflected on the sample surface exposed to an external magnetic field. This method delivers complementary information to spectroscopic ellipsometry regarding the origin of the optical transitions (see, e.g., refs 23 and 63). We used a custom-built MOKE spectrometer in polar configuration operated at RT equipped with a xenon white light lamp, a 1.7 T electromagnet, and a polarization analysis path based on a photoelastic modulator and a lock-in amplifier.64,65 This allows the independent determination of the rotation of the light polarization plane θKerr and the ellipticity of the light polarization ηKerr. For MOKE spectroscopy, the complex Kerr angle ΦKerr = θKerr + iηKerr is measured for photon energies from 1.7 to 5.0 eV. Similar to spectroscopic ellipsometry, a complex optical modeling procedure is applied to the experimental results which accounts for the layer thickness dependent interference effects and allows to determine the off-diagonal elements of the dielectric tensor εxy of CFO as a quantitative measure for its magneto-optical activity. Using MOKE magnetometry at RT, the Kerr rotation θKerr was recorded for varying magnetic fields at a fixed photon energy of 2.2 eV under the variation of the external magnetic field (±1.7 T). Since θKerr relates linearly to the out-of-plane magnetization of the sample, it can be considered as a direct representation thereof.

electronic properties that are not competitive with epitaxially grown films. To overcome the limitations of previous wet-chemical approaches, we utilize a method to effectively prepare smooth thin films of metal oxides by spin coating them from aqueous precursor solutions with subsequent annealing. This method, recently coined “Prompt Inorganic Condensation”, or PIC, has been used to make high quality films of a number of oxides, including dielectrics,53−58 transparent conducting oxides,59,60 and high-resolution inorganic resists.61,62 In these examples using PIC, dehydration and nitrate removal are shown to occur without negatively affecting film morphology, allowing the preparation of dense, smooth films at relatively low temperatures. We show that the electronic, optical, and magnetic properties of the nanocrystalline films prepared in this work are comparable to those achieved by vapor phase deposition techniques. The fabrication process is reliable and easily tunable for different material compositions while the use of purified water instead of organic or halogenated solvents reduces the environmental impact.

2. EXPERIMENTAL SECTION The synthetic procedure used to prepare thin films of CoFe2O4 (CFO) is illustrated in Figure 1. NiFe2O4, ZnFe2O4, and zinc-doped cobalt iron oxide (Co0.95Zn0.05Fe2O4) films were prepared using the same procedure with appropriate adjustment of the solution composition. 2.1. Precursor Solution. Aqueous precursor solutions were prepared by dissolving the suitable metal nitrates in ultrapure water (18.2 MΩ·cm). For CoFe2O4, 40.402 g of Fe(III)(NO3)3·9H2O (99 %, VWR Chemicals) and 14.552 g of Co(II)(NO3)2·6H2O (99 %, Merck Millipore) were added to about 30 mL of 18.2 MΩ·cm H2O under constant stirring and heating at 70 °C. Once the dark brown-red solution was free of any solid phase agglomerations, it was diluted to 100 mL using 18.2 MΩ·cm H2O. The resulting precursor solution was 1.5 M metal ion concentration with the necessary Fe3+ and Co2+ ions at a 2:1 ratio and a nitrate counterion concentration of 4 M. The latter is important in order to keep the pH value of the solution below 3 to prevent precipitation of metal oxides or hydroxides. Analogously prepared precursor solutions with lower metal ion concentration result in proportionally thinner films upon spin coating. 2.2. Film Preparation. Up to 4 × 4 cm2 large B-doped p-type Si(100) wafers (5−30 Ω·cm) with native SiO2 were used as substrates. Prior to solution deposition, the substrates were cleaned by sonication in a 10 % Contrad 70 solution at 40 °C. After thorough rinsing with 18.2 MΩ·cm H2O, the Si surface was highly hydrophilic and suitable for spin coating. Additionally, we found that the substrates work best for spin coating when they are heated to 200 °C immediately prior to use. For solution deposition, the entire substrate surface was covered with precursor solution dropped from a syringe through a 0.45 μm PTFE filter. The spin coating process began with an acceleration of 4918


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Chemistry of Materials Additionally, in-plane and out-of-plane RT magnetization curves, as well as a magnetization vs temperature (MR vs T) measurement were recorded using a Quantum Design MPMS Superconducting Quantum Interference Device-Vibrating Sample Magnetometer (SQUID-VSM). The magnetization vs temperature scan was performed in the in-plane direction with a quartz sample holder according to the following procedure: starting at RT, the magnetic field was set to 7 T followed by a magnet reset to bring the magnetic field to 0 T with minimum SQUID magnet remanence. With 0 T field, the sample was cooled down to 4 at 20 K/min and the moment vs temperature was measured up to 400 K. The structure of the magnetic domains of the CFO surface was visualized using an in situ high resolution-magnetic force microscope (HR-MFM) from NanoScan. A Team Nanotec high resolution tip with low-moment and a spring constant of D = 0.7 N/m was used in this experiment. To minimize the topographic interference in the magnetic contrast, the scan was performed in noncontact mode with a tip-to-sample distance of approximately 25 ± 5 nm. The sample was carefully demagnetized before the MFM scan was performed. SQUID and MFM results are provided in the Supporting Information. The surfaces of the CFO samples were studied by atomic force microscopy (AFM) in AC mode (SmartSPM, AIST-NT, Novato, CA, USA) with cantilevers with nominal tip size of 8 nm (HQ:NSC14/AL BS, μMasch, Bulgaria). The grain size distribution and RMS roughness values were determined with the help of the Gwyddion software.66 Cross sections of the films were investigated by scanning and transmission electron microscopy. Classical methods of ion beam milling were used to prepare electron transparent samples. Images taken with a FEI Nova NanoSEM 200 at 15 kV and a FEI CM 20 TEM at 200 keV clearly show the morphology of the films, while the obtained diffraction patterns verify the crystalline structure of the nanoparticles. Both reveal the evolution of the lattice parameter with annealing temperature. The crystallinity of the 800 °C annealed CFO was verified by an Xray diffractometry 2θ scan in grazing incidence geometry (0.3 ° angle of incidence) carried out by Bruker AXS using a BRUKER D8 Discover diffractometer (see Supporting Information). Electrical conductivity of the ferrite films was determined using samples deposited onto quartz substrates while keeping all other deposition and annealing parameters unchanged. The resistivity for each film was calculated from 2 point probe I−V measurements (averaged over at least 10 different positions on the film surface) using a Keithley 2636A two channel source meter. Due to the generally large resistivities of the films, contact resistance can be neglected and a 4 point probe I−V measurement procedure was not feasible.

FTIR was used to monitor the loss of water and nitrates from the film as a function of annealing temperature and to assess metal oxide formation. As can be seen in Figure 2, a drastic reduction in the OH stretching band (3500 cm−1) and NO bands (1550, 1300, 1100, 800, and 750 cm−1) occurs at temperatures below 200 °C, indicating that water and nitrate loss occur at relatively low temperatures. By 200 °C, the OH and NO modes are nearly undetectable, indicating that the film mainly consists of a metal oxide network. The appearance of a distinct peak at 550 cm−1 after a 400 °C anneal is likely due to the onset of crystallization. Sharpening of this absorption at higher annealing temperatures is due to crystallite growth.

Figure 2. Transmission FTIR spectra of annealed CFO films deposited from 1.5 M precursor solution on Si normalized to a Si reference. Inverted SiO2 absorption at temperatures ≥600 °C indicate that the blank Si wafer used for background correction oxidizes more rapidly than the wafer covered with the CFO film. The spectra were vertically shifted for clarity.

The initial temperature at which the spin-cast hydrated gel is heated is an important parameter that affects the subsequent film evolution. If it is too low, sequential decomposition of differing nitrates may lead to elemental segregation through the thickness of the film. This can lead to a phase segregated material after crystallization occurs. Alternatively, if the initial temperature is too high, gas expulsion from counterion decomposition can disrupt the rapidly forming oxide network. We found that, in accordance with the FTIR spectra, an initial annealing temperature of 200 °C allowed prompt condensation without precipitation or disruptive gaseous expulsion. At this temperature, most of the water and nitrates have been expelled from the film. Accordingly, films were first heated to 200 °C before ramping to the final annealing temperature. 3.2. Film Thickness, Morphology, and Crystallinity of CoFe2O4 Films. CoFe2O4 (CFO) film thicknesses were determined using spectroscopic ellipsometry measurements and verified using scanning and transmission electron microscopy (SEM and TEM) cross section images (Figures 3 and 4). Film thickness is predominantly defined by the metal ion concentration of the precursor solution and the annealing temperature and can be fine-tuned by adjusting the spin coating parameters.56 Using a 1.5 M precursor solution, CFO film thicknesses for films annealed at 400, 600, and 800 °C were 68 ± 1, 59 ± 1, and 48 ± 1 nm, respectively.

3. RESULTS AND DISCUSSION 3.1. Evolution of CoFe2O4 Films with Annealing Temperature. PIC thin films are prepared by spin coating an aqueous precursor solution onto a hydrophilic substrate to form a thin film of a hydrated gel of metal nitrates. While PIC is a chemical solution deposition route, it is distinct from traditional sol−gel methods in that there are no organic ligands or solvents employed. The rapid water evaporation induced by the spin coating process prevents crystallization of nitrate salts, which could be problematic under slower evaporation conditions.48 The hydrated nitrate gel is then rapidly heated to a temperature high enough to drive off H2O and to expel or decompose NO3− counterions. As these processes occur, an extended metal oxide/hydroxide network is formed. The relatively unstable initial bonding configurations of this network allows for the facile release of the byproducts of counterion decomposition and condensation reactions between neighboring metal hydroxides. While these reactions proceed, the network settles into a relatively stable amorphous oxide. As the temperature is increased even further, the amorphous network relaxes into the thermodynamically stable crystalline phase. 4919


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Figure 3. Bright field TEM images (a, d, g), high resolution TEM images (b, e, h), and electron diffractograms (c, f, i) of CFO film cross sections after 400, 600, and 800 °C annealing. Areas with parallel lattice planes have been marked in the HR-TEM images for clarity. Fast Fourier transformation (FFT) patterns of the HR-TEM images used for determination of the lattice parameters are not shown here. Electron diffractograms show reflection rings based on bulk spinel CFO crystallographic XRD data for the cubic Fd-3m lattice.67

condensation of remaining hydroxyl groups. In addition, the film thickness decrease is also correlated with changes in the morphology and nanostructural ordering, especially at the higher annealing temperatures. TEM cross sectional imaging and electron diffraction experiments illustrate this behavior (see Figure 3). The CFO film annealed at 400 °C consists of small irregular grains (Figure 3a). The high resolution TEM (HR-TEM) image (Figure 3b) shows crystallites no larger than 10 nm randomly arranged in an amorphous matrix. A fast Fourier transformation (FFT) pattern of the crystalline areas of the HR-TEM image allows estimation of a (cubic, Fd-3m) lattice parameter a = 7.8 Å. The electron diffraction pattern (Figure 3c) shows broad rings with very weak crystal reflections, indicating poor crystallinity. The few weak reflections lie outside of the reference diffraction rings calculated on the basis of literature crystallographic information typical for bulk spinel

Figure 4. SEM cross sectional image of the CFO film annealed at 800 °C.

Decrease in film thickness is related to chemical processes that occur as the temperature is increased. The thickness decrease between 400 and 600 °C may be partly due to loss of residual water and nitrate counterions and/or additional 4920


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Chemistry of Materials CFO.67 This is consistent with a lattice parameter smaller than the literature value for the spinel phase of a = 8.392 Å. With further annealing, the CFO films become denser and more compact. This trend coincides with the increase of crystallite size. After annealing at 600 °C, the crystallites are as large as 30 nm, with a relatively small sample volume remaining amorphous (see Figure 3d,e). A corresponding fast Fourier transformation (FFT) pattern of the crystallline areas of the images gives an estimated lattice parameter a = 8.2 Å, which is still below the literature value for bulk CFO. The electron diffraction pattern shows many reflections stemming from a large number of randomly arranged crystallites in the film. They can all be attributed to the CFO inverse spinel structure67 (Figure 3f); their location slightly outside the reference rings is consistent with a smaller lattice parameter. The crystallinity of the CFO film annealed at 800 °C is even more pronounced. The TEM image (Figure 3g) shows spherical or cubic crystallites with many of them extending over the entire film thickness. A very similar behavior was previously observed for a 50 nm thick sputtered CFO film annealed at 900 °C.26 Also clearly visible in Figure 3g is the 22 nm thick SiO2 film that has been thermally grown underneath the CFO film during the annealing process. An SEM cross section image (Figure 4) shows a narrow grain size distribution (∼32.5 nm) on a large scale. The representative “HR-TEM” cross sectional image of the film (Figure 3h) reveals the structure of such a particle to be single crystalline with a lattice spacing d = 4.9 Å for a (111) oriented crystallite and d = 3.0 Å for a (220) orientation. These values generate an estimated lattice parameter of a = 8.5 Å. All reflections in the electron diffractogram (Figure 3i) can be assigned to the known CFO spinel diffraction pattern,67 while the location of some reflections slightly inside of the reference rings are consistent with a lattice parameter exceeding the reference value of a = 8.392 Å. The high crystalline ordering within the 800 °C annealed CFO film is corroborated by XRD (see Figure S1), where the reflection pattern matches well with the bulk values for the inverse spinel phase of CFO.67 The growth of CFO crystallites with increased annealing temperature was observed in other studies.68−71 For our films, this effect can also be visualized using atomic force microscopy (AFM) (Figure 5). The grain size distribution was determined by applying a grain segmentation method (using Gwyddion66). It should be noted that the results of the grain size analysis are not influenced by the apparent drift occurring in some of the AFM images. For the 800 °C annealed CFO film, we found at least 50 % of the nanoparticles to have an equivalent disk diameter of 33 ± 7 nm. The average grain sizes correspond well to the values that can be estimated from the cross sectional electron microscopy images. Values for the root-mean-square roughness (RMS) from the AFM data increase with annealing temperature, from less than 0.5 nm up to about 2.2 nm (Figure 5d). This is similar to the trend observed for sol−gel prepared CFO thin films.44 In general, the surface roughness increase is strongly correlated to the formation and the growth of the CFO nanocrystallites. The surface roughness must be considered when evaluating the results from spectroscopic ellipsometry. The modeling procedure applied to the raw ellipsometric data accounts for scattering effects at the rough surface by introducing an additional roughness layer consisting of an effective medium approximation mixture of 50 % ferrite film and 50 % void.

Figure 5. AFM topography images of the CFO films annealed at 400 °C (a), 600 °C (b), and 800 °C (c). The average grain size (equivalent disk diameter) and the RMS value as a function of the annealing temperature (d). 4921


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Chemistry of Materials However, the fitted thickness of this roughness layer cannot be easily compared to the RMS roughness values determined by AFM because spectroscopic ellipsometry has a different sensitivity toward surface roughness. In ellipsometry, the profile of the surface, i.e., the granular structure of the CFO film, has a significant contribution to the overall surface roughness. Accordingly, the ellipsometry modeling did not require any surface roughness layer for the 400 °C annealed CFO film, while the roughness of the 600 and 800 °C annealed films was determined to be 3.4 ± 0.2 and 3.9 ± 0.2 nm, respectively. This is consistent with the trend of increased surface roughness for higher annealing temperatures observed by AFM. 3.3. Optical and Magneto-Optical Properties of CoFe2O4 Films. Spectroscopic ellipsometry modeling allows determination of the dielectric function of the ferrite films, which can serve as a measure of quality when compared to a high-quality crystalline single-phase reference sample. For our reference, we used a 450 nm CFO film prepared by pulsed laser deposition (PLD) on a MgO substrate at 720 °C. This PLD reference film is in fact the same sample as reported in ref 23. For the real part of the dielectric function (Figure 6a), the spectral line shapes of the films annealed at 600 and 800 °C agree well with ref 23. The magnitude of the εxx′ values is significantly smaller for the sample annealed at 600 °C but approaches the reference values for the film annealed at 800 °C. The reason for the reduced εxx′ value is the lower optical density of the films, which results from their granular structure

with small voids or amorphous regions remaining between the CFO crystallites. The spectral εxx′ line shape of the 400 °Cannealed sample differs from those of the high temperature annealed films with an apparent red shift of certain features and with different contributions toward both ends of the spectral region. This is not unexpected for the low temperature annealed film, as it is has a higher degree of disorder and associated defects, cavities, and amorphous regions than the films annealed at higher temperature. The differences between the 400 °C sample and the other samples become even more prominent when comparing the imaginary part of the dielectric function εxx″ (Figure 6b). CFO is known to be transparent in the IR with an indirect optical band gap ranging from 1.1872 to 1.43 eV,73 depending on sample preparation and postdeposition treatment. It was also shown that the degree of inversion in the spinel structure, i.e., the cation distribution, influences the optical band gap energy.74,75 From Figure 6b, this absorption band onset can be determined for all three CFO films to be around 1.4 eV. As expected for films with increasing crystallinity, there is a slight blue shift of the absorption onset as the annealing temperature is increased from 400 to 600 °C and further to 800 °C. The weak absorption features occurring at energies below this onset for the 400 °C annealed film diminish and vanish for the films annealed at 600 and 800 °C, respectively. Furthermore, an additional absorption feature at around 1.75 eV can be identified for the 400 °C annealed film. These absorption features are attributed to the d to d on-site transitions of the Co2+ (3d7) ions in octahedral configuration,73 i.e., the crystal field split transitions 4T1g(F) → 4T2g(F) at 0.75 eV, 4T1g(F) → 4 A2g(F) at 1.25 eV, and 4T1g(F) → 4T1g(P) at 1.75 eV. Generally, these transitions are spin-allowed but parityforbidden. These transitions are observed for the low temperature annealed films most likely because they are more disordered, and distortion of the local structure breaks the crystal symmetry of the Oh field, making these transitions allowed,76 albeit with comparably small oscillator strengths. As the crystal quality increases with higher annealing temperature, the contribution of these features decreases. With regard to the full spectral εxx″ line shape, we find good agreement for the 800 °C annealed spin coated film with the PLD reference. Because the absorption above the band gap appears as an arrangement of very broad contributions, it is difficult to assign distinct optical transitions based solely on spectroscopic ellipsometry data. Since these films are magnetic, more detailed insight into the electronic transitions can be obtained using MOKE spectroscopy. Figure 7 shows the off-diagonal elements of the dielectric tensor εxy obtained from modeling of the MOKE spectroscopy data for the spin coated CFO films and for the PLD reference film. As a measure of the magneto-optical activity, the overall magnitude of the real and imaginary εxy values can be seen to increase with higher annealing temperatures while the spectral line shape remains nearly the same. The reason for this is the increasing crystallinity and particle size with annealing temperature. The real and the imaginary εxy spectra of the 800 °C annealed CFO film show very good agreement with the results for the PLD reference sample, with the exception of the εxy′ magnitude exceeding the reference values in high and low energy regions while showing smaller values in the spectral range between 2.5 and 3.8 eV. Since the observed spectral features arise from magnetooptically active optical transitions involving specific crystal sites,

Figure 6. Annealing temperature dependence of the real (a) and imaginary (b) parts of the diagonal elements of the dielectric tensor for the spin coated CFO films in comparison to the PLD reference.23 4922


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prominent for the 800 °C annealed sample. The apparent splitting can be attributed either to the occurrence of spin− orbit coupling effects87 or to the contribution of the normally spin-forbidden crystal field transition (4A2(F) → 2T1(G)) which is very close in energy (2.07 eV).88 3.4. Magnetic and Electrical Properties of CoFe2O4 Films. The magnetization hysteresis of the CFO films (Figure 8) was probed using MOKE magnetometry and shows a remarkable improvement in the magnetic properties with increasing annealing temperature. The observed increase in coercivity and remanence is related to the improved crystallinity of the annealed films along with crystallite growth. The magnetic remanence R of 17 % ± 5 % for the 400 °C annealed CFO film increases to 51 % ± 1 % with 600 °C annealing and further to 63 % ± 1 % for 800 °C. Here, it should be noted that the remanence values can only be estimated since the Kerr rotation is not fully saturated at the highest applied field (B = 1.7 T). The coercive field HC increases from 47 ± 14 mT for the 400 °C annealed film to 408 ± 3 mT after the 600 °C treatment and to an impressive value of 668 ± 2 mT for the 800 °C annealed CFO. The large remanence and coercivity values for the 800 °C annealed CFO film were verified also by SQUID magnetometry (see Figure S2). Such large HC values are consistent with the fact that the size of the magnetic domains is more than ten times larger (∼600 nm) than the average crystallite size, as shown by MFM imaging (for discussion, see Figure S3).

Figure 7. Real (a) and imaginary (b) part of the off-diagonal elements of the dielectric tensor for the spin coated CFO films annealed at different temperatures and for the PLD reference sample.23

the differences indicate a deviation from the inverse cation distribution in the spinel. For a fully inverse spinel, the tetrahedral sites would be occupied by Fe3+ ions, and the remaining Fe3+ and Co2+ ions would occupy octahedral sites. According to previous magneto-optical studies on CFO,23,63,77−79 the magneto-optically active electronic transitions can be assigned either to intervalence charge transfer (IVCT) between octahedrally coordinated Co2+ and Fe3+ or to intersublattice charge transfer involving Fe3+ ions occupying tetrahedral and octahedral crystal sites (see indication of transitions in Figure 7a). Co2+ “misplaced” in tetrahedral sites gives rise to the crystal field transition 4A2(F) → 4T1(P), which was observed in all previous magneto-optical studies23,63,77−79 at photon energies of 1.8−2.0 eV. The strong magneto-optical activity of this transition was also reported for investigations on other substituted cobalt ferrites.80−83 As can be seen from Figure 7a (real part εxy′), the crystal field transition at 1.9 eV is strongly pronounced for the CFO film annealed at 800 °C, even exceeding that of the high quality PLD film. This suggests the 800 °C annealed film is only partially inverse, similar to the observations of Zhang et al.84 The fact that the cation distribution of a CFO sample may change with thermal history was previously observed by Mössbauer spectroscopy.85,86 A further interesting finding relates to the splitting of the 1.9 eV crystal field transition feature in the εxy′ spectrum. For all data sets, the optical modeling required the application of two separate Gaussian oscillators in order to fit the line shapes around 1.9 eV, but the presence of the shoulder is most

Figure 8. Hysteresis loops from MOKE magnetometry for the differently annealed spin coated CFO films (a) and the obtained coercivity and remanence values (b). 4923


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Chemistry of Materials The coercivity values for the 800 °C annealed CFO film exceed those reported for other wet chemical fabrication processes and are close to those of epitaxially grown films. For CFO powders, the reported HC values are no larger than 175 mT.68−70 Sol−gel preparation routes can deliver CFO thin films with coercivities of about 270 mT after 950 °C annealing,44 while spin coating of CFO/BiTiO3 layer stacks leads to out of plane coercivities of up to 510 mT.51 A few studies on samples prepared via vapor phase deposition methods found RT coercivity values larger than the one reported here for the 800 °C annealed CFO sample. Coercivity values of up to 930 mT26 were found for CFO films prepared by radio frequency sputtering with subsequent annealing, while high temperature PLD was used to obtain epitaxially grown CFO thin films with coercivities of 780 mT71 or up to 1250 mT.89 These unusually large coercivity values along with a pronounced perpendicular anisotropy of the CFO films are explained by their nanocrystalline structure (shape anisotropy) and a large residual strain. Also, a magnetocrystalline anisotropy contribution of octahedral Co2+, originating from its large spin orbit coupling,90 enhances HC. Consequently, a higher degree of inversion of the CFO spinel structure, i.e., more octahedrally coordinated Co2+, is known to lead to higher coercivity.91 The electrical conductivity of the films, as determined by 2 point probe I−V characterization, is found to be strongly dependent on annealing temperature. The CFO films show RT resistivities as low as (1.4 ± 0.6) × 105 Ω·cm for the film prepared at 400 °C and up to (6.1 ± 0.8) × 106 Ω·cm for the 800 °C annealed film. These values are comparable or superior to nanocrystalline CFO films prepared by other wet-chemical routes (108 Ω·cm,92 106 Ω·cm28) and are only surpassed by electron beam evaporated CFO films (104 Ω·cm).73 The conductivity was found to be larger for the low temperature prepared films, which have much smaller grain sizes, more defects, and local inhomogeneities in composition and cation distribution. This coincides with previous observations,93 where nanocrystalline CFO showed larger conductivity arising from tunneling of polarons between localized states following the Mott variable range hopping model.94 It was furthermore suggested that the conductivity in CFO could be enhanced by incorporating divalent and trivalent iron and cobalt ions and thereby promoting electron hopping (Fe2+ ↔ Fe3+) and/or hole hopping (Co2+ ↔ Co3+).95,96 3.5. Chemical Flexibility Allows Tuning of Magnetic and Electrical Film Properties. The PIC aqueous preparation route is broadly applicable to many metal oxide compositions and stoichiometries and can be exploited to tune the magnetic and electrical properties of the ferrite films. For example, a pure nickel ferrite film (NiFe2O4) prepared under the same conditions as CFO exhibits a coercivity of only 63 ± 7 mT and a remanence of 21 % ± 5 % (Figure S4). Doping CFO with 5 % zinc (Co0.95Zn0.05Fe2O4) while maintaining the same 800 °C annealing procedure leads to a decrease of the RT magnetic coercivity (408 ± 2 mT) and of the remanence (51 % ± 1 %) (see Figure S4). On the other hand, zinc doping significantly improves the electrical properties. The 5 % zinc doped CFO film (Co0.95Zn0.05Fe2O4) shows a resistivity that is 2 orders of magnitude smaller (

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