Properties of Zirconium Oxide and Cobalt Ferrite Layered Nanocomposite

CoFe2O4 nanoparticles with 3–30 nm in diameter were synthesized by sol-gel method. The particles were spread as a solid discontinuous layer over planar silicon and TiN substrates by spin coating and covered by 15 nm thick ZrO2 films by atomic layer deposition. Crystal structures distinctively characteristic of CoFe2O4 and ZrO2 constituents were preserved. The nanocomposite CoFe2O4-ZrO2 layers demonstrated dielectric polarization, saturative magnetization, and implications of resistive switching behavior. Behavior most clearly attributed to memory materials was observed in the field admittance characteristic with two distinct states in susceptibility of the nanocomposite. © The Author(s) 2017. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited. [DOI: 10.1149/2.0331712jss] All rights reserved.

There is permanent and growing interest in complex compound materials for a variety of applications in which the components tailor their best structural, mechanical, optical or electronic properties making the material versatile for the planned applications. Metal oxide based layered systems can demonstrate a variety of resistive switching, ferroelectric, ferromagnetic and other properties useful in prospective devices. 1 Cobalt-iron-oxygen ternary system has been of considerable interest, driven perhaps by the necessity of use it as a constituent compound in more complex structures exploited in solid oxide fuel cells. 2 Cobalt ferrite has been studied 3 as an attractive magnetic material suited to magneto-optical recording, integrated optics, or waveguides, due to its potential combination of semiconducting and magnetic properties including high coercivity, anisotropy, and magnetostriction. CoFe 2 O 4 particles have been earlier synthesized by mechanical milling and sintering of Fe 2 O 3 and Co 3 O 4 powders, 4 forced hydrolysis of ionic Co(II) and Fe(III) salts at 160 • C in polyol medium, 5 thermal decomposition and/or direct micellar routes from cobalt(II) acetylacetonate and iron(III)acetylacetonate, 6,7 co-precipitation and/or micelles methods from iron and cobalt chlorides, 8,9 hydrothermally from chlorides 10,11 or nitrates, 12 solvothermal from Fe chloride and Co acetate, 13 solution combustion 14 and sol-gel from cobalt and iron nitrates, 15,16 or cobalt and iron acetylacetonates. 3 Regarding other 3D structures based on cobalt ferrite, Co x Fe 3−x O 4 magnetic thin films have been grown on the pore walls of alumina membranes by atomic layer deposition. 17 Cobalt ferrite in thin film form has also been prepared by spray pyrolysis using cobalt and iron nitrate precursors. 18 Magnetization-field or Kerr loops with certain coercive force has earlier been recorded in CoFe 2 O 4 particles [3][4][5][6]8,10,14,16 and thin films, 18 in most cases at room temperature. Ferromagnetic CoFe 2 O 4 particles have been added to YBa 2 Cu 3 O 7-δ thin films to enhance flux pinning in potentially superconducting materials which, however, led to the formation of Y(Fe,Co)O 3 , leaving the host material yttrium-deficient. 19 On the other hand, dilute Fe and Co codoping into ZrO 2 has resulted in ferromagnetic behavior at room temperature for annealing temperatures above 900 • C, suggesting that partial formation of secondary phases, such as cobalt ferrite, promoted ferromagnetism. 20 Very small CoFe 2 O 4 particles with diameter 4-6 nm alone have also demonstrated magnetization decreasing toward higher temperatures and have thus been described as superparamagnetic. 6,9 Coercive field decreased substantially with increasing temperature, although saturative magnetization could be achieved at room temperature as well. 5 z E-mail: aile.tamm@ut.ee CoFe 2 O 4 particles embedded in SiO 2 /ZrO 2 host matrix have been studied and characterized as magneto-optical isolators and described as promising candidates for this application theoretically, with proposition to deposit corresponding composites by sol-gel method. 21,22 CoFe 2 O 4 nanoparticles forming a ferrofluid led through sol could therewith be prepared by co-precipitation of corresponding metal hydroxides. 23 The latter composite materials layers would be able to demonstrate considerable Faraday rotation with the degree increasing with the content of CoFe 2 O 4 particles. Zirconium oxide was herewith used to adjust the refractive index of the structure to meet the fundamental waveguiding conditions. It seems that nanocomposites consisting of CoFe 2 O 4 particles and ZrO 2 thin films grown by atomic layer deposition (ALD) have not yet been prepared. On the other hand, CoFe 2 O 4 -TiO 2 nanostructures have been elaborated using colloidal chemistry and ALD. 24 Electrospun CoFe 2 O 4 microfibers have also conformally coated by ALD-grown TiO 2 . 25 However, cobalt ferrite thin films have also been grown by ALD on porous monoclinic ZrO 2 supporting substrates. 26 This study was devoted to the application of atomic layer deposition for coating the cobalt ferrite nanoparticles with thin zirconium dioxide films. The structure, electrical and magnetic performances of the resulting composites were addressed. Potential of the composite material to demonstrate both electrical and magnetic polarization was evaluated.

Experimental
The particles were synthesized analogously to the method described by Ammar et al. 5 with slight modifications to the synthesis procedure. The synthesis procedure was following: 1.62 g (0.006 mol) of FeCl 3 · 6 H 2 O, 0.747 g (0.003 mol) of Co(CH 3 COO) 2 · 4 H 2 O and 2.214 g (0.027 mol) of CH 3 COONa were dissolved in 30.0 g propylene glycol. One gram of acetyl acetone was added, and the mixture was refluxed for five hours. After the completion of the reaction, the mixture was cooled and centrifuged. The particles were washed twice with acetone and dispersed in butanol.
X-ray photoelectron spectroscopy (XPS) was used for investigating the chemical state and elemental composition of cobalt iron oxide nanoparticle-based films after different treatments. XPS measurements were conducted using a surface station equipped with an electron energy analyzer (SCIENTA SES 100) and a non-monochromatic twin anode X-ray tube (Thermo XR3E2), with characteristic energy of 1253.6 eV (Mg K α1,2 FWHM 0.68 eV). All XPS measurements were conducted in Ultra-High Vacuum (UHV) with a base-pressure better than 8 × 10 −10 mbar. Size of CoFe 2 O 4 particles was characterized by small angle X-ray scattering (SAXS) analysis on diffractometer SmartLab (Rigaku) by using SAXS optics. Concentration of CoFe 2 O 4 particles in butanol was 4.5 mass % for all three samples (nr. 2,3,4). Butanol was used as a buffer for background correction. Mark capillary tubes (0.7 mm diameter) were used for sample holders. Scattering data analysis was performed by program NanoSolver (Rigaku).
ZrO 2 films were grown in a low-pressure (200-260 Pa) flowtype in-house built hot-wall ALD reactor 27 at 300 • C from zirconium tetrachloride, ZrCl 4 (Aldrich, 99.99%) and ozone, O 3 . Nitrogen, N 2 (99.999% purity, AGA), was applied as the carrier and purging gas. ZrCl 4 was evaporated at 170 • C from open boats inside the reactor. Ozone was produced from O 2 (99.999% purity, AGA) using BMT Messtechnik 802 N generator. The ozone concentration, measured by BMT Messtechnik 964 analyzer, was 245-250 g/m 3 in the experiments. The estimated ozone flow rate from the generator was about 67 sccm, while the carrier gas flow rate was kept at about 220 sccm. The ALD cycle times were 5 s for Zr precursor and ozone pulses as well as purge periods.
ZrO 2 films were grown on spin-coated CoFe 2 O 4 particles as well as reference Si(100) substrates covered by chemically grown 1-2 nm thick SiO 2 . In additon, highly-doped conductive Si substrates covered by 10 nm thick TiN film grown by chemical vapor deposition were used as bottom electrodes. For electrical measurements, the particles, covered with ZrO 2 film after spreading on TiN substrates, were supplied with platinum electrodes electron-beam evaporated on top of the films.
The crystal structure was evaluated by grazing incidence X-ray diffractometry (GIXRD), using an X-ray diffractometer Smart-Lab (Rigaku). X-ray fluorescence (XRF) spectrometer Rigaku ZSX 400 and program ZSX Version 5.55 was used to evaluate the elemental composition of films. Surface morphology of films and a crosssection of an ALD coated stack was evaluated by scanning electron microscopy (SEM) using a Dual Beam equipment (FEI Company) Helios NanoLab 600.
Electrical measurements were carried out by means of a semiconductor analyzer (Keithley 4200SCS), with samples put in a light-tight and electrically shielded box. The DC voltage was applied to the top electrode, being the bottom one grounded. To carry out the admittance parameters recording, a small signal of 30 mV r. m. s. was overlapped to the DC bias voltage. A parallel admittance model was selected which directly provides the real component value of the admittance (conductance, G) and the imaginary one (capacitance, C). The measurement frequency was varied between 1 kHz and 1 MHz. Additional electrical measurements were also carried out by means of an Agilent DXO-X 3104 digital oscilloscope with a built-in wave generator. Herewith the standard Sawyer-Tower experiment was carried out by applying a periodic triangular-shaped stimulus and recording the voltage loops data from the oscilloscope. Charge values were obtained from the sensed voltage across a stated capacitance.
Magnetometry was performed using Vibrating Sample Magnetometer (VSM) option of the Physical Property Measurement System 14T (Quantum Design) by scanning the magnetic field from −1 to 1 T parallel to the film surface at room temperature.

Results and Discussion
The thickness of ZrO 2 thin films grown using 150 ALD cycles was 15 nm, allowing one to consider average growth rate for the zirconium oxide as high as 0.1 nm/cycle. The average thickness of layer consisting of CoFe 2 O 4 particles spread on silicon substrate by spin-coating was 120 nm. Figure 1 presents SEM images of noncoated particles as well as ZrO 2 film deposited on particles.
Visual inspection of the nanocomposites revealed that the layer consisting of the ferrite particles only is, evidently, rather rough with features implying porous surface. ALD process of ZrO 2 films tended to conformally coat the particles without complete filling of the pores. However, the whole composite is not to be considered as porous throughout the whole 100-150 nm thickness, as the cross-section image has revealed appreciably uniform and continuous layer coating the silicon substrate (Fig. 1).
The small-angle X-ray scattering patterns of all samples were almost identical showing a fast decrease of intensity at low scattering angles (2θ < 0.6 • ) and a broad halo around scattering angles 0.7-2.0 (not shown). This shape of SAXS pattern did not allow approximations by using mono-modal size distribution function. Instead, a bimodal size distribution gave the best match to the observed scattering patterns with residual error R = 1.5%. The average diameters of particles corresponding to the first and second size distributions were 3.6(3) nm and 32(2) nm, respectively. Relative volume factions of both distributions were equal to 0.5(1) although the broadening of the first distribution was about two times smaller compared to the second distribution (Fig. 2).
It is not clear what exactly has caused the bimodal division of the ferrite particles between sets of two different average sizes. It might, however, be possible, that the smallest particles are those grown to the minimum sizes enabling short-range ordering and thus forming the nanocrystallites, whereas the larger particles may be formed because of the agglomeration of smaller ones neighboring each other.
In the XPS spectra of the CoFe 2 O 4 nanoparticles, Co 2p, and Fe 2p lines were clearly visible (Fig. 3). Also, O 1s and residual C 1s lines were recorded at 530 and 280 eV, respectively, and some slight traces of residual chlorine could be observed at 285 eV (not shown).
In the Co 2p spectrum, Co 2p3/2 peak was identified at 780.7 eV and Co 2p1/2 at 796.1 eV, in addition, satellite peaks were identified at 786.2 and 802.3 eV respectively. Based on the location of the Co 2p photo lines and the satellite structures, cobalt was identified as Co 2+ . 28,29 Fe 2p lines indicated that iron is in Fe 3+ oxidation state, while the existence of the satellite peak at 719.3 eV serves as clear evidence of Fe 3+ in the structures deposited. 30 On the ALD covered nanoparticle films, Zr 3d lines at 182.3 and 184.5 eV appeared (not shown), but Fe 2p and Co 2p lines were not visible any more (Fig. 2). Figure 4 depicts X-ray diffraction patterns from reference ZrO 2 films, CoFe 2 O 4 particles alone, and ZrO 2 /CoFe 2 O 4 composite layers.
The GIXRD patterns taken from reference ZrO 2 films (Fig. 4) 31 However, in the same study considerable permittivity dispersion was revealed, indicating the permittivity values as high as 200 and above that at measurement frequency at 1 kHz. This was proposedly arising from nonintrinsic polarization processes, such as interfacial polarization due to internal leakage and consequent formation of large capacitance on Schottky barriers at the oxide-electrode interfaces. 31 At frequencies exceeding 100 kHz the permittivity was reduced below 25, which was regarded as a reasonable value. Analogously, very large values of dielectric permittivity reaching and exceeding several hundreds were measured for powders consisting of CoFe 2 O 4 nanoparticles with sizes ranging from 6 to 30 nm. 32 The apparent permittivity values were found to increase markedly with the decrease in both particle size and measurement frequency, being indicative of the role for grain boundary conduction and accompanying interfacial polarization. Further, in another study on CoFe 2 O 4 sintered at 400 • C from sol-gel synthesized powders, initially consisting of particles with size 6-50 nm, the room-temperature permittivity was 18.6, measured at 1 kHz. 33 In the present study, one can, based on permittivity values obtained, thus also consider quite dense layer of ferrite particles formed with marked increment of effective permittivity at relatively low measurement frequencies due to considerable leakage currents in the composite. Figure 6 depicts the electroforming process and the subsequent bipolar switching of the current.
The application of a voltage of around 6.4 V with a current compliance of 2 mA, to avoid the irreversible breakdown of the dielectric films, probably led to the formation of conductive filaments. Then, the conduction between top and bottom electrodes was enabled and the switching process activated. I-V cycles could excellently be repeated but with a rather narrow window remained between ON and OFF states, less than one order of magnitude. The low resistance state (ON state) was achieved by applying a negative voltage bias of −2 V (Fig.  6), whereas the high resistance state (OFF state) was reached when applying a positive voltage of +1.5 V. Studies on CoFe 2 O 4 as resistive switching material have been relatively scarce compared to several other metal oxide films. Resistive switching behavior has been observed and described, e.g., in CoFe 2 O 4 films electrodeposited on porous alumina templates 34 and chemical solution deposited on planar platinum electrodes. 35 Figure 7 demonstrates capacitance-voltage and admittance (conductance) -voltage loops measured from the same composite sample. The measured magnitudes in there are conductance as the real part of the admittance, and capacitance as the imaginary part of the admittance.
Small signal parameters, both conductance and capacitance, also show switching loops with good repetitiveness, as it is shown in Fig. 7 for 20 consecutive cycles obtained at 100 kHz. As it has been recently demonstrated, 36 in samples exhibiting bipolar resistive switching, both the phase (conductance, G) and positive quadrature (capacitance, C) components of admittance show hysteretic behavior. Similarly, to set and reset current loops, G-V and C-V curves constitute two different lobes at positive and negative voltages, respectively. These loops were, in the present study, reproducible with an appreciable window for 20 consecutive cycles measured at 100 kHz. Figure 8 demonstrates capacitance and conductance loops as the components of admittance, measured separately against positive and negative sweeps of applied voltage. Both values were registered after returning to zero voltage after each programming voltage pulse. 37 Memory cycles were obtained by using a return-to-zero pulse voltage sequence: 0 V (1 ms) → programming voltage V of 1 ms → 0 V (1 ms), with the programming voltage V following a double ramp ranging from +3.5 V to −2 V with −0.1 V steps. Admittance signal was recorded at reading voltage of 0 V to discard any disturbance of the measurement process to possible conductive filaments responsible of the conduction. Figure 7 depicts the conductance and capacitance  values, respectively, measured at 0 V as functions of the programming voltage. The ON and OFF states are clearly distinguished. One can thus see, that two different states of memory can be sensed and fixed at 0 V after each programming cycle. Figure 9 depicts an electrical charge versus voltage loop measured using Sawyer-Tower circuit.
One can see in Fig. 9, that, despite the obvious hysteresis in the charge-voltage loop, saturation regime for the electrical polarization is not quite achieved at either polarity of the external field, as the polarized charge kept increasing with the voltage. This kind of charge behavior is hardly to be regarded as that characteristic of defined ferroelectric material, although very weak implications of certain tendency to saturative charge polarization might be recognized at the highest voltages used. The currents through the materials layer, as measured, have indeed been considerably high (Fig. 5). Most probably, most of the charge, responsible for the effectively measurable polarization in the material deposited, was that drifting in the electric field from an electrode to the counter electrode. That charge, either electronic or ionic, could become trapped at the interfaces between metal oxide layer and the electrode evaporated, giving rise to the interfacial polarization. Consequently, the electrical charge becomes carried to and trapped at the interface layer under certain polarity, and an opposite polarity with increasing, oppositely directed, field is required to release the charge from the traps and for the following drift toward the counter electrode. On the other hand, somewhat analogous polarization-field behavior has been observed and recorded in Ti-doped CoFe 2 O 4 ceramics 38 or in nanostructures containing thin layers Fe 2 O 3 and Co 3 O 4 , 39 or Er 2 O 3 and Fe 2 O 3, 40 and still considered promising in terms of multiferroic performance of the material.
The ferrite nanoparticles as well as the layer of nanoparticles covered by ZrO 2 film demonstrated saturate behavior in magnetizationfield curves, together with narrow hysteresis otherwise characteristic of soft ferromagnetic materials (Fig. 10).
CoFe 2 O 4 is a well-known magnetic material also possessing noticeable magnetocrystalline anisotropy within its cubic lattice. 41 In our study, the coercive fields, H C , for CoFe 2 O 4 nanopowder and CoFe 2 O 4 nanoparticle layer covered by ZrO 2 film were 130 and 70 Oe, respectively (Fig. 10). For comparison with literature data, saturation magnetization, remnant magnetization, and coercive field measured from powders of CoFe 2 O 4 nanoparticles with average size of 34 nm, synthesized by hydrothermal method have been 56.88 emu/g, 21.44 emu/g, and 507.8 Oe, respectively. 42 In another study, the corresponding values measured from hydrothermally synthesized and annealed powders consisting of particles with average size of 20 ± 2 nm reached 59-60 emu/g, 23 emu/g, and 570-650 Oe, respectively. 12 Figure 11 depicts magnetization-field curves measured from the nanocomposite CoFe 2 O 4 -ZrO 2 layer at room temperature (300 K) and at as low temperature as 5 K.
The saturation magnetization, M sat , in ferromagnetic polarization regime at 5.12 × 10 −5 emu. Coercivity value, H C , reached 17.3 (5) kOe. Remanent magnetization, M r , was estimated as high as 4.293 × 10 −5 emu. Thus, the squareness of the M-H loop, M r /M sat , was as high as 0.838. For comparison, in pellets consisting of Co 1.5 Fe 1.5 O 4 nanoparticles with the size ranging from 9 to 40 nm fabricated in co-precipitation process from Co(NO 3 ) 2 ·6H 2 O and Fe(NO 3 ) 3 ·9H 2 O salts, coercivity and squareness values of 11-16 kOe and 0.70-0.92, quite comparable to those obtained in the present study, were measured at 4 K. 45 The maximum values were obtained in the case of average nanograin size of approximately 16  It is to be considered, that both CoFe 2 O 4 and ZrO 2 layers may be magnetized separately. Also ZrO 2 , especially when stabilized dominantly in its metastable tetragonal polymorph, can demonstrate saturative magnetization and hysteresis in magnetization-field curves. 46 In our sample, humps apparent in the M-H loop during the increasing and decreasing magnetization (Fig. 10) may be indicative of the exchange coupling between magnetizing layers, i.e. bias effect due to the interaction between base CoFe 2 O 4 particle layer and the ZrO 2 film deposited on top of it, resulting in the re-orientation of the polarization in base and vicinal layers at different times during the overall polarization process under the increasing external field.

Summary
CoFe 2 O 4 nanoparticles with bimodal size distribution (mean sizes 3.6 and 32 nm), synthesized by sol-gel method, were spread by spincoating as uniform ca. 100 nm thick layer of particles on silicon substrates. The layer of particles was homogeneously coated by 15 nm thick, dominantly tetragonal/cubic, ZrO 2 film grown by atomic layer deposition. The nanocomposite layered structures formed were characterized in terms of their capacitive properties, current switching and magnetoelectric behavior. The capacitor-like samples demonstrated marked dielectric dispersion with relative permittivity 25 of the double CoFe 2 O 4 -ZrO 2 layer at 100 kHz and above that measurement frequency. Significant role for leakage currents through the material could be considered. The composite layer also demonstrated clear tendency to resistive switching behavior via alternate formation of states of two different current density values. Measurements in resistive switching regime became feasible after application of a forming voltage of around 6.4 V. Loops between extremal charge values of 300 nC, approximately, on capacitors of 0.204 mm 2 area were recorded also in the electrical polarization charge -voltage/field behavior, presumably also affected by the interfacial polarization during voltage sweeps. In addition, the composite layers could also be saturatively magnetized in external magnetic fields, demonstrating magnetization-field hysteresis loops with low coercivity characteristic of soft ferromagnetic like materials. At low temperatures, the coercivity exceeded 17 kOe. The squareness of the M -H loop was higher than 0.8. The clearest behavior attributable to that of potential memory materials was, however, achieved and demonstrated in the capacitance-conductance measurement regime, allowing one to achieve well-defined, highly quadratic, loops in admittance-programming voltage behavior, revealing two distinct functional states in the susceptibility of the external electric field.