CFO Films Research Articles

Structural, Magnetic, and Magnetostriction Properties of Flexible, Nanocrystalline CoFe2O4 Films Made by Chemical Processing.

We report on the simple, single-step, and cost-effective fabrication, characterization, and performance evaluation of cobalt ferrite (CoFe2O4; CFO) nanocrystalline (NC) thin films on a flexible mica substrate. The chemical solution-based drop-casting method employed to fabricate crystalline CFO films and their characterization was performed by studying the phase formation, surface morphology, and magnetic parameters, while sensor applicability was evaluated using combined magnetic and magnetostrictive properties. X-ray diffraction (XRD) indicates the single-phase and nanocrystalline nature of CFO films, where the crystallite size is ∼60 nm. The optimum conditions employed resulted in CFO NC films with surface particles exhibiting a spherical shape morphology with a homogeneous size distribution, as revealed by scanning electron microscopy analyses. Raman spectroscopic characterization of the chemical bonding indicates all of the active bands that are characteristic of the ferrite phase confirm the spinel structure, which is in agreement with XRD studies. The saturation magnetization (M S) and coercivity (H C), which are extracted from the field-dependent magnetization data, of CFO NC films were found to be 15.8 emu/g and 1.6 kOe, respectively, while the first-order magnetocrystalline anisotropy constant K 1 was ∼1.07 × 106 erg/cm3. The magnetostriction strain curve indicates that the CFO NC films exhibit a strain value of ∼86 ppm at an applied magnetic field of 8 kOe, indicating their suitability for flexible sensor devices.

Magnetoelastic constant of thin films determined by a four-point bending apparatus

We have developed a method to variably induce lattice strains and to quantitatively evaluate the induced magnetic anisotropy. Both tensile and compressive strains were introduced into epitaxial films of cobalt ferrite (CFO) grown on a single crystal MgO(001) substrate using a four-point bending apparatus made of a plastic material fabricated by a 3D printer. The change in magnetic anisotropy due to bending strain can be measured quantitatively by using the conventional magneto-torque meter. The strain-induced magnetic anisotropy increased with the tensile strain and decreased with the compressive strain as expected from a phenomenological magnetoelastic theory. The magnetoelastic constant obtained from the changes in bending strains shows quantitatively good agreement with that of the CFO films with a uniaxial epitaxial strain. This signifies that the magnetoelastic constant can be evaluated by measuring only one film sample with strains applied by using the bending apparatus.

Small magnetless integrated optical isolator using a magnetized cobalt ferrite film

We experimentally demonstrated a silicon optical isolator using a monolithically integrated cobalt ferrite (CFO) film as magneto-optical material, achieving an isolation ratio of 9.6dB near 1550nm wavelength. By virtue of the large Faraday rotation coefficient of CFO, the device has a compact footprint comparing to the current yttrium iron garnet-based isolators. Furthermore, we demonstrated self-biased isolation by the strong remanence of CFO films, eliminating the tedious demand of an external magnet. Cobalt ferrite represents a potential alternative approach to yttrium iron garnets to realize a practical device for on-chip isolation in silicon photonic integrated circuits because of the small footprint.

Standalone photoconversion of CO2 using Ti and TiOx-sandwiched heterojunction photocatalyst of CuO and CuFeO2 films

Artificial photoconversion of carbon dioxide to value-added chemicals remains a challenge. In this work, we synthesize heterojunction copper oxide and copper iron oxide (CuO/CuFeO2; CFO) films on transparent conducting substrates with metallic Ti layers, and deposit disordered TiOx particles on the CFO films. The Ti/CFO/TiOx samples convert CO2 into formate with ∼100% selectivity and drive O2 evolution under simulated sunlight in the absence of any electrical and chemical biases, achieving the overall solar-to-chemical energy conversion efficiency of ∼5%. The primary roles of the Ti underlayer are to create a robust contact between CFO and substrates and facilitate hole transfer. The TiOx (with unsaturated Ti atoms) delocalizes the electron density of the CFO and enhances electron transfer to the adsorbed CO2. Density functional theory calculations reveal that Ti/CFO/TiOx is the most suitable among the bare and modified CFO samples for efficient CO2 adsorption, formiloxyl intermediate formation, and formate desorption.

Magneto-ionic control of magnetism in two-oxide nanocomposite thin films comprising mesoporous cobalt ferrite conformally nanocoated with HfO2.

Advances in nanotechnology require of robust methods to fabricate new types of nanostructured materials whose properties can be controlled at will using simple procedures. Nanoscale composites can benefit from actuation protocols that involve mutual interfacial interactions on the nanoscale. Herein, a method to create nanoscale composite thin films consisting of mesoporous cobalt ferrite (CFO) whose pore walls are nanocoated with HfO2 is presented. Porous CFO films are first prepared by sol-gel. Atomic layer deposition is subsequently used to conformally grow a HfO2 layer at the surface of the pore walls, throughout the thickness of the films. The magnetic properties of uncoated and HfO2-coated CFO mesoporous films are then modulated by applying external voltage, via magneto-ionic effects. The CFO-HfO2 composite films exhibit an enhanced magnetoelectric response. The magnetic moment at saturation of the composite increases 56% upon the application of -50 V (compared to 24% for CFO alone). Furthermore, dissimilar trends in coercivity are observed: after applying -50 V, the coercivity of the composite film increases by 69% while the coercivity of the CFO alone decreases by 25%. The effects can be reversed applying suitable positive voltages. This two-oxide nanocomposite material differs from archetypical magneto-ionic architectures, in which voltage-driven ion migration is induced between fully-metallic and oxide counterparts. The synthesized material is particularly appealing to develop new types of magnetoelectric devices with a highly tunable magnetic response.

In situ observation of electric-field induced magnetic domain evolution in (Ba,Ca)(Ti,Zr)O3–CoFe2O4 multiferroic films

The integration of ferroelectric and ferromagnetic promises an essential strategy of obtaining high performance electronic devices. In this work, we demonstrate in situ observation of electric field induced magnetic domain structure evolution for 0.5Ba(Ti0.8Zr0.2)O3–0.5(Ba0.7Ca0.3)TiO3–CoFe2O4 (BZT–0.5BCT/CFO) films, which manifests the magnetoelectric (ME) coupling between ferroelectric BZT–0.5BCT and ferrimagnetic CFO. The multiferroic behaviors of BZT–0.5BCT/CFO bilayers thin films were characterized by measuring ferroelectric domains, ferroelectric and ferrimagnetic hysteresis loops. The magnetic domain structure were investigated as functions of electric field, when the sample is applied with a voltage of 3 V, approximately 49.2% of the magnetization domain was varied in CFO thin films. The modulation of the domain structure could be attributed to the strain-induced mechanical transduction between the ferroelectric and magnetic films and modulation of the electron density of the CFO films. Direct observation of electric field induced magnetic domain evolution is significant since it gives a direct evidence of magnetoelectric coupling effect.

Large negative uniaxial magnetic anisotropy in highly distorted Co-ferrite thin films

The strain induced magnetic anisotropy (MA) of epitaxial Co-ferrite (CFO) thin films grown on MgAl2O4(001) (MAO) by reactive sputtering was studied. These films underwent large tetragonal compressive strain due to the lattice mismatch between the substrates and films, resulting in tetragonalities of up to −0.04. Scanning transmission electron microscopy observation combined with fast Fourier transform analysis revealed that the lattice distortion monotonically relaxed with the increasing film thickness. Unlike the CFO(001) films on MgO(001) substrates, a magnetically enhanced layer exists at the interface between CFO and MAO. A large negative uniaxial MA energy of −5.9 MJ/m3 was confirmed for the thinnest film of 12.9 nm at 300 K by magneto-torque measurements. The induced uniaxial anisotropy decreased with the increasing film thickness owing to misfit relaxation. The magneto-elastic (ME) constant, which was determined by a fitting line for films with different thicknesses and therefore different tetragonalities, was 0.15±0.01 GJ/m3. This result is consistent with the value of 0.14 GJ/m3 for the bulk and suggests that the framework of the phenomenological ME theory for CFO is valid for a wide range of tetragonalities, at least up to −0.04. Our results also indicate that the misfit engineering of CFO thin films has great potential in inducing enhanced uniaxial MA.

Influence of oxygen vacancies and La0.7Sr0.3MnO3 layer on the structure and magnetic properties of cobalt ferrite thin films

CoFe2O4 thin film is expected to be used as the spin filter in spintronics. In order to prepare high-quality CoFe2O4 thin films for spin filtering, we studied the influence of oxygen vacancies on the structure and magnetic properties of cobalt ferrite (Co1−xFe2+xO4, CFO) films prepared in different atmosphere, and obtained the CFO films without significant soft magnetic phase under optimum Ar and O2 atmosphere condition. We further investigated the magnetic properties of the CFO/La0.7Sr0.3MnO3 (CFO/LSMO) bilayers containing various thicknesses of CFO films, and found that the CFO/LSMO bilayers with the ultrathin CFO films primarily display the soft magnetic properties.

Optical properties of spin coated Co0.6Zn0.4Mn0.3Fe1.7O4 thin films deposited on silicon and platinum coated silicon substrates

ABSTRACTThis work presents optical properties of Zn and Mn co-doped CFO (Co0.6Zn0.4Mn0.3Fe1.7O4) films deposited on Si and Pt coated Si substrates by spin coating technique. X-ray diffraction pattern of the films exhibited spinel cubic structure with polycrystalline phase. Optical characteristics of the CZFMO films are studied using photoluminscence and UV-visible spectroscopy. With the rise in annealing temperature, quenching in PL emission was observed for films grown on silicon; whereas a marginal enhancement in emission was observed for films deposited on the Pt / Si substrate. Optical band gap of the films are found to vary between 2.3 – 2.7 eV range.

Annealing pressure induced ions transfer in Cobalt-Ferrite thin films on amorphous SiO2/Si substrates

In this work, Cobalt-Ferrite (CFO) films were grown on silicon substrates with 300nm amorphous silicon dioxide by Pulsed Laser Deposition (PLD) with different annealing conditions. The results of structural analysis prove that the CFO films have high crystalline quality with (111) preferred orientation. The Raman spectra and X-ray absorption spectra (XAS) indicate that the Co ions can transfer from tetrahedral sites to octahedral sites with increasing the annealing pressure. The site exchange of Co and Fe ions leads to the change of saturation magnetization in the CFO films. Our experiments provide not only a way to control the magnetism of CFO films, but also a suitable magnetic layer to develop silicon and semiconductor based spintronic devices.

Flexible Heteroepitaxy of CoFe2O4/Muscovite Bimorph with Large Magnetostriction.

A bimorph composed of ferrimagnetic cobalt ferrite (CoFe2O4, CFO) and flexible muscovite was fabricated via van der Waals epitaxy. The combination of X-ray diffraction and transmission electron microscopy was conducted to reveal the heteroepitaxy of the CFO/muscovite system. The robust magnetic behaviors against mechanical bending were characterized by hysteresis measurements and magnetic force microscopy, which maintain a saturation magnetization (Ms) of ∼120-150 emu/cm3 under different bending states. The large magnetostrictive response of the CFO film was then determined by digital holographic microscopy, where the difference of magnetostrction coefficient (Δλ) is -104 ppm. The superior performance of this bimorph is attributed to the nature of weak interaction between film and substrate. Such a flexible CFO/muscovite bimorph provides a new platform to develop next-generation flexible magnetic devices.

Strain and Ferroelectric-Field Effects Co-mediated Magnetism in (011)-CoFe2O4/Pb(Mg1/3Nb2/3)0.7Ti0.3O3 Multiferroic Heterostructures.

Electric-field mediated magnetism was investigated in CoFe2O4 (CFO, deposited by reactive cosputtering under different oxygen flow rates) films fabricated on (011)-Pb(Mg1/3Nb2/3)0.7Ti0.3O3 (PMN-PT) substrates. Ascribed to the volatile strain effect of PMN-PT, the magnetization of the CFO films decreases along the [01-1] direction whereas it increases along the [100] direction under the electric field, which is attributed to the octahedron distortion in the spinel ferrite. Moreover, a nonvolatile mediation was obtained in the CFO film with low oxygen flow rate (4 sccm), deriving from the ferroelectric-field effect, in which the magnetization is different after removing the positive and negative fields. The cooperation of the two effects produces four different magnetization states in the CFO film with low oxygen flow rate (4 sccm), compared to the only two different states in the CFO film with high oxygen flow rate (10 sccm). It is suggested that the ferroelectric-field effect is related to the oxygen vacancies in CFO films.

Study on the multiferroic properties of 2-2 type of BiFeO3-CuFe2O4 thin films

The 2-2 type BiFeO3-CuFe2O4(BFO-CFO) bilayer thin films were deposited on FTO/glass substrates by sol-gel method, using Bi(NO3)3·5H2O, Fe(NO3)3·9H2O and Cu(NO3)3·3H2O as the raw materials. The structure, surface morphologies and electric properties of the thin films were investigated. The XRD patterns show that the structure of BFO is the distorted rhombohedral perovskite with R3c:H (161) space lattice, CFO is the tetragonal spinel with type I41/amd (141) space lattice. The interface is obvious between the CFO and BFO films, which shows that the BFO and CFO have no inter-diffusion phenomena. When the frequency is 10kHz, the dielectric loss peak is consistent with the dielectric relaxation of Wagner Maxwell. The leakage current density of BFO-CFO under positive and negative bias voltages is asymmetric, and there is a hysteresis phenomenon in the positive bias voltage. The results of XPS show that the Fe2+ of the BFO-CFO is decreased and the leakage current of the film is improved. The saturation magnetization (Ms) of BFO-CFO is 25.8emu/cm3 at room temperature, which is about 40 times of the pure BFO.

Spin transport and temperature-dependent giant positive junction magnetoresistance in CoFe2O4/SiO2/p-Si heterostructure

The observation of giant positive junction magnetoresistance (JMR) in our cobalt ferrite (CFO)/p-Si heterojunction has been reported here. The pulsed laser deposition technique has been used for fabrication of the heterojunction. The junction confirms a very good rectifying magnetic diode like behavior at low temperature, whereas at high temperatures the junction shows nonlinear I–V characteristics. The magnetic field-dependent magnetoresistance (MR) of both CFO film and across the heterojunction have been studied in detail. The CFO film shows negative MR behavior, whereas the junction shows large positive JMR behavior throughout the temperature range. The spin lifetime (142 ps) and spin diffusion length (331 nm) have been estimated of the heterostructure at 10 K. The highest JMR value (~1600 %) has been observed at 10 K, and it gradually decreases at higher temperature range. The origin of positive JMR in our heterojunction has been best explained by the standard spin injection theory.

Magnetic Properties of Cobalt Ferrite (001) Films Grown on Spinel-Type Buffer Layers

To improve the interface magnetism of Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3-x</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> (CFO) films that are grown on MgO substrates and exhibit extremely large perpendicular magnetic anisotropy, we attempted to insert two different spinel-type oxides [CoCr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> (CCO) and MgTi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> (MTO)] between the CFO films and their MgO(001) substrates. Although both buffer layers were epitaxially grown on MgO(001), the CFO films grown on CCO/MgO(001) became polycrystalline with a significantly lower level of magnetization, while the magnetization of the CFO films on MTO/MgO(001) fell as a function of the thickness of the MTO layer. A secondary ion mass spectroscopy analysis revealed a remarkable interdiffusion of Co and Mg.

Epitaxial Growth of Co&lt;sub&gt;0.75&lt;/sub&gt;Fe&lt;sub&gt;2.25&lt;/sub&gt;O&lt;sub&gt;4&lt;/sub&gt;/NiO Bilayer on MgO(001) Substrate

Co x Fe 3−x O 4 (CFO) films epitaxially grown on MgO(001) substrates show perpendicular magnetic anisotropy as high as 10 Merg/cm3[1]. However, magnetization is dependent on thickness. The magnetization per unit area linearly increases with increasing thickness of the films. Although the slope corresponds to the bulk magnetization, the slope intersects the horizontal (thickness) axis, suggesting that a magnetic dead layer exists at the interface/surface of the films. There are several possible reasons for the decrease in magnetization at the interface such as diffusion of Mg from the substrates, existence of anti-phase boundaries (APBs), and so on. In order to prevent the formation of the dead layer and improve the magnetic properties at the interface, we investigated the effect of a NiO insertion layer on the magnetization of CFO films. It has been reported that the insertion layer of NiO prevents Mg diffusion from MgO substrates in Fe 3 O 4 /MgO(001)[2]. Since NiO is isostructural with MgO, we can expect that APBs in CFO films are formed on the NiO insertion layers as well as on the MgO surfaces.

Magnetic field-dependent polarization of (111)-oriented PZT–Co ferrite nanobilayer: Effect of Co ferrite composition

The perfect (111)-oriented PZT/CFO (CFO=CoFe2O4, Co0.8Fe2.2O4 and Co0.6Mn0.2Fe2.2O4) bilayer multiferroic thin films were grown on Pt(111)/Si substrate at 600°C using pulsed laser deposition technique. The precision X-ray diffraction analysis (avoiding the shift of peak due to the sample misalignment) revealed that the CFO films on Pt(111)/Si substrate were under an out-of-plane contraction and deposition of PZT top layer led to more increase in the out-of-plane contraction, i.e. increase in the residual stresses. The PZT and CFO layers have significant effects on magnetic and ferroelectric properties of PZT/CFO bilayer films, respectively, leading to an enhanced in-plane magnetic anisotropy as well as increased and asymmetric polarization. The effect of composition of CFO layer on magnetic field-dependent polarization of PZT/CFO bilayer films was investigated by applying the magnetic field during P-E measurement. The polarization of PZT films were increased by applying the magnetic field as a result of strain transferred from magnetostrictive CFO underlayer. This increase in polarization for PZT/Co0.6Mn0.2Fe2.2O4 was higher than that for PZT/Co0.8Fe2.2O4 and both of them were significantly higher than that for PZT/CoFe2O4 bilayer film, which was discussed based on their magnetostriction properties.

Effect of thickness on the stress and magnetoelectric coupling in bilayered Pb(Zr0.52Ti0.48)O3-CoFe2O4 films

Magnetoelectric bilayered Pb(Zr0.52Ti0.48)O3-CoFe2O4(PZT-CFO) films with different PZT thicknesses were grown on (111)Pt/Ti/SiO2/Si substrates using chemical solution spin-coating. Structural characterization by X-ray diffraction and electron microscopy shows pure phases and well-defined interfaces between the PZT and CFO films. The CFO-PZT-substrate structure effectively alleviates the substrate clamping effect for the CFO layer, showing appreciable magnetoelectric responses in the composite films. Both the direct magnetoelectric effect and the magnetic field-induced Raman shifts in the A1(TO1) soft mode of PZT demonstrate the magnetic-mechanical-electric coupling in the films. The results also indicate that with a constant CFO layer thickness, the thickness of the PZT layer plays an important role in the stress relaxation and strong magnetoelectric coupling. The coupling could be further enhanced by increasing the CFO thickness, optimizing the volume (thickness) fraction of the PZT thickness, and releasing the clamping effect from the substrate.

Enhancement of in-plane magnetic anisotropy in (111)-oriented Co0.8Fe2.2O4 thin film by deposition of PZT top layer

The CoFe2O4 and Co0.8Fe2.2O4 single layer (CFO) as well as PZT/CoFe2O4 and PZT/Co0.8Fe2.2O4 bilayer thin films were grown using the pulsed laser deposition technique on Pt(111)/Si substrates at 600 °C. All films had a perfect (111)-orientation and the degree of orientation of CFO films was improved by the deposition of a PZT top layer. Precision X-ray diffraction analysis (avoiding the shift of peaks due to sample misalignment) revealed that the CFO films on Pt(111)/Si substrate were under an out-of-plane contraction and the deposition of a PZT top layer led to the increase in the out-of-plane contraction. The (111)-oriented CFO single layer films had a strong in-plane magnetic anisotropy as a result of orientation as well as the stress-induced magnetic anisotropy. The magnetic properties of CFO film were altered by the deposition of a PZT top layer leading to the enhancement of in-plane magnetic anisotropy. The enhanced in-plane magnetic anisotropy was more detectable in PZT/Co0.8Fe2.2O4 rather than PZT/CoFe2O4 bilayer film, which could be expected from its higher magnetocrystalline as well as magnetostriction constants.

Preparation and multiferroic properties of 2-2 type CoFe2O4/Pb(Zr,Ti)O3 composite films with different structures

2-2 Type layered CFO/PZT (CoFe2O4/Pb(Zr0.52Ti0.48)O3) magnetoelectric composite films with four different structures were prepared on Pt/Ti/SiO2/Si substrates via a sol–gel method. These films annealed at 700°C contain PZT and CFO phase without impurity phases. The prepared composite films exhibit 2-2 type layered structures with obvious interfaces and no diffusions exist between CFO and PZT films. Ferromagnetic and ferroelectric responses were simultaneously observed in the composite films. These composite films exhibit good magnetoelectric coupling effects and the magnetoelectric voltage coefficients (αE) increase with increasing the volume contents of CFO films in composite films. The αE value of composite film (2PZT/4CFO/2PZT/4CFO/2PZT) reaches a maximum (227mVcm−1Oe−1) among all the prepared composite films.