CoFe2O4 Layer Research Articles

Strain-insensitive ferromagnetic SrRuO3 thin films with ferrimagnetic CoFe2O4 buffer layer

Flexible electronics, such as wearable devices and biosensors, require materials that maintain their properties under mechanical stress. A recent study addresses this by focusing on SrRuO3 (SRO) thin films, which typically suffer reduced coercivity under strain. Herein, we introduce a novel approach by using CoFe2O4 (CFO) as a buffer layer in SRO/CFO/F-mica heterostructures to address this issue. When subjected to a strain of up to ±0.553 %, these heterostructures displayed a mere 11 % variation in saturation magnetic moment and coercive field, significantly outperforming SRO/BaTiO3 configurations, which showed a 95 % reduction in coercivity at only −0.3 % strain. This result demonstrates the effectiveness of the CFO layer in stabilizing the magnetic properties of SRO films against external mechanical deformations. These findings mark a significant advancement in the development of mechanically robust thin films for complex oxide heterostructures in flexible device applications.

Simulation of the resistance switching performance and synaptic behavior of TiO2-based RRAM devices with CoFe2O4 insertion layers.

The electrothermal coupling model of Pt/CoFe2O4/TiO2/TiN devices was established to study their resistive switching characteristics and basic biological synaptic properties in our research. The processes of set and reset are simulated, and the distribution of the temperature, the electric field and the concentration of oxygen vacancies in the dielectric layer are obtained. The switching performance of the TiO2-based device is significantly improved after the CoFe2O4 layer is inserted, with the switching voltage, working current and power consumption being reduced, while the switching ratio is increased. By changing the thermal conductivity of the top electrode, the rupture position of the conductive filament can be controlled. The I-V characteristics of the Pt/CoFe2O4/TiO2/TiN device during the reset and set processes are fitted linearly in logarithmic coordinates, and the ohmic conduction mechanism or the space-limited charge conduction mechanism is mainly satisfied in the high and low resistance states. Finally, the application of dual-layer devices on biological synapses is studied, and the basic biological characteristics of enhancement, inhibition and paired pulse promotion are simulated successfully. In addition, the redox reaction induced by oxygen vacancy migration also promotes the formation and rupture of the conductive filament. Results of the study show this ferrite material as an insertion layer in a resistive random-access memory structure that offers potential for future information storage and bioneuromorphic computation devices.

The oxidation of an equimolar FeCoNiAl medium-entropy alloy at 900 °C in various oxygen pressures

The oxidation of an equimolar FeCoNiAl medium-entropy alloy was studied at 900 °C in various O2-containing atmospheres over the oxygen pressure range of 10–105 Pa. The alloy oxidized parabolically, having its oxidation rates steadily increasing with oxygen pressure. Duplex scales formed on the oxidized substrate, consisting of an outer CoFe2O4 layer and an inner α-Al2O3 layer. The position of the Pt-marker was located at the outer- and inner-layer boundary, demonstrating that the outward cation diffusion controlled the formation of CoFe2O4, while the growth of Al2O3 was contributed by the inward oxygen diffusion.

Influence of LiNbO3 layer thickness on structural and dielectric properties of CoFe2O4\\LiNbO3 multiferroic bilayers prepared by laser ablation

Multiferroic bilayer thin films composed by LiNbO3 and CoFe2O4 were prepared by laser ablation over (100) platinum-covered Si substrates, to understand the influence of the LiNbO3 layer thickness on their structural, electric and dielectric properties. The films were dense and polycrystalline, presenting rhombohedral LiNbO3 and cubic spinel CoFe2O4 layers. The CoFe2O4 unit cell experienced compression along the growth direction of the films, whereas the LiNbO3 layer exhibited a structural distortion in its unit cell. These strain effects were due to the mechanical interaction between both layers and to the presence of a higher amount of point defects in the thinner LiNbO3 samples, which gradually relaxed as the LiNbO3 layer thickness increased. The DC conductivity and activation energies of LiNbO3, determined from low-frequency dielectric permittivity behavior, were found to be dependent on the layer thickness. Thicker LiNbO3 layers displayed bulk-like characteristics, with a transition from polaronic to ionic conduction with increasing temperature. In contrast, thinner LiNbO3 layer films displayed polaronic behavior up to higher temperatures, due to the formation of bound polarons associated with lithium defects during deposition. AC conductivity analysis revealed that the CoFe2O4 layer conductivity mechanism primarily involved hopping between ions located at octahedral sites in the ferrite. These findings provide valuable insights into structure-function relationships and dynamical behavior within multiferroic bilayer thin films, promoting their applicability for emerging magneto-electric technologies.

Photoelectrocatalytic peroxymonosulfate activation over CoFe2O4-BiVO4 photoanode for environmental purification: Unveiling of multi-active sites, interfacial engineering and degradation pathways

This work reported on the development of CoFe2O4-BiVO4 photoanode based photoelectrocatalytic system collaborating with peroxymonosulfate activation for organic contaminants removal. CoFe2O4 layer not only provided active sites for direct peroxymonosulfate activation but also accelerated charge separation process for the enhancement of photocurrent density and photoelectrocatalytic performance. Junction of CoFe2O4 layer on BiVO4 photoanode led to the improvement of photocurrent density to 4.43 mA/cm2 at 1.23 VRHE, which was approximately 4.06 times higher than that of pure BiVO4. Subsequently, the corresponding optimal degradation efficiency toward the tetracycline model contaminant achieved to be 89.1% with total organic carbon removal value of about 43.7% within 60 min. Moreover, the degradation rate constant of CoFe2O4-BiVO4 photoanode in photoelectrocatalytic system was 0.037 min−1, which was about 1.23, 2.64 and 3.70 times higher than the values in photocatalysis, electrocatalysis and PMS only based systems, respectively. In addition, radical scavenging experiments and electron spin resonance spectra indicated a synergy of radical and nonradical coupling process where •OH and 1O2 played vital roles during tetracycline degradation. Plausible photoelectrocatalytic mechanism and degradation pathway were proposed. This work provided an effective strategy to construct peroxymonosulfate assisted photoelectrocatalytic system toward green environmental applications.

Microstructure and diffusion behavior of Co-Ni-W conversion coating for metallic interconnect of solid oxide fuel cell

To improve the oxidation resistance and electrical conductivity of the interconnects, a new Co-Ni-W alloy layer containing 5.0 at.% W electroplated on the surface of SUS 430 stainless steel and oxidized at 800°C in air for 1000 h was studied. It was confirmed that the oxide layer structure eventually evolves into six oxide layers after oxidation, namely Co3O4, CoFe2O4, NiCo2O4/CoWO4, (Co,Fe,Ni)3O4, (Cr,Fe,Co)3O4, and Cr2O3 layer by scanning electron microscope, transmission electron microscope and X-ray diffraction. The four Cr-free oxide layers completely prevented the outward diffusion of Cr. Furthermore, the Co-Ni-W coating significantly reduced the area specific resistance of sample.

All‐alkoxide based deposition and properties of a multilayer La0.67Sr0.33MnO3/CoFe2O4/La0.67Sr0.33MnO3 film

AbstractSingle and multilayer films of La0.67Sr0.33MnO3 and CoFe2O4 were deposited by spin‐coating. The all‐alkoxide precursors allowed inorganic gel films of extreme homogeneity to be formed and converted to phase pure complex oxides at low temperatures. La0.67Sr0.33MnO3 films were made with La‐ and Ca‐methoxy‐ethoxides and Mn19O12(moe)14(moeH)10 as precursors at 800 °C. The CoFe2O4 films were obtained at extremely low 275 °C, using a new CoFe2‐methoxyethoxide precursor. The decomposition and microstructural development on heating was described by TG, TEM, XRD and IR spectroscopy. XRD showed no spurious phases and the unit‐cell dimensions coincided quite well with literature values of the targeted phases. The structural, magnetic and electronic properties of these films established their phase purity and high quality with physical properties comparable to films deposited by physical deposition methods. The magnetic and magneto transport results are presented for single, bi‐ and tri‐ layer structures. The magnetically soft La0.67Sr0.33MnO3 layer was exchange coupled to the magnetically hard CoFe2O4 layer, giving rise to interesting switching behaviour in magnetism and magneto‐transport properties.

CoFe2O4 nanocrystals for interface engineering to enhance performance of perovskite solar cells

Interface engineering has been widely applied to perovskite solar cells (PSCs) for solving the problems caused by high density of parasitic traps and the defects that exist at grain boundaries and surfaces. Various inorganic metal oxides play roles in PSCs, helping to achieve higher efficiency and better long-term stability. In this work, two-phase hydrothermal method is used for CoFe2O4 nanocrystals (NCs) synthesis and the obtained NCs were used to modify the surface between perovskite (PVK) and hole transport layer (HTL). Results show that CoFe2O4 modified devices have enhanced hole extraction, reduced trap density, and hydrophobic ligand connected to CoFe2O4 NCs provide better moisture resistance to PVK layer. Devices with CoFe2O4 layer exhibit a best PCE of 19.65% with improved open circuit voltage (VOC), fill factor (FF) and reproducibility. At steady-state, CoFe2O4 modified devices show a current density of 20.16 mA/cm2 and PCE of 18.97%.

Dynamical Strain-Driven Phase Separation in Flexible CoFe2O4/CoO Exchange Coupling System.

Epitaxial CoFe2O4(CFO)/CoO bilayers were fabricated by pulsed laser deposition on flexible muscovite mica substrate. Samples with different CFO thicknesses were employed to study the phenomenon of exchange bias involving strongly anisotropic ferromagnet. Magnetic measurements exhibited great enhancement in the features of exchange bias. Raman and X-ray absorption spectroscopies indicated that a new phase emerged within the CFO layer because of the cation charge redistribution in CFO layer under bending, which in turn gave rise to anomalous hysteresis loops exhibited in the bent bilayers. These results provide a fundamental understanding about the mechanisms of exchange bias prevailing in these bilayers and call attention to the implementation of spintronic devices using flexible heterostructures such as the present CFO/CoO bilayers.

Magnetic coupling modulation of flexible Fe3O4/CoFe2O4 heterojunction grown on mica

Recently, flexible wearable devices have attracted enough attention due to their outstanding potential in practical applications. As two classic ferrite materials, Fe3O4 and CoFe2O4 (CFO) have been introduced in many applications such as catalyst, magnetic information and communication technologies. In this paper, a layer of CFO is epitaxially grown on a mica substrate deposited by pulsed laser deposition (PLD), and then a Fe3O4 films is further deposited, using sintered CFO and Fe3O4 poly crystalline targets. The as-deposited film material and surface roughness were characterized by X-ray diffraction (XRD) and atomic force microscopy (AFM). By measuring the hysteresis loop of Fe3O4/CFO film, the magnetic properties of films with different curvatures were studied. The relationship between the thickness ratio of the CFO layer to the Fe3O4 layer and the magnetic properties of the thin film crystals was studied, and the tunability of the function was proved.

Fabrication of reactive flat-sheet ceramic membranes for oxidative degradation of ofloxacin by peroxymonosulfate

A novel CoFe2O4-decorated flat-sheet ceramic membrane (CFCM) was prepared via a facile one-step hydrothermal method and utilized for peroxymonosulfate (PMS) activation in the catalytic degradation of ofloxacin (OFX) in a dead-end filtration mode. Several characterization methods confirmed the successful deposition of a CoFe2O4 layer on the surface of a pristine Al2O3 ceramic membrane. The CFCM possessed a considerably smaller average pore size (45 nm) and contact angle (20°) than a pristine Al2O3 flat-sheet ceramic membrane (80 nm and 50°) after modification. The catalytic degradation results revealed that nearly 100% removal of 40 μM OFX could be achieved within 20 min at pH 6.0, with 2 mM PMS and 100 kPa of transmembrane pressure (TMP). Moreover, the CFCM suffered little interference from co-existing SO42− and Cl− in the water matrix, but was significantly hindered by HCO 3− and humic acid. The low concentration of metal-leaching by CFCM will makes it reliable for catalytic degradation processes. Sulfate radicals were found to be the predominant reactive radicals that drove OFX degradation, and according to density functional theory (DFT) calculations they were generated via electron transfer from CoFe2O4 to chemisorbed PMS.

Annealing heating rate dependence of microstructure and multiferroic properties in Bi4Ti2.9Fe0.1O12/CoFe2O4 layered magnetoelectric composite films prepared by chemical solution deposition method

Lead-free magnetoelectric composite films of CoFe2O4 (CFO) and Bi4Ti2.9Fe0.1O12 (BTF) were prepared on the LaNiO3-coated Si substrate by chemical solution deposition. Though the CFO layer randomly grows on the LaNiO3 layer, the oriented growth of the BTF layer can be regulated by rapid annealing treatment. The results indicated that the annealing heating rate significantly affects the c-axis orientation of the BTF layer, thereby resulting in obvious changes in microstructure, leakage behavior, dielectricity and ferroelectricity of the BTF/CFO films. The BTF/CFO film prepared at the heating rate of 40 °C/s presents the most compact structure, minimum leakage current, maximum dielectric constant and the highest residual polarization of 28.8 μC/cm2. Moreover, the excellent magnetoelectric coupling performance along with the highest magnetoelectric coupling voltage coefficient αE of 20.1 V/cm∙Oe can be achieved in these conditions.

Fabrication and thickness-dependent magnetic studies of tunable multiferroic heterostructures (CFO/LSMO/LAO)

This study investigates the structural and magnetic properties of LaAlO3(LAO)/La0.7Sr0.3MnO3 (LSMO)/CoFe2O4(CFO) heterostructures grown by pulsed laser deposition. X-ray diffraction analysis illustrates the crystalline oxide growth and the rocking curve measurement along the (200)-peak of LSMO show a small FWHM (0.37°), which indicates an excellent out-of-plane orientation of the film. Transmission electron and scanning electron energy-dispersive X-ray spectroscopy confirms the homogeneous distribution of structural and elemental composition of the heterostructure. From the static magnetic measurements in the LSMO films at room temperature, we report the presence of a ferromagnetic hysteresis loop with a coercive field of 327 Oe. In addition, we have examined the effect of CFO layer thickness on the ferromagnetism of LSMO and obtained improved magnetic properties with a maximum coercive field of 792 Oe in CFO/LSMO heterostructure for an optimum CFO thickness of ~ 40 nm. From the dynamic magnetic measurements using ferromagnetic resonance spectroscopy, we have obtained a low Gilbert damping parameter of ~ 0.037 for the LSMO/LAO structure due to spin–orbit coupling. Besides, the small gyromagnetic ratio (0.002 GHz/Oe) of the same film is quite promising to design spin valve and magnetic tunneling-based devices.

Structural transition, defect complexes and improved ferroelectric behaviors of Bi0.88Sr0.03Gd0.09Fe0.94Mn0.04Co0.02O3/Co1-xMnxFe2O4 bilayer thin films

Bi0.88Sr0.03Gd0.09Fe0.94Mn0.04Co0.02O3/Co1-xMnxFe2O4 (BSGFMC/CMxFO) bilayer thin films were successfully prepared by the sol-gel method. There exists a structural transition from the co-existence of trigonal-R3c:H and trigonal-R3m:R phases to a single trigonal-R3m:R phase in the BSGFMC layer, which is induced by Mn-doping in the CoFe2O4 layer. The amount of oxygen vacancies in the bilayer films is reduced by Mn-doping, leading to a decreased amount of defect complexes. The dielectric dispersion of the BSGFMC/CMxFO bilayer films also disappears with the increase of Mn content. The BSGFMC/CM0.3FO bilayer film exhibits saturated polarization at a lower applied voltage compared with the other samples, while, a large remnant polarization (106 μC cm−2) and a small coercive field (263 kV cm−1) are obtained in this bilayer film. The capacitance-voltage behaviors further confirm its impressive ferroelectric performance. The excellent ferroelectric properties of the BSGFMC/CM0.3FO bilayer film mainly result from the presence of fewer defect complexes and the structural transition in the BSGFMC layer. In addition, the BSGFMC/CM0.3FO bilayer film also displays a giant saturation magnetization with 45.91 emu cm−3. The superior multiferroic properties provide a potentially enable application in novel multiferroic devices.

Room temperature magnetoelectric and magnetodielectric properties of 2–2 bilayer 0.50Pb (Ni1/3Nb2/3) O3–0.35PbTiO3–0.15PbZrO3/CoFe2O4 thin film

A 2–2 bilayer composite 0.50Pb(Ni1/3Nb2/3)O3–0.35PbTiO3–0.15PbZrO3(PNNZT)/CoFe2O4 (CFO) thin film has been fabricated using a pulsed laser deposition technique. Due to the coexistence of ferromagnetic (FM) order in the CFO layer and ferroelectric (FE) order in the PNNZT layer, the product property, i.e., the magnetoelectric (ME) coupling has been realized with a ME voltage coefficient (αE) of ~500mV/cm.Oe and a magnetodielectric response (MD(%) of 0.27 at room temperature. The present new composite thin film with its enhanced ME and MD properties could be a useful material for developing ME-based devices.

Magnetoelectricity of CoFe2O4 and tetragonal phase BiFeO3 nanocomposites prepared by pulsed laser deposition

The coupling between the tetragonal phase (T-phase) of BiFeO3 (BFO) and CoFe2O4 (CFO) in magnetoelectric heterostructures has been studied. Bilayers of CFO and BFO were deposited on (001) LaAlO3 single crystal substrates by pulsed laser deposition. After 30 min of annealing, the CFO top layer exhibited a T-phase-like structure, developing a platform-like morphology with BFO. Magnetic hysteresis loops exhibited a strong thickness effect of the CFO layer on the coercive field, in particular along the out-of-plane direction. Magnetic force microscopy images revealed that the T-phase CFO platform contained multiple magnetic domains, which could be tuned by applying a tip bias. A combination of shape, strain, and exchange coupling effects are used to explain the observations.

Communication—Reactive Sintering of a CoFe2O4 Coating on Ferritic Stainless Steels for SOFC Interconnect Application

A mixed Fe+CoO precursor was successfully utilized to synthesize a dense CoFe2O4 spinel coating on the Crofer 22 APU interconnect via reactive sintering at 900°C for 2 h in air without the use of any reduction treatment. Both the uncoated and coated samples were subject to thermal oxidation at 850°C for 500 h in air. Phase constituents, microstructure, and area specific resistance (ASR) of the uncoated and coated samples before and after the oxidation testing were evaluated. The CoFe2O4 layer effectively reduced the Cr2O3 scale growth and lowered the scale ASR.

Electronic structure and magnetic properties of magnetically dead layers in epitaxial CoFe2O4/Al2O3/Si(111) films studied by x-ray magnetic circular dichroism

Epitaxial CoFe2O4/Al2O3 bilayers are expected to be highly efficient spin injectors into Si owing to the spin filter effect of CoFe2O4. To exploit the full potential of this system, understanding the microscopic origin of magnetically dead layers at the CoFe2O4/Al2O3 interface is necessary. In this paper, we study the crystallographic and electronic structures and the magnetic properties of CoFe2O4(111) layers with various thicknesses (thickness d = 1.4, 2.3, 4, and 11 nm) in the epitaxial CoFe2O4(111)/Al2O3(111)/Si(111) structures using soft X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) combined with cluster-model calculation. The magnetization of CoFe2O4 measured by XMCD gradually decreases with decreasing thickness d and finally a magnetically dead layer is clearly detected at d = 1.4 nm. The magnetically dead layer has frustration of magnetic interactions which is revealed from comparison between the magnetizations at 300 and 6 K. From analysis using configuration-interaction cluster-model calculation, the decrease of d leads to a decrease in the inverse-to-normal spinel structure ratio and also a decrease in the average valence of Fe at the octahedral sites. These results strongly indicate that the magnetically dead layer at the CoFe2O4/Al2O3 interface originates from various complex networks of superexchange interactions through the change in the crystallographic and electronic structures. Furthermore, from comparison of the magnetic properties between d = 1.4 and 2.3 nm, it is found that ferrimagnetic order of the magnetically dead layer at d = 1.4 nm is restored by the additional growth of the 0.9-nm-thick CoFe2O4 layer on it.

Absence of magnetic proximity effects in magnetoresistivePt/CoFe2O4hybrid interfaces

Ultra-thin Pt films grown on insulating ferrimagnetic CoFe2O4 (111) epitaxial films display a magnetoresistance upon rotating the magnetization of the magnetic layer. We report here X-ray magnetic circular dichroism (XMCD) recorded at Pt-L2,3 and Pt-M3 edges. The results indicate that the Pt magnetic moment, if any, is below the detection limit (< 0.001 {\mu}$_B$/Pt), thus strongly favoring the view that the presence of CoFe2O4 does not induce the formation of magnetic moments in Pt. Therefore, the observed magnetoresistance cannot be attributed to some sort of proximity-induced magnetic moments at Pt ions and subsequent magnetic-field dependent scattering. It thus follows that either bulk (spin Hall and Inverse spin Hall Effects) or interface (Rashba) spin-orbit related effects dominate the observed magnetoresistance. Furthermore, comparison of bulk magnetization and XMCD data at (Fe,Co)-L2,3 edges suggests the presence of some spin disorder in the CoFe2O4 layer which may be relevant for the observed anomalous non-saturating field-dependence of spin Hall magnetoresistance.

The phase transition and superior multiferroic properties of (Mn, Co) co-doped BiFeO3/CoFe2O4 double-layer films

(Mn, Co) co-doped BiFeO3/CoFe2O4 (BFMCO/CFO) double-layer films were prepared on fluorine doped tin oxide (FTO)/glass substrates by the chemical solution deposition method. The effect of (Mn, Co) co-doping on crystal structure, ferroelectric and magnetic properties of BFO/CFO double-layer films was investigated. Mn and Co co-doping on the BiFeO3 layer induced the tendency of phase transformation from rhombohedral to tetragonal. Well saturated polarization hysteresis loops were observed in BFMCO/CFO films, meanwhile a giant remanent polarization of the double-layer films (Pr ∼ 123 μC/cm2, after compensation) is obtained. Furthermore, an enhanced ferromagnetic behavior of Ms ∼ 113 emu/cm3, Mr ∼ 67 emu/cm3 and Hc ∼ 1200 Oe is observed in BFMCO/CFO films. The reduction of Fe2+ and the existence of high valence iron ions with co-doping Mn and Co were confirmed by XPS. Therefore, the difference of magnetic moment of TM2+/3+ and Fe3+ may play a more important role in the suppression of the spiral spin modulation. As a result, the introduction of (Mn, Co) and CoFe2O4 layer is an effective way to enhance the multiferroic behavior of the films.