CoFe2O4 Films Research Articles

Spinel/perovskite cobaltite nanocomposites synthesized by combinatorial pulsed laser deposition

Perovskite/spinel nanocomposites have attracted great attention due to the novel properties that originate from the coupling between two oxides through their interfaces. Combinatorial pulsed laser deposition was used in this work to grow La0.8Sr0.2CoO3 (LSC)/CoFe2O4 (CFO) nanocomposites on (001) SrTiO3 (STO) under various growth conditions. At a substrate temperature of 680 °C, the LSC/CFO consisted of columns of CFO and a third phase, likely to be CoOx, within a highly textured LSC matrix. Lowering the temperature to 600 °C resulted in highly textured LSC/CFO nanocomposites consisting of grains of LSC and CFO a few nanometers in diameter. The grain size increased with decreasing growth rate. In the nanocomposites, redistribution of Fe into the perovskite phase is expected. The strain in the CFO was impacted by the lattice match at the interfaces with the LSC, and changes in composition as well as the strain led to lower magnetic anisotropy compared with CFO films on STO. This approach can also be used to produce other novel nanocrystalline spinel/perovskite systems.

Magneto-dielectric and magnetoelectric anisotropies of CoFe2O4/Bi5Ti3FeO15 bilayer composite heterostructural films

Bilayer composite heterostructural films consisting of magnetic CoFe2O4 and multiferroic Bi5Ti3FeO15 films were prepared by the chemical solution deposition method.

Study of resistive switching and magnetism modulation in the Pt/CoFe2O4/Nb:SrTiO3 heterostructures

CoFe2O4 (CFO) thin films are epitaxially grown on Nb doped (001) SrTiO3 (NSTO) single-crystal substrates by pulsed laser deposition to form Pt/CFO/NSTO heterostructures. These heterostructures exhibit typical bipolar resistive switching effect with maximum switching ratio of 5 × 104, multi-level resistance states, excellent retention, and anti-fatigue properties. When the resistance states of the heterostructures are switched between low resistance state and high resistance state upon applying bias voltages, the saturation magnetization of the CFO films shows corresponding changes associated with the resistive switching. These close correlations between the resistive switching and the magnetization can be attributed to the electrons filling into and releasing from the defect energy levels introduced by oxygen vacancies in the CFO film. These results show potential application in the multi-functional magnetoelectric sensor and non-volatile multi-level resistive switching memory.

Modification of structure and magnetic anisotropy of epitaxial CoFe2O4 films by hydrogen reduction

Heteroepitaxial CoFe2O4 (CFO) thin films with different thicknesses were deposited on MgO (001) substrates. The as-deposited CFO films show a clear switching of magnetic anisotropy with increasing film thickness. The thinner films (<100 nm) show a perpendicular magnetic anisotropy due to the out-of-plane compressive strain. The thicker films exhibit an in-plane easy axis owing to the dominating shape anisotropy effect. The magnetostriction coefficient of CFO films is estimated to be λ[001] = −188 × 10−6. Metallic CoFe2 films were obtained by annealing the as-deposited CFO films in forming gas (Ar 93% + H2 7%) at 450 °C. XRD shows that CoFe2 films are textured out-of-plane and aligned in-plane, owing to lattice matching between CoFe2 and MgO substrate. TEM results indicate that as-deposited films are continuous while the annealed films exhibit a nanopore mushroom structure. The magnetic anisotropy of CoFe2 films is dominated by the shape effect. The results demonstrate that hydrogen reduction can be effectively used to modify microstructures and physical properties of complex metal oxide materials.

Orientation-Dependent Properties of CoFe2O4-Bi3.15Nd0.85Ti3O12 Bilayer Multiferroic Films Prepared by a Sol–Gel Method

Lead-free bilayer multiferroic films of CoFe2O4 (CFO) and oriented Bi3.15Nd0.85Ti3O12 (BNT) were deposited on a conventional Pt(111)/Ti/SiO2/Si(100) substrate by use of a sol–gel method. BNT layers with different preferred orientations were grown by use of different spinning and annealing processes. The ferroelectric, ferromagnetic, and magnetoelectric (ME) properties of the CFO–BNT bilayer thin films were investigated at room temperature. a-Axis-oriented bilayer films had larger ME voltage coefficients and larger converse ME response than c-axis-oriented films. This study shows that the orientation of the BNT layer has a stress-mediated interfacial effect which can substantially affect the magnetoelectric coupling behavior of the bilayer structures.

Magnetic CoFe2O4 films with controllable dendritic arrays by a combined method of electrodeposition and anode activation

Magnetic CoFe2O4 films with controllable dendritic arrays were successfully prepared by a combined method of electrodeposition and anode activation.

Functional spinel oxide heterostructures on silicon

Novel spinel heteroepitaxial structures composed of ferrimagnetic spinel CoFe2O4 (CFO) and ultrathin γ-Al2O3 buffer layers are integrated on Si(111). Flat (111)-oriented epitaxial CFO films with only two in-plane domains induced by γ-Al2O3 are ferromagnetic at room temperature with high saturation magnetization. These results open the prospect of using functional spinel layers on silicon for monolithic devices.

Impacts of amorphous and crystalline cobalt ferrite layers on the giant magneto-impedance response of a soft ferromagnetic amorphous ribbon

A systematic study of the effect of depositing CoFe2O4 (CFO) films of various thicknesses (d = 0–600 nm) on the giant magneto-impedance (GMI) response of a soft ferromagnetic amorphous ribbon Co65Fe4Ni2Si15B14 has been performed. The CFO films were grown on the amorphous ribbons by the pulsed laser deposition technique. X-ray diffraction and transmission electron microscopy revealed a structural variation of the CFO film from amorphous to polycrystalline as the thickness of the CFO film exceeded a critical value of 300 nm. Atomic force microscopy evidenced the increase in surface roughness of the CFO film as the thickness of the CFO film was increased. These changes in the crystallinity and morphology of the CFO film were found to have a distinct impact on the GMI response of the ribbon. Relative to the bare ribbon, coating of amorphous CFO films significantly enhanced the GMI response of the ribbon, while polycrystalline CFO films decreased it considerably. The maximum GMI response was achieved near the onset of the structural transition of the CFO film. These findings are of practical importance in developing high-sensitivity magnetic sensors.

Tuning of optical bandgap and magnetization of CoFe2O4 thin films

Single phase CoFe2O4 thin films are deposited on quartz substrates at 400 °C using ultrasonic assisted chemical vapor deposition, and the tuning of optical bandgap and saturation magnetization of films is demonstrated by varying the post deposition annealing temperature. The optical band gap varies from 1.58 to 1.41 eV and saturation magnetization increases from 4 to 46 emu/g as the post deposition annealing temperature is increased from 500 °C to 700 °C. The observed change in optical bandgap and the magnetic properties is attributed of the shifting of Co2+ from the octahedral to the tetrahedral site with the increase in the annealing temperature. Raman studies of the films support the redistribution of Co2+ among the octahedral and tetrahedral sites in CoFe2O4 films with the increase in the annealing temperature.

Epitaxial ferromagnetic oxide thin films on silicon with atomically sharp interfaces

A bottleneck in the integration of functional oxides with silicon, either directly grown or using a buffer, is the usual formation of an amorphous interfacial layer. Here, we demonstrate that ferromagnetic CoFe2O4 films can be grown epitaxially on Si(111) using a Y2O3 buffer layer, and remarkably the Y2O3/Si(111) interface is stable and remains atomically sharp. CoFe2O4 films present high crystal quality and high saturation magnetization.

Effect of non-magnetic ion substitution on the magnetoelastic properties of CoFe2O4 films

We report on the structural and magnetic properties of CoFe1.9Zr0.1O4 films under epitaxial strain. The films are grown with high structural quality on MgO, SrTiO3, and piezoelectric Pb(Mg1∕3Nb2∕3)0.72Ti0.28O3 substrates by pulsed laser deposition. The substrate-induced strain is found effective to alter the magnetic anisotropy of the films. Reversible strain measurements using the piezoelectric substrates are providing evidence that nearly bulk-like magnetoelastic properties are maintained for films of down to at least 25 nm thickness. This is in contrast to unsubstituted cobalt ferrite films and suggests that a chemical control of the spinel lattice provides a promising route to optimize the tunability of the magnetic properties of thin ferrite films.

Crossover of uniaxial magnetic anisotropy direction mediated by interfacial strain of CoFe2O4 films

This study examined the deposition temperature-dependent magnetic anisotropy of CoFe2O4 films grown on Al2O3(0001) substrates using pulsed-laser deposition. X-ray diffraction revealed all films to have a 〈111〉 orientation except for the films grown at room temperature, which exhibited amorphous characteristics. Furthermore, the films deposited between 350°C and 550°C exhibited out-of-plane tensile strain even though, which was relieved as the deposition temperature increased. On the other hand, film deposited at 650°C showed out-of-plane compressive strain. The in-plane and out-of-plane magnetic hysteresis loops, which were measured at room temperature, showed a decreased out-of-plane anisotropy when the deposition temperature was increased. Simple uniaxial magnetic anisotropy energy calculations based on the experimental data showed a direct correlation between the uniaxial magnetic anisotropy direction and stress of the films. X-ray photoelectron spectroscopy revealed variations in the cation distribution according to the deposition temperature.

The synthesis of Co1−xDyxFe2O4 nanoparticles and thin films as well as investigating their magnetic and magneto-optical properties

The modification and optimization of the magnetic and magneto-optical properties of cobalt ferrite are of great importance due to their various applications in well-known scientific and industrial categories. In order to observe the effect of adding Dy3+ to the cobalt ferrite composition, Co1−xDyxFe2O4 (0≤x≤0.1) ceramic nanoparticles were synthesized by the co-precipitation chemical method and then their microstructure and magnetism were investigated through x-ray diffractometry, TEM micrography, IR spectroscopy and VSM magnetometry. The polar magneto-optical Kerr effect of the thin film specimens was also studied. The results reveal that the doping of Dy3+ ions could effectively alter the inversion degree of the spinel structure and the following magnetic and magneto-optical features. The ferrite coercivity was enhanced by 150% after adding Dy. The perceptible shifts of peak rotations were observed in the Kerr spectra for the Dy-doped CoFe2O4 films. Furthermore, TC had a descending trend with the addition of Dy from 440°C to 420°C for Co0.95Dy0.05Fe2O4.

Electrically controlled magnetization switching in CoFe2O4/Pb(Mg1/3Nb2/3)O3–PbTiO3 heterostructure

CoFe2O4 (CFO) films with different thicknesses are grown on (001) Pb(Mg1/3Nb2/3)O3–PbTiO3(PMN–PT) substrate using pulsed laser deposition. The manipulation of magnetization is observed in CFO films by magnetic force microscopy (MFM) images and magnetization measurement with an electric field applying on the piezoelectric substrate. Taking advantage of the change in coercivity induced by the electric field, the magnetization reversal is realized in this heterostructure at room temperature. The mechanism and potential applications for electric control of magnetism in CFO/PMN–PT layered films are discussed.

Nonlinear strain dependence of magnetic anisotropy in CoFe2O4 films on MgO(001) substrates

CoFe2O4 films were deposited on MgO(001) substrates using pulsed laser deposition with various laser energy densities. We found that the CoFe2O4 films were grown in the (001) orientation and that the lattice constant of the CoFe2O4 films was dependent on the laser energy densities. Perpendicular magnetic anisotropy (PMA) was observed, but the lattice strain dependence was not consistent with the phenomenological model in which the PMA is in proportion to the lattice strain. The lattice strain dependence of PMA was compared with an electron theory in which the spin-orbit interaction and the tetragonal crystal field in the electronic state of a single Co ion are considered. The experimental result does not contradict with the calculation, which shows that the magnetic anisotropy is not proportional to the lattice strain and asymmetric in respect to negative and positive lattice strain.

Annealing control of magnetic anisotropy and phase separation in CoFe2O4-BaTiO3 nanocomposite films

Multiferroic heteroepitaxial nanocomposite films of BaTiO3 and CoFe2O4 (CFO) have been grown by pulsed laser deposition employing alternating ablation of two ceramic targets. Films grown at temperatures between 650 °C and 710 °C contain columnar CFO grains about 10–20 nm in diameter embedded in a BaTiO3 matrix. The very strong vertical compression of these grains causes large perpendicular magnetic anisotropy. Post-growth annealing treatments above the growth temperature gradually release the compression. This allows one to tune the stress-induced magnetic anisotropy. Additionally, annealing leads to substantial enhancement of the saturation magnetization MS. Since MS of a pure CFO film remains unchanged by a similar annealing procedure, MS is proposed to depend on the volume fraction of the obtained CFO phase. We suggest that MS can be utilized to monitor the degree of phase separation in nanocomposite films.

Cobalt ferrite dispersion in organic solvents for electrophoretic deposition: Influence of suspension parameters on the film microstructure

An electrophoretic deposition (EPD) method was applied for the preparation of CoFe2O4 (CFO) films on Al2O3/Pt substrates. A coprecipitation process was used to synthesize fine CFO powders with an average particle size of ∼40 nm. Influences of suspension parameters such as solvents, iodine additive, and charged polymer on the suspension stability and film microstructure were investigated in detail. Three suspensions including CFO–acetylacetone, CFO–acetylacetone–0.08 wt% I2 and CFO–acetylacetone–0.2 wt% polyethylenimine (PEI) were optimized, respectively. It was found that CFO was deposited uniformly and the potential required for the deposition was small for the three optimized suspensions. After sintering at 1250 °C for 2 h, the film from CFO–acetylacetone–0.08 wt% I2 showed many cracks, which indicates this suspension is not suitable for preparing high quality CFO ceramic films. While the sintered films fabricated from the other two optimized suspensions exhibited dense structures. Based on the electric and magnetic properties of CFO ceramic films, it can be concluded that CFO–acetylacetone–0.2 wt% PEI is the proper suspension to prepare films with better microstructures and properties.

Volatile and nonvolatile magnetic easy-axis rotation in epitaxial ferromagnetic thin films on ferroelectric single crystal substrates

We explored the relationship between phase transformation and magnetoelectric effect by depositing epitaxial CoFe2O4 films on ⟨110⟩ oriented Pb(Mg,Nb)O3-PbTiO3 (PMN-PT) with three different PbTiO3 contents (PMN-28PT, PMN-29.5PT, and PMN-30PT). Electric-field controlled rhombohedral to orthorhombic phase transformation was confirmed by both piezoelectric and dielectric constant measurements. A giant in-plane (IP) uniaxial strain in CoFe2O4 film was induced due to dramatic lattice parameter change trigged by phase transition. Magnetic easy axis can be rotated from IP⟨110⟩ to IP⟨001⟩. More importantly, the phase transformation could be either reversible or irreversible, resulting in either volatile or nonvolatile magnetic easy axis rotations.

Thickness dependence of the magnetoelastic effect of CoFe2O4 films grown on piezoelectric substrates

Epitaxial CoFe2O4 (CFO) films of varying thickness were grown on piezoelectric Pb(Mg1∕3Nb2∕3)0.72Ti0.28O3 substrates. The magnetic anisotropy of the CFO films is controlled by the piezoelectric in-plane strain imposed by the substrate constraint during application of an electric field. We find that the strain-induced change of the remanent magnetization is constant at large thickness, but drops significantly below ∼100 nm. This thickness dependence of the magnetoelastic effect is shown not to be caused by a variation of the as-grown strain state.

(111)-Oriented Co0.8Fe2.2O4+δ thin film grown by pulsed laser deposition: structural and magnetic properties

The perfect (111)-oriented Co0.8Fe2.2O4+δ thin films were grown on Pt(111)/Si substrate by pulsed laser deposition technique. Co0.8Fe2.2O4+δ film grown at oxygen pressure of 10 mTorr (optimum condition) has the highest (111)-orientation degree, the lowest surface roughness, uniformly compacted nanosize grain-feature structure (50–80 nm), and the highest magnetization. The Fe K-edge X-ray absorption near edge structure analyses revealed that, in Fe-doped CoFe2O4 film, the Fe also exists as Fe3+ (Co0.8Fe2.2O4+δ), which affects the lattice parameter as well as magnetic properties. The magnetic properties (saturation magnetization, coercivity, and squareness) of Co0.8Fe2.2O4+δ thin film are significantly higher than those of CoFe2O4 film. Moreover, the (111)-oriented Co0.8Fe2.2O4+δ thin film demonstrates strong in-plane magnetic anisotropy, which results from orientation, as well as the stress-induced magnetic anisotropy.