CFO Films Research Articles

Electrical and thermal control of magnetic coercive field in ferromagnetic/ferroelectric heterostructures

We report the growth of ferromagnetic epitaxial CoFe${}_{2}$O${}_{4}$ (CFO) thin films on 011-oriented tetragonal-structured 0.62Pb(Mg,Nb)O${}_{3}$-0.38PbTiO${}_{3}$ single crystal substrates. In the substrates, electric field-induced polarization reorientation resulted in maximum strains of \ensuremath{-}1.3% and 0.64% along in-plane 100 and along in-plane $0\overline{1}1$, respectively. As a consequence, in the CFO films, irreversible magnetic coercive field changes of 580, \ensuremath{-}510, and \ensuremath{-}220 Oe were observed along in-plane 100 and $0\overline{1}1$ and out-of-plane 011, respectively, after electric field poling. After annealing at 470 K, the substrate depoled, and both the magnetic coercive fields and the remnant magnetizations along the three directions recovered their original values.

Magnetoelectric properties of Ba0.8Sr0.2TiO3–CoFe2O4 multilayered composite film via sol–gel method

Ba0.8Sr0.2TiO3–CoFe2O4 (BST–CFO) multilayered composite films were prepared on Pt/Ti/SiO2/Si substrates via a sol–gel method and spin-coating technique. Microstructures, electric property, magnetic property and magnetoelectric (ME) property of the composites were studied. Results show that the composite films calcined at 750 °C have BST and CFO phases and no obvious impurity phases were detected. Further, the composite films exhibit layered structures and a transition layer which is composed of interfacial delamination exists at the interface between BST and CFO layers. Ferroelectric and ferromagnetic properties were simultaneously observed in the films, evidencing the coexistence of the ferroelectric and ferromagnetic properties. Furthermore, the saturation magnetization value of the composite film is lower than that of the pure CFO film derived by the same processing as a result of the effect of the nonferromagnetic BST layers. Also, ferroelectric hysteresis loops reveal that the saturated polarization and remanent polarization of the composite film are lower than those of the pure BST films. In addition, the composite film exhibits a strong ME effect, which makes the composite film attractive for technological applications as devices.

Nanocolumnar Interfaces and Enhanced Magnetic Coercivity in Preferentially oriented Cobalt Ferrite Thin Films Grown Using Oblique-Angle Pulsed Laser Deposition

Highly textured cobalt ferrite (CFO) thin films were grown on Si (100) substrates using oblique-angle pulsed laser deposition (α-PLD). X-ray diffraction and in-depth strain analysis showed that the obliquely deposited CFO films had both enhanced orientation in the (111) crystal direction as well as tunable compressive strains as a function of the film thicknesses, in contrast to the almost strain-free polycrystalline CFO films grown using normal-incidence PLD under the same conditions. Using in situ optical plume diagnostics the growth parameters in the α-PLD process were optimized to achieve smoother film surfaces with roughness values as low as 1-2 nm as compared to the typical values of 10-12 nm in the normal-incidence PLD grown films. Cross-sectional high resolution transmission electron microscope images revealed nanocolumnar growth of single-crystals of CFO along the (111) crystallographic plane at the film-substrate interface. Magnetic measurements showed larger coercive fields (∼10 times) with similar saturation magnetization in the α-PLD-grown CFO thin films as compared to those deposited using normal-incidence PLD. Such significantly enhanced magnetic coercivity observed in CFO thin films make them ideally suited for magnetic data storage applications. A growth mechanism based on the atomic shadowing effect and strain compression-relaxation mechanism was proposed for the obliquely grown CFO thin films.

Structural, electric and magnetic properties of Pb(Zr0.2Ti0.8)O3–CoFe2O4 heterostructures

Electric and magnetic properties of symmetric and asymmetric Pb(Zr0.2 Ti0.8)O3–CoFe2O4 (PZT/CFO) heterostructures, grown by pulsed laser deposition on SrTiO3 (100) substrates with a 25nm SrRuO3 (SRO) buffer layer as bottom electrode, were investigated by using hysteresis and capacitance measurements. X-ray diffraction, and transmission electron microscopy investigations reveal the high quality crystalline structure and the epitaxial relationship between SRO, PZT and CFO. The electric polarization–voltage hysteresis reveals that the remnant polarization and the coercive field are significantly affected by the CFO layer. The frequency dependence of capacitance suggests a Maxwell–Wagner type relaxation at low frequencies and is also affected by the presence of the PZT/CFO interface(s). The magnetic hysteresis measurements infer the possible presence of another spinel phase (Co3O4) in the CFO film, due to the lattice mismatch at the PZT/CFO interfaces, and with direct influence on the magnetic response of the structure. According to the electric and magnetic characterization, better room temperature multiferroic properties would be expected for the symmetric heterostructure.

Influence of Rapid Thermal Annealing on the Structure and Magnetic Properties of CoFe2O4 Films Prepared by Sol-Gel Method

CoFe2O4 thin films were prepared on the Si (001) substrates using the sol-gel method. In this study, the effects of annealing temperature, annealing time and annealing atmosphere on the structure and magnetic properties of CoFe2O4 films have been examined. X-ray diffraction analysis revealed that the films formed single spinel structures when the annealing temperature was above 450°C. The crystallization of the CFO thin films increased as the annealing temperature increased. Atomic force microscopy analysis showed that the film grain size increased with annealing temperature and that there was a single domain structure of critical size at 650°C. Vibrating sample magnetometer measurements showed that the saturation magnetization (Ms) reached a maximum and that the coercivity (Hc) varied with annealing conditions. When the CFO film was annealed at 650°C and 40 seconds, the ratio of O2:Ar was 1:1. The values for Ms and Hc had a maximum of 47.6 Am2.Kg−1 and a minimum of 86.8 KA.m−1, respectively.

Magnetoelectric properties of CoFe2O4–Pb(Zr0.52Ti0.48)O3 multilayered composite film via sol–gel method

CoFe2O4–Pb(Zr0.52Ti0.48)O3 (CFO–PZT) multilayered composite film was prepared on Pt/Ti/SiO2/Si substrate via a sol–gel method and spin-coating technique. Results show that PZT and CFO phases exist in the composite film, calcined at 700 °C, besides substrate phase, and no obvious impurity phases can be detected. The composite film exhibits layered structure with obvious boundary between CFO and PZT films. Ferroelectric and ferromagnetic properties were simultaneously observed in the composite film, evidencing the ferroelectric and ferromagnetic properties in the composite film. The composite film exhibits both good magnetic and electric properties, as well as, magnetoelectric (ME) effect. The saturation magnetization value of the composite film is lower than that of the pure CFO film derived by the same processing as a result of the effect of the nonferromagnetic PZT layers. Ferroelectric hysteresis loops reveal that saturated polarization and remanent polarization of the composite film are lower than those of the pure PZT films. The composite film exhibits a very large ME effect, which makes the composite film attractive for technological applications as devices.

Influence of reactive atmosphere on properties of cobalt ferrite thin films prepared using pulsed-laser deposition

A series of cobalt ferrite (CFO) thin films were grown on SiO2/Si(100) substrates using pulsed-laser deposition (PLD) at substrate temperature (TDEP) of 250 °C and oxygen pressures (PO2) from 0.67 to 6.7 Pa. The influence of PO2 on crystal structure, phase mixture, deposition rate, and magnetic properties was investigated. It is shown in this study that there is a window of PO2 for optimized growth of nanograined CFO films at low TDEP and that either higher or lower values of PO2 produce undesirable multiphase mixtures. CFO films grown at such low substrate temperature and optimized oxygen pressure on thermal expansion matched substrates can be applied in multilayer sensors or MEMS devices.

Magnetoresistance curves of spin valves using the (1 1 0) cobalt ferrite pinning layer

Magnetic and magnetoresistance (MR) properties of spin valves composed of Co/Cu/Co/(1 1 0) cobalt ferrite (CFO) were studied, which showed complicated MR properties. It was confirmed from the in-plane magnetization curves that the epitaxial (1 1 0) CFO films have easy axes along the ⟨1 1 0⟩ direction and hard axes along the ⟨0 0 1⟩ direction due to in-plane tensile stress. The spin valves showed quite different MR curves depending on the direction of the magnetic field applied in the plane of (1 1 0) CFO. Butterfly-shaped MR curves, which implied the difference in the coercive fields between the top and bottom Co layers, were observed when the field was applied along the ⟨0 0 1⟩ direction of CFO. On the other hand, when the field was applied along the ⟨1 1 0⟩, unusually shaped MR curves, which showed large MR ratio in spite of the hard direction of the Co layer, were observed. This MR behaviour might indicate the parallel and anti-parallel magnetization configuration between the top and bottom Co layers at zero fields. These magnetic and MR properties could be explained using the micromagnetic calculation of magnetization processes by assuming strong 90° coupling between the bottom Co and CFO layers, and CFO pinning layers having continuous spin rotation at the interface near the bottom Co layer with an exchange stiffness coupling as a spring magnet.

SYNTHESIS OF CoFe2O4/Pb(Zr0.53Ti0.47)O3 MULTIFERROIC COMPOSITE THICK FILMS BY LOW-SINTERING-TEMPERATURE SCREEN PRINTING METHOD

CoFe 2 O 4/ Pb ( Zr 0.53 Ti 0.47) O 3 (abbreviated as CFO/PZT) multiferroic composite thick films were successfully fabricated on alumina substrate with gold bottom electrode by screen printing method at a low-sintering temperature. The processing included the modification and dispersion of ferromagnetic CFO powder and ferroelectric PZT powder, the preparation of uniform pastes, and the selection of proper annealing temperature for composite thick films. Transmission electron microscopic pictures (TEM) indicated the submicron meter of particles size for both CFO and PZT particles. After annealing at 900°C for 1 h in air, tape test confirmed the quality of multiferroic thick films as well as pure CFO and PZT films. X-ray diffraction (XRD) showed a coexistence of CFO and PZT phases; furthermore, a smooth surface was observed through scanning electron microscopic (SEM) pictures along with the sharp cross-sectional picture, indicative of 100 μm of film thickness. Ferromagnetic and ferroelectric properties were observed in CFO/PZT films simultaneously at room temperature. Compared with the reported CFO/PZT multiferrroic thin films, the present ferromagnetic property was closing to that of the chemical sol-gel synthesized film and even that from the physical pulsed laser deposition technique. However, the ferroelectric property showed a degenerated behavior, possible reasons for this was discussed and further optimization was also proposed for the potential multifunctional application.

Magnetic anisotropy and field switching in cobalt ferrite thin films deposited by pulsed laser ablation

We report the observation of contrasting magnetic behavior in cobalt ferrite (CFO) thin films deposited on single crystalline magnesium oxide (MgO) and strontium titanate (STO). Epitaxial films on MgO (100) with a lattice mismatch of 0.35% showed out-of-plane anisotropy whereas the films on STO (100) with a lattice mismatch of 7.4% displayed in-plane anisotropy. Stress anisotropy calculated from angle-dependent x-ray diffraction analysis confirmed that the change in anisotropy originates from the lattice mismatch. An additional low-field switching characteristic is observed in the M-H loops of the CFO films, which became prominent with lowering temperature as also evidenced from the rf transverse susceptibility measurements. The obtained results revealed that the low field switching is associated with the film-substrate interface.

Electric-field-induced magnetic easy-axis reorientation in ferromagnetic/ferroelectric layered heterostructures

A phenomenological model is presented for the electric-field-induced reorientation of magnetic easy axis in ferromagnetic (FM)/ferroelectric (FE) layered heterostructures, where the FM layers include either polycrystalline or (001) preferentially oriented films of widely used Fe, ${\text{CoFe}}_{2}{\text{O}}_{4}$ (CFO), Ni, and ${\text{Fe}}_{3}{\text{O}}_{4}$. The magnetic easy axes could rotate within the film plane at relatively low applied electric fields in CFO, Ni, and ${\text{Fe}}_{3}{\text{O}}_{4}$ films. As the applied electric field increases, the easy axes tend to switch from an in-plane direction to an out-of-plane one in CFO and Ni films. Such electric-field-induced easy-axis reorientations (EARs) can be abrupt and gradual, defined as first-order and second-order transitions, respectively. The influences of residual strain, different electric-field orientations, and nonlinear strain vs electric-field behavior of the FE substrates, on the EARs are also addressed.

Study on the growth and interfacial strain of CoFe2O4/BaTiO3 bilayer films

CoFe2O4/BaTiO3 bilayer films were epitaxially deposited on SrTiO3 substrates by laser molecular beam epitaxy (LMBE). The growth process of the bilayer films was in-situ monitored by reflection high-energy electron diffraction (RHEED). Sixty nanometer thick-BTO layer was firstly fabricated in a layer-by-layer growth mode with an atomic smooth surface. CFO films with a varying thickness ranging from 5 to 60nm were subsequently deposited on BTO-coated STO substrates. The different growth behaviors of CFO films were observed due to the lattice mismatch strain. Between two short stages of the growth mode transforming, a long duration with Stransky and Krastonov growth mode was maintained. Strainfully relaxed CFO film in the island growth mode was finally formed. High-resolution X-ray diffraction (HRXRD) was used to further analyze the strain effect. It was found that the tensile stress imposed on BTO by CFO was strengthened with increasing the thickness of CFO films, which could lessen the distortion of BTO by counteracting the compressive stress caused by STO substrates. The strengthened tensile stress weakened the ferroelectric property of BTO films by reducing structural tetragonality, which was demonstrated by polarization–electric (P–E) measurement.

Epitaxial growth of the CoFe 2O 4 film on SrTiO 3 and its characterization

Cobalt ferrite (CoFe 2O 4) thin film is epitaxially grown on (0 0 1) SrTiO 3 (STO) by laser molecular beam epitaxy (LMBE). The growth modes of CoFe 2O 4 (CFO) film are found to be sensitive to laser repetition, the transitions from layer-by-layer mode to Stranski–Krastanov (SK) mode and then to island mode occur at the laser repetition of 3 and 5 Hz at 700 °C, respectively. The X-ray diffraction (XRD) results show that the CFO film on (0 0 1) SrTiO 3 is compressively strained by the underlying substrate and exhibits high crystallinity with a full-width at half-maximum of 0.86°. Microstructural studies indicate that the as-deposited CFO film is c-oriented island structure with rough surface morphology and the magnetic measurements reveal that the compressive strained CoFe 2O 4 film exhibits an enhanced out-of-plane magnetization (190 emu/cm 3) with a large coercivity (3.8 kOe).

Oxygen permeability of nanocrystalline Ce 0.8Gd 0.2O 1.9–CoFe 2O 4 mixed-conductive films

Electronic and oxygen ionic conductive (mixed-conductive) oxides allow oxygen permeation under oxygen partial pressure differences at elevated temperatures of typically 800–1000 °C. The oxygen permeation performance decreases abruptly at lower temperatures as a result of the high activation energy required for solid-state transport of O 2− ions and electrons. This study is intended to improve the oxygen permeation flux of a mixed-conductive solid oxide at lower temperatures. Dense nanocrystalline Ce 0.8Gd 0.2O 1.9 (CGO)–CoFe 2O 4 (CFO) mixed-conductive films were prepared on porous Ce 0.8Gd 0.2O 1.9 substrates using a chemical solution spin-coating technique. The oxygen permeation flux and morphology of the CGO–CFO films were investigated along with the crystallinity of the CGO–CFO powder samples. The oxygen permeation flux of the CGO–CFO films was improved through the use of a reduced preparation firing temperature, suggesting that the membrane's oxygen permeation properties are correlated closely with the material's nanocrystalline structure. This result is important for developing oxygen permeable devices with enhanced performance at lower temperatures of approximately 600 °C.