CoFe2O4 Films Research Articles

Phase-field simulation of electric-field-induced in-plane magnetic domain switching in magnetic/ferroelectric layered heterostructures

The electric-field-induced in-plane magnetic domain switching in magnetic/ferroelectric (FE) layered heterostructures was studied using phase-field simulations. In particular, we chose the CoFe2O4 (CFO) magnetic film and the Pb(Zn1/3Nb2/3)O3–PbTiO3 (PZN-PT) FE layer as a representative example due to their strong respective magnetoelastic and piezoelectric couplings. In-plane 90° magnetic domain switching in the CFO film was observed when a transverse electric field was applied to the PZN-PT layer. The detailed switching behaviors as well as the corresponding magnetic domain structures are presented for CFO films with different geometric sizes and initial magnetization configurations. The effect of a dynamic electric field on the switching process, i.e., a time-dependent electric-field-induced magnetic domain switching, is also discussed.

Role of crystal orientation on the magnetic properties of CoFe2O4 thin films grown on Si (1 0 0) and Al2O3 (0 0 0 1) substrates using pulsed laser deposition

We report the influence of crystal orientation on the magnetic properties of CoFe2O4 (CFO) thin films grown on single crystal Si (100) and c-cut sapphire (Al2O3) (0001) substrates using pulsed laser deposition technique. The thickness was varied from 200 to 50nm for CFO films grown on Si substrates, while it was fixed at 200nm for CFO films grown on Al2O3 substrates. We observed that the 200 and 100nm thick CFO–Si films grew in both (111) and (311) directions and displayed out-of-plane anisotropy, whereas the 50nm thick CFO–Si film showed only an (111) orientation and an in-plane anisotropy. The 200nm thick CFO film grown on an Al2O3 substrate was also found to show a complete (111) orientation and a strong in-plane anisotropy. These observations pointed to a definite relation between the crystalline orientation and the observed magnetic anisotropy in the CFO thin films.

Perpendicular magnetic anisotropy in CoFe2O4(001) films epitaxially grown on MgO(001)

We report on the magnetic properties of epitaxial cobalt-ferrite films with orientations parallel to [001] and [111] grown by a reactive molecular beam epitaxy method using pure ozone gas as an oxidation agent. Both Mössbauer spectroscopy and magnetization measurement of the CoFe2O4(001) film grown on MgO(001) indicate that the film has perpendicular magnetic anisotropy (PMA) with high coercivity, whereas the film of CoFe2O4(111) grown on α-Al2O3(0001) appears to be paramagnetic. The maximum uniaxial anisotropy energy for CoFe2O4(001) estimated from the magnetization and coercivity at room temperature is ≈3×106 erg/cm3.

Anomalous Enhancement in Magnetization of BiFeO3 – CoFe2O4 Heterostructure Multilayer Thin Films Deposited on SrTiO3 with Different Orientation

The magnetic properties of BiFeO3(BFO) – CoFe2O4(CFO) heterostructure thin films, grown on (100), (110) and (111) SrTiO3 (STO) substrates by employing pulsed laser deposition (PLD) method, have been studied. The XRD patterns of BFO-CFO multilayer thin films compared with the pure CFO and BFO thin films grown using under identical conditions evidenced that all peaks present correspond to CFO and BFO structure in two separate phases. Micro-Raman spectra analysis also reveals the presence of BFO and CFO in two separate phases in the heterostructures thin films. The anomalous enhancement in magnetization has been observed for BFO−CFO multilayer thin films, relative to pure BFO thin films in all three different orientations. The saturation magnetization for (100), (111) and (110) oriented BFO-CFO thin films are 44, 62 and 80 emu/cc suggesting that (110) is most preferable direction to achieve the magneto electric (ME) coupling effect.

Nontunnel transport through CoFe2O4 nanometric barriers

Electric transport through ultrathin CoFe2O4 (CFO) films of different thicknesses is studied using current sensing atomic force microscopy. Analysis of current distribution maps and I-V characteristics reveals anomalous thickness dependence. Results indicate the existence of an Ohmic conduction channel in parallel with the tunnel one. The origin of the nontunneling, likely non-spin-preserving, channel is discussed in the context of recent results on spin-filtering CFO-based devices.

Magnetoelectric effect in Pb(Zr0.95Ti0.05)O3 and CoFe2O4 heteroepitaxial thin film composite

Multiferroic epitaxial films, include SrRuO3/Pb(Zr0.95Ti0.05)O3/CoFe2O4 has been successfully deposited on SrTiO3 substrate by pulsed-laser deposition technique. The results show that the prepared films exhibit a single phase. The Pb(Zr0.95Ti0.05)O3 (PZT) film was highly textured with (100) orientation and gives good ferroelectric properties with saturated polarization of 15μC/cm2. The magnetic coercivity of CoFe2O4 film on Pb(Zr0.95Ti0.05)O3 has been dampened to 0.9kOe. The anisotropic magnetically behavior of CoFe2O4 film was changed to isotropic by using high Zr concentrated PZT as underneath layer. Heterostructure films show a good ferromagnetic and ferroelectric coupling that lead to the large magnetoelectricity of 287mV/cmOe.

A robust approach for the growth of epitaxial spinel ferrite films

Heteroepitaxial spinel ferrites NiFe2O4 and CoFe2O4 films have been prepared by pulsed laser deposition at various temperatures (175–690 °C) under ozone/oxygen pressure of 10 mTorr. Due to enhanced kinetic energy of ablated species at low pressure and enhanced oxidation power of ozone, epitaxy has been achieved at significantly lower temperatures than previously reported. Films grown at temperature below 550 °C show a novel growth mode, which we term “vertical step-flow” growth mode. Epitaxial spinel ferrite films with atomically flat surface over large areas and enhanced magnetic moment can be routinely obtained. Interestingly, the growth mode is independent of the nature of substrate (spinel MgAl2O4, perovskite SrTiO3, and rock-salt MgO) and film thickness. The underlying growth mechanism is discussed.

Electric-field control of strain-mediated magnetoelectric random access memory

A strain-mediated magnetoelectric random access memory with electric-field-writing is presented, which consists of a magnetic tunnel junction (MTJ) in intimate contact with a ferroelectric (FE) layer. The calculations show that the magnetization vector in the free layer of the MTJ unit can switch in-plane by 90° upon applying an appropriate electric field to the FE layer, as compared to the common 180° reversal induced by magnetic field or spin-current. A perfect interface between the FE layer and the MTJ is assumed. The free layers used for illustration include either (001)-oriented or polycrystalline magnetic films of Fe–Co alloy, CoFe2O4 (CFO), Ni, and Fe3O4. Among them, the (001)-oriented FeCo and CFO films with positive magnetocrystalline anisotropy constant (i.e., K1>0) show an abrupt magnetization switching, while a gradual magnetization switching takes place in the (001)-oriented Ni and Fe3O4 films with K1<0 as well as the polycrystalline films. Such electric-field-induced in-plane magnetization switching can result in a remarkable change in the MTJ’s electric resistance. In particular, hysteretic dependence of the device resistance on the applied electric field is obtained for the cases of the (001)-oriented FeCo and CFO free layers that exhibit the abrupt magnetization switching, whereby a nonvolatile information storage process can be achieved. The influence of the shape of the free layer on both magnetization and resistance switching features is discussed.

Magnetic and structural properties of cobalt ferrite thin films and structures

Co-ferrite (CoFe2O4) films and patterned arrays were prepared using ion-beam sputtering at elevated substrate temperatures on two types of substrate; MgO (001) and SrTiO3(001). The influence of structure, epitaxy, and interface chemistry on the magnetic properties is analyzed as a function of deposition temperature and annealing. Whereas epitaxial growth and high crystallinity of the CoFe2O4 films is obtained for optimized deposition conditions on both types of substrates, magnetic properties are significantly depleted in films on MgO due to interfacial diffusion of Mg. Although lattice mismatch is much higher, a saturation magnetization of μ0MS = 420 mT not far below bulk values and a remanent magnetization of 142 mT is obtained in 30 nm thin films on SrTiO3. Patterned arrays of CoFe2O4 show an in-plane easy magnetization axis in contrast to homogeneous films before patterning.

Switching of magnetic anisotropy in epitaxial CoFe2O4 thin films induced by SrRuO3 buffer layer

The magnetic anisotropy of epitaxial spinel ferrite CoFe2O4 films grown on SrTiO3 substrates by pulsed laser deposition can be reoriented by inserting a thin SrRuO3 buffer layer. Without SrRuO3, the CoFe2O4 films show a uniaxial anisotropy with the easy axis perpendicular to the film plane, while inserting a SrRuO3 buffer layer results in the switching of the easy axis into the in-plane orientation. This is associated with a tensile and a compressive strain for the films without and with buffer layer, respectively, which is also correlated with the different density of interfacial misfit dislocations. As a result, the ferrite films can be effectively tailored by using SrRuO3.

Electric field manipulation of magnetization at room temperature in multiferroic CoFe2O4/Pb(Mg1/3Nb2/3)0.7Ti0.3O3 heterostructures

Multiferroic heterostructures were fabricated by growing ferrimagnetic CoFe2O4 films on ferroelectric Pb(Mg1/3Nb2/3)0.7Ti0.3O3 substrates using pulsed laser deposition. Upon applying an electric field, the in-plane magnetization of the heterostructures increases and the out-of-plane magnetization decreases. Sharp and reversible changes in magnetization under electric field were also observed for the poled sample. The relative change in magnetization-electric field hysteresis loops were obtained for both the in-plane and out-of-plane magnetizations. Analysis of the results suggests that the electric field induced change in magnetic anisotropy via strain plays an important role in the magnetoelectric coupling in the heterostructures.

Epitaxial self-assembly of multiferroic nanostructures

The morphology and structure of interfaces in PbTiO3–CoFe2O4 films on SrTiO3 substrates of various orientations are studied. It is found that for film with thickness more than 100 nm with columnar two-phase morphology, the average orientations of the interfaces between the phases reside on the {110}, {111}, and {112} planes for films normal to substrates oriented along ⟨001⟩, ⟨110⟩, and ⟨111⟩ correspondingly. These macroscopic interfaces consist of {111} nanofacets. The study of very thin film less than 30 nm shows that the interfaces between nanosize phases are {111} planes. Available theoretical results on morphology of multiferroic thin film nanostructure allow us to conclude that for thick films (>100 nm), two-phase columnar morphology (shape of constituent phases, their arrangement, and average crystallographic orientations of the interfaces) is essentially determined by the trend for minimization of elastic energy, while the trend to minimize the energy of interfaces results in faceting interfaces along the {111} planes. Possibility of long range ordering in phase arrangement is discussed.

High perpendicular coercive field of (100)-oriented CoFe2O4 thin films on Si (100) with MgO buffer layer

CoFe 2 O 4 thin films with large perpendicular magnetic anisotropy were obtained by pulsed laser deposition on Si(100) substrates with MgO buffer layers. Transmission electron microscopy study reveals the columnar structure of these CoFe2O4 films and confirms their (100) texture. Magnetic properties of these films have been investigated in the function of substrate temperature and film thickness. A perpendicular coercivity as high as 7.8 kOe has been achieved in the CoFe2O4 film deposited at 700 °C, with a thickness of 50 nm and a grain size of 30 nm. The high coercivity mechanism is possibly associated with the magnetocrystalline anisotropy, the strain anisotropy, the shape anisotropy due to the columnar structure, and also the appropriate grain size approaching the single-domain critical value.

Strain induced magnetic anisotropy in highly epitaxial CoFe2O4 thin films

Cobalt ferrite (CoFe2O4) thin films were epitaxially grown on (001) SrTiO3 and (001) MgO by laser molecular beam epitaxy. Microstructural studies indicate that the CoFe2O4 grown on (001) SrTiO3 with compressive strain are c-oriented island growth mode with rough surface morphology, whereas the films on (001) MgO with tensile strain become c oriented with layer-by-layer mode. Magnetic property studies reveal that the compressive strained CoFe2O4 films on (001) SrTiO3 can significantly enhance out-of-plane magnetization (190emu∕cm3) with a large coercivity (3.8kOe). In contrast, the tensile strained CoFe2O4 films on (001) MgO exhibit weak magnetic anisotropy. These results suggest that strong magnetic anisotropy is highly dependent on the lattice mismatch induced strain.

Magnetoelectric coupling in epitaxial CoFe2O4 on BaTiO3

The authors have synthesized epitaxial CoFe2O4 films on piezoelectric BaTiO3 single crystal substrates as a model magnetoelectric system. The BaTiO3 substrate provides a surface lattice that can be dynamically changed in an attempt to alter the strain state and hence the magnetization of the CoFe2O4 film. Magnetization measurements indicate that the magnetic anisotropy of CoFe2O4 is dominated by compressive epitaxial strain effects and can be understood in terms of the symmetry of the substrate surface unit cell.

Growth and Characterization of Epitaxial Barium Titanate and Cobalt Ferrite Composite Film

The deposition condition for the multiferroic 0.65BaTiO3−0.35CoFe2O4 composite film was tuned by optimizing the condition for parent BaTiO3 and CoFe2O4 film on SrTiO3 by varying the oxygen partial pressure and substrate temperature. The grown composite film shows good epitaxy on SrRuO3/SrTiO3 conducting substrate. The composite films also show expected ferroelectric and ferromagnetic properties which are necessary to get good magnetoelectric coupling in this material.

Preparation and magnetic properties of the CoFe2O4 thin films on Si substrate by sol-gel technique

A stable precursor for CoFe2O4 thin film was prepared by sol-gel technique from the aqueous solution of FeCl3·6H2O and CoCl2·6H2O. Sol was deposited on a naturally oxidized silicon-substrate by spinning technique (2000 rpm) and heat treated at different temperatures ranging from 700 to 1100 °C. Thickness of the films was controlled in the range of 400–500 nm and all the films were characterized by using XRD and SEM. The effects of temperature and the composition on the formation of CoFe2O4 thin film were also studied. Films obtained at relatively lower temperature showed multi-phases of α-Fe2O3, CoFe2O4 and CoO while the formation of CoFe2O4 phase increases with increasing temperature. Furthermore, the composition of the solution in mol% has great role on the formation of CoFe2O4 films and the film containing 50 mol% of Co2+ exhibited CoFe2O4 mono-phase. Surface morphology of the films was studied by scanning electron microscope (SEM). Magnetic properties of the films, studied by using vibrating sample magnetometer (VSM), showed relatively high saturation magnetization (8.04–22.21 kWb/m2) as well as high coercivity (44.59–63.30 kA/m). Saturation magnetization also increases with increasing heat treatment temperature.

Pulsed laser deposition grown CoFe2O4∕Fe3O4 bilayers and their tunneling characteristics

We have investigated the electrical and magnetic properties of thin epitaxial films of CoFe2O4 on Fe3O4 under layers. CoFe2O4 may be a promising barrier for use in spin dependent tunneling devices using half-metallic Fe3O4 as an electrode due to the small lattice mismatch between the two layers. We were able to characterize the electrical properties of the CoFe2O4 layer by measuring the local tunneling current with an atomic force microscope equipped with a conductive tip. The measured barrier height as determined by fitting the current–voltage curves to Simmons' formula was 0.29eV.

Transmission Mössbauer Spectroscopy on Pulsed-Laser-Deposited Thick CoFe2O4Films

57Fe Mossbauer spectra of pulsed-laser deposited (PLD) films of CoFe2O4 of 0.3 µm thickness is investigated using transmission geometry is reported. Mossbauer parameters were determined for the tetrahedral (A) and octahedral (B) sites. The PLD processed films gave measurable spectra with no visible evidence of clustering or multiple phases present. Results on the films agreed with those of the bulk material. The films exhibited magnetic hyperfine and quadruple splittings similar to that of bulk CoFe2O4. This work demonstrates that measurable transmission Mossbauer spectra may be obtained for PLD deposited CoFe2O4 thick films.

Microstructure of Sputtered CoFe2O4 Film

The growth morphology of CoFe2O4 (Co ferrite) film deposited by the sputtering method was investigated at substrate temperatures of 330–870 K. Co ferrite films were grown on (001) MgO single crystals epitaxially. Results, however, showed that the increase of substrate temperatures leads to the formation of Co precipitates in a Co ferrite matrix. Careful inspection of X-ray diffraction patterns revealed that γ-Fe2O3 formed in the films deposited at the substrate temperature of 330 K, which indicates that decomposition of Co ferrite had took place during deposition. The crystal structures of Co in the matrix were the hcp and fcc structures at the substrate temperatures of 670 and 870 K, respectively. Co was elongated in a filament-like fashion in the direction of growth in the film deposited at 670 K, but had a particle-like appearance in the film deposited at 870 K. The growth morphology of Co in the Co ferrite matrix can be understood by the interfacial energy and the anisotropic structure of hcp Co.