Ferrite Films Research Articles

Topotactic fluorination induced stable structure and tunable electronic transport in perovskite barium ferrite thin films

The synthesis of perovskite (ABO3) films with mixed anions has enabled us to constitute a novel class of materials exhibiting intriguing functionalities. In contrast to traditional metallic cation doping, anion doping within easy-to-prepare conditions usually gives rise to tremendous modulations of physical properties with tunable crystal structures. Here, oxyfluoride BaFeO3-xFy (BFO–F) thin films were successfully synthesized via a low-temperature reaction with polyvinylidene fluoride, which exhibit extremely structural stability over time. The existence of fluorine was confirmed by the combination of X-ray photoelectron depth profile and soft X-ray absorption spectroscopy. The oxyfluoride BFO–F films demonstrate a sequence of structural evolutions with reduced out-of-plane lattices, by means of elevating fluorination temperatures. The valence state of iron in BFO–F is gradually changed from 3+ to 4+ for higher fluorination temperatures. Moreover, the resistivity of BFO–F dramatically decreases, compared to that of pristine oxygen-deficient BaFeO3-δ film. Our results provide insight into the modulations on the structures and physical properties of perovskite oxyfluorides through the facile fluorination process and contribute to the fundamental understanding of fluorination.

Quantitative analysis of the direct piezoelectric response of bismuth ferrite films by scanning probe microscopy

Polarisation domain structure is a microstructure specific to ferroelectrics and plays a role in their various fascinating characteristics. The piezoelectric properties of ferroelectrics are influenced by the domain wall contribution. This study provides a direct observation of the contribution of domain walls to the direct piezoelectric response of bismuth ferrite (BiFeO3) films, which have been widely studied as lead-free piezoelectrics. To achieve this purpose, we developed a scanning probe microscopy-based measurement technique, termed direct piezoelectric response microscopy (DPRM), to observe the domain structure of BiFeO3 films via the direct piezoelectric response. Quantitative analysis of the direct piezoelectric response obtained by DPRM, detailed analysis of the domain structure by conventional piezoelectric force microscopy, and microscopic characterisation of the direct piezoelectric properties of BiFeO3 films with different domain structures revealed that their direct piezoelectric response is enhanced by the walls between the domains of spontaneous polarisation in the same out-of-plane direction.

PZT\u2013NZF/CF ferrite flexible thick films: Structural, dielectric, ferroelectric, and magnetic characterization

The preparation and properties of thick flexible three-phase composite films based on lead zirconium titanate (PZT) and various ferrites (nickel zinc ferrite (NZF) and cobalt ferrite (CF)) were reported in this study. Properties of three-phase composite films were compared with pure polyvinylidene fluoride (PVDF) and PZT–PVDF films. X-ray diffraction data indicated the formation of well crystallized structure of PZT and NZF/CF phases, without the presence of undesirable phases. Scanning electron micrographs showed that the ceramic particles were dispersed homogeneously in the PVDF matrix and atomic force microscopy confirmed that the size of the particles is around 30 nm. Non-saturated hysteresis loops were evident in all samples due to the presence of highly conductive ferrite phases. Under magnetic field of 10 kOe, composite films exhibited a typical ferromagnetic response. Dielectric properties were investigated in the temperature range from −128 to 250 °C and frequency range of 400 Hz–1 MHz. The results showed that the value of dielectric constant of the PVDF/PZT/ferrite composites increased about 25% above the one obtained for pure PVDF.

Effect of Structural Inhomogeneities on the Spin-Wave and Magnetic Properties of Monocrystalline Spinel Ferrite Films in the Microwave Range

We have studied magnetic parameters of monocrystalline manganese spinel ferrite films differing in composition and structural perfection and assessed the effect of dislocation distribution on the frequency dependence of the ferromagnetic resonance (FMR) linewidth in the microwave range. The results demonstrate that, if an external magnetic field is perpendicular to the film surface, electron paramagnetic resonance (EPR) absorption lines correspond to ferromagnetic and spin wave resonances (SWRs). SWR is due to nanostructural surface inhomogeneities. The EPR spectrum of a tangently magnetized film represents a single dipole exchange oscillation spectrum. Parameters of dipole and exchange modes have been determined.

Enhancement of magnetization in substituted Zn ferrite thin films

We report the effect of substitution of 10% divalent ion (Mn2+, Co2+ and Ni2+) on the magnetization of Zn ferrite thin films. M0.1Zn0.9Fe2O4 (M = Mn2+, Co2+ and Ni2+) thin films were pulse laser deposited on to fused quartz substrate at substrate temperatures (TS) ranging from room temperature (RT) to 750 °C. Some of the films deposited with substrate at room temperature were also annealed at different temperatures (TA). While the bulk samples of similar composition are paramagnetic at room temperature, the films were found to be magnetically ordered. The values of magnetization obtained were dependent on TS and TA, similar to pure Zn ferrite films. However, the magnetization values for substituted films were stable over a much larger range of substrate temperatures than pure Zn ferrite films. The thin films of Co0.1Zn0.9Fe2O4, Mn0.1Zn0.9Fe2O4 and Ni0.1Zn0.9Fe2O4 deposited at TS = 750 °C showed the 10 K magnetization values of 5.8 kG, 4.4 kG and 4.5 kG respectively and 0.1 kG, 0.3 kG and 0.2 kG at RT respectively. These values are to be compared with the reported values of ZnFe2O4 thin films deposited at TS = 750 °C, which showed magnetization value of 0.7 kG and 0.2 kG at 10 K and RT respectively. The magnetic properties of bulk and thin films of M0.1Zn0.9Fe2O4 were explained based on the cation occupancy of A and B-sites.

Tape casting and characterization of yttrium iron garnet ferrite thick film for microwave substrate application

In this study, the tape casting and characterization of a yttrium iron garnet (YIG) ferrite thick film was carried out for microwave substrate applications. A high-performance YIG ferrite thick film was developed using a well-dispersed non-aqueous YIG slurry. The resulting green YIG film exhibited a smooth surface and a uniform particle-packing structure with a green density of 3.12 g/cm3. The YIG substrate sintered at 1480 °C exhibited a dense microstructure and good properties with a bulk density of 5.06 g/cm3, dielectric constant (εr) of 14.2, dielectric loss (tanδe) of 1.8 × 10−4 (10 GHz), saturate magnetization (4πMs) of 168.2 mT, coercive force (Hc) of 0.47 Oe, and ferromagnetic resonance linewidth (ΔH) of 5.2 KA/m. Thus, the YIG casting film developed in this study was found to be suitable for microwave substrate applications.

Dynamics and manipulation of ferroelectric domain walls in bismuth ferrite thin films.

Ferroelectric domain walls differ from domains not only in their crystalline and discrete symmetry, but also in their electronic, magnetic, and mechanical properties. Although domain walls provide a degree of freedom to regulate the physical properties at the nanoscale, the relatively lower controllability prevents their practical applications in nano-devices. In this work, with the advantages of 3D domain configuration detection based on piezoresponse force microscopy, we find that the mobility of three types of domain walls (tail-to-tail, head-to-tail, head-to-head) in (001) BiFeO3 films varies with the applied electrical field. Under low voltages, head-to-tail domain walls are more mobile than other domain walls, while, under high voltages, tail-to-tail domain walls become rather active and possess relatively long average lengths. This is due to the high nucleation energy and relatively low growth energy for charged domain walls. Finally, we demonstrate the manipulation of domain walls through successive electric writings, resulting in well-aligned conduction paths as designed, paving the way for their application in advanced spintronic, memory and communication nano-devices.

Microwave Tunable Devices on the YIG-VO2 structures

The theory describing a tunability of the spin-wave spectrum for the ferrite/vanadium dioxide layered structures through changing the VO2 conductivity is suggested. An influence of the various parameters on the spin-wave wavenumber variations and damping decrement is studied. We show that an effective wavenumber variation with minimal losses is achieved by matching the thicknesses of the ferrite and vanadium dioxide films, as well as the value of external magnetic field. We show also that owing to the electrodynamic interaction, a quality of contact between the ferrite and vanadium dioxide layers is not critical.

Magnetooptical properties of epitaxial strained ferrite films grown by MOCVD method

CoFe2O4 (111) epitaxial films on a single crystalline (100) ZrO2(Y2O3) substrates and non-oriented CoFe2O4 films on a metal tape were grown by the metalloorganic chemical vapour deposition (MOCVD) method. The CoFe2O4/ZrO2(Y2O3) films contain the coherent high-angle boundaries between variant domains due to the complex epitaxial distortions compared to polycrystalline state of the CoFe2O4/(metal tape) film. The structural peculiarities of films with variant structure lead to the strong anisotropy of its magnetooptical properties. Moreover a significant magnetorefractive effect, up to 1 % at 6 kOe, is revealed in both the polycrystalline and epitaxial films with variant structure in the near infrared spectral range.

Formation of ultrathin cobalt ferrite films by interdiffusion ofFe3O4/CoObilayers

In this work an alternate pathway is demonstrated to form ultrathin cobalt ferrite (${\mathrm{Co}}_{x}{\mathrm{Fe}}_{3\ensuremath{-}x}{\mathrm{O}}_{4}$) films by interdiffusion of ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$/CoO bilayers. Bilayer samples with different ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$/CoO thickness ratios have been prepared by reactive molecular beam epitaxy on Nb-doped ${\mathrm{SrTiO}}_{3}$(001) substrates to obtain cobalt ferrite films of varied stoichiometry. Subsequently, oxygen-assisted postdeposition annealing experiments for consecutive temperature steps between $300{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ and $600{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ have been conducted monitoring the interdiffusion process by means of high-resolution x-ray reflectivity, soft and angle-resolved hard x-ray photoelectron, and x-ray absorption spectroscopy. Magnetic properties were characterized using superconducting quantum interference device magnetometry. The interdiffusion process starts from $300{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ annealing temperature and is completed for temperatures above $500{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$. For completely interdiffused films with Co:Fe ratios larger than 0.84:2 a thin segregated CoO layer on top of the ferrite is formed. This CoO segregation is attributed to surface and interface effects. In addition, multiplet calculations of x-ray absorption spectra are performed to determine the occupancy of different sublattices. These results are correlated with the magnetic properties of the ferrite films. A stoichiometric ${\mathrm{CoFe}}_{2}{\mathrm{O}}_{4}$ film with partial inversion has been formed exhibiting homogeneously distributed ${\mathrm{Co}}^{2+}$ and mainly ${\mathrm{Fe}}^{3+}$ valence states if the initial Co:Fe content is 1.09:2. Thus, for the formation of stoichiometric cobalt ferrite by the proposed postdeposition annealing technique an initial Co excess has to be provided as the formation of a top CoO layer is inevitable.

Tunable enhanced sensing of ferrite film using meander-shaped spoof surface plasmon polariton waveguide

In this letter, the magnetic field enhancement characteristics of the spoof surface plasmon polaritons (SSPPs) is utilized to enhance the sensing of thin ferrite film. The operating sensing frequency can be tuned by adjusting the bias static magnetic field. For demonstration, a magnetic yttrium iron garnet film (i.e., YIG) with small thickness of 2 μm over a simple SSPP-based sensing structure is designed, fabricated and measured, showing improvement of higher than 20 dB to the absorption (i.e., sensing ability). The proposed method has significant potentials in the application of plasmonic integrated devices using thin ferrite materials.

Effect of atomic replacement on the magnetic anisotropy in epitaxially grown ferrite thin films

In the present work, NiFe2O4 (NFO) and MgFe2O4 (MFO) thin films are deposited on MgAl2O4 (MAO)(1 0 0) substrate using pulsed laser deposition technique to investigate the atomic replacement effects on the magnetic anisotropy of the films. X-ray diffraction analysis confirms the c-oriented growth of the films. Magnetic field dependent magnetization discloses the soft magnetic nature, lower saturation magnetic field and large magnetic anisotropy in MFO(25 and 50 nm) films as compared to NFO(27 and 47 nm). In particular, MFO(50 nm) film exhibits a symmetrical ferromagnetic resonance spectrum in contrast to the bulk counterpart. The angular dependence of the resonance field reveals the uniaxial magnetic anisotropy in MFO(50 nm) film, making it one of the potential candidates for energy-efficient information and communication based spintronic devices. An induced magnetic anisotropy in MFO may be ascribed to the magneto-elastic Ni2+ ions replacement with non-magneto-elastic Mg2+ ions.

Effect of hydrothermal annealing on structure and magnetic properties of RF sputtered Mn-Zn Ferrite thin films

The Mn0.9Zn0.1Fe2O4 (MZF) films were deposited by RF-magnetron sputtering on quartz substrates in argon atmosphere at 12 mTorr of partial gas pressure using RF power of 100 W. Thickness of the as-grown MZF films were 300, 650 and 970 nm. X-ray diffraction patterns confirm the presence of single-phase cubic spinel structure in all the as-grown films. The magnetization at 300 K is found to decrease with increasing film thickness. Whereas, the magnetization at 80 K is found to increase with increasing film thickness. The strong angular dependence of M(H) plots indicate the presence of anisotropy in the as-grown MZF films. The studies on the hydrothermally heat treated film of 970 nm thickness reveals an increase in crystallite size, homogeneity and magnetization as compared to the as-grown film. The magnetic hysteresis, M(H) curve for the hydrothermally heated film show more isotropic behavior with improved magnetic properties as compared to the as-grown film. The ferromagnetic resonance data complements the magnetization results. The optical transmission studies show an increase in the bandgap of the film from 1.9 to 2.1 eV on hydrothermal annealing.

Stoichiometry, phase, and texture evolution in PLD-Grown hexagonal barium ferrite films as a function of laser process parameters

Barium hexaferrite (BaFe12O19 or BaM) films were grown on c-plane sapphire (0001) substrates by pulsed laser deposition (PLD) to evaluate the effects of laser fluence on their composition, structure, and magnetic properties. A Continuum Surelite pulsed 266 nm Nd:YAG laser was employed, and the laser fluence was varied systemically between 1 and 5.7 [J/cm2]. As the laser fluence changed the stoichiometric transfer between the BaM targets and deposited thin films were affected. The Fe to Ba ratio in the films increased with laser fluence. The films deposited at laser fluences below 4 J/cm2 showed undesirable 3-dimensional island growth. Moreover, an insufficient laser energy resulted in deposition of secondary phases corresponding to Barium monoferrite (BaFe2O4) and Magnetite (Fe3O4). Alternatively, laser fluences above 5 J/cm2 promoted resputtering of the growing film at the substrate and degraded film quality, structure, and magnetic properties. BaM films deposited at 4.8 J/cm2 - the optimal laser fluence - showed excellent c-axis orientation perpendicular to the film plane with an anisotropy field of 16 kOe and saturation magnetization of 4.39 kG. These results clearly demonstrate a strong influence of laser parameters on PLD-grown hexaferrite films and provide a path for high-yield production using pulsed laser depositions systems.

Synergetic effect of thermal and electromagnetic energy on electrical properties of zinc ferrite thin film

In this study, the nanocrystalline ZnFe2O4 thin films were deposited using spray pyrolysis technique. The structure, morphology, and optical and electrical properties of the sample were analyzed. The film shows higher conductivity for selective illumination having wavelength corresponding to the bang gap energy. In case of thermal excitation the rapid increase in conductivity was observed after 500 K. The combined exposure of heat and light affects the conductivity without changing activation energy of the material.

Stabilization of ε−Fe2O3 epitaxial layer on MgO(111)/GaN via an intermediate γ -phase

In the present study we have demonstrated epitaxial stabilization of the metastable magnetically-hard $\varepsilon$-Fe$_2$O$_3$ phase on top of a thin MgO(111) buffer layer grown onto the GaN (0001) surface. The primary purpose to introduce a 4\,nm-thick buffer layer of MgO in between Fe$_2$O$_3$ and GaN was to stop thermal migration of Ga into the iron oxide layer. Though such migration and successive formation of the orthorhombic GaFeO$_3$ was supposed earlier to be a potential trigger of the nucleation of the isostructural $\varepsilon$-Fe$_2$O$_3$, the present work demonstrates that the growth of single crystalline uniform films of epsilon ferrite by pulsed laser deposition is possible even on the MgO capped GaN. The structural properties of the 60\,nm thick Fe$_2$O$_3$ layer on MgO / GaN were probed by electron and x-ray diffraction, both suggesting that the growth of $\varepsilon$-Fe$_2$O$_3$ is preceded by formation of a thin layer of $\gamma$-Fe$_2$O$_3$. The presence of the magnetically hard epsilon ferrite was independently confirmed by temperature dependent magnetometry measurements. The depth-resolved x-ray and polarized neutron reflectometry reveal that the 10\,nm iron oxide layer at the interface has a lower density and a higher magnetization than the main volume of the $\varepsilon$-Fe$_2$O$_3$ film. The density and magnetic moment depth profiles derived from fitting the reflectometry data are in a good agreement with the presence of the magnetically degraded $\gamma$-Fe$_2$O$_3$ transition layer between MgO and $\varepsilon$-Fe$_2$O$_3$. The natural occurrence of the interface between magnetoelectric $\varepsilon$- and spin caloritronic $\gamma$- iron oxide phases can enable further opportunities to design novel all-oxide-on-semiconductor devices.

Magnetoelectric response and tunneling magnetoresistance behavior of flexible P(VDF-H FP)/Cobalt ferrite nanofiber composite films

Fabrication of magnetoelectric (ME) polymer composite films by embedding ferromagnetic cobalt ferrite (CoFe2O4) nanofibers into electroactive poly(vinylidene fluoride–hexafluoropropylene) (P(VDF-HFP)) matrix is reported. Single-phase CoFe2O4 nanofibers made of cubic spinel nano crystallites are synthesized by using electro-spinning method, whereas the solution-casting technique is adapted to prepare flexible polymer composite films. The influence of CoFe2O4 nanofiber on structural, functional, magnetic, ferroelectric, and magnetoresistance properties of the composite films is investigated. The cross-coupling between ferroelectric and ferromagnetic orderings is ensured, by the variations of ferroelectric response at different magnetic fields. These magnetoelectric films are found to exhibit a negative tunneling magnetoresistance (TMR) effect with maximum TMR% of 28 for the film with 20 wt% of CoFe2O4 loading. The dielectric constant and electrical energy storage density of the films are increased with the addition of CoFe2O4 nanofiber. The ME films exhibiting TMR and high energy density can be the potential candidates for multifunctional device applications such as memory and spintronics devices, magnetic sensor, and bio-sensor.

Role of interface quality for the spin Hall magnetoresistance in nickel ferrite thin films with bulk-like magnetic properties

We utilized spin Hall magnetoresistance (SMR) measurements to experimentally investigate pure spin current transport in thin film heterostructures of nickel ferrite (NiFe2O4,NFO) and normal metals (NM) Ta and Pt. We grew (001)-oriented NFO thin films by pulsed laser deposition on lattice-matched magnesium gallate (MgGa2O4) substrates, thereby significantly improving their magnetic and structural properties. We performed SMR measurements at room temperature in patterned Hall bar structures for charge currents applied in the [100]- and [110]-directions of NFO. We found that the extracted SMR magnitude for NFO/Pt heterostructures depends crucially on the Pt resistivity of the investigated Hall bar structure. We further study this resistivity scaling of the SMR effect at different temperatures for NFO/Pt. Our results suggest that the spin mixing conductance of the NFO/Pt interface and the Pt resistivity depend on the interface quality and thus a correlation between these two quantities exists.

Spin-wave diagnostics of the magnetization distribution over the thickness of a ferrite film

High-intensity microwave excitation of short-wavelength exchange spin waves in a normally magnetized epitaxial film of an yttrium iron garnet (YIG) has been found. The waves were excited in a thin diffusion layer at the boundary between the YIG film and a nonmagnetic gadolinium gallium garnet substrate. It propagates into the YIG film. In the pulsed excitation mode, a series of delayed echo pulses of the exchange waves reflected from the opposite surface of the film was observed. The distribution function of the magnetization over the thickness of the ferrite film was calculated based on the measurements of the time delay of the echo pulses. The dispersion characteristics of the detected waves were calculated.

Design of zinc ferrite thin films with excess tetrahedrally coordinated Fe3+ ions and their magnetic interactions

In the present work, zinc ferrite thin films on glass substrates were grown using radio-frequency sputtering method in oxygen environment at base pressure of 2 × 10−6 Torr. As-grown films were annealed at 200 °C for 1, 3 and 5 h. X-ray diffraction study envisaged the existence of peaks corresponding to (311) and (440) planes of cubic spinel structure. Both the lattice parameter and crystallite size increased with annealing time leading to crystal perfection with annealing time. Fourier transform infrared spectroscopic study showed presence of bands corresponding to metal-oxygen vibrations in the spinel lattice. Fe K-edge extended X-ray absorption fine structure (EXAFS) measurements revealed that the tetrahedral site occupancy of Fe3+ ions are 1.2 for as-grown zinc ferrite thin film. This occupancy is 1.18, 1.12 and 1.09 for annealing times of 1, 3 and 5 h, respectively. These observations were supported by Zn K-edge EXAFS investigations. X-ray magnetic circular dichroism measurements performed for these films show negative exchange interaction among the Fe3+ ions situtated at different sites. Thus, zinc ferrite thin films having excess occupancy at tetrahedral site was successfully designed on glass substrate.