Hexagonal Ferrite Thin Films Research Articles

Spin Seebeck effect in W-type and Z-type hexagonal ferrite thin films

The spin Seebeck effect was studied in thin films of hexagonal ferrites of W-type SrCo2−xZnxFe16O27 with x = 0, 1, and 2 and Z-type with the composition Sr3Co2Fe24O41. The present data were compared with the previously obtained data on Y-type Ba2Zn2−xCoxFe12O22 with x = 0 and 2. Our work showed that the SSE signal can also be generated in materials with complicated crystal and magnetic structures. Hexaferrites with the magnetic easy plane were selected except for uniaxial W-type with x = 2, for which the highest saturated SSE 0.144 μV/K was observed; however, magnetic field above 1 T must be applied to achieve saturation. For low field application, hexaferrites with easy magnetization in ab-plane are more suitable, namely, Y-type with x = 2 or W-type with x = 1. SSE is suppressed in Co-substituted hexaferrites due to the random distribution of Co among various sites. Therefore, the Co cations interfere with the long-range magnetic ordering and diminish the spin-wave propagation. Additional suppression of SSE might be invoked by the higher magnetic anisotropy of the Co-substituted hexaferrites, which opens a gap in the magnon spectrum and, thus, reduces the contribution of lower energy magnons with longer diffusion length.

Manipulation of charged domain walls in geometric improper ferroelectric thin films: A phase-field study

Using phase-field simulations, we show how interfaces acting on the geometric-improper ferroelectric polarization of hexagonal manganite and ferrite thin films can be used to control the formation of charged domain walls. We modify the Landau expansion of the free energy valid in bulk to emulate interface effects known from previous cross-sectional experiments, and we verify our model by comparing our results with images obtained in these experiments. We then show how the interface affects the orientation of ferroelectric domain walls in the fully three-dimensional case. Furthermore, we demonstrate that interface effects combined with an external electric field enable us to specifically choose the dominant domain-wall type (head-to-head, tail-to-tail, or neutral). We also find that an electric field can stabilize a novel domain-wall type which only emerges in the improper ferroelectric order but not in the primary structural distortion. Since the domain walls have a conductivity that is different from the interior of the domains, the influence of the interfaces of a thin film on the type and distribution of the walls gives us the possibility to control the transport properties of a material by appropriate thin-film engineering.

Enhanced Switchable Ferroelectric Photovoltaic Effects in Hexagonal Ferrite Thin Films via Strain Engineering.

Ferroelectric photovoltaics (FPVs) are being extensively investigated by virtue of switchable photovoltaic responses and anomalously high photovoltages of ∼104 V. However, FPVs suffer from extremely low photocurrents due to their wide band gaps (Eg). Here, we present a promising FPV based on hexagonal YbFeO3 (h-YbFO) thin-film heterostructure by exploiting its narrow Eg. More importantly, we demonstrate enhanced FPV effects by suitably exploiting the substrate-induced film strain in these h-YbFO-based photovoltaics. A compressive-strained h-YbFO/Pt/MgO heterojunction device shows ∼3 times enhanced photovoltaic efficiency than that of a tensile-strained h-YbFO/Pt/Al2O3 device. We have shown that the enhanced photovoltaic efficiency mainly stems from the enhanced photon absorption over a wide range of the photon energy, coupled with the enhanced polarization under a compressive strain. Density functional theory studies indicate that the compressive strain reduces Eg substantially and enhances the strength of d-d transitions. This study will set a new standard for determining substrates toward thin-film photovoltaics and optoelectronic devices.

Switchable ferroelectric photovoltaic effects in epitaxial h-RFeO3 thin films.

Ferroelectric photovoltaics (FPVs) have drawn much attention owing to their high stability, environmental safety, and anomalously high photovoltages, coupled with reversibly switchable photovoltaic responses. However, FPVs suffer from extremely low photocurrents, which is primarily due to their wide band gaps. Here, we present a new class of FPVs by demonstrating switchable ferroelectric photovoltaic effects and narrow band-gap properties using hexagonal ferrite (h-RFeO3) thin films, where R denotes rare-earth ions. FPVs with narrow band gaps suggest their potential applicability as photovoltaic and optoelectronic devices. The h-RFeO3 films further exhibit reasonably large ferroelectric polarizations (4.7-8.5 μC cm-2), which possibly reduces a rapid recombination rate of the photo-generated electron-hole pairs. The power conversion efficiency (PCE) of h-RFeO3 thin-film devices is sensitive to the magnitude of polarization. In the case of the h-TmFeO3 (h-TFO) thin film, the measured PCE is twice as large as that of the BiFeO3 thin film, a prototypic FPV. The effect of electrical fatigue on FPV responses has been further investigated. This work thus demonstrates a new class of FPVs towards high-efficiency solar cell and optoelectronic applications.

Spin Seebeck effect in Y-type hexagonal ferrite thin films

Spin Seebeck effect (SSE) has been investigated in thin films of two Y-hexagonal ferrites Ba$_2$Zn$_{2}$Fe$_{12}$O$_{22}$ (Zn2Y) and Ba$_2$Co$_{2}$Fe$_{12}$O$_{22}$ (Co2Y) deposited by a spin-coating method on SrTiO$_3$(111) substrate. The selected hexagonal ferrites are both ferrimagnetic with similar magnetic moments at room temperature and both exhibit easy magnetization plane normal to $c$-axis. Despite that, SSE signal was only observed for Zn2Y, whereas no significant SSE signal was detected for Co2Y. We tentatively explain this different behavior by a presence of two different magnetic ions in Co2Y, whose random distribution over octahedral sites interferes the long range ordering and enhances the Gilbert damping constant. The temperature dependence of SSE for Zn2Y was measured and analyzed with regard to the heat flux and temperature gradient relevant to the SSE signal.

SrAl12O19 thin films by chemical solution deposition and their use as buffer layers for oriented growth of hexagonal ferrites

Thin films of SrAl12O19 were prepared on α-Al2O3(0001) substrates through the chemical solution deposition method and thermal treatment. Two types of precursor systems were used: stoichiometric mixture of strontium methoxyethoxide with aluminium iso-butoxide, and strontium methoxyethoxide topotactically reacting with Al2O3 substrate. Two distinctly different film-substrate orientation relationships were observed. In the latter case the orientation can described as perfect hexagon-on-hexagon epitaxy with 112−0SrAl12O19 || 101−0Al2O3 and (0001)Al2O3 || (0001)SrAl12O19. This relationship has 1.17% lattice misfit in the substrate plane. In the former case SrAl12O19 grains are rotated about the substrate normal and form more orientation variants that can be explained by the coincident-site lattice model for grain boundaries. We demonstrated that both types of SrAl12O19 films can be used as a chemical buffer and structural template layer for the growth of oriented hexagonal ferrite thin films. In both types of films the (BaSr)Fe12O19 phase cope the in-plane orientation of their respective templates.

Generation of pure spin currents via spin Seebeck effect in self-biased hexagonal ferrite thin films

Light-induced generation of pure spin currents in a Pt(2.5 nm)/BaFe12O19(1.2 μm)/sapphire(0.5 mm) structure is reported. The BaFe12O19 film had strong in-plane uniaxial anisotropy and was therefore self-biased. Upon exposure to light, a temperature difference (ΔT) was established across the BaFe12O19 thickness that gave rise to a pure spin current in the Pt via the spin Seebeck effect. Via the inverse spin Hall effect, the spin current produced an electric voltage across one of the Pt lateral dimensions. The voltage varied with time in the same manner as ΔT and flipped its sign when the magnetization in BaFe12O19 was reversed.

Oriented hexagonal ferrite thin films prepared by chemical solution deposition

Hexagonal ferrites (M, Y, Z-type) represent a new diverse class of magnetoelectric (ME) multiferroics, where ME effect is driven by complex magnetic order. Integration of ME materials with standard semiconductor technology is important for ultimate realization of ME functionalities. They have the potential to display ME coupling under low magnetic field bias and at temperatures close to room temperature. Methods based on sol–gel transition offer possibility of low cost and efficient way for the evaluation of new material system. The single phase, epitaxial thin films of Y-type hexagonal ferrite has been prepared and studied. Thin films of Ba2Zn2Fe12O22(Y) hexaferrite were prepared through the chemical solution deposition method on SrTiO3(111)(ST) single crystal substrates using epitaxial SrFe12O19(M) hexaferrite thin layer as a seed template layer. The process of crystallization was mainly investigated by means of X-ray diffraction and atomic force microscopy. A detailed inspection revealed that growth of seed layer starts through the break-up of initially continuous film into high density of well-oriented isolated grains with expressive shape anisotropy and hexagonal habit.The vital parameters of the seed layer, i.e. thickness, substrate coverage,crystallization conditions and temperature ramp were optimized with the aim to obtain epitaxially crystallized Y phase. By overcoating this seed layer, Y phase prepared under optimum deposition and heat treatment conditions presents a (001) orientation perpendicular to the substrate. Perfect parallel in-plane alignment of the hexagonal cells of SrTiO3substrate and both hexaferrite phases was proved by fast ω and φ scan measurements on sets of several diffraction planes at asymmetric orientations, and also by pole figures. The soft magnetic character and existence of pronounced magnetic anisotropy in Y films were confirmed by room temperature measurements of magnetization.

Oriented Y-type hexagonal ferrite thin films prepared by chemical solution deposition

Abstract Thin films of Ba2Zn2Fe12O22 (Y) hexaferrite were prepared through the chemical solution deposition method on SrTiO3(1 1 1) (ST) single crystal substrates using epitaxial SrFe12O19 (M) hexaferrite thin layer as a seed template layer. The process of crystallization was mainly investigated by means of X-ray diffraction and atomic force microscopy. A detailed inspection revealed that growth of seed layer starts through the break-up of initially continuous film into isolated grains with expressive shape anisotropy and hexagonal habit. The vital parameters of the seed layer, i.e. thickness, substrate coverage, crystallization conditions and temperature ramp were optimized with the aim to obtain epitaxially crystallized Y phase. X-ray diffraction Pole figure measurements and Φ scans reveal perfect parallel in-plane alignment of SrTiO3 substrate and both hexaferrite phases.

Physical properties of Al doped Ba hexagonal ferrite thin films

We developed the thin film microwave magnetic material, M-type barium hexagonal ferrite (BaM) doped with Al, for signal processing devices operating above 40 GHz with little to no applied magnetic field. Al was chosen as the dopant material because it significantly increases the already strong anisotropy field of BaM. A series of thin film BaAlxFe12-xO19 samples, x ranging from 0 to 2 in 0.25 steps, were deposited on Pt templates using a metal-organic decomposition growth technique. The resulting films are polycrystalline and highly textured, with the hexagonal c-axis directed out of plane. These films are also self-biasing; easy axis hysteresis loops have a high squareness ratio, s, in the 0.83-0.92 range. As expected, the anisotropy field increases with x, ranging from 1.34 to 2.19 × 106 A/m (16.9-27.5 kOe) for x = 0-2, while the saturation magnetization Ms decreases with x, ranging from 0.334 to 0.175 × 106 A/m (4πMs = 4.2-2.2 kG) for x = 0-2. These values were measured at room temperature, but the temperature dependence of these quantities was also measured below room temperature, down to 30 K. The measured ferromagnetic resonance linewidths, on the order of 12-30 × 103 A/m (140–370 Oe) for compositions below x = 1, indicate device-quality films. Above a certain threshold, the linewidth increases linearly with frequency at a rate of 0.2-0.64 × 103 (A/m)/GHz (2.5-8 Oe/GHz) for x = 0–1, respectively. The behavior of the linewidth is correlated with the structural properties of the films measured using x-ray diffraction and atomic force microscopy. The results of magnetic force microscopy, Curie point measurements, spectral ellipsometry (index of refraction), and magneto-optical measurements are also included and discussed.

Planar millimeter wave band-stop filters based on the excitation of confined magnetostatic waves in barium hexagonal ferrite thin film strips

A planar millimeter wave band-stop filter based on confined magnetostatic wave (MSW) excitations in an M-type barium hexagonal ferrite (BaM) film strip was demonstrated. The device consists of a BaM film strip on the top of a coplanar waveguide with the strip length along the signal line. For zero magnetic fields, the device shows a band-stop filtering response at 53 GHz. This response originates from the excitation of confined MSW modes across the BaM strip width. The filter operation frequency is tunable with low fields. This tuning relies on the change in the MSW dispersion with field.

Self-biased planar millimeter wave notch filters based on magnetostatic wave excitation in barium hexagonal ferrite thin films

The use of M-type barium hexagonal ferrite (BaM) thin films for self-biased planar millimeter wave notch filters was demonstrated for the first time. The BaM film was grown by pulsed laser deposition and showed a remanent to saturation magnetization ratio of 0.99 and a 60 GHz ferromagnetic resonance linewidth of about 300 Oe. The filter consisted of a BaM film element positioned on the top of a coplanar waveguide and showed a band-stop response at 53 GHz for zero external fields. This filtering response resulted from power absorption due to magnetostatic wave excitation in the BaM film.

Millimeter wave phase shifter based on ferromagnetic resonance in a hexagonal barium ferrite thin film

A hexagonal ferrite thin film-based planar millimeter-wave phase shifter was demonstrated. The device made use of an M-type barium ferrite (BaM) thin film prepared by pulsed laser deposition and a coplanar waveguide geometry. The phase tuning relied on ferromagnetic resonance in the BaM film. The device showed a phase tuning rate of 43°/(mm kOe) and an insertion loss of 3.1 dB/mm in the on-resonance regime. In off-resonance regimes, the device showed smaller loss and smaller tuning rates. The experimental results were confirmed by theoretical calculations.

Preparation and characterization of barium hexagonal ferrite thin films on a Pt template

M-type barium hexagonal ferrite films with the crystallographic c axis out of plane were successfully deposited onto a Pt template using a metallo-organic decomposition technique. For the best film, x-ray diffraction patterns revealed strong (00l) reflections and a texture fraction of 0.953, confirming the out of plane c axis orientation. Atomic force microscopy images confirm hexagonal grains in this film with an average lateral size of ∼500 nm. Hysteresis loops revealed a high effective out of plane anisotropy field, high perpendicular remanent magnetization Mr=0.93 Ms, and out of plane coercivity of 4.5 kOe. Out of plane Ferromagnetic Resonance measurements determined the values of γ=2.79 GHz/kOe and effective anisotropy field. The full width at half maximum FMR linewidth was 338 Oe at 60 GHz. These properties are suitable for possible use in on-wafer millimeter wave devices.

Preparation of barium ferrite films with high Fe/Ba ratio by sol–gel method

Hexagonal BaFe12O19 ferrite (BaM) thin films were prepared on Si (100) substrate successfully by sol–gel technology and post annealing. The results showed the BaM phase can be formed and crystallized into c-axis textured grains even when the Fe/Ba ratio of the precursor varied from 6.5 to 9.5. However, the behavior of the saturation magnetization (Ms) and intrinsic coercivity (Hc) depended strongly on the Fe/Ba ratio and annealing temperature (Ta) varied from 700 to 900 °C. The Ms and Hc values deceased with an increase of Fe/Ba ratio, for instance, were about 290 emu/cm3 and 4,200 Oe for the Fe/Ba = 6.5 film but only 116 emu/cm3 and 2,300 Oe for the Fe/Ba = 9.5 film. The Ms and Hc values of the Fe/Ba ratio = 6.5 film increased monotonously with increasing Ta, were about 120 emu/cm3 and 2,500 Oe at Ta = 800 °C, and reached to 345 emu/cm3 and 4,600 Oe at Ta = 950 °C.

Ba Fe 12 O 19 thin films grown at the atomic scale from BaFe2O4 and α-Fe2O3 targets

Hexagonal barium ferrite (BaFe12O19) thin films were grown at the atomic scale by alternating target laser ablation deposition (ATLAD) of orthorhombic BaFe2O4 and rhombohedral α-Fe2O3 on ⟨00l⟩ Al2O3 substrates. Crystallographic, magnetic, and microwave properties of the ATLAD films were determined to be comparable with single crystal quality bulk and films of BaFe12O19 produced by other techniques. The ability to deposit high quality hexagonal ferrite thin films by utilizing multiple targets of different chemical compositions in the deposition routine provides unique opportunities to control the ionic distribution in the unit cell of this important class of ferrite materials.

Perpendicular orientation of Ba-ferrite thin film with Al top layer and underlayer

The effectiveness of Al top layer and Al underlayer on the crystallization temperature of hexagonal Barium ferrite (BaM) thin film was studied. All of the BaM films were deposited on Si (111) substrate without any substrate heating. Then, flash annealing was carried out to crystallize the BaM film. A very ultrathin Al top layer of 5 nm was helpful for improving the crystallographic properties of BaM thin film, but the effect was more intensified when Al was used as an underlayer instead of a top layer. From the X-ray diffraction (XRD) diagram, the c axis orientation of BaM film with an Al underlayer was confirmed even though the annealing temperature was reduced to 650/spl deg/C. The saturation magnetization value also supports the XRD data. From the experimental data, it was found that the Al underlayer can reduce the crystallization temperature of BaM in the range of 600/spl deg/C to 625/spl deg/C by flash annealing for 1 min. The nanocrystalline structure of the Al underlayer promotes the crystallization of BaM very remarkably and hence reduces the crystallization temperature.

Crystallization of Ba-Me hexagonal ferrite thin films

Thin films of Ba-Me ferrites are synthesized by reactive rf diode sputtering of a BaO · nFe2O3 ceramic target. Quartz plates subjected to preliminary annealing are used as substrates. The influence of the barium ion content on the crystalline and magnetic properties and the microstructure of the prepared films is investigated, and the interrelation between the quantity dH c /dT and the microstructure of the film is considered. The prepared films satisfy the requirements for materials used as information carriers with a superhigh recording density.

Perpendicular magnetic recording media using hexagonal ferrite thin films deposited on Pt underlayers and interlayers

Ba ferrite (BaM) thin films with excellent c-axis orientation were deposited on Pt(1 1 1) underlayers using Facing Targets Sputtering system. BaM thin films with extremely low thickness, as thin as 8 nm, exhibited apparent c-axis orientation. Such thin BaM layers exhibited small grain size. BaM/Pt multilayered structure was also effective to introduce exchange interaction among BaM crystallites.

Low-temperature growth of c-axis-oriented Y-type hexagonal ferrite thin films by the polymeric precursor method

Well-crystallized Ba2Zn2Fe12O22 (Zn2–Y) films with high c-axis oritentation were successfully formed on Ag substrates at low temperature by the polymeric precursor method. A precursor solution with stoichiometric Ba2+, Zn2+, and Fe3+ ions was deposited on the substrates by a dip-coating. The films were then heat-treated at temperatures ranging from 700 to 900 °C. The crystallization process of c-axis-oriented Zn2–Y films occurred at the considerably low temperature of 750 °C, though a small amount of spinel oxides contaminated them. The films had hexagonal grain structures which were developed by increasing the heat-treatment temperature. Magnetization curves of the Zn2–Y film heated at 900 °C clearly indicated that the film had large in-plane magnetic anisotropy and had small in-plane coercivity.