The temperature dependence of the Seebeck coefficient of homoepitaxial metal organic vapor phase (MOVPE) grown, silicon doped $β$-Ga$_2$O$_3$ thin films was measured relative to aluminum. For room temperature we found the relative Seebeck coefficient of $S_{β\text{-Ga}_2\text{O}_3\text{-Al}}=(-300\pm20)\;μ$V/K. At high bath temperatures $T>240$ K, the scattering is determined by electron-phonon-interaction. At lower bath temperatures between $T=100$ K and $T=300$ K, an increase in the magnitude of the Seebeck coefficient is explained in the frame of Strattons formula. The influence of the different scattering mechanisms on the magnitude of the Seebeck coefficient is discussed and compared with Hall measurement results.
We report the first experimental observation of the spin Seebeck effect in magnetite thin films. The signal observed at temperatures above the Verwey transition is a contribution from both the anomalous Nernst (ANE) and spin Seebeck effects (SSE). The contribution from the ANE of the Fe3O4 layer to the SSE is found to be negligible due to the resistivity difference between Fe3O4 and Pt layers. Below the Verwey transition the SSE is free from the ANE of the ferromagnetic layer and it is also found to dominate over the ANE due to magnetic proximity effect on the Pt layer.
We report a longitudinal spin Seebeck effect (SSE) study in epitaxially grown FeF2(110) antiferromagnetic (AFM) thin films with strong uniaxial anisotropy over the temperature range of 3.8 - 250 K. Both the magnetic field- and temperature-dependent SSE signals below the Néel temperature (TN=70 K) of the FeF2 films are consistent with a theoretical model based on the excitations of AFM magnons without any net induced static magnetic moment. In addition to the characteristic low-temperature SSE peak associated with the AFM magnons, there is another SSE peak at TN which extends well into the para…
The longitudinal spin Seebeck effect (LSSE) has been investigated in high-quality epitaxial CoFe2O4 (CFO) thin films. The thermally excited spin currents in the CFO films are electrically detected in adjacent Pt layers due to the inverse spin Hall effect (ISHE). The LSSE signal exhibits a linear increase with increasing temperature gradient, yielding a LSSE coefficient of ~100 nV/K at room temperature. The temperature dependence of the LSSE is investigated from room temperature down to 30 K, showing a significant reduction at low temperatures, revealing that the total amount of thermally gener…
We demonstrated a flexible thermoelectric (TE) sheet based on the longitudinal spin Seebeck effect (LSSE) that is especially suitable for heat-flow sensing applications. This TE sheet contained a Ni0.2Zn0.3Fe2.5O4 film which was formed on a flexible plastic sheet using a spray-coating method known as ferrite plating. The experimental results suggest that the ferrite-plated film, which has a columnar crystal structure aligned perpendicular to the film plane, functions as a unique one-dimensional spin-current conductor suitable for bendable LSSE-based sensors.
We report a systematic study on the thermoelectric performance of spin Seebeck devices based on Fe3O4/Pt junction systems. We explore two types of device geometries: a spin Hall thermopile and spin Seebeck multilayer structures. The spin Hall thermopile increases the sensitivity of the spin Seebeck effect, while the increase in the sample internal resistance has a detrimental effect on the output power. We found that the spin Seebeck multilayers can overcome this limitation since the multilayers exhibit the enhancement of the thermoelectric voltage and the reduction of the internal resistance …
The electronic transport and spincaloric properties of epitaxial magnetic tunnel junctions with half-metallic Co$_2$MnSi Heusler electrodes, MgO tunneling barriers, and different interface terminations are investigated by using first-principles calculations. A new approach to spincaloric properties is presented that circumvents the linear response approximation inherent in the Seebeck coefficient and compared to the method of Sivan and Imry. This approach supports two different temperatures in the two electrodes and provides the exact current and/or voltage response of the system. Moreover, it…
We have investigated hybridized magnon-phonon excitation (magnon polarons) in spin Seebeck effects (SSEs) in Bi$_x$Y$_{3-x}$Fe$_{5}$O$_{12}$ (Bi$_x$Y$_{3-x}$IG; $x=0$, $0.5$, and $0.9$) films with Pt contact. We observed sharp peak structures in the magnetic field $H$ dependence of the longitudinal SSE (LSSE) voltages, which appear when the phonon dispersions are tangential to the magnon dispersion curve in Bi$_x$Y$_{3-x}$IG. By increasing the Bi amount $x$, the peak fields in the LSSE shift toward lower $H$ values due to the reduction of the sound velocities in Bi$_x$Y$_{3-x}$IG. We also meas…
We report the experimental observation of the spin Seebeck effect (SSE) in Ag/CoFe noble metal/magnetic metal bilayers with a longitudinal structure. Thermal voltages jointly generated by the anomalous Nernst effect (ANE) and the SSE were detected across the Ag/CoFe/Cu strip with a perpendicular thermal gradient. To effectively separate the SSE and the ANE part of the thermal voltages, we compared the experimental results between the Ag/CoFe/Cu strip and Cu/CoFe/Cu strip, where two samples processed with the heating power instead of the temperature difference through the thin CoFe film. The re…
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 …
We evaluated the thermoelectric properties of longitudinal spin Seebeck devices by using ten different transition metals (TMs). Both the intensity and sign of spin Seebeck coefficients were noticeably dependent on the degree of the inverse spin Hall effect and the resistivity of each TM film. Spin dependent behaviors were also observed under ferromagnetic resonance. These results indicate that the output of the spin Seebeck devices originates in the spin current.
The lattice of (001)-oriented BiFeO$_3$ epitaxial thin film has been identified by synchrotron x-ray diffraction. By choosing proper scattering zones containing the fixed (001) reflection, we have shown that low-symmetry phases similar to a $M_A$ phase exist in the thin film at room temperature. These results demonstrate a change in phase stability from rhombohedral in bulk single crystals, to a modified monoclinic structure in epitaxial thin films.
About ten years after ferroelectricity was first reported in doped HfO2 polycrystalline films, there is tremendous interest in this material and ferroelectric oxides are once again in the spotlight of the memories industry. Great efforts are being made to understand and control ferroelectric properties. Epitaxial films, which have fewer defects and a more controlled microstructure than polycrystalline films, can be very useful for this purpose. Epitaxial films of ferroelectric HfO2 have been much less investigated, but after the first report in 2015 significant progress has been achieved. This…
We report on the spin-Hall magnetoresistance (SMR) and spin Seebeck effect (SSE) in multiferroic CoCr2O4 (CCO) spinel thin films with Pt contacts. We observe a large enhancement of both signals below the spin-spiral (Ts = 28 K) and the spin lock-in transitions (T_{lock_in} = 14 K). The SMR and SSE response in the spin lock-in phase are one order of magnitude larger than those observed at the ferrimagnetic transition temperature (Tc = 94 K), which indicates that the interaction between spins at the Pt|CCO interface is more efficient in the non-collinear magnetic state below Ts and T_{lock-in}. …
In general, it is difficult to access the thermal conductivity of thin insulating films experimentally just by electrical means. Here, we present a new approach utilizing the tunnel magneto-Seebeck effect (TMS) in combination with finite-element modeling (FEM). We detect the laser-induced TMS and the absolute thermovoltage of laser-heated magnetic tunnel junctions with 2.6 nm thin barriers of MgAl$_2$O$_4$ (MAO) and MgO, respectively. A second measurement of the absolute thermovoltage after a dielectric breakdown of the barrier grants insight into the remaining thermovoltage of the stack. Thus…
A thorough understanding of the mean-free-path (MFP) distribution of the energy carriers is crucial to engineer and tune the transport properties of materials. In this context, a significant body of work has investigated the phonon and electron MFP distribution, however, similar studies of the magnon MFP distribution have not been carried out so far. In this work, we used thickness-dependence measurements of the longitudinal spin Seebeck (LSSE) effect of yttrium iron garnet films to reconstruct the cumulative distribution of a SSE related effective magnon MFP. By using the experimental data re…
Doping ferroelectric Hf0.5Zr0.5O2 with La is a promising route to improve endurance. However, the beneficial effect of La on the endurance of polycrystalline films may be accompanied by degradation of the retention. We have investigated the endurance - retention dilemma in La-doped epitaxial films. Compared to undoped epitaxial films, large values of polarization are obtained in a wider thickness range, whereas the coercive fields are similar, and the leakage current is substantially reduced. Compared to polycrystalline La-doped films, epitaxial La-doped films show more fatigue but there is no…
The first epitaxial ferroelectric wurtzite film with clear polarization-electric field hysteresis behavior is presented. The coercive field of this epitaxial Al0.7Sc0.3N film on W/c-sapphire substrate is 0.4 +- 0.3 MV cm-1 (8 %) smaller than that of a conventional fiber textured film on a Pt/TiOx/SiO2/Si substrate, attributed to the 0.01 +- 0.007 Å smaller c-axis lattice parameter in the epitaxial film. The strain and decrease of the coercive field most likely originate from epitaxial strain rather than the mismatch in thermal coefficient of expansion. These results provide an insight for furt…
The rare-earth nickelates (RNiO3) exhibit interesting phenomena such as unusual antiferromagnetic order at wavevector q = (1/2, 0, 1/2) and a tunable insulator-metal transition that are subjects of active research. Here we present temperature-dependent transport measurements of the resistivity, magnetoresistance, Seebeck coefficient, and Hall coefficient (RH) of epitaxial SmNiO3 thin films with varying oxygen stoichiometry. We find that from room temperature through the high temperature insulator-metal transition, the Hall coefficient is hole-like and the Seebeck coefficient is electron-like. …
Realization of intrinsic surface dominant transport in a wide temperature region for a topological insulators (TIs) is an important frontier research in order to promote the progresses of TIs towards the future electronics. We report here systematic measurements of longitudinal electrical transport, Shubnikov-de-Haas (SdH) quantum oscillations, Hall coefficient (R_H^2D), and Seebeck coefficient as a function of film thickness (d) and temperature using high quality Bi2-xSbxTe3-ySey (BSTS) single crystal thin films grown by physical vapor-phase deposition. The thickness dependence of sheet condu…
At the ultra-thin film limit, quantum confinement strongly improves thermoelectric figure of merit in materials such as Sb$_2$Te$_3$ and Bi$_2$Te$_3$. These high quality films have only been realized using well controlled techniques such as molecular beam epitaxy. We report a two fold increase in the Seebeck coefficient for both p-type Sb$_2$Te$_3$ and n-type Bi$_2$Te$_3$ using thermal co-evaporation, an affordable approach. At the thick film limit greater than 100 nm, their Seebeck coefficients are around 100 $μV/K$, similar to results obtained in other work. When the films are thinner than 5…
We found a giant Seebeck effect in semiconducting single-wall carbon nanotube (SWCNT) films, which exhibited a performance comparable to that of commercial Bi2Te3 alloys. Carrier doping of semiconducting SWCNT films further improved the thermoelectric performance. These results were reproduced well by first-principles transport simulations based on a simple SWCNT junction model. These findings suggest strategies that pave the way for emerging printed, all-carbon, flexible thermoelectric devices.
In order to obtain Ni-Mn-Ga epitaxial films crystallized in martensite structures showing Magnetic-Induced Rearrangement (MIR) of martensite variants, a fine control of the composition is required. Here we present how the co-sputtering process might be helpful in the development of Ni-Mn-Ga epitaxial films. A batch of epitaxial Ni-Mn-Ga films deposited by co-sputtering of a Ni-Mn-Ga ternary target and a pure manganese target has been studied. The co-sputtering process allows a precise control of the film compositions and enables keeping the epitaxial growth of Ni-Mn-Ga austenite during deposit…
The authors demonstrated that a full-Heusler Co2FeGe (CFG) alloy thin film was epitaxially grown by rapid-thermal-annealing-induced germanidation of an Fe/Co/pseudo-Ge(001)-on-insulator (GOI) multilayer formed on a Si-on-insulator (SOI) substrate. X-ray diffraction (XRD) measurements with the out-of-plane and in-plane configurations revealed that the CFG film was epitaxially grown along the [001] direction with the in-plane epitaxial relation of CFG[100]||GOI[100], although the film slightly contained a texture component. The strong (111) and (200) superlattice diffraction intensities indicate…
Thin epitaxial GaAs films, with thickness varying from 140 to 1000 nm and different Si doping levels, were grown at 650C by organometallic vapor phase epitaxy (OMVPE) on Ge substrates and extensively analyzed by low-temperature photoluminescence (PL) spectroscopy.
Epitaxial copper thin films were deposited by magnetron sputtering. The adatoms during deposition are influenced by deposition parameters which cause variations in thin film properties. XRD and FESEM studies were done to get an insight into the growth mechanisms of the films. A modeling has been done on the epitaxial thin film growth with sputtering process. The parameters during sputtering like, sputtering yield, pressure, temperature, current density, deposition time were related and an attempt has been made to analyze the sputtering process.
We present results on growth of large area epitaxial ReS2 thin film both on c plane sapphire substrate and MoS2 template by pulsed laser deposition (PLD). Films tend to grow with (0001) ReS2 perpendicular to (0001) Al2O3 and (0001) ReS2 perpendicular to (0001) MoS2 parallel to (0001) Al2O3 at deposition temperature below 300 deg C. Films are polycrystalline grown at temperature above 300 deg C. The smoothness and quality of the films are significantly improved when grown on MoS2 template compared to sapphire substrate. The results show that PLD is suitable to grow ReS2 epitaxial thin film over…
A method is developed which allows to determine the first-order and the second-order magnetoelastic coefficients of a magnetic bulk material from the ab-initio calculation of the magnetocrystalline anisotropy energy as function of a prestrain. Excplicit results are given for bcc Fe, and they agree well with experimental data obtained from the magnetostrictive stress measurements for epitaxial Fe films.
Transition metal oxide thin films show versatile electrical, magnetic, and thermal properties which can be tailored by deliberately introducing macroscopic grain boundaries via polycrystalline solids. In this study, we focus on the modification of the magnetic and thermal transport properties by fabricating single- and polycrystalline epitaxial SrRuO3 thin films using pulsed laser epitaxy. Using epitaxial stabilization technique with atomically flat polycrystalline SrTiO3 substrate, epitaxial polycrystalline SrRuO3 thin film with crystalline quality of each grain comparable to that of single-c…
We report on the effect of epitaxial strain on magnetic and optical properties of perovskite LaCrO3 (LCO) single crystal thin films. Epitaxial LCO thin films are grown by pulsed laser deposition on proper choice of substrates to impose different strain states. A combined experimental and theoretical approach is used to demonstrate the direct correlation between lattice-strain and functional properties. The magnetization results show that the lattice anisotropy plays a critical role in controlling the magnetic behavior of LCO films. The strain induced tetragonality in the film lattice strongly …
We demonstrate high-quality epitaxial germanium (Ge) films on a metallic silicide, Fe3Si, grown directly on a Ge(111) substrate. Using molecular beam epitaxy techniques, we can obtain an artificially controlled arrangement of silicon (Si) or iron (Fe) atoms at the surface on Fe3Si(111). The Si-terminated Fe3Si(111) surface enables us to grow two-dimensional epitaxial Ge films, whereas the Fe-terminated one causes the three-dimensional epitaxial growth of Ge films. The high-quality Ge grown on the Si-terminated surface has almost no strain, meaning that the Ge films are not grown on the low-tem…
Epitaxial films have the potential to be used as model systems for fundamental investigations on the martensitic transformation in binary NiTi. In this paper, we discuss growth of binary NiTi thin films on single crystalline MgO substrates. Sputter deposition is used to grow NiTi films. Films prepared by complementary preparation routes (with different deposition temperatures and subsequent heat treatments) are investigated by X-ray diffraction, electron microscopy, atomic force microscopy, and electrical resistivity measurements, with the aim of optimizing film properties, particularly to obt…
Temperature-dependent spin-Seebeck effect data on Pt|YIG (Y$_3$Fe$_5$O$_{12}$)|GGG (Gd$_3$Ga$_5$O$_{12}$) are reported for YIG films of various thicknesses. The effect is reported as a spin-Seebeck resistivity (SSR), the inverse spin-Hall field divided by the heat flux, to circumvent uncertainties about temperature gradients inside the films. The SSR is a non-monotonic function of YIG thickness. A diffusive model for magnon transport demonstrates how these data give evidence for the existence of two distinct length scales in thermal spin transport, a spin diffusion length and a magnon energy r…
We showed that a wetting layer in epitaxially strained thin films which decreases with increasing lattice mismatch strain arises due to the variation of nonlinear elastic free energy with film thickness. We calculated how and at what thickness a flat film becomes unstable to perturbations of varying size for films with both isotropic and anisotropic surface tension. We showed that anisotropic surface tension gives rise to a metastable enlarged wetting layer. The perturbation amplitude needed to destabilize this wetting layer decreases with increasing lattice mismatch. We also studied the early…
We analyze X-ray diffraction data used to extract cell parameters of ultrathin films on closely matching substrates. We focus on epitaxial La2/3Sr1/3MnO3 films grown on (001) SrTiO3 single crystalline substrates. It will be shown that, due to extremely high structural similarity of film and substrate, data analysis must explicitly consider the distinct phase of the diffracted waves by substrate and films to extract reliable unit cell parameters. The implications of this finding for the understanding of strain effects in ultrathin films and interfaces will be underlined
Chalcogenide perovskites have emerged as a new class of electronic materials, but fundamental properties and applications of chalcogenide perovskites remain limited by the lack of high quality epitaxial thin films. We report epitaxial thin film growth of BaZrS3, a prototypical chalcogenide, by pulsed laser deposition. X-ray diffraction studies show that the films are strongly textured out of plane and have a clear in-plane epitaxial relationship with the substrate. Electron microscopy studies confirm the presence of epitaxy for the first few layers of the film at the interface, even though awa…
Epitaxial films of bismuth telluride topological insulators have received increasing attention due to potential applications in spintronic and quantum computation. One of the most important properties of epitaxial films is the presence of interface defects due to lateral lattice mismatch since electrically active defects can drastically compromise device performance. By describing hybrid reflections in hexagonal bismuth telluride films on cubic substrates, in-plane lattice mismatches were characterized with accuracy at least 20 times better than using other X-ray diffraction methods, providing…
I present here theory of PZT thin film - MgO substrate system thermal expansion effect. I show that elastic properties of the substrate have to be taken into account to describe this effect.
We investigated the ferroelectric domain wall propagation in epitaxial Pb(Zr,Ti)O3 thin films over a wide temperature range (3 - 300 K). We measured the domain wall velocity under various electric fields and found that the velocity data is strongly nonlinear with electric fields, especially at low temperature. We found that, as one of surface growth problems, our domain wall velocity data from ferroelectric epitaxial film could be classified into the creep, depinning, and flow regimes due to competition between disorder and elasticity. The measured values of velocity and dynamical exponents in…
Thin epitaxial films of metals on insulating substrates are essential for many applications, as conducting layers, in magnetic devices or as templates for further growth. In this work, we report on the growth of epitaxial Ru films on single-crystalline Al$_2$O$_3$(0001) substrates by magnetron sputtering and their subsequent systematic characterization using Rutherford backscattering spectrometry of He ions both in random and in channeling conditions. We include results of a Ru(0001) single crystal for comparison. Analysis of channeling shows that films thicker than 35 nm grow with (0001) orie…
We investigated the origin of ferromagnetism in epitaxially grown Mn-doped Ge thin films. Using low-temperature molecular beam epitaxy, Mn-doped Ge films were successfully grown without precipitation of ferromagnetic Ge-Mn intermetallic compounds, such as Mn5Ge3. Magnetic circular dichroism measurements revealed that the epitaxially grown Mn-doped Ge films exhibited clear ferromagnetic behavior, but the Zeeman splitting observed at the critical points was not induced by the s,p-d exchange interactions. High-resolution transmission electron microscopy and energy dispersive X-ray spectroscopy an…
Properties of complex oxide thin films can be tuned over a range of values as a function of mismatch, composition, orientation, and structure. Here, we report a strategy for growing structured epitaxial thermoelectric thin films leading to improved Seebeck coefficient. Instead of using single-crystal sapphire substrates to support epitaxial growth, Ca$_3$Co$_4$O$_9$ films are deposited, using the Pulsed Laser Deposition technique, onto Al$_2$O$_3$ polycrystalline substrates textured by Spark Plasma Sintering. The structural quality of the 2000 Åthin film was investigated by Transmission Electr…
The experimental data on the thermal properties of the nanostructured perovskite ferroelectrics are presented and analysed. The ability of the modified 3w method for specific heat measurements is discussed and illustrated by the thermal properties of the thin polycrystalline (BaTiO3 on fused SiO2, 50-1100 nm) and epitaxial (BaTiO3 on MgO, 50-500 nm) films. The similarity of specific heat, phase transition excess entropy evolution in polycrystalline films and nanograined ceramics is revealed. The data on the specific heat of epitaxial films show the extremely diffused anomaly near 400 K, the ph…
Magnetite epitaxial thin films have been prepared by pulsed laser deposition at 340 C on MgO and Si substrates. One key result is that the thin film properties are almost identical to the properties of bulk material. For 40 - 50 nm thick films, the saturation magnetization and conductivity are respectively 453 emu/cm^3 and 225 1/(Ohm cm) at room temperature. The Verwey transition is at 117 K. The Hall effect indicates an electron concentration corresponding to 0.22 electrons per formula unit at room temperature. Normal and anomalous Hall effect both have a negative sign.
X-ray diffraction (XRD) data and analysis for epitaxial iron selenide thin films grown by pulsed laser deposition (PLD) are presented. The films contain $β$-FeSe and Fe$_7$Se$_8$ phases in a double epitaxy configuration with the $β$-FeSe phase (001) oriented on the (001) MgO growth substrate. Fe$_7$Se$_8$ simultaneously takes on two different epitaxial orientations in certain growth conditions, exhibiting both (101)- and (001)- orientations. Each of these orientations are verified with the presented XRD data. Additionally, XRD data used to determine the PLD target composition as well as mosaic…
Mechanical restoring forces acting on ferroelastic domain walls displaced from the equilibrium positions in epitaxial films are calculated for various modes of their cooperative translational oscillations. For vibrations of the domain-wall superlattice with the wave vectors corresponding to the center and boundaries of the first Brillouin zone, the soft modes are singled out that are distinguished by a minimum magnitude of the restoring force. It is shown that, in polydomain ferroelectric thin films, the soft modes of wall vibrations may create enormously large contribution to the film permitt…
We report on the generation and transport of thermal spin currents in fully epitaxial γ-Fe$_2$O$_3$/NiO(001)/Pt and Fe$_3$O$_4$/NiO(001)/Pt trilayers. A thermal gradient, perpendicular to the plane of the sample, generates a magnonic spin current in the ferrimagnetic maghemite (γ-Fe$_2$O$_3$) and magnetite (Fe$_3$O$_4$) thin films by means of the spin Seebeck effect. The spin current propagates across the epitaxial, antiferromagnetic insulating NiO layer, before being detected in the Pt layer by the inverse spin Hall effect. The transport of the spin signal is studied as a function of the NiO …
Domain structures of 320 nm thin epitaxial films of ferroelectric PbTiO3 grown by MOCVD technique in identical conditions on SmScO3 and TbScO3 perovskite sub- strates have been investigated by Raman spectroscopy and piezoresponse force microscopy techniques. Phonon frequency shifts and typical domain structure motifs are discussed. The results reveal strikingly different domain structure architecture: domain structures of the PbTiO3 film grown on SmScO3 have dominantly a-domain orientation while strongly preferential c-domain orientation was found in the PbTiO3 film grown on the TbScO3 substra…
We present a study of the synthesis of epitaxial V$_2$O$_3$ films on $c$-plane Al$_2$O$_3$ substrates by reactive dc-magnetron sputtering. The results reveal a temperature window, at substantially lower values than previously reported, wherein epitaxial films can be obtained when deposited on [0001] oriented surfaces. The films display a metal-insulator transition with a change in resistance of up to four orders of magnitude, strongly dependent on the O$_2$ partial pressure during deposition. While the electronic properties of the films show sensitivity to the amount of O$_2$ present during de…
In this paper we report the deposition of epitaxial thin films of Nd1-xSrxCoO3 with x=0, 0.2 and 0.5 on single crystalline substrates (SrTiO3 and LaAlO3) carried out by means of rf-magnetron sputtering. The deposited films are all completely oriented and epitaxial and characterized by a nanocrystalline morphology. As-deposited films have an average roughness around 1 nm while after the thermal treatment this increases up to 20 nm while preserving the nanocrystalline morphology. All the films deposited on SrTiO3 have shown to be under a certain degree of tensile strain while those on the LaAlO3…
Flexoelectricity describes the coupling between polarization and strain/stress gradients in insulating crystals. In this paper, using the Landau-Ginsburg-Devonshire phenomenological approach, we found that flexoelectricity could increase the theoretical critical thickness in epitaxial BaTiO3 thin films, below which the switchable spontaneous polarization vanishes. This increase is remarkable in tensile films while trivial in compressive films due to the electrostriction caused decrease of potential barrier, which can be easily destroyed by the flexoelectricity, between the ferroelectric state …
The metastable orthorhombic phase of hafnia is generally obtained in polycrystalline films, whereas in epitaxial films, its formation has been much less investigated. We have grown Hf0.5Zr0.5O2 films by pulsed laser deposition, and the growth window (temperature and oxygen pressure during deposition and film thickness) for epitaxial stabilization of the ferroelectric phase is mapped. The remnant ferroelectric polarization, up to around 24 uC/cm2, depends on the amount of orthorhombic phase and interplanar spacing and increases with temperature and pressure for a fixed film thickness. The leaka…
We demonstrate room temperature ferroelectricity in the epitaxial thin films of magnetoelectric GaFeO3. Piezo-force measurements show a 180o phase shift of piezoresponse upon switching the electric field indicating nanoscale ferroelectricity in epitaxial thin films of gallium ferrite. Further, temperature dependent impedance analysis with and without the presence of an external magnetic field clearly reveals a pronounced magneto-dielectric effect across the magnetic transition temperature. In addition, our first principles calculations show that Fe ions are not only responsible for ferrimagnet…
We develop a phenomenological thermodynamic theory of ferroelectric BaTiO3 (BT) thin films epitaxially grown on cubic substrates using the Landau-Devonshire eight-order potential. The constructed "misfit-temperature" phase diagram is asymmetrical. We found that, overall view of the phase diagram depends on the values of compliances used in calculations and provide two qualitatively different diagrams. A thermodynamic path for BT film grown onto particular substrate can be found using a plot of the room-temperature tetragonal distortion (c-a)/a as a function of misfit strain.
The mean-field Landau-type theory is used to analyze the polarization properties of epitaxial ferroelectric thin films grown on dissimilar cubic substrates, which induce biaxial compressive stress in the film plane. The intrinsic effect of the film surfaces on the spontaneous polarization is taken into account via the concept of the extrapolation length. The theory simultaneously allows for the influence of the misfit strain imposed on the film lattice by a thick substrate. Numerical calculations are performed for PbTiO3 and BaTiO3 films under an assumption of the polarization reduction in sur…
We demonstrate that SrRuO3 and CaRuO3 thin films undergo a room temperature structural phase transition driven by the substrate imposed epitaxial biaxial strain. As tensile strain increases, ARuO3 (A=Ca, Sr) films transform from the orthorhombic phase which is usually observed in bulk SrRuO3 and CaRuO3 at room temperature, into a tetragonal phase which in bulk samples is only stable at higher temperatures. More importantly, we show that the observed phenomenon strongly affects the electronic and magnetic properties of ARuO3 thin films that are grown on different single crystal substrates which…
A micromechanical model based on the theory of elasticity has been developed to study the configuration of self-assembled impurity nanostructures in high temperature superconducting YBCO films. With the calculated equilibrium strain and elastic energy of the impurity doped film, a phase diagram of lattice mismatches $vs.$ elastic constants of the dopant was obtained for the energetically-preferred orientation of impurity nanorods. The calculation of the nanorod orientation and the film lattice deformation has yielded an excellent agreement with experimental measurements.
