Wafer‐Scale Single‐Crystalline Ferroelectric Perovskite Nanorod Arrays

1D ferroelectric nanostructures are promising for enhanced ferroelectric and piezoelectric performance on the nanoscale, however, their synthesis at the wafer scale using industrially compatible processes is challenging. In order to advance the nanostructure‐based electronics, it is imperative to develop a silicon‐compatible growth technique yielding high volumetric density and an ordered arrangement. Here, a major breakthrough is provided in addressing this need and ordered and close‐packed single crystalline ferroelectric nanorod arrays, of composition PbZr_0.52Ti_0.48O₃ (PZT), grown on commercial grade 3 in. silicon wafer are demonstrated. PZT nanorods exhibit enhanced piezoelectric and ferroelectric performance compared to thin films of similar dimensions. Sandwich structured architecture utilizing 1D PZT nanorod arrays and 2D reduced graphene oxide thin film electrodes is fabricated to provide electrical connection. Combined, these results offer a clear pathway toward integration of ferroelectric nanodevices with commercial silicon electronics.     

Highly (1 0 0)-oriented Pb(Zr0.20Ti0.80)O3/(Pb1−xLax)Ti1−x/4O3 multilayered thin films prepared by RF magnetron sputtering

The Pb(Zr_0.20Ti_0.80)O₃/(Pb_1−xLa_x)Ti_1−x/4O₃ (x = 0, 0.10, 0.15, 0.20) (PZT/PLT_x) multilayered thin films were in situ deposited on the Pt(1 1 1)/Ti/SiO₂/Si(1 0 0) substrates by RF magnetron sputtering technique with a PbO_x buffer layer. With this method, all PZT/PLT_x multilayered thin films possess highly (1 0 0) orientation. The PbO_x buffer layer leads to the (1 0 0) orientation of the multilayered thin films. The effect of the La content in PLT_x layers on the dielectric and ferroelectric properties of the PZT multilayered thin films was systematically investigated. The enhanced dielectric and ferroelectric properties are observed in the PZT/PLT_x (x = 0.15) multilayered thin films. The dielectric constant reaches maximum value of 365 at 1 KHz for x = 0.15 with a low loss tangent of 0.0301. Along with enhanced dielectric properties, the multilayered thin films also exhibit large remnant polarization value of 2P_r = 76.5 μC/cm², and low coercive field of 2E_c = 238 KV/cm.     

Hierarchically Ordered Nano‐Heterostructured PZT Thin Films with Enhanced Ferroelectric Properties

Realization of ferroelectric (FE) devices based on the polarization effects of Pb(Zr_0.52Ti_0.48)O₃ (PZT) has reinforced the investigation of this material in multiple dimensions and length scales. Multi‐level hierarchical nanostructure engineering in PZT thin films offer dual advantages of variable length‐scale and dimensionality. Here, the growth of hierarchically ordered PZT nano‐hetero­structures (Nhs) from PZT seed‐layer deposited on SrTiO₃:Nb (100) substrates, using a physical/chemical combined methodology involving pulsed laser deposition (PLD) and hydrothermal processes, is reported. Systematic SEM, TEM, and Raman spectroscopy studies reveal mixed hetero‐ and homo‐epitaxial growth mechanism. In the final stage, 3D Nh units cross‐link and form a dense network‐like Nh PZT thin‐film. FE polarizations are measured without using any polymer fill‐layer which otherwise introduces huge dielectric losses and lowers the polarization values for a FE device. In benefit, well saturated and symmetric FE hysteresis loops are observed with high degree of squareness and a high remnant polarization (54 μC cm^‐2 at a coercive field of 237 kV cm^‐1). This work provides a pathway towards preparing hierarchical PZT Nhs offering coherent design of high‐performance FE capacitors for data storage technologies in future.     

Influences of embossing technology on Pb(Zr0.3,Ti0.7)O3 ferroelectric thin film

The ferroelectric random access memory (FRAM) which uses ferroelectric thin film as memory material is considered to be a candidate for the next generation memory application. In this work, we apply nano-embossing technology to fabricate Pb(Zr_0.3,Ti_0.7)O₃ (PZT) ferroelectric thin film nanostructures and investigate the influence of the patterning process on the material and ferroelectric properties by using SEM, XRD and Precision Ferroelectric Tester. Embossing process has been optimized for embossing depth and pattern profile. It was found that embossing will result in (1 0 0) preferred orientation of the PZT thin film. The electrical characteristics of patterned and un-patterned PZT films have been also studied for comparison.     

Relaxor Behavior in Ordered Lead Magnesium Niobate (PbMg1/3Nb2/3O3) Thin Films

The local compositional heterogeneity associated with the short‐range ordering of Mg and Nb in PbMg_1/3Nb_2/3O₃ (PMN) is correlated with its characteristic relaxor ferroelectric behavior. Fully ordered PMN is not prepared as a bulk material. This work examines the relaxor behavior in PMN thin films grown at temperatures below 1073 K by artificially reducing the degree of disorder via synthesis of heterostructures with alternate layers of Pb(Mg_2/3Nb_1/3)O₃ and PbNbO₃, as suggested by the random‐site model. 100 nm thick, phase‐pure films are grown epitaxially on (111) SrTiO₃ substrates using alternate target timed pulsed‐laser deposition of Pb(Mg_2/3Nb_1/3)O₃ and PbNbO₃ targets with 20% excess Pb. Selected area electron diffraction confirms the emergence of (1/2, 1/2, 1/2) superlattice spots with randomly distributed ordered domains as large as ≈150 nm. These heterostructures exhibit a dielectric constant of 800, loss tangents of ≈0.03 and 2× remanent polarization of ≈11 µC cm^?2 at room temperature. Polarization–electric field hysteresis loops, Rayleigh data, and optical second‐harmonic generation measurements are consistent with the development of ferroelectric domains below 140 K. Temperature‐dependent permittivity measurements demonstrate reduced frequency dispersion compared to short range ordered PMN films. This work suggests a continuum between normal and relaxor ferroelectric behavior in the engineered PMN thin films.     

Synthesis and characterization of Pb(Zr0.54Ti0.46)O3 thin films on (100)Si using textured YBa2Cu3O7−δ and yttria-stabilized zirconia buffer layers by laser physical vapor deposition technique

We have fabricated high-quality <001> textured Pb(Zr_0.54Ti_0.46)O₃ (PZT) thin films on (00l)Si with interposing <001> textured YBa₂Cu₃O_7−δ (YBCO) and yttria-stabilized zirconia (YSZ) buffer layers using pulsed laser deposition (KrF excimer laser, λ, = 248 nm, τ = 20 nanosecs). The YBCO layer provides a seed for PZT growth and can also act as an electrode for the PZT films, whereas YSZ provides a diffusion barrier as well as a seed for the growth of YBCO films on (001)Si. These heterostructures were characterized using x-ray diffraction, high-resolution transmission electron microscopy, and Rutherford backscattering techniques. The YSZ films were deposited in oxygen ambient (∼9 × 10⁻⁴ Torr) at 775°C on (001)Si substrate having <001>YSZ // <001>Si texture. The YBCO thin films were deposited in-situ in oxygen ambient (200 mTorr) at 650°C. The temperature and oxygen ambient for the PZT deposition were optimized to be 530°C and 0.4-0.6 Torr, respectively. The laser fluence to deposit this multilayer structure was 2.5-5.0 J/cm². The <001> textured perovskite PZT films showed a dielectric constant of 800-1000, a saturation polarization of 37.81 μC/cm², remnant polarization of 24.38 μC/cm² and a coercive field of 125 kV/cm. The effects of processing parameters on microstructure and ferroelectric properties of PZT films and device implications of these structures are discussed.     

Transient Negative Capacitance Effect in Atomic‐Layer‐Deposited Al2O3/Hf0.3Zr0.7O2 Bilayer Thin Film

The negative capacitance (NC) effect is now attracting a great deal of attention in work towards low‐power operation of field effect transistors and extremely large capacitance density in dynamic random access memory. However, to date, observation of the NC effect in dielectric/ferroelectric bilayer capacitors has been limited to the use of epitaxial ferroelectric thin films based on perovskite crystal structures, such as Pb(Zr,Ti)O₃ and BaTiO₃, which is not compatible with current complementary metal oxide semiconductor technology. This work, therefore, reports on the transient NC effect in amorphous‐Al₂O₃/polycrystalline‐Hf_0.3Zr_0.7O₂ bilayer systems prepared using atomic layer deposition. The thin film processing conditions are carefully tuned to achieve the appropriate ferroelectric performances that are a prerequisite for the examination of the transient NC effect. Capacitance enhancement is observed in a wide voltage range in 5–10 nm thick Al₂O₃/Hf_0.3Zr_0.7O₂ bilayer thin films. It is found that the capacitance of the dielectric layer plays a critical role in the determination of additional charge density induced by the NC effect. In addition, inhibition of the leakage current is important for stabilization of nonhysteretic charge–discharge behavior of the bilayers. The mean‐field approximation combined with classical Landau formalism precisely reproduces the experimental results.     

Probing Local and Global Ferroelectric Phase Stability and Polarization Switching in Ordered Macroporous PZT

We describe the characterization, ferroelectric phase stability and polarization switching in strain‐free assemblies of PbZr_0.3Ti_0.7O₃ (PZT) nanostructures. The 3‐dimensionally ordered macroporous structures present uniquely large areas and volumes of PZT where the microstructure is spatially modulated and the composition is homogeneous. Variable temperature powder X‐ray diffraction (XRD) studies show that the global structure is crystalline and tetragonal at room temperature and undergoes a reversible tetragonal to cubic phase transition on heating/cooling. The measured phase‐transition temperature is 50–60 °C lower than bulk PZT of the same composition. The local ferroelectric properties were assessed using piezoresponse force spectroscopy that reveal an enhanced piezoresponse from the nanostructured films and demonstrate that the switching polarization can be spatially mapped across these structures. An enhanced piezoresponse is observed in the nanostructured films which we attribute to the formation of strain free films, thus for the first time we are able to assess the effects of crystallite‐size independently of internal stress. Corresponding polarization distributions have been calculated for the bulk and nanostructured materials using a direct variational method and Landau‐Ginzburg‐Devonshire (LGD) theory. By correlating local and global characterization techniques we have for the first time unambiguously demonstrated the formation of tetragonal and ferroelectric PZT in large volume nanostructured architectures. With the wide range of materials available that can be formed into such controlled architectures we conclude that this study opens a pathway for the effective studies of nanoscale ferroelectrics in uniquely large volumes.     

Microstructure and domain configurations in ferroelectric PbTiO3 and Pb(Zr,Ti)O3 thin fims

Ferroelectric domain configurations in PbTiO₃ and Pb(Zr_xT_1−x)O₃ (PZT, x = 0.3 or 0.5) thin films have been studied by transmission electron microscopy. The PbTiOg and PZT thin films have been deposited by the ionized cluster beam technique and radio frequency sputtering, respectively. The grain size in these thin films is typically less than 0.5 μm. Lamellar 90°-domain features have been observed in both PbTiO₃ and PZT (30/70) samples. The domain walls correspond to the {011} twin boundaries. La-doping and Ca-modification are shown to affect the microstructure of the PZT films. No clear domain feature occurs in the PZT thin film that has composition near the morphotropic phase boundary. The effects of grain sizes are briefly discussed.     

Ferroelectric Materials: Probing Local and Global Ferroelectric Phase Stability and Polarization Switching in Ordered Macroporous PZT (Adv. Funct. Mater. 5/2011)

We describe the characterization, ferroelectric phase stability and polarization switching in strain‐free assemblies of PbZr_0.3Ti_0.7O₃ (PZT) nanostructures. The 3‐dimensionally ordered macroporous structures present uniquely large areas and volumes of PZT where the microstructure is spatially modulated and the composition is homogeneous. Variable temperature powder X‐ray diffraction (XRD) studies show that the global structure is crystalline and tetragonal at room temperature and undergoes a reversible tetragonal to cubic phase transition on heating/cooling. The measured phase‐transition temperature is 50–60 °C lower than bulk PZT of the same composition. The local ferroelectric properties were assessed using piezoresponse force spectroscopy that reveal an enhanced piezoresponse from the nanostructured films and demonstrate that the switching polarization can be spatially mapped across these structures. An enhanced piezoresponse is observed in the nanostructured films which we attribute to the formation of strain free films, thus for the first time we are able to assess the effects of crystallite‐size independently of internal stress. Corresponding polarization distributions have been calculated for the bulk and nanostructured materials using a direct variational method and Landau‐Ginzburg‐Devonshire (LGD) theory. By correlating local and global characterization techniques we have for the first time unambiguously demonstrated the formation of tetragonal and ferroelectric PZT in large volume nanostructured architectures. With the wide range of materials available that can be formed into such controlled architectures we conclude that this study opens a pathway for the effective studies of nanoscale ferroelectrics in uniquely large volumes.     

Fabrication of Microcantilever Sensors Actuated by Piezoelectric Pb(Zr0.52Ti0.48)O3 Thick Films and Determination of Their Electromechanical Characteristics

The integration and the device realization of Pb(Zr, Ti)O₃ (PZT) thick films on Si substrates are known to be extremely difficult because the processing temperature of the PZT thick film is close to the melting point of Si. However, PZT thick‐film devices on Si warrant attention as they are appropriate for biological transducers; they generate large actuating forces and have a relatively high sensitivity for mass detection, especially in liquids. In this study, Pb(Zr_0.52Ti_0.48)O₃ thick‐film cantilever devices are successfully fabricated on a Pt/TiO₂/SiN_x/Si substrate using a screen‐printing method and microelectromechanical systems (MEMS) process. Elastic and electromechanical properties such as the Young's modulus and transverse piezoelectric coefficient are determined from microstructural and electrical analyses for further mechanical study. The calculated Young's modulus of the thick film, 53.9 ± 3.85 GPa, corresponds to the resonant frequency obtained from the measured harmonic oscillation response. The transverse piezoelectric constant, d₃₁, of –20.7 to –18.8 pC N^–1 is comparable to that of a dense thin film. These values promise the possibility of determining the resonance properties of a thick‐film cantilever by designing its structure and then simulating the harmonic oscillation response. Using the PZT thick‐film cantilever, a strong harmonic oscillation with a quality (Q) factor of about 23 is demonstrated in water. The observation of strong harmonic oscillation in liquid implies the feasibility of precise real‐time recognition of biomolecules using PZT thick‐film cantilevers.     

Sol-Gel processing of Nb-doped Pb(Zr,Ti)O3 thin films for ferroelectric memory applications

The ferroelectric properties of Nb-doped PZT thin films prepared by a sol-gel method were evaluated relative to memory device application requirements. Within the range of 0 to 4 mol %, Nb-doping of PZT compositions near the morphotropic phase boundary region (i.e. PZT 53/47) enhanced overall ferroelectric properties by reducing the te-tragonal distortion of the unit cell. A 4 mol % Nb-doped PZT 53/47 thin film (0.26 μm) had a coercivity of 8 V/ μm, a remanence ratio of 0.54, a switchable polarization of 45 μC/cm², and a specific resistivity of 3 x 10⁹ Ω-cm. Nb-doping levels in excess of 5 mol had a detrimental effect on the resulting thin film ferroelectric properties. X-ray diffraction (XRD) analysis of highly doped films showed development of a significant PbO phase accompanied by diffraction line broadening of the perovskite phase. As such, it was postulated that the creation of excessive lead vacancies in the PNZT lattice resulted in PbO accumulation at the grain boundaries which impeded grain growth, and hence, adversely affected ferroelectric switching performance. The fatigue performance of the sol-gel derived thin film capacitor system was a function of switching voltage. At switching fields sufficient to saturate the polarization, the endurance of the thin film capacitor was greater than 10⁹ cycles. Cycling with lower fields reduced endurance values, but in all cases, the switchable polarization decreased linearly with the logarithm of cycles. Nb-doping did not have a significant effect on the fatigue performance.     

Perfect Bi4Ti3O12 Single‐Crystal Films via Flux‐Mediated Epitaxy

Excellent crystallinity of material films and atomic control of their surface/interface, sufficient for the realization of their optimal physical properties, are technological premises for modern functional‐device applications. Bi₄Ti₃O₁₂ and related compounds attract much interest as highly insulating, ferroelectric materials for use in ferroelectric random‐access memories. However, it has been difficult thus far for Bi₄Ti₃O₁₂ films to satisfy such requirements when formed using vapor‐phase epitaxy, owing to the high volatility of Bi in a vacuum. Here, we demonstrate that flux‐mediated epitaxy is one of the most promising and widely applicable concepts to overcome this inevitable problem. The key point of this process is the appropriate selection of a multi‐component flux system. A combinatorial approach has led to the successful discovery of the novel flux composition of Bi–Cu–O for Bi₄Ti₃O₁₂ single‐crystal film growth. The perfect single‐crystal nature of the stoichiometric Bi₄Ti₃O₁₂ film formed has been verified through its giant grain size and electric properties, equivalent to those of bulk single crystals. This demonstration has broad implications, opening up the possibility of preparing stoichiometric single‐crystal oxide films via vapor‐phase epitaxy, even if volatile constituents are required.     

Synthesis of the PZT films deposited on pt-coated (100) Si substrates for nonvolatile memory applications

Ferroelectric lead-zirconate-titanate (PZT) thin films were deposited by the pulsed laser deposition technique on Pt-coated (100) Si substrates. This study was focused on the investigation of the PZT film growth on (100) Si substrate at varying deposition parameters and electrical characterization of the films including hysteresis loop and fatigue properties by RT66A Standardized Ferroelectric Test System. PZT deposited at higher temperature (575°C in 450 mTorr O₂ partial pressure) showed the best crystalline structure. The remnant polarization and the retained polarization of the ferroelectric capacitors were 13 μC/cm² and 20 μC/cm², respectively. The crystallographic properties of the films were determined using the x-ray diffractometer method. The cross-sectional transmission electron microscope results showed very smooth interfaces among different layers of films.     

A Giant Electrocaloric Effect in Nanoscale Antiferroelectric and Ferroelectric Phases Coexisting in a Relaxor Pb0.8Ba0.2ZrO3 Thin Film at Room Temperature

Recently, large electrocaloric effects (ECE) in antiferroelectric sol‐gel PbZr_0.95Ti_0.05O₃ thin films and in ferroelectric polymer P(VDF‐TrFE)55/45 thin films were observed near the ferroelectric Curie temperatures of these materials (495 K and 353 K, respectively). Here a giant ECE (ΔT = 45.3 K and ΔS = 46.9 J K^?1 kg^?1 at 598 kV cm^?1) is obtained in relaxor ferroelectric Pb_0.8Ba_0.2ZrO₃ (PBZ) thin films fabricated on Pt(111)/TiO_x/SiO₂/Si substrates using a sol‐gel method. Nanoscale antiferroelectric (AFE) and ferroelectric (FE) phases coexist at room temperature (290 K) rather than at the Curie temperature (408 K) of the material. The giant ECE in such a system is attributed to the coexistence of AFE and FE phases and a field‐induced nanoscale AFE to FE phase transition. The giant ECE of the thin film makes this a promising material for applications in cooling systems near room temperature.     

Inside Front Cover: Perfect Bi4Ti3O12 Single‐Crystal Films via Flux‐Mediated Epitaxy (Adv. Funct. Mater. 4/2006)

Flux‐mediated epitaxy has been developed for ferroelectric Bi₄Ti₃O₁₂ single‐crystal film growth, as shown on the inside cover. The key point is the selection of an appropriate flux material. A combinatorial high‐throughput screening technique reported by Matsumoto and co‐workers on p. 485 has led to the successful discovery of the novel flux composition, Bi–Cu–O, for Bi₄Ti₃O₁₂ single‐crystal film growth. This flux‐mediated epitaxy is not limited to oxide epitaxy, but is also widely applicable to various promising materials for the realization of non‐Si‐based electronics, such as nitrides, carbides, and halides. Excellent crystallinity of material films and atomic control of their surface/interface, sufficient for the realization of their optimal physical properties, are technological premises for modern functional‐device applications. Bi₄Ti₃O₁₂ and related compounds attract much interest as highly insulating, ferroelectric materials for use in ferroelectric random‐access memories. However, it has been difficult thus far for Bi₄Ti₃O₁₂ films to satisfy such requirements when formed using vapor‐phase epitaxy, owing to the high volatility of Bi in a vacuum. Here, we demonstrate that flux‐mediated epitaxy is one of the most promising and widely applicable concepts to overcome this inevitable problem. The key point of this process is the appropriate selection of a multi‐component flux system. A combinatorial approach has led to the successful discovery of the novel flux composition of Bi–Cu–O for Bi₄Ti₃O₁₂ single‐crystal film growth. The perfect single‐crystal nature of the stoichiometric Bi₄Ti₃O₁₂ film formed has been verified through its giant grain size and electric properties, equivalent to those of bulk single crystals. This demonstration has broad implications, opening up the possibility of preparing stoichiometric single‐crystal oxide films via vapor‐phase epitaxy, even if volatile constituents are required.     

Multifunctional Nanocrystalline Thin Films of Er2O3: Interplay between Nucleation Kinetics and Film Characteristics

In this study, thin films of Er₂O₃ are deposited by low‐pressure metal–organic chemical vapor deposition (MOCVD) using a tris(isopropylcyclopentadienyl)erbium precursor and O₂ on various substrates, including p‐type Si(100), Si(111), Corning glass, and c‐axis‐oriented α‐Al₂O₃(0001). The resulting films are extensively characterized in order to demonstrate their applicability as antireflective and protective coatings and as high‐k gate dielectrics. The interplay existing among the substrate, the nucleation kinetics, and the resulting structural, morphological, optical, and electrical properties of Er₂O₃ thin films is explored. Fast nucleation governed by surface energy minimization characterizes the growth of (111)‐oriented Er₂O₃ on Si(100), glass, and α‐Al₂O_3. Conversely, nonhomogeneous nucleation leads to polycrystalline Er₂O₃ on Si(111) substrates. Er₂O₃ films grown on Si(100) possess superior characteristics. A high refractive index of 2.1 at 589.3 nm, comparable to the value for bulk single crystalline Er₂O₃, a high transparency in the near UV‐vis range, and an optical bandgap of 6.5 eV make Er₂O₃ interesting as an antireflective and protective coating. A static dielectric constant of 12–13 and a density of interface traps as low as 4.2 × 10¹⁰ cm² eV^–1 for 5–10 nm thick Er₂O₃ layers grown on Si(100) render the present Er₂O₃ films interesting also as high‐k dielectrics in complementary metal oxide semiconductor (CMOS) devices.     

PZT,PT干凝胶的制备及应用

采用减压抽滤的方法成功制备了Pb(Zr0.5Ti0.5)O3,PbTiO3干凝胶,并用STA449C差热分析仪表征了干凝胶的性能。干凝胶溶解后得到了性能优良的PZT薄膜和PZT/PT复合膜。采用X射线衍射技术表征了两种薄膜的微观结构及成相特征。薄膜的介电性能及漏电流性能由HP4284ALCR及Keithley6517A来确定。试验结果表明:用减压抽滤得到的干凝胶的方法,可以彻底解决溶胶凝胶中先体存放的问题,得到的铁电薄膜有优良的介电与铁电性能。PZT的相对介电常数与介质损耗分别为424,0.033,PT作为中间层的复合膜的相对介电常数和介质损耗分别为261,0.014;PT薄膜可以调整和改进PZT薄膜的性能,使之达到应用于热释电探测器的要求。     

铁电薄膜非制冷红外探测器的技术发展

介绍了课题组在铁电薄膜的制备、特性研究以及非制冷红外探测技术方面的研究结果。研究获得了几种控制铁电薄膜微结构(如晶粒尺寸、晶粒形状、Ba_1-xSr_xTiO₃(BST)和PbZr_xTi_1-xO₃ (PZT)铁电薄膜的择优取向生长等)的新技术;研制出具有高度自极化特性的0.7Pb(Mg_1/3Nb_2/3)O₃-0.3PbTiO₃ (PMNT)弛豫铁电薄膜;探索了一种具有将红外转化为可见光机制的新型光读出方式;发明了PZT薄膜的低温(400 °C)生长方法。此外,还介绍了256×1线列铁电薄膜非制冷红外探测器的研制及部分成像结果。     

The Effects of Multiphase Formation on Strain Relaxation and Magnetization in Multiferroic BiFeO3 Thin Films

Multiferroic epitaxial Bi‐Fe‐O thin films of different thicknesses (15–500 nm) were grown on SrTiO₃ (001) substrates by pulsed laser deposition under various oxygen partial pressures to investigate the microstructural evolution in the Bi‐Fe‐O system and its effect on misfit strain relaxation and on the magnetic properties of the films. Films grown at low oxygen partial pressure show the canted antiferromagnetic phase α‐Fe₂O₃ embedded in a matrix of BiFeO₃. The ferromagnetic phase, γ‐Fe₂O₃ is found to precipitate inside the α‐Fe₂O₃ grains. The formation of these phases changes the magnetic properties of the films and the misfit strain relaxation mechanism. The multiphase films exhibit both highly strained and fully relaxed BiFeO₃ regions in the same film. The magnetization in the multiphase Bi‐Fe‐O films is controlled by the presence of the γ‐Fe₂O₃ phase rather than heteroepitaxial strain as it is the case in pure single phase BiFeO₃. Also, our results show that this unique accommodation of misfit strain by the formation of α‐Fe₂O₃ gives rise to significant enhancement of the piezo electric properties of BiFeO_3.