Growth control of RF magnetron sputtered SrRuO3 thin films through the thickness of LaNiO3 seed layers

SrRuO₃ (SRO) thin films were grown on SiO₂/Si substrates with different thickness of LaNiO₃ (LNO) seed layers by RF magnetron sputtering. Effects of LNO thickness on the grain orientation, surface morphology, magnetic behavior and electrical transport properties of SRO films were investigated. The orientation of SRO films transformed from (110)_pc to (001)_pc and the residual stress was released gradually with increasing the thickness (pc refers to the pseudo-cubic unit cell of SrRuO₃). SRO films with higher orientation grown on LNO exhibited more flat surface, higher saturation magnetization, and lower coercive field. The magnetic anisotropy was enhanced on thicker LNO due to the different states of residual stress. In addition, the temperature dependence of resistivity was promoted by the microstructural disorder. (110)_pc-oriented SRO monolayer electrode and (001)_pc-oriented SRO/LNO300 bilayer electrode own low room temperature sheet resistance of 0.38 Ω/□ and 0.26 Ω/□, respectively. The results indicate that the controllable SRO films can be used as not only good bottom electrodes but also promising templates to control the crystallographic orientations of various other perovskite-based functional materials.     

Microstructures and ferroelectric properties of PbTiO3/PbZrO3 superlattices deposited by pulse laser deposition

In general, the strain effect and the electrostatic effect are important factors influencing the electrical properties of ferroelectric superlattices. However, the interfacial diffusion may also greatly influence the electrical properties of ferroelectric superlattices. Here, we deposited PbTiO₃/PbZrO₃ (PTO/PZO) superlattices on Nb-doped SrTiO₃ (NSTO) single-crystal substrates by pulse laser deposition with the same deposition processes but different cooling processes to explore the effects of interface diffusion on the structural and electrical properties of the superlattices. The experimental results showed that with increasing the holding time in the cooling process after deposition, the ferroelectric and dielectric properties were enhanced, meanwhile, the leakage current density was reduced. Transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS) results showed that the PTO/PZO interfaces became blurred with increasing cooling time. Therefore, the increase in ferroelectric and dielectric properties and the decrease in leakage current density may be due to the interdiffusion of Ti and Zr ions at the PTO/PZO interfaces resulting in the formation of Pb(Zr_xTi_1-x)O₃ (PZT) and the decrease of interface defects at the PTO/PZO interfaces. Our results demonstrate that besides the strain and electrostatic effects, the interdiffusion of the elements at the interfaces is also an important factor that influences the electrical properties of ferroelectric superlattices.     

Metastable SrRuO3 phases with lattice-dependent magnetic anisotropy by tailoring interfacial oxygen octahedral coupling

Tunable magnetic anisotropy can greatly facilitate the integration of perovskite oxides in emerging magnetic memory devices. Tailoring the metastable structures is a powerful and effective method to tune the magnetic anisotropy of perovskite oxides possessing a large spin–lattice correlation. Herein, we stabilized SrRuO₃ (SRO) at two metastable phases by tailoring the interfacial octahedral coupling between SRO and LaCoO₃ (LCO). Two metastable phases, distorted-rhombohedral and tetragonal structures, were stabilized in turn as the SRO thickness increased. Through the stabilization of these metastable phases, we realized a strongly lattice-dependent magnetic anisotropy and magnetic structure symmetry of SRO, employing its large spin–orbit coupling. Detailed octahedral rotation analysis demonstrated that when the SRO film was thin, the rotation phase mismatch between SRO and LCO played a decisive role in stabilizing the distorted-rhombohedral phase, while the competition between the rotation phase mismatch and tilt propagation relaxation induced the tetragonal phase when the SRO film was thick. These results demonstrate the feasibility of tailoring various structural symmetries and exotic functionalities through interface coupling.     

Optical properties of epitaxial BiFeO3 thin film grown on SrRuO3-buffered SrTiO3 substrate

The BiFeO₃ (BFO) thin film was deposited by pulsed-laser deposition on SrRuO₃ (SRO)-buffered (111) SrTiO₃ (STO) substrate. X-ray diffraction pattern reveals a well-grown epitaxial BFO thin film. Atomic force microscopy study indicates that the BFO film is rather dense with a smooth surface. The ellipsometric spectra of the STO substrate, the SRO buffer layer, and the BFO thin film were measured, respectively, in the photon energy range 1.55 to 5.40 eV. Following the dielectric functions of STO and SRO, the ones of BFO described by the Lorentz model are received by fitting the spectra data to a five-medium optical model consisting of a semi-infinite STO substrate/SRO layer/BFO film/surface roughness/air ambient structure. The thickness and the optical constants of the BFO film are obtained. Then a direct bandgap is calculated at 2.68 eV, which is believed to be influenced by near-bandgap transitions. Compared to BFO films on other substrates, the dependence of the bandgap for the BFO thin film on in-plane compressive strain from epitaxial structure is received. Moreover, the bandgap and the transition revealed by the Lorentz model also provide a ground for the assessment of the bandgap for BFO single crystals.     

All-inorganic flexible high-temperature strain sensor based on SrRuO3/muscovite heteroepitaxy

Design and fabrication of flexible strain sensors are still challenging for high-temperature application in the aerospace, metallurgical industry, and underground energy exploration. In the present work, an all-inorganic flexible strain sensor has been fabricated based on single-crystal SrRuO₃ (SRO) thin films epitaxially grown on muscovite. A high resistive tunability ((∆R/R₀%) ∼ 26.05%), excellent linearity (correlation coefficient of 0.93), and large gauge factor (GF ∼ 101) have been achieved in the as-prepared SRO-based sensors with great bending durability. The mechanism for the tunability of the electrical transport in flexible SRO thin films under mechanical bending is proposed based on the Raman and X-ray photoelectron spectroscopy results. The sensors have been confirmed to work properly from ambient temperatures up to ∼425 K, showing the potential application in high-temperature environments. Moreover, the flexible sensors have the ability to monitor the hand joint movements.     

Fabrication of Pb(Zr,Ti)O3 thin films utilizing unconventional powder magnetron sputtering (PMS)

Pb(Zr,Ti)O3 (PZT) ferroelectric ceramic films exhibit highly superior ferroelectric, pyroelectric and piezoelectric properties which are promising for a number of applications including non-volatile random access memory devices, non-linear optics, motion and thermal sensors, tunable microwave systems and in energy harvesting (EH) use. In this research, a thin layer of PZT was deposited on two different substrates of Strontium Titanate (STO) and Strontium ruthenate (SRO) by powder magnetron sputtering (PMS) system. The preliminary powders, consisting of PbO, ZrO₂ and TiO_2, were manually mixed and placed into the target holder of the PMS. The deposition was performed at an elevated temperature reaching up to 600 °C via a ceramic heater. This high temperature is required for PZT thin film crystallinity, which is never achieved in conventional physical vapour deposition processes. The phase structure, crystallite size, stress-strain and surface morphology of deposited thin films were characterized using X-ray Diffraction (XRD) and Field Emission Scanning Electron Microscopy (FESEM). The composition of the PZT thin films were also analysed by X-ray photoelectron spectroscopy (XPS). The mechanical properties of the thin films were evaluated with micro-scratch adhesive strength and micro hardness equipment. FESEM results showed that the PZT thin films were successfully deposited on both SRO and STO substrates. The surfaces of the coated samples were free from cracks, relatively smooth, uniform and dense. The profile of X-ray diffraction confirmed the formation of single-c-domain/single crystal perovskite phase grown on both substrates. The XPS analysis have shown that the PZT thin film grown by this method and that a target of PZT+10% PbO is a proper target for growing nominal PZT thin films. The adhesion strength and micro hardness results have confirmed the stability and durability of the thin film on the substrates, although higher values have been reported for thin film of PZT deposited on SRO surfaces.     

Effects of frequency on electrical fatigue behavior of ZnO-modified Pb(Mg1/3Nb2/3)0.65Ti0.35O3 ceramics

This work aims to study the effects of frequency on the electrical fatigue behavior of ZnO-modified Pb(Mg_1/3Nb_2/3)_0.65Ti_0.35O₃ (PMNT) ceramics. Changes in microstructures, ferroelectric and piezoelectric properties of the ceramics at bipolar electrical fatigue frequencies of 5, 10, 50 and 100 Hz were observed. The thickness of damaged surface of the ceramics decreased with increasing frequency. The degradation of properties of the ceramics fatigued at low frequency was greater than those fatigued at high frequency. The degradation by electrical fatigue at lower frequencies, 5 and 10 Hz, could be caused by the effects of both field screening and domain pinning, while at higher frequencies the fatigue was mainly a result of the field screening effect. The fatigue properties of ZnO-modified PMNT ceramics was compared to Pb-based and Pb-free ferroelectric ceramics. It was found that the fatigue endurance of ZnO-modified PMNT ceramic was greater than that of hard PZT ceramic but less than that of Pb-free ferroelectric ceramic.     

Growth of the ternary Pb(Mn,Nb)O3–Pb(Zr,Ti)O3 thin film with high piezoelectric coefficient on Si by RF sputtering

In this study, ternary ferroelectric 0.06Pb(Mn_1/3Nb_2/3)O₃–0.94Pb(Zr_0.48Ti_0.52)O₃ (PMN–PZT) thin film with high piezoelectric coefficient were grown on La_0.6Sr_0.4CoO₃-buffered Pt/Ti/SiO₂/Si substrate by RF magnetron sputtering method. The phase and domain structure along with the macroscopic electrical properties were obtained. Under the optimized temperature of 550°C and sputtering pressure 0.9 Pa, the PMN–PZT film owned large remnant ferroelectric polarization of 62 μC/cm². In addition, the PMN–PZT film had polydomain structures with fingerprint-type nanosized domain patterns and typical local piezoelectric response. Through piezoelectric force microscopy, the PMN–PZT thin film at nanoscale exhibited obvious domain reversal when subjected to in situ poling field. It was further found that the quasi-static piezoelectric coefficient of the PMN–PZT thin film reached 267 pC/N, which was about twice to that of the commercial PbZrO₃–PbTiO₃ (PZT) thin film. The optimized relaxor ferroelectric thin film PMN–PZT on silicon with global electrical properties shows great potential in the piezoelectric micro-electro-mechanical systems applications.     

Electrical properties of sputtered PZT films on stabilized platinum electrode

PZT (54/46) thin films were deposited by r.f. magnetron sputtering followed by a post-annealing treatment on silicon substrates. The crucial role of a Ti adhesion layer on the Ti/Pt bottom electrode is presented. The deposition conditions and the thickness of Ti have dominant effects on the interactions between Ti and Pt during the annealing treatment. The Pt layer, whatever its thickness, did not act as a barrier against Ti-out diffusion. The stability of the bottom electrode was achieved by using a TiO_x layer instead of a pure metallic Ti adhesion layer. The electrical properties of PZT films in terms of dielectric and ferroelectric performance were evaluated, particularly as a function of the PZT film thickness.     

Hysteretic properties of Mn-doped Pb(Zr,Ti)O3 thin films

Pb(Zr,Ti)O₃ (PZT 30/70) and Mn-doped Pb(Zr,Ti)O₃ (PMZT 30/70) thin films have been fabricated on Pt/Ti/SiO₂/Si substrates by a chemical solution deposition technique. The experiments found that the addition of Mn in PZT thin films greatly improves the ferroelectric properties of thin films. It is demonstrated that the Mn-doped (1 mol%) PZT showed fatigue-free characteristics at least up to 10¹⁰ switching bipolar pulse cycles under 10 V and excellent retention properties. The Mn-doped PZT thin films also exhibited well-defined hysteresis loops with a remnant polarization (Pr) of 34 μC/cm² and a coercive field (Ec) of 100 kV/cm for the thickness of 300 nm. Dielectric constant and loss (tanδ) for Mn doped PZT thin films are 214 and 0.008, respectively. These figures compare well with or exceed the values reported previously. In this paper, the mechanism by which Mn influences on the ferroelectric properties of PZT thin films has also been discussed.     

Large Piezoelectricity and Ferroelectricity in Mn‐Doped (Bi0.5Na0.5)TiO3‐BaTiO3 Thin Film Prepared by Pulsed Laser Deposition

Mn‐doped (Bi_0.5Na_0.5)_0.94Ba_0.06TiO₃ (MnBNBT) thin films were prepared on SrRuO₃ (SRO)‐coated (001) SrTiO₃ (STO) single crystal substrates by pulsed laser deposition under different processing conditions. Structural characterization (i.e., XRD and TEM) confirms the epitaxial growth of STO/SRO/MnBNBT heterostructures. Through the judicious control of deposition temperature, the defect level within the films can be finely tuned. The MnBNBT thin film deposited at the optimized temperature exhibits superior ferroelectric and piezoelectric responses with remanent polarization P_r of 33.0 μC/cm² and piezoelectric coefficient d₃₃ of 120.0 ± 20 pm/V.     

Microstructure,electrical and mechanical properties of MgO nanoparticles-reinforced porous PZT 95/5 ferroelectric ceramics

Porous Pb (Zr_0.95Ti_0.05) O₃/xMgO (PZT/MgO, x=0, 0.1, 0.2, 0.5 and 1.0 wt%) ferroelectric ceramics were prepared with MgO nanoparticles as reinforcing phase. The effects of MgO nanoparticles on the phase, microstructure, electrical and mechanical properties of as-prepared ceramics were investigated. The results show that the grain size is reduced obviously when increasing the amount of MgO. Compared with pure porous PZT, T_C of MgO-added PZT ceramics shifts to higher temperature. Moreover, dielectric, ferroelectric and piezoelectric properties show no much degradation. Further, PZT/MgO ceramics possess enhanced mechanical properties compared to pure porous PZT ceramics, the largest increment of the fracture toughness and hardness being 31.3% and 19.8%, respectively. The optimal electrical and mechanical properties are obtained with the addition of less than 0.5 wt% MgO nanoparticles.     

Fatigue improvement in modified lead zirconate titanate ceramics through employment of La0.8Sr0.2MnO3 buffer layers

The dielectric, ferroelectric and fatigue properties of modified lead zirconate titanate (PZT) ceramics were investigated in terms of the effect of La_0.8Sr_0.2MnO₃ (LSM) buffer layers. The double sided LSM buffer layers resulted in a lower dielectric loss, a weaker frequency dependence of dielectric constant, a lower leakage current density, and an increase in the saturation polarization. Moreover, it was found that up to 1.4×10⁷ cycle numbers, the Ag||LSM/PZT/LSM||Ag capacitor, with remanent polarization decreased by 55%, was superior to the Ag||PZT||Ag capacitor by 85%. The results indicate that the LSM buffer layers can improve the fatigue endurance of the PZT ceramics with Ag electrodes, mainly because the accumulated charges were compensated at the interface junctions between the LSM buffer layers and the Ag electrodes. We fit the polarization fatigue data using a modified model and calculated the characteristic decay time of oxygen vacancy migration in the Ag||LSM/PZT/LSM||Ag and the Ag||PZT||Ag capacitors, respectively.     

Structural and electrical properties of Pb(Zr0.53Ti0.47)O3 films prepared on La0.5Sr0.5CoO3 coated Si substrates

PbZr_0.53Ti_0.47O₃ (PZT) thin films with thickness of 0.9 μm were prepared on La_0.5Sr_0.5CoO₃ (LSCO) coated Si substrates. Both PZT and LSCO were prepared by the sol–gel method. The concentration of LSCO sol was varied from 0.3 to 0.1 mol/L, which could modify the preferential orientation of PZT thin films and consequently affect the dielectric and ferroelectric properties. The LSCO electrode layers derived from lower sol concentration of 0.1 mol/L have much more densified structure, which facilitates the formation of (1 0 0) textured PZT films with smooth and compact columnar grains. PZT thin films prepared on the optimized LSCO films exhibit the enhanced dielectric constant and remnant polarization of 980 and 20 μC/cm², respectively.     

Large domain-wall currents in epitaxial Au/BiFeO3/SrRuO3 thin-film capacitors with modulated oxygen vacancy and wall densities

Large domain wall (DW) conductivity in an insulating ferroelectric plays an important role in the future nanosensors and nonvolatile memories. However, the wall current was usually too small to drive high-speed memory circuits and other agile nanodevices requiring high output-powers. Here, a large domain-wall current of 67.8 μA in a high on/off ratio of ~4460 was observed in an epitaxial Au/BiFeO₃/SrRuO₃ thin-film capacitor with the minimized oxygen vacancy concentration. The studies from read current-write voltage hysteresis loops and piezo-response force microscope images consistently showed remaining of partially unswitched domains after application of an opposite poling voltage that increased domain wall density and wall current greatly. A theoretical model was proposed to explain the large wall current. According to this model, the domain reversal occurs with the appearance of head-to-head and tail-to-tail 180° domain walls (DWs), resulting in the formation of highly conductive wall paths. As the applied voltage increased, the domain-wall number increased to enhance the on-state current, in agreement with the measurements of current-voltage curves. This work paves a way to modulate DW currents within epitaxial Au/BiFeO₃/SrRuO₃ thin-film capacitors through the optimization of both oxygen vacancy and domain wall densities to achieve large output powers of modern domain-wall nanodevices.     

Tailoring the Magnetoelectric Properties of Pb(Zr,Ti)O3 Film Deposited on Amorphous Metglas Foil by Laser Annealing

This study demonstrates the modulation of off‐resonance magnetoelectric (ME) response of the Pb(Zr,Ti)O₃ (PZT)/ Metglas (FeBSi) bilayered composite by laser annealing. A continuous‐wave 532 nm Nd:YAG laser with varying fluences (210–390 J/mm²) was utilized to anneal the 2 μm thick PZT film deposited using granule spray in vacuum (GSV) technique on magnetostrictive amorphous Metglas foil. It was found that the dielectric and ferroelectric properties of the PZT film are strongly affected by the exposure to laser fluence. The ME voltage coefficient of PZT/Metglas increased with the fluence up to 345 J/mm², reaching a high value of 880 mV/cm·Oe. The electrical and ME properties were correlated with the changes observed in crystallinity and grain size of the PZT film as well as with the alterations in microstructure and magnetic behavior of Metglas. Our results demonstrate that enhanced ME coupling can be realized in PZT/Metglas film composites by controlling the laser fluence.     

Continuous ferrimagnetic Y3Fe5O12 layers on the ceramic PbZr0·45Ti0·55O3 substrates

For the first time, continuous layers of yttrium iron garnet (YIG, Y₃Fe₅O₁₂) with a thickness of about 2 μm were synthesized on ferroelectric ceramic substrates based on lead titanate zirconate (PZT, PbZr_0·45Ti_0·55O₃). The Y₃Fe₅O₁₂ layer was deposited by ion-beam sputtering – deposition on PZT substrates of 400 μm thick by sputtering a polycrystalline target of the composition Y₃Fe₅O₁₂ with a mixture of argon and oxygen ions. Due to preliminary planarization of the PZT surface with a TiO₂ layer, a high-quality plane-parallel YIG/PZT interface was obtained, which is confirmed by scanning electron microscopy in combination with the focused ion beam technique. Atomic force microscopy showed that planarization makes it possible to achieve surface smoothness of 10 nm.The YIG/PZT heterostructures obtained in this work are potentially attractive for use in logic circuits based on low-scattering spin waves, memory elements, as well as electrically controlled microwave devices.     

Effects of pore shape and porosity on the properties of porous PZT 95/5 ceramics

Porous lead zirconate titanate (PZT 95/5) ferroelectric ceramics were prepared by sintering compacts consisting of PZT and pore formers. The piezoelectric, dielectric and ferroelectric properties of porous PZT ceramics were investigated as a function of pore shape and porosity. Piezoelectric coefficient (d₃₃), dielectric constant (ɛ₃₃) and remnant polarization (P_r) decreased with an increase in porosity, and the porous PZT ceramics with spherical pores exhibited better properties than that with irregular pores. Furthermore, the electrical conductivities of PZT ceramics were investigated to explain the phenomena that porous PZT ceramics exhibited lower dielectric loss (tan δ) than dense PZT ceramics in the temperature range from 250 to 500 °C.     

Growth and electrical properties of high-Curie point rhombohedral Mn-Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 thin films

High-quality ternary relaxor ferroelectric (100)-oriented Mn-doped 0.36Pb(In_1/2Nb_1/2)O₃-0.36Pb(Mg_1/3Nb_2/3)O₃-0.28PbTiO₃ (Mn-PIMNT) thin films were grown on SrRuO₃-buffered SrTiO₃ single-crystal substrate in a wide deposition temperature range of 550-620°C using the pulsed laser deposition method. The phase structure, ferroelectric, dielectric, piezoelectric properties, and nanoscale domain evolution were studied. Under the deposition temperature of 620°C, the ferroelectric hysteresis loops and current-voltage curves showed that the film owned significantly enhanced remnant ferroelectric polarization of 34.5 μC/cm² and low leakage current density of 2.7 × 10⁻¹⁰ A/cm². Moreover fingerprint-type nanosized domain patterns with polydomain structures and well-defined macroscopic piezoelectric properties with a high normalized strain constant of 40 pm/V was obtained. Under in situ DC electric field, the domain evolution was investigated and 180° domain reversal was observed through piezoelectric force microscope. These global electrical properties make the current Mn-PIMNT thin films very promising in piezoelectric MEMS applications.     

Structural and electronic properties of germanene/MoS2 monolayer and silicene/MoS2 monolayer superlattices

Superlattice provides a new approach to enrich the class of materials with novel properties. Here, we report the structural and electronic properties of superlattices made with alternate stacking of two-dimensional hexagonal germanene (or silicene) and a MoS₂ monolayer using the first principles approach. The results are compared with those of graphene/MoS₂ superlattice. The distortions of the geometry of germanene, silicene, and MoS₂ layers due to the formation of the superlattices are all relatively small, resulting from the relatively weak interactions between the stacking layers. Our results show that both the germanene/MoS₂ and silicene/MoS₂ superlattices are manifestly metallic, with the linear bands around the Dirac points of the pristine germanene and silicene seem to be preserved. However, small band gaps are opened up at the Dirac points for both the superlattices due to the symmetry breaking in the germanene and silicene layers caused by the introduction of the MoS₂ sheets. Moreover, charge transfer happened mainly within the germanene (or silicene) and the MoS₂ layers (intra-layer transfer), as well as some part of the intermediate regions between the germanene (or silicene) and the MoS₂ layers (inter-layer transfer), suggesting more than just the van der Waals interactions between the stacking sheets in the superlattices.