Ultrahigh Tunability of Room Temperature Electronic Transport and Ferromagnetism in Dilute Magnetic Semiconductor and PMN‐PT Single‐Crystal‐Based Field Effect Transistors via Electric Charge Mediation

Multiferroic heterostructures composed of complex oxide thin films and ferroelectric single crystals have aroused considerable interest due to the electrically switchable strain and charge elements of oxide films by the polarization reversal of ferroelectrics. Previous studies have demonstrated that the electric‐field‐control of physical properties of such heterostructures is exclusively due to the ferroelectric domain switching‐induced lattice strain effects. Here, the first successful integration of the hexagonal ZnO:Mn dilute magnetic semiconductor thin films with high performance (111)‐oriented perovskite Pb(Mg_1/3Nb_2/3)O₃‐PbTiO₃ (PMN‐PT) single crystals is reported, and unprecedented charge‐mediated electric‐field control of both electronic transport and ferromagnetism at room temperature for PMN‐PT single crystal‐based oxide heterostructures is realized. A significant carrier concentration‐tunability of resistance and magnetization by ≈400% and ≈257% is achieved at room temperature. The electric‐field controlled bistable resistance and ferromagnetism switching at room temperature via interfacial electric charge presents a potential strategy for designing prototype devices for information storage. The results also disclose that the relative importance of the strain effect and interfacial charge effect in oxide film/ferroelectric crystal heterostructures can be tuned by appropriately adjusting the charge carrier density of oxide films.     

Giant Piezoelectric Coefficients in Relaxor Piezoelectric Ceramic PNN‐PZT for Vibration Energy Harvesting

It is well known that the piezoelectric performance of ferroelectric Pb(Zr,Ti)O₃ (PZT) based ceramics is far inferior to that of ferroelectric single crystals due to ceramics' polycrystalline nature. Herein, it is reported that piezoelectric stress coefficient e₃₃ = 39.24 C m^?2 (induced electric displacement under applied strain) in the relaxor piezoelectric ceramic 0.55Pb(Ni_1/3Nb_2/3)O₃–0.135PbZrO₃–0.315PbTiO₃ (PNN‐PZT) prepared by the solid state reaction method exhibits the highest value among various reported ferroelectric ceramic and single crystal materials. In addition, its piezoelectric coefficient d₃₃* = 1753 pm V^?1 is also comparable with that of the commercial Pb(Mg_1/3Nb_2/3)O₃‐PbTiO₃ (PMN‐PT) piezoelectric single crystal. The PNN‐PZT ceramic is then assembled into a cymbal energy harvester. Notably, its maximum output current at the acceleration of 3.5 g is 2.5 mA_pp, which is four times of the PMN‐PT single crystal due to the large piezoelectric e₃₃ constants; while the maximum output power is 14.0 mW, which is almost the same as the PMN‐PT single crystal harvester. The theoretical analysis on force‐induced power output is also presented, which indicates PNN‐PZT ceramic has great potential for energy device application.     

Long‐Range Nonvolatile Electric Field Effect in Epitaxial Fe/Pb(Mg1/3Nb2/3)0.7Ti0.3O3 Heterostructures

One of the ideal candidates of using electric field to manipulate magnetism is the recently developed multiferroics with emergent coupling of magnetism and electricity, particularly in synthesizing artificial nanoscale ferroelectric and ferromagnetic materials. Here, a long‐range nonvolatile electric field effect is investigated in Fe/Pb(Mg_1/3Nb_2/3)_0.7Ti_0.3O₃ heterostructure using the dependence of the magnon‐driven magnetoelectric coupling on the epitaxial Fe thin film (4–30 nm) thickness at room temperature using measurements based on the ferromagnetic resonance. The magnon‐driven magnetoelectric coupling tuning of the ferromagnetic resonance field shows a linear response to the electric field, with a resonance field shift that occurs under both positive and negative remanent polarizations, and demonstrates nonvolatile behavior. Moreover, the spin diffusion length of the epitaxial Fe thin film of ≈9 nm is obtained from the results that the change of the cubic magnetocrystalline anisotropy field under different electric fields varies with Fe thickness. These results are promising for the design of future multiferroic devices.     

The Contributions of Polar Nanoregions to the Dielectric and Piezoelectric Responses in Domain‐Engineered Relaxor‐PbTiO3 Crystals

The existence of polar nanoregions is the most important characteristic of relaxor‐based ferroelectric materials. Recently, the contributions of polar nanoregions to the shear piezoelectric property of relaxor‐PbTiO₃ (PT) crystals are confirmed in a single domain state, accounting for 50%–80% of room temperature values. For electromechanical applications, however, the outstanding longitudinal piezoelectricity in domain‐engineered relaxor‐PT crystals is of the most significance. In this paper, the contributions of polar nanoregions to the longitudinal properties in [001]‐poled Pb(Mg_1/3Nb_2/3)O₃‐0.30PbTiO₃ and [110]‐poled Pb(Zn_1/3Nb_2/3)O₃‐0.15PbTiO₃ (PZN‐0.15PT) domain‐engineered crystals are studied. Taking the [110]‐poled tetragonal PZN‐0.15PT crystal as an example, phase‐field simulations of the domain structures and the longitudinal dielectric/piezoelectric responses are performed. According to the experimental results and phase‐field simulations, the contributions of polar nanoregions (PNRs) to the longitudinal properties of relaxor‐PT crystals are successfully explained on the mesoscale, where the PNRs behave as “seeds” to facilitate macroscopic polarization rotation and enhance electric‐field‐induced strain. The results reveal the importance of local structures to the macroscopic properties, where a modest structural variation on the nanoscale greatly impacts the macroscopic properties.     

Mechanical Confinement: An Effective Way of Tuning Properties of Piezoelectric Crystals

Using <001>‐oriented Pb(Mg_1/3Nb_2/3)O₃–PbTiO₃ ferroelectric single crystals as a model material, the impact of mechanical confinements on polarization hysteresis, coercive field, and remanent polarization of relaxor‐based piezocrystals is investigated. Comparative studies are made among rhombohedral and tetragonal single crystals, as well as a polycrystalline ceramic, under uniaxial and radial compressive pre‐stresses. The dramatic changes observed are interpreted in terms of the piezoelectric effect and possible phase transitions for rhombohedral crystals, and ferroelastic domain switching and the piezoelectric effect for tetragonal crystals. Under radial compressive stresses, the coercive field for the rhombohedral crystal is observed to increase to 0.67 kV/mm and that for the tetragonal crystal is increased to 0.78 kV/mm. This is a 200% increase relative to the unstressed condition. The results demonstrate a general and effective approach to overcome the drawback of low coercive fields in these relaxor‐based ferroelectric crystals, which could help facilitate widespread implementation of these piezocrystals in engineering devices.     

Pulsed laser deposition of PMN thin films

Lead magnesium niobate (Pb(Mg_1/3Nb_2/3)O₃ or PMN) thin films have been deposited on different substrates by pulsed laser deposition. Film composition was studied by secondary ion mass spectroscopy and their crystallinity by X-ray diffraction techniques. Small-signal dielectric constant and loss factor of the films have been measured as a function of temperature and frequency on structures with 3 mm diameter evaporated Au top electrodes, by using an impedance analyser. The obtained PMN films have been found polycrystalline and partially oriented, with a certain amount of secondary pyrochlore phases and interface phases which are detrimental for the overall dielectric constant and modifies the behaviour of the heterostructures. Polarization hysteresis measurements performed at different temperatures below and above the dielectric maximum temperature T_m showed the persistence of polarization above T_m.     

Giant Electric Field Tuning of Magnetic Properties in Multiferroic Ferrite/Ferroelectric Heterostructures

Multiferroic heterostructures of Fe₃O₄/PZT (lead zirconium titanate), Fe₃O₄/PMN‐PT (lead magnesium niobate‐lead titanate) and Fe₃O₄/PZN‐PT (lead zinc niobate‐lead titanate) are prepared by spin‐spray depositing Fe₃O₄ ferrite film on ferroelectric PZT, PMN‐PT and PZN‐PT substrates at a low temperature of 90 °C. Strong magnetoelectric coupling (ME) and giant microwave tunability are demonstrated by a electrostatic field induced magnetic anisotropic field change in these heterostructures. A high electrostatically tunable ferromagnetic resonance (FMR) field shift up to 600 Oe, corresponding to a large microwave ME coefficient of 67 Oe cm kV^?1, is observed in Fe₃O₄/PMN‐PT heterostructures. A record‐high electrostatically tunable FMR field range of 860 Oe with a linewidth of 330–380 Oe is demonstrated in Fe₃O₄/PZN‐PT heterostructure, corresponding to a ME coefficient of 108 Oe cm kV^?1. Static ME interaction is also investigated and a maximum electric field induced squareness ratio change of 40% is observed in Fe₃O₄/PZN‐PT. In addition, a new concept that the external magnetic orientation and the electric field cooperate to determine microwave magnetic tunability is brought forth to significantly enhance the microwave tunable range up to 1000 Oe. These low temperature synthesized multiferroic heterostructures exhibiting giant electrostatically induced tunable magnetic resonance field at microwave frequencies provide great opportunities for electrostatically tunable microwave multiferroic devices.     

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。     

In Vivo Self‐Powered Wireless Transmission Using Biocompatible Flexible Energy Harvesters

Additional surgeries for implantable biomedical devices are inevitable to replace discharged batteries, but repeated surgeries can be a risk to patients, causing bleeding, inflammation, and infection. Therefore, developing self‐powered implantable devices is essential to reduce the patient's physical/psychological pain and financial burden. Although wireless communication plays a critical role in implantable biomedical devices that contain the function of data transmitting, it has never been integrated with in vivo piezoelectric self‐powered system due to its high‐level power consumption (microwatt‐scale). Here, wireless communication, which is essential for a ubiquitous healthcare system, is successfully driven with in vivo energy harvesting enabled by high‐performance single‐crystalline (1 ? x )Pb(Mg_1/3Nb_2/3)O₃?(x )Pb(Zr,Ti)O₃ (PMN‐PZT). The PMN‐PZT energy harvester generates an open‐circuit voltage of 17.8 V and a short‐circuit current of 1.74 µA from porcine heartbeats, which are greater by a factor of 4.45 and 17.5 than those of previously reported in vivo piezoelectric energy harvesting. The energy harvester exhibits excellent biocompatibility, which implies the possibility for applying the device to biomedical applications.     

Complete Set of Material Constants of ?011?-Poled Rhombohedral Single-Crystal 0.25Pb(In1/2Nb1/2)O3-0.47Pb(Mg1/3Nb2/3)O3-0.28PbTiO3

The complete set of elastic, dielectric, and piezoelectric constants of a 0.25Pb(In_1/2Nb_1/2)O₃-0.47Pb(Mg_1/3Nb_2/3)O₃-0.28PbTiO₃ ternary relaxor ferroelectric single crystal after 〈011〉 poling was determined by ultrasonic and resonance methods. It exhibited a rhombohedral structure as determined by powder x-ray diffraction (XRD) and x-ray fluorescence (XRF) analysis. The electromechanical coupling factor k ₁₅ and piezoelectric coefficient d ₁₅ for shear mode are 0.93 pC/N and 2658 pC/N, and k ₃₂ and d ₃₂ for transverse mode are 0.91 pC/N and -1337 pC/N, respectively. More importantly, the rhombohedral to orthorhombic phase-transition temperature is much higher than that of PZN-0.07PT or PMN-0.29PT.     

Fabrication of Relaxer-Based Piezoelectric Energy Harvesters Using a Sacrificial Poly-Si Seeding Layer

The effect of a polycrystalline silicon (poly-Si) seeding layer on the properties of relaxor Pb(Zr_0.53,Ti_0.47)O₃–Pb(Zn_1/3,Nb_2/3)O₃ (PZT–PZN) thin films and energy-harvesting cantilevers was studied. We deposited thin films of the relaxor on two substrates, with and without a poly-Si seeding layer. The seeding layer, which also served as a sacrificial layer to facilitate cantilever release, was found to improve morphology, phase purity, crystal orientation, and electrical properties. We attributed these results to reduction of the number of nucleation sites and, therefore, to an increase in relaxor film grain size. The areal power density of the wet-based released harvester was measured. The power density output of the energy harvester with this relaxor composition and the poly-Si seeding layer was 325 μW/cm².     

Low-Voltage Magnetoelectric Coupling in Fe0.5Rh0.5/0.68PbMg1/3Nb2/3O3-0.32PbTiO3 Thin-Film Heterostructures

The rapid development of computing applications demands novel low-energy consumption devices for information processing. Among various candidates, magnetoelectric heterostructures hold promise for meeting the required voltage and power goals. Here, a route to low-voltage control of magnetism in 30 nm Fe_0.5Rh_0.5/100 nm 0.68PbMg_1/3Nb_2/3O₃-0.32PbTiO₃ (PMN-PT) heterostructures is demonstrated wherein the magnetoelectric coupling is achieved via strain-induced changes in the Fe_0.5Rh_0.5 mediated by voltages applied to the PMN-PT. We describe approaches to achieve high-quality, epitaxial growth of Fe_0.5Rh_0.5 on the PMN-PT films and, a methodology to probe and quantify magnetoelectric coupling in small thin-film devices via studies of the anomalous Hall effect. By comparing the spin-flop field change induced by temperature and external voltage, the magnetoelectric coupling coefficient is estimated to reach ≈7 × 10⁻⁸ s m⁻¹ at 325 K while applying a −0.75 V bias.     

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.     

Piezoelectric Study of 0.65Pb(Mg1/3,Nb2/3)O3-0.35PTO3 Nanopoder Ceramics

The piezoelectric properties of relaxor ferroelectric 0.65Pb(Mg1/3Nb2/3)O3-0.35PbTiO3 ceramic prepared by a sol-gel combustion method have been investigated as function of sintering temperature. The results show that its phase structure is near the morphoteropic phase boundary (MPB), and outstanding electrical properties are obtained with this composition. The highest piezoelectric coefficients were observed for the samples sintered at temperature of 1200oC. In comparison with pure PMN ((1-x)Pb(Mg1/3Nb2/3)O3(x)PbTiO3), the substitution of 35% PT results in the decrease of sintered temperature and improved the relaxation behavior.     

Modulation of Spin Dynamics via Voltage Control of Spin‐Lattice Coupling in Multiferroics

Motivated by the most recent progresses in both magnonics (spin dynamics) and multiferroics fields, this work aims at magnonics manipulation by the magnetoelectric coupling effect. Here, voltage control of magnonics, particularly the surface spin waves, is achieved in La_0.7Sr_0.3MnO₃/0.7Pb(Mg_1/3Nb_2/3)O₃‐0.3PbTiO₃ multiferroic heterostructures. With the electron spin resonance method, a large 135 Oe shift of surface spin wave resonance (≈7 times greater than conventional voltage‐induced ferromagnetic resonance shift of 20 Oe) is determined. A model of the spin‐lattice coupling effect, i.e., varying exchange stiffness due to voltage‐induced anisotropic lattice changes, has been established to explain experiment results with good agreement. Additionally, an “on” and “off” spin wave state switch near the critical angle upon applying a voltage is created. The modulation of spin dynamics by spin‐lattice coupling effect provides a platform for realizing energy‐efficient, tunable magnonics devices.     

Oxygen Stoichiometry Effect on Polar Properties of LaAlO3/SrTiO3

Discovery of a ferroelectric‐like behavior of the LaAlO₃/SrTiO₃ (LAO/STO) interfaces provides an attractive platform for the development of nanoelectronic devices with functionality that can be tuned by electrical or mechanical means. However, further progress in this direction critically depends on deeper understanding of the physicochemical mechanism of this phenomenon. In this report, this problem by testing the electronic properties of the LAO/STO heterostructures with oxygen stoichiometry used as a variable is addressed. Local probe measurements in conjunction with interface electrical characterization allow to establish the field‐driven reversible migration of oxygen vacancies as the origin of the ferroelectric‐like behavior in LAO/STO. In addition, it is shown that oxygen deficiency gives rise to the formation of micrometer‐long atomically sharp boundaries with robust piezoelectricity stemming from a significant strain gradient across the boundary region. These boundaries are not ferroelectric but they can modulate the local electronic characteristics at the interface. The obtained results open a possibility to design and engineer electromechanical functionality in a wide variety of nominally nonpolar and non‐piezoelectric complex oxide heterostructures and thin films.     

Beam-Mode Piezoelectric Properties of Ternary Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 Single Crystals for Medical Linear Array Applications

In this work, the dielectric and beam-mode piezoelectric properties of ternary 0.35Pb(In_1/2Nb_1/2)O₃-0.35Pb(Mg_1/3Nb_2/3)O₃-0.30PbTiO₃ (PIMNT35/35/30) piezoelectric single crystals were investigated. The Curie temperature (T _C) and rhombohedral-to-tetragonal phase-transition temperature (T _rt) are 187°C and 127°C, about 30°C higher than those of PMNT crystals. The beam-mode coupling coefficient k ₃₃ ^w was found to be 90.3%. Furthermore, 3.5-MHz linear arrays based on PIMNT35/35/30 crystals and Pb(Zr_1−x Ti _x )O₃ ceramic (PZT-5H) were simulated using PiezoCAD software. The results indicate that the sensitivity and −6 dB bandwidth of a PIMNT35/35/30 transducer would be approximately 4 dB and 20% higher, respectively, compared with a traditional PZT transducer.     

On the Feasibility of Growing Pb[(Zn1/3Nb2/3)0.91Ti0.09]O3 Single Crystals

Pb[(Zn_1/3Nb_2/3)_0.91Ti_0.09]O₃ (PZNT91/9) single crystals were grown by two methods: from solution using PbO as a self‐fluxing agent (SC method) and directly from the melt without fluxing (MC method). In both growth methods, an allomeric Pb[(Mg_1/3‐Nb_2/3)_0.69Ti_0.31]O₃ (PMNT69/31) single crystal was used as a seed. X‐ray diffraction patterns of ground crystals showed that phase‐pure perovskite PZNT91/9 single crystals were successfully fabricated by the above two methods. The composition of the crystals obtained by both the SC and MC methods was analyzed using X‐ray fluorescence, which confirmed that the crystal composition is close to the nominal value, although volatilization of PbO and segregation during crystal growth are inevitable. The MC PZNT91/9 crystals exhibit excellent piezoelectric properties, with the piezoelectric constant, d₃₃, in the range of 1800–2200 pC N^–1. This value is comparable to that of the SC crystals. However, the MC crystals show an abnormal dielectric behavior. In contrast with the SC crystals, in the MC crystals a much broader dielectric peak appears in the dielectric response curves, accompanied by a much lower peak temperature of around 105 °C. Furthermore, frequency dispersion is apparent over a much wider temperature range (even more apparent than in pure relaxors), where a large, i.e., about 70 °C, full width at half maximum (FWHM) for the dielectric peaks is observed in the dielectric response. It is speculated that such an unusual phenomenon correlates with defects, microinhomogeneities, and polar regions in the as‐grown MC crystals. The origins of this abnormality have not been interpreted in detail until now. However, optical observation of the domain structure confirms that both the SC and MC crystals possess complex structural states.     

Growth of Epitaxial (110) 0.7Pb(Mg<Subscript>1/3</Subscript>Nb<Subscript>2/3</Subscript>)O<Subscript>3</Subscript>-0.3PbTiO<Subscript>3</Subscript> Thin Films on r-Plane Sapphire Substrates by RF Magnetron Sputtering

The role of substrate temperature and substrate surface geometry in determining the crystal structure and crystallinity of 0.7Pb(Mg_1/3Nb_2/3)O₃-0.3PbTiO₃ (PMN-PT) thin films grown on r-plane sapphire substrates is examined. A 30-nm-thick amorphous PMN-PT seed layer deposited at 250°C and subjected to rapid thermal annealing at 850°C results in the formation of an epitaxial (110) perovskite PMN-PT growth template that can be used for subsequent growth of single-crystal (110) perovskite PMN-PT films at elevated temperature. The data show that single-crystal perovskite is promoted when the films nucleate with the \( \langle \overline{1} 11 \rangle \) PMN-PT direction parallel to the \( \langle 0\bar{2}21 \rangle \) Al₂O₃ direction.     

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.