Ren-Kui Zheng

Substrate-induced strain effect in La0.875Ba0.125MnO3 thin films grown on ferroelectric single-crystal substrates

The authors have studied the substrate-induced strain effect in La0.875Ba0.125MnO3 (LBMO) thin films grown on ferroelectric 0.67Pb(Mg1∕3Nb2∕3)O3–0.33PbTiO3 (PMN-PT) single-crystal substrates. Both the strain and resistance of the films can be in situ varied by applying an electric field across the PMN-PT substrates. X-ray diffraction analysis indicates that the variations of strain and resistance result from the induced strain in the PMN-PT substrate due to the ferroelectric polarization or the converse piezoelectric effect. The relationships between the resistance and the induced strain in the LBMO film and PMN-PT substrate have been quantitatively analyzed.

Strain-mediated electric-field control of resistance in the La0.85Sr0.15MnO3∕0.7Pb(Mg1∕3Nb2∕3)O3–0.3PbTiO3 structure

The authors have deposited thin films of La0.85Sr0.15MnO3 (LSMO) on 0.7Pb(Mg1∕3Nb2∕3)O3–0.3PbTiO3 (PMN-PT) single-crystal substrates and have achieved modulation of the resistance of the LSMO film by applying an electric field across the PMN-PT substrate whether the LSMO film is in the paramagnetic, ferromagnetic, or charge-ordered state. Piezoelectric measurements show that the electric field gives rise to a lattice strain in the PMN-PT substrate via the converse piezoelectric effect, which then induces a lattice strain and hence a resistance change in the LSMO film. Analysis of the data indicates that the electric-field-induced lattice strain effect dominates over the field effect in the LSMO/PMN-PT structure.

Investigation of substrate-induced strain effects in La0.7Ca0.15Sr0.15MnO3 thin films using ferroelectric polarization and the converse piezoelectric effect

We have fabricated manganite film/ferroelectric crystal heterostructures by growing La0.7Ca0.15Sr0.15MnO3 (LCSMO) films on ferroelectric 0.67Pb(Mg1/3Nb2/3)O3−0.33PbTiO3 (PMN-PT) single-crystal substrates. The efficient mechanical coupling at the interface, originated from ferroelectric polarization or the converse piezoelectric effect in the PMN-PT substrate, gives rise to large changes in the strain state, electrical resistance, magnetoresistance, and insulator-to-metal transition temperature (TP) of the film. We interpreted all these changes in terms of substrate-induced strain, which modifies the tetragonal distortion of MnO6 octahedra and the electron-lattice coupling strength in the film. Quantitative relationships between TP and induced strain in the LCSMO film have been established.

Photo-induced enhancement of the power factor of Cu2S thermoelectric films

Element doping is commonly used to adjust the carrier concentrations in semiconductors such as thermoelectric materials. However, the doping process unavoidably brings in defects or distortions in crystal lattices, which further strongly affects the physical properties of the materials. In this work, high energy photons have been used to activate the carriers in Cu2S thermoelectric films. As a result, the carrier concentrations, and the respective electrical conductivity as well as Seebeck coefficient are further changed. The photon-induced electrical transport properties are further analyzed utilizing a Parallel circuit model. Due to the realization of optimized carrier concentrations by photon activation, the power factor of Cu2S film is improved more than 900 times as compared with the dark data. As compared to the traditional doping process, the approach using photon activation can realize the tuning of carrier concentrations without affecting crystal lattice. This method provides an opportunity to investigate the intrinsic physical properties of semiconductor materials without involving traditional element doping process that usually brings in additional lattice defects or distortions.

Ferroelectric, electrical, and magnetic properties of BiFe0.95Mn0.05O3 thin films epitaxially grown on conductive Nb:SrTiO3 and La0.7Sr0.3MnO3-buffered Nb:SrTiO3 substrates

BiFe0.95Mn0.05O3 (BFMO) thin films with different thicknesses have been epitaxially grown on <001>-oriented Nb-doped SrTiO3 (NbSTO) and La0.7Sr0.3MnO3(LSMO)-buffered NbSTO substrates by pulsed laser deposition. At high bias field the space-charge-limited current (SCLC) is the dominant conduction mechanism for all BFMO films while at low bias field the Ohmic conduction is the predominant mechanism. An analysis of leakage current characteristics reveals that the ferroelectric properties are critically dependent on the density of defects in BFMO films. For the BFMO/LSMO/NbSTO structure, the coercive field of the BFMO film is much smaller than that of the BFMO film directly grown on the NbSTO substrate, which is attributed to the suppression of the substrate-induced clamping effect. For both BFMO/NbSTO and BFMO/LSMO/NbSTO structures, the ferroelectric hysteresis loops show no change under a magnetic field up to 9 T, which is explained in terms of weak ferromagnetic-ferroelectric coupling in the BFMO film and the very low magnetic-field-induced electric voltage drop across the LSMO layer.

Tunable strain effect and ferroelectric field effect on the electronic transport properties of La0.5Sr0.5CoO3 thin films

Tensiled La0.5Sr0.5CoO3 (LSCO) thin films were epitaxially grown on piezoelectric 0.67Pb (Mg1/3Nb2/3)O3-0.33PbTiO3 (PMN-PT) single-crystal substrates. Due to the epitaxial nature of the interface, the lattice strain induced by ferroelectric poling or the converse piezoelectric effect in the PMN-PT substrate is effectively transferred to the LSCO film and thus reduces the tensile strain of the film, giving rise to a decrease in the resistivity of the LSCO film. We discuss these strain effects within the framework of the spin state transition of Co3+ ions and modification of the electronic bandwidth that is relevant to the induced strain. By simultaneously measuring the strain and the resistivity, quantitative relationship between the resistivity and the strain was established for the LSCO film. Both theoretical calculation and experimental results demonstrate that the ferroelectric field effect at room temperature in the LSCO/PMN-PT field-effect transistor is minor and could be neglected. Nevertheless, wit...

Interface correlated exchange bias effect in epitaxial Fe3O4 thin films grown on SrTiO3 substrates

We report exchange bias effect in Fe3O4 films epitaxially grown on SrTiO3 substrates. This effect is related to the formation of Ti3+-vacancy complexes at the surface of SrTiO3 in ultrahigh vacuum that in turn triggers the growth of a thin antiferromagnetic (AFM) FeO layer (∼5 nm) at the interface. The picture of antiferromagnetic FeO interacting with native ferrimagnetic Fe3O4 matrix reasonably accounts for this anomalous magnetic behavior. With increasing film thickness from 17 to 43 nm, the exchange bias effect and the magnetization anomaly associated with the AFM phase transition of the FeO layer are progressively weakened due to the increase in the volume fraction of the Fe3O4 phase, indicating the interfacial nature of the exchange coupling. Our results highlight the important role of interface engineering in controlling the magnetic properties of iron oxide thin films.

Intrinsic and quantitative effects of in-plane strain on ferroelectric properties of Mn-doped BiFeO3 epitaxial films by in situ inducing strain in substrates

We report in situ manipulation of the in-plane strain exx(BFMO) and coercive field EC(BFMO) of BiFe0.95Mn0.05O3 (BFMO) films epitaxially grown on La0.7Sr0.3MnO3 film buffered 0.71Pb(Mg1/3Nb2/3)O3-0.29PbTiO3 (PMN-PT) substrates. PMN-PT poling-induced strain is effectively transferred to BiFe0.95Mn0.05O3 films and enhances exx(BFMO) and EC(BFMO), with a gauge factor (ΔEC(BFMO)/EC(BFMO))/(δexx) ∼−25 and −326 for the BFMO(001) and BFMO(111) films, respectively. Based on the strain dependence of EC(BFMO), we established a quantitative relationship between EC(BFMO) and exx(BFMO). Using ferroelastic strain of PMN-PT, we achieved reversible and non-volatile modulation of strain and EC(BFMO) of BFMO films, providing an approach for non-volatile and reversible turning of strain and physical properties of ferroelectric films.

Effects of ferroelectric-poling-induced strain on the transport and magnetic properties of La7/8Ba1/8MnO3 thin films

We have investigated the effects of the strain induced by ferroelectric poling on the transport and magnetic properties of La7/8Ba1/8MnO3 (LBMO) thin films epitaxially grown on ferroelectric 0.67Pb(Mg1/3Nb2/3)O3–0.33PbTiO3 (PMN–PT) single-crystal substrates. The ferroelectric poling reduces the in-plane tensile strain of the film, giving rise to a decrease in the resistivity and an increase in the magnetization, Curie temperature, and magnetoresistance of the LBMO film. These strain effects are explained within the framework of coexisting phases whose volume fractions are modified as a result of the reduction in the tetragonal distortion of MnO6 octahedra induced by ferroelectric poling. An investigation of the effects of polarization reversal on the transport properties of the LBMO film indicates that the ferroelectric-poling-induced strain effects dominate over the ferroelectric field effects in the LBMO/PMN–PT structure.

Magnetic and electrical properties of three-dimensional (La,Pr,Ca)MnO3 nanofilm/ZnO nanorod p–n junctions

Three-dimensional (3D) nanostructured p–n junctions have been fabricated by growing p-type La0.5Pr0.17Ca0.33MnO3 (LPCMO) manganite thin film on n-type ZnO nanorod arrays using pulsed laser deposition. The 3D LPCMO nanofilm/ZnO nanorod p–n junctions exhibit excellent room temperature rectification performance with a high rectification factor of ∼1650 at ±5.0 V, approximately 14 times larger than that of manganite film- and ZnO nanowires (or film)-based layered p–n junctions. The ferromagnetic phase transition temperature TC for the LPCMO nanofilm is significantly (∼28 K) higher than that of 2D LPCMO films directly grown on Si substrates, which is interpreted in terms of the nanograin-induced surface effect and lattice strain effect. The large portion of magnetically frozen phase establishes the existence of strong electronic phase separation in the 3D nanofilm.