Q. X. Zhu

Coupling of magnetic field and lattice strain and its impact on electronic phase separation in La0.335Pr0.335Ca0.33MnO3/ferroelectric crystal heterostructures

Phase-separated La0.335Pr0.335Ca0.33MnO3 films were epitaxially grown on (001)- and (111)-oriented ferroelectric single-crystal substrates. Upon poling along the [001] or [111] direction, dramatic decrease in resistance, up to 99.98%, and complete melting of the charge-ordered phase were observed, caused by poling-induced strain rather than accumulation of electrostatic charge at interface. Such poling-induced strain effects can be effectively tuned by a magnetic field and mediated by electronic phase separation. In particular, our findings show that the evolution of the strength of electronic phase separation against temperature and magnetic field can be determined by measuring the strain-tunability of resistance [(ΔR/R)strain] under magnetic fields.

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.

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.

Effects of ferroelectric polarization switching on the electronic transport and magnetic properties of La0.8Ce0.2MnO3 epitaxial thin films

The authors report the electronic transport and magnetic properties of the La0.8Ce0.2MnO3 (LCEMO) thin film epitaxially grown on the ferroelectric 0.67Pb(Mg1/3Nb2/3)O3-0.33PbTiO3 (PMN-PT) single-crystal substrate and their dependence on the polarization state of the PMN-PT substrate. Upon electric-field-induced polarization switching, the electrical resistance and magnetization of the LCEMO film were modulated reversibly. The underlying coupling mechanism that is responsible for the electric-field-control of the resistance and magnetization strongly depends on temperature, being strain-mediated type at relatively high temperatures but becoming charge-mediated type with decreasing temperature. The knowledge about the evolution of the coupling mechanism with temperature not only helps to understand the drive force for multiferroic properties but also is important for theoretical modeling and device fabrication.

Electric-field-controlled interface strain coupling and non-volatile resistance switching of La1-xBaxMnO3 thin films epitaxially grown on relaxor-based ferroelectric single crystals

We have fabricated magnetoelectric heterostructures by growing ferromagnetic La1-xBaxMnO3 (x = 0.2, 0.4) thin films on (001)-, (110)-, and (111)-oriented 0.31Pb(In1/2Nb1/2)O3-0.35Pb(Mg1/3Nb1/2)O3-0.34PbTiO3 (PINT) ferroelectric single-crystal substrates. Upon poling along the [001], [110], or [111] crystal direction, the electric-field-induced non-180° domain switching gives rise to a decrease in the resistance and an enhancement of the metal-to-insulator transition temperature TC of the films. By taking advantage of the 180° ferroelectric domain switching, we identify that such changes in the resistance and TC are caused by domain switching-induced strain but not domain switching-induced accumulation or depletion of charge carriers at the interface. Further, we found that the domain switching-induced strain effects can be efficiently controlled by a magnetic field, mediated by the electronic phase separation. Moreover, we determined the evolution of the strength of the electronic phase separation against...