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Quaternary alloy ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}{\mathrm{As}}_{1\ensuremath{-}y}{\mathrm{P}}_{y}$ hosts magnetic and electronic properties that can be tuned by varying the P concentration ``$y$'', Mn concentration ``$x$'' and by annealing. In this work we make use of this tunability to probe the origin of the anomalous Hall effect (AHE) in ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}{\mathrm{As}}_{1\ensuremath{-}y}{\mathrm{P}}_{y}$ thin films grown on GaAs that host perpendicular magnetic anisotropy. Specifically, we find that AHE in this class of materials is determined primarily by two contributions: an intrinsic band component arising from the Berry curvature, and a component determined by hopping conduction. As we vary the properties of ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}{\mathrm{As}}_{1\ensuremath{-}y}{\mathrm{P}}_{y}$ from the metallic to the semi-insulating regime by changing the value of $y$ and by postgrowth annealing, we observe a clear crossover from a Berry-curvature-induced AHE to one that is caused by hopping. The transition occurs approximately at the point where the numbers of localized and itinerant holes become comparable. In this hopping regime, the conductivity follows the Efros-Shklovskii scaling law versus temperature indicating the presence of a Coulomb gap, but the AHE remains robustly present. These results indicate that ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}{\mathrm{As}}_{1\ensuremath{-}y}{\mathrm{P}}_{y}$ can host an interesting interplay between magnetism and Coulomb interactions.

The distribution and manipulation of oxygen defects, including oxygen vacancies $({O}_{v})$ and interstitial oxygen $({O}_{i})$, directly results in modifications in a variety of emergent functionalities in transition metal oxides. This is applicable in nanoscale control, which is a key issue in data and energy storage devices, such as resistive switching memory. Among all the oxygen-defect-related topics, nanoscale oxygen defect migration paths have a direct impact on the performance of memory devices, and are intrinsically determined by the structural anisotropy. Here, we use layered single-crystalline ferroelectric, hexagonal manganites $(h\ensuremath{-}\mathrm{REMn}{\mathrm{O}}_{3})$ to demonstrate ${\mathrm{O}}_{\mathrm{i}}$-migration-induced nanoscale manipulation of conductance in the $ab$ plane. Conversely, this unique phenomenon cannot be achieved along the $c$ axis. Furthermore, a density functional theory calculation reveals that the energy barriers are lower for planar migrations of ${\mathrm{O}}_{\mathrm{i}}$, when compared to out-of-plane migration, and are responsible for such anisotropic resistance switching.

A comprehensive understanding of strain coupling across heterointerfaces and its impact on physical properties of oxide heterostructures is important for elucidating the mechanisms of certain novel physical phenomena occurring at heterointerfaces, such as magnetoelectric coupling, tunneling electroresistance effects, and strain-driven exchange bias. Using the $\mathrm{L}{\mathrm{a}}_{0.5}\mathrm{C}{\mathrm{a}}_{0.5}\mathrm{Mn}{\mathrm{O}}_{3}/\mathrm{Pb}(\mathrm{I}{\mathrm{n}}_{1/2}\mathrm{N}{\mathrm{b}}_{1/2}){\mathrm{O}}_{3}\text{\ensuremath{-}}\mathrm{Pb}(\mathrm{M}{\mathrm{g}}_{1/3}\mathrm{N}{\mathrm{b}}_{2/3}){\mathrm{O}}_{3}\text{\ensuremath{-}}\mathrm{PbTi}{\mathrm{O}}_{3}$ (PINT) multiferroic heterostructure as a model system, we systematically investigated the influences of interface strain coupling on the electronic transport and magnetic properties as well as the electronic phase separation of charge-ordered $\mathrm{L}{\mathrm{a}}_{0.5}\mathrm{C}{\mathrm{a}}_{0.5}\mathrm{Mn}{\mathrm{O}}_{3}$ thin films through electric-field-induced ferroelectric domain switching. Upon the irreversible initial poling of the PMNT substrate, the induced in-plane compressive strain $({\ensuremath{\varepsilon}}_{xx(\mathrm{film})}\phantom{\rule{0.16em}{0ex}}=\ensuremath{-}0.045%)$ causes a decrease in ${T}_{\mathrm{CO}}(\ensuremath{\Delta}{T}_{\mathrm{CO}}=180\phantom{\rule{0.16em}{0ex}}\mathrm{K})$ and resistance $[{(\ensuremath{\Delta}R/R)}_{\mathrm{strain}}\ensuremath{\sim}\ensuremath{-}99.4%]$, resulting in a gauge factor ${(\ensuremath{\Delta}R/R)}_{\mathrm{strain}}/{\ensuremath{\varepsilon}}_{xx(\mathrm{film})}\phantom{\rule{0.16em}{0ex}}\ensuremath{\sim}220\phantom{\rule{0.16em}{0ex}}800%$. Such a large strain-tunability of resistance is unprecedented and magnetic-field tunable. This, together with the strain-tunable magnetoresistance (MR) and magnetization of the films, demonstrates strong coupling between the strain and the magnetic field. Further analysis indicates that this coupling is essentially mediated by the electronic phase separation, whose relative strength could be monitored by measuring ${(\ensuremath{\Delta}R/R)}_{\mathrm{strain}}$ against magnetic field and temperature. By combining 180\ifmmode^\circ\else\textdegree\fi{} ferroelectric domain switching and x-ray diffraction and transport measurements, we identify that this electric-field modulation of the physical properties is strain-mediated but not interface charge-mediated. In addition, we observed that the non-180\ifmmode^\circ\else\textdegree\fi{} ferroelastic domain switching-induced in-plane tensile strain $({\ensuremath{\varepsilon}}_{xx(\mathrm{film})}\phantom{\rule{0.16em}{0ex}}=0.1%)$ induces a large increase in the resistance (up to \ensuremath{\sim}87.4%) and ${T}_{\text{CO}}$ and a drop in MR, signaling the stabilizing of the charge-ordered phase. Our findings provide further insight into the strain effect and essential physics of perovskite manganites, particularly the electronic phase separation.

R. K. Zheng,1,2,* H.-U. Habermeier,1 H. L. W. Chan,2 C. L. Choy,2 and H. S. Luo3 1Max Planck Institute for Solid State Research, Heisenbergstrasse 1, D-70569 Stuttgart, Germany 2Department of Applied Physics and Materials Research Center, The Hong Kong Polytechnic University, Hong Kong, China 3State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 201800, China Received 9 May 2009; revised manuscript received 9 July 2009; published 25 September 2009

Using ferroelectric $0.67\text{Pb}({\text{Mg}}_{1/3}{\text{Nb}}_{2/3}){\text{O}}_{3}\text{\ensuremath{-}}0.33{\text{PbTiO}}_{3}$ single crystals as substrates, we studied the effects of the ferroelectric poling and the converse piezoelectric effect on the strain state, resistance, insulator-to-metal transition temperature $({T}_{C})$, and magnetoresistance (MR) of ${\text{La}}_{0.7}{\text{Ba}}_{0.3}{\text{MnO}}_{3}$ (LBMO) thin films. In situ x-ray diffraction measurements indicate that the ferroelectric poling (or the converse piezoelectric effect) induces a substantial reduction in the in-plane tensile strain in the LBMO film, giving rise to a decrease in the resistance and an increase in ${T}_{C}$. The relative changes of the resistance and ${T}_{C}$ are proportional to the induced reduction in the in-plane tensile strain $(\ensuremath{\delta}{\ensuremath{\epsilon}}_{xx})$ in the film. The reduction in the in-plane tensile strain leads to opposite effects on MR below and above ${T}_{C}$, namely, MR is reduced for $Tl{T}_{C}$ while MR is enhanced for $Tg{T}_{C}$. We discuss these strain effects within the framework of the Jahn-Teller (JT) electron-lattice coupling and phase separation scenario that are relevant to the induced strain. Similar studies on ${\text{CaMnO}}_{3}$ thin films, for which there is no JT distortion of ${\text{MnO}}_{6}$ octahedra, show that the resistance of the films also decreases when the tensile strain is reduced, indicating that the resistance change arising from the reduction in Mn-O bond length dominates over that arising from the reduction in Mn-O-Mn bond angle.

We have deposited ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}\mathrm{Mn}{\mathrm{O}}_{3}$ (LSMO) thin films on piezoelectric $(1\ensuremath{-}x)\mathrm{Pb}({\mathrm{Mg}}_{1∕3}{\mathrm{Nb}}_{2∕3}){\mathrm{O}}_{3}\ensuremath{-}x\mathrm{Pb}\mathrm{Ti}{\mathrm{O}}_{3}$ $(x\ensuremath{\sim}0.3)$ (PMN-PT) single-crystal substrates and studied the effects of substrate-induced lattice strain on the resistance of the LSMO films. By applying electric fields across the PMN-PT substrate, we in situ induce an in-plane compressive (tensile) strain in the PMN-PT substrate via the converse piezoelectric effect. The induced strain is transferred to the LSMO film, which then imposes an in-plane compressive (tensile) strain on the LSMO film, thereby causing a linear decrease (increase) in the resistance of the LSMO film. Based on the experimental results, we establish quantitative relationships between the resistance of the LSMO film and the converse piezoelectric-effect-induced out-of-plane and in-plane strains in the PMN-PT substrate and LSMO film, respectively.

Thin films of ${\mathrm{La}}_{0.75}{\mathrm{Ca}}_{0.25}\mathrm{Mn}{\mathrm{O}}_{3}$ (LCMO) have been grown on ferroelectric $(1\ensuremath{-}x)\mathrm{Pb}({\mathrm{Mg}}_{1∕3}{\mathrm{Nb}}_{2∕3}){\mathrm{O}}_{3}\text{\ensuremath{-}}x\mathrm{Pb}\mathrm{Ti}{\mathrm{O}}_{3}$ ($x\ensuremath{\sim}0.33$) single-crystal substrates. The ferroelectric polarization in the PMN-PT substrates gives rise to a decrease in resistance and an upward shift in Curie temperature of the LCMO films, which was interpreted in terms of the ferroelectric polarization induced lattice strain effect. The resistance of the LCMO films can be modulated by applying dc/ac electric fields across the polarized PMN-PT substrates. The electric field induces lattice strains in the PMN-PT substrates via the converse piezoelectric effect, which subsequently changes the strain state and resistance of the LCMO films. A quantitative relation between the resistance in the LCMO films and the induced lattice strains in the PMN-PT substrates has been established. Moreover, we identified that the ferroelectric polarization and converse piezoelectric effect induced lattice strain effects dominate over the field effect in the LCMO/PMN-PT systems.