Andreas Herklotz

Strain-induced insulator state and giant gauge factor of La0.7Sr0.3CoO3 films.

We report on the observation of a strain-induced insulator state in ferromagnetic La0.7Sr0.3CoO3 films. Tensile strain above 1% is found to enhance the resistivity by several orders of magnitude. Reversible strain of 0.15% applied using a piezoelectric substrate triggers huge resistance modulations, including a change by a factor of 10 in the paramagnetic regime at 300 K. However, below the ferromagnetic ordering temperature, the magnetization data indicate weak dependence on strain for the spin state of the Co ions. We interpret the changes observed in the transport properties in terms of a strain-induced splitting of the Co eg levels and reduced double exchange, combined with a percolation-type conduction in an electronic cluster state.

Differentiating Ferroelectric and Nonferroelectric Electromechanical Effects with Scanning Probe Microscopy

Ferroelectricity in functional materials remains one of the most fascinating areas of modern science in the past several decades. In the last several years, the rapid development of piezoresponse force microscopy (PFM) and spectroscopy revealed the presence of electromechanical hysteresis loops and bias-induced remnant polar states in a broad variety of materials including many inorganic oxides, polymers, and biosystems. In many cases, this behavior was interpreted as the ample evidence for ferroelectric nature of the system. Here, we systematically analyze PFM responses on ferroelectric and nonferroelectric materials and demonstrate that mechanisms unrelated to ferroelectricity can induce ferroelectric-like characteristics through charge injection and electrostatic forces on the tip. We will focus on similarities and differences in various PFM measurement characteristics to provide an experimental guideline to differentiate between ferroelectric material properties and charge injection. In the end, we apply the developed measurement protocols to an unknown ferroelectric material.

Reversible strain effect on the magnetization of LaCoO3 films

films grown epitaxially on piezoelectric substrateshas been found to systematically decrease with the reduction of tensile strain. The magnetizationchange induced by the reversible strain variation reveals an increase of the Co magnetic momentwith tensile strain. The biaxial strain dependence of the Curie temperature is estimated to bebelow 4 K/% in the as-grown tensile strain state of our films. This is in agreement with results fromstatically strained films on various substrates.

Electrical characterization of PMN–28%PT(001) crystals used as thin-film substrates

Ferroelectric and piezoelectric properties of (001) 0.72PbMg1/3Nb2/3O3–0.28PbTiO3 (PMN–28%PT) single crystals have been investigated from cryogenic temperatures to 475 K. PMN–28%PT is used as piezoelectric substrate, e.g., in multiferroic heterostructures. Electric field-induced phase transformations have been examined by electrical characterization including measurements of polarization loops, dielectric permitivitty, and the resistance change in La0.7Sr0.3MnO3 films deposited on the (001) face. The relaxor ferroelectric transition behavior was studied by means of time-dependent current measurements. A phase diagram is set up. PMN–28%PT is found to be at the border of the appearance of the monoclinc phase (MC) bridging the rhombohedral-tetragonal (R-T) transformation at higher PbTiO3 contents. Measurements of the lattice expansion reveal that a high piezoelectric effect persists down to low temperatures. Therefore, PMN–28%PT single crystals are found to be appropriate substrates for application of piezoe...

Emerging magnetism and anomalous Hall effect in iridate–manganite heterostructures

Strong Coulomb repulsion and spin–orbit coupling are known to give rise to exotic physical phenomena in transition metal oxides. Initial attempts to investigate systems, where both of these fundamental interactions are comparably strong, such as 3d and 5d complex oxide superlattices, have revealed properties that only slightly differ from the bulk ones of the constituent materials. Here we observe that the interfacial coupling between the 3d antiferromagnetic insulator SrMnO3 and the 5d paramagnetic metal SrIrO3 is enormously strong, yielding an anomalous Hall response as the result of charge transfer driven interfacial ferromagnetism. These findings show that low dimensional spin–orbit entangled 3d–5d interfaces provide an avenue to uncover technologically relevant physical phenomena unattainable in bulk materials.

Continuously Controlled Optical Band Gap in Oxide Semiconductor Thin Films

The optical band gap of the prototypical semiconducting oxide SnO2 is shown to be continuously controlled through single axis lattice expansion of nanometric films induced by low-energy helium implantation. While traditional epitaxy-induced strain results in Poisson driven multidirectional lattice changes shown to only allow discrete increases in bandgap, we find that a downward shift in the band gap can be linearly dictated as a function of out-of-plane lattice expansion. Our experimental observations closely match density functional theory that demonstrates that uniaxial strain provides a fundamentally different effect on the band structure than traditional epitaxy-induced multiaxes strain effects. Charge density calculations further support these findings and provide evidence that uniaxial strain can be used to drive orbital hybridization inaccessible with traditional strain engineering techniques.

Controlling octahedral rotations in a perovskite via strain doping

The perovskite unit cell is the fundamental building block of many functional materials. The manipulation of this crystal structure is known to be of central importance to controlling many technologically promising phenomena related to superconductivity, multiferroicity, mangetoresistivity, and photovoltaics. The broad range of properties that this structure can exhibit is in part due to the centrally coordinated octahedra bond flexibility, which allows for a multitude of distortions from the ideal highly symmetric structure. However, continuous and fine manipulation of these distortions has never been possible. Here, we show that controlled insertion of He atoms into an epitaxial perovskite film can be used to finely tune the lattice symmetry by modifying the local distortions, i.e., octahedral bonding angle and length. Orthorhombic SrRuO3 films coherently grown on SrTiO3 substrates are used as a model system. Implanted He atoms are confirmed to induce out-of-plane strain, which provides the ability to controllably shift the bulk-like orthorhombically distorted phase to a tetragonal structure by shifting the oxygen octahedra rotation pattern. These results demonstrate that He implantation offers an entirely new pathway to strain engineering of perovskite-based complex oxide thin films, useful for creating new functionalities or properties in perovskite materials.

SrTiO3 on piezoelectric PMN-PT(001) for application of variable strain

SrTiO3 (STO) is the most frequently used substrate material for complex oxide films. In this work, STO is explored as a buffer layer on piezoelectric pseudocubic Pb(Mg1/3Nb2/3)0.72Ti0.28O3(001) (PMN-PT) substrates which serve to reversibly strain thin films. The STO buffer layer reduces the in-plane lattice parameter and allows for better lattice matching to broader range of thin film materials. STO films (30 nm) have been grown with epitaxial orientation on PMN-PT with an in-plane lattice parameter close to that of bulk STO. The substrate`s rhombohedral domain structure has been imaged by Atomic Force Microscopy (AFM). The related ferroelectric domain structure has been investigated by Piezoresponse Force Microscopy (PFM). Within a domain, STO grows with rather low roughness (rms < 0.2 nm). The transfer of the piezoelectric substrate strain to the STO film and its variation with an applied electric field is studied by x-ray diffraction. The strain dependence of the electrical resistance is measured for a ferromagnetic manganite film grown on top of the STO. Both experiments confirm qualitatively that the STO buffer transfers the substrate strain into a functional film deposited on top.

Enhancing interfacial magnetization with a ferroelectric

Ferroelectric control of the electronic and magnetic properties of a correlated oxide provides new opportunities for fundamental science and practical device applications. However, the exploding interest in ferroelectric control of magnetic interfaces, which typically happens in a few nanometers, has been inhibited by the lack of appropriate characterization techniques. Here, the authors have used polarized neutron reflectivity (PNR), a nondestructive yet powerful technique, to directly probe the evolution of the interfacial magnetism at the interface between ferromagnetic La

Thermal generation of spin current in epitaxial CoFe2O4 thin films

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