Yaron Segal

Origin of the magnetoelectric coupling effect in Pb(Zr0.2Ti0.8)O{3}/La{0.8}Sr{0.2}MnO{3} Multiferroic heterostructures.

The electronic valence state of Mn in Pb(Zr0.2Ti0.8)O{3}/La{0.8}Sr{0.2}MnO{3} multiferroic heterostructures is probed by near edge x-ray absorption spectroscopy as a function of the ferroelectric polarization. We observe a temperature independent shift in the absorption edge of Mn associated with a change in valency induced by charge carrier modulation in the La0.8Sr0.2MnO3, demonstrating the electronic origin of the magnetoelectric effect. Spectroscopic, magnetic, and electric characterization shows that the large magnetoelectric response originates from a modified interfacial spin configuration, opening a new pathway to the electronic control of spin in complex oxide materials.

Crystalline oxides on silicon.

This review outlines developments in the growth of crystalline oxides on the ubiquitous silicon semiconductor platform. The overall goal of this endeavor is the integration of multifunctional complex oxides with advanced semiconductor technology. Oxide epitaxy in materials systems achieved through conventional deposition techniques is described first, followed by a description of the science and technology of using atomic layer-by-layer deposition with molecular beam epitaxy (MBE) to systematically construct the oxide-silicon interface. An interdisciplinary approach involving MBE, advanced real-space structural characterization, and first-principles theory has led to a detailed understanding of the process by which the interface between crystalline oxides and silicon forms, the resulting structure of the interface, and the link between structure and functionality. Potential applications in electronics and photonics are also discussed.

Temperature dependence of the magnetoelectric effect in Pb(Zr0.2Ti0.8)O3/La0.8Sr0.2MnO3 multiferroic heterostructures

The magnetoelectric response of Pb(Zr0.2Ti0.8)O3/La0.8Sr0.2MnO3 (PZT/LSMO) artificial multiferroic heterostructures as a function of temperature, electric, and magnetic field, shows that the largest magnetoelectric coupling is attained at temperatures near the magnetic critical point of LSMO, at ∼180 K (−13.5 Oe cm kV−1). The magnetoelectric coupling displays a strong temperature dependence, changing sign at 150 K and saturating to positive values below ∼100 K (+6 Oe cm kV−1). The magnetoelectric curve switches hysteretically between two states in response to the ferroelectric switching. The peak in the magnetoelectric response coincides with the observation of on/off switching of magnetism in LSMO near the critical region, where the sensitivity to electric field is largest, making it a promising approach for device applications.

Origin of 90° domain wall pinning in Pb(Zr0.2Ti0.8)O3 heteroepitaxial thin films

We describe a transmission-electron-microscopy study of the ferroelectric domains in an epitaxial Pb(Zr0.2Ti0.8)O3 (PZT) film grown on La0.7Sr0.3MnO3/SrTiO3(001). We directly observe the pinning of 90° domain walls by pairs of misfit dislocations, respectively, with Burgers vectors a [100] and a [001]. Model calculations based on the elastic theory confirm our finding that, in addition to the depolarization field surrounding the dislocation, the strain field of misfit dislocation-pairs plays the primary role in the formation and pinning of a domains.

Achieving A‐Site Termination on La0.18Sr0.82Al0.59Ta0.41O3 Substrates

Smooth substrate surfaces terminated with a single atomic plane are essential for perfecting the epitaxial growth of transition metal oxide fi lms. SrTiO 3 (STO), which is closely lattice matched to many transition metal oxides, is extensively used in part because TiO 2 -terminated surfaces exhibiting singleunit-cell steps can be achieved through etching and annealing techniques. [ 1–5 ] In addition to improving the epitaxial growth of fi lms, substrates terminated by a single atomic plane enable interfaces between transition metal oxides to be studied. For example, TiO 2 -terminated substrates have enabled the interface between LaAlO 3 and STO to be probed where metallicity and superconductivity have been unexpectedly observed. [ 6 , 7 ]

Control of magnetism in Pb(Zr0.2Ti0.8)O3/La0.8Sr0.2MnO3 multiferroic heterostructures (invited)

We present an overview of our results demonstrating a large, charge-driven, magnetoelectric coupling in epitaxial Pb(Zr0.2Ti0.8)O3/La0.8Sr0.2MnO3 (PZT/LSMO) multiferroic heterostructures. Measurements of the magnetization as a function of temperature and applied electric field using magneto-optic Kerr effect magnetometry show a large change in the magnetic critical temperature and magnetic moment of the LSMO layer for the two states of the PZT ferroelectric polarization, which modulates the charge-carrier concentration at the LSMO interface. Near-edge x-ray absorption spectroscopy measurements show directly that the valence state of Mn is modulated by the PZT polarization state, demonstrating that the magnetoelectric coupling in these PZT/LSMO multiferroic heterostructures is purely electronic in origin. From the combined spectroscopic, magnetic, and electric characterization, we conclude that both the interfacial spin state and spin configuration are modulated electrostatically. This ability of controlli...

Dynamic Evanescent Phonon Coupling Across the La 1 − x Sr x MnO 3 / SrTiO 3 Interface

The transport and magnetic properties of correlated La0.53Sr0.47MnO3 ultrathin films, grown epitaxially on SrTiO3, show a sharp cusp at the structural transition temperature of the substrate. Using a combination of experiment and first principles theory we show that the cusp is a result of evanescent cross-interface coupling between the charge carriers in the film and a soft phonon mode in the SrTiO3, mediated through linked oxygen octahedral motions. The amplitude of the mode diverges at the transition temperature, and phonons are launched into the first few atomic layers of the film, affecting its electronic state.

The atomic structure and polarization of strained SrTiO3/Si

For thin film devices based on coupling ferroelectric polarization to charge carriers in semiconductors, the role of the interface is critical. To elucidate this role, we use synchrotron x-ray diffraction to determine the interface structure of epitaxial SrTiO3 grown on the (001) surface of Si. The average displacement of the O octahedral sublattice relative to the Sr sublattice determines the film polarization and is measured to be about 0.05 nm toward the Si, with Ti off-center displacements 0.009 nm away from the substrate. Measurements of films with different boundary conditions on the top of the SrTiO3 show that the polarization at the SrTiO3/Si interface is dominated by oxide-Si chemical interactions.

Scanning SQUID susceptometry of a paramagnetic superconductor

Scanning SQUID susceptometry images the local magnetization and susceptibility of a sample. By accurately modeling the SQUID signal we can determine the physical properties such as the penetration depth and permeability of superconducting samples. We calculate the scanning SQUID susceptometry signal for a superconducting slab of arbitrary thickness with isotropic London penetration depth, on a non-superconducting substrate, where both slab and substrate can have a paramagnetic response that is linear in the applied field. We derive analytical approximations to our general expression in a number of limits. Using our results, we fit experimental susceptibility data as a function of the sample-sensor spacing for three samples: 1) delta-doped SrTiO3, which has a predominantly diamagnetic response, 2) a thin film of LaNiO3, which has a predominantly paramagnetic response, and 3) a two-dimensional electron layer (2-DEL) at a SrTiO3/AlAlO3 interface, which exhibits both types of response. These formulas will allow the determination of the concentrations of paramagnetic spins and superconducting carriers from fits to scanning SQUID susceptibility measurements.

Growth and characterization of PZT/LSMO multiferroic heterostructures

The control of the correlated state in complex oxides offers the opportunity to harness the multifunctional properties characteristic of these systems, one example of which is the recent observation of multiferroic behavior in Pb(Zr0.2Ti0.8)O3/La1−xSrxMnO3 (PZT/LSMO) heterostructures. Here, the authors present a detailed study of the growth and structural characterization of epitaxial PZT/LSMO structures grown by a combination of molecular beam epitaxy and off-axis rf magnetron sputtering. In situ and ex situ structural characterizations demonstrate the high quality and single crystalline nature of the films over a wide range of film compositions and thicknesses.