Showing papers by "Stéphane Fusil published in 2007"

Tunnel junctions with multiferroic barriers

Abstract: Multiferroics are singular materials that can exhibit simultaneously electric and magnetic orders. Some are ferroelectric and ferromagnetic and provide the opportunity to encode information in electric polarization and magnetization to obtain four logic states. However, such materials are rare and schemes allowing a simple electrical readout of these states have not been demonstrated in the same device. Here, we show that films of La0.1Bi0.9MnO3 (LBMO) are ferromagnetic and ferroelectric, and retain both ferroic properties down to a thickness of 2 nm. We have integrated such ultrathin multiferroic films as barriers in spin-filter-type tunnel junctions that exploit the magnetic and ferroelectric degrees of freedom of LBMO. Whereas ferromagnetism permits read operations reminiscent of magnetic random access memories (MRAM), the electrical switching evokes a ferroelectric RAM write operation. Significantly, our device does not require the destructive ferroelectric readout, and therefore represents an advance over the original four-state memory concept based on multiferroics.

Room-temperature coexistence of large electric polarization and magnetic order in Bi Fe O 3 single crystals

Abstract: From an experimental point of view, room-temperature ferroelectricity in $\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_{3}$ is raising many questions. Electric measurements made a long time ago on solid solutions of $\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_{3}$ with $\mathrm{Pb}(\mathrm{Ti},\mathrm{Zr}){\mathrm{O}}_{3}$ indicate that a spontaneous electric polarization exists in $\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_{3}$ below the Curie temperature ${T}_{C}=1143\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Yet in most reported works, the synthesized samples are too conductive at room temperature to get a clear polarization loop in the bulk without any effects of extrinsic physical or chemical parameters. Surprisingly, up to now there has been no report of a $P(E)$ (polarization versus electric field) loop at room temperature on single crystals of $\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_{3}$. We describe here our procedure to synthesize ceramics and to grow good quality sizeable single crystals by a flux method. We demonstrate that $\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_{3}$ is indeed ferroelectric at room temperature through evidence by piezoresponse force microscopy and $P(E)$ loops. The polarization is found to be large, around $60\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{C}∕{\mathrm{cm}}^{2}$, a value that has only been reached in thin films. Magnetic measurements using a superconducting quantum interference device magnetometer and M\"ossbauer spectroscopy are also presented. The latter confirms the results of nuclear magnetic resonance measurements concerning the anisotropy of the hyperfine field attributed to the magnetic cycloidal structure.

Template-grown NiFe/Cu/NiFe nanowires for spin transfer devices

Abstract: We have developed a new reliable method combining template synthesis and nanolithography-based contacting technique to elaborate current perpendicular-to-plane giant magnetoresistance spin valve nanowires, which are very promising for the exploration of electrical spin transfer phenomena. The method allows the electrical connection of one single nanowire in a large assembly of wires embedded in anodic porous alumina supported on Si substrate with diameters and periodicities to be controllable to a large extent. Both magnetic excitations and switching phenomena driven by a spin-polarized current were clearly demonstrated in our electrodeposited NiFe/Cu/ NiFe trilayer nanowires. This novel approach promises to be of strong interest for subsequent fabrication of phase-locked arrays of spin transfer nano-oscillators with increased output power for microwave applications.

Bias dependence of tunnel magnetoresistance in spin filtering tunnel junctions: Experiment and theory

Abstract: A spin filter is a type of magnetic tunnel junction in which only one of the electrodes is magnetic and the insulating barrier is ferro- or ferrimagnetic. We report on spin-dependent transport measurements and their theoretical analysis in epitaxial spin filters integrating a tunnel barrier of the high-Curie-temperature ferrimagnetic spinel $\mathrm{Ni}{\mathrm{Fe}}_{2}{\mathrm{O}}_{4}$, with half-metallic ${\mathrm{La}}_{2∕3}{\mathrm{Sr}}_{1∕3}\mathrm{Mn}{\mathrm{O}}_{3}$ and Au electrodes. A positive tunnel magnetoresonance of up to $\ensuremath{\sim}50%$ is obtained at low temperature, which we find decreases with bias voltage. In view of these experimental results, we propose a theoretical treatment of the transport properties of spin filters with epitaxial magnetic barriers, based on an elliptical variation of the decay rates within the spin-dependent gaps in analogy with what was calculated for nonmagnetic barrier materials such as MgO or $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3}$. Whereas the spin filtering efficiency for zero bias is of one sign, we show that this can easily change with bias; the degree of change hinges on the energy variation of the majority and minority spin decay rates of the transmission across the barrier. We point out some shortcomings of approaches based on models in which the transmission is related to spin-dependent barrier heights, and some implications for future experimental and theoretical research on spin filters.

Electron energy loss spectroscopy determination of Ti oxidation state at the (001) LaAlO3/SrTiO3 interface as a function of LaAlO3 growth conditions

Abstract: At the (001) interface between the two band-insulators LaAlO3 and SrTiO3, a high-mobility electron gas may appear, which has been the object of numerous works over the last four years. Its origin is a subject of debate between the interface polarity and unintended doping. Here we use electron energy loss 'spectrum images', recorded in cross-section in a scanning transmission electron microscope, to analyse the Ti3+ ratio, characteristic of extra electrons. We find an interface concentration of Ti3+ that depends on growth conditions.

Magnetic patterning of (Ga,Mn)As by hydrogen passivation

Abstract: We present a method to create magnetic patterns in thin layers of (Ga,Mn)As. It relies on local hydrogen passivation to significantly lower the hole density, and thereby locally suppress the carrier-mediated ferromagnetic phase. The sample surface is thus maintained continuous, and the minimal structure size is of about 200nm. In micron-sized ferromagnetic dots fabricated by hydrogen passivation on perpendicularly magnetized layers, the switching fields can be maintained closer to the continuous film coercivity, compared to dots made by usual dry etch techniques.

Charge imbalance at oxide interfaces: How nature deals with it

Abstract: Some interfaces in semiconductors or insulators structurally cause a valence mismatch, which leads to a two-dimensional space charge that must be balanced by localised or mobile charge carriers. Screening by mobile electrons presents a lot of theoretical as well as practical interests. However it is extremely rare, so that we are aware of only one case, on which we focus here: the (0 0 1) interface between LaAlO 3 and TiO 2 -terminated SrTiO 3 . Theoretically, this interface between two insulators is positively charged. Electron conductivity is observed in this system, but whether it is associated with the interface screening or an extrinsic unintended doping is not yet settled. Here, we use the literature and our own numerical and practical experiments to discuss the physics of this system.

PROOF COPY 091722JAP Spin momentum transfer effects observed in electrodeposited Co/Cu/Co nanowires

Abstract: Spin-transfer torque effects are reported in nanowires consisting in Co/Cu/Co trilayers electrodeposited on an anodic alumina template. Using a nanolithography process based on electrically controlled nanoindentation of the alumina template, we are able to investigate the spin transport properties of single nanowires at room temperature. For small applied magnetic fields, we have measured resistance changes above a critical direct-current (dc) injected current that corresponds to the change in resistance observed in the magnetoresistance curves at low current. We conclude that magnetic reversals are driven by a spin-polarized current. The critical current densities needed for the magnetization reversals are in the 10(7) A/cm(2) range and the dependence of the critical currents with the applied field is consistent with the spin-transfer mechanism. For large applied magnetic fields, the differential resistance exhibits some peaks that we attribute to the onset of high-frequency excitations of the free-layer magnetization. According to the high density of electrodeposited nanowires in alumina templates, our results are promising for synchronized spin-transfer oscillators. (c) 2007 American Institute of Physics.

Mapping the Spatial Distribution of Charge Carriers in LaAlO3/SrTiO3 Heterostructures

Abstract: At the interface between complex insulating oxides, novel phases with interesting properties may occur, such as the metallic state reported in the LaAlO3/SrTiO3 system. While this state has been predicted and reported to be confined at the interface, some works indicate a much broader spatial extension, thereby questioning its origin. Here we provide for the first time a direct determination of the carrier density profile of this system through resistance profile mappings collected in cross-section LaAlO3/SrTiO3 samples with a conducting-tip atomic force microscope (CT-AFM). We find that, depending upon specific growth protocols, the spatial extension of the high-mobility electron gas can be varied from hundreds of microns into SrTiO3 to a few nanometers next to the LaAlO3/SrTiO3 interface. Our results emphasize the potential of CT-AFM as a novel tool to characterize complex oxide interfaces and provide us with a definitive and conclusive way to reconcile the body of experimental data in this system.