Electroresistance Effect Research Articles

Giant electrode effect on tunnelling electroresistance in ferroelectric tunnel junctions.

Among recently discovered ferroelectricity-related phenomena, the tunnelling electroresistance (TER) effect in ferroelectric tunnel junctions (FTJs) has been attracting rapidly increasing attention owing to the emerging possibilities of non-volatile memory, logic and neuromorphic computing applications of these quantum nanostructures. Despite recent advances in experimental and theoretical studies of FTJs, many questions concerning their electrical behaviour still remain open. In particular, the role of ferroelectric/electrode interfaces and the separation of the ferroelectric-driven TER effect from electrochemical ('redox'-based) resistance-switching effects have to be clarified. Here we report the results of a comprehensive study of epitaxial junctions comprising BaTiO(3) barrier, La(0.7)Sr(0.3)MnO(3) bottom electrode and Au or Cu top electrodes. Our results demonstrate a giant electrode effect on the TER of these asymmetric FTJs. The revealed phenomena are attributed to the microscopic interfacial effect of ferroelectric origin, which is supported by the observation of redox-based resistance switching at much higher voltages.

Gate tuning of anomalous Hall effect in ferromagnetic metal SrRuO3

The electric field effect on ferromagnetism offers a new dimension in the recent advancement of spintronics. We report on the gate control of transport properties in thin films of oxide-based ferromagnetic metal, SrRuO3. An electric double layer transistor configuration was utilized with an ionic liquid dielectric to apply a strong electric field on a SrRuO3 thin film of 5 monolayers in thickness. The application of gate voltage induced a clear electroresistance effect, despite a considerably-large initial carrier density of the order of 1022 cm−3. Furthermore, we found that the gate modulation of the anomalous Hall conductivity σxy, which was as large as ∼±40% at low temperatures, was about three times larger than that of the longitudinal conductivity σxx. The variation of σxy is characterized by the power-law scaling relation with σxx, which is widely observed in a bad metal regime of the charge transport.

Bias-dependence of the tunneling electroresistance and magnetoresistance in multiferroic tunnel junctions

We predict that the tunneling electroresistance effect is present at finite bias even in multiferroic tunnel junctions (MFTJs) with inversion symmetry. The effect is highly sensitive to the relative magnetization orientation in the electrodes. In addition, we demonstrate control of the bias-dependence of the tunneling magnetoresistance (TMR) in MFTJs via switching of the ferroelectric polarization of the barrier. The polarization induces a monotonic bias behavior in TMR which can be reversed by polarization switching. The magnitude of both effects is proportional to the polarization. The underlying mechanism is the inversion symmetry breaking due to the polarization combined with the interplay of the bias-induced and polarization-induced spin-dependent interfacial screening. These results expand the possibilities for the next-generation multifunctional devices.

Proximity coupling of a granular film with a ferroelectric substrate and giant electroresistance effect

We study electron transport in granular film placed above the ferroelectric substrate. We show that the conductivity of granular film strongly depends on the ferroelectric state due to screening effects which modify the Coulomb blockade in granular film. In particular, the electric current in granular film is controlled by the direction of ferroelectric polarization. We show that the ferroelectric/granular film system has a large electro-resistance effect. This effect can be utilized in memory and electric field sensor applications.

Interface-modification-enhanced tunnel electroresistance in multiferroic tunnel junctions

We report a large tunnel electroresistance (TER) effect up to ∼104% in La0.67Sr0.33MnO3/BaTiO3/Co (LSMO/BTO/Co) multiferroic tunnel junctions (MFTJs), which couples with well-defined tunnel magnetoresistance. The large TER is related to (LaAlO3)0.3(LaSrTaO6)0.7 substrates which guarantee a high-quality LSMO/BTO interface and robust ferroelectricity in BTO. The insert of 0.5 nm-thick Pt between the Co electrode and BTO barrier further enhances the TER value to 105% and improves the endurance of the MFTJs, ascribed to the shortened screening length and reduced oxidation of BTO/Co interface. Their use would advance the process towards practical MFTJs with four resistance states.

Fast Two-stage Protector Against Electromagnetic Pulse Based on Electroresistance Effect in Polycrystalline La-Sr(Ca)-Mn-O Films

The electroresistance (ER) effect in polycrystalline films of La0.83Sr0.17MnO3 and La0.7Ca0.3MnO3 was investigated in the temperature range of (5 – 290) K using high power sub-nanosecond rise time electrical pulses with amplitude up to 1 kV. It was obtained that conductance vs. voltage dependences are nonlinear and could be well fitted by empirical formula G = G0 + Gα · Uα; where G is conductance, U is the voltage applied across the sample, G0 is the conductance at low voltage, and Gα and α are the parameters related to the electrical transport mechanism. Parameters α for La-Ca-Mn-O and La-Sr-Mn-O were 1.5 and 1.33 respectively. It was obtained that there are two regions of the electroresistance vs. temperature dependence for both films: low temperature region where ER exhibits very slow dependence on temperature and high temperature region where ER significantly decreases with temperature. It was demonstrated that polycrystalline manganite films can be used for the development of protectors against short electromagnetic pulse (EMP), and fast two-stage protector operating at cryogenic temperatures (80 K) is proposed. DOI: http://dx.doi.org/10.5755/j01.ms.20.2.6325

Ferroelectric tunnel junctions with multi-quantum well structures

Ferroelectric tunnel junctions (FTJs) with multi-quantum well structures are proposed and the tunneling electroresistance (TER) effect is investigated theoretically. Compared with conventional FTJs with monolayer ferroelectric barriers, FTJs with single-well structures provide TER ratio improvements of one order of magnitude, while FTJs with optimized multi-well structures can enhance this improvement by another order of magnitude. It is believed that the increased resonant tunneling strength combined with appropriate asymmetry in these FTJs contributes to the improvement. These studies may help to fabricate FTJs with large TER ratio experimentally and put them into practice.

Current induced abnormal electroresistance effect observed in epitaxial La0.9Hf0.1MnO3 thin films

La0.9Hf0.1MnO3 thin films with thickness 100 nm were prepared by using a pulsed laser deposition technique. Transport behaviors were investigated under various applied currents without an applied magnetic field. When the applied current is not too large, the peak value of the resistance gradually decreases with increasing current, demonstrating a normal electroresistance (ER) effect. However, when the current reaches a critical value, a high-resistance state appears at a lower temperature below the Curie temperature. And the appeared resistance peak at low temperature turns out to be extremely sensitive to a weak current. Even a very small current could greatly depress the height of the peak, an abnormal ER effect appears. Maximum resistance ratio ER, defined as [R(1 μA)-R(100 μA)]/R(100 μA), is about 1257% at 50 K. Physics related to the appearance of the novel state and the abnormal ER effect is discussed.

Giant Electroresistance Induced by Electric Currents in Epitaxial Thin Films of Pr<sub>0.7</sub>Sr<sub>0.3</sub>MnO<sub>3</sub>

Electric-current induced electroresistance effect has been investigated in epitaxial Pr0.7Sr0.3MnO3thin film grown on the (100) SrTiO3substrate. A significant change ~38% in the ratio of the peak resistance at different currents with a current density up to ~3.3×103A/cm2was achieved. Such an ER effect is more remarkable in comparing with that reported in other manganite oxides with similar current densities. Compared with that of the as-grown films, the electroresistance of the post-annealed films is smaller, while the ER effect (~14%) is obtained. Although the nature behind such an electroresistance effect has not been well understood, the field tunability of the metal/insulator transition and the electroresistance effect induced by currents might be of potential for various applications such as filed effect devices.

Tunneling Electroresistance Induced by Interfacial Phase Transitions in Ultrathin Oxide Heterostructures

The ferroelectric (FE) control of electronic transport is one of the emerging technologies in oxide heterostructures. Many previous studies in FE tunnel junctions (FTJs) exploited solely the differences in the electrostatic potential across the FTJs that are induced by changes in the FE polarization direction. Here, we show that in practice the junction current ratios between the two polarization states can be further enhanced by the electrostatic modification in the correlated electron oxide electrodes, and that FTJs with nanometer thin layers can effectively produce a considerably large electroresistance ratio at room temperature. To understand these surprising results, we employed an additional control parameter, which is related to the crossing of electronic and magnetic phase boundaries of the correlated electron oxide. The FE-induced phase modulation at the heterointerface ultimately results in an enhanced electroresistance effect. Our study highlights that the strong coupling between degrees of freedom across heterointerfaces could yield versatile and novel applications in oxide electronics.

Interplay of cross-plane polaronic transport and resistive switching in Pt–Pr0.67Ca0.33MnO3–Pt heterostructures

The identification of the cross-plane electric transport mechanisms in different resistance states of metal–oxide sandwich structures is essential for gaining insights into the mechanisms of resistive switching (RS). Here, we present a systematic study of cross-plane electric transport properties of Pr0.67Ca0.33MnO3 (PCMO) thin films sandwiched by precious Pt metal electrodes. We observe three different transport regimes: ohmic, nonlinear and RS. The nonlinear regime is associated with colossal magneto-resistance (CMR) and colossal electro-resistance (CER) effects. In contrast to RS, the CMR and CER are volatile resistance effects which persist only during application of strong magnetic or electric fields and they are restricted to low temperatures. At low current densities, the device resistance is dominated by small polaron hopping transport of the PCMO film. At higher electric current densities near the switching threshold, the interface resistance starts to dominate and remarkably also exhibits thermally activated transport properties. Our studies also shed light onto the interplay of colossal resistance effects and RS: at low temperatures, RS can be only induced by reduction of the PCMO resistivity through CMR and CER. This clearly demonstrates the key role of the current density for controlling the amplitude of non-volatile resistive changes. Conversely, the CMR can be used as a probe for the switching induced changes in disorder and correlations. At small switching amplitudes, we observe slight changes in polaron activation energy which can be attributed to changes at the interface. If the switching amplitude exceeds 1000% and more, the CMR effect in the device can be reversibly changed. This indicates persistent changes in electronic or lattice structure of large regions within the PCMO film.

Electro-photo double modulation on the resistive switching behavior and switchable photoelectric effect in BiFeO3 films

We present an electro-photo double modulation on the resistive switching behavior in an Au/BiFeO3/La0.7Sr0.3MnO3/SrTiO3 heterostructure, combining the electro-resistance effect and the photo-resistance effect. The pulse voltages can lead to nonvolatile resistance variations in this heterostructure, and the laser illumination can also modulate the high and low resistance states. Consequently, four stable resistance states are achieved. Furthermore, a switchable photoelectric effect-the direction of the photocurrent can be switched by polarization reversal, is also observed in this heterostructure. The present results should have potential applications to develop multi-state memory devices based on perovskite oxides.

Prediction of ferroelectric stability and magnetoelectric effect of asymmetric multiferroic tunnel junctions

Taking into account size effect of the ferroelectric tunnel barrier and interface effect determined by two dissimilar magnetic electrodes, the ferroelectric stability and magnetoelectric effect of asymmetric multiferroic tunnel junctions have been comprehensively calculated. Results of the stable structure, polarization, electrostatic potential, charge density, spin density, and magnetic moments demonstrated a series of the great hidden features in asymmetric multiferroic tunnel junctions and showed significant differences between two polarization states. The potential and magnetic moment are strongly affected by electric and magnetic polarizations, suggesting the possibility of ferroelectric control of magnetization and coexistence of tunnel magnetoresistance and giant electroresistance effects.

Electroresistance and current-induced metastable states in the thin film of half-doped manganite Pr0.5Sr0.5MnO3

Electroresistance (ER) effects and current-induced metastable states in the thin film of Pr0.5Sr0.5MnO3 were investigated. The initial state was insensitive to weak currents and susceptible to high-density currents. As the current density reached a certain value, metastable states, which were very sensitive to weak currents at low temperatures, were excited. It was found that as the excited current increased, the induced metastable state showed a larger electroresistance in a wider temperature range. Interfacial effects related to electrodes could be ruled out. The observed effects might be related to the coexistence and instability of the multiphases in manganites.

Electric field modulation of magnetism and electric properties in La-Ca-MnO3/Pb(Zr0.52Ti0.48)O3 magnetoelectric laminate

We investigated the effects of electric field on the magnetization and electric transport properties in La0.6Ca0.4MnO3/Pb(Zr0.52Ti0.48)O3 (LCMO/PZT) thin film prepared by sol-gel method and simple spin coating technology. Obvious converse magnetoelectric effect (the relative change in magnetization, ΔM ∼ 15%), as well as large electroresistance effect (the relative change in resistance, ER ∼ 61%), has been obtained in LCMO thin film with applying electric field on the PZT ceramic. The efficient mechanical coupling, which originated from ferroelectric polarization and inverse piezoelectric effect in PZT substrate, leads to large changes in the magnetism and resistance of LCMO thin film. The experimental results can be interpreted by the changes of both spin exchange coupling and electron-lattice coupling strengths in LCMO film, which result from the strain at the ferromagnetic/ferroelectric interface.

Enhanced tunnelling electroresistance effect due to a ferroelectrically induced phase transition at a magnetic complex oxide interface

The range of recently discovered phenomena in complex oxide heterostructures, made possible owing to advances in fabrication techniques, promise new functionalities and device concepts. One issue that has received attention is the bistable electrical modulation of conductivity in ferroelectric tunnel junctions (FTJs) in response to a ferroelectric polarization of the tunnelling barrier, a phenomenon known as the tunnelling electroresistance (TER) effect. Ferroelectric tunnel junctions with ferromagnetic electrodes allow ferroelectric control of the tunnelling spin polarization through the magnetoelectric coupling at the ferromagnet/ferroelectric interface. Here we demonstrate a significant enhancement of TER due to a ferroelectrically induced phase transition at a magnetic complex oxide interface. Ferroelectric tunnel junctions consisting of BaTiO3 tunnelling barriers and La(0.7)Sr(0.3)MnO3 electrodes exhibit a TER enhanced by up to ~10,000% by a nanometre-thick La(0.5)Ca(0.5)MnO3 interlayer inserted at one of the interfaces. The observed phenomenon originates from the metal-to-insulator phase transition in La(0.5)Ca(0.5)MnO3, driven by the modulation of carrier density through ferroelectric polarization switching. Electrical, ferroelectric and magnetoresistive measurements combined with first-principles calculations provide evidence for a magnetoelectric origin of the enhanced TER, and indicate the presence of defect-mediated conduction in the FTJs. The effect is robust and may serve as a viable route for electronic and spintronic applications.

Electroresistance and Joule heating effects in manganite thin films

Electroresistance phenomena have been investigated in La2/3Sr1/3MnO3 (LSMO) microbridges through the analysis of I-V characteristic curves. I-V curves are nonlinear in all the range of temperatures explored, but are almost reversible and exhibit origin symmetry which suggest the existence of Joule self-heating effects. However, comparing the resistance determined from the I-V curves (R = V/I) at the actual sample temperature, measured through a Pt thermometer microfabricated on top of the LSMO microbridges, with the resistance measured in thermal equilibrium, i.e., in the low current regime, it is evident that Joule self-heating effects alone cannot account for the experimental results. We demonstrate that oxygen depletion is the cause for the observed increase of resistance and the reduction of TC.

DC- and pulse electroresistive effects in mixed manganites—a new view

A new concept is introduced which can develop interface electroresistive (ER-) and pulse electroresistive (EPIR-) effects far from phase transition temperatures and phase separation regions. It is based on strong self-trapping effects in mixed manganites, but this self-trapping is rather connected with larger defect trap states, not just with trapped electrons or holes. Due to a peculiar electrical field induced feedback, this self-trapping does not occur at random, but rather tends to build up space charge accumulation layers which are not at equilibrium with the matrix free carriers.

Colossal electroresistance and magnetoresistance effect in polycrystalline perovskite cobaltites Nd1−xSrxCoO3 (x = 0.1, 0.2, 0.3)

Polycrystalline samples of cobaltites Nd1−xSrxCoO3 (x=0.1, 0.2, 0.3) with the perovskite structure were synthesized by conventional solid state reaction method. Different currents ranging from 1μA to 1mA were applied to study the electroresistance (ER) effect. Temperature dependence of resistivity for the samples decrease dramatically with increase of the currents in the low temperature and non-linear current–voltage (I–V) characteristics are observed. The maximum ER ratio (defined as ER=(ρ1mA−ρL)/ρL×100%) of the all samples is more than −99%, which indicates a colossal electroresistance (CER) effect has found in these compounds. What is more, we also found that electric current can have influence on the low field magnetoresistance (LFMR) of the samples. LFMR decrease with the increase of applied current. The experimental results were discussed by spin polarized carriers modified ferromagnetic (FM)/nonmagnetic (NM) interface and grain interior phase separation scenario.

Coexistance of Giant Tunneling Electroresistance and Magnetoresistance in an All-Oxide Composite Magnetic Tunnel Junction

We propose, by performing advanced ab initio electron transport calculations, an all-oxide composite magnetic tunnel junction, within which both large tunneling magnetoresistance (TMR) and tunneling electroresistance (TER) effects can coexist. The TMR originates from the symmetry-driven spin filtering provided by an insulating BaTiO(3) barrier to the electrons injected from the SrRuO(3) electrodes. Following recent theoretical suggestions, the TER effect is achieved by intercalating a thin insulating layer, here SrTiO(3), at one of the SrRuO(3)/BaTiO(3) interfaces. As the complex band structure of SrTiO(3) has the same symmetry as that of BaTiO(3), the inclusion of such an intercalated layer does not negatively alter the TMR and in fact increases it. Crucially, the magnitude of the TER also scales with the thickness of the SrTiO(3) layer. The SrTiO(3) thickness becomes then a single control parameter for both the TMR and the TER effect. This protocol offers a practical way to the fabrication of four-state memory cells.