Evgeny Tsymbal Publications

Ferroelectric and multiferroic tunnel junctions

E.Y. Tsymbal et al. — Mon, 26 Nov 2012 12:09:20 PST

The phenomenon of electron tunneling has been known since the advent of quantum mechanics, but continues to enrich our understanding of many fi elds of physics, as well as creating sub-fi elds on its own. Spin-dependent tunneling in magnetic tunnel junctions has aroused considerable interest and development. In parallel with this endeavor, recent advances in thin-fi lm ferroelectrics have demonstrated the possibility of achieving stable and switchable ferroelectric polarization in nanometer-thick fi lms. This discovery opened the possibility of using thin-fi lm ferroelectrics as barriers in magnetic tunnel junctions, thus merging the fi elds of magnetism, ferroelectricity, and spin-polarized transport into an exciting and promising area of novel research. Nowadays, this research has become an important constituent of a broader effort in multiferroic materials and heterostructures that involves rich fundamental science and offers a potential for applications in novel multifunctional devices. The purpose of this article is to review recent developments in ferroelectric and multiferroic tunnel junctions. Starting from the concept of electron tunneling, we fi rst discuss the key properties of magnetic tunnel junctions and then assess key functional characteristics of ferroelectric and multiferroic tunnel junctions. We discuss the recent demonstrations of giant resistive switching observed in ferroelectric tunnel junctions and the new concept of electrically controlling the spin polarization in magnetic tunnel junctions with a ferroelectric tunnel barrier.

Perspectives of Giant Magnetoresistance

E. Y. Tsymbal et al. — Tue, 21 Aug 2012 10:17:54 PDT

Giant magnetoresistance (GMR) is one of the most fascinating discoveries in thin-film magnetism, which combines both tremendous technological potential and deep fundamental physics. Within a decade of GMR being discovered in 1988 commercial devices based on this phenomenon, such as hard-disk read-heads, magnetic field sensors and magnetic memory chips, had become available in the market. These achievements would not have been possible without a detailed understanding of the physics of GMR, which requires a quantum-mechanical insight into the electronic spin-dependent transport in magnetic structures.

A giant tunneling electroresistance effect driven by electrically controlled spin valve at a complex oxide interface

J.D. Burton et al. — Fri, 20 Jul 2012 13:01:55 PDT

A giant tunneling electroresistance effect may be achieved in a ferroelectric tunnel junction by exploiting the magnetoelectric effect at the interface between the ferroelectric barrier and a magnetic La_1-xSr_xMnO₃ electrode. Using ﬁrst-principles density-functional theory we demonstrate that a few magnetic monolayers of La_1-xSr_xMnO₃ near the interface act, in response to ferroelectric polarization reversal, as an atomic-scale spin valve by ﬁltering spin-dependent current. This produces more than an order of magnitude change in conductance, and thus constitutes a giant resistive switching effect.

Transport Spin Polarization of High-Curie Temperature MnBi Films

P. Kharel et al. — Fri, 20 Jul 2012 12:21:11 PDT

We report on the study of the structural, magnetic, and transport properties of highly textured MnBi ﬁlms with the Curie temperature of 628 K. In addition to detailed measurements of resistivity and magnetization, we measure transport spin polarization of MnBi by Andreev reﬂection spectroscopy and perform fully relativistic band-structure calculations of MnBi. A spin polarization from 51% ± 1% to 63% ± 1% is observed, consistent with the calculations and with an observation of a large magnetoresistance in MnBi contacts. The band-structure calculations indicate that in spite of almost identical densities of states at the Fermi energy, the large disparity in the Fermi velocities leads to high transport spin polarization of MnBi. The correlation between the values of magnetization and spin polarization is discussed.

Evolution of the band alignment at polar oxide interfaces

J.D. Burton et al. — Fri, 20 Jul 2012 12:06:44 PDT

First-principles calculations demonstrate the evolution of the band alignment at La_0.7A_0.3MnO₃/ La_1−xA_xO / TiO₂/ SrTiO₃ (001) heterointerfaces, where A=Ca, Sr, or Ba, as the interfacial A-site composition, La_1−xA_x, is varied from x=0.5 to x=1.0. This variation leads to a linear change in the SrTiO₃ valence-band offset with respect to the Fermi level of the La_0.7A_0.3MnO₃ metal electrode and hence to a linear change in the Schottky barrier height at this interface. The effect arises due to electrostatic screening of the polar interface, altering the interfacial dipole and hence the electrostatic potential step at this interface. We ﬁnd that both the La_0.7A_0.3MnO₃ and SrTiO₃ layers contribute to screening with both electronic and ionic screening being important for the change in the interface dipole. The results are in agreement with the recent experimental data.

Ferroelectric dead layer driven by a polar interface

Y. Wang et al. — Fri, 20 Jul 2012 11:21:58 PDT

Based on ﬁrst-principles and model calculations we investigate the effect of polar interfaces on the ferroelectric stability of thin-ﬁlm ferroelectrics. As a representative model, we consider a TiO₂ -terminated BaTiO₃ ﬁlm with LaO monolayers at the two interfaces that serve as doping layers. We ﬁnd that the polar interfaces create an intrinsic electric ﬁeld that is screened by the electron charge leaking into the BaTiO₃ layer. The amount of the leaking charge is controlled by the boundary conditions which are different for three heterostructures considered, namely, vacuum/LaO/BaTiO₃ /LaO, LaO/BaTiO₃ , and SrRuO₃ /LaO/BaTiO₃ /LaO. The intrinsic electric ﬁeld forces ionic displacements in BaTiO₃ to produce the electric polarization directed into the interior of the BaTiO₃ layer. This creates a ferroelectric dead layer near the interfaces that is nonswitchable and thus detrimental to ferroelectricity. Our ﬁrst-principles and model calculations demonstrate that the effect is stronger for a larger effective ionic charge at the interface and longer screening length due to a stronger intrinsic electric ﬁeld that penetrates deeper into the ferroelectric. The predicted mechanism for a ferroelectric dead layer at the interface controls the critical thickness for ferroelectricity in systems with polar interfaces.

Tailoring a two-dimensional electron gas at the LaAlO₃ ∕ SrTiO₃ (001) interface by epitaxial strain

C. W. Bark et al. — Wed, 02 May 2012 15:22:03 PDT

Recently a metallic state was discovered at the interface between insulating oxides, most notably LaAlO₃ and SrTiO₃. Properties of this two-dimensional electron gas (2DEG) have attracted significant interest due to its potential applications in nanoelectronics. Control over this carrier density and mobility of the 2DEG is essential for applications of these unique systems, and may be achieved by epitaxial strain. However, despite the rich nature of strain effects on oxide materials properties, such as ferroelectricity, magnetism, and superconductivity, the relationship between the strain and electrical properties of the 2DEG at the LaAlO₃∕SrTiO₃ heterointerface remains largely unexplored. Here, we use different lattice constant single-crystal substrates to produce LaAlO₃∕SrTiO₃ interfaces with controlled levels of biaxial epitaxial strain. We have found that tensile-strained SrTiO₃ destroys the conducting 2DEG, while compressively strained SrTiO₃ retains the 2DEG, but with a carrier concentration reduced in comparison to the unstrained LaAlO₃∕SrTiO₃ interface. We have also found that the critical LaAlO₃ overlayer thickness for 2DEG formation increases with SrTiO₃ compressive strain. Our first-principles calculations suggest that a strain-induced electric polarization in the SrTiO₃ layer is responsible for this behavior. The polarization is directed away from the interface and hence creates a negative polarization charge opposing that of the polar LaAlO₃ layer. This behavior both increases the critical thickness of the LaAlO₃ layer, and reduces carrier concentration above the critical thickness, in agreement with our experimental results. Our findings suggest that epitaxial strain can be used to tailor 2DEGs properties of the LaAlO₃∕SrTiO₃ heterointerface.

Highly Spin-Polarized Conducting State at the Interface between Nonmagnetic Band Insulators: LaAlO₃/FeS₂ (001)

J. D. Burton et al. — Wed, 02 May 2012 15:13:01 PDT

First-principles density functional calculations demonstrate that a spin-polarized two-dimensional conducting state can be realized at the interface between two nonmagnetic band insulators. The (001) surface of the diamagnetic insulator FeS₂ (pyrite) supports a localized surface state deriving from Fe d orbitals near the conduction band minimum. The deposition of a few unit cells of the polar perovskite oxide LaAlO₃ leads to electron transfer into these surface bands, thereby creating a conducting interface. The occupation of these narrow bands leads to an exchange splitting between the spin subbands, yielding a highly spin-polarized conducting state distinct from the rest of the nonmagnetic, insulating bulk. Such an interface presents intriguing possibilities for spintronics applications.

Interface dipole effect on thin film ferroelectric stability: First-principles and phenomenological modeling

Xiaohui Liu et al. — Wed, 02 May 2012 15:09:13 PDT

Utilization of the switchable spontaneous polarization of ferroelectric materials offers a promising avenue for the future of nanoelectronic memories and logic devices provided that nanoscale metal-ferroelectric-metal heterostructures can be engineered to maintain a bi-stable polarization switchable by an applied electric field. The most challenging aspect of this approach is to overcome the deleterious interface effects which tend to render ferroelectric polarization either unstable or unswitchable and which become ever more important as ferroelectric materials are produced thinner and thinner. Here we use first-principles density functional calculations and phenomenological modeling to demonstrate that a BaO/RuO₂ interface termination sequence in SrRuO₃/BaTiO₃/SrRuO₃ epitaxial heterostructures grown on SrTiO₃ can lead to a nonswitchable polarization state for thin BaTiO₃ films due to a fixed interface dipole. The unfavorable interface dipole at the BaO/RuO₂ interface leads to a strong preference for one polarization state and, in thin film structures, leads to instability of the second state below a certain critical thickness, thereby making the polarization unswitchable.We analyze the contribution of this interface dipole to the energetic stability of these heterostructures. Furthermore, we propose and demonstrate that this unfavorable interface dipole effect can be alleviated by deposition of a thin layer of SrTiO₃ at the BaO/RuO₂ terminated interface. Our first-principles and phenomenological modeling predict that the associated change of the interface termination sequence to SrO/TiO₂ on both sides of the heterostructure leads to a restoration of bi-stability with a smaller critical thickness, along with an enhancement of the barrier for polarization reversal. These results demonstrate that interface engineering is a viable approach to enhance ferroelectric properties at the nanoscale.

Multiferroic tunnel junctions with poly(vinylidene fluoride)

Julian P. Velev et al. — Wed, 02 May 2012 15:02:27 PDT

We perform first-principles calculations based on density functional theory of the spin-resolved conductance of poly(vinylidene fluoride)- (PVDF) based multiferroic tunnel junctions (MFTJs).We consider Co/PVDF/O/Co (0001) MFTJs with one oxidized interface, representing the different experimental growth conditions for the two interfaces. We demonstrate that this natural asymmetry leads to multiple resistance states associated with different magnetization configurations of the electrodes and ferroelectric polarization orientations of the barrier. Our results indicate very high tunability of the tunneling magnetoresistance and electroresistance effects, which could be useful for logic and memory applications.

Coherent potential approximation as a voltage probe

Mikhail Y. Zhuravlev et al. — Wed, 02 May 2012 14:58:11 PDT

Coherent potential approximation (CPA) haswidely been used for studying the residual resistivity of bulk alloys and electrical conductivity in systems with structural disorder. Here, we revisit the single-site CPA within the Landauer-Büttiker approach applied to the electronic transport in layered structures and show that thismethod can be interpreted in terms of the B¨uttiker voltage-probe model that has been developed for treating phase-breaking scattering in mesoscopic systems. We demonstrate that the on-site vertex function, which appears within the single-site CPA formalism, plays the role of the local chemical potential within the voltage-probe approach. This interpretation allows the determination of the chemical potential profile across a disordered conductor, which is useful for analyzing results of transport calculations within the CPA. We illustrate this method by providing several examples. In particular, for layered systems with translational periodicity in the plane of the layers, we introduce the local resistivity and calculate the interface resistance between disordered layers.

Intrinsic defects in multiferroic BiFeO₃ and their effect on magnetism

Tula R. Paudel et al. — Wed, 02 May 2012 14:54:27 PDT

We investigate the energetics of the intrinsic defects in bulk multiferroic BiFeO₃ and explore their implication for magnetization using a first-principles approach based on density functional theory. We find that the dominant defects in oxidizing (oxygen-rich) conditions are Bi and Fe vacancies and in reducing (oxygen-poor) conditions are O and Bi vacancies. The calculated carrier concentration shows that the BiFeO₃ grown in oxidizing conditions has p-type conductivity. The conductivity decreases with oxygen partial pressure, and the material becomes insulating with a tendency for n-type conductivity. We find that the Bi and Fe vacancies produce a magnetic moment of ∼1μ_B and 5μ_B per vacancy, respectively, for p-type BiFeO₃ and none for insulating BiFeO₃. O vacancies do not introduce any moment for both p-type and insulating BiFeO₃. Calculated magnetic moments due to intrinsic defects are consistent with those reported experimentally for bulk BiFeO₃.

Magnetic and superconducting phases at the LaAlO₃/SrTiO₃ interface: The role of interfacial Ti 3d electrons

N. Pavlenko et al. — Wed, 02 May 2012 14:48:37 PDT

Ferromagnetism and superconductivity are, in most cases, adverse. However, recent experiments reveal that they coexist at interfaces of LaAlO₃ and SrTiO₃. We analyze the magnetic state within density functional theory and provide evidence that magnetism is not an intrinsic property of the two-dimensional electron liquid at the interface. We demonstrate that the robust ferromagnetic state is induced by the oxygen vacancies in SrTiO₃ or in the LaAlO₃ layer. This allows for the notion that areas with increased density of oxygen vacancies produce ferromagnetic puddles and account for the previous observation of a superparamagnetic behavior in the superconducting state.

Metallic and Insulating Oxide Interfaces Controlled by Electronic Correlations

H. W. Jang et al. — Wed, 23 Feb 2011 13:41:41 PST

The formation of two-dimensional electron gases (2DEGs) at complex oxide interfaces is directly influenced by the oxide electronic properties. We investigated how local electron correlations control the 2DEG by inserting a single atomic layer of a rare-earth oxide (RO) [(R is lanthanum (La), praseodymium (Pr), neodymium (Nd), samarium (Sm), or yttrium (Y)] into an epitaxial strontium titanate oxide (SrTiO₃) matrix using pulsed-laser deposition with atomic layer control. We find that structures with La, Pr, and Nd ions result in conducting 2DEGs at the inserted layer, whereas the structures with Sm or Y ions are insulating. Our local spectroscopic and theoretical results indicate that the interfacial conductivity is dependent on electronic correlations that decay spatially into the SrTiO₃ matrix. Such correlation effects can lead to new functionalities in designed heterostructures.

Interlayer exchange coupling across a ferroelectric barrier

M Ye Zhuravlev et al. — Wed, 23 Feb 2011 13:32:13 PST

A new magnetoelectric effect is predicted originating from the interlayer exchange coupling between two ferromagnetic layers separated by an ultrathin ferroelectric barrier. It is demonstrated that ferroelectric polarization switching driven by an external electric field leads to a sizable change in the interlayer exchange coupling. The effect occurs in asymmetric ferromagnet/ferroelectric/ferromagnet junctions due to a change in the electrostatic potential profile across the junction affecting the interlayer coupling. The predicted phenomenon indicates the possibility of switching the magnetic configuration by reversing the polarization of the ferroelectric barrier layer.

Magnetoelectric effect at the SrRuO₃/BaTiO₃ (001) interface: An ab initio study

Manish K. Niranjan et al. — Mon, 19 Jul 2010 13:58:07 PDT

Ferromagnet/ferroelectric interface materials have emerged as structures with strong magnetoelectric coupling that may exist due to unconventional physical mechanisms. Here we present a first-principles study of the magnetoelectric effect at the ferromagnet/ferroelectric SrRuO3 /BaTiO3 (001) interface. We find that the exchange splitting of the spin-polarized band structure, and therefore the magnetization, at the interface can be altered substantially by reversal of the ferroelectric polarization in the BaTiO3. These magnetoelectric effects originate from the screening of polarization charges at the SrRuO3 /BaTiO3 interface and are consistent with the Stoner model for itinerant magnetism.

Tunneling electroresistance in ferroelectric tunnel junctions with a composite barrier

M. Ye. Zhuravlev et al. — Mon, 19 Jul 2010 13:03:17 PDT

Tunneling electroresistance (TER) effect is the change in the electrical resistance of a ferroelectric tunnel junction (FTJ) associated with polarization reversal in the ferroelectric barrier layer. Here we predict that a FTJ with a composite barrier that combines a functional ferroelectric film and a thin layer of a nonpolar dielectric can exhibit a significantly enhanced TER. Due to the change in the electrostatic potential with polarization reversal, the nonpolar dielectric barrier acts as a switch that changes its barrier height from a low to high value. The predicted values of TER are giant and indicate that the resistance of the FTJ can be changed by several orders in magnitude at the coercive electric field of ferroelectric.

Effect of spin-dependent screening on tunneling electroresistance and tunneling magnetoresistance in multiferroic tunnel junctions

M. Ye. Zhuravlev et al. — Mon, 07 Jun 2010 13:56:12 PDT

Using a ferroelectric barrier as a functional material in a (magnetic) tunnel junction has recently attracted significant interest due to new functionalities not available in conventional tunnel junctions. Switching a ferroelectric polarization of the barrier alters conductance resulting in a tunneling electroresistance (TER) effect. Using a ferroelectric barrier in a magnetic tunnel junction makes it mutiferroic where TER coexists with tunneling magnetoresistance (TMR). Here we develop a simple model for a multiferroic tunnel junction (MFTJ) which consists of two ferromagnetic electrodes separated by a ferroelectric barrier layer. The model explicitly includes the spin-dependent screening potential and thus extends previously developed models for FTJs and MFTJs. Our results demonstrate that the effect of spin-dependent screening may be sizable and may provide significant contributions to TMR and TER in MFTJs. We find that, similar to FTJs with a composite (ferroelectric/dielectric) barrier layer, the TER in a MFTJ with such a barrier is dramatically enhanced indicating that the resistance ratio between the states corresponding to the opposite polarization orientations may be as high as 10⁴ and even higher. Our results demonstrate the possibility of four resistance states in MFTJs with a pronounced difference in resistance and a possibility to control these resistance by an electric field (through ferroelectric polarization of the barrier) and by a magnetic field (through magnetization configuration of the electrodes). These functionalities may be interesting to device applications of MFTJs.

Electrically driven magnetism on a Pd thin film

Y. Sun et al. — Mon, 07 Jun 2010 13:56:11 PDT

Using first-principles density-functional calculations, we demonstrate that ferromagnetism can be induced and modulated on an otherwise paramagnetic Pd metal thin-film surface through application of an external electric field. As free charges are either accumulated or depleted at the Pd surface to screen the applied electric field, there is a corresponding change in the surface density of states. This change can be made sufficient for the Fermi-level density of states to satisfy the Stoner criterion, driving a transition locally at the surface from a paramagnetic state to an itinerant ferromagnetic state above a critical applied electric field, E_c. Furthermore, due to the second-order nature of this transition, the surface magnetization of the ferromagnetic state just above the transition exhibits a substantial dependence on electric field, as the result of an enhanced magnetoelectric susceptibility. Using a linearized Stoner model, we explain the occurrence of the itinerant ferromagnetism and demonstrate that the magnetic moment on the Pd surface follows a square-root variation with electric field, m ∞ (E - E_c)^1/2 consistent with our first-principles calculations.

Prediction of a Switchable Two-Dimensional Electron Gas at Ferroelectric Oxide Interfaces

Manish K. Niranjan et al. — Thu, 03 Dec 2009 12:06:59 PST

The demonstration of a quasi-two-dimensional electron gas (2DEG) in LaAlO₃=SrTiO₃ heterostructures has stimulated intense research activity in recent years. The 2DEG has unique properties that are promising for applications in all-oxide electronic devices. For such applications it is desirable to have the ability to control 2DEG properties by external stimulus. Here, based on first-principles calculations we predict that all-oxide heterostructures incorporating ferroelectric constituents, such as KNbO₃=ATiO₃ (A = Sr, Ba, Pb), allow creating a 2DEG switchable between two conduction states by ferroelectric polarization reversal. The effect occurs due to the screening charge at the interface that counteracts the depolarizing electric field and depends on polarization orientation. The proposed concept of ferroelectrically controlled interface conductivity offers the possibility to design novel electronic devices.