S. S. Jaswal

Giant Electroresistance in Ferroelectric Tunnel Junctions

The interplay between the electron transport in metal-ferroelectric-metal junctions with ultrathin ferroelectric barriers and the polarization state of a barrier is investigated. Using a model which takes into account screening of polarization charges in metallic electrodes and direct quantum tunneling across a ferroelectric barrier, we calculate the change in the tunneling conductance associated with the polarization switching. We find the conductance change of a few orders of magnitude for metallic electrodes with significantly different screening lengths. This giant electroresistance effect is the consequence of a different potential profile seen by transport electrons for the two opposite polarization orientations.

Surface Magnetoelectric Effect in Ferromagnetic Metal Films

A surface magnetoelectric effect is revealed by density-functional calculations that are applied to ferromagnetic Fe(001), Ni(001), and Co(0001) films in the presence of an external electric field. The effect originates from spin-dependent screening of the electric field which leads to notable changes in the surface magnetization and the surface magnetocrystalline anisotropy. These results are of considerable interest in the area of electrically controlled magnetism and magnetoelectric phenomena.

Predicted Magnetoelectric Effect in Fe/BaTiO 3 Multilayers: Ferroelectric Control of Magnetism

An unexplored physical mechanism which produces a magnetoelectric effect in ferroelectric/ferromagnetic multilayers is studied based on first-principles calculations. Its origin is a change in bonding at the ferroelectric/ferromagnet interface that alters the interface magnetization when the electric polarization reverses. Using Fe/BaTiO3 multilayers as a representative model we show a sizable difference in magnetic moments of Fe and Ti atoms at the two interfaces dissimilar by the orientation of the local electric dipole moment. The predicted magnetoelectric effect is comparable in magnitude with that observed in elastically-coupled composites and opens a new direction to control magnetic properties of thin-film layered structures by electric fields.

Magnetic Tunnel Junctions with Ferroelectric Barriers: Prediction of Four Resistance States from First Principles

Magnetic tunnel junctions (MTJs), composed of two ferromagnetic electrodes separated by a thin insulating barrier layer, are currently used in spintronic devices, such as magnetic sensors and magnetic random access memories. Recently, driven by demonstrations of ferroelectricity at the nanoscale, thin-film ferroelectric barriers were proposed to extend the functionality of MTJs. Due to the sensitivity of conductance to the magnetization alignment of the electrodes (tunneling magnetoresistance) and the polarization orientation in the ferroelectric barrier (tunneling electroresistance), these multiferroic tunnel junctions (MFTJs) may serve as four-state resistance devices. On the basis of first-principles calculations, we demonstrate four resistance states in SrRuO(3)/BaTiO(3)/SrRuO(3) MFTJs with asymmetric interfaces. We find that the resistance of such a MFTJ is significantly changed when the electric polarization of the barrier is reversed and/or when the magnetizations of the electrodes are switched from parallel to antiparallel. These results reveal the exciting prospects of MFTJs for application as multifunctional spintronic devices.

Electric field effect on magnetization at the Fe/MgO(001) interface

Density-functional calculations are performed to explore magnetoelectric effects originating from the influence of an external electric field on magnetic properties of the Fe/MgO(001) interface. It is shown that the effect on the interface magnetization and magnetocrystalline anisotropy can be substantially enhanced if the electric field is applied across a dielectric material with a large dielectric constant. In particular, we predict an enhancement of the interface magnetoelectric susceptibility by a factor of the dielectric constant of MgO over that of the free standing Fe (001) surface. We also predict a significant effect of electric field on the interface magnetocrystalline anisotropy due to the change in the relative occupancy of the 3d-orbitals of Fe atoms at the Fe/MgO interface. These results may be interesting for technological applications such as electrically controlled magnetic data storage.

Multi-ferroic and magnetoelectric materials and interfaces

The existence of multiple ferroic orders in the same material and the coupling between them have been known for decades. However, these phenomena have mostly remained the theoretical domain owing to the fact that in single-phase materials such couplings are rare and weak. This situation has changed dramatically recently for at least two reasons: first, advances in materials fabrication have made it possible to manufacture these materials in structures of lower dimensionality, such as thin films or wires, or in compound structures such as laminates and epitaxial-layered heterostructures. In these designed materials, new degrees of freedom are accessible in which the coupling between ferroic orders can be greatly enhanced. Second, the miniaturization trend in conventional electronics is approaching the limits beyond which the reduction of the electronic element is becoming more and more difficult. One way to continue the current trends in computer power and storage increase, without further size reduction, is to use multi-functional materials that would enable new device capabilities. Here, we review the field of multi-ferroic (MF) and magnetoelectric (ME) materials, putting the emphasis on electronic effects at ME interfaces and MF tunnel junctions.

Interface Effect on Ferroelectricity at the Nanoscale

Interfaces play a critical role in nanoscale ferroelectricity. We perform a first-principles study of ultrathin KNbO(3) ferroelectric films placed between two metal electrodes, either SrRuO(3) or Pt. We show that bonding at the ferroelectric-metal interfaces imposes severe constraints on the displacement of atoms, destroying the bulk tetragonal soft mode. If the interface bonding is sufficiently strong, the ground-state represents a ferroelectric domain with an interface domain wall, driven by the intrinsic oppositely oriented dipole moments at the two interfaces. The critical thickness for the net polarization of the KNbO(3) film is predicted to be about 1 nm for Pt and 1.8 nm for SrRuO(3) electrodes.

Tailoring magnetic anisotropy at the ferromagnetic/ferroelectric interface

It is predicted that magnetic anisotropy of a thin magnetic film may be affected by the polarization of a ferroelectric material. Using a Fe∕BaTiO3 bilayer as a representative model and performing first-principles calculations, we demonstrate that a reversal of the electric polarization of BaTiO3 produces a sizable change in magnetic anisotropy energy of Fe films. Tailoring the magnetic anisotropy of a nanomagnet by an adjacent ferroelectric material may yield entirely new device concepts, such as electric-field controlled magnetic data storage.

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

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.

Electronic polarisabilities of ions in alkali halide crystals

Abstract Tessman, Kahn and Shockley calculated the electronic polarisabilities of ions in alkali halide crystals using the long wavelength limiting values of the visible light dielectric constants. We have recalculated these widely used polarisabilities using the more accurate room-temperature dielectric constant data of Lowndes and Martin and a better minimisation procedure of Pirenne and Kartheuser. We have also calculated for the first time the low temperature values of these polarisabilities. The computed values of the polarisability in A 3 are Li + 0·029, Na + 0·285, K + 1·149, Rb + 1·707, Cs + 2·789, F − 0·876, Cl − 3·005, Br − 4·168, I − 6·294 at 300°K and Li + 0·029, Na + 0·290, K + 1·133, Rb + 1·679, Cs + 2·743, F − 0·858, Cl − 2·947, Br − 4·091, I − 6·116 at 4°K. The relative standard deviations for all the alkali halides are 1·20 and 1·43 per cent at 300°K and 4°K respectively justifying the additive nature of the individual ion polarisabilities.