Ferroelectric

1. Ballistic Anisotropic Magnetoresistance (BAMR)E.Y. Tsymbal and S.S. Jaswal,University of NebraskaDMR-0203359 The resistivity of bulk ferromagnetic metals depends on the angle between the magnetization and the electric current. This phenomenon was discovered by Thomson in 1857 and was called Anisotropic Magneto-resistance (AMR). In this work we show that in the ballistic regime of conductance altering the magnetization direction leads to a change in the number of bands crossing the Fermi energy due to the spin-orbit interaction. This affects the ballistic conductance and is the origin of Ballistic Anisotropic Magnetoresistance (BAMR). We illustrate the significance of the BAMR by performing ab-initio calculations of the ballistic conductance of ferromagnetic nanowires for magnetization parallel and perpendicular to the axis of the wire. Physical Review Letters 94, 127203 (2005) Atomic structure of a nanowire and the predicted variation of the conductance as a function of the angle between the magnetization and the axis of the wire.

2. Giant Electroresistance in Ferroelectric Tunnel JunctionsE.Y. Tsymbal and S.S. Jaswal,University of NebraskaDMR-0203359 E Potential P Metal Metal – – – – – + + + + + Ferroelectric Recent experimental and theoretical findings suggest that ferroelectricity persists down to vanishingly small sizes, which opens a possibility to further miniaturize electronic devices based on ferroelectric materials. The existence of ferroelectricity at such a small film thickness makes it possible to use ferroelectrics as tunnel barriers in metal/ferroelectric/metal (M/FE/M) junctions. In this work we show that the electrical resistance in the ferroelectric tunnel junctions (FTJs) depends strongly on the orientation of the electric polarization which can be switched by an applied electric field. This result is encouraging in view of potential applications of FTJs as binary data storage media in nonvolatile random access memories. Physical Review Letters 94, 246802 (2005) Ferroelectric tunnel junction and electrostatic potential profile for two opposite directions of the polarization.

3. Impurity-Assisted Interlayer Exchange CouplingE.Y. Tsymbal and S.S. Jaswal,University of NebraskaDMR-0203359 Magnetizations of two ferromagnetic films separated by a thin insulating barrier layer are exchange-coupled due to the tunneling spin polarization propagating across the barrier. In this work  we show that impurities or defects creating localized states within the band gap of the insulator lead to a significant enhancement in the interlayer exchange coupling (IEC). We demonstrate that the impurity-assisted IEC becomes antiferromagnetic if the energy of the impurity state matches the Fermi energy and that the coupling can change sign as a function of barrier thickness. These results explain available experimental data on the IEC across tunnel barriers. Physical Review Letters 94, 026806 (2005) Interlayer exchange coupling as a function of impurity energy (left panel) and of barrier layer thickness (right panel).

4. EducationE.Y. Tsymbal and S.S. Jaswal,University of NebraskaDMR-0203359 This NSF grant supports J. D. Burton who joined our theory group in spring 2003 as an undergraduate student. Now he is a graduate student progressing toward the PhD degree. In a very short period of time J. D. mastered micromagnetic modeling, sophisticated transport theories, and first-principles band structure methods. For his excellent performance J. D. was awarded an Othmer Fellowship. The NSF funding also supported Dr. Kirill Belashchenko who had made major contributions to the research on spin-dependent tunneling. Deservedly recognized for his outstanding achievements, Kirill joined our department faculty as a tenure-track assistant professor, following an extremely competitive search. J. D. Burton (on the left) and Dr. Kirill Belashchenko(on the right) exhausted by scientific battles at our theory group meeting.