S. Mathi Jaya

Resonant tunnel magnetoresistance in double-barrier magnetic tunnel junctions: A non-equilibrium Green's function study

The non-equilibrium Greens function technique is used to study the transport characteristics of double-barrier magnetic tunnel junctions. The exchange coupling strength of the electrodes is found to be crucial in deciding the magnetoresistance characteristics of these devices. At sufficiently large values of the magnetic coupling strength, the device is found to exhibit resonant tunnel magnetoresistance and its magnitude is found to be large. The existence of pure spin currents in these devices when there is antiferromagnetic coupling between the end electrodes is found to be the primary cause of resonant tunnel magnetoresistance. The influence of the band occupation of the electrodes and the many-body interaction present in the electrode regions on the spin current and magnetoresistance are also studied.

Quasiparticle spectrum of ferro- and antiferromagnetic local moment film with finite band occupation

Abstract The quasiparticle spectrum of a local moment film with two layers is calculated using the s–f model considering both ferro- and antiferromagnetic coupling between the layers. The conduction band of the film is assumed to have a band occupation of 0.2 and at this band occupation the quasiparticle spectrum is determined at different temperatures. We find striking correlation effects in the quasiparticle spectrum because of the s–f interaction and the spectrum is found to split into two parts. The splitting is found to persist even in the paramagnetic region and such a splitting cannot be obtained from one electron approaches.

Temperature-dependent quasiparticle conduction band structure of the EuS monolayer

The temperature-dependent quasiparticle band structure of a ferromagnetic EuS monolayer is calculated using the s-f model which combines the one-electron band structure and a many-body model evaluation. The one-electron part required for the many-body calculation was obtained from a TB-LMTO (tight-binding linear muffin-tin orbital) band-structure calculation. A suitable supercell with five layers of empty spheres was used in the band-structure calculations to obtain the Bloch energies of the monolayer. We find striking correlation effects in the quasiparticle spectrum induced by the s-f exchange interaction and also observe significant temperature effects in the spectrum. We have further calculated the quasiparticle spectrum of the bulk EuS using the same s-f model in order to compare the results with those for the monolayer and to analyse the influences of the low dimensionality on the quasiparticle spectrum of this system. The bulk spectrum also exhibits significant correlation and temperature effects. The conduction bands of the monolayer as well as the bulk EuS show red-shift with respect to temperature and the magnitude of the red-shift is found to be larger for some bands in the bulk EuS compared to that of the monolayer.

Middle-layer ferromagnetism-induced transition of the tunnel magneto-resistance in double-barrier magnetic tunnel junctions: A non-equilibrium Green's function study

Using the non-equilibrium Greens function modeling of the transport characteristics of tunnel devices, we have found the middle-layer ferromagnetism-induced transition of the tunnel magneto-resistance in double-barrier magnetic tunnel junctions. It is observed from our study that even a weak ferromagnetism of the middle metallic layers is capable of promoting resonant tunnel magneto-resistance in these devices and the strength of the ferromagnetism is found to have strong influence on the bias dependence of the resonant tunnel magneto-resistance. The spin-up and spin-down currents flow in opposite directions for certain band occupancies and at certain bias voltage ranges when there is antiferromagnetic coupling between the electrodes of the tunnel junction. Resonant tunnel magneto-resistance occurs when the net current (sum of spin-up and spin-down currents) becomes very small at situations mentioned above. We have further studied the influence of band occupation of the electrode layers and the many-body interactions present in the electrode region on the spin current and magneto-resistance of these devices.

Molecular dynamics studies of displacement cascades in Fe-Y2TiO5 system

The effect of displacement cascade on Fe-Y2TiO5 bilayer is studied using classical molecular dynamics simulations. Different PKA species – Fe, Y, Ti and O – with the same PKA energy of 8 keV are used to produce displacement cascades that encompass the interface. It is shown that Ti atom has the highest movement in the ballistic regime of cascades which can lead to Ti atoms moving out of the oxide clusters into the Fe matrix in ODS alloys.

Compositional variation of magnetic moments in Fe(1-x)Mx (M=Co/Ni) nanowires: Ab initio study

Ab initio simulations are used to investigate the spin magnetic moments of free standing alloy nanowires of Fe(1-x)Mx (M=Co/Ni; x=0 to 1) in the bcc (110) orientation. The variation of average magnetic moments shows a systematic reduction with the addition of Co/Ni that is at variance with the Slater-Pauling curve, well known for bulk transition metal alloys. These results are rationalized on the basis of spin-polarized density of states.

Temperature-dependent quasiparticle band structure of an antiferromagnetic two-layer EuTe film

Abstract The temperature-dependent quasiparticle band structure of an antiferromagnetic two layer EuTe film is calculated using the s–f model. The one electron part required for the many body evaluation was obtained from TB-LMTO band structure calculation. A suitable supercell with five layers of empty spheres was used in the band structure calculation to obtain the Bloch energies of the film. The quasiparticle band structure of the (1 0 0) film is calculated at different temperatures and we find significant correlation effects in the spectrum. Some bands exhibit wave-vector-dependent splitting and the magnitude of the split-gap increases with increasing temperature. It is clearly seen as the combined effects of correlation and temperature. We find a red-shift of the conduction band edge of this EuTe film with respect to temperature whereas the bulk EuTe exhibits blue shift.

Interlayer exchange coupling in M/N/M multilayers

The interlayer exchange coupling (IEC) of two local moment ferromagnetic layers separated by a non-magnetic spacer layer (a M/N/M multilayer) is studied using the modified Ruderman-Kittel-Kasuya-Yosida (RKKY) method along with the s-f model. The IEC exhibits oscillatory behaviour with respect to the spacer layer thickness and it oscillates between ferromagnetic and antiferromagnetic configurations. The conventional RKKY method is also used to obtain the IEC and the results are compared with those obtained from the modified RKKY method which incorporates the electron correlation effects. We find significant correlation effects on the IEC and in fact the correlations alter the nature and magnitude of the magnetic coupling. Hence this study indicates the importance of the inclusion of correlation effects to the understanding of the IEC in multilayer systems with local moment ferromagnetic sublayers and to offering a satisfactory explanation for the experimental results.