Partha Mitra

Weak-Localization Correction to the Anomalous Hall Effect in Polycrystalline Fe Films

In situ transport measurements have been made on ultrathin (<100 A thick) polycrystalline Fe films as a function of temperature and magnetic field for a wide range of disorder strengths. For sheet resistances Rxx less than approximately 3kOmega, we find a logarithmic temperature dependence of the anomalous Hall conductivity sigmaxy, which is shown for the first time to be due to a universal scale dependent weak-localization correction within the skew-scattering model. For higher sheet resistance, granularity becomes important and the break down of universal behavior becomes manifest as the prefactors of the lnT correction term to sigmaxx and sigmaxy decrease at different rates with increasing disorder.

Spin valve effect in self-exchange biased ferromagnetic metal/semiconductor bilayers

We report magnetization and magetoresistance measurements in hybrid ferromagnetic metal/semiconductor heterostructures comprised of MnAs∕(Ga,Mn)As bilayers. Our measurements show that the (metallic) MnAs and (semiconducting) (Ga,Mn)As layers are exchange coupled, resulting in an exchange biasing of the magnetically softer (Ga,Mn)As layer that weakens with layer thickness. Magnetoresistance measurements in the current-perpendicular-to-the-plane geometry show a spin valve effect in these self-exchange biased bilayers. Similar measurements in MnAs∕p-GaAs∕(Ga,Mn)As trilayers show that the exchange coupling diminishes with spatial separation between the layers.

Polarization-Tailored Fano Interference in Plasmonic Crystals: A Mueller Matrix Model of Anisotropic Fano Resonance

Fano resonance is observed in a wide range of micro- and nano-optical systems and has been a subject of intensive investigations due to its numerous potential applications. Methods that can control or modulate Fano resonance by tuning some experimentally accessible parameters are highly desirable for realistic applications. Here we present a simple yet elegant approach using the Mueller matrix formalism for controlling the Fano interference effect and engineering the resulting asymmetric spectral line shape in an anisotropic optical system. The approach is founded on a generalized model of anisotropic Fano resonance, which relates the spectral asymmetry to physically meaningful and experimentally accessible parameters of interference, namely, the Fano phase shift and the relative amplitudes of the interfering modes. The differences in these parameters between orthogonal linear polarizations in an anisotropic system are exploited to desirably tune the Fano spectral asymmetry using pre- and postselection of optimized polarization states. The concept is demonstrated on waveguided plasmonic crystals using Mueller matrix-based polarization analysis. The approach enabled tailoring of several exotic regimes of Fano resonance in a single device, including the complete reversal of the spectral asymmetry, and shows potential for applications involving control and manipulation of electromagnetic waves at the nanoscale.

Ion-beam-induced nanosmoothening and conductivity enhancement in ultrathin metal films

We present a systematic in situ study of the effect of postdeposition low-energy (200eV) ion bombardment on resistance and surface topography of ultrathin iron (<50A) and copper (<130A) films. The ion-beam-induced nanosmoothening occurs while material is being removed and gives rise to an initial decrease in resistance followed by a steady increase as the film is subsequently uniformly eroded. The shunt resistance associated with the resistance decrease is found to be independent of the thickness of the underlying film, thus indicating that the conductivity enhancement is due primarily to surface modification.

In situ self-assembly and photopolymerization for hetero-phase synthesis and patterning of conducting materials using soft oxometalates in thermo-optical tweezers

We demonstrate a novel method of simultaneous photoassisted hetero-phase synthesis, doping, and micro-scale patterning of conductive materials. The patterning is performed by controlled self-assembly mediated by a micro-bubble induced in an optical tweezers configuration. The high temperature generated due to the light field of the tweezers also drives diverse chemical reactions that lead to the in situ formation of conducting metal-oxides and polymers due to a charge transfer mechanism with soft oxometalates (SOMs). We synthesize two conducting polymers – polypyrrole and polyaniline – doped by the metal oxides Mo–O2 and Mo–O3, from dispersions of the respective organic compounds with the SOMs, and form permanent patterns out of them by continuous self-assembly arising from manipulation of the micro-bubble using Marangoni flows generated by the tweezers. The electrically conducting patterns of width varying between around 4–50 μm, are written in the form of simple Hall-bar geometries, and a four-probe measurement technique yields conductivities on the order of ∼450–600 Siemens cm−1 – which is much higher than that reported for both polypyrrole and polyaniline in earlier work. This technique can easily be used in patterning complicated electrical circuits in mesoscopic length scales, and can also be extended to solution processed electronic device development by green chemical routes.

Tunneling Magnetoresistance in Exchange Biased Ferromagnetic Semiconductor Tunnel Junctions

Summary form only given.Here, we demonstrate the observation of TMR in exchange-biased MTJs derived from the ferromagnetic semiconductor (Ga,Mn)As. Although still limited to operation at low temperatures, these devices provide an important step forward in exploring proof-of-concept semiconductor spintronic tunneling devices.

Low Temperature Quantum Corrections to the Anomalous Hall Conductivity in Ultra‐thin Fe Films

In situ transport measurements on disordered polycrystalline Fe films reveal a logarithmic temperature dependence of the longitudinal Rxx and anomalous Hall RxxAH resistances at low temperatures and a heretofore‐unobserved scaling behavior in which the relative changes in Rxx and Rxy are found to be equal. Accordingly, the anomalous Hall conductivity is non‐zero with a logarithmic temperature dependence that is believed to be a manifestation of quantum effects due to enhanced e‐e interactions and magnetic scattering in itinerant ferromagnets.