Kondo effect and impurity band conduction in Co:TiO2 magnetic semiconductor

The nature of charge carriers and their interaction with local magnetic moments in an oxide magnetic semiconductor is established. For cobalt-doped anatase TiO2 films, we demonstrate conduction in a metallic donor-impurity band. Moreover, we observe a clear signature of the Kondo effect in electrical transport data with remarkably high Kondo temperatures of up to 120 K. This indicates a strong coupling between local Co moments and delocalized electrons in the impurity band

Anomalous Hall effect in anatase Co:TiO2 ferromagnetic semiconductor

We have investigated the effects of modification of the SrTiO3 /Co interface as well as the SrTiO3 barrier on\ud the tunnel magnetoresistance TMR of La0.67Sr0.33MnO3 /SrTiO3 /Co junctions. Modification was realized by\ud the introduction of one atomic layer of either TiO2 or SrO at the SrTiO3 /Co interface. Barriers with different\ud oxygen content were also studied. In these structures we have observed positive as well as negative TMR, with\ud a trend towards positive TMR for junctions with interfacial SrO and/or more oxygen-deficient barriers. This\ud work offers more insight into the SrTiO3 /Co tunnel spin polarization and its sign

Intrinsic and Ionic Conduction in Humidity-Sensitive Sulfonated Polyaniline

The influence of humidity on the conductivity of sulfonated polyaniline (SPANI) and polyaniline (PANI) is investigated with electrochemical impedance spectroscopy (EIS). Separation of intrinsic (q) and ionic charge (i) mobility was observed using combination of ac and dc impedance measurements at relative humidity concentrations (RH) ranging from 5% to 90%. Nyquist plots for both acid-doped PANI and SPANI show single semicircles at all humidity levels, indicating pure intrinsic charge conduction. These semicircles are well-described with a fairly constant capacitance (C1 = 10-10 F) of the interdigitated electrodes and with a large varying resistance (10 kO - 10 MO) of the polymer bulk with RC times in the order of 10-3-10-6 sec. In all samples the intrinsic charge conductivity increases for increasing humidity, with emeraldine base PANI showing the strongest (2 orders of magnitude) increase upon raising the RH from 5 - 90%. A partial exchange of protons with sodium ions in SPANI (SPAN-Na) induces a second semicircle in the Nyquist plot at humidity levels above 40% with large RC (t) values (10-1 - 10-3 sec), which is attributed to a contribution of ionic mobility at the polymer/electrode interface. Blends of SPAN-Na with poly(vinyl alcohol) (PVA) and polyacrylamide (PAA) also show a second semicircle at slightly higher humidity levels from 50% RH onwards. In all cases this second semicircle is best described by a Constant Phase Element (CPE) with a value (order typical 10-9 - 10-7 F) increasing at higher humidity, and a varying exponential coefficient (0.5 <n <0.8), possibly due to a changing dispersion in the representative relaxation times for migration and accumulation of the ionic species. In addition, for SPAN-Na/SPANI samples blended with PVA a third semicircle is seen which can be described by an additional RC element representing a second phase or grain boundary contribution in the film

Epitaxial diodes of a half-metallic ferromagnet on an oxide semiconductor

We report on the fabrication and electrical characterization of epitaxial Schottky diodes of a half-metallic ferromagnet on an oxide semiconductor. La0.67Sr0.33MnO3 thin films are grown by pulsed laser deposition on niobium-doped SrTiO3 semiconductor substrates with two doping concentrations and a TiO2 surface termination. The current across the diodes is dominated by thermionic emission and shows high rectification and low reverse bias leakage. At room temperature, the Schottky barrier height is 0.95 eV (0.65 eV) and the ideality factor is 1.08 (1.18) for the diodes with a low (high) doped semiconductor. With decreasing temperature the Schottky barrier height decreases and the ideality factor increases.

Intrinsic and Ionic Conduction in Humidity-Sensitive Sulfonated Polyaniline

The influence of humidity on the conductivity of sulfonated polyaniline (SPANI) and polyaniline (PANI) is investigated with electrochemical impedance spectroscopy (EIS). Separation of intrinsic (q) and ionic charge (i) mobility was observed using combination of ac and dc impedance measurements at relative humidity concentrations (RH) ranging from 5% to 90%. Nyquist plots for both acid-doped PANI and SPANI show single semicircles at all humidity levels, indicating pure intrinsic charge conduction. These semicircles are well-described with a fairly constant capacitance (C1 = 10-10 F) of the interdigitated electrodes and with a large varying resistance (10 kO - 10 MO) of the polymer bulk with RC times in the order of 10-3-10-6 sec. In all samples the intrinsic charge conductivity increases for increasing humidity, with emeraldine base PANI showing the strongest (2 orders of magnitude) increase upon raising the RH from 5 - 90%. A partial exchange of protons with sodium ions in SPANI (SPAN-Na) induces a second semicircle in the Nyquist plot at humidity levels above 40% with large RC (t) values (10-1 - 10-3 sec), which is attributed to a contribution of ionic mobility at the polymer/electrode interface. Blends of SPAN-Na with poly(vinyl alcohol) (PVA) and polyacrylamide (PAA) also show a second semicircle at slightly higher humidity levels from 50% RH onwards. In all cases this second semicircle is best described by a Constant Phase Element (CPE) with a value (order typical 10-9 - 10-7 F) increasing at higher humidity, and a varying exponential coefficient (0.5 <n <0.8), possibly due to a changing dispersion in the representative relaxation times for migration and accumulation of the ionic species. In addition, for SPAN-Na/SPANI samples blended with PVA a third semicircle is seen which can be described by an additional RC element representing a second phase or grain boundary contribution in the film