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

Sign of tunnel spin polarization of low-work-function Gd/Co nanolayers in a magnetic tunnel junction

Magnetic tunnel junctions having a low-work-function Gd/Co nanolayer at the interface with an Al2O3 tunnel barrier are shown to exhibit both positive and negative values of the tunnel magnetoresistance. The sign of the tunnel spin polarization of the Gd/Co nanolayer electrode depends on the thickness of the Gd and Co layers, temperature, and applied voltage. This reflects the nature of the interaction between the conduction electrons of the rare-earth and transition metals. \u

Tunnel Spin Polarization Versus Energy for Clean and Doped Al2O3 Barriers

The variation of the tunnel spin-polarization (TSP) with energy is determined using a magnetic tunnel transistor, allowing quantification of the energy dependent TSP separately for both ferromagnet/insulator interfaces and direct correlation with the tunnel magnetoresistance (TMR) measured in the same device. The intrinsic TSP is reduced below the Fermi level, and more strongly so for tunneling into empty states above the Fermi level. For artificially doped barriers, the low bias TMR decreases due to defect-assisted tunneling. Yet, this mechanism becomes ineffective at large bias, where instead inelastic spin scattering causes a strong TMR decay

Tunnel spin polarization of Ni80Fe20/SiO2 probed with a magnetic tunnel transistor

The tunnel spin polarization of Ni80Fe20/SiO2 interfaces has been investigated using a magnetic tunnel transistor (MTT). The MTT with a Ni80Fe20/SiO2 emitter shows a magnetocurrent of 74% at 100 K, corresponding to a tunnel spin polarization of the Ni80Fe20/SiO2 interface of 27%. This is only slightly lower than the value of 34% for Ni80Fe20/Al2O3 interfaces determined in similar MTT structures. This suggests that SiO2 can be applied in semiconductor spintronic devices, for example in ferromagnet/SiO2/Si tunnel contacts for spin injection.\ud \u

Opposite Spin Asymmetry of Elastic and Inelastic Scattering of Nonequilibrium Holes Injected into a Ferromagnet

The spin asymmetry of elastic and inelastic scattering of nonequilibrium holes injected into Co thin films is examined using a p-type magnetic tunnel transistor. Spin-dependent transmission yields a positive or negative magnetocurrent depending on Co thickness and hole energy. Up to a critical thickness of about 3 nm, (quasi)elastic scattering dominates with a short attenuation length (<1 nm) and preferential attenuation of holes in the majority spin bands, consistent with spin-wave emission. At a larger Co thickness, inelastic scattering dominates with a larger attenuation length (~4 nm) and opposite spin asymmetry

Size Dependence of the Magnetic and Electrical Properties of the Spin-Valve Transistor

The electrical and magnetic properties of the spin-valve transistor (SVT) are investigated as a function of transistor size. A new fabrication process, designed to study the size dependence of the SVT properties, uses: silicon-on-insulator (SOI) wafers, a combination of ion beam and wet etching and a negative tone photoresist (SU8) as an insulating layer. The Si/Pt emitter and Si/Au collector Schottky barrier height do not depend on the transistor dimensions. The parasitic leakage current of the Si/Au collector is, however, proportional to its area. The relative collector current change with magnetic field is 240%, independent of size, while the transfer ratio starts to decrease for SVTs with an emitter area below 25 × 25 ¿m2. The maximum input current is found to be limited by the maximum current density allowed in the base (1.7 × 107 A/cm2), which is in agreement with the maximum current density for spin valve