Spin-Valves Incorporating Magnetic and Nonmagnetic Organic Semiconductors

A clear spin-valve effect is reported for stacked thin film devices constructed of two ferromagnets with differing coercivities, iron (Fe) (100nm) and iron-cobalt (Fe$_{50}$:Co$_{50})$ (30nm), which are magnetically decoupled by a layer of $\alpha $-sexithiophene ($\alpha $6T) (120nm). Coherent spin transport is expected to be facilitated by relatively low spin orbit coupling in $\pi $-conjugated materials$^{1-3}$. Spin-injection is aided by tunnel barriers at metal/semiconductor interfaces. A spin-valve effect of up to 20{\%}, with switching at the expected coercive fields, is observed at 4.5K and the effect persists up to 150K. The conduction electrons in vanadium tetracyanoethylene (V[TCNE]$_{x})$, an organic-based magnetic semiconductor with T$_{C}>$ 350K, are fully spin-polarized$^{4}$. In addition to the low field `conventional' spin-valve switching from 10 to 100K, an unusual background high field magnetoresistance is reported for the spin-valve device structure where $\alpha $6T (50nm) is the nonmagnetic spacing layer between V[TCNE]$_{x}$ ($<$1um), and cobalt (Co) (25nm). Supported by DOE Grant No. DE-FG02-01ER45931 and DARPA (ONR Grant No. N00014-02-1-0593). 1. Dediu, et al., Solid State Comm. \textbf{122} 181 (2002) 2. Epstein, MRS Bull. \textbf{28} 492 (2003) 3. Xiong, et al., Nature \textbf{427} 821 (2004) 4. Prigodin, et al., Adv. Mater. \textbf{14} 1230 (2002), Raju, et al., J. Appl. Phys. \textbf{93} 6799 (2003)