Magnetotransport Studies of Organic Spin Valves A. M. Goldman, University of Minnesota, DMR Graph of the resistance versus magnetic field at 100K. Gate and drain voltages were -80V and -4V, respectively. The horizontal arrows indicate the direction of the applied magnetic field. The vertical arrows indicate the relative magnetizations of the two ferromagnetic contacts. Spin transport through organic semiconductors (OSCs) is of interest because OSCs are expected to have longer spin diffusion lengths than inorganic semiconductors. Organic field effect transistors with ferromagnetic source/drain contacts are being used to observe spin tran- sport. The gate voltage induces carriers in the OSC, and by changing the relative magne- tizations of the two ferromagnetic contacts the current flow can be modulated. We have ob- served big changes of resistance when negative gate voltages are applied to induce holes and the magnetic field is swept to change the relative orientations of the magnetizations of the source and drain between parallel and anti-parallel orientations. This technique is being used to study the mechanism of spin transport through organic semiconductors. The measurements were carried out using the Physical Properties Measurement System purchased with this grant. Cartoon of organic field effect transistor. The source/drain contacts, the gate contact and the dielectric are Co(400Å)/Au(20Å), doped Si(500μm)/Al(100Å)/Au(750Å) and SiO 2 (2850Å), respectively. The thickness of the tetracene single crystal is several thousand angstroms. The channel width and length are 1μm and 1000Å, respectively. The source/drain contacts have different geometries so as to have different coercive fields. VDVD VGVG Source Drain Channel (tetracene single crystal) DielectricGate ( R - R 0 )/ R 0 (%) Magnetic Field (gauss)

Education: This work is being carried out by Mr. Masaya Nishioka, a graduate student in the Physics Graduate Program at the University of Minnesota. The materials and devices are prepared with the support of the University of Minnesota MRSEC and the measurements are carried out using the Physical Properties Measurement System purchased under this program. Undergraduates also used this instrument. An REU student, Emily Beauvois, from Augsberg College retrofitted a UV source to the apparatus this past summer and was able to modulate the conductivity of an OSC. This will be incorporated in future magnetotransport measurements. Societal Impact: The device being studied is, in principle, a new transistor which could have a big impact on technology if it could be operated at room temperature. There are three new properties: first, the current can be modulated in two ways, by changing either the gate voltage or the relative magnetizations of the contacts. Second, the presence of two magnetic contacts gives the device memory. Third, the device could be on a flexible substrate. Because traditional transistors do not have these properties, variants of this device could change engineering practice. Magnetotransport Studies of Organic Spin Valves A. M. Goldman, University of Minnesota, DMR