Electrical detection of spin accumulation in ferromagnet-semiconductor devices

insemiconductors isfarless advanced. We focus hereontheimportant caseofan interface between aferromagnetic metal (iron) andGaAs.Although isolated successes havebeenreported previously2, ithasbeenextremely difficult toisolate small signals fromthebackground contributions that exist duetothepresence offerromagnetic electrodes. We address this particular problem byusing an approach inwhichwedetect asignal that issensitive onlytothedephasing ofspins byprecession ina transverse magnetic field. We showthat theaccumulation ofspin-polarized electrons ataforward-biased Schottky tunnel barrier between Feandn-GaAscanbedetected electrically. Thespinaccumulation, whichhasbeenobserved previously usingoptical techniques,3'4 leads toanadditional voltage dropacross thebarrier thatis suppressed byasmall transverse magnetic field. Thiseffect, whichisduetothedepolarization ofspins in thetransverse field, isequivalent totheHanleeffect observed inoptical pumping experiments. The experiment5 iscarried outonlateral devices withtwoepitaxial Fe/GaAsSchottky tunnel barriers at opposite endsofann-doped (n= 3.6x1016 cm-3) GaAschannel. Additional contacts along thechannel are usedtomeasure thevoltages across thesource anddrain independently. Thelateral geometry allows usto carry outtransport andKerrmicroscopy measurements onthesamedevice. Asverified bytheKerr measurements,4 spins caneither beinjected atthesource contact oraccumulated atthedrain duetothe spin-dependent reflectivity oftheFe/GaAs interface. A magnetic field isapplied perpendicular tothe plane, sothatthemagnetizations oftheelectrodes remain fixed throughout theelectrical transport measurements. Thefield-dependence ofthevoltage across thereverse-biased source contact isnearly featureless overthefield range oftheexperiment. Incontrast, apronounced peakatzerofield isobserved inthevoltage measured across theforward-biased drain contact. Thisasymmetry withrespect tothe current direction isobserved inevery device overawidechannel doping range andisconsistent witha density ofstates argument basedontunnelling. We modelthefield dependence ofthevoltage by considering spinprecession, drift, diffusion, andrelaxation. Theapproach issimilar tothat used previously formetals, except that inthis casespins aregenerated anddetected atasingle ferromagnetic contact. Withtheexception oftheamplitude, all parameters required tofit thevoltage peakatzerofield areobtained fromindependent measurements. Athightemperatures (T> 30K),thewidth ofthepeakis determined predominantly bythespin lifetime. Inthis temperature range, themagnetic field dependence of theKerrrotation canbefitwiththesameparameters usedformodelling thevoltage peak, providing compelling support fortheinterpretation ofthetransport data. Thedemonstration ofaHanle effect intransport isasignificant step. Inthecasereported here, wehave usedonlyasingle electrode, whichfunctions asbotha"source" anda"detector." Ourcurrent workis focusing onthenon-local geometry inwhichthespin polarization isdetected withanadjacent electrode. Thisisthenextstep inmoving towards atruesource-drain FET-like device inwhichthetransconductance depends ontherelative magnetizations ofthetwocontacts. Thisworkwassupported byONR,theDARPA SpinS program, theNSFMRSECprogram under DMR 02-12032, andtheLANLLDRD program.