Observation of Spin Injection at a Ferromagnet-Semiconductor Interface
Physical Review Letters
Spin injection at a ferromagnet-semiconductor interface is observed by projecting the spinpolarized current in the ferromagnet onto the spin-split density of states of a high mobility twodimensional electron gas (2DEG). For a given polarization of carriers in the 2DEG, reversing the magnetization orientation of the ferromagnet modulates the interface resistance. Equivalently, reversing the polarization of the 2DEG carriers by reversing the bias polarity gives the same resistance modulation.
... nce modulation. Interface resistance changes of order 1% at room temperature indicate interfacial current polarizations of order 20%. PACS numbers: 75.25. + z, 73.40.Qv, The injection and detection of spin-polarized carriers at a ferromagnet-semiconductor interface is recognized as an important problem in condensed matter physics , with one focus of interest centered on the creation of a spininjected high electron mobility transistor (HEMT)  . Spin injection across a ferromagnet-nonmagnetic metal interface  provided a cornerstone for the field of spindependent transport in metals , and spin injection at a ferromagnet-superconductor interface has recently been demonstrated  . Despite considerable effort, however, the goal of generalizing spin injection to a ferromagnetsemiconductor system has remained elusive. Spin-dependent transport in a two-dimensional electron gas (2DEG) is uniquely interesting because a spin-orbit effect causes a spin splitting of the density of states. The Fermi wave vector k F p 2pn s of the 2DEG carriers is of order 10 6 cm 21 for typical carrier densities of order n s ϳ 10 12 cm 22 , and the Fermi velocity y F ϳ 10 7 cm͞s is weakly relativistic. An electric field E E zẑ perpendicular to the plane of the 2DEG transforms, in the frame of the carriers, as an effective magnetic field H ء with components in the x-y plane. This magnetic field can interact with the spin magnetic moment of the carriers, m B , and split the conduction band into spin subbands separated by an effective Zeeman energy D SOm B H ء . When the source of E z is a gradient of the confining potential, the appropriate spin-orbit Hamiltonian where s are the Pauli matrices, and a describes the strength of the spin-orbit coupling. Values of D SO ഠ 2ak 2 to 5 meV have been measured by analyzing the beat pattern of Shubnikov-De Haas oscillations [6, 7] . A variable field E z can be applied using a gate voltage  , and the corresponding changes in the value of a were experimentally measured in an In x Ga 12x As͞InAl 12y As HEMT  . Recently, the excitation of spin-polarized carriers into the conduction band of GaAs, and their subsequent detection, was demonstrated by a synchronous optical pump-probe technique. Pulses of circularly polarized light were used to generate a population of spin-polarized carriers, and polarization analysis of reflected probe pulses gave a measurement of the carrier spin lifetime  . Spin diffusion lengths of order several mm were also measured, and it was shown that an electric field could push the spin-polarized carriers over distances of order 100 mm, larger than typical source-drain separations. Similar photo excitation experiments used a polarization analysis of photoluminescence in a GaInAs quantum well to measure spin diffusion in a GaAs overlayer  . While these experiments address aspects of spindependent transport in semiconductor systems, the missing ingredient is the use of a ferromagnetic film to inject and/or detect spin-polarized carriers. In this Letter, we describe a direct observation of the injection and detection of a spin-polarized current from a ferromagnetic film into a 2DEG. A recent theory  predicted that spin-polarized current flow across a ferromagnet-2DEG interface could be detected by an appropriate measurement of the interface resistance R i of a simple diode structure [Figs.