While modern electronics relies on using the electron's charge to do various operations (transistors, capacitors, etc.), ‘spintronics’ describes devices that rely on the electron's magnetic moment, or spin, for functionality. The energy cost associated with changing the electron's spin state is much lower than the cost of manipulating its charge. In addition, it is predicted to be much faster to manipulate the spin (think of flipping a tiny bar magnet from ‘north’ to ‘south’). Given these characteristics, scientists have for some time focused on the potential of spintronics to deliver fast, low-power computing. However, progress in the area of computer logic and information processing has been delayed by the lack of appropriate materials and the difficulty in developing a spin-dependent analog of the gain---or signal amplification---found in traditional charge-based computing.
Figure 1. Diagram showing the layer structure of the hybrid spin-LED. Al, Au: Aluminum and gold. PR: Photoresist (an insulating polymer used in standard device fabrication). n-AlGaAs, p-AlGaAs: Electron-doped and hole-doped aluminum gallium arsenide, respectively. QW: Quantum well (in our case, QW is a thin slice of GaAs in between the AlGaAs layers). γ: Photon.
In parallel, the field of organic-based ferromagnets has seen remarkable progress in the development of plastics with magnetic ordering (that is, with electrons in a material all lined up with their spins pointing in one direction) that extends well above room temperature. The reason such a property is important in spintronics has to do with the ability to generate a ‘spin current’ that plays the same role as the charge current for conventional electronics. If we pass an electronic current through a magnetically ordered material, then the electron current that comes out the other end has a preferred spin orientation, a spin current. By, for example, linking the small molecule tetracyanoethylene (TCNE) with various transition metals (iron, cobalt, vanadium), researchers have demonstrated the ability to synthesize plastic films that have room temperature magnetic ordering and semiconducting electronic properties.1 These characteristics are an excellent match to the materials requirements for spintronic logic, and it is that opportunity that we are exploiting in the development of hybrid organic/inorganic spintronic devices (see Figure 1).2
The specific organic-based magnet that we selected is vanadium tetracyanoethylene (V[TCNE]∼2). This material is an organic semiconductor with a conductivity of 10−2S/cm2 and a magnetic ordering temperature greater than 400K (the highest temperature reported in the metal-TCNE family of compounds).1 One of the biggest challenges in fabricating these devices is the constraint that once the V[TCNE]∼2 is deposited, no further solvent exposure or elevated temperature (above ∼100°C) can be tolerated. The result is a bottom-up approach wherein we first pattern the inorganic layers using traditional photolithography, etching and metallization, and only deposit the organic material at the second to last step (see schematic in Figure 1; the top metal contact is deposited using a shadow mask).
Once completed, the hybrid devices are transferred in an air-free sample mount (see inset to Figure 1) into our magneto-optical cryostat. We independently verify the quality of our p-n diode, the spectrum of the light-emitting diode (LED), and the magnetic properties of the V[TCNE]∼2 layer. The electroluminescence from the device reveals two peaks from the gallium arsenide quantum well embedded in the intrinsic region of the LED: see panel (a) of Figure 2
(a) Electroluminescence (EL) of spin-LED. (b) Circular polarization of spin-LED (red triangles) and magnetization of V[TCNE]∼2
(green line). HH, LH: heavy-hole and light-hole, respectively. i-GaAs: Undoped aluminum gallium arsenide. Adapted from Fang et al. 2
Under typical operating conditions, an electrical bias is applied between the top contact and the p-layer of the LED (see Figure 1). This drives a spin-polarized electron current from the V[TCNE]∼2 layer into the LED, where the electrons recombine with either the heavy- or light-holes injected from the p-contact. The optical polarization of the light emitted from the LED is defined as P=(IRCP−ILCP)/(IRCP+ILCP), where ILCP and IRCP refer to the intensity of left-circular polarized and right-circular polarized light. This quantity directly measures the number of spin up versus spin down electrons created by the V[TCNE]∼2. Panel (b) of Figure 2 shows this polarization as a function of applied magnetic field for light emitted by the heavy-holes (red triangles) on the same graph as the magnetization of the V[TCNE]∼2 layer (green line). The heavy-hole polarization tracks exactly with the magnetization, demonstrating successful spin injection across the hybrid organic/inorganic interface.
While the polarization signal in these first-generation devices is relatively weak, it is easily detected by our sensitive spin-detector. More importantly, this proof of principle demonstration opens the door to a new era of hybrid organic/inorganic spintronics. The hybrid interface in these early studies has not yet been optimized, and we expect improvement over the next several years as we apply various strategies (based on both chemistry and semiconductor engineering) to improve the spin transmission efficiency. These aspects represent the focus of our future work. We anticipate that the flexibility offered by these hybrid structures will contribute to the development of spintronic technologies ranging from next-generation electronics to potential applications such as flexible electronics and chemical sensing.
Ezekiel Johnston-Halperin, Lei Fang
Ohio State University
Johnston-Halperin received his BS in physics from Case Western Reserve University in 1996, followed in 2003 by his PhD at the University of California at Santa Barbara working with David D. Awschalom. He is currently an assistant professor of physics.
Fang received her BS in physics in 2004 from the University of Science and Technology in China. In March 2011, she obtained her PhD in physics at the Ohio State University, where she worked in various projects related to novel spintronic materials.
1. J. M. Manriquez, T. Y. Gordon, R. S. Mclean, A. J. Epstein, J. S. Miller, A room-temperature molecular/organic-based magnet, Science 252
, no. 5011, pp. 1415-1417, 1991. doi:10.1126/science.252.5011.1415
2. L. Fang, K. D. Bozdag, C.-Y. Chen, P. A. Truitt, A. J. Epstein, E. Johnston-Halperin, Electrical spin injection from an organic-based ferrimagnet in a hybrid organic-inorganic heterostructure, Phys. Rev. Lett.
106, no. 15, pp. 156602-1, 2011. doi:10.1103/PhysRevLett.106.156602