PORTLAND, Ore. A research technique for making organic spin-valve electronics could herald the coming of nonvolatile semiconductors melded with optical emitters, transducers and sensors.
By using the spin of electrons, organic spin valves not only offer nonvolatile storage, but also enable emitters and sensors to share a chip with a processor, rather than requiring separate inorganic silicon or gallium arsenide chips for transducers.
The proof-of-concept spin valve could enable "organic semiconductors that can not only store and process information, but can also emit light and detect radiation, air pollutants and magnetic fields," said Jing Shi, associate professor of physics at the University of Utah. He performed the work with Utah physics professor Valy Vardeny and postdoctoral researchers Zuhong Xiong and Di Wu.
Conventional spin valves are made from superthin alternating layers of conducting metal and nonconducting insulators, rather than semiconductors, making them difficult to integrate onto conventional silicon or GaAs.
However, by integrating so-called spintronics-based devices into organic semiconductors, Shi and Vardeny foresee an era of organic chips that put memory, processing, emitters and sensors on the same device.
"As physicists, we are not interested in building chips for any particular application, but rather in advancing the understanding of spintronics in inorganic semiconductors," said Shi. "Now that we have proven the concept we hope that semiconductor engineers will begin using spintronics in their organic semiconductors."
The Utah team's spin valves use a three-layer structure in which an organic compound called Alq3 (technically, 8-hydroxyquinoline aluminum) substitutes for the middle insulating layer. Alq3 is already used in the fabrication of organic light-emitting diodes for flat-panel televisions.
The structure's two outside metallic layers are made from cobalt and lanthanum-strontium-manganese- oxide, acting as electrodes that inject electrons into the organic layer in response to the externally applied magnetic field.
The researchers report successfully aligning the spins of the electrons in the organic layer, resulting in giant-magnetoresistance current changes of up to 40 percent, which were verified to be nonvolatile when the power was shut off.
Flip and spin
Magnetic fields are generated when the spins of a substance's electrons their magnetic "direction" point the same way. In bar magnets, the spin of the metallic atoms' electrons are all fixed in the same direction by convention called up or down thereby enabling the positive and negative ends of the magnet. In electromagnets the unidirection spin effect is induced by an electric field that flips all the spins one way or the other.
In spin valves, the effect is reversed. Instead of an electric field inducing the magnetism, the magnetism induces the electrical field. For instance, a spin valve that alternates very thin layers of conductors/nonconductors can flip between a high-current and a low-current mode almost instantly in response to a weak magnetic field.
Such inorganic spin valves have already revolutionized read-head electronics for hard-disk drives, where they have been used since the mid-1990s. Spin valves are more sensitive to magnetic fields than conventional read heads because only a very slight change in magnetism on the hard disk induces a sharp change in current through the read head.
Spintronics was born in the late 1980s with the discovery of the giant-magnetoresistance effect. By applying an external magnetic field to a material with alternating layers of metal/insulator, the spin of all the electrons can be switched, instantly changing the resistance of the material from high to low.
Since the mid-1990s spin valves have become the standard method of creating high-density read heads. Spin valves sense smaller, denser magnetic domains, thereby enabling disks to carry more information and to be read and written more quickly.
Shi and Vardeny claim that the low-temperature processing and flexibility to make memory, processors, sensors and emitters on the same chip will eventually popularize organic-semiconductor spintronics for even mainstream chip makers.