A New Step In Spintronics
University of Utah physicists have taken an important step toward a new generation of faster, cheaper computers and electronics by building the first "organic spin valves" – electrical switches that integrate two emerging fields of technology: organic semiconductor electronics and spin electronics, or spintronics. In a study published Feb. 26 in the journal Nature, the researchers report they used a semiconductor made of organic material – instead of a conventional semiconductor such as silicon – to make switch-like valves that can control the flow of electrical current. They were able to change the flow of electricity through the valves by 40 percent. "It's an early step toward a new generation of miniature electronic devices: computer chips, light-emitting devices for displays, and sensors to detect radiation, air pollutants, light and magnetic fields," says Z. Valy Vardeny, a professor of physics and coauthor of the study. Jing Shi, an associate professor and the study's principal author, adds: "We are making progress toward devices that are made with organic materials and utilize a different property of electrons [their spin rather than their electrical charge] for things like computer memory, computer processors and sensors of various sorts." More research and engineering are needed to produce such devices that are spintronic as well as electronic, but "we have done an important proof-of-concept experiment," says Shi. Shi and Vardeny conducted the study with two University of Utah postdoctoral researchers: Zuhong Xiong and Di Wu. A Primer on Spintronics and Spin Valves In electronic devices, information is stored and transmitted by the flow of electricity in the form of negatively charged subatomic particles called electrons. The zeroes and ones of computer binary code are represented by the presence or absence of electrons within a semiconductor or other material. In spintronics, information is stored and transmitted using another property of electrons: their spin. Spin is a difficult concept to explain. Technically, spin is the intrinsic angular momentum of a particle. But an easier way to describe spin is to imagine that each electron contains a tiny bar magnet, like a compass needle, that points either up or down to represent the electron's spin. Electrons moving through a nonmagnetic material normally have random spins (half are up and half are down) so the net effect is zero. But magnetic fields can be applied so that the spins are aligned (all up or all down), allowing a new way to store binary data in the form of ones (spins all up) and zeroes (spins all down). Shi says the field of spintronics was born in the late 1980s with the discovery of the "giant magnetoresistance effect." Resistance is a measure of how much a material resists the flow of electrical current or electrons. The giant magnetoresistance effect occurs when a magnetic field is used to align the spin of electrons in the material, inducing a large change in a material's resistance. The effect first was discovered in a device made of multiple layers of electrically conducting material: alternating magnetic and nonmagnetic layers. The device was known as a "spin valve" because when a magnetic field was applied to the device, the spin of its electrons went from all up to all down, changing its resistance so that the device acted like a valve to increase or decrease the flow of electrical current. Conventional spin valves have been widely used in computers since the mid 1990s. In older computers, electrical current was used by the "read head" to decipher data stored magnetically on the hard drive. Modern computer read heads are spin valves that are far more sensitive at reading data stored on a hard drive, allowing high-density, high-speed hard drives that store more data and can be read more quickly. Spintronics "has quickly revolutionized magnetic recording technology and is going to revolutionize random access memory (RAM) made of semiconductors," Shi says. Compared with purely electronic computers, computers with spintronic memory should be able to store more data, consume less power and process data more quickly. Conventional computer memory has transistors that use electric charges to store data as zeroes and ones. Spintronic memory will use up and down electron spins to represent such data. Spintronics also should make instant-on computers possible. Once the spins are aligned, they stay that way until changed by a magnetic field – even if a computer is shut off. As a result, data will be available the moment a computer is turned back on, with no need to boot up the computer to move data from the hard drive to the memory. Shi says major electronics companies now are developing spin-valve memory chips, which will show up first in cellular phones and digital cameras. The Study: Spintronics and Organic Semiconductors Get Married The next step in spintronics is to combine the advantages of spin-based devices with the qualities of semiconductors, such as their ability to be "doped" with substances that make them carry more or less electricity, or make them able to emit light, Shi says. But he says researchers have made little progress so far in integrating the magnetic materials of spintronics with conventional semiconductors such as silicon or gallium arsenide. A major problem is that conventional semiconductors must be fabricated at high temperatures, making it difficult to produce the ultra-thin layers necessary to make a spin valve. So Shi and fellow researchers set out to show that it is possible to create a spin valve made with an organic semiconductor rather than a conventional semiconductor. Compared with conventional semiconductors, organic semiconductors are inexpensive and simpler to make, can be manufactured at lower temperatures with fewer toxic wastes, have electronic properties that can be adjusted, and are flexible so they can be molded to desired shapes. Organic semiconductors already are used as light-emitting diodes for some flat-screen TVs, cell phone displays, some billboards and a few computer display screens. Shi, Vardeny, Xiong and Wu built three-layer organic spin valves using a middle layer made from an organic semiconductor named 8-hydroxyquinoline aluminum, or Alq3, which now is used in certain light-emitting diodes and is being developed for use in TV screens. The organic semiconductor was sandwiched between two metallic layers: one made of cobalt and the other a compound named lanthanum strontium manganese oxide. The two metals acted as electrodes, injecting electrons with the desired spin into the middle, organic semiconductor layer. The spin valve is on a chip that measures about one-third inch square. The physicists successfully injected electrons with aligned spins into the organic semiconductor and showed that the spins stayed aligned as the electrons moved through the semiconductor. By applying a weak magnetic field to the organic spin valve, the physicists caused a 40 percent change in the electrical current flowing through the valve. That qualifies as giant magnetoresistance. The researchers also showed the spin-up or spin-down alignment of electrons was maintained when power was shut off – a property essential for spintronic computer memory. More work is needed to develop organic spin valves that operate at higher temperatures, something that might be accomplished by removing impurities from the organic semiconductor. The spin valves in the study operated at temperatures ranging from minus 440 degrees Fahrenheit to minus 40 degrees Fahrenheit (minus 262 degrees Celsius to minus 40 degrees Celsius). Nevertheless, the experiment "sets a stage for more practical applications," Shi says. "Organic semiconductors can be used for spintronic devices such as spin valves, spin light-emitting diodes and spin transistors." Those devices can be used in computer memory chips and sensors to detect air pollution, magnetic fields, radiation or light, Vardeny says. For example, a conventional semiconductor transistor amplifies electric current and that's about it. But an organic semiconductor can be designed so that its electron spins go from aligned to nonaligned when it is exposed to light, air pollution or radiation, changing the flow of electric current to trigger an alarm, Vardeny says.