Conventional electronics depends on the electrical charges on electrons. The emerging field of spintronics aims to produce devices that also make use of the quantum spin states, or internal angular momentum, of electrons. Researchers at the University of Tokyo, the RIKEN Frontier Research System in Wako and New York University have developed a spintronic technique for accumulating ordered spin states that could have valuable applications in information storage1.
Spintronics requires ‘spin-polarized’ non-magnetic materials in which the electron spins all point in the same direction. This has previously been achieved by injecting a spin-polarized current into the material from a magnetic needle. However the direction of polarization can only be changed by rotating the needle, which is not practical at nanometer scales.
The research team has solved this problem by using two injection needles, allowing them to control the polarization direction simply by varying electrical currents. Their experimental device, called a lateral spin valve, consists of a copper strip in contact with two 80-nanometer-wide injection needles at right angles to one another, made of Permalloy—a magnetic alloy of nickel and iron (Fig. 1).
Electrons can only have spins in ‘up’ or ‘down’ states. In magnetic materials, spins can align so that when the material is observed from the ‘polarization direction’, we observe only one type of spin. “Permalloy is a strong ferromagnet that provides spin polarized electrical currents because the population[s] of up and down spins are spontaneously different,” explains RIKEN scientist YoshiChika Otani.
The researchers found that by changing the electrical currents in the Permalloy needles, they had complete control over the angle of spin polarization in the copper strip. Furthermore, the number of polarized spins accumulated in the copper strip obeyed a simple cosine relationship with the angle of polarization.
“Normally spin polarization is along the magnetization direction,” says Otani. “By combining spin polarized currents with two different quantized axes, we can rotate the resultant quantized axis.”
The technique could be used to reverse the magnetization of certain materials. “We believe that our electrical means of controlling the spin polarization is important for realizing efficient spin-injection-induced magnetization reversal,” explains Otani.
The magnetization reversal process could lead to new types of random access memory for computers. In future the researchers also hope to apply their technique to induce a quantum spin version of the Hall Effect which is exploited in several types of sensors.
Reference
1. Kimura, T., Otani, Y.-C. & Levy, P.M. Electrical control of the direction of spin accumulation. Physical Review Letters 99, 166601 (2007). | article |
