Spin Torque Nano Oscillators (STNO)

Many wireless communication devices, e.g., mobile phones use an oscillator that receives a signal from the transmitting tower. At present, the oscillator frequency of these oscillators can be varied in a rather limited range. These oscillators are typically composed of inductors and capacitors. However, inductors are usually larger in size and hence are a bottleneck for miniaturization of wireless communications devices. A new type of oscillator technology that is significantly smaller in size and can provide additional advantages of large tuning range as well as low power consumption. This oscillator is based on a new quantum mechanical concept called, “spin transfer torque" (STT), which was first predicted by John Slonczewski and L. Berger in the year 1996 [1,2]. Spin transfer torque can be exerted on the magnetization in a magnetic material when sufficiently high current passes through the magnetic material. This torque arises due to the interaction of electrons with the magnets. The electrons transfer some of their angular momentum to the magnetization. As a result, the magnetization can rotate, when the current density is sufficiently high. Experimentally this can be achieved, if the size of the magnetic material is very small, of the order of hundreds of nanometers or less. Such as nano-scale device is known as spin torque nano-oscillator (STNOs), which can generate high-frequency voltage signals in the GHz range. STNO have remarkable potential for various communication applications, e.g., radio frequency (RF) signal generator, modulator and RF detectors. However, the major limitation of such devices is their large linewidth and low power output. Our group addresses some of the fundamental issues on how to minimize linewidth and consequently to explore the suitability of these devices for wireless communication application. We focuses on magnetic tunnel junction (MTJ) based nanopillar devices with diameters <300 nm. An MTJ is a multilayer stack comprising of two magnetic layers separated by a non-magnetic and insulating spacer e.g., magnesium oxide (MgO).

[1] J. C. Slonczewski, J. Magn. Magn. Mater. 159, L1 (1996).

[2] L. Berger, Phys. Rev. B 54, 9353 (1996).