Modulation of single and double spin torque oscillators

Abstract

We discuss modulation of metallic spin torque oscillators (STOs) based on single and double nano-contacts. The modulation behavior of both types of devices is characterized by equally spaced multiple sidebands, which can be ascribed mostly to a frequency modulation phenomenon. However, quantitative examination reveals that amplitude modulation is also present in both types of devices. We also show the feasibility of modulating the synchronized state of a double nano-contact STO. The synchronized state can be treated as a single oscillator, which is a very promising feature for future applications of STO arrays.A spin torque oscillator (STO) is a magneto-resistive nano-device composed of a fixed layer that serves as a polarizer (spin polarizing the conduction electrons) and a free layer whose magnetization is excited into steady state oscillations [1,2]. The oscillation of the magnetization is then detected as an AC voltage by virtue of the giant magnetoresistance (GMR) or the tunneling magnetoresistance effect. STOs are potential next generation oscillators for telecommunication and radar applications. Major advantages of STOs include extremely wide-tunability [3, 4] and small size compared to a quartz crystal resonators or LC tank-based voltage controlled oscillators.Modulation is the process of varying the characteristic parameters ( frequency and amplitude) of a periodic high-frequency carrier wave, with a low-frequency modulating signal. Recently it has been shown that the signal generated from STOs can be modulated [5,6]. This has opened new applications for STOs as analog modulators. However, studies of modulation in STOs are very much limited, and studies of double nano-contact STOs are mostly focused on synchronization phenomena [7,8,9,10,11,12]. There is no literature on theoretical studies of modulation of synchronized STOs. In this work, we report on experimental studies of the modulation of double nano-conatct STOs in synchronized conditions and compare the results with those of a single STO. We show that in both types of devices frequency modulation dominates the overall behavior. However, amplitude modulation also plays a role. We demonstrate that during modulation, the synchronized condition of double STOs can be treated as single oscillator.The sputter-deposited layer structure of our devices consists of: Si/SiO 2 /Cu(25 nm)/Co 81 Fe 19 (20 nm)/Cu(6 nm)/Ni 80 Fe 20 (4.5 nm)/Cu(3 nm)/Pd(2 nm).

Recent Advances in Nanocontact Spin-Torque Oscillators

“…Recently a more thorough analysis provided the first experimental evidence [12] of amplitude modulation. It also showed excellent quantitative agreement with a nonlinear frequency and amplitude modulation (NFAM) theory [11] as well as modulation up to 3.2 GHz [13,14,52]. It has also been shown that modulation has a general averaging effect on the non-linearity of the STO signal where modulation over regions of high frequency non-linearity results in a significant reduction (up to 90%) of the signal linewidth [16].…”

Spin-Torque and Spin-Hall Nano-Oscillators

“…In 2011, modulation of a pair of nanocontact STNOs was successfully demonstrated [159], [160], [157]. In Fig.6, we reproduce the results from [159] which show that the synchronized state of a pair of nanocontact STNOs (at 57 mA) can be maintained while modulating.…”

“…In 2011, modulation of a pair of nanocontact STNOs was successfully demonstrated [159], [160], [157]. In Fig.…”