Switching based Spin Transfer Torque Oscillator with zero-bias field and large tuning-ratio

Abstract

We propose a novel concept of obtaining oscillations with frequencies in very-high frequency (VHF) and ultra-high frequency (UHF) bands. A traditional spin torque nano-oscillator (STNO) consists of at least one pinned layer (PL) and one free layer (FL) which precesses in a fixed orbit, defined by a precession angle, which results in magneto-resistance (MR) oscillations. In STNO, with aligned or even orthogonal easy-axis of the magnetic layers and with or without external bias magnetic field, it is not possible to attain full MR swing. The constringed MR swing jeopardizes the extracted output power. Furthermore, the orbit is strongly disturbed by the thermal fluctuations resulting in strong magnetic noise. In stark contrast to the operation principle of a STNO, we theoretically demonstrate, with the practical parameters from the experiments, that with a unidirectional current in a dual asymmetric free-layers (with no pinned layer) based perpendicular magnetic tunnel junction (pMTJ), both of the free layers can attain a complete and out-of-phase self-sustained switching without the aid of any external magnetic field. This design facilitates a switching based spin torque oscillator (SW-STO) with a full MR swing, and hence a larger output power, for stable and more thermally robust free-running oscillations. Furthermore, the integration with the n-type metal-oxide-semiconductor (NMOS) field-effect transistors at 130, 65 and 14 nm node is appraised to expound its effect on the oscillator performance, controllability with DC bias and the design constraints, to demonstrate the viability of the design as a dynamically controllable oscillator for practical on-chip implementation.