Intrinsic Phase Shift and Novel Dynamic Magnetization States of a Spin Torque Oscillator under ac Current Injection
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0998-J02-02
Intrinsic Phase Shift and Novel Dynamic Magnetization States of a Spin Torque Oscillator under ac Current Injection Yan Zhou, Johan Persson, and Johan Akerman Microelectronics and Applied Physics, Information and Communication Technology, Electrum 229, Stockholm, 16440, Sweden
ABSTRACT We report on a preferred phase shift ∆Φ0 between a spin torque oscillator (STO) and an ac current (Iac) injected at the intrinsic frequency (fSTO) of the STO. In the in-plane precession mode (IP) the STO adjusts to a state where its resistance (or voltage) lags Iac about a quarter of a wave length (∆Φ0=87°-94°). In the IP mode ∆Φ0 increases somewhat with the dc current. As the precession changes into the Out-Of-Plane (OOP) mode, ∆Φ0 jumps by about 180°, i.e. the STO resistance now precedes Iac by about a quarter of a wave length (|∆Φ0|=86°). At the IP/OOP boundary, the ac current mixes the two oscillation modes and both periodic and chaotic oscillations are observed. As a consequence of mixing, subharmonic terms appear in the STO signal. ∆Φ0 can furthermore be tuned by changing one or more of the anisotropy field, the demagnetizing field or the applied field. At the IP/OOP boundary, Iac mixes the two oscillation modes. The intrinsic ∆Φ0 will impact any circuit design based on STO technology and will e.g. have direct consequences for phase locking in networks of serially connected STOs.
INTRODUCTION In recent years, tremendous interest in current-driven steady-state precessional motion of magnetization in nanometer-sized magnetic devices has been caused by theoretical predictions and experimental observation [1-15]. The spin transfer-induced magnetization precession has many intriguing properties, namely, the precessional frequency is in the range from 5 to 40 GHz and is tunable via both current and field, has a very narrow room temperature line width (1.5-50 MHz) leading to high Q up to 18000, to name a few [11-13]. These intriguing properties suggest that spin transfer oscillators (STO) based on the above mechanism are very promising candidates in microwave signal processing applications and telecommunications [11,15]. The main drawback of the spin transfer oscillator is its low output power of at present typically less than 1 nW [13-15]. The successful synchronization could overcome this difficulty and thus greatly increase the potential for realistic STO applications. Recently, it has been experimentally demonstrated that STOs can phase-lock to an injected ac current [16]. The phase-locking of the oscillator to the external source causes frequency pulling/pushing and provides very significant means for frequency mixing, phase control, as well as coherent enhancement of the output power in
serially connected STOs [13-15]. However, the detailed theoretical study of the phase-locking is still in great need in order to utilize the electrical control of these nanosized microwave devices to the fullest. In this paper, we study the phase shift between the oscillation of the STO and the applied ac current. We find a prefer
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