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In this paper, we formulate a generalized theoretical model to describe the nonlinear dynamics observed in combined frequency-amplitude modulators whose characteristic parameters exhibit a nonlinear dependence on the input modulating signal. The derived analytical solution may give a satisfactory explanation of recent laboratory observations on magnetic spin-transfer oscillators and fully agrees with results of micromagnetic calculations. Since the theory has been developed independently of the mechanism causing the nonlinearities, it may encompass the description of modulation processes of any physical nature, a promising feature for potential applications in the field of communication systems. [ABSTRACT FROM AUTHOR]

A previous universal model of "combined" frequency-amplitude nonlinear modulation has been used to describe the spin-wave dynamics observed in spin-transfer oscillators. In that case, it was shown that it is not possible to obtain a pure frequency modulation process because the strict nonlinear relationship existing between the characteristic parameters of the oscillator yields a non-negligible amplitude modulation process to take place simultaneously. Here, we demonstrate that there exists a particular configuration in which a spin-torque oscillator behaves as a "pure" nonlinear amplitude modulator. For the case of a nano-contact geometry, for example, such a condition implies the usage of an external out-of-plane bias field directed along the critical angle at which the frequency tunability coefficient T = ∂ƒ/∂I vanishes. In all those cases in which this coefficient is not zero, the previous combined process takes place. The phenomenon is first analyzed by developing an analytical model which describes the structure of the frequency spectrum of a nonlinearly amplitude modulated signal. Results of this approach are finally compared with those of micromagnetic investigations, carried out in time and frequency domains, deriving an excellent agreement. [ABSTRACT FROM AUTHOR]

The simultaneous action of dc and ac spin-polarized current traversing a magnetic nanocontact device results in a combined amplitude-frequency nonlinear modulation process, a promising application of spin-transfer oscillators. A micromagnetic analysis of the nonlinear magnetization dynamics occurring in such devices is carried out in both spatial and frequency domain. Numerical results show that the proposed combined modulation scheme affects the amplitude of the sidebands as well as their phase, and remark a substantial difference with respect to the properties exhibited by a classical linear modulator. Further investigations performed as function of the frequency of the modulating signal allow to establish that, when the modulating frequency approaches the carrier one, no modulation occurs and a frequency pulling, typical of injection locking phenomena, takes place instead. [ABSTRACT FROM AUTHOR]