We study the current-controlled modulation of a nanocontact spin-torque oscillator. Three principally different cases of frequency nonlinearity (d²f/dIdc² being zero, positive, and negative) are investigated. Standard nonlinear frequency-modulation theory is able to accurately describe the frequency shifts during modulation. However, the power of the modulated sidebands only agrees with calculations based on a recent theory of combined nonlinear frequency and amplitude modulation.
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Muduli et al. (2010) studied this question.