Eur. Phys. J. B (2012) 85: 47
https://doi.org/10.1140/epjb/e2011-20505-3

Regular Article

Analysis of magnetization instability patterns in spin-transfer nano-oscillators

¹ Laboratorio di Meccatronica, Politecnico di Torino, 11029 Verrès, Aosta, Italy
² Dip. Tecnologie, Università “Parthenope”, 80143 Napoli, Italy
³ INRIM – Istituto Nazionale di Ricerca Metrologica, 10135 Torino, Italy
⁴ Dip. Ing. Elettrica, Università “Federico II”, 80125 Napoli, Italy
⁵ ECE Dept., UMIACS, AppEl Center, University of Maryland, College Park MD 20742, USA

^a e-mail: bonin.roberto@gmail.com

Received: 23 June 2011
Received in final form: 2 December 2011
Published online: 30 January 2012

Abstract

The stability of large precessional magnetization motions induced by spin-polarized currents in spin-transfer nano-oscillators is discussed. Quantitative analytical predictions are obtained for the critical values of spin-polarized injected current and external magnetic field at which the oscillator magnetization precession becomes unstable. It is shown that the mechanism leading to instability is parametric resonance of well-defined pairs of magnetostatically coupled perturbation modes. The amplitude of these modes grows to large non-thermal values when the oscillator frequency matches the mean of the natural frequencies of the two coupled modes. Analytical predictions are obtained for the space-time structure and symmetry of the magnetization patterns that are formed at the instability. Analytical results are compared with numerical simulations of spin-transfer-driven magnetization dynamics.