High frequency magnetic eigen excitations in a spin valve submitted to CPP DC current

We study the magnetization dynamics induced at low field by spin-transfer in a pillar-shaped spin valve. The spin valve is a square of 150 nm* 150 nm patterned from a film of IrMn 7 nm/CoFe, 2.4 nm/Ru, 0.8 nm/CoFe, 4.4 nm/Cu, 2.6 nm/CoFe, and 3.6 nm. The spin valve is studied in its anti-parallel state at 50 K. The high frequency voltage noise generated by the DC current flowing through the magneto-resistive device is used to identify the excitations induced by spin-transfer. Between an instability current of 1.72 mA and the switching current of 1.89 mA, we demonstrate the existence of pre-switch steady-state excitations, i.e. low amplitude precession. We study the frequency (10 GHz, red shift -1.46 GHz/mA) of this excitation, its line width (78-246 MHz), the power it carries (113 nW), and the current dependance thereof. We discuss those experimental findings using the formalism of Sun et al. and Valet et al., and show that the experimental behavior can be described by a macrospin approximation only at the very onset of the pre-switch excitations. The early saturation of the excitation power and the non-monotonic switching probability with the current are experimental indications that the pre-switch excitations are strongly non-uniform when approaching the switching current.