  Slow magnetic relaxation and two level fluctuations measurements under high
current injection is performed in single-contacted ferromagnetic
nanostructures. The magnetic configurations of the samples are described by two
metastable states of the uniform magnetization. The current-dependent effective
energy barrier due to spin-transfer from the current to the magnetic layer is
measured. The comparison between the results obtained with Ni nanowires of 6
$\mu $m length and 60 nm diameter, and Co (10 nm)/Cu (10 nm)/Co(30 nm)
nanometric pillars of about 40 nm in diameter refined the characterization of
this effect. It is shown that all observed features cannot be reduced to the
action of a current dependent effective field. Instead, all measurements can be
described in terms of an effective temperature, which depends on the current
amplitude and direction. The system is then analogous to an unstable open
system. The effect of current induced magnetization reversal is interpreted as
the balance of spin injection between both interfaces of the ferromagnetic
layer.
