  We study whether a direct measurement of the absolute temperature of a
Magnetic Tunnel Junction (MTJ) can be performed using the high frequency
electrical noise that it delivers under a finite voltage bias. Our method
includes quasi-static hysteresis loop measurements of the MTJ, together with
the field-dependence of its spin wave noise spectra. We rely on an analytical
modeling of the spectra by assuming independent fluctuations of the different
sub-systems of the tunnel junction that are described as macrospin fluctuators.
We illustrate our method on perpendicularly magnetized MgO-based MTJs patterned
in 50*100 nm2 nanopillars. We apply hard axis (in-plane) fields to let the
magnetic thermal fluctuations yield finite conductance fluctuations of the MTJ.
Instead of the free layer fluctuations that are observed to be affected by both
spin-torque and temperature, we use the magnetization fluctuations of the sole
reference layers. Their much stronger anisotropy and their much heavier damping
render them essentially immune to spin-torque. We illustrate our method by
determining current-induced heating of the perpendicularly magnetized tunnel
junction at voltages similar to those used in spin-torque memory applications.
The absolute temperature can be deduced with a precision of +/- 60 K and we can
exclude any substantial heating at the spin-torque switching voltage.
