  Perpendicular magnetic tunnel junctions (MTJ) with a bottom pinned reference
layer and a composite free layer (FL) are investigated. Different thicknesses
of the FL were tested to obtain an optimal balance between tunneling
magnetoresistance (TMR) ratio and perpendicular magnetic anisotropy. After
annealing at 400 $^\circ$C, the TMR ratio for 1.5 nm thick CoFeB sublayer
reached 180 % at room temperature and 280 % at 20 K with an MgO tunnel barrier
thickness corresponding to the resistance area product RA = 10
Ohm$\mathrm{\mu}$m$^2$. The voltage vs. magnetic field stability diagrams
measured in pillar-shaped MTJs with 130 nm diameter indicate the competition
between spin transfer torque (STT), voltage controlled magnetic anisotropy
(VCMA) and temperature effects in the switching process. An extended stability
phase diagram model that takes into account all three parameters and the
effective damping measured independently using broadband ferromagnetic
resonance technique enabled the determination of both STT and VCMA coefficients
that are responsible for the FL magnetization switching.
