  Power consumption is the main limitation in the development of new high
performance random access memory for portable electronic devices. Magnetic RAM
(MRAM) with CoFeB/MgO based magnetic tunnel junctions (MTJs) is a promising
candidate for reducing the power consumption given its non-volatile nature
while achieveing high performance. The dynamic properties and switching
mechanisms of MTJs are critical to understanding device operation and to enable
scaling of devices below 30 nm in diameter. Here we show that the magnetic
reversal mechanism is incoherent and that the switching is thermally nucleated
at device operating temperatures. Moreover, we find an intrinsic thermal
switching field distribution arising on the sub-nanosecond timescale even in
the absence of size and anisotropy distributions or material defects. These
features represent the characteristic signature of the dynamic properties in
MTJs and give an intrinsic limit to reversal reliability in small magnetic
nanodevices.
