  As a room-temperature multiferroic, BiFeO3 has been intensively investigated
for both magnetoelectric devices and non-volatile ferroelectric memory
applications. Both magnetoelectric and ferroelectric memory devices have the
same control knob: polarization switching by an applied electric field. Due to
the rhombohedral symmetry of BiFeO3, there are four ferroelastic variances and
three different polarization switching events: (1) 71{\deg} switching from r1-
to r3+, (2) 109{\deg} switching from r1- to r2+ (or r4+), and (3) 180o
switching from r1- to r1+ (the superscript + and - stand for up and down
polarization, respectively). Each switching path is coupled to a different
reorientation of the BiFeO3 unit cell, and hence different coupling to the
magnetic order as well as different magnitudes of switchable polarization. A
degradation of the ferroelectric properties of BiFeO3 will result in losing
controllability of magnetic order switching in magnetoelectric devices and
capacity for information storage in ferroelectric memory devices. Especially,
polarization fatigue will directly restrict the reliability of the actual
devices. Hence it is important to understand the intrinsic fatigue behavior of
each polarization switching path in BiFeO3 thin films. In this communication,
we report polarization fatigue in BiFeO3 depending on switching path, and
propose a fatigue model which will broaden our understanding of the fatigue
phenomenon in low-symmetry materials.
