  Tuning the lattice degree of freedom in nanoscale functional crystals is
critical to exploit the emerging functionalities such as piezoelectricity,
shape-memory effect, or piezomagnetism, which are attributed to the intrinsic
lattice-polar or lattice-spin coupling. Here it is reported that a mechanical
probe can be a dynamic tool to switch the ferroic orders at the nanoscale
multiferroic phase boundaries in BiFeO 3 with a phase mixture, where the
material can be reversibly transformed between the "soft" tetragonal-like and
the "hard" rhombohedrallike structures. The microscopic origin of the
nonvolatile mechanical switching of the multiferroic phase boundaries, coupled
with a reversible 180{\deg} rotation of the in-plane ferroelectric
polarization, is the nanoscale pressure-induced elastic deformation and
reconstruction of the spontaneous strain gradient across the multiferroic phase
boundaries. The reversible control of the room-temperature multiple ferroic
orders using a pure mechanical stimulus may bring us a new pathway to achieve
the potential energy conversion and sensing applications.
