  The electronic phase separation (EPS) of optimally doped La2/3Ca1/3MnO3
(LCMO) thin films under various degrees of anisotropic strain is investigated
by static magnetotransport and dynamic relaxation measurements. Three LCMO
films were grown simultaneously on (001) NdGaO3 (NGO) substrates by pulsed
laser deposition, and then post-growth annealed at 780 oC in O2 for different
durations of time. With increasing annealing time, the films developed
significant strains of opposite signs along the two orthogonal in-plane
directions. The static temperature-dependent resistivity, R(T), was measured
simultaneously along the two orthogonal directions. With increasing annealing
time, both zero-field-cooled and field-cooled R(T) show significant increases,
suggesting strain-triggered EPS and appearance of antiferromagnetic insulating
(AFI) phases in a ferromagnetic metallic (FMM) ground state. Meanwhile, R(T)
along the tensile-strained [010] direction becomes progressively larger than
that along the compressive-strained [100]. The enhanced resistivity anisotropy
indicates that the EPS is characterized by phase-separated FMM entities with a
preferred orientation along [100], possibly due to the cooperative deformation
and rotation/tilting of the MnO6 octahedra under the enhanced anisotropic
strain. The anisotropic EPS can also be tuned by an external magnetic field.
During a field-cycle at several fixed temperatures, the AFI phases are melted
at high fields and recovered at low fields, resulting in sharp resistance
changes of the ratio as high as 104. Furthermore, the resistivity was found to
exhibit glass-like behavior, relaxing logarithmically in the phase-separated
states. Fitting the data to a phenomenological model, the resulting resistive
viscosity and characteristic relaxation time are found to evolve with
temperature, showing a close correlation with the static measurements in the
EPS states.
