Static and dynamic signatures of anisotropic electronic phase separation in La2/3Ca1/3MnO3 thin films under anisotropic strain

Abstract

The electronic phase separation (EPS) of optimally doped ${\mathrm{La}}_{2/3}{\mathrm{Ca}}_{1/3}\mathrm{Mn}{\mathrm{O}}_{3}$ (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) $\mathrm{NdGa}{\mathrm{O}}_{3}$ substrates by pulsed laser deposition, and then postgrowth annealed at $780{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ in ${\mathrm{O}}_{2}$ 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, $\ensuremath{\rho}(T)$, was measured simultaneously along the two orthogonal directions. With increasing annealing time, both zero-field-cooled and field-cooled $\ensuremath{\rho}(T)$ show significant increases, suggesting strain-triggered EPS and appearance of antiferromagnetic insulating (AFI) phases in a ferromagnetic metallic (FMM) ground state. Meanwhile, $\ensuremath{\rho}(T)$ 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 $\mathrm{Mn}{\mathrm{O}}_{6}$ 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 ratio as high as ${10}^{4}$. Furthermore, the resistivity was found to exhibit glasslike 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.