  Interfaces formed by correlated oxides offer a critical avenue for
discovering emergent phenomena and quantum states. However, the fabrication of
oxide interfaces with variable crystallographic orientations and strain states
integrated along a film plane is extremely challenge by conventional
layer-by-layer stacking or self-assembling. Here, we report the creation of
morphotropic grain boundaries (GBs) in laterally interconnected cobaltite
homostructures. Single-crystalline substrates and suspended ultrathin
freestanding membranes provide independent templates for coherent epitaxy and
constraint on the growth orientation, resulting in seamless and atomically
sharp GBs. Electronic states and magnetic behavior in hybrid structures are
laterally modulated and isolated by GBs, enabling artificially engineered
functionalities in the planar matrix. Our work offers a simple and scalable
method for fabricating unprecedented innovative interfaces through controlled
synthesis routes as well as provides a platform for exploring potential
applications in neuromorphics, solid state batteries, and catalysis.
