  We demonstrate that magnetic phase separation and competing spin order in the
colossal magnetoresistive (CMR) manganites can be directly explored via tuning
strain in bulk samples of nanocrystalline La$_{1-x}$Ca$_x$MnO$_3$. Our results
show that strain can be reversibly frozen into the lattice in order to
stabilize coexisting antiferromagnetic domains within the nominally
ferromagnetic metallic state of La$_{5/8}$Ca$_{3/8}$MnO$_3$. The measurement of
tunable phase separation via magnetic neutron powder diffraction presents a
direct route of exploring the correlated spin properties of phase separated
charge/magnetic order in highly strained CMR materials and opens a potential
avenue for realizing intergrain spin tunnel junction networks with enhanced CMR
behavior in a chemically homogeneous material.
