  The Pr$_{0.50}$Sr$_{0.5}$0CoO$_3$ perovskite exhibits unique
magnetostructural properties among the rest of ferromagnetic/metallic
Ln$_{0.50}$Sr$_{0.50}$CoO$_3$ compounds. The sudden orthorhombic-tetragonal
(Imma $\to$ I4/mcm) structural transition produces an unusual magnetic behavior
versus temperature and external magnetic fields. In particular, the symmetry
change is responsible for a spontaneous spin rotation in this metallic oxide.
We have studied half-doped Ln$_{0.50}$(Sr$_{1-x}$A$_x$)$_{0.50}$CoO$_3$
cobaltites varying the ionic radius rA of A-site cations (divalent cations and
lanthanides) in order to complete the T-rA phase diagram. The influence of the
structural distortion and the A-cations size for the occurrence of a
spontaneous spin reorientation in the metallic state has been investigated. As
the reorientation of the magnetization is driven by the temperature induced
collapse of the orthorhombic distortion, a careful investigation of the
structural symmetry is presented varying the structural distortion of the
Sr-rich half-doped cobaltites by means of both compositional and temperature
changes. The region in the phase diagram of these perovskites where the phase
of magnetic symmetry Fm'm'm replaces that of Im'm'a symmetry was determined in
this family of ferromagnetic/metallic cobaltites. In that region the
magnetization direction has rotated 45 degrees within the a-b plane with
respect to the second.
