  Strontium doping transforms manganites of type La(1-x)Sr(x)MnO(3) from an
insulating antiferromagnet (x=0) to a metallic ferromagnet (x>0.16) due to the
induced charge carriers (holes). Neutron scattering experiments were employed
to investigate the effect of Sr doping on a tailor-made compound of composition
La(0.7)Sr(0.3)Mn(0.1)Ti(0.3)Ga(0.6)O(3). By the simultaneous doping with Sr2+
and Ti4+ ions the compound remains in the insulating state, so that the
magnetic interactions for large Sr doping can be studied in the absence of
charge carriers. At TC=215 K there is a first-order reconstructive phase
transition from the trigonal R-3c structure to the orthorhombic Pnma structure
via an intermediate virtual configuration described by the common monoclinic
subgroup P21/c. The magnetic excitations associated with Mn3+ dimers give
evidence for two different nearest-neighbor ferromagnetic exchange
interactions, in contrast to the undoped compound LaMnyA(1-y)O(3) where both
ferromagnetic and antiferromagnetic interactions are present. The doping
induced changes of the exchange coupling originates from different Mn-O-Mn bond
angles determined by neutron diffraction. The large fourth-nearest-neighbor
interaction found for metallic manganites is absent in the insulating state. We
argue that the Ruderman-Kittel-Kasuya-Yosida interaction reasonably accounts
for all the exchange couplings derived from the spin-wave dispersion in
metallic manganites.
