Effect of Sr doping on the magnetic exchange interactions in manganites of type La1−xSrxMnyA1−yO3(A=Ga,Ti;0.1≤y≤1)

Abstract

Strontium doping transforms manganites of type $\mathrm{L}{\mathrm{a}}_{1\ensuremath{-}x}\mathrm{S}{\mathrm{r}}_{x}\mathrm{Mn}{\mathrm{O}}_{3}$ 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 $\mathrm{L}{\mathrm{a}}_{0.7}\mathrm{S}{\mathrm{r}}_{0.3}\mathrm{M}{\mathrm{n}}_{0.1}\mathrm{T}{\mathrm{i}}_{0.3}\mathrm{G}{\mathrm{a}}_{0.6}{\mathrm{O}}_{3}$. By the simultaneous doping with $\mathrm{S}{\mathrm{r}}^{2+}$ and $\mathrm{T}{\mathrm{i}}^{4+}$ 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 ${T}_{\mathrm{C}}=215\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, there is a first-order reconstructive phase transition from the trigonal $R\ensuremath{-}3c$ structure to the orthorhombic Pnma structure via an intermediate virtual configuration described by the common monoclinic subgroup $P{2}_{1}/c$. The magnetic excitations associated with $\mathrm{M}{\mathrm{n}}^{3+}$ dimers give evidence for two different nearest-neighbor ferromagnetic exchange interactions, in contrast to the undoped compound $\mathrm{LaM}{\mathrm{n}}_{y}{\mathrm{A}}_{1\ensuremath{-}y}{\mathrm{O}}_{3}$ 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.