Effect of Sr doping on the magnetic exchange interactions in manganites

TL;DR

This study investigates how Sr doping affects magnetic exchange interactions in manganites, revealing changes in magnetic coupling and structural phases, especially in insulating states without charge carriers, using neutron scattering techniques.

Contribution

It provides new insights into the magnetic interactions in Sr-doped manganites, especially in insulating states, by analyzing neutron scattering data and structural changes.

Findings

Two different ferromagnetic exchange interactions identified

Large fourth-nearest-neighbor interaction absent in insulating state

RKKY interaction explains exchange couplings in metallic manganites

Abstract

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.