Band filling effect on polaron localization in LaCaSr)MnO manganites
arXiv:1309.7308
Abstract
We report on an a SR and Mn NMR investigation of the magnetic order parameter as a function of temperature in the optimally doped La(CaSr)MnO and in the underdoped LaSrMnO and LaCaMnO metallic manganite families. The study is aimed at unraveling the effect of lattice distortions, implicitly controlled by the Ca-Sr isoelectronic substitution, from that of hole doping on the Curie temperature and the order of the magnetic transition. At optimal doping, the transitions are second order at all values, including the (LaCaMnO) end member. In contrast, they are first order in the underdoped samples, which show a finite (truncated) order parameter at the Curie point, including LaSrMnO whose is much higher than that of LaCaMnO. The order parameter curves, on the other hand, exhibit a very minor dependence on , if truncation is excepted. This suggests that the effective exchange interaction between Mn ions is essentially governed by local distortions, in agreement with the original double-exchange model, while truncation is primarily, if not entirely, an effect of under- or overdoping. A phase diagram, separating in the plane polaron-driven first order transitions from regular second order transitions governed by critical fluctuations, is proposed for the LaCaSr)MnO system.
submitted to Phys. Rev. B