Structure, magnetism, electrical transport, and optical properties of the electron-doped quasi-2D manganates LaxCa3-xMn2O7

Abstract

We have successfully synthesized LaxCa3-xMn2O7 (x = 0, 0.3, 0.5, 0.7, 0.9) samples with Ruddlesden-Popper (RP) structure in Cmcm space group. Meanwhile, X-ray photoelectron spectroscopy (XPS) results indicate that the concentrations of Mn3+ and oxygen vacancy defects VO are enhanced with increasing doping. As a result, the magnetic and electrical transport properties can be tuned accordingly. With increasing doping, the antiferromagnetic (AFM) interaction was gradually weakened along with the enhancement of ferromagnetic (FM) interaction, and eventually the magnetic structure evolves from G-type to C-type. Our comprehensive experimental studies all propose that these manganates tend to intrinsically separate into AFM matrix state, spin glass regions, and FM cluster regions at lower temperature, which results from the competition between FM (Mn3+-O2--Mn4+) and AFM (Mn3+-O2--Mn3+ or/and Mn4+-O2--Mn4+) domains. Especially, this behavior can be modulated by La doping, i.e. the spin glass regions and FM cluster regions tend to be enhanced with increasing La doping. According to molecular field theory, it indicates that the stronger Dzyaloshinskii-Moriya (DM) interaction effect and canted antiferromagnetism (CAFM) only present in C-type structure. The electrical transport properties can be also modulated by La doping, attributed to the changes of the local states around the mobility edge. Those results are quite essential for understanding the evolution of the complicated magnetoelectric properties in the RP LaxCa3-xMn2O7 ceramics.