Anisotropic spin correlations in bilayered La1.1Sr1.9Mn2O7 investigated by electron spin resonance

Electron spin resonance (ESR) and magnetization measurements were performed on a bilayered single crystal of La1.1Sr1.9Mn2O7. A coexistence of anisotropic paramagnetic resonance (PMR) line and ferromagnetic resonance (FMR) line was observed in a wide temperature range. Temperature dependence of the ESR intensity reveals the competition between local ferromagnetic (FM) correlations and antiferromagnetic (AFM) correlations in the PM state. The antiferromagnetic ordering transition was identified from temperature dependence of resonance field and linewidth of the PMR line in both the ab plane and c -axis directions. The observed FMR line suggests the existence of local intralayer FM clusters. Below 200 K, the FMR line is found to be split and an optical mode appears in addition to the main acoustic mode, indicating the appearance of AFM correlation across MnO2 bilayers below the A-type AFM ordering temperature T N .