Phase coexistence and magnetic anisotropy in La1-xCaxMnO3 (0< x ≤0.23) studied via electron spin resonance

We report an electron-spin-resonance (ESR) study of stoichiometric La1-xCaxMnO3 for 0 < x <= 0.23. The room-temperature ESR linewidth shows a marked decrease with x that is mainly due to the variation of the magnetic susceptibility and a much slower evolution of spin relaxation, associated with the interplay of crystal-field and superexchange interactions. In the canted antiferromagnetic regime, the ESR parameters show initially an x-dependent ferromagnetic (FM) behavior that evolves to antiferromagnetic in the vicinity of T-N, reflecting the develpment of spin canting. A weak ferromagnetic-resonance (FMR) mode is observed below T-N that becomes enhanced near the phase boundary to the ferromagnetic insulating (FMI) phase. This mode persists at higher doping levels (x <= 0.16) and is attributed to residual FM domains due to the release of crystal strain near the structural transformation at T < 100 K. In the FMI regime, FMR measurements show strong uniaxial magnetic anisotropy up to x=0.16 with K=(1-2)x10(6) erg/cm(3) at 100 K. The temperature dependence of the magnetic anisotropy reveals power-law scaling with the magnetization, K(T)proportional to M(T)(3), indicative of single-ion magnetocrystalline anisotropy and the persistence of orbital order in the FMI state. As the FM metallic phase is approached, the magnetic anisotropy decreases significantly and an additional FMR mode systematically emerges in the paramagnetic regime at T less than or similar to 260 K for x=0.19-0.23. Its relative spectral weight grows with x indicating an increase of the underlying FM fraction that correlates with the enhanced metallic behavior.