Magnetic frustration in Y-doped manganites: Electron spin resonance and magnetization

Abstract

Magnetic susceptibility (χdc) and electron spin resonance (ESR) measurements on La0.67−xYxCa0.33MnO3 in the paramagnetic (PM) regime Tc⩽T⩽1000 K are presented, for 0⩽x⩽0.20. For T>750 K,χdc follows a Curie–Weiss (CW) law with C=2.95(15) emu K/mol, and a CW temperature, Θ, showing a monotonic variation as a function of x. The ESR linewidth follows a T dependence given by ΔHpp(T)=[C/Tχdc(T)]ΔH∞. The parameter ΔH∞ increases from 2600 G for x=0 to 3300 G for x=0.20. The correlation with expected variation of the anisotropic spin–spin interactions is discussed. The results appear to be insensitive to the changes of the double-exchange isotropic interaction. A comparison of our experimental results in the PM regime with reported data on the ordered phase indicates a complex magnetic behavior associated with Y substitution. The CW temperature, a measure of the average strength of the magnetic interactions, decreases slowly with x (17% for x=0.20) reflecting the softening of the double exchange due to the bending of the Mn–O–Mn bond. At low temperatures the materials order ferromagnetically (FM) in all cases. The ordering temperature, defined as the point of the maximum slope for M vs T, decreases rapidly with x and the transition becomes increasingly broad. For x=0, we found Tc/Θ≈0.68, value close to 0.78 which is expected within a constant coupling model. This ratio, usually a measure of magnetic frustration, decreases rapidly with x reaching Tc/Θ≈0.26 for x=0.20. However, for this concentration the FM transition is already extremely broad and the relation of this behavior with structural disorder and possible phase separation is discussed.