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

Magnetic susceptibility (chi(dc)) and electron spin resonance (ESR) measure ments on La0.67-xYxCa0.33MnO3 in the paramagnetic (PM) regime T-c less than or equal to T less than or equal to 1000 K are presented, for 0 less than or equal to x less than or equal to 0.20. For T > 750 K,chi(dc) follows a C urie-Weiss (CW) law with C=2.95(15) emu K/mol, and a CW temperature, Theta, showing a monotonic variation as a function of x. The ESR linewidth follow s a T dependence given by Delta H-pp(T) = [C/T chi(dc)(T)]Delta H-infinity. The parameter Delta H-infinity increases from 2600 G for x = 0 to 3300 G f or x = 0.20. The correlation with expected variation of the anisotropic spi n-spin interactions is discussed. The results appear to be insensitive to t he changes of the double-exchange isotropic interaction. A comparison of ou r experimental results in the PM regime with reported data on the ordered p hase indicates a complex magnetic behavior associated with Y substitution. The CW temperature, a measure of the average strength of the magnetic inter actions, decreases slowly with x (17% for x = 0.20) reflecting the softenin g of the double exchange due to the bending of the Mn-O-Mn bond. At low tem peratures the materials order ferromagnetically (FM) in all cases. The orde ring temperature, defined as the point of the maximum slope for M vs T, dec reases rapidly with x and the transition becomes increasingly broad. For x = 0, we found T-c/Theta approximate to 0.68, value close to 0.78 which is e xpected within a constant coupling model. This ratio, usually a measure of magnetic frustration, decreases rapidly with x reaching T-c/Theta approxima te to 0.26 for x = 0.20. However, for this concentration the FM transition is already extremely broad and the relation of this behavior with structura l disorder and possible phase separation is discussed. (C) 2000 American In stitute of Physics. [S0021-8979(00)41808-6].