Origin of the electron spin resonance signal in the manganites: from polarons to phase separation

In the manganites L1-xMxMnO3 (L = La, Nd, Pr, ...; M = Sr, Ba, Ca, ...), th e doping concentration introduces a mixed valence (Mn3+, Mn4+) which govern s the magnetic and electric properties of the compound. Mn3+ (S = 2) is sca rcely observed in electron spin resonance (ESR). In contrast, Mn4- (S = 3/2 ), is a good ESR probe. However, X-band measurements show an enhanced Mn4susceptibility, which is the signature of some kind of coupling of the Mn4 ions with the Mn3+ ions, but its exact nature is still controversial. We p resent multifrequency ESR experiments (9-385 GHz) obtained on different sys tems (La1-deltaMnO3, La1-xMnO3, La1-xCaxMnO3, and Nd1-xCaxMnO3) in the low- concentration range (0 less than or equal to x less than or equal to 0.33). In the paramagnetic regime, the Mn3+ spectrum cannot be observed because o f fast relaxation. The signal arises from polarons, whose sizer temperature and magnetic field dependences vary with M and x. The single line observed in the metallic compound evolves towards a double-peak structure visible a t high frequency in La0.97MnO3. Its evolution with temperature below the ma gnetic transition reveals the presence of manganese ions in a different mag netic environment, i.e., phase separation. The magnetic order of the separa ted phase is not ferromagnetic. It is a more complex order, which depends s ubstantially on the nature of the cation M.