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

Abstract

In the manganites L1−xMxMnO3 (L = La, Nd, Pr, …; M = Sr, Ba, Ca, …), the doping concentration introduces a mixed valence (Mn3+, Mn4+) which governs the magnetic and electric properties of the compound. Mn3+ (S = 2) is scarcely observed in electron spin resonance (ESR). In contrast, Mn4+ (S = 3/2), is a good ESR probe. However, X-band measurements show an enhanced Mn4+ susceptibility, 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 present multifrequency ESR experiments (9–385 GHz) obtained on different systems (La1−δMnO3, La1−xMnO3, La1−xCaxMnO3, and Nd1−xCaxMnO3) in the low-concentration range (0 <x< 0.33). In the paramagnetic regime, the Mn3+ spectrum cannot be observed because of fast relaxation. The signal arises from polarons, whose size, temperature and magnetic field dependences vary with M andx. The single line observed in the metallic compound evolves towards a double-peak structure visible at high frequency in La0.97MnO3. Its evolution with temperature below the magnetic transition reveals the presence of manganese ions in a different magnetic environment, i.e., phase separation. The magnetic order of the separated phase is not ferromagnetic. It is a more complex order, which depends substantially on the nature of the cation M.