Magnetic properties of(Pr(Ca, Sr))MnO3 studied by nuclear magnetic resonance

The valence state of Mn in magnetoresistive perovskites has been widely rel ated to the level of alkaline metal substitution of the rare earth, by assu ming that the Mn exists in 3 + or 4+ valence states. Measurements have been made on a series of samples in the system Pr1-(x+y)(CaxSryMnO3. Mn-55 nucl ear magnetic resonance (NMR) zero field spin echo measurements on a single crystal with x=0.1 and y = 0 show distinct resonances around 310 MHz (Mn4+) and 400 MHz (Mn3+) but also at 590 MHz (Mn2+). An increase in the Mn2+ res onance frequency in an applied field implies that the Mn2+ moment is antife rromagnetically coupled to the net magnetization. Spectra from powder sampl es are much broader. The resonance frequencies increase as the Pr content i s reduced. For systems with x + y = 0.4, there is little change in the NMP spectrum as the proportions of Ca and Sr are altered. (C) 1998 American Ins titute of Physics. [S0021-8979(98)26511-X].