A 55Mn nuclear magnetic resonance study of mixed-valence manganites

The temperature dependence of the Mn-55 nuclear magnetic resonance of some manganese perovskites of the form [La(1-x)A(x)]MnO3 (A = Ca, Sr) for x of 1/3 is reported. The Mn-55 spin-echo spectra were measured in zero field and in an applied magnetic field up to 6 T for polycrystalline samples of [La0.67-yTbyCa0.33]MnO3 (y = 0, 0.1, 0.22), [La0.6Y0.07Ca0.33]MnO3 and [La0.7Sr0.3]MnO3. The single resonance line observed for all compounds, for temperatures between 4.2 K and 293 K, is attributed to a fast carrier motion (<10(-9) s) between the Mn3+ and Mn4+ sites resulting from the double-exchange mechanism. For some of the samples the zero-field nuclear magnetic resonance signal has been detected at temperatures far above the magnetic ordering temperature, providing direct evidence of magnetic correlations whose lifetime is greater than 10(-5) s. The correlations can possibly be related to magnetic polarons.