This report presents a ⁵⁵Mn NMR study of ¹⁶O and ¹⁸O containing La_0.33Nd_0.33Ca_0.34MnO₃. Previous bulk measurements reported in the literature reveal that at low temperatures the ¹⁶O based compound shows charge ordered (CO) and ferromagnetic (FM) insulating phases, whereas the ¹⁸O based compound exhibits a charge ordered insulating behavior [1]. In both compounds, application of a magnetic field can change the low temperature state to a metallic one and the field required for inducing the transitions is two times higher for ¹⁸O based compounds. The ⁵⁵Mn spin-echo spectra were measured at 4.2K. The spectra, for both compounds, consist of a single line corresponding to the double exchange (DE) state of manganese, which is attributed to the existence of ferromagnetic metallic (FMM) regions. The intensity of the resonant line is more than an order of magnitude lower for the ¹⁸O based compound compared with the ¹⁶O based compound, which indicates an order of magnitude lower content of the FMM phase in the ¹⁸O based compound. The Mn resonance of the CO or FM insulating phases are not observed, possibly due to a fast nuclear relaxation of the Mn³⁺ and Mn⁴⁺ cations. The measurements of the spectra for large pulse spacing show a pronounced minimum at the centre of the DE line. This is related to a minimum of the spin-spin relaxation time at the line centre caused by the Suhl-Nakamura interaction between nuclear spins. This interaction is effective in the large regions of uniform magnetisation, which indicates that the FMM regions are at least tens of nanometres in size for both compounds. The results are discussed in terms of phase segregation in the compounds and the influence of oxygen isotopic substitution on it.

[1] M.R.Ibarra et al, Phys. Rev. B, 57 (1998) 7446

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