A Mn-55 nuclear magnetic resonance (NMR) study of the La1-xCaxMnO3 system,
0 < x < 1, is reported. For x < 0.3 and x > 0.5 the spin echo spectra conta
in resonance lines corresponding to both the microscopic ferromagnetic and
antiferromagnetic regions. A resonance line near 380 MHz, which is similar
to that of the spectrum of the ferromagnetic and metallic compounds (0.2 <
x < 0.5) is observed for all x. This indicates the presence of clusters in
which the Mn magnetic moments are double exchange coupled in the whole rang
e of doping. Their hyperfine field exhibits a much weaker temperature depen
dence than that of the lines of the host. This reveals the greater strength
of the magnetic coupling within the clusters than in the antiferromagnetic
regions and indicates a weak magnetic coupling of the clusters to the host
matrix. A comparison with the NMR results on the hole doped nonstoichiomet
ric compounds, LaMnO3+delta is made. The temperature dependence of the line
intensities corresponding to the double exchange clusters and to the host
matrix is attributed to the evolution of the magnetic phases in the compoun
ds. (C) 2000 American Institute of Physics. [S0021-8979(00)66908-6].