Colossal magnetoresistance and hyperfine interactions in iron-dopedLa0.75Ca0.25MnO3

Detailed measurements and analysis of hyperfine properties of iron-doped La 0.75 Ca 0.25 Mn 1-x Fe x O 3 are reported. In a previous paper [Phys. Rev. B 57, 7841 (1998)] it was shown that a very small increase of x from 0.0425 to 0.045 causes dramatic changes in the transport properties, whereas no changes are detected either in the structural phase or the Mossbauer data. In the present study a complete analysis of Mossbauer data registered in the temperature range of 300-4 K is presented. From 300 K down to 190 K the samples are in a paramagnetic state. Below 190 K a magnetic order begins to develop, although a paramagnetic contribution persists down to 77 K. This existence of paramagneticlike domains deep into the so-called ferromagnetic metallic regime provides evidence of the two-phase character of the metallic state in the mixed-valent manganite system. The quadrupole splitting of the major spectral component shows a jump at the para-ferro transition. This reflects lowering of the local point-charge symmetry in the remaining paramagneticlike phase in the ferromagnetic regime, presumably because of the changes in its shape function or its character, as the ferromagnetic clusters stabilize in space. Another small component of the spectrum, which appears to represent the grain-boundary region, exhibits ferromagnetic ordering at around 100 K and also a behavior opposite to that of the major fraction under external magnetic field.