The evolution of Griffiths-phase-like features and colossal magnetoresistance inLa1−xCaxMnO3 (0.18 ≤ x ≤ 0.27) across the compositional metal–insulator boundary

Abstract

Detailed measurements of the magnetic and transport properties of single crystals ofLa1−xCaxMnO3 (0.18 ≤ x ≤ 0.27) are summarized, and lead to the following conclusions. Whiletemperature-dependent (magneto-) resistance measurements narrow thecompositionally modulated metal–insulator (M–I) transition to lie between0.19 ≤ xc ≤ 0.20 in the series studied, comparisons between the latter magnetic data provide thefirst unequivocal demonstration that (i) the presence of Griffiths-phase-like (GP)features do not guarantee colossal magnetoresistance (CMR), while confirming(ii) that neither are the appearance of such features a prerequisite for CMR.These data also reveal that (iii) whereas continuous magnetic transitions occur for0.18 ≤ x ≤ 0.25, theuniversality class of these transitions belongs to that of a nearest-neighbour 3D Heisenberg model onlyfor x≤0.20, beyond which complications due to GP-like behaviour occur. The implications of thevariation (or lack thereof) in critical exponents and particularly critical amplitudes andtemperatures across the compositionally mediated M–I transition support theassertion that the dominant mechanism underlying ferromagnetism across theM–I transition changes from ferromagnetic super-exchange (SE) stabilized byorbital ordering in the insulating phase to double-exchange (DE) in the orbitallydisordered metallic regime. The variations in the acoustic spin-wave stiffness,D, and thecoercive field, HC, support this conclusion. These SE and DE interaction mechanisms are demonstrated tonot only belong to the same universality class but are also characterized by comparablecoupling strengths. Nevertheless, their percolation thresholds are manifestly different inthis system.