Magnetic and transport behavior of electron-dopedLa1−xMgxMnO3(0.45<~x<~0.6)

Detailed measurements of the linear and nonlinear magnetic responses, the zero-field cooled and field-cooled magnetization, and the resistivity of electron-doped ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Mg}}_{x}{\mathrm{MnO}}_{3},$ $0.45l~xl~0.6,$ are summarized. For $0.05l~xl~0.6$ this system exhibits a single paramagnetic to ferromagnetic phase transition on cooling with no thermodynamic reentrant transition to a spin-glass-like phase. Nevertheless, both a detailed analysis of the critical response and the low temperature saturation moment show clear evidence of competing interactions. The latter could arise either as a result of spontaneous electronic phase separation or from conventional noncollinearity (homogeneous, or---due to the random substitution process---inhomogeneous). These results argue against a simple double exchange picture for this system and a uniform ferromagnetic ground state. However, features evident in the zero-field-cooled behavior (and the nonlinear response) at temperatures below ${T}_{c}$ originate from technical sources viz. the increase in coercive field, so it is possible that a gradual onset of competing interactions might arise from a related source. The resistivity data confirm that the complete suppression of a metal-insulator transition in these systems with small average A-site radius extends into the electron-doped regime $(xg~0.5).$ Thus ferromagnetism dominates but the system remains insulating, contrasting with an emerging ferromagnetic metallic state for which a spontaneous electronic phase-separation approach has been proposed. Indeed, the transport data conform with model expressions for charge transport by nonadiabatic small polarons in the paramagnetic phase, but they are quantitatively inconsistent with the expression derived recently by Rahkmanov et al. [Phys. Rev. B $63,$ 174429 (2001)] for the specific model of polaronic hopping in a nonmetallic, phase separated picture.