Magnetotransport properties of La${}_{1-x}$Ca${}_x$MnO${}_3$ (0.52 $\le x\le$ 0.75): signature of phase coexistence

Abstract

We report the temperature and magnetic field dependence of transport properties in epitaxial films of the manganite La${}_{1-x}$Ca${}_x$MnO${}_3$ in the overdoped region of the phase diagram for $x>0.5$, where a charge--ordered (CO) and an antiferromagnetic (AF) phase are present. Resistivity, magnetoresistance and angular dependence of magnetoresistance were measured in the temperature interval $4.2\mathrm{K}<T<300\mathrm{K}$, for three concentrations $x=0.52,0.58$ and $0.75$ and in magnetic fields up to 5 T. The semiconductor/insulator--like behavior in zero field was observed in the entire temperature range for all three concentrations \textit{x} and the electric conduction, at lower temperatures, in the CO state obeys 3D Mott's variable--range hopping model. A huge negative magnetoresistance for $x=0.52$ and $x=0.58$, a metal--insulator transition for $B>3\mathrm{T}$ for $x=0.52$ and the presence of anisotropy in magnetoresistance for $x=0.52$ and $x=0.58$ show the fingerprints of colossal magnetoresistance (CMR) behavior implying the existence of ferromagnetic (FM) clusters. The declining influence of the FM clusters in the CO/AF part of the phase diagram with increasing $x$ contributes to a possible explanation that a phase coexistence is the origin of the CMR phenomenon.

33 pages, 7 figures, 1 table, accepted in Thin Solid Films