Nonlinear properties of ferromagnetic La _1-xCa _xMnO ₃ single crystals

The resistive and magnetic properties of La _1-x Ca _x MnO ₃ single crystals are investigated. In particular, properties of close-to-critically doped crystals with x = 0.22 are confronted with those of underdoped crystals with x = 0.18 and x = 0.2 and optimally doped crystals with x = 0.3. A systematic investigation of nonlinear transport shows that the critically doped crystal does not simply constitute an intermediate case between strongly nonlinear underdoped and linear, optimally doped one. The observed low-temperature resistivity increase can be interpreted in terms of orbital ordering or as a manifestation of an intrinsic tunnelling mechanism. The transport measurements have been completed by magneto-optic investigations which suggest that the low temperature resistivity of the underdoped x = 0.18 compound is dominated by tunnelling through intrinsic barriers associated with twin domains, while phase separation dominates in the critically doped x = 0.22 compound.