Nonlinear properties of ferromagnetic La <InlineEquation ID="Equ1"> <EquationSource Format="TEX">$\mathsf{_{1-x}}$</EquationSource> </InlineEquation>Ca<InlineEquation ID="Equ2"> <EquationSource Format="TEX">$\mathsf{_{x}}$</EquationSource> </InlineEquation>MnO<InlineEquation ID="Equ3"> <EquationSource Format="TEX">$\mathsf{_3}$</EquationSource> </InlineEquation> single crystals

The resistive and magnetic properties of La<Subscript>1-x </Subscript>Ca<Subscript> x </Subscript>MnO<Subscript>3</Subscript> 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. Copyright Springer-Verlag Berlin/Heidelberg 2003