The transport and magnetic properties of single crystals of ${\mathrm{La}}_{1-x}{\mathrm{Ca}}_x{\mathrm{MnO}}_3$ $(x=0.2,$ 0.3) have been investigated. The transport and magnetization results of the ferromagnetic insulator, ${\mathrm{La}}_{0.8}{\mathrm{Ca}}_{0.2}{\mathrm{MnO}}_3,$ are quite different from those of ${\mathrm{La}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_3$ that falls into a ferromagnetic metal. We have found that the paramagnetic to ferromagnetic transitions observed in ${\mathrm{La}}_{0.8}{\mathrm{Ca}}_{0.2}{\mathrm{MnO}}_3$ and ${\mathrm{La}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_3$ are second and first order using the Banerjee criterion, respectively. The observation of magnetic frustration and Heisenberg exponents derived by the modified Arrott plot favors the existence of short-range charge/orbital ordering in ${\mathrm{La}}_{0.8}{\mathrm{Ca}}_{0.2}{\mathrm{MnO}}_3$ and even in ${\mathrm{La}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_3.$

• Received 2 June 2000

DOI:https://doi.org/10.1103/PhysRevB.63.092504