Structural and magnetic properties in bidimensional manganites

The Lny-xCa2-xMnO4 series, with a K2NiF4 structure, has been studied by neutron powder diffraction and electron microscopy in connection with the magnetic and transport properties. We report here the results of the structural and magnetic study of two compounds, Pr0.25Ca1.75MnO4 and Ho0.08Ca1.92MnO4, for which the Mn valency is 3.75 and 3.92 respectively and consequently that show different magnetic and transport behaviours. At room temperature (RT), the average structure of both compounds is orthorhombic with lattice parameters ap\(\sqrt\Box2\Box\), 12 A, ap\(\sqrt\Box2\Box\), and Cmca space group. With decreasing temperature, the Ho0.08Ca1.92MnO4 compound exhibits two magnetic phase transitions: antiferromagnetism appears at TN≈165 K and ferromagnetism at TC≈75 K. The two phases coexist below TC without any structural transition. The Pr0.25Ca1.75MnO4 compound exhibits charge ordering below TCO≈275 K, characterised by superlattice reflections clearly evidenced by electron diffraction. Neutron diffraction pattern refinements show a partial structural transition from Cmca (RT) to C2/c (low temperature) around the charge-ordering temperature and antiferromagnetism below TN≈150 K.