Investigations of the La 1− xCa xMn 2Ge 2 (0⩽ x⩽1) solid solution by magnetic measurements and neutron diffraction

Abstract

The La 1− x Ca x Mn 2Ge 2 (0⩽ x⩽1) solid solution has been studied by means of X-ray powder diffraction, magnetic measurements and neutron diffraction study. For x⩽0.7, all samples exhibit an easy-axis ferromagnetic behaviour below T C (320 K> T C>80 K). The Curie temperature and the maximum magnetisation values (4.2 K, H appl.=17 kOe) decrease continuously with the La substitution rate from 320 K and 1.45 μ B per Mn atom for LaMn 2Ge 2, to 80 K and 0.4 μ B for La 0.3Ca 0.7Mn 2Ge 2. Futhermore, above T C, neutron diffraction study of the La 1− x Ca x Mn 2Ge 2 ( x=0.2, 0.5, 0.7 and 1.0) reveals the occurrence of an additional antiferromagnetic region (not detected by magnetometric measurements) characterised by antiferromagnetic (0 0 1) Mn planes. The Néel temperature increases from 420 K for LaMn 2Ge 2 up to 670 K for CaMn 2Ge 2. The magnetic structures of La 0.8Ca 0.2Mn 2Ge 2, La 0.5Ca 0.5Mn 2Ge 2 and La 0.3Ca 0.7Mn 2Ge 2 have been determined between 2 and 600 K. A new AF DC magnetic type ordering is found in La 0.5Ca 0.5Mn 2Ge 2 between T C and T t. The results lead to the construction of the x– T magnetic phase diagram and are discussed and compared with the results obtained for the La 1− x Y x Mn 2Ge 2 solid solution.