INVESTIGATIONS OF THE LA1-XCAXMN2GE2 0-LESS-THAN-OR-EQUAL-TO-X-LESS-THAN-OR-EQUAL-TO-1) SOLID-SOLUTION BY MAGNETIC MEASUREMENTS AND NEUTRON-DIFFRACTION

The La1-xCaxMn2Ge2 (0 less than or equal to x less than or equal to 1) solid solution has been studied by means of X-ray powder diffraction, magnetic measurements and neutron diffraction study. For x less than or equal to 0.7, all samples exhibit an easy-axis ferromagnetic behavi our below T-C (320 K > Te > 80 K). The Curie temperature and the maxim um magnetisation values (4.2 K, H-appl. = 17 kOe) decrease continuousl y with the La substitution rate from 320 K and 1.45 mu(beta), per Mn a tom for LaMn2Ce2, to 80 K and 0.4 mu(beta) for La0.3Ca0.7Mn2Ge2. Furth ermore, above T-c, neutron diffraction study of the La1-xCaxMn2Ge2, (x = 0,2, 0,5, 0.7 and 1.0) reveals the occurrence of an additional anti ferromagnetic region (not detected by magnetometric measurements) char acterised by antiferromagnetic (0 0 1) Mn planes. The Neel temperature increases from 420 K for LaMn2Ge2 up to 670 K for CaMn2Ge2. The magne tic structures of La0.8Ca0.2Mn2Ge2, La0.5Ca0.5Mn2Ge2 and La0.3Ca0.7Mn2 Ge2 La0.3Ca0.7Mn2Ge2 have been determined between 2 and 600 K. A new A F DC magnetic type ordering is found in La0.5Ca0.5Mn2Ge2, between T-c and T-t. The results lead to the construction of the x-T magnetic phas e diagram and are discussed and compared with the results obtained for the La1-xYxMn2Ge2 solid solution. (C) 1998 Elsevier Science B.V. All rights reserved.