NEUTRON-DIFFRACTION STUDIES ON THE MAGNETIC-ORDERING PROCESS IN THE LAYERED MN PEROVSKITE LA2-2XSR1+2XMN2O7 (X = 0.40, 0.45 AND 0.48)

Comprehensive neutron-diffraction studies on La2-2xSr1+2xMn2O7 (x = 0. 40, 0.45 and 0.48) single crystals have revealed that the low-temperat ure magnetic phase consists of planar ferromagnetic (FM) and A-type an tiferromagnetic (AFM) components, indicating a canted AFM ordering. Up on increasing the hole concentration x, the canting angle between plan es changes from 6.3 degrees (nearly planar FM) at x = 0.40 to 180 degr ees (A-type AFM) at x = 0.48, while the ordering temperature of the FM component, T-C, decreases from 120 K to 0 K correspondingly. We have also discovered that the A-type AFM ordering remains above Te and show s an anomalous exponential decrease to T-N similar to 200 K. This newl y found intermediate A-type AFM phase may play a significant role in t he enhancement of CMR effects in this layered Mn perovskite system.