⁵⁵Mn NMR in Ferromagnetic Perovskites RENi_0.5Mn_0.5O₃ (RE = Rare Earth Element)

Abstract

The hyperfine magnetic fields, H HF , at 55 Mn were measured by NMR at 2 K for ferromagnetically ordered double perovskites REMn 0.5 Ni 0.5 O 3 (RE = rare earth element). The field H HF tends to decrease as RE goes from La (-258 kOe) to Lu (-282 kOe). In addition to the overall RE dependence a break is found between RE = Tb and Dy. The field H HF is interpreted as the sum of H HF,Mn 4+ arising from the unpaired electrons in Mn 4+ to which 55 Mn belongs and H STHF supertransferred from the neighboring Ni 2+ ions. The present result shows that H STHF decreases by 30–55%, as RE goes from La to Lu, due to the reduction of the covalency of Ni 2+ –O 2- and Mn 4+ –O 2- bonds, in addition to a 25% decrease arising from the cos 2 θ-type bond angle dependence of H STHF . The reduction of the covalency as RE goes from La to heavier elements reasonably explains the non-linear cos 2 θ dependence of T C of the present oxides. The magnetic moments of RE ions do not contribute to the "average" H HF but give rise to large transverse relaxation rates by their fluctuation, which is suppressed by an applied magnetic field.