^<55>Mn NMR in Ferromagnetic Perovskites RENi_<0.5>Mn_<0.5>O_3 (RE=Rare Earth Element)(Condensed Matter : Electronic Structure, Electrical, Magnetic and Optical Properties)

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The hyperfine magnetic fields, H_<HF>, at ^<55>Mn were measured by NMR at 2K 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 (-258kOe) to Lu (-282kOe). 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.
2005-04-15