Hyperfine Magnetic Field at 55Mn in Ferromagnetic Perovskites REMn0.5(Ni1-xMgx)0.5O3 with RE = La, Pr, Nd, and Eu : RE Dependence of Supertransferred Hyperfine Interaction

The hyperfine magnetic field at 55 Mn in ferromagnetically ordered double perovskites of the form REMn 0.5 (Ni 1- x Mg x ) 0.5 O 3 with RE=La, Pr, Nd, and Eu were measured by NMR at 1.8 K. The hyperfine magnetic field H HF,Mn 4+ , which arises from (3 d e ) 3 unpaired electrons in Mn 4+ to which 55 Mn belongs, is -294.5 kOe independent of RE species. On the other hand, H STHF supertransferred from 3 d 3 z 2 - r 2 unpaired electrons in the neighboring Ni 2+ ions decreases as RE goes from La (+35.8 kOe) to Eu (+20.1 kOe). The RE dependence arises from the reduction in the covalency of the Mn 4+ –O 2- and Ni 2+ –O 2- bonds as RE goes from La to Eu, as well as from the cos 2 θ-type bond angle dependence of the STHF interaction. With the RE dependence of covalency determined from H STHF , the superexchange interaction between Mn 4+ and Ni 2+ depending on RE in the present system has been well interpreted.