Large polarons in rare-earth doped hexaferrites observed by NMR

We investigated the charge distribution in rare-earth (R=La, Nd, Sm) doped hexaferrites R x Sr 1-x Fe 12 O 19 at x≤0.33, T = 4.2 K and in magnetic fields up to 7 T with the local probe of 57 Fe- nuclear magnetic resonance (NMR). All five crystallographic Fe-sites in the doped material are subject to an additional internal field ΔB hf , independent of R. ΔB hf increases to 0.4 T with x≤0.06, then decreases again. For x>0.15, the Fe-spectrum is nearly indistinguishable from the one observed in undoped samples. The homogeneous shift of all Fe-lines is ascribed to the formation of large polarons with a magnetic polarization increasing at small x, and decreasing again when additional states are involved in their formation.