Large polarons in rare-earth doped hexaferrites observed by NMR

We investigated the charge distribution in rare-earth (R=La, Nd, Sm) doped hexaferrites RxSr1-xFe12O19 at x less than or equal to 0.33 T = 4.2 K and in magnetic fields up to 7 T with the local probe of Fe-57- nuclear magnetic resonance (NMR). All five crystallographic Fe-sites in the doped material are subject to an additional internal field DeltaB(hf), independent of R. DeltaB(hf) increases to 0.4 T with x less than or equal to 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. (C) 2003 Elsevier B.V. All rights reserved.