Anti-phase boundaries pinned abnormal positive magnetoresistance in Mg doped nanocrystalline zinc spinel ferrite

Ni and Mg substituted Zn1-xMxFe2O4 (M = Ni, Mg) nanocrystals with variable concentration were synthesized by thermal decomposition of EDTA precursor complexes at 600 degrees C. The crossover from negative to positive magnetoresistance (MR) is realized by replacing the doping ions from Ni to Mg in nanocrystalline zinc spinel ferrites. In Zn1-xNixFe2O4 nanocrystalline ferrite, the negative MR is originated from spin-polarized electron tunneling across the grain boundaries. In Zn1-xMgxFe2O4 nanocrystalline ferrite, the electron hopping between the sublattices of A- and B-site is deeply decreased due to Mg doping, which hence suppresses the spin-polarized electron tunneling across the neighbouring grains. Moreover, the enhanced magnetic scattering across the antiferromagnetic-coupled antiphase grain boundaries favors the abnormal positive MR effect. The combination of both the effects causes the abnormal positive MR effect. (c) 2008 Elsevier B.V. All rights reserved.