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 °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 anti-phase grain boundaries favors the abnormal positive MR effect. The combination of both the effects causes the abnormal positive MR effect.