Scanning tunneling spectroscopy of high-resistance grain boundaries in sintered Mn-Zn ferrite

It is well-known that the grain boundary resistivity is several orders higher than the intragranular one for the low-loss sintered Mn-Zn ferrites containing small amount of Ca and Si. Observations by analytical transmission microscopes have revealed that Si and Ca atoms are segregated at the grain boundaries over a few nm wide. However, a correlation between the resistivity and microstructure remains unestablished. In this study, the scanning tunneling spectroscopy was applied for the first time to a polycrystalline Mn-Zn ferrite to study the thickness of the grain boundaries from the standpoint of electronic structure. Measurements have revealed that the region about 110 nm in width with small density of states is spreading across the grain boundaries. It follows that the bandgap at the region should be wide, resulting in a higher resistivity compared to the interior of the grains. The value of 110 nm, the width of high resistivity region, is much larger than the previously reported values. This suggests that Fe²⁺ vacancies are involved in the electronic structure. The vacancies may be generated at A or B sites in the spinel structure by the diffusion of Ca ions from the interior of grains to the grain boundary region during the cooling from the sintering temperature. This mechanism is in accordance with the model proposed by Paulus.