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 high er than the intragranular one for the low-loss sintered Mn-Zn ferrites cont aining small amount of Ca and Si. Observations by analytical transmission m icroscopes have revealed that Si and Ca atoms are segregated at the grain b oundaries over a few nm wide. However, a correlation between the resistivit y and microstructure remains unestablished. In this study, the scanning tun neling spectroscopy was applied for the first time to a polycrystalline Mn- Zn ferrite to study the thickness of the grain boundaries from the standpoi nt 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 tha t the bandgap at the region should be wide, resulting in a higher resistivi ty compared to the interior of the grains. The value of 110 nm, the width o f high resistivity region, is much larger than the previously reported valu es. This suggests that Fe2+ vacancies are involved in the electronic struct ure. 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 bound ary region during the cooling from the sintering temperature. This mechanis m is in accordance with the model proposed by Paulus.