Defect induced d0 ferromagnetism in a ZnO grain boundary

TL;DR

This study uses computational methods to confirm that zinc vacancies at grain boundaries induce ferromagnetism in ZnO, revealing a defect-driven mechanism for magnetic properties in this material.

Contribution

It provides the first detailed atomic-level analysis showing how zinc vacancies at grain boundaries cause ferromagnetism in ZnO, confirming the defect-induced hypothesis.

Findings

Zinc vacancies preferentially aggregate at grain boundaries.

VZn induces significant spin polarization at oxygen sites.

Ferromagnetic exchange energy increases with VZn concentration.

Abstract

Several experimental studies have referred to the grain boundary (GB) defect as the origin of the ferromagnetism in ZnO. However, the mechanism of this hypothesis has never been confirmed. Present study investigates the atomic structure and the effect of point defects in a ZnO GB using the GGA+U approximation. The relaxed GB possesses large periodicity and channels with 8 and 10 numbered atoms having 4 and 3 fold coordination. Unlike O vacancy (VO), Zn vacancy (VZn) is preferentially aggregated at the GB, relative to the bulk-like region, indicating the possibility of obtaining p-type conductivity in polycrystalline ZnO. Although no magnetization is obtained from point defect-free GB, VZn induces spin polarization as large as 0.68 {\mu}B/atom to the O sites at the GB. Ferromagnetic exchange energy > 150 eV is obtained by increasing the concentration of VZn and by the injection of holes into the system. Electronic structure analysis indicates that the spin polarization without external dopants originates from the O 2p orbitals, a common feature of d0 semiconductors.