Search for ferromagnetism in transition-metal-doped piezoelectric ZnO

Abstract

We present the results of a computational study of ZnO in the presence of Co and Mn substitutional impurities. The goal of our work is to identify potential ferromagnetic ground states within the (Zn,Co)O or (Zn,Mn)O material systems that are also good candidates for piezoelectricity. We find that, in contrast to previous results, robust ferromagnetism is not obtained by substitution of Co or Mn on the Zn site, unless additional carriers (holes) are also incorporated. We propose a practical scheme for achieving such p-type doping in ZnO.

Investigation of pure and Co2+-doped ZnO quantum dot electronic structures using the density functional theory: choosing the right functional

“…This agrees with earlier theoretical studies of bulk Co 2+ -doped ZnO [52], [54]- [56]. The crystal field splitting between the occupied Co 2+ e and t 2 d-orbitals is about 0.5 eV for spin-up electrons in the LSDA and pure PBE theories.…”

“…This description is consistent with experimental results showing no measurable change in the Co 2+ ligand field transition energies between nanocrystalline and bulk Co 2+ -doped ZnO [10,14,59,60], and recent experimental MCD observations and theoretical LSDA results showing pinned Co 2+ d levels but size-dependent band edge energies in Co 2+ -doped ZnSe QDs [14]. An interesting observation can be made regarding the separation between the occupied and unoccupied spin-down levels, an energy that is often related to the tetrahedral crystal field splitting parameter, Dq [52,53]. The energy between the occupied Co 2+ d spin-down e and unoccupied Co 2+ t 2 levels is less than 1 eV in LSDA calculations, and grows to ∼6 eV in the calculations performed with the hybrid functional.…”

Role of defects in ferromagnetism inZn1−xCoxO: A hybrid density-functional study

“…16 LSDA calculations of the density of states for Zn 1−x Co x O show Co 3d levels at the top of the majority-spin valence band. The highest occupied level is a minority-spin 3d state approximately 1 eV above the majority-spin 3d states.…”

“…4 Here we report results of all-electron B3LYP ͑Refs. [15][16][17][18][19] and two of these 16,18 have considered the role of defects in promoting ferromagnetism. One of the difficulties in using DFT to study defects in an insulating material with defect-induced electronic states in midgap is severe underestimation of the band gap and overestimation of exchange coupling constants.…”

Magnetism of (Zn,Co)O thin films probed by x-ray absorption spectroscopies

“…2 On the contrary, other papers claimed that this ferromagnetic phase is stabilized by p-type doping. 3 In addition to this theoretical debate, the experimental results are also controversial. A number of reports categorically attribute the observed magnetic signal ͑ferromagnetic or paramagnetic͒ to the dilute phase, whereas others claim that ferromagnetism originates from cobalt clusters.…”