Crystallographically oriented Co and Ni nanocrystals inside ZnO formed by ion implantation and postannealing

TL;DR

This study investigates the formation and magnetic properties of Co and Ni nanocrystals in ZnO, revealing their role in ferromagnetism and how annealing influences their magnetic behavior.

Contribution

It provides a detailed structural and magnetic characterization of Co and Ni nanocrystals in ZnO, clarifying their origin of ferromagnetism in doped ZnO.

Findings

Co and Ni nanocrystals are the main source of ferromagnetism.

Nanocrystals are crystallographically oriented within ZnO.

Magnetic properties can be tuned by annealing.

Abstract

In the last decade, transition-metal-doped ZnO has been intensively investigated as a route to room-temperature diluted magnetic semiconductors (DMSs). However, the origin for the reported ferromagnetism in ZnO-based DMS remains questionable. Possible options are diluted magnetic semiconductors, spinodal decomposition, or secondary phases. In order to clarify this question, we have performed a thorough characterization of the structural and magnetic properties of Co- and Ni-implanted ZnO single crystals. Our measurements reveal that Co or Ni nanocrystals (NCs) are the major contribution of the measured ferromagnetism. Already in the as-implanted samples, Co or Ni NCs have formed and they exhibit superparamagnetic properties. The Co or Ni NCs are crystallographically oriented with respect to the ZnO matrix. Their magnetic properties, e.g., the anisotropy and the superparamagnetic blocking temperature, can be tuned by annealing. We discuss the magnetic anisotropy of Ni NCs embedded in ZnO concerning the strain anisotropy.