Homogenous and heterogeneous magnetism in (Zn,Co)O

TL;DR

This study investigates the magnetic properties of (Zn,Co)O films grown by atomic layer deposition, revealing that magnetic behavior depends on Co distribution, with dilute magnetic semiconductor characteristics at low growth temperatures and ferromagnetism linked to metallic Co granules at higher temperatures.

Contribution

The paper provides a comprehensive analysis of how Co distribution and interdiffusion in (Zn,Co)O layers influence their magnetic properties, distinguishing between dilute magnetic semiconductor behavior and ferromagnetism due to metallic inclusions.

Findings

Dilute magnetic semiconductor behavior with spin-glass freezing at 160°C.

Ferromagnetic-like features linked to metallic Co granules at 200°C and above.

Co interdiffusion leads to truly random (Zn,Co)O alloys with up to 40% Co.

Abstract

A series of (ZnO)m(CoO)n digital alloys and superlattices grown by atomic layer deposition has been investigated by a range of experimental methods. The data provide evidences that the Co interdiffusion in the digital alloy structures is sufficient to produce truly random Zn1-xCoxO mixed crystals with x up to 40%. Conversely, in the superlattice structures the interdiffusion is not strong enough to homogenize the Co content along the growth direction results in the formation of (Zn,Co)O films with spatially modulated Co concentrations. All structures deposited at 160\circC show magnetic properties specific to dilute magnetic semiconductors with localized spins S = 3/2 coupled by strong but short range antiferromagnetic interactions that lead to low temperature spin-glass freezing. It is demonstrated that ferromagnetic-like features, visible exclusively in layers grown at 200\circC and above, are associated with an interfacial mesh of metallic Co granules residing between the substrate and the (Zn,Co)O layer. This explains why the magnitude of ferromagnetic signal is virtually independent of the film thickness as well as elucidates the origin of magnetic anisotropy. Our conclusions have been derived for layers in which the Co concentration, distribution, and aggregation have been determined by: secondary-ion mass spectroscopy, electron probe micro-analysis, high-resolution transmission electron microscopy with capabilities allowing for chemical analysis; x-ray absorption near-edge structure; extended x-ray absorption fine-structure; x-ray photoemission spectroscopy, and x-ray circular magnetic dichroism. Macroscopic properties of these layers have been investigated by superconducting quantum interference device magnetometery and microwave dielectric losses allowing to confirm the important role of metallic inclusions.