Homogeneous and heterogeneous magnetism in (Zn,Co)O: From a random antiferromagnet to a dipolar superferromagnet by changing the growth temperature

Abstract

A series of (Zn,Co)O layers with Co contents x up to 40% grown by atomic layer deposition have been investigated. All structures deposited at 160 • C show magnetic properties specific to II-VI dilute magnetic semiconductors with localized spins S = 3/2 coupled by strong but short-range antiferromagnetic interactions resulting in low-temperature spin-glass freezing for x = 0.16 and 0.4. At higher growth temperature (200 • C) metallic Co nanocrystals precipitate in two locations giving rise to two different magnetic responses: (i) a superparamagnetic contribution coming from volume disperse nanocrystals; (ii) a ferromagneticlike behavior brought about by nanocrystals residing at the (Zn,Co)O/substrate interface. It is shown that the dipolar coupling within the interfacial two-dimensional dense dispersion of nanocrystals is responsible for the ferromagneticlike behavior.

Formation of ferromagnetic Co–H–Co complex and spin-polarized conduction band in Co-doped ZnO

“…Several researchers have even suggested that pure ZnCoO 10–14 is not ferromagnetic. These results have raised doubts concerning the possible effects of unintentional defects and/or impurities, which are likely to affect the magnetic properties of ZnCoO 15–28 .…”

Photoionization of cobalt impurities in zinc oxide

“…This effect was not observed for samples with a reduced Co fraction and grown at low temperature (e.g., 160 °C) . This is why the present ESR investigations are performed for samples with a low Co concentration—bulk samples obtained by Physical Vapor Transport (PVT) method with Co concentration less than 10 17 cm −3 and selected ZnCoO layers described in the references and . Only in the former case we could detect a photosensitive ESR signal of Co 2+ ions, which will be explained further on.…”

“…The origin of the observed magnetic ordering in ZnCoO alloys was investigated in many works. In our studies such investigations were performed for alloys with different Co fractions obtained at low temperature . From these studies we concluded that RT FM (observed for some of our samples) is due to metal accumulations at ZnCoO/Si interface, rather than to “volume properties” of the alloys.…”

“…From these studies we concluded that RT FM (observed for some of our samples) is due to metal accumulations at ZnCoO/Si interface, rather than to “volume properties” of the alloys. This effect was not observed for samples with a reduced Co fraction and grown at low temperature (e.g., 160 °C) . This is why the present ESR investigations are performed for samples with a low Co concentration—bulk samples obtained by Physical Vapor Transport (PVT) method with Co concentration less than 10 17 cm −3 and selected ZnCoO layers described in the references and .…”

Magnetic Semiconductors

“…Soon after this report, paramagnetic behavior of homogeneous Zn 0.75 Co 0.25 O prepared by PLD on Al 2 O 3 (0001) was also reported [184]. Since then, there have been a large number of reports on Co-doped ZnO prepared by various methods, such as chemical synthesis, chemical vapor transport, solid-state reaction, sputtering, ion implantation, atomic layer deposition, MOCVD, and MBE; some report ferromagnetism at room temperature and others paramagnetism [185][186][187][188][189][190][191][192][193]. There are also reports that suggest that the observed ferromagnetism is related to carrier-induced mechanism [185,186,194,195].…”