Homogenous and heterogeneous magnetism in (Zn,Co)O

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.

ZnO, ZnMnO and ZnCoO films grown by atomic layer deposition

“…It was found that formation of most of the oxides is enhanced at increased temperatures. Thus, a low growth temperature is highly profitable, as already indicated by Sharma and co-workers [5], and by us for both ZnMnO [15,28,29] and ZnCoO [24,30]. This is why in further studies we lowered a growth temperature by replacing zinc acetate with DEZn or DMZn precursors.…”

“…Knowledge of the growth mode is not only academic. We found that this information is crucial to explain magnitude and anisotropy of the observed magnetization, as is discussed separately [30]. However, if low temperature processes of ZnO and ZnTMO deposition are required (to get uniform TM distribution, material suitable for hybrid structures with organic films, etc...), control of the growth mode is fairly difficult.…”

“…This information is important to account for very contradicting reports on strength of magnetic interactions in e.g. ZnCoO [30].…”

“…These investigations, and also magnetic ones, led to the following conclusion: even though we reduced substrates temperature to 300 ºC, this temperature was still too high to obtain uniform ZnMnO (we then found the similar situation for ZnCoO) alloys. Thus, in the further studies we replaced zinc acetate with DEZn and DMZn [24,28,30], which allowed further reduction of a growth temperature. We also replaced a manganese precursor.…”

“…For ZnCoO alloys the XAS study is discussed in details in the reference [30]. Empty circles represent experimental results, full lines -obtained fits.…”