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${\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}$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${}^{\ensuremath{\circ}}$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.