Defect‐Induced Ferromagnetism in Co‐doped ZnO

Abstract

Dilute magnetic semiconductors (DMSs) that are formed by the partial replacement of cations in semiconductors by magnetic transition metal ions have drawn considerable attention because of their potential use in spintronic devices. [1,2] Many reports on the observation of ferromagnetism (FM) at room temperature in transition-metal-doped ZnO, TiO 2 , and other semiconducting oxides have been published.[3±14] However, the origin of FM in these materials is still not well understood. Mechanisms based on carrier-mediated FM [15,16] and percolation of bound magnetic polarons [3,17,18] have been proposed. The presence of a certain level of free carriers is required for carrier-mediated FM, whereas the bound magnetic polaron model is also applicable to the insulating state. In order to explore the mechanism of FM, epitaxial thin films of Co-doped ZnO have been grown using ultrasonic-assisted solution chemical vapor deposition (UASCVD). The technique is a simple, soft, combinatorial process that allows for growth at low temperatures using either inorganic [19] or organic precursors. [20] Deposition can be carried out in the absence of a vacuum and this is particularly useful for preventing evaporation of cations of high volatility. First, the influence of growth temperature on the presence or absence of a well-known broad feature in undoped ZnO films at ca. 500 nm (green band), which arises because of structural defects, namely Zn interstitials (Zn_ i ) [21] or oxygen vacancies (V_ o ), [22] is investigated. While simple convention would suggest doubly charged Zn_ i _ and V_ o _, here the convention of singly charged defects is used because the conductivity of ZnO due to native defects is found to be correlated to singly charged defects of Zn_ i [23] Figure 2 shows magnetization (M) versus field (H) curves at room temperature for Zn 0.98 Co 0.02 O films prepared at 400 and 500 C. The film prepared at 400 C shows a saturation M value (M s ) of over 0.4 l B /Co (l B : Bohr magneton), whereas the film grown at 500 C has barely a trace signal above the background. Both the films are highly resistive (resistivity q » 10 4 X cm), which is indicative of near-oxygen stoichiometry and suggests that FM does not depend upon the presence of a significant carrier concentration. The net moment per Co atom, although an order of magnitude lower than the absolute Co moment, is comparable to that found in many previous reports and can be interpreted either in terms of a predominately paramagnetic response (most Co ions are decoupled) or whether Co ions can couple antiferromagnetically or ferromagnetically depending on the local environment. Venkatesan et al. [24] present clear evidence for the latter, which implies that the magnetism is likely to be macroscopically homogenous.

Carrier-mediated nonlocal ferromagnetic coupling between local magnetic polarons in Fe-doped In2O3and Co-doped ZnO

“…We show that samples lose their magnetism after the number of oxygen vacancies are reduced by annealing in air and this occurs even when the samples still show metallic conductivity due to ionized donors. This contrast with earlier studies in which both the magnetism and the metallic conductivity vanished after annealing in air 27,28 .These findings help to enrich our understanding of the microscopic origin of ferromagnetism 5 in oxides DMSs.…”

Reversible ferromagnetic switching in ZnO:(Co, Mn) powders

“…For instance, it has been shown that the ferromagnetism of both epitaxial 4,12 and nanoparticle thin films 13,4 of Co:ZnO can be reinforced by exposing them to Zn vapor, which was recently shown to generate zinc interstitials ͑Zn i ͒ n-type defects. 14 On the other hand, fast annealings of Mn:ZnO nanocrystals capped with N-rich molecules 4 have been shown to enhance their ferromagnetism.…”

Advantageous use of metallic cobalt in the target for pulsed laser deposition of cobalt-doped ZnO films

“…In these n-type samples, the magnetization decreases sharply as oxygen is added. 4,[8][9][10] An undoped ZnO film has Zn interstitials and O vacancies when oxygen deficient and Zn vacancies and O interstitials when grown with excess oxygen. The magnetism in the films with excess oxygen is due to Zn vacancies.…”

“…These include the stoichiometric transfer of material from the target to the film and the control of oxygenation during the film growth. A general trend for Co-doped ZnO has been to find that a deficiency of oxygen in the growth chamber enhances the ferromagnetism through the formation of donor states [4][5][6][7] and that annealing films in air or oxygen quenches any magnetization [8][9][10] although there are also a few conflicting results in which magnetism is observed in ZnO oxygenated films with and without doping with Co. 11,12 The role of metallic Co precipitates is less clear, with some authors claiming that they are responsible for all of the observed magnetic properties [13][14][15] ; however, other authors have identified ferromagnetic phases in ZnCoO films that lack any metallic component. 16,17 The PLD growth of ZnCoO starts from a target that has been made to the desired composition by a solid-state reaction of a Co-containing compound and zinc oxide.…”