Structural Evidence for Zn Intersititials in Ferromagnetic Zn_1–xCo_xO Films

Abstract

The field of spintronics has attracted significant attention since it promises new semiconductor device functionality through functionalization of both electron charge and spin degrees of freedom. Dilute magnetic semiconductors (DMS) which are formed by partial replacement of cations in a nonmagnetic semiconductor by magnetic transition metal ions are promising materials for spintronic devices since they possess charge and spin degrees of freedom in a single substance. [1] Earlier work on Mn-substituted GaAs demonstrated the potentiality of DMS materials.[2] However, the ferromagnetic transition temperature was low (T c ∼ 150 K). Theoretical predictions [3,4] of room temperature ferromagnetism (FM) in DMS has generated considerable interest in studying transition metal doped ZnO. The origin of FM in these transition metal doped oxides is not yet fully understood. Mechanisms based on carrier-induced ferromagnetism [3,4] and percolation of bound magnetic polarons [5][6][7] have been proposed. Cobaltdoped ZnO is considered to be very promising for DMS. [8][9][10][11][12][13][14][15][16][17] While there have been reports of irreproducibility of sample properties [8] as well as uncertainties about whether Co clusters are responsible for ferromagnetism, [12] there is also very strong evidence for intrinsic, ferromagnetism, and in some cases switchable, in samples made by careful chemical synthesis routes. [18][19][20] The role of defects in mediating ferromagnetism in cobaltdoped ZnO has been demonstrated by several groups. [16,20,21] The defect chemistry of ZnO is very complex, with Zn interstitials, Zn vacancies, oxygen interstitials and vacancies all known to be present in varying concentrations. [22,23] Hence, experiments to change single variables are not trivial. On the other hand, depending on processing conditions, the predominant defects are Zn interstitals and/or oxygen vacancies [24,25] and so from careful annealing studies it should be possible to distinguish the lattice variations arising from these two defect types. This paper reports on the properties of as-grown and postannealed pure ZnO and Co-doped ZnO epitaxial films grown by ultrasonic assisted solution chemical vapour deposition (UASCVD) at low temperatures. Using this method, a wide variety of doping levels are easily explored simply through changing solution composition. We show that after only one short, low temperature anneal of Zn 0.98 Co 0.02 O, M s is reduced, and at the same time so is the 'c' axis lattice parameter. The results can only be explained by a reduction of Zn concentration. Hence, measurement of the 'c' parameter provides a simple diagnostic tool for assessing whether Zn interstitials are present or not. Considering the very wide range of properties of doped-ZnO (e.g., high transparency, piezoelectricity, wide bangap semiconductivity, magnetoresistivity, magnetooptic, electro-optic and chemical sensing properties) and the need to control defects and associated charge carriers in these applications, the result of this work has br...

Enhanced ferromagnetism in single crystalline Co-doped ZnO thin films by Al codoping

“…Theoretically, Dietl et al first predicted that p-type Mn doped ZnO would show ferromagnetism with T C well above RT [3]. In contrast, Co-doped ZnO was theoretically predicated to become ferromagnetic with n-type doping [4] and a flood of experimental studies of this subject have been reported upon [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24]. However, the presented data were plagued by instability and a lack of reproducibility.…”

Antiferromagnetic interaction between paramagnetic Co ions in the diluted magnetic semiconductorZn1−xCoxO

“…21,22 Annealing of Zn 1−x Co x O in reducing atmosphere such as in high vacuum, which may produce oxygen vacancies, have been reported to enhance the ferromagnetism, while annealing in oxidizing atmosphere such as air decreases the magnetic moments of Zn 1−x Co x O. [23][24][25][26] In addition, we note that ferromagnetism has been reported in some oxide thin films without magnetic ions, [27][28][29] probably due to defects such as oxygen vacancies.…”

Comparative Study of Room‐Temperature Ferromagnetism in Cu‐Doped ZnO Nanowires Enhanced by Structural Inhomogeneity

“…As the temperatures goes down from RT, the magnetization of S2 increases continuously, in agreement with the previous results in DMS thin films. [14,37] Cu doping decreases (increases) the mobile carrier concentration (resistivity), [9,15] detrimental to the formation of conventional threedimensional spin wave. [38] At lower temperatures, both samples exhibit the Curie-Weiss type behavior, suggesting the existence of a paramagnetic phase.…”

“…[8] The absence of superparamagnetism also manifests in the resemblance between the ZFC and FC curves. [37] Finally, the anomalous feature near 50 K in the M-T curves likely originates from the magnetic transition of oxygen contaminants. [41] The enhanced RTFM was further confirmed by the room-temperature magnetic force microscopy (MFM) measurements.…”