Anisotropic Ferromagnetism in Substituted Zinc Oxide

Abstract

Abstract. Room-temperature ferromagnetism is observed in (110) oriented ZnO films containing 5 at % of Sc, Ti, V, Fe, Co or Ni, but not Cr, Mn or Cu ions. There are large moments, 1.9 and 0.5µ B /atom for Co-and Ti-substituted oxides, respectively. Scsubstituted ZnO shows also a moment of 0.3 µ B /Sc. Magnetization is very anisotropic, with variations of up to a factor three depending on the orientation of the applied field relative to the R-cut sapphire substrates. Results are interpreted in terms of a spin-split donor impurity band model, which can account for ferromagnetism in insulating or conducting high-k oxides with concentrations of magnetic ions that lie far below the percolation threshold. The variation of the ferromagnetism with oxygen pressure used during film growth is evidence of a link between ferromagnetism and defect concentration.PACS Numbers: 75.50.Pp; 75.30.Hx;75.30.Gw;75.70 [2][3][4][5][6] or another transition element [7][8][9][10]. The results are sensitive to the form of the sample and preparation method. Other studies found lower magnetic ordering temperatures [11][12][13][14], or no ferromagnetism at all above 3 K for any 3d dopant [15]. In the absence of an exchange mechanism which could account for a high Curie temperature at doping levels far below the percolation threshold, these reports have been received with skepticism, and the belief that the ferromagnetism must somehow be associated with clustering or incipient formation of secondary phases. But there is spectroscopic evidence that divalent cobalt does indeed substitute on the tetrahedral sites of the wurtzite structure [1,11,17], with a wide solid solubility range [15]. A search by Rode et al [4] revealed no evidence for phase segregation in Co-doped ZnO films.Nevertheless, until a clear connection between the magnetic properties and electronic structure can be shown, doubts that doped zinc oxide is truly a magnetic semiconductor will persist.We recently proposed a model for high-temperature ferromagnetism in dilute n-type

Role of defects in ferromagnetism inZn1−xCoxO: A hybrid density-functional study

“…Experiment 4 shows that ferromagnetism of Zn 1−x Co x O in n-type ZnO is destroyed by annealing in oxygen. Spinless V O and V O 2+ defects do not provide the necessary magnetic coupling of transition-metal ions; since there is evidence that V O + spontaneously disproportionates into these spinless vacancies from several theoretical studies, 36,51 including this work, it seems unlikely that simple O vacancies are the promoters of ferromagnetism in Zn 1−x Co x O unless magnetic V O + or similar defects are stabilized by some mechanism.…”

“…Venkatesan et al have shown that the magnetic moment per Co ion in Zn 1−x Co x O at room temperature is reduced from 2.8 B per Co ion to zero as the oxygen pressure is varied during annealing. 4 Here we report results of all-electron B3LYP ͑Refs. [15][16][17][18][19] and two of these 16,18 have considered the role of defects in promoting ferromagnetism.…”

“…[2][3][4][5][6] Electron paramagnetic resonance ͑EPR͒ measurements show that it is possible to produce Zn 1−x Co x O with x ഛ 0.1 ͑Refs. 7-9͒ in which Co is substituted for Zn rather than forming clusters.…”

“…Epitaxial growth of thin films with ͑110͒ orientation has been achieved, 12 and there is considerable anisotropy of the magnetism in the thin film. 4 The remainder of this paper is organized as follows: Details of calculations are described in Sec. II; results of B3LYP calculations on the defects mentioned above, Zn 1−x Co x O as well as two Co ions in the same unit cell as V O , Zn i or N O and two V O vacancies in the same supercell as a V Zn vacancy, are described in Sec.…”

Structure and magnetic properties of Mn-doped ZnO thin films

“…Carrier-mediated ferromagnetism was initially proposed, and it is believed that p-type conductivity favor s this mechanism [1,2]. An alternative mechanism is the "F-center" model, in which the oxygen vacancy plays an important role [14,15]. In summary, we have systemically studied the structure and magnetic properties of …”