Ferromagnetic transition metal implanted ZnO: a diluted magnetic semiconductor?

Traditional electronics can be greatly stimulated by a combination of magnetic and semiconducting properties where spins provide an additional degree of freedom. Recently theoretical works predict that some semiconductors (e.g. ZnO) doped with transition metal are diluted magnetic semiconductors (DMS) [1]. In binary DMS materials, transition metal ions substitute cation sites of the host semiconductor and are coupled by free carriers resulting in ferromagnetism. One of the main obstacles in creating DMS materials is the formation of secondary phases because of the solid-solubility limit. Ion implantation is a non-equilibrium method, and has been successfully used to create GaAs-based DMS [2]. In our study transition metal ions were implanted into ZnO single crystals with the peak concentrations of 0.5-10 at.%. We established a correlation between structural and magnetic properties. By synchrotron radiation X-ray diffraction secondary phases (Fe, Ni, Co and ferrite nanocrystals) were observed and have been identified as the source for ferromagnetism [3-6]. Due to their different crystallographic orientation with respect to the host crystal these nanocrystals in some cases are very difficult to be detected by a simple Bragg-Brentano scan. This results in the pitfall of using XRD to exclude secondary phase formation in DMS materials. For comparison, the solubility of Co diluted in ZnO films amounts to 10 at.% using pulsed laser deposition (PLD) [7]. Such diluted, Co-doped ZnO films show paramagnetic behaviour. However, only the magnetoresistance of Co-doped ZnO films reveals possible s-d exchange interaction as compared to Co-implanted ZnO single crystals.
References
[1] T. Dietl, et al., Science 2000; 287:1019.
[2] M. A. Scarpulla, et al, Appl. Phys. Lett. 2003; 82:1251.
[3] K. Potzger, et al., Appl. Phys. Lett. 2006; 88 :052508.
[4] Shengqiang Zhou, et al., J. Appl. Phys. 2008; 103:023902.
[5] Shengqiang Zhou, et al., Phys. Rev. B 2008 ; 77:035209.
[6] Qingyu Xu, et al., Phys. Rev. B 2006; 73:205342.