Granular magnetic ZnO: structure, magnetism and transport properties

Transition metal (TM) doped ZnO has been extensively investigated due to its potential application as a diluted magnetic semiconductor with Curie temperature above room temperature (RT). After one decade effort, however the research community realized that (i) ZnO diluted with TM ions only shows paramagnetism [1], and (ii) the observed ferromagnetic signal mostly originates from secondary phases [2,3]. The aim of our research is now to investigate the application potential of granular structures which are created by TM ion implantation into ZnO single crystals. By varying implantation and post-annealing temperatures, we can control the chemical state of TM ions. ZnO with dispersed TM ions can be obtained by ion implantation at temperatures below RT or by using defective ZnO substrates. In this case, TM ions are in ionic states, and only show paramagnetism. Concerning the nature of phase separation, three regimes have been established. (I) ZnO embedded with TM nanocrystals can be obtained by ion implantation at elevated temperatures (e.g. 350oC) and by post-annealing at mild temperatures (below 350oC). In this regime, TM ions are mostly in metallic states (i.e. Fe, Co, Ni). Co and Ni nanocrystals have crystallographic orientation relationship with the ZnO matrix [2]. (II) ZnO embedded with nanocrystalline spinel ferrites AFe2O4 (A=Zn, Co, Ni) can be obtained by co-implantation pulsing post-annealing at 800 oC [4,5]. (III) ZnO embedded with disordered nanosized regions can be obtained by ion implantation with very large ion-fluences. The heavily disordered nanosized regions consists of large Co concentration [6]. Although ferromagnetism has been observed in all the three regimes, magneto-transport properties are drastically different. Only ordinary magneto-resistance (MR) has been observed in regimes I and II, while the samples in regime III reveal negative MR and anomalous Hall effect simultaneously. The anomalous Hall resistivity is saturated at low field giving hope for applicability in spintronics.

[1] A. Ney, , et al., Phys. Rev. Lett. 100, 157201 (2008)
[2] S. Zhou, et al., Phys. Rev. B 77, 035209 (2008).
[3] K. Potzger, and S. Zhou, phys. stat. sol. (a), submitted (2008).
[4] S. Zhou, et al., J. Phy. D-Appl. Phys., 40, 964 (2007).
[5] S. Zhou, et al., Phys. Rev. B, submitted (2008).
[6] K. Potzger, et al., Appl. Phys. Lett., submitted (2008).