Correlation between structures and magnetism in transition metal implanted ZnO
Key researcher: S. Zhou (s(dot)zhou(at)hzdr.de)
Dietl et al. [Science 287, 1019 (2000)] proposed the mean-field Zener model to understand the ferromagnetism in DMS materials. It has been successfully applied in (Ga,Mn)As and (Zn,Mn)Te materials. This model predicts that p-type ZnO or GaN doped with Mn can exhibit critical temperatures above 300 K. With this prediction, the mining for the room temperature DMS based on ZnO and GaN has been drastically boosted. However, the origin for the reported ferromagnetism in ZnO-based DMS remains questionable. Possible options are diluted magnetic semiconductors, spinodal decomposition, or secondary phases. This can only be answered by a careful correlation of the measured magnetic properties with analysis methods that are capable of detecting precipitates and preferably at nanometer scales. For this purpose, we have performed a thorough characterization of the structural and magnetic properties of transition metal implanted ZnO as well as Ge single crystals. Our investigation reveals the necessarity of using synchrotron radiation as the light source for X-ray diffraction characterization of DMS materials. We found that
- Fe, Co and Ni nanocrystals can form upon ion implantation into ZnO bulk crystals. Those nanocrystals can be ferromagnetic even above room temperature. More importantly, Fe nanocrystals are only detectable using SR-XRD, while below the detection limit of the conventional lab-based XRD.
- Upon different thermal process, magnetic nanocrystals in the ZnO matrix undergo a phase transformation. Fe nanocrystals can grow, oxidize and form ferrites by post annealing, while Co nanocrystals can be transformed from hcp to fcc phases. Depending on the crystalline structure, the magnetic nanocrystals can be crystallographically oriented within the ZnO matrix.
Fig. 1 (Left) shows the XRD results for Fe implanted ZnO single crystals. The Fe concentration is around 5 at.%. By conventional XRD one cannot detect any secondary phases while using synchrotron XRD one clearly observes the bcc-Fe(110) peak. The bcc-Fe nanocrystals are responsible for the observed ferromagnetism. Figure is from Appl. Phys. Lett. 88, 052508 (2006).
(1) Crystallographically oriented Co and Ni nanocrystals inside ZnO formed by ion implantation and post-annealing
S. Zhou, K. Potzger, J. von Borany, R. Grötzschel, W. Skorupa, M. Helm, J. Fassbender
Phys. Rev. B 77, 035209 (2008) and Virt. J. Nanoscale Sci. and Technol., Feb. 2008.
(2) Using x-ray diffraction to identify precipitates in transition metal doped semiconductors
Shengqiang Zhou, K. Potzger, G. Talut, J. von Borany, W. Skorupa, M. Helm, and J. Fassbender,
J. Appl. Phys. 103, 07D530 (2008).
(3) Crystallographically oriented magnetic ZnFe2O4 nanoparticles synthesized by Fe implantation into ZnO
Shengqiang Zhou, K. Potzger, H. Reuther, G. Talut, F. Eichhorn, J. von Borany, W. Skorupa, M. Helm, J. Fassbender
J. Phys. D: Appl. Phys, 40, 964 (2007)
(4) Fe implanted ferromagnetic ZnO
K. Potzger, Shengqiang Zhou, H. Reuther, A. Mücklich, F. Eichhorn, N. Schell, W. Skorupa, M. Helm, and J. Fassbender, T. Herrmannsdörfer and T. P. Papageorgiou
Appl. Phys. Lett. 88, 052508 (2006).