Dilute magnetic semiconductors are envisioned as sources of spin-polarised carriers for future semiconductor devices which simultaneously utilise spin and charge degrees of freedom. Zn1–xCoxO (Co:ZnO) was thought a promising n-type dilute magnetic semiconductor for spintronics. Although Co:ZnO materials have been investigated intensively, their respective magnetic behaviours remain controversial from experimental and theoretical points of view. Claims of room-temperature ferromagnetism, or its absence, have been made by experimentalists and theoreticians [1]. Thus, the possibility that metallic Co nanoclusters account for the observed magnetic order has to be ruled out with great care.

Co:ZnO with 10.8% Co was grown by pulsed laser deposition onto c-plane sapphire substrates [1], and studied with SQUID magnetometry and synchrotron radiation at the Zn and Co K-edges. X-ray absorption near-edge spectra (XANES) were recorded at beamline ID12 in total fluorescence yield both with linear and circular polarised light under 10° grazing incidence. To record the X-ray linear dichroism (XLD) the polarisation was flipped using a quarter wave plate. The X-ray magnetic circular dichroism (XMCD) was recorded by reversing the circular polarisation as well as the magnetic field direction.

Figure 102 shows the XANES and their respective XLD spectra recorded at the Zn and Co K-edges using linear polarised light. The XLD spectra show features typical for the wurtzite lattice of the ZnO host. In addition, simulations of the XANES and XLD spectra were made using FDMNES [2], and the lattice parameters of ZnO for a Co:ZnO supercell with Co on Zn substitutional sites. The good agreement between experiment and simulation suggests that more than 95% of the Co dopant atoms are located on Zn substitutional sites. This result underlines the excellent structural quality of the Co:ZnO, devoid of metallic Co, small Co clusters or antisite disorder.

Fig. 102: XANES spectra at the Zn and Co K-edges recorded with two orthogonal linear polarisations and the respective XLD spectra. Simulations using the FDMNES code are also shown.

Figure 103 summarises the magnetic properties of the Co:ZnO sample. M(H)-curves with H perpendicular to the c-axis were recorded using SQUID (full symbols) and XMCD at the pre-edge feature of the Co K-edge as indicated in the inset. This pre-edge feature is characteristic for Co2+ in tetrahedral coordination [1] confirming the XLD results. The XMCD data were scaled to the SQUID data and both techniques consistently show paramagnetic behaviour with a comparable curvature of M(H). Three different fits using Brillouin functions for S = 3/2 and various L are scaled to the SQUID data. L = 3 reflects the maximum possible value of the magnetic moment for Co2+ (3d7) resulting in too strong a curvature, whereas the L = 0 fit has too weak a curvature to reproduce the data. Using L = 1.07, (µz = 4.1 µB/Co atom), which results in an L/S-value of 0.7 that was already reported by means of XMCD data recorded at the Co L3/2-edges, the curvature of the M(H) curves can be reproduced well. However, if one calculates the effective magnetic moment per Co atom from the measured magnetisation by SQUID, one finds only 1.44 µB/Co atom which results in a linear M(H)-curve using the respective Brillouin function in disagreement with the data. In other words, only 28(3)% of the magnetisation that can be expected from the curvature of M(H), are measured by SQUID. If one calculates the abundance of isolated Co atoms, Co-O-Co pairs, triples etc on Zn cation sites, i.e. an hcp sublattice with 12 next neighbours [3], this results in 28% of isolated Co atoms, 18% of pairs, 11% of triples etc. for 10.8% of Co in ZnO. Therefore, the magnetisation measured by SQUID can be readily explained by a paramagnetic behaviour of the isolated Co atoms, an antiparallel alignment of the Co-O-Co pairs and negligible contributions from larger cation clusters.

Fig. 103: M(H)-curves recorded at 5 K by SQUID and XMCD at the Co K-edge, respectively. The inset shows the respective XMCD spectrum. Lines are Brillouin fits (see text).

In conclusion we were able to demonstrate by means of XLD that the Co dopant atoms occupy exclusively substitutional Zn sites in high-quality Co:ZnO samples. In such samples, the isolated Co atoms behave as paramagnetic whereas the Co-O-Co pairs couple antiferromagnetically. Thus we find no signs of intrinsic ferromagnetic interactions.


Principal publication and authors

A. Ney (a), K. Ollefs (a), S. Ye (a), T. Kammermeier (a), V. Ney (a), T.C. Kaspar (b), S.A. Chambers (b), F. Wilhelm (c), and A. Rogalev (c), Phys. Rev. Lett. 100, 157201 (2008).
(a) Experimentalphysik, Universität Duisburg-Essen, Duisburg (Germany)
(b) Pacific Northwest National Laboratory, Richland, Washington, (USA)
(c) ESRF


[1] T.C. Kaspar, T. Droubay, S.M. Heald, P. Nachimuthu, C.M. Wang, V. Shutthanandan, C.A. Johnson, D.R. Gamelin, and S.A. Chambers, New J. Phys. 10, 055010 (2008).
[2] Y. Joly, Phys. Rev. B 63, 125120 (2001).
[3] R.E. Behringer, J. Chem. Phys. 29, 537 (1958).