Defect induced low temperature ferromagnetism in Zn1−xCoxO films

Abstract

We present a detailed study on the structural, magnetic, and optical properties, as well as the electronic structure of epitaxial Co-doped ZnO films prepared by magnetron sputtering. Different preparation conditions were implemented in order to control the concentration of oxygen vacancies in the ZnO host lattice. Magnetization measurements indicate ferromagnetic behavior at low temperature for samples prepared at oxygen-poor conditions whereas the samples prepared at oxygen-rich conditions show extremely small ferromagnetic signal corroborating that ferromagnetism in Zn 1−x Co x O correlates with the presence of the oxygen-related defects. X-ray absorption spectroscopy ͑XAS͒ at the Co L 2, 3 edge together with optical transmittance measurements show that Co ions are present in the high-spin Co 2+ ͑d 7 ͒ state under tetrahedral symmetry indicating a proper incorporation in the ZnO host lattice. Comparison of the O K edge XAS spectra of the samples prepared at different conditions show substantial changes in the spectral line shape which are attributed to the presence of lattice defects such as oxygen vacancies in the ferromagnetic oxygen-poor Co-doped ZnO samples. Our findings indicate that the ferromagnetic properties of Co-doped ZnO samples are strongly correlated with the presence of oxygen vacancies in the ZnO lattice supporting the spin-split impurity band model.

X-Ray Spectroscopic Investigations of Zn$_{0.94}$Co$_{0.06}$O Thin Films

“…The spectrum agrees well with those from Krishnamurthy et al [33] and Chang et al [34] measured at undoped ZnO. No visible change of the spectral features between 530 and 544 eV due to the presence of oxygen vacancies [16], [33], [34] can be observed, which leads to the conclusion that an effective introduction of oxygen vacancies has not been achieved. XMCD measurements by Tietze et al [31] also showed that oxygen itself is not the origin of ferromagnetism in Co-doped ZnO.…”

“…The interpretation is complicated due to spreading results, as not only ferromagnetism, but also paramagnetism [10], [11] and antiferromagnetism [12] were found. The reports on ferromagnetic behavior range from strong RT ferromagnetism [13]- [15] to weak ferromagnetism only at low temperatures [16], [17]. Several groups even found ferromagnetic behavior with a superconducting quantum interference device (SQUID), while X-ray magnetic circular dichroism (XMCD) showed paramagnetic behavior of the Co atoms [18], [19].…”

Defect-trapped electrons and ferromagnetic exchange in ZnO

“…2,[32][33][34] A strong correlation has been established between vacuum annealing of samples and ferromagnetism in films. This has been reported in Zn 1-x Co x O, 3,11,[14][15][16]34,35 Zn 1-x Mn x O, 36 ZnO, 36 TiO 2 , 12,13 and HfO 2 . 10 In one case, 15 cycling between ferromagnetic and non-magnetic states by vacuum or air annealing was clearly demonstrated.…”

The local structure of Co-doped ZnO films studied by X-ray absorption spectroscopy

“…Further more, Dietl and Sato predicted that above room-temperature ferromagnetism could be realized by doping a few percentages of Co or Mn magnetic ions [6,7]. Until now, much attention has been paid to the magnetic properties of ZnO-based DMS materials fabricated by a variety of method [8][9][10][11][12]. However, the magnetic properties still remain an issue since the observed magnetism ranging from paramagnetic, spinglass and antiferromagnetic behavior to ferromagnetism below or even at high temperature [13][14][15][16][17].…”