Polycrystalline Zn1−xCoxO diluted magnetic semiconductors have been prepared by coprecipitation technique in the concentration range 0⩽x⩽0.1. Structure, composition analysis, and optical absorption measurements revealed that cobalt is incorporated into the lattice, as Co2+ substituting Zn2+ ions, forming a solid solution with wurtzite structure instead of Co precipitates. Room- and low-temperature magnetization measurements reveal a paramagnetic behavior for the Co-doped ZnO samples with a paramagnetic Co amount smaller than the nominal concentration. χT versus T evidenced that the remaining Co is antiferromagnetically coupled through oxygen. This is further supported by a simple model that shows that as the Co concentration increases the amount of nearest neighbors Co atoms increases thus giving antiferromagnetic coupling and reducing the paramagnetic contribution.

We acknowledge the support of the French Agence Nationale de la Recherche (ANR) under the reference ANR-09-BLAN-0187-03.

We have used the coprecipitation technique to synthesize polycrystalline Co-doped SnO 2 diluted magnetic semiconductors with Co concentrations of 0, 0.5, and 2.0%. X-ray diffraction patterns showed for all samples the expected SnO 2 tetragonal structure with no additional peaks corresponding to parasitic phases. Transmission electron microscopy (TEM) did not indicate the presence of magnetic parasitic phases and confirmed that Co ions are uniformly distributed inside the samples. Optical absorption measurements showed an energy band gap which decreases when increasing the Co concentration. Raman spectroscopy shows an intensity loss of classical cassiterite SnO 2 vibration lines, which is an indication of significant structural modifications and disorder of the SnO 2 lattice. Magnetization measurements revealed a mixture of paramagnetic and antiferromagnetic behavior for Co-doped SnO 2 with no sign of ferromagnetism.

Using the spectroscopies based upon x-ray absorption, we have studied the structural and magnetic properties of Zn1−xCoxO films (x = 0.1 and 0.25) produced by reactive magnetron sputtering. These films show ferromagnetism with a Curie temperature TC above room temperature in bulk magnetization measurements. Our results show that the Co atoms are in a divalent state and in tetrahedral coordination, thus substituting Zn in the wurtzite-type structure of ZnO. However, x-ray magnetic circular dichroism at the Co L2,3 edges reveals that the Co 3d sublattice is paramagnetic at all temperatures down to 2 K, both at the surface and in the bulk of the films. The Co 3d magnetic moment at room temperature is considerably smaller than that inferred from bulk magnetisation measurements, suggesting that the Co 3d electrons are not directly at the origin of the observed ferromagnetism.PACS numbers: 75.50. Dd, 75.30.Hx, 61.10.Ht Among the most investigated topics in the field of spin electronics, dilute magnetic semiconductors (DMSs) occupy a prominent position, because they would allow one to exploit efficiently the spin and the charge of the electrons in the same device. In fact electronic devices have been working for decades omitting the spin of the electron. In 1990 Datta and Das proposed a new magnetoelectronic device (a field effect transistor), 1 whose practical realisation has been hindered by the weak spin injection efficiency from a ferromagnet to a semiconductor. A ferromagnetic semiconductor would constitute therefore an alternative route towards the efficient spin injection into normal semiconductors. Up to very recently, however, the main concern was related to the low Curie temperature of the known DMSs, which is well below room temperature and precludes therefore potential applications.2 A significant breakthrough was achieved recently, since room temperature ferromagnetism was predicted 3 and observed 4,5,6,7,8,9 for semiconductors such as GaN and ZnO doped with Co, Mn or other transition metals. However, many reports remain controversial and the nature of the magnetic coupling has not been revealed yet. In fact, the original prediction of high T C ferromagnetism in these systems by Dietl et al.3 lies on the assumption that they can be properly doped with p-type carriers, which would mediate the magnetic interactions. However, in order to account for the numerous experimental observations of ferromagnetism in n-type ZnO, alternative models have been proposed, which are mainly relying on the presence of defects (like for example vacancies or interstitials).10,11 In most of these models, the presence of a magnetic impurity such as Co or Mn is a necessary ingredient for the appearance of ferromagnetism, but other models show that this might not be needed and that ferromagnetism can appear also in undoped oxides.12 To date, however, there has not been any clear experimental proof of the validity of any of these models and of the role of the magnetic dopants.In order to tackle these problems, we have performed e...

The electronic structure of Zn 1−x Co x O ͑x = 0.02, 0.06, and 0.10͒ diluted magnetic semiconductors is investigated using soft x-ray emission spectroscopy and first-principles calculations. X-ray absorption and emission measurements reveal that most Co dopants are incorporated at the Zn sites and that free charge carriers are absent over a wide range of Co concentrations. The excess Co interstitials appear in the samples with high Co concentration ͑10 at. %͒ but are isolated without any direct exchange interaction with substitutional Co atoms. The lack of free charge carriers and the direct Co-Co interactions is responsible for the absence of ferromagnetism in the samples. First-principles calculations suggest that the exchange interaction between substitutional Co atoms induces only an antiferromagnetic coupling, and strong ferromagnetism in Co-doped ZnO requires not only free charge carriers but also the Co interstitials directly interacting with substitutional Co atoms.

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We report on the origin of ferromagnetism in ZnO and Co-doped ZnO magnetic semiconductor films prepared by sol-gel technique on Si(100) and SiO2 substrates. X-ray diffraction measurements have shown that ZnO and Co-doped ZnO have the expected wurtzite structure and a preferential orientation along the c axis. Optical transmittance measurements revealed that Co is incorporated into the lattice, as Co2+ substituting Zn2+ ions. Magnetization measurements have shown room-temperature ferromagnetism for both ZnO and ZnO:Co films. After annealing, the magnetization curves recorded on free substrates show the same ferromagnetic behavior as the ZnO:Co film indicating that the ferromagnetism is extrinsic and due to external pollution.

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