We report on the magnetic properties of thoroughly-characterized Zn1−xCoxO epitaxial thin films, with low Co concentration, x = 0.003 − 0.005. Magnetic and EPR measurements, combined with crystal field theory, reveal that isolated Co 2+ ions in ZnO possess a strong single ion anisotropy which leads to an "easy plane" ferromagnetic state when the ferromagnetic Co-Co interaction is considered. We suggest that the peculiarities of the magnetization process of this state can be viewed as a signature of intrinsic ferromagnetism in ZnO:Co materials.PACS numbers: 71.20. Be, 75.30.Gw, 76.30.Fc Spintronics, an emerging branch of micro-and nanoelectronics which manipulates the electron spin rather than its charge, has need for spin polarization components. In most spintronic devices, metallic ferromagnetic (FM) materials are used to this end. However the physics of metal-semiconductor injection is incompatible with the concept of semiconductor devices, preventing their application [1]. A suitable solution would be a FM semiconductor at room-temperature.The magnetic properties of diluted magnetic semiconductors are due to the substitution of cations by transition-metal (TM) ions, and have been extensively studied for at least five decades [2]. Co-doped ZnO -a possible candidate for high-T c FM semiconductors -has attracted much interest from both theoretical and experimental points of view. Yet, there is an ongoing debate about its magnetic properties. Early theoretical studies using the local spin density approximation (LSDA) for Zn 1−x Co x O found it to be a FM semimetal [3]. Contrary to this, more recent LSDA calculations [4,5] on large supercells detected a competition between FM and antiferromagnetic (AFM) interactions, i.e. an AFM or spinglass groundstate.Experimentally, high-T c FM phases in Zn 1−x Co x O (x = 0.1-0.25) were found in thin films produced by pulsed laser deposition [6], by the sol-gel method [7], and by rf magnetron co-sputtering [8]. They were also found in bulk single crystals prepared by implantation [9]. Controversially, AFM correlations between TM ions and the absence of any FM bulk phases were observed in Zn 1−x Co x O (x = 0.005-0.15, 0.2) samples fabricated by precursor decomposition [4], in polycrystalline powder samples [10] as well as in thin films [11].In this rather contradictory situation a major question which arises is whether a reliable identification of an intrinsic FM phase of ZnO doped by Co is possible at all.Here we address this question on both experimental and theoretical grounds. We argue that such an identification requires a thorough examination of the magnetic properties of Co 2+ ions in the ZnO lattice, and in particular the magnetic anisotropy of cobalt. By EPR and magnetic measurements, we first prove that Co 2+ , which has a spin S = 3/2, shows a huge single ion anisotropy of DS z 2 type, with D = 2.76 cm −1 . We then validate this result theoretically by combining crystal field theory with an estimate of the crystal field parameters. Theory and experiment clearly d...

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In a rather contradictory situation regarding magnetic data on Co-doped ZnO, we have succeeded in fabricating high-quality single crystalline Zn(1-x)Co(x)O (x=0.003-0.07) thin films. This gives us the possibility, for the first time, to examine the intrinsic magnetic properties of ZnO:Co at a quantitative level and therefore to address several unsolved problems, the major one being the nature of the Co-Co interaction in the ZnO structure.