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...

We present an inelastic neutron scattering investigation of Li2CuO2 detecting the long sought quasi-1D magnetic excitations with a large dispersion along the CuO2-chains studied up to 25 meV. The total dispersion is governed by a surprisingly large ferromagnetic (FM) nearestneighbor exchange integral J1 = −228 K. An anomalous quartic dispersion near the zone center and a pronounced minimum near (0,0.11,0.5) r.l.u. (corresponding to a spiral excitation with a pitch angle about 41 •) point to the vicinity of a 3D FM-spiral critical point. The leading exchange couplings are obtained applying standard linear spin-wave theory. The 2 nd neighbor inter-chain interaction suppresses a spiral state and drives the FM in-chain ordering below the Néel temperature. The obtained exchange parameters are in agreement with the results for a realistic five-band extended Hubbard Cu 3d O 2p model and LSDA+U predictions.

The two-magnon problem for the frustrated XXZ spin-1/2 Heisenberg Hamiltonian and external magnetic fields exceeding the saturation field B s is considered. We show that the problem can be exactly mapped onto an effective tight-binding impurity problem. It allows to obtain explicit exact expressions for the two-magnon Green's functions for arbitrary dimension and number of interactions. We apply this theory to a quasi-one dimensional helimagnet with ferromagnetic nearest neighbor J 1 < 0 and antiferromagnetic next nearest neighbor J 2 > 0 interactions. An outstanding feature of the excitation spectrum is the existence of two-magnon bound states. This leads to deviations of the saturation field B s from its classical value B cl s which coincides with the one-magnon instability. For the refined frustration ratio |J 2 /J 1 | > 0.374661 the minimum of the two-magnon spectrum occurs at the boundary of the Brillouin zone. Based on the two-magnon approach, we propose general analytic expressions for the saturation field B s , confirming known previous results for one-dimensional isotropic systems, but explore also the role of interchain and long-ranged intrachain interactions as well as of the exchange anisotropy.