The structural, electronic and magnetic properties of Con clusters (n =2−20) have been investigated using density functional theory within the pseudopotential plane wave method. An unusual hexagonal growth pattern has been observed in the intermediate size range, n =15−20. The cobalt atoms are ferromagnetically ordered and the calculated magnetic moments are found to be higher than that of corresponding hcp bulk value, which are in good agreement with the recent SternGerlach experiments. The average coordination number is found to dominate over the average bond length to determine the effective hybridization and consequently the cluster magnetic moment.

In this paper we have obtained the exact eigenstates of a two dimensional damped harmonic oscillator in the presence of an external magnetic field varying with respect to time in time dependent noncommutative space. It has been observed that for some specific choices of the damping factor, the time dependent frequency of the oscillator and the time dependent external magnetic field, there exists interesting solutions of the time dependent noncommutative parameters following from the solutions of the Ermakov-Pinney equation. Further, these solutions enable us to get exact analytic forms for the phase which relates the eigenstates of the Hamiltonian with the eigenstates of the Lewis invariant. Then we compute the expectation value of the Hamiltonian.The expectation values of the energy are found to vary with time for different solutions of the Ermakov-Pinney equation corresponding to different choices of the damping factor, the time dependent frequency of the oscillator and the time dependent applied magnetic field. We also compare our results with those in the absence of the magnetic field obtained earlier.

We report an unusual evolution of structure and magnetism in stoichiometric MnO clusters based on an extensive and unbiased search through the potential energy surface within density functional theory. The smaller clusters, containing up to five MnO units, adopt two-dimensional structures; and regardless of the size of the cluster, magnetic coupling is found to be antiferromagnetic in contrast to previous theoretical findings. Predicted structure and magnetism are strikingly different from the magnetic core of Mn-based molecular magnets, whereas they were previously argued to be similar. Both of these features are explained through the inherent electronic structures of the clusters.PACS numbers: 36.40.Cg, 71.15.Mb Transition metal oxide, especially MnO 1-5 , clusters have recently attracted extensive multidisciplinary research activity because of their diverse and tunable magnetic and catalytic properties. Generally, as compared to the bulk, the local magnetic moment is enhanced in smaller dimensions due to reduction in the number of neighboring atoms. This results in either an overall enhancement of the total moment for the ferromagnetic (FM) case or lead to a finite moment even for the antiferromagnetic (AFM) case due to unequal compensation of spin up and down electrons. Magnetic coupling also evolves with particle size, and such size evolution for MnO clusters is non-monotonic. MnO clusters with a diameter of 5-10 nm show FM behavior 6 even though the bulk phase is AFM 7 . In contrast, Mn-based single molecular magnets (with magnetic core < 1.5 nm) show a 'layered' AFM/ferrimagnetic structure within the mixedvalent Mn centers, resulting in a large magnetic moment and spin anisotropy 1,2 . Moreover, the MnO clusters take essential part in a variety of biological (catalytic) processes from photosynthesis to bacterially mediated organic matter decomposition. The active inorganic center of the oxygen evolving photosystem II contains a manganese oxide cluster (Mn 4 O 4 Ca), which catalyzes the light-driven oxidation of water 3 . Indeed, synthetic complexes containing cuboidal Mn 4 O 4 cores have been found to exhibit unique reactivity in water oxidation/O 2 evolution 4 .The prediction of geometry at the atomic level is one of the most fundamental challenges in condensed matter science. The magnetic and catalytic properties (i.e., broadly speaking: the electronic structure) are strongly coupled to the 'inherent structure' (corresponding to minima of the potential energy surface (PES)) of the cluster. Experimental evidence of structure and magnetic coupling, and their size evolution for the transition metal oxide clusters in the gas phase are scarce. However, the structure and (FM) magnetism of (MnO) x clusters have been predicted theoretically 8-10 . Such a theoretical prediction is complex, and requires a systematic and rigorous search through the PES. This is essential to predict the deepest minima. The complexity of the PES search increases with increasing cluster size. Possible geometrical structures increas...

Effects of alloying on the electronic and magnetic properties of MnxCoy (x + y=n=2-5; x=0-n) and Mn2Co11 nanoalloy clusters are investigated using the density functional theory (DFT). Unlike the bulk alloy, the Co-rich clusters are found to be ferromagnetic and the magnetic moment increases with Mn-concentration, and is larger than the moment of pure Con clusters of same size. For a particular sized cluster the magnetic moment increases by 2 µB/Mn-substitution, which is found to be independent of the size and composition. All these results are in good agreement with recent Stern-Gerlach (SG) experiments [Phys. Rev. B 75, 014401 (2007) and Phys. Rev. Lett. 98, 113401 (2007)]. Likewise in bulk MnxCo1−x alloy, the local Co-moment decreases with increasing Mn-concentration.

Manipulation of intrinsic magnetic and electronic structures of graphene nanoflakes is of technological importance. Here we carry out systematic study of the magnetic and electronic phases, and its manipulation in graphene nanoflakes employing first-principles calculation. We illustrate the intricate shape and size dependence on the magnetic and electronic properties, and further investigate the effects of carrier doping, which could be tuned by gate voltage. A transition from nonmagnetic to magnetic phase is observed at a critical flake size for the flakes without sublattice imbalance, which we identify to be originated from the armchair defect at the junctions of two sublattices on the edge. Electron, or hole doping simultaneously influences the magnetic and electronic structures, and triggers phase changes. Beyond a critical doping, crossover from antiferromagnetic to ferromagnetic phase is observed for the flakes without sublattice imbalance, while suppression of magnetism, and a possible crossover from magnetic to nonmagnetic phase is observed for flakes with sublattice imbalance. Simultaneous to magnetic phase changes, a semiconductor to (half) metal transition is observed, upon carrier doping.

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