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Based on a combination of many-body potentials, an analysis of the inertia tensors and a Density Functional Theory framework, we use a method to harvest the lowest energy states of any set of cluster systems. Then, this methodology is applied to the Pt6Cu6 cluster case and the structural, chemical, electronic, anisotropy, magnetic and vibrational properties of the lowest energy isomers are studied. Unexpectedly, some tens of isomers with much lower energy than the precedent believed ground state [J. Chem. Phys., 131(4):044701] are found, which indicates the goodness of this methodology. Some of the isomers obtained present the point groups Cs, C2v according to Schoenflies notation, while others do not exhibit specific symmetry operations. The global chemical descriptors as the ionization potential, the electron affinity and the chemical hardness have oscillating behaviors with overall decreasing trends as the energy of the isomer grows up, indicating a higher rate of deactivation by sintering processes and a higher strength of the adsorption of small molecules on these systems. We present interesting results of the electronic, magnetic, anisotropy, vibrational and thermal properties of these clusters and discuss them; what can be useful information for future experiments and technical applications in varied fields as catalysis, spintronics, molecular magnetism or magnetic storage information.
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A classical interatomic potential for iron/iron-fluoride systems is developed in the framework of the charge optimized many-body (COMB) potential. This interatomic potential takes into consideration the effects of charge transfer and many-body interactions depending on the chemical environment. The potential is fitted to a training set composed of both experimental and ab initio results of the cohesive energies of several Fe and FeF2 crystal phases, the two fluorine molecules F2 and the F2-1 dissociation energy curve, the Fe and FeF2 lattice parameters of the ground state crystalline phase, and the elastic constants of the body centered cubic Fe structure. The potential is tested in an NVT ensemble for different initial structural configurations as the crystal ground state phases, F2 molecules, iron clusters, and iron nanospheres. In particular, we model the FeF2/Fe bilayer and multilayer interfaces, as well as a system of square FeF2 nanowires immersed in an iron solid. It has been shown that there exists a reordering of the atomic positions for F and Fe atoms at the interface zone; this rearrangement leads to an increase in the charge transfer among the atoms that make the interface and put forward a possible mechanism of the exchange bias origin based on asymmetric electric charge transfer in the different spin channels.
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Interest in low dimensional magnetic systems has been growing due to the novel and dramatically differentiated effects of their physical properties, which give them special behaviors and uses in biomedical, environmental and technological fields. In this study we report extensive first-principles calculations on the geometric optimization as well as electronic, magnetic, mechanical and thermal properties of several quasi one-dimensional core/shell nanowires: Cu/Fe3O4, Co/Fe3O4, and CoO/Fe3O4. The main focus lies on the quantum confinement effects as well as on the effect of the interaction between the ferrimagnetic semiconductor shell material (magnetite nanotube) and core compounds with differentiated magnetic behavior such as (i) a ferromagnetic material (Co), (ii) an antiferromagnetic transition metal oxide (CoO) and (iii) a non-magnetic simple metal (Cu). The mechanical properties of the related nanosystems are studied through the effects of axial deformations, and their thermal behavior is evaluated by considering the electronic contribution of each sample to the heat capacity, and some potential technological applications are suggested.
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Along with the growing precision in the control of matter at increasingly smaller size scales, a field of research, based onto magnetic materials of technical interest, such as bimetallic clusters, has been developed in very recent years. Thereby, here, we report on a complete study of bimetallic clusters composed of cobalt and nickel with up to 7 atoms using ab initio methods in the GGA approach. We applied an unrestricted search method based on the tensor of inertia eigenvalues to find the most stable configurations of the clusters, obtaining a diverse set of structures with different geometric properties. We explored the effect of composition on the structural properties, the chemical stability, the magnetization and the magnetic anisotropy energy (MAE) of the so-obtained systems. Our results indicate that the behavior of the clusters is mainly governed by the Co-Co interaction and to a lesser extent by the Co-Ni and Ni-Ni interactions. Furthermore, for a given cluster size the magnetic moment increases by 2 µB/Co-substitution plus 1 µB/Ni-substitution coming from the cobalt and nickel core d-states, while in some cases unpaired hybrid s-electrons can also give rise to itinerant magnetism. These features have been analyzed with the help of a Jellium model and have important consequences for the magnetism and the magnetic anisotropy of the clusters. The magnetic behavior and MAE present complex and intriguing landscapes, which suggests the possibility of finely controlling the magnetic states, by tuning the cluster composition, aiming at technical implementation in fields such as molecular magnetism or quantum computation. In particular, cases such as Co6Ni, Co4Ni3, CoNi6 Co2Ni5 and Co3Ni3 present high relative stability and enhanced magnetic moments (around 10 µB), what makes them promising candidates for applications such as subnanometer magnetic information storage.
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By means of Monte Carlo simulations we studied field driven nucleation and propagation of transverse domain walls (DWs) in magnetic nanowires subjected to temperature gradients. Simulations identified the existence of critical thermal gradients that allow the existence of reversal processes driven by a single DW. Critical thermal gradients depend on external parameters such as temperature, magnetic field and wire length, and can be experimentally obtained through the measurement of the mean velocity of the magnetization reversal as a function of the temperature gradient. Our results show that temperature gradients provide a high degree of control over DW propagation, which is of great importance for technological applications.
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In recent years, construction and characterization of core-shell structures have attracted great attention because of their unique functional properties and their integration into technological devices. However, some aspects of their basic physics still remain to be explored. In this study, we report on an extensive hierarchical multiscale modeling methodology applied to Fe-Ni core/shell nanostructures of technological interest. As a first step, supported on a first-principles study, we develop a methodology to compute primordial but unprecedented parameters such as the exchange coupling and the equilibrium bond distances at the interface, namely JFe-Ni = 35.48 meV and d = 2.5 Å. This methodology can be used for computing fundamental parameters in mixed systems by knowing the parameters in the bulk samples, and the so-obtained results can be used in higher size scale simulations. As a proof, the results obtained are used as input parameters for atomistic simulations on Fe-Ni samples made out of a Fe core surrounded by a Ni shell whose external diameter varies finely in the range 60-110 nm. The inner diameter and height are fixed to be 40 and 50 nm, respectively. We address the structural, electronic, static magnetic and hysteresis properties of the Fe-Ni core/shell cylindrical nanostructures in different size ranges. These nanostructures reveal different magnetic properties with novel complex states, which are studied in detail.
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In this work we report a complete structural and magnetic characterization of crystalline MnF2 under pressure obtained using first principle calculations. Density functional theory was used as the theoretical framework, within the generalized gradient approximation plus the Hubbard formalism (GGA+U) necessary to describe the strong correlations present in this material. The vibrational, the magnetic exchange couplings and the structural characterization of MnF2 in the rutile ground state structure and potential high pressure phases are reported. The quasiharmonic approximation has been used to obtain the free energy, which at the same time is used to evaluate the different structural transitions at 300 K. Based on previous theoretical and experimental studies on AF2 compounds, ten different structural candidates were considered for the high pressure regime, which led us to propose a path for the MnF2 structural transitions under pressure. As experimental pressure settings can lead to non-hydrostatic conditions, we consider hydrostatic and non-hydrostatic strains in our calculations. According to our results we found the following sequence for the pressure-driven structural phase transition in MnF2: rutile (P42/mnm) â α-PbO2-type (Pbcn) â dist. HP PdF2-type (Pbca) â dist. fluorite (I4/mmm) â cotunnite (Pnma). This structural path is correlated with other phase transitions reported on other metal rutile fluorides. In particular, we found that our proposed structural phase transition sequence offers an explanation of the different paths observed in the literature by taking into account the role of the hydrostatic conditions. In order to get a deep understanding of the modifications of MnF2 under pressure, we have analyzed the pressure evolution of the structural, vibrational, electronic, and magnetic properties for rutile and for each of the high pressure phases.
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Accurate first-principle calculations on bimetallic cobalt-copper clusters of up to six atoms (Pérez et al 2012 J. Nanopart. Res. 14 933) revealed a close similarity of the ground-state magnetic properties to the ultimate jellium model, provided that a 2D to 3D geometric transition was invoked. We discuss this relationship in terms of partial occupancies of the valence electrons in both cases, with the jellium results described by nonperturbative spherical wavefunctions. Based upon this, we propose a scheme to predict magnetic properties of cobalt-copper clusters of up to twenty atoms using arguments of dimensionality and charge localization, and confirm some of these results with other independent density-functional calculations and experimental available data. The comparison with experiments is carried out for neutral and singly ionized cobalt clusters. Furthermore, a many-body tight-binding pseudopotential is used with Monte Carlo techniques to verify the stability of these new first-principle solutions.