Atomic clusters provide an ideal system where the evolution of magnetism from atoms to the bulk can be studied. Clusters are characterized by unique geometries and reduced symmetry and dimensionality. Thus, the studies of the magnetic moment on cluster size can demonstrate how the bulk and surface magnetism evolve. Studies of magnetic properties of clusters on metallic substrates provide an additional dimension to this problem. Using first principles state-of-the-art theoretical techniques, the evolution of magnetic moments of transition metal clusters such as those of Ni, V, Mn, and Rh will be presented. The role of substrate on cluster geometry, stability, and magnetic properties will also be discussed. A critical comparison between theory and experiment will be made.
[F19.02] Magnetic Properties of Clusters of Transition Metal Atoms
Antonis Andriotis, Nectarios Lathiotakis (University of Crete), Madhu Menon (University of Kentucky)
We present our results of tight-binding molecular dynamics method in the study of magnetic clusters of transition metal atoms, namely Ni(n), Fe(n) and Co(n), n \leq 56. Our results indicate that the average magnetic moment per atom is significantly higher in the cluster than in the bulk, in agreement with recent experimental data for Fe and Ni clusters. Furthermore, it is found that magnetic effects stabilize the clusters in geometries that were found to be completely unstable when magnetism is ignored. In general, magnetic effects drive Fe (and to a lesser extent Ni) clusters into geometries of higher symmetry than that of the corresponding singlet states.
[F19.03] The Effect of Temperature Cycling on Magnetic Properties of Graphite Coated Ni and Co Nanoparticles
K. Parvin, J.A. Block, J.L. Alpers (Physics Dept., San Jose State University), J. Host, V.P. Dravid (Materials Science and Engineering Dept., Northwestern Univer= sity)
We report the structure and magnetic studies of carbon coated nanoparticles of Ni
and Co synthesized in a special low carbon: metal ratio arc chamber. Powder x-ray
diffraction (XRD) profiles and lattice plane spacing measurements from high
resolution TEM indicate particles of FCC Ni or Co and graphitic coating with no
evidence of carbides or solid solutions of carbon in the metal. Measurements of
magnetization as a function of temperature in the range 20 - 900 C suggest the
existence of an additional phase, possibly carbides below detection limit for XRD.
Upon heating and recooling, enhanced magnetization and decreased coercive field
at all temperatures were measured. The effect of annealing on other magnetic
parameters will also be presented.
[F19.04] Magnetic Properties of Supported Transition Metal Clusters
V.S. Stepanyuk, W. Hergert (Martin Luther Univ.,Halle,Germany), K. Wildberger, R. Zeller, P.H. Dederichs (Inst. Festkorp.,Julich,Germany), S.K. Nayak, P. Jena (Virginia Commonwealth University)
Magnetic properties of V_n, Mn_n, and Ni_n clusters supported on a Ag(001) substrate have been studied self-consistently for n up to 9 using first principles Korringa-Kohn-Rostoker method. Multiple magnetic solutions with nearly equal binding energies have been obtained for several of these clusters. Although the substrate has been found to have significant effect on the magnetic moments of supported clusters, it is the reduced dimensionality that makes these clusters magnetic. The existence of magnetic bistability may have important technological applications. \$ Work supported in part from a grant from the Army Research Office (DAAL03-92-G-0106).
[F19.05] Antiferromagnetic Nanoparticles: Anomalous Behavior and Modeling
Richard H. Kodama (U.C. San Diego, La Jolla, CA)
We have performed an experimental study of the magnetic behavior of a series of samples of NiO nanoparticles with average sizes varying from 50Å\ to 800ÅRemarkable properties include coercivities and loops shifts of up to 10 kOe, open loops in 50 kOe, and time dependent moment in 70 kOe. This behavior is difficult to understand in terms of normal 2-sublattice antiferromagnetic ordering. Based on exchange constants measured for bulk NiO,(M. T. Samuelson, E. J. Hutchings, Phys. Rev. B6, 3447 (1972).) we note that the 2-sublattice spin configuration is only slightly energetically favored over 4,6, or 8 sublattices. Numerical modeling of spin configurations in these nanoparticles demonstrates that 8-sublattice configurations are energetically favored due to reduced coordination at surface cations. The relatively weak coupling between the sublattices allows drastic changes in their relative orientations, as well as a variety of reversal paths for the spins upon cycling the applied field. We find that the simultaneous shifted loop and irreversibility can be explained by this model which includes bulk and surface anisotropies.
[F19.06] Relationship Between Magnetism, Topology, and Reactivity of Rh Clusters
S.E. Weber, S.K. Nayak, P. Jena (Virginia Commonwealth University), K. Wildberger, R. Zeller, P.H. Dederichs (Inst. Festkorp.,Julich,Germany), V.S. Stepanyuk, W. Hergert (Martin Luther Univ.,Halle,Germany)
Self-consistent first principles calculations based on the molecular orbital theory and the Korringa-Kohn- Rostoker (KKR) Green's function method have been used to demonstrate that the magnetism of Rh clusters can be altered by either modifying their structure or depositing them on a suitable substrate. This ability to change the magnetic properties of clusters can also have significant effect on their chemical reactivity, thus linking the field of magnetism and catalysis of atomically engineered materials. \$ Work supported in part from a grant from the Army Research Office (DAAL03-92-G-0106).
[F19.07] Classical Heisenberg model of a ring nanostructure: Correlation functions and susceptibility
J.H. Luscombe (Department of Physics, Naval Postgraduate School, Monterey, CA 93943), M. Luban, F. Borsa (Ames Laboratory)
We present approximate analytic expressions for the two-spin correlation function C_N(n)=\langle \vec s_i\cdot\vec s_i+n \rangle_N and susceptibility \chi_N for a ring of N classical Heisenberg spins that are valid for all but very low temperatures. While these quantities are known exactly,(G.S. Joyce, Phys. Rev. 155\rm, 478 (1967).) they involve complicated infinite series of Bessel functions. Compared with exact results, the simple formula C_N(n)=(u^n+u^N-n)/(1+u^N), where u(K)= cothK-K^-1, provides an excellent approximation for K<2, with K=J/(k_BT) the nearest-neighbor coupling constant. This expression combines the expected exponential decay of correlations with the periodic boundary condition for a ring, C_N(n)=C_N(N-n). The susceptibility \chi_N obtained from our correlation function is in excellent agreement with the exact quantity and with data from the molecular ferric wheel(K. Taft, ! et al\rm, J. Am. Chem. Soc. \b f 116\rm, 823 (1994).) for T>50 K.
[F19.08] On the magnetic behaviour of 13-atom Rhodium clusters
C.F.J. Flipse, J.D. O'Mahony (Dept. of Physics, Eindhoven University of Technoloy, The Netherlands), H. van Leuken, R.A. de Groot (Electronic Structure of Materials, Catholic University of Nij\-megen, The Netherlands)
A number of theoretical investigations have recently predicted high magnetic moments for small, highly-symmetric, 13-atom clusters of Rh. Yet the magnitude of the magnetic moments predicted exhibit considerable variation and are consistently larger, by a factor of two or more, than the experimental values. A comparison between a fully relativistic electronic structure calculation and the atomic multiplet program, based on Cowan's atomic Hartree-Fock program, will be made. It is shown that high magnetic moments for these clusters, depending on their structure, are expected because of symmetry required orbital degeneracy of the electrons.
[F19.09] Magnetic Properties of Ni Clusters
P. Jena, S.K. Nayak, B.V. Reddy, S.N. Khanna, B.K. Rao (Virginia Commonwealth University)
character.
The equilibrium geometries of Ni_n (n \le 23) clusters have been obtained using classical molecular dynamics simulation. Using these geometries and the local spin density approximation in the molecular orbital theory, we have calculated the magnetic moment/atom as a function of size self-consistently. The results vary strongly with size and relate nicely to the average coordination number in the clusters. A critical comparison of the theoretical results with experiment is provided. Improvements in both theory and experiment are needed to achieve a quantitative understanding of the evolution of the magnetic properties of transition metal clusters. \$ Work supported in part from a grant from the Army Research Office (DAAL03-92-G-0106).
[F19.10] Are Pd Clusters Magnetic?
B.V. Reddy, S.N. Khanna, B.K. Rao, P. Jena (Virginia Commonwealth University)
Recent experimental suggestion that Pd_n clusters for n=3-6 exhibit Ni-like magnetic electronic structure has been examined theoretically by calculating their equilibrium geometries, binding energies, and electronic structure. Self-consistent molecular orbital theory based on spin-density functional formulation and numerical atomic bases were used. All clusters were found to be non-magnetic. Our results will be compared with previous available theoretical studies. \$ Work supported in part from a grant from the Army Research Office (DAAL03-92-G-0106).