Session J32: Artificial Structured or Self-Assembled Magnetic Materials

Surface-state mediated indirect exchange interaction between magnetic nanodots on metallic substrates

We investigate theoretically the ferromagnetic ordering of magnetic nanodots grown on flat or vicinal metal substrates. We first show that, on a flat substrate, the surface state-mediated indirect exchange interaction between the nanodots can be significant enough to account for the high ferromagnetic transition temperature observed in recent experiments. We obtain the quantitative coupling strength and characteristic length scale of the magnetic interaction via detailed Monte Carlo simulations. We then study how the reduced dimensionality of the surface state on vicinal surfaces affects the collective magnetic behavior of the systems, and discuss the findings in connection with latest experimental observations.   

Artificial Nanomagnet With Lateral Confinement

We introduce a novel way---curved Cu(111) substrate---to smoothly modify the surface states by introducing a miscut angle and study the impact of modifying vicinal surface states on the ferromagnetic behavior of Fe dots. With this curved substrate, the same growth parameter can be ensured in the whole miscut angle studied. When the Fe dot assemblies have an in-plane easy axis, two distinct regimes and a critical terrace width, separating these two regimes, can be identified. There are three contributing factors: the vicinal surface state, the competition between the Fe-dots diameter and the terrace width, and the in-plane uniaxial magnetic anisotropy. The couplings between these three factors lead to the interesting behavior observed in the Fe/vicinal Cu(111) nanodot assemblies.   

Multiple Quantum Transitions In Magnetic Nanoparticles

Absorption at multiple resonance frequencies is observed in magnetic nanoparticles in strong similarity with forbidden multiple quantum transitions known for paramagnetic ions. The detailed studies of these low-field signals in the dependence on temperature, concentration and orientation of the texturized samples will be presented and discussed using a ``quantization'' approach, considering resonance transitions between energy levels of a giant spin corresponding to the total magnetic moment of a nanoparticle.   

SEMPA Measurements of Ferromagnetic Nanodisk Phase Diagrams

We use Scanning Electron Microscopy with Polarization Analysis (SEMPA) to image the magnetic domain structures of ferromagnetic nanodisks with different diameters and thicknesses, and thereby determine the phase diagram of the ground state in these technologically important magnetic structures. Depending on the nanodisk dimensions, one of three distinct ground state magnetic configurations is observed: a single domain in-plane, a single domain out-of-plane, or a vortex state. In contrast to previous work, the magnetic states of \textit{individual} nanodisks are determined using simultaneous SEMPA measurements of both the in-plane and out-of-plane magnetization components. By systematically imaging Permalloy nanodisks with diameters that range from 35 nm to 190 nm and with thicknesses that range from 10 nm to 65 nm, we are able to locate phase boundaries and the triple point between the three phases. Near the phase boundaries and triple point we observe a mixture of the different phases. A model magnetic phase diagram generated by using the OOMMF micromagnetic simulator is found to agree well with the phase diagram determined by the SEMPA measurements. This work is supported by the NIST-CNST/UMD-NanoCenter Cooperative Agreement.   

Ordering, Texture and Magnetism in Ultrathin FePt Films

Non-epitaxially grown $L$1$_{0}$ FePt ultrathin films have been fabricated and investigated. All films were magnetron sputtered onto SiO$_{2}$ substrates in the form [Fe/Pt]$_{n}$, with individual layer thicknesses from about 0.1 to 0.4 nm. The films' nominal total thickness ranged from 5 to 20 nm and was controlled by varying the number of bi-layer repetitions. The $L$1$_{0}$ phase and (001) texture were obtained by post-deposition annealing for 300 seconds at 600 \r{ }C. Transmission electron microscopy showed all as-deposited films as continuous; but after annealing, all films except the thickest ones showed agglomeration into a connected island-like morphology. $L$1$_{0}$-ordering and texture were confirmed by x-ray and electron diffraction, and the degree of order tended to decrease with increasing nominal film thickness. The (001) texture was greatest when the nominal film thickness was 12.5 nm, coinciding with an island thickness of the same value. SQUID magnetometry shows a relatively unusual trend of coercivity increasing with island thickness, with the highest value corresponding to the most ordered sample. The process of film agglomeration and the effects of bi-layer thickness and annealing temperature and time are also discussed. -- This research is supported by NSF-MRSEC (RS), INSIC, DOE (DJS), and NCMN.   

Numerical Studies of Magnetization Reversal in Thin Annular Nanorings

The rate of thermally activated magnetization reversal in thin ferromagnetic nanorings has been found analytically in a 1D model in which the demagnetization energy is approximated by a local surface term [1]. Numerical micromagnetic calculations confirm all aspects of the analytic model for narrow thin rings, such as permalloy rings of 200 nm mean radius, 40 nm width and 2 nm thickness [2]. However, the model breaks down in for extremely wide rings, when the ring width approaches its mean diameter. Here we present numerical micromagnetic results for the transition states between the clockwise and counterclockwise state in this limit. We describe how the two transition configurations of narrow rings cease to be saddles of the energy functional. Also, a new low energy metastable state is found to exist for a narrow range of fields. We discuss the results of applying the String Method [3] to determine the transition states and energy barriers between the lowest magnetization configurations of rings. [1] K. Martens, D.L. Stein, and A.D. Kent, PRB 73, 054413 (2006) [2] G. D. Chaves-O'Flynn, D.L. Stein, and A.D. Kent, arXiv:0811.0440 (2008) [3] W. E, W. Ren, E. Vanden-Eijnden, J. Chem. Phys 126, 164103 (2007)   

Onset of magnetism in supported transition metal encapsulated silicon cages

In the past few years, silicon based clusters have attracted a lot of attention as building blocks of nanomaterials. Some of the most promising candidates are the transition metal encapsulated silicon cages, which have been shown to be specially stable, both experimentally and theoretically. However, for the use of these materials in fields like spintronics, it is not only necessary to be semiconductor based, but also that they present a finite magnetic moment. However, it has been shown that the magnetic moment of the transition metal atom encapsulated in silicon cages is quenched due to the hybridization with silicon. By performing density functional calculations in the generalized gradient approximation, we show that the magnetic moment of these clusters can be recovered by depositing then on a surface. Using CrSi$_{12}$ on Si(111) as an example, we have deposited the cluster in different orientations. The studies show that, for most of them, a finite magnetic moment is preserved in the system after a geometrical relaxation. The origin of this behavior is discussed in terms of hybridization, comparing to the unsupported situation.   

Metamaterials with tunable refractive index fabricated from amorphous ferromagnetic microwires and optical Magnus effect

For homogeneous NPVM (negative phase--velocity mediums) [V. G. Veselago, Soviet Physics - Uspekhi \textbf{10} (1968) 509; T. G. Mackay, A. Lakhtakia, Phys. Rev. E \textbf{69} (2004) 026602] anomalous effects such as negative refraction, light pressure, Doppler shift, Cherenkov-Vavilov radiation, $Goos-H\ddot {a}nchen$ effect have been discovered in different frequency ranges. In this presentation the optical circular polarized effect is calculated for inhomogeneous mediums (optical Magnus effect) and it is shown that it is anomalous in NPVM with respect to ``right-handed'' materials. The proposed metamaterials fabricated from glass coated amorphous ferromagnetic Co-Fe-Cr-B-Si microwires are shown to exhibit a negative refractive index for electromagnetic waves over scale of GHz frequencies [A.V. Ivanov, A.N. Shalygin, A.V. Vedyayev, V.A. Ivanov, JETP Letters \textbf{85} (2007) 565]. The magnetostatic interaction between microwires has been taken into account. The phase and group velocities in proposed metamaterial have been calculated. The ratio of thereof depends monotonically on the size of the microwires. Optical properties of such metamaterials are tunable by an external magnetic field and mechanical stress.   

Magnetism in 1D Cobalt-Cyclopentadienyl Sandwich Molecular Wire

A challenge for technological applications at the nanometer scale is to find magnetic materials with reduced dimensionality. Recent theoretical studies have predicted ferromagnetic and half-metallic behavior for the 1D-organometallic benzen vanadium wire. Here we discuss a variety of magnetic orderings such as anti-ferromagnetic and ferrimagnetic half-metallicity in the cobaltocen Co2(C5H5)2 nanowire. We performed DFT-calculations to optimize its geometry and used the NMTO downfolding technique to construct the real-space low energy Hamiltonian. To describe electronic correlations beyond the mean-field, we used the developed Variational Cluster Approach. Our preliminary results show that non-quasiparticle states appear in the half-metallic gap, which reduce considerably the spin polarization of such a wire. Ab-initio electron transport calculations are in progress to establish the role of cobaltocen nanowire as part of a future spin filter.   

Comparative Density Functional study of Ti on CuN/Cu(100)

We have performed a Density Functional Theory (DFT) calculation using the Projector Augmented Wave (PAW) technique to study the electronic structure of adatoms of Ti placed on a single layer of copper nitride (CuN) surface grown on top of Cu(100). The insulating CuN surface mediates superexchange interactions between the magnetic adatoms, and also can strongly affect electronic properties. The PAW technique allows us to have elements of the precision of an all-electron (AE) calculation and the performance of an Ultrasoft Pseudopotential (USPPs) calculation. We compare results obtained of the magnetic moment, atomic positions, and charge densities of Ti adatoms with both USPPs and PAW methods, and discuss the similarities and differences between the methods. We also report studies of the effect of the Coulombic repulsion U in the PAW methology, and compare some of these results to an all-electron calculation.   

Characteristics of Co islets on Cu(111) from first principles calculations

Through first principles electronic calculations, based on the spin-polarized density functional theory using the generalized gradient approximation and the ultrasoft pseudopotential method in the plane wave representation, we have examined the structure and magnetic properties of Co monomer, dimer and several n-mers on Cu(111). We find that the monomer has slight preference for the fcc site as compared to the hcp (about 0.02eV) while there is no such preference in the case of the Co dimer. The dimer bond length is found to be about 2.15?. For the 6 atoms cluster, we find that it prefers to be antiferromagnetic and absolute magnetic moment of each Co atom is about 0.07-0.08$\mu $B. The monomer is non-magnetic while a high magnetic moment of 1.94$\mu $B per Co atom is found in the case of dimer. We discuss our results in the context of recent experimental and theoretical findings [1,2] \\[3pt] [1] S. Borisova \textit{et al}, Phys. Rev. B \textbf{78}, 075428 (2008) \\[0pt] [2] O. Mironets \textit{et al}, Phys. Rev. Lett. \textbf{100}, 096103 (2008)   

Growth and characterization of MnAs on HOPG

MnAs thin films exhibit room temperature ferromagnetism and have been extensively studied on substrates such as GaAs and Si, for spintronic applications. Film properties, such as vertical transport measurements, are often hindered by the presence of underlying substrates. The non-reactive HOPG surface provides an ideal environment for studies of MnAs with minimal efffect from the substrate. We grew MnAs on HOPG by MBE. AFM measurements indicated that the MnAs particles self-assemble on step edges of the HOPG surface, to form highly ordered wire-like structures. Magnetization measurements showed the MnAs/HOPG sample to be ferromagnetic at room temperature. A temperature dependent AFM/MFM study yielded a Curie temperature of 330K. Individual particles are found to be ferromagnetic and the neighboring particles are antiferromagnetically coupled. Modulation dI/dV measurements showed a clear difference between the electronic states in MnAs and in the HOPG substrate used as a reference.   

Aging and {\it in-situ\/} annealing reduction of magnetite (Fe$_3$O$_4$) thin films grown on the polar MgO(111) surface

Previous transmission electron microscopy and diffraction studies of Fe$_3$O$_4$(111)/MgO(111) polar oxide interfaces found \footnote{ V. K. Lazarov, et al., Phys. Rev. Lett. {\bf 90}, 216108 (2003).} the formation of (110)-oriented metallic Fe nano-crystals at the interface and within the magnetite film under oxidizing conditions that result in pure magnetite growth on the neutral MgO(001) surface. The question arises whether these iron nano-crystals oxidize with prolonged aging in air. We find, instead, that they not only persist but grow in average thickness within the magnetite film. We have also explored whether reduction can be achieved by {\it in-situ\/} annealing in vacuum starting from pure phase Fe$_3$O$_4$(111)/MgO(111) samples. We find a phase transformation from Fe$_3$O$_4$ to FeO at 720$^\circ$C and a second phase transformation at 800$^\circ$C from FeO into Fe nanoparticles that tend to nucleate along the surface.   

Magnetic and Optical Anisotropy in Epitaxial Cobalt Films

Cobalt films were grown by molecular beam epitaxy on CaF2 buffer layers on silicon and showed a variety of magnetic and optical effects. When the substrate is Si(111), CaF2 grows as flat layer with (111) surface and Co grows on it in face centered cubic lattice. Magneto-optical Kerr effect (MOKE) from these structures is isotropic in plane. When the substrate is Si(100), CaF2 growth direction is [110] due to unique properties of CaF2/Si(100) interface. Then CaF2 surface has grooves with {\{}111{\}} facets and cobalt grown on it has in-plane magnetic anisotropy with easy axis along the grooves. The dependence of remanence magnetisation and coercivity on azimuthal angle (between the grooves and field) follows single domain model except the range between 80 to 90 degrees - there is a peak related to crystalographic anisotropy. MOKE from these structures can have different sign depending on incidence angle; this is consistent with calculations of multilayer model. We found that the sign of MOKE can change also versus azimuthal angle, which was unexpected. Physics behind this phenomenon will be presented.   

Structure and magnetism of Mn$_{0.5}$Ni$_{0.5}$/Ni(001) system

The dependence of the magnetic structure on the geometry of materials is well-established for decades. Most of the theoretical and experimental investigations have focused on transition metals adsorbed on (001) noble metal surfaces. These systems can be considered as two-dimensional magnets with negligible influence of the substrate. Several experimental investigations have reported a c(2$\times $2) ordered alloy structure for MnCu/Cu(001) and MnNi/Ni(001) systems. In this work, I present a theoretical study of the magnetic structure for the c(2$\times $2) ordered Mn$_{0.5}$Ni$_{0.5}$/Ni(001) alloyed system. The calculations were performed using the density functional theory (DFT) and the exchange-correlation potential was treated by the generalized gradient approximation (GGA). In this study, an in-plane ferromagnetic structure was obtained, which is more stable than the antiferromagnetic by 398 meV. The Mn and Ni atoms exhibit local magnetic moments of 3.998$\mu _{B}$ and 0.250 $\mu _{B }$, respectively. I obtained relaxations of +1.5{\%} and -1.05{\%} for Mn and Ni surface atoms, respectively.