Session W43: Low Dimensional Magnetism: Surfaces and Molecules

Magneto-optical second-harmonic generation from a structurally optimized Fe/W(110) surface

The nonlinear magneto-optical response from a structurally optimized W(110) surface with a magnetic Fe overlayer is calculated from first principles. The electronic structure of the ground state is calculated using the full-potential LAPW method with first-order relativistic corrections, and the magneto-optical properties are obtained in the electric dipole approximation. We present results for the magnetic anisotropy energy as well as the nonlinear optical properties, including the susceptibility tensor, intensities in various optical configurations, angular dependences, and the nonlinear optical Kerr effect. We find that both the easy axis as well as the hard axis are in the surface plane, with the easy along the $1\bar{1}0$-direction and the hard axis along the $001$-direction. We show how the direction of magnetization changes the magneto-optical properties.   

Step Decorated Ferromagnetic Fe Stripes on Pt(997)

We investigate the step decoration and magnetic properties of Fe grown on a Pt(997) vicinal single crystal by means of reflection high energy electron diffraction (RHEED), scanning tunneling microscopy (STM) and the surface magneto-optical Kerr effect (SMOKE). RHEED and STM indicate that Fe stripes form at low coverage due to step decoration. SMOKE data taken along a wedge indicate that the Fe is ferromagnetic above 0.2 ML with its magnetic easy axis canted out of plane. As temperature is increased near the Curie temperature, there is an irreversible spin reorientation to the perpendicular out-of-plane direction. The canting can be understood as arising from a competition between an in-plane, step-induced magnetic anisotropy and a perpendicular surface anisotropy associated with extended terraces due to the polarization of the proximal Pt. The spin reorientation can be understood as arising from the different temperature dependences of the two competing anisotropies, with the two-dimensional surface anisotropy overcoming the one-dimensional step anisotropy at elevated temperature. Above 1.7 ML the perpendicular easy-axis reorients in plane due to the emergence of the shape anisotropy.   

Surface Magnetism of Fe overlayers on Pt(110)

Fe overlayers on Pt(110) surface have been studied using surface magneto-optic Kerr effect(SMOKE). Using the Pt(110) substrate is based on the possibility of making Fe nano wires at low coverage(0.5ML) since this surface shows missing row (2x1) reconstruction. If the Fe atoms are filled in this missing row, mono-atomic Fe wires can be formed though one Fe atom is surrounded by three neighboring Pt atoms. At room temperature (growth and measurement), the SMOKE signal starts to show up at the coverage of 1.0ML only at longitudinal geometry. When we grow at low temperature, weak polar signal has been observed down to 0.5ML with different LEED pattern. Annealing process after the 1ML deposition of Fe can leads to the formation of antiferromagnet so that further deposition of Fe layers can be exchange biased upon field cooling. We will show this interesting exchange bias in monolayer limit and the possible origin of this exchange bias in our system.   

Ferromagnetic Stability in Fe Nanodot Assemblies on Cu(111) Induced by Indirect Coupling through the Substrate

To first order, assemblies of nano-scale magnetic dots are superparamagnetic. In these systems, thermal energy, which causes fluctuation of the dots' magnetic moments, becomes significant enough to overcome the anisotropy energy barrier and randomize their orientation at the blocking temperature. This typically occurs far bellow room temperature. We report collective ferromagnetic behavior in two- dimensional Fe dot assemblies on the Cu(111) surface that persists above room temperature. Our ability to tune the average size and spacing of the dots enables us to investigate the relative contributions of the mechanisms that support this unexpectedly robust magnetic order. Our experimental results and simulations indicate that the high-Tc ferromagnetism cannot be explained by either magnetic anisotropy or dipolar interaction. Direct comparison of the Curie temperatures (Tc) of similar dots prepared on various substrates including Cu(100) and Ge(111) allows us to conclude that the observed high-Tc ferromagnetism for Fe dots on Cu(111) is a result of an indirect exchange interaction via the surface states of Cu(111) substrate.   

Enhanced hyperfine magnetic fields for face-centered tetragonal Fe(110) ultrathin films on vicinal Pd(110)

The structure and hyperfine magnetic properties of epitaxial Fe ultrathin films on a vicinal Pd(110) surface have been investigated by means of Low Energy Electron Diffraction (LEED), Reflection High Energy Electron Diffraction (RHEED) and $^{57}$Fe Conversion Electron M\"{o}ssbauer Spectroscopy (CEMS). LEED and RHEED provide evidence for initial pseudomorphic film growth. The RHEED determination of the in-plane atomic distance versus Fe film thickness demonstrates the stabilization of the metastable fcc-like Fe structure on Pd(110). This interpretation is supported by\textit{ in-situ} $^{57}$Fe CEMS measurements which indicate an enhanced saturation hyperfine field of $\approx $ 39T for a 3 monolayers thick Fe film at 25 K. This is the highest value ever measured for Fe on a metallic substrate. Our results suggest that ultrathin fcc-like (face-centered tetragonal) Fe films on Pd(110) are in a ferromagnetic high-moment state, and show a considerably enhanced Fe magnetic moment due to electronic 3$d$-4$d$ hybridization at the Fe/Pd interface.   

SHG on Transition Metal Oxides

Optical second harmonic generation (SHG), within the electric-dipole approximation, is known to be sensitive to the surface electronic structure of inversion symmetric media. Besides, it allows to distinguish magneto-optically ferromagnetic from antiferromagnetic order of cubic (001) surfaces. Here, we compute the SHG from NiO(001) and CoO(001) surfaces from first principles. To address the notoriously difficult electronic correlations of transition metal monoxides we use a high-level quantum-chemical \textit{ab initio} approach. We find that the gap states of CoO(001), due to level bunching, are equally well suited for ultrafast manipulation of the antiferromagnetic state as those of NiO(001), the latter having fewer resonances. Furthermore, we present calculations of the nonlinear optical susceptibility tensor $\chi^{(2\omega)}$ for the NiO(001) and CoO(001) surfaces, where we, besides the electric-dipole contributions, for the first time include both magnetic-dipole and electric-quadrupole contributions computed from our first principles theory. For a detailed disentanglement of bulk and surface contributions we additionally calculate the nonlinear optical susceptibility of bulk NiO and perform a detailed comparison of the computed nonlinear SHG spectra with recent experimental findings. In this way we offer an alternative explanation of the experimental results in terms of surface electric-dipole and bulk nonlocal electric-quadrupole contributions.   

Magnetic-moment-induced local superstructure on a graphite surface observed by SP-STM

Magnetic properties of carbon materials such as graphite, fullerenes and carbon nanotubes are of great interest for new device materials. We have investigated local magnetic characteristic on graphite surfaces by spin-polarized (SP) STM. The SP measurements were performed with magnetized Ni tips at 77K. In order to extract the magnetic property from the electronic property we referred STM data obtained with W and non-magnetized Ni tips. The SP-STM image showed coexistence of a typical graphite structure and a local superstructure which had not been observed with the W or non-magnetized Ni tips. The SP-STM spectra exhibited ~40meV shift to the higher energies, and in the superstructure region a characteristic peak appeared near Fermi level. Plausibly the superstructure corresponds to spatial distribution of the local magnetic moment induced by adsorbates.   

Ultra-thin transition-metal silicide films on Si: trends with chemical composition and substrate orientation

Ferromagnetic films of monosilicides, if grown epitaxially on Si, could be an interesting materials system for spin injection into silicon. While the natural P2$_1$3 structure of the compounds $M$Si ($M$=Mn,Fe,Co,Ni) is incompatible with Si surfaces, silicides with a CsCl-like crystal structure could be grown epitaxially. We study such films on Si(001) systematically by varying the transition metal species, and find ultrathin (2 or 3 monolayer) MnSi and CoSi films to be ferromagnetic, while FeSi and NiSi films turn out to be non-magnetic. For all films, capping by a Si layer is found to be energetically favorable. While MnSi shows a layered magnetic structure with sizable magnetic moments ($\sim 2\mu_B$) in the surface and interface layers, the CoSi films show homogeneous magnetization with magnetic moments of Co $\sim 0.5\mu_B$. Both findings are interpreted in terms of the electronic structure of the films. Comparing CsCl-like MnSi films on either Si(001) or Si(111), we find both to be thermodynamically stable with respect to bulk Mn, but metastable with respect to bulk P2$_1$3-MnSi. The increase in stability with film thickness is monotonic for Si(111), but passes through less stable intermediate structures at $<1$~ML Mn for Si(001). Hexagonal phases of Mn-silicides, which could conceivably be grown on Si(111) as well, are found to be energetically less favorable than the CsCl-like MnSi films proposed by us.   

Quantum Fluctuations and Excitations in Magnetic Quasicrystals

We study the effects of quantum fluctuations and the excitation spectrum for the antiferromagnetic Heisenberg model on the octagonal tiling, a two-dimensional quasicrystal structure. Using a combination of quantum Monte Carlo and numerically solved spin-wave theory, a non-trivial inhomogeneous magnetic ground state is found. A hierarchical structure in the values of the staggered moments is observed which arises from the self- similarity of the quasiperiodic lattice. The magnetic excitation spectrum consists of magnon-like low-energy modes, as well as dispersionless high-energy states of multifractal nature. The dynamical spin structure factor exhibits linear soft- modes at low energies, self-similar structures with bifurcations emerging at intermediate energies, and flat bands in high-energy regions. This generic model is a first step towards understanding magnetic quasicrystals such as the recently discovered Zn-Mg-Ho icosahedral structure.   

Uniaxial Magnetic Anisotropy Induced By Steps in La0.67Sr0.33MnO3 Thin Films

Epitaxial thin films of La0.67Sr0.33MnO3 (LSMO) were grown by pulsed laser deposition on TiO2 terminated SrTiO3 (STO) vicinal substrates having regular arrays of monatomic steps. The LSMO films grow epitaxial, thereby replicating the step-terrace structure of the substrate up to the film surface, as confirmed by atomic force microscopy. Magnetic studies using a Vibrating Sample Magnetometer show that the films have an in-plane uniaxial magnetic anisotropy at room temperature. Interestingly, we observe that the easy axis depends on the direction of steps on the film surface and is always oriented along the direction of the steps. We attribute the uniaxial anisotropy to magneto-crystalline anisotropy due to defect planes induced by the step-terrace structure of the substrate and film. At low temperatures, biaxial anisotropy dominates with [110] as easy and [100] direction as hard axes and with no significant dependence on step orientation. The biaxial anisotropy at low temperatures is thought to arise from the four-fold tensile strain in the LSMO film caused by its lattice mismatch with the STO substrate.   

Quantum Corral Wave-function Engineering

We present a theoretical method for the design and optimization of quantum corrals[1] with specific electronic properties. Taking advantage that spins are subject to a RKKY interaction that is directly controlled by the scattering of the quantum corral, we design corral structures that reproduce spin Hamiltonians with coupling constants determined a priori[2]. We solve exactly the bi-dimensional scattering problem for each corral configuration within the s-wave approximation[3] and subsequently the geometry of the quantum corral is optimized by means of simulated annealing[4] and genetic algorithms[5]. We demonstrate the possibility of automatic design of structures with complicated target electronic properties[6]. This work was performed under the auspices of the US Department of Energy by the University of California at the LLNL under contract no W-7405-Eng-48. [1] M. F. Crommie, C. P. Lutz and D. M. Eigler, Nature 403, 512 (2000) [2] D. P. DiVincenzo et al., Nature 408, 339 (2000) [3] G. A. Fiete and E. J. Heller, Rev. Mod. Phys. 75, 933 (2003) [4] M. R. A. T. N. Metropolis et al., J. Chem. Phys. 1087 (1953) [5] E. Aarts and J. K. Lenstra, eds. Local search in combinatorial problems (Princeton University Press, 1997) [6] A. A. Correa, F. Reboredo and C. Balseiro, Phys. Rev. B (in press).   

Tuning of Magnetic Ordering in Single-Molecule Magnets

Single-molecule magnets (SMMs) are usually characterized by superparamagnetic blocking of the cluster spins. Magnetic interactions between cluster spins may, however, drive to long- range order when the cluster magnetic anisotropy is sufficiently small. Our experiments on the Fe$_{17}$ SMM [Angew. Chem. Int. Ed. \textbf{43}, 5772 (2004)] show that this is an unique example of system where both limiting cases are observable. By chemical substitution of intercluster ligands, we are able to change the space group symmetry without affecting the individual molecules, a.o. keeping the surrounding ligands, the cluster spin $S=35/2$ ground-state and magnetic anisotropy (uniaxial with $D\simeq 0.02$~K) unaltered. Antiferromagnetic long-range order at $T_{N}=0.8$~K is found for $R\bar{3}$ symmetry, whereas any sign of ordering is wiped out for $Pa\bar{3}$ symmetry leaving a superparamagnetic blocking at $T_{B}\simeq 0.5$~K as the prevailing mechanism.   

Heisenberg S=1/2 Spin Ladders: New Molecular-Based Examples

Heisenberg spin ladders have a cooperative spin singlet ground state and two critical fields. The ratios of the two fields depend on the ratio of the interaction strengths along the rungs and along the rails of the ladder. Coordination polymers provide examples of spin ladders with exchange strengths small enough to induce critical fields that are less than 60 tesla. To date these molecular-based compounds consist either of lattices of CuBr$_{4}$ dianions packed into ladders by large organic molecules [1,2] or neutral Cu$_{2}$X$_{4}$ dimers (X = Cl, Br) linked into ladders by bridging diazine molecules such as quinoxaline [3]. We present the structures and results of magnetic measurements of three new spin ladders, (quinolinium)$_{2}$CuBr$_{4}$, Cu(methylpyrazine)Cl$_{2}$, and Cu(2,3-dimethylpyrazine)Cl$_{2}$. The susceptibility of each of the compounds possesses a characteristic rounded maximum near 20 K, with a rapid decrease at lower temperatures. Comparison of the data for Cu(2,3-dimethylpyrazine)Cl$_{2}$ to a spin ladder model reveals the rung and rail exchange constants to be 29.0 K and 17.2 K, respectively. [1] C. P. Landee et al, \textit{Phys. Rev. B} \textbf{63}, R100402 (2001). [2] R D. Willett et al, \textit{Inorganic Chemistry} \textbf{43}, 3804-3811 2004). [3] C. P. Landee et al, \textit{Polyhedron }\textbf{22}, 2325-2329 (2003).