Session X37: Focus Session: Nanomagnetism -- Molecules

Magnetic Properties of Ultrathin Metal-Organic Layers on Metal Surfaces: Supramolecules and Macrocycles

The design of molecular films with tunable electronic and magnetic properties has become a major goal of current research. Metal-organic complexes at surfaces provide versatile ways to achieve this goal by controlling the metal-ligand and metal-substrate hybridization. In this talk, we discuss two approaches to lay out the structure and magnetic properties of molecular layers on metallic substrates based on transition metal-directed supramolecular assembly [1] and the deposition of macrocycle molecular films [2]. Using a combination of scanning tunnelling microscopy, x-ray magnetic circular dichroism, ligand field and density functional calculations we study the interplay of molecular and metal states, addressing the spin and orbital moments of the molecules as well as their magnetic anisotropy. We show that, under certain conditions, these quantities can be controlled by the number and symmetry of the organic ligands independently from the substrate. We reveal unusual phenomena such as 500{\%} giant orbital moment anisotropy in metal-phthalocyanine complexes and molecular Kondo states, which we contrast with measurements of metal atoms and nanoparticles at surfaces [3,4]. The charge state of the metal ions and their electronic coupling with the surface is analyzed in terms of multiplet simulations of x-ray absorption spectra and compared to density functional theory results. \\[4pt] [1] P. Gambardella et al., Nature Mater. \textbf{8}, 189 (2009). \\[0pt] [2] S. Stepanow et al., to be published. \\[0pt] [3] H. Brune and P. Gambardella, Surf. Sci. \textbf{603}, 1830 (2009) \\[0pt] [4] P. Gambardella et al., Science \textbf{300}, 1130 (2003).   

Quantum Levels Spectroscopy of Magnetic Molecules Using Tunnel-diode Resonator

Many magnetic molecules form crystals in which intermolecular interactions can be ignored and at suitably low temperatures the description of these finite -- spin systems can be reduced to the analysis of a discrete spectrum of quantum energy levels within an individual molecule. By applying external magnetic field one can access information about low-lying energy levels that cross due to Zeeman splitting. Conventional experimental techniques are only sensitive to the ground state level crossings. Tunnel-diode resonator is a radio-frequency technique to measure the differential magnetic susceptibility in the millikelvin range and in high static external field. Having 1 ppb sensitivity, it can probe the magnetic energy spectrum in both the ground state and low-lying excited states. As a successful example, the TDR technique has recently been used to investigate the magnetic molecules Cr$_{12}$Cu$_{2}$ and Cr$_{10}$Cu$_{2}$. When compared with theoretical quantum Monte Carlo (QMC) simulations, we find the TDR results to be in extraordinary agreement with the predicted energy spectrum. This shows that the TDR technique is a unique and powerful spectroscopic tool for studies of magnetic molecules.   

New Molecular-based quasi-2D S=1/2 Heisenberg Antiferromagnet

We report on the magnetic properties of a newly synthesized molecular-based quasi-2D S=1/2 Heisenberg antiferromagnet, Cu(pz)$_{2}$(HOpy)$_{2}$(ClO$_{4})_{2}$, where pz = pyrazine and HOpy = 2-hydroxypyridine. Powder susceptibility studies show the intralayer exchange J to be 7 K, with the ordering temperature less than 1.8 K. The compound consists of layers of Cu(II) ions bridged by pyrazine molecules into a square lattice, with the bulky HOpy molecules coordinated in the axial sites, providing exceptional isolation between layers (layer-layer distance = 12.18 {\AA}). Unlike all other members of the copper/pyrazine family of 2D antiferromagnets, no anions are coordinated to the copper sites, a factor that may influence the exchange anisotropy that can dominate the low-temperature behavior in this family. Recent results will be reported.   

Magnetic field dependent electronic transport of Mn$_{4}$ single-molecule magnet.

We have performed single-electron transport measurements on a Mn4 single-molecule magnet (SMM) in where amino groups were added to electrically protect the magnetic core and to increase the stability of the molecule when deposited on the single-electron transistor (SET) chip. A three-terminal SET with nano-gap electro-migrated gold electrodes and a naturally oxidized Aluminum back gate. Experiments were conducted at temperatures down to 230mK in the presence of high magnetic fields generated by a superconducting vector magnet. Mn$_{4}$ molecules were deposited from solution to form a mono-layer. The optimum deposition time was determined by AFM analysis on atomically flat gold surfaces. We have observed Coulomb blockade an electronic excitations that curve with the magnetic field and present zero-field splitting, which represents evidence of magnetic anisotropy. Level anticrossings and large excitations slopes are associated with the behavior of molecular states with high spin values ($S$~$\sim $~9), as expected from Mn$_{4}$.   

Coherent manipulation of mononuclear lanthanide-based single-molecule magnets

Using electron spin echo (ESE) spectroscopy, we report measurements of the longitudinal (T$_{1})$ and transverse (T$_{2})$ relaxation times of diluted single-crystals containing recently discovered mononuclear lanthanide-based single-molecule magnets (SMMs) encapsulated in polyoxometallate cages [AlDamen\textit{ et al. J. Am. Chem. Soc. }\underline {130}, 8874 -- 8875 (2008)]. This encapsulation offers the potential for preserving bulk SMM properties outside of a crystal, e.g. in molecular spintronic devices. The magnetic anisotropy in these complexes arises from the spin-orbit splitting of the ground state $J$ multiplet of the lanthanide ion in the presence of a ligand field. At low frequencies only hyperfine-split transitions within the lowest ground state $\pm m_{J}$ doublet are observed. Spin relaxation times were measured for a holmium complex, and the results were compared for different hyperfine transitions and crystal dilutions. Clear Rabi oscillations were also observed, indicating that one can manipulate the spin coherently in these complexes.   

Collective coupling of $\sim10^{16}$ Fe$_8$ single-molecule magnets to a resonant cavity

When a spin resonantly couples to the electromagnetic mode of a cavity, spin and photon states hybridize producing entangled states and a ``vacuum Rabi splitting'' between the states. The magnetic-dipole transition between the coupled states results in a splitting $<$100 s$^{-1}$, practically unmeasurable. The well-known model of Tavis and Cummings [1] predicts that for $N$ spins coupled to the cavity, the vacuum Rabi splitting is enhanced by $\sqrt{N}$. We performed a reflection spectroscopy experiment at 1.75 K on a single crystal of the Fe$_8$ single-molecule magnet (SMM) in a cylindrical cavity with a bare frequency of 147.6 GHz. An applied magnetic field brings the SMMs to resonance with this mode, resulting in an observed splitting of $\sim3.4 \times 10^{9}$ s$^{-1}$. This value corresponds to $N\sim 2 \times 10^{16}$ Fe$_8$ SMMs collectively coupled to the cavity. From the dimensions of the crystal, we estimate a total of $N\sim 6 \times 10^{16}$ SMMs in the sample, suggesting that a substantial fraction of the crystal's molecules are simultaneously coupled to the cavity. As temperature is increased, $N$ decreases, reducing the observed splitting. [1] Phys. Rev., {\bf 170}, 379 (1968).   

Surface Supported Formation of Magnetic Molecular Chains

Further progress towards device development requires molecular classes with more flexible structures for functionalization. Salens are a promising molecular class of volatile metal-organic complexes with a metallic ion caged from three sides. Salens can be chemically modified to tune interactions with a substrate and neighboring molecules, or to establish intermolecular electronic and magnetic communication between two metallic centers through the organic periphery. Modification of Co-Salen structure to Co-Salophene-Br$_2$ provides more stable molecules which can form, by utilizing Ullmann type ring coupling reactions, surface supported covalently bound chains. Chain manipulations with a Low Temperature Scanning Tunneling Microscope (LT-STM) tip clearly identify covalent linking between the molecular units. Kondo effect of the engineered molecular chains is studied by local probe spectroscopy (dI/dV) and spectroscopic mapping. This work presents a novel avenue of molecular magnetism where new molecular systems are designed from basic units and assembled on a surface.   

Spin-Delocalization in Molecular Orbital Kondo Resonance

Molecules with transition-metal complexes have great potentials in spintronics and molecular electronics. Controlling their spin states and spin polarization is a key challenge for future applications. Here, we report an extensive redistribution of spin density for self-assembled TBrPP-Co [5, 10, 15, 20 --Tetrakis -(4-bromophenyl)-porphyrin-cobalt] molecules adsorbed on a Cu(111) surface. The TBrPP-Co molecule has a spin-active cobalt atom caged at the center of porphyrin unit and four bromo-phenyl groups are attached to its four corners. These molecules readily self-assemble and form ordered, ribbon-like monolayer islands on Cu(111). We probe the spatially extended Kondo resonance of the molecules by means of tunneling spectroscopy and spectroscopic mapping. The origin of this effect is explained by means of first-principles and numerical renormalization group calculations.   

Unexpected charge effects in Mott insulators: Magnetoelectric behavior of Cr-trimer complexes

In this work we present measurements of the dielectric response as a function of magnetic field of Cr-trimer systems, which combined with recent theoretical developments indicates a magnetoelectric behavior evidenced by a purely electronic mechanism. Magnetic field strengths of the order of the exchange interaction (J $\sim $ t$^{2}$/U) strongly perturb the spin texture, which is evident as steps and plateaus in the magnetization behavior. The corresponding shifts in dielectric properties reveal the role of the charge degrees of freedom. Electron Spin Resonance (ESR) results indicates which terms in the effective spin Hamiltonian are responsible for the magnetoelectric coupling. Furthermore, we observe novel dipole-active ESR give rise to the possibility of negative refractive indices under special conditions.   

Electronic Structures of Magnetic Ion Containing Porphyrin Molecules on Au(111)

Orderings and electronic structures of organic molecules on metal substrates have been studied due to possible applications in electronic devices. In molecular systems, delocalized pi-electrons play important roles in the adsorption behaviors and electronic structures. We studied the adsorption and electronic structures of Co-Porphyrin molecules on Au(111) using scanning tunneling microscopy (STM) and spectroscopy (STS) at low temperature. Molecules form closely packed two-dimensional islands on Au(111) surface with two different types, having different shape evolutions in our energy-dependent STM observations. The Kondo resonance state, occurred by spin exchange interaction between the Co center atom and conduction electrons in the metal substrate, was observed in one type, while it was absent in the other type in scanning tunneling spectroscopy measurements. Possible origins of two molecular shapes will be discussed.   

Identifying Charge States of Molecules with Spin-flip Spectroscopy

The charge states of single magnetic CoPc chains on Pb(111) film were manipulated with a scanning tunneling microscope and identified by spin-flip inelastic tunneling spectroscopy. We show that the tunneling spectra of the charged and neutral states of the molecular chain exhibit different features associated with the spin-flip processes. Therefore we are able to identify the charge states by the spin-flip spectroscopy. This approach does not rely on the indirect information induced by the additional charges on molecules. The experiment demonstrates a general method for detecting the charge states at the nanometer scale in a more straightforward manner.   

Entangled states decoherence in coupled molecular spin clusters

Localized electron spins in solid-state systems are widely investigated as potential building blocks of quantum devices and computers. While most efforts in the field have been focused on semiconductor low-dimensional structures, molecular antiferromagnets were recently recognized as alternative implementations of effective few-level spin systems. Heterometallic, Cr-based spin rings behave as effective spin-1/2 systems at low temperature and show long decoherence times [1]; besides, they can be chemically linked and magnetically coupled in a controllable fascion [2]. Here, we theoretically investigate the decoherence of the Bell states in such ring dimers, resulting from hyperfine interactions with nuclear spins. Based on a microscopic description of the molecules [3], we simulate the effect of inhomogeneous broadening, spectral diffusion and electron-nuclear entanglement on the electron-spin coherence, estimating the role of the different nuclei (and of possible chemical substitutions), as well as the effect of simple spin-echo sequences. References: [1] F. Troiani, et al., Phys. Rev. Lett. 94, 207208 (2005). [2] G. A. Timco, S: Carretta, F. Troiani et al., Nature Nanotech. 4, 173 (2009). [3] F. Troiani, V. Bellini, and M. Affronte, Phys. Rev. B 77, 054428 (2008).   

Successful Deposition of Cr$_{7}$Ni Molecular Clusters on Au and HOPG

Molecular nanoMagnets (MnM) are quantum objects with great potentialities as molecular devices for storing or processing information [1]. Key issues are to understand to which extent we are able to control interaction with the surface and if/how MnM preserve their properties when grafted on surfaces.$^{ }$Here we deeply investigated the properties of submonolayer distributions of isolated molecular Cr$_{7}$Ni rings, excellent candidate for encoding qubits, deposited by liquid phase on Au(111) [2,3] and HOPG surfaces [4], exploiting STM, XPS, X-ray Absorption Spectroscopy (XAS) and Magnetic Circular Dichroism (XMCD) techniques. Overall, our results show that complex magnetic molecules can be successfully grafted onto different surfaces, and that possible changes in their magnetic behavior must be considered case by case. \\[4pt] [1] M. Affronte . J. Mater. Chem., 19, 1731 -- 1737 (2009); G.Timco et al. Nature Nanotechnology 4, (2009) 173. \\[0pt] [2] V. Corradini et al., Phys. Rev. B 79, 144419 (2009) \\[0pt] [3] V. Corradini et al., Phys. Rev. B 77, 014402 (2008) \\[0pt] [4] A. Ghirri et al. submitted (2009).