Session J18: Focus Session: Low D/Frustrated Magnetism - Molecular Magnets I

Magnetic field-induced XY-AFM in 2D Heisenberg Antiferromagnet [Cu(pyz)$_{2}$(pyO)$_{2}$](PF$_{6})$

Specific heat ($C_{p})$ and magnetic susceptibility (\textit{$\chi $}) measurements were performed on the two-dimensional spin-1/2 Heisenberg antiferromagnet [Cu(pyz)$_{2}$(pyO)$_{2}$](PF$_{6})_{2}$ in DC and pulsed magnetic fields up to H = 15 T and 40 T, respectively [1]. We observe no long-range magnetic order down to 500 mK in zero applied magnetic field, suggesting that [Cu(pyz)$_{2}$(pyO)$_{2}$](PF$_{6})_{2}$ is close to an ideal 2D AFM and instead undergoes a Berezinskii-Kosterlitz-Thouless (BKT) transition. However, the application of a finite magnetic field induces a clear anomaly in $C_{p}$, although not in \textit{$\chi $}. This behavior is known to be a remarkable signature of magnetic field induced XY-AFM [2]. In addition, $C_{p}$(H) measurements in pulsed fields, performed down to T = 1.5 K and up to H = 40 T, were used to map out the asymmetric Field-Temperature phase diagram which provides additional support for an ideal realization of field-induced XY-AFM in [Cu(pyz)$_{2}$(pyO)$_{2}$](PF$_{6})_{2}$.\\[0pt] [1] Y. Kohama et al., \textit{Rev. Sci. Instrum.} \textbf{81}, 104902 (2010).\\[0pt] [2] A. Cuccoli et al., \textit{Phys. Rev. }\textbf{B68}, 060402(R) (2003).   

Pressure-induced Jahn-Teller axis switching in Cu(pyz)F$_2$(H$_2$O)$_2$?

We employed infrared spectroscopy along with complementary lattice dynamics and spin density calculations to investigate local structure and magnetism through the series of pressure-driven transitions in Cu(pyz)F$_2$(H$_2$O)$_2$. Rather than frequency shifts that dovetail with the recently proposed pressure-induced Jahn-Teller switching model, we overall mode hardening, particularly in the Cu--OH$_2$ bending mode. We combine these findings with a reanalysis of the crystal structure to reveal the series of pressure-induced transitions as a combination of $a$-axis rotation, $c$-directed compression that acts to weaken O-H...F hydrogen bonds, and pyrazine ring buckling. The magnetic dimensionality crossover can be understood in terms of changes in magnetic orbital overlap.   

Lost and found: The missing diabolical points in the Fe$_8$ molecular magnet

The tunneling spectrum of the single-molecule-magnet Fe$_8$ is known to have diabolical points (DP's). For magnetic fields along the hard axis, there are four such points for tunneling between the ground pair of levels, whereas the simplest model including only second-order anisotropy would predict ten DP's. The difference is due to a very weak fourth-order anisotropy, which in a semiclassical picture generates instantons with endpoint discontinuities, one of which dominates for large enough fields, and having no interfering partner, causes six of the underlying DP's to go away. However, as shown by Bruno, the six missing DP's do not truly disappear, but merely move off the hard axis into the hard-medium plane. In this talk, we report on a numerical search for these ``missing" DP's. This search is nontrivial because the energy surface is like a smooth golf course, on which the DP's are extremely localized and deep holes. We therefore locate the DP's by following the lines of the Berry curvature which have monopole singulairities at the DP's. This exercise is performed for tunneling between excited pairs of levels also. An experimental observation of the rediscovered DP's would be an important test of the underlying spin Hamiltonian for Fe$_8$. (Submitted to the arxiv: Nov.~18, 2010.)   

Magnetic Superatoms

The electronic states in metal clusters are grouped in shells much in the same way as in atoms. Filling of the electronic shells leads to stable species called magic numbers. This has led to the preposition that selected stable metal clusters can mimic chemical properties of atoms on the periodic table and can be classified as superatoms. Here, we propose an extension of the superatom concept to magnetic species by invoking systems that hybridize localized and delocalized electronic states. Through first principles studies focusing on the electronic structure and magnetic moment, we show that TMMg$_{n}$ (TM = Sc, Ti, V, Cr, Mn, Fe, Co, and Ni) clusters exhibit a new class of magnetic superatoms stabilized by magnetic supershells. The talk will include possible applications of the new building blocks.   

Tunnel-diode Resonator Spectroscopy of Quantum Levels in Cr$_{12}$Ln$_{4}$ (Ln=Y,Eu,Gd,Tb,Dy,Ho,Yb) Magnetic Molecules

The differential magnetic susceptibility for a series of Cr$_{12}$Ln$_{4}$ (Ln=Y,Eu,Gd,Tb,Dy,Ho,Yb) magnetic molecules was measured in static (up to 16 T) and pulsed (up to 45 T) magnetic fields using a rf tunnel-diode resonator (TDR). At low temperatures, the behavior of these finite spin systems is governed by discrete energy spectra of the individual molecules. In magnetic field, low-energy quantum levels Zeeman-split, crossing at field values where magnetization exhibits a step corresponding to switching between different spin states. In high fields, we detect multiple level crossings which allow for a detailed mapping of the energy diagram. We then perform quantum Monte Carlo (QMC) using a Heisenberg Hamiltonian with three adjustable exchange constants whose values are chosen so as to optimize agreement with the experimental energy spectrum. The variations in results for the studied molecules are correlated to the magnetic properties of the lanthanide ions.   

Spin-electric coupling in Cu$_3$, V$_{15}$ and other frustrated molecular magnet rings: a first principle study

Frustrated triangular single-molecule magnets (SMMs) without inversion symmetry, such as Cu$_3$ and V$_{15}$, are characterized by a doubly degenerate S=1/2 ground-state (GS) with opposite chirality. Recently it has been proposed theoretically [1] and verified by ab-initio calculations [2] that an external electric field can couple these two chiral spin states, even in the absence of spin-orbit interaction (SOI). The efficiency of these coupling depends on the electric dipole moment between chiral states. In this talk we report on first-principle calculations of the coupling strength for the triangular SMMs Cu$_3$ and V$_{15}$. The spin-electric coupling is found to be considerably stronger in V$_{15}$ than in Cu$_3$. We discuss the mechanism leading to an enhanced spin-electric coupling, which can be used as a convenient guide to synthesize SMMs that can respond more efficiently to an external electric field. \\[4pt] [1] M. Trif et.al. Phys. Rev. B 82, 045429 (2010). Mircea Trif et.al. Phys. Rev. Lett. 101, 217201 (2008) \\[0pt] [2] M.F. Islam et.al. Phys. Rev. B 82, 155446 (2010)   

Magnetic anisotropy and high-spin effects in single-molecule transistors

Fabrication of single-molecule transistors where electron transport occurs through an individual molecule has become possible due to the recent progress in molecular electronics. Three-terminal configuration allows charging molecules and performing transport spectroscopy in multiple redox states. Single-molecule magnets combining large spin with uniaxial anisotropy are of special interest as appealing candidates for high density memory applications and quantum information processing. We study single-molecule magnets Fe$_4$. Three-terminal junctions are fabricated using electromigration of gold nanowires followed by a self-breaking. High-spin Kondo effect and inelastic cotunneling excitations show up in transport measurements. Several excitations feature the energy close to the energy of zero-field splitting (ZFS) of a ground spin multiplet in bulk. This splitting is caused by the anisotropy and is a hallmark of single-molecule magnets. We observe nonlinear Zeeman effect due to a misalignment of an anisotropy axis and a magnetic field direction. The ZFS energy is increased in oxidized and reduced states of the molecule indicating enhancement of the anisotropy in these states.   

Magneto-optical spectroscopic studies of solid and solution-phase tetra-phenyl porphyrin

Tetraphenylporphyrin (TPP) is a synthetic heterocyclic compound that serves as a model system for heme proteins and cytochromes. TPP can accomodate a metal ion in the center; D-shell ion porphyrin complexes with a crystalline solid phase are of interest for magnetic studies because of the possibility of macroscopic long range magnetic order of the ion spins. We have investigated the 5K magnetic properties of poly-crystalline thin films of the heme protoporphyrin IX analogue tetra-phenyl porphyrin, complexed with Zn and Mn, deposited through a capillary pen technique that produces 100um to 1 mm sized grains. Our novel experimental setup measures the UV/VIS, linear dichroism and magnetic circular dichorism simultaneously, incorporates a photoelastic modulator and a microscopy superconducting magnet for high-field (5T) measurements. We present solution and crystalline data on metal-complexed TPP; data are analyzed in terms of A and B-type MCD using a perimeter model. We find good agreement with previous solution data, and novel crystalline phase spectra that are correlated to the long range ordering.   

Chiral molecule for spin filtering purposes: the study of L- and D-Alanine

The field of molecular electronics has attracted scientists by the great opportunities and versatility it offers as a replacement for standard semiconductor electronics with organic materials, thus bringing down the cost, and opening endless possibilities for chemical synthesis, and scientific breakthrough. Of particular interest is the use of chiral molecules, such as alanine, for spin filtering studies in hope of creating highly spin-polarized charge carriers for spintronics applications. Preliminary studies of both L- and D-alanine on Cu(111) were conducted using scanning tunneling microscopy and spectroscopy, revealing the formation of a 2-dimensional phase at low coverage, a hexagonal ``flower'' pattern at intermediate coverage, and a chain and ring superstructures at high coverage. A model is proposed to explain the surface chemistry and bonding of the molecules on the metallic surface. Current studies of L- and D-alanine on Fe/W show promises in the intermediate coverage regime.   

DFT and STM studies of magnetism in single Co(TCNE) complexes on an ultrathin insulating film

We present results from large-scale ab initio DFT calculations for geometry and electronic structure of Co(TCNE) complexes on a c($2\times 2)$Cu$_2$N substrate. The long-term aim is to study charge and spin transport in molecular systems. The work is done in concert with STM experiments. In particular, we perform calculations to help explain STM observations indicating that the electronic and magnetic properties of Co(TCNE) complexes vary with apparent molecular orientation. Using plane wave DFT with a GGA functional and spin polarization, we perform geometry optimization to identify the most commonly seen orientations of Co(TCNE). We further study the resulting electronic structure, using calculated LDOS and simulated STM images, to compare with observations. To study the strong in-plane magnetic anisotropy suggestd by spin-flip spectroscopy, we do noncollinear magnetic calculations on the relaxed structure, including spin-orbit coupling effects.   

Non-equilibrium transport through a single molecular Kondo impurity in a scanning tunneling microscope junction

An unpaired spin from a single electron trapped in a molecular orbital in a double barrier scanning tunneling microscope (STM) junction at sub-Kelvin temperature and high magnetic field showed a non-equilibrium transport through a Kondo impurity. Hysteresis and switching in a conductance allows the spin and charge state of the molecule in the junction to be controlled. Mechanically tuning the coupling of the single spin to STM tip showed a gradual change from lowest order spin-flip inelastic tunneling spectroscopy (IETS) to the Kondo resonance. Using the imaging capability of STM, we observed clear sub-molecular node structures of the spin-flip IETS and the Kondo resonance.   

Electronic transport through single-molecule magnets by scanning tunneling spectroscopy

Atomic structure of molecules and electrodes are expected to sensitively influence the properties of molecular spintronics devices. We have studied the transport properties of individual Mn$_{4}$ single-molecule magnets bound to a surface using atomic force and scanning tunneling microscopy at cryogenic temperatures Unlike previous scanning probe microscopy experiments, we are able to continuously tune the density of states of individual molecules using novel device geometries in-situ We will discuss transport properties of single-molecule magnets as a function of their atomic structure, coupling to electrodes and the Fermi levels.   

Magnetic excitations from an $S$=1/2 antiferromagnetic tetramer system Cu$_2$PO$_4$OH

Cu$_2$PO$_4$OH is a candidate material for the $S$=1/2 diamond-shaped antiferromagnetic tetramer system.\footnote{A. A. Belik $et$ $al.$, Inorg. Chem. 46, 8684 (2007).} The magnetic susceptibility shows a spin-gap behavior and the exchange interaction $J$ was estimated to be 138 K. Since there have not been so many experimental studies in the spin tetramer systems, it is important to clarify the magnetism in this compound. We have performed inelastic neutron scattering experiments on a powder sample of Cu$_2$PO$_4$OH on a chopper neutron spectrometer ARCS installed at SNS at ORNL in order to study the magnetic excitations from the tetramer spin system. We have clearly observed two magnetic excitations at $\sim$12 and $\sim$20 meV, whose widths in energy are broader than the instrumental resolution. It was found that the energy levels cannot be explained with the simple antiferromagnetic tetramer model with only nearest-neighbor interaction. We will discuss the results including further-neighbor interactions.   

High-field EPR study of a ReCl$_{4}$(CN)$_{2}$ molecular magnet building block

Slow magnetic relaxation has been observed in the single-chain magnet (DMF)$_{4}$MReCl$_{4}$(CN)$_{2}$ (M = Mn, Fe, Co, Ni) [D. Harris \textit{et al}., J. Am. Chem. Soc. \textbf{132}, 3980 (2010)]. The ReCl$_{4}$(CN)$_{2}$ \textbf{(1)} molecule has been synthesized in which the local environment of the Re$^{IV}$ ion is same as in the single-chain magnet. Electron Paramagnetic Resonance (EPR) measurements have been performed on single crystal of complex \textbf{1} to determine the magnetic anisotropy of the Re$^{IV}$ ions. Both intra and inter Kramer's doublet transitions are observed in high-field (up to 36T) EPR experiments. The data indicate a significant axial anisotropy of the easy-plane type ($D>$ 0), with sizeable rhombic $E$ term. In light of these findings, we are developing a theoretical model to account for the slow relaxation in the single-chain magnet.