Session W13: Focus Session: Low-Dimensional and Molecular Magnetism - Metallorganics and Spincrossover Molecular Magnets

Magnetic properties of selected Prussian Blue Analogs (PBAs)

Prussian Blue Analogs consists of MC$_{6}$ and AN$_{6}$ octahedra connected by cyanide ligands (M, A= metals). They typically crystallize in cubic structures. We have studied temperature and field dependence of the magnetization and the susceptibility of selected Prussian Blue Analogs such as hexacyanocobaltates, -ferrates and -chromates. All compounds exhibit modified Curie --Weiss behavior in the paramagnetic region. The observed effective moments of those compounds were compared with the ones of the respective free-ion values. Furthermore, we find evidence that a few of the compounds exhibit a transition to long-range magnetic order at low temperatures.   

Persistent Photocontrolled Magnetism in Core-Shell Prussian Blue Analogues

Cubic heterostructured (\textbf{BA}) particles of Prussian blue analogues, composed of shells of ferromagnetic K$_j$Ni$_k$[Cr(CN)$_6$]$_l\cdot n$H$_2$O (\textbf{A}), \mbox{$T_c\sim70$~K}, surrounding bulk cores ($\sim350$~nm) of photoactive ferrimagnetic Rb$_a$Co$_b$[Fe(CN)$_6$]$_c\cdot m$H$_2$O (\textbf{B}), $T_c\sim20$~K, have been studied. \mbox{Below} $T_c\sim 70$~K, these samples exhibit a persistent photoinduced decrease in low-field magnetization, resembling results from previous core-shell particles\footnote{M.F. Dumont \emph{et al.}, Inorg. Chem. \textbf{50} (2011) 4295.} and analogous \textbf{ABA} films.\footnote{D.M. Pajerowski \emph{et al.}, J. Am. Chem. Soc. \textbf{132} (2010) 4058.} This net decrease suggests that the photoinduced lattice expansion in the \textbf{B} layer generates a strain-induced decrease in the magnetization of the \textbf{A} layer, similar to a pressure-induced decrease observed by others in a pure \textbf{A} material\footnote{M. Zentkov\'a \emph{et al.}, J. Phys.: Condens. Matter \textbf{19} (2007) 266217.} and by us in the \textbf{BA} cubes. To quantify the length scale over which the photoinduced strain dissipates into the \textbf{A} layer, a series of \textbf{B} and \textbf{BA} cubes of varying shell thickness have been characterized.   

Pressure-induced local lattice distortions in Co(dca)$_2$

We employed vibrational spectroscopy along with complementary lattice dynamics and spin density calculations to investigate local structure and magnetism through the series of pressure-induced transitions in Co(dca)$_2$. Analysis of several ligand bending modes reveals compression and distortion of molecular linkages and a major change in the crystal lattice through the 1 GPa transition, whereas a modified local structure (due to a change in molecular symmetry) but similar crystal lattice is anticipated above 3 GPa. We discuss our findings in terms of the competition between antiferromagnetic and ferromagnetic exchange interactions.   

Magnetotransport Properties of Switchable Valence Tautomer Films

We report tunneling magnetoresistance (TMR) and spectroscopy results in trilayer stacks of spincoated molecular films sandwiched between two metallic thin film electrodes. The molecules, cobalt dioxolene complexes, exhibit switchable bistable paramagnetic states as a result of intramolecular electron transfer accompanied by a spin-crossover of the Co ion, the so-called valence tautomerism (VT). Temperature dependent differential conductance measurements show changes in the density of states that correspond to the temperature dependent VT transition. The observed electronic transition is shown to be switchable by light exposure, which correlates the counterpart in the magnetic susceptibility. In trilayers with two ferromagnetic electrodes, e.g. Permalloy and Co, spin valve effects have been observed above and below the VT transition, including room temperature. The dependence of the TMR effect on bias voltage, light exposure, and molecular film thickness has been examined systematically as a function of temperature, aimed at exploring the effects of spin dependent states in the VT molecules.   

Computational studies of the Fe(II) spin crossover compound Fe[H$_2$B(pz)$_2$]$_2$(bpy)

Using calculations from first principles, we studied the electronic and transport properties of the Fe(II) spin crossover (SCO) compound Fe[H$_2$B(pz)$_2$]$_2$(bpy). The magnetic transition has been imposed by constrained magnetization calculations and the computed electronic structure agrees with available experimental data. Besides the characterization of the single molecule, we constructed a ?-stacking molecular chain of the compound and evaluated electronic transport in the direction of the chain for both the low-spin and the high-spin configurations. We found the high-spin configuration to be more conductive than the low-spin case, in agreement with experimental measurements of corresponding currents through disordered thin films. Molecule-molecule interactions are taken into account by the London dispersion forces. The spin-switchable electronic transport properties of this kind of Fe(II) SCO compound systems provide viable proofs for future switchable molecular spintronic devices and applications.   

Highly Unquenched Orbital Moment In Fe Phthalocyanine

Metal-Phthalocyanine molecules (MPc) form a family of compounds with a wide range of commercial application such as catalysts or dyes, and more recently in thin film technology. In an early work we found that in the $\alpha $-phase of FePc, where the FePc molecules are stacked in a herringbone structure, the Fe atoms are strongly magnetically coupled into ferromagnetic Ising chains with very weak antiferromagnetic interchain coupling. The chains achieve 3D long range ordering at T$_{N}$=10 K, and strong irreversibility (slow relaxation) below 5K. The Fe(II) is in a S=1 state and the hyperfine field in the ordered phase reaches a record value in Fe(II) of B$_{hf}$=66.2 T. This result is consistent with a large, unquenched orbital moment. It has been measured directly in a X-ray Magnetic Circular Dichroism (XMCD) spectroscopic study on FePc thin films deposited parallel on a Au surface predeposited on a Si substrate. The XMCD spectra at the L$_{3}$ and L$_{2}$ edges were measured as a function of incident angle $\gamma $. The orbital moment is $\left| {m_L } \right|=0.53\pm 0.04\mu _B $ and the isotropic spin component is $m_S =0.64\pm 0.05\mu _B $. The origin of this unusually high orbital moment is the incompletely filled e$_{g}$ level lying close to the Fermi energy. The ferromagnetically coupled Fe moments show strong, in-plane anisotropy [1]. Angular dependent measurements at the Fe K-edge also show strong quadrupolar excitations associated to a strong orbital moment, confirming the above result of the existence of a large, unquenched orbital moment in this molecule. Submonolayer FePc thin films deposited on Au, recently studied my XAS and XMCD have shown that there is charge transfer from the substrate to the Fe atom, modifying the electronic structure and magnetic properties [2] \\[4pt] [1] J. Bartolom\'{e} et al., Phys. Rev. B 81, 195405 (2010) \\[0pt] [2] S. Stepanow et al., Phys. Rev. B 83, 220401(R) (2011).   

Theoretical modelling of exchange interactions in metal-phthalocyanines

The theoretical understanding of exchange interactions in organics provides a key foundation for quantum molecular magnetism. Recent SQUID magnetometry of a well know organic semiconductor, copper-phthalocyanine [1,2] (CuPc) shows that it forms quasi-one-dimensional spin chains. Green's function perturbation theory calculation [3] is used to find the dominant exchange mechanism. Hybrid density functional theory simulations [4] give a quantitative insight to exchange interactions and electronic structures. Both calculations are performed for different stacking and sliding angles for lithium-Pc, cobalt-Pc, chromium-Pc, and copper-Pc. The exchange interactions depend strongly on stacking angles, but weakly on sliding angles. Our results qualitatively agree with the experiments, and remarkably $\alpha $-cobalt-Pc has a very large exchange above liquid-Nitrogen temperature. Our theoretical predictions on the exchange interactions can guide experimentalists to design novel organic semiconductors. \\[0pt] [1] S. Heutz, et. al., Adv. Mat., 19, 3618 (2007) [2] Hai Wang, et. al., ACS Nano, 4, 3921 (2010) [3] Wei Wu, et. al., Phys. Rev. B 77, 184403 (2008) [4] Wei Wu, et. al., Phys. Rev. B 84, 024427 (2011)   

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

Tetraphenylporphyrin (TPP) is a heterocyclic model system for porphyrins found in heme proteins, cytochromes and photosynthetic cofactors. TPP can accommodate 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 TPP complexed with Zn, Mn and Cu and deposited through a room temperature capillary pen technique that produces grain size in the 100 micron to 1mm range. Our novel setup measures the UV/VIS, linear dichroism and MCD simultaneously and incorporates a photoelastic modulator and a microscopy superconducting magnet for high-field (5T) measurements. In addition, we present 25T data on samples from the new split magnet at NHMFL. 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 of the solid state.   

Spin exchange interaction in quasi-1D Cu-phthalocyanine crystalline thin film measured by Magnetic Circular Dichroism (MCD) spectroscopy

Highly-oriented Cu-phthalocyanine (PC) pen-written crystalline thin films can be viewed as quasi-1D spin 1/2 magnetic chains. In order to reveal the nature of spin exchange between localized S=1/2 Cu spins, MCD spectroscopy was performed on films with millimeter-sized grains fabricated from a soluble CuPc derivative in magnetic fields up to 10 Tesla at temperatures ranging from 0.4 K to 77K. At T$<$2K and B$<$4T the MCD associated with transitions between Q-band $\pi$ electron states exhibits a non-linear temperature-dependent Brillouin-like increase with magnetic field. For B$>$4T the MCD evolves linearly with magnetic field, as expected from diamagnetic carbon-based systems. Theoretical modeling\footnote{W. Wu et. al.,PRB 84,024427(2011)} of electronic structure and exchange interactions in this system predicts an indirect exchange mechanism mediated by delocalized ligand states. Our MCD measurements identified the states responsible for this exchange.   

Introducing copper phthalocyanine as a qubit

Quantum information processing (QIP) has been shown to solve certain useful problems faster than its classical counterpart. However finding a physical system upon which to execute these algorithms is a challenging task. One promising implementation is to use an electron spin in a magnetic field as the information bearing quantum system. Numerous options have been proposed along these lines. Here I discuss a new candidate qubit, copper phthalocyanine. The copper atom at the centre of the molecule carries an unpaired electron. Pulsed electron paramagnetic resonance measurements of relaxation times reveal that it has potential for QIP. We measure the spin-lattice and spin-spin relaxation times of this electron and demonstrate single qubit manipulations. Solid-state electronic devices can be built with this low cost material, which is optically active, and offers great opportunities for chemical and physical modification, leading to significant control of magnetic and other properties.   

Structure-Controlled Coercivity of Low-Dimensional Metallo-Organic Thin Films

Metallo-organic thin films based on iron phthalocyanine were prepared to form self-assembled structures. Based on the substrate, the metal-ion chains can be grown perpendicular or parallel to the substrate. Individual iron chains are separated by 1.3 nm and interact weakly at low temperatures. The magnetic response of these structurally templated thin films is studied for a fixed film thickness and varied deposition temperatures. The magnetic hysteresis loops are wasp-shaped and mark long-range ferromagnetic interaction below a temperature of 4.5 K. The hysteresis loop coercivity can be correlated with the grain size of the iron phthalocyanine thin film. The enhanced coercivity is attributed to longer iron chains that are formed in larger grains.   

The Otto thermodynamic cycle using the magnetic molecule Ni$_2$

In order to design realistic molecular heat engines, the study of quantum thermodynamics is essential since classical thermodynamics does not apply in this extreme miniaturization limit [1,2]. Realizing a thermodynamic cycle on an existing magnetic molecule embodies a novel and unique approach to understand and exploit the thermodynamic properties of spin at the molecular level.\\ Here we propose an Otto cycle in the Ni$_2$ dimer based on a fully ab-initio calculation of the electronic states and the perturbative inclusion of spin-orbit coupling. A laser pulse, described by the time-dependent Schr\"{o}dinger equation, is used to heat the Ni$_2$ dimer. The pulse not only excites the electrons to higher, many-body electronic states, but also influences the spin of the system due to spin-orbit coupling. Using a low-temperature thermal bath the system is cooled back to the ground state. The adiabatic work exchange between the Ni$_2$ and the environment is described by the quasi-static expansion or compression of the bond length of the dimer. The calculated efficiency of the cycle is up to 34\%.\\ $[1]$ T. D. Kieu, Phys. Rev. Lett. {\bf 93 } 140403 (2004)\\ $[2]$ H. T. Quan, Phys. Rev. E {\bf 79} 041129 (2009)\\ $[3]$ T. Zhang \emph{et al.}, Phys. Rev. A {\bf 75} 062102 (2007)