Session J33: Focus Session: Spin Dependent Physics in Organic-Based Materials I

Characterization of Novel Double-Chain Metal Monophosphonates as Magnetic Dimers

The magnetic properties of three iso\-struc\-tural metal mono\-phos\-pho\-nates, \mbox{M\{(2-C$_5$H$_4$NO)CH$_2$PO$_3$\}(H$_2$O)$_2$} with M~=~Co~({\bf 1}), Ni~({\bf 2}), Mn~({\bf 3}), were investigated as potential anti\-ferro\-mag\-netic spin ladders whose magnetic moment can be tuned by the metal ion. Along with a diamagnetic Cd analog, these compounds possess a double-chain crystal structure where the \mbox{M$_2$($\mu$-O)$_2$} dimers are bridged by \mbox{O-P-O} chains. The low-field magnetic susceptibilities for compounds {\bf 1} and {\bf 2} indicate that the systems are non-interacting dimers. During isothermal magnetic field sweeps at temperatures down to 50~mK, a signature associated with the spin-flop transition is observed. On the other hand, compound {\bf 3} appears to have a dimer-like superexchange that is similar in strength to three dimensional magnetic interactions present in the material.   

Upper Critical Fields of Moleular-Based Spin Ladders

The upper critical (saturation) fields of several spin ladders was found using a high field, short pulse magnet at LANL. These compounds include $Cu(Quinoxaline)Cl_2$, $Cu(Quinoxaline)Br_2$, $Cu(2,3 dimethylpyrazine)Cl_2$, $Cu(2,3 dimethylpyrazine)Br_2$, $Cu(methylpyrazine)Br_2$, and $Cu(methylpyrazine)Cl_2$. The data were taken at temperatures as low as $460$ mK with a pulsed field strength as high as 57 tesla. The upper critical fields were estimated by considerations of overcoming the exchange energies associated with all the compounds. These energies were extracted from model fits to the susceptibility data. These estimations agree reasonably well with the experimentally observed upper critical fields. We report on these results and compare the data to simulations of the magnetization as a function of field as a further check to confirm that these compounds obey the associated model systems.   

Giant Antiferromagnetically-Coupled Moments in a Diruthenium Molecule-Based Magnet with Interpenetrating Sublattices*

The molecule-based magnet [Ru$_{2}$(O$_{2}$CMe)$_{4}$]$_{3}$[Cr(CN)$_{6}$] contains two interpenetrating cubic sublattices. Each sublattice is magnetically frustrated by the easy-plane anisotropy of the spin-3/2 diruthenium paddlewheel complexes, which are antiferromagnetically coupled to spin-3/2 Cr ions at the cube corners. Consequently, each cubic sublattice has a non-collinear spin state with net moment along one of the cubic diagonals. The moments of the two interpenetrating sublattices behave like giant moments that are antiferromagnetically coupled at small fields and become aligned above a critical field of about 1000 Oe $\sim $ K$_{c}$/$\mu _{B}$,$_{ }$where K$_{c }\sim $ 2 x 10$^{-3 }$meV is the weak coupling between sublattices. Due to the small critical field, the magnetic correlation length can be directly estimated from the field and temperature dependence of the magnetization while a polycrystalline sample undergoes the metamagnetic transition. *R.S. Fishman, S. Okamoto, W.W. Shum, and J.S. Miller, \textit{Physical Review B} \textbf{80}, 064401 (2009).   

Coexistence of ordered and disordered magnetic moments in [Cu(pyz)$_{2}$(VF$_{6})$]\textbullet 4H$_{2}$O and comparison to [Cu(HF$_{2})$(pyz)$_{2}$]SbF$_{6}$ (pyz = pyrazine)

Quasi-2D [Cu(pyz)$_{2}$(VF$_{6})$]\textbullet 4H$_{2}$O (\textbf{1}) and [Cu(HF$_{2})$(pyz)$_{2}$]SbF$_{6}$ (\textbf{2}) have tetragonal symmetry and consist of 2D [Cu(pyz)$_{2}$]$^{2+}$ square lattices that are linked in 3D by bridging VF$_{6}^{2-}$ (\textbf{1}) or HF$_{2}^{-}$ (\textbf{2}) anions. Magnetic susceptibility shows apparent paramagnetism in \textbf{1} whereas a broad maximum at 12.5 K and sharp kink at 4.3 K indicate short- (SRO) and long-range (LRO) magnetic ordering, respectively, for \textbf{2}. Additional experimental data however, indicate that a LRO state occurs below 3.6 K for \textbf{1}. The observed LRO in \textbf{1} is confined to the Cu-sublattice while the V$^{4+}$ magnetic moments remain disordered. The structural and magnetic behavior of \textbf{1} and \textbf{2} will be described.   

The Spin Dependent Near Fermi-Edge Structure of M[TCNE] Magnets

M[TCNE] organic-based magnets are an important class of solids for both application and magnetic exchange and correlation study. The detailed spin polarized occupied electronic structure of M[TCNE] magnets has eluded description from conventional ligand field theory, the results of elementally- or spin-sensitive photon and electron spectroscopies as well as spin resolved density functional calculations. This talk will present heuristic models for M=V, Fe and Ni in the context of the local physical structure and all electronic structure studies completed to date, but with a new twist to the onsite and nearest neighbor Coulomb repulsion based correlation.   

A comparison of the spin polarized unoccupied electronic structure of [Fe$^{II}$(TCNE$^{-})$(NCMe)$_{2}$]$^{+}$[Fe$^{III}$Cl$_{4}$]$^{- }$determined by XMCD vs. UV-Vis MCD

A direct comparison between XMCD and UV-Vis MCD of the energy resolution and spin polarization of the unoccupied electronic structure of [Fe$^{II}$(TCNE$^{-})$(NCMe)$_{2}$]$^{+}$[Fe$^{III}$Cl$_{4}$]$^{-}_{ }$will be presented. Our studies suggest that the enhanced m$_{j}$ selectivity in UV-Vis MCD, from partially occupied initial states, leads to a greater energy resolution of the unoccupied electronic structure of Fe[TCNE] vs. that allowed by XMCD and may be true for magnetic solids in general. Further, we will show evidence of the spin polarization of the lowest unoccupied state for the Fe[TCNE] organic-based magnet in the context of it's applied use in magnetoelectronics.   

Bonding and Magnetic Exchange in Metal-[TCNE] Magnets

The correlation between indirect magnetic exchange and bonding is an outstanding question in condensed matter physics and is also directly related to understanding the mechanism that governs magnetic ordering temperature, electron delocalization and Coulomb correlations resulting in high carriers' spin-polarization in molecular-based magnetic solids. The relationship between this magnetic exchange pathway and the chemical bonding that holds the solid together is yet unclear. In the first room-temperature molecule-based magnetic semiconductor V[TCNE]$_{x}$ (TCNE = tetracyanoethylene; $x \quad \sim $ 2), ferrimagnetism was attributed to a strong antiferromagnetic exchange between unpaired V$^{II }$3$d \quad t_{2g}$ electrons and a delocalized unpaired electron residing on the singly occupied TCNE\textit{ $\pi $}* molecular orbital, while both these states do not participate in chemical bonding, yet they exhibit a strong indirect (e.g., super, double) magnetic exchange. Resolving this problem can provide an indispensible information allowing to achieve a long term goal of direct spin polarization injection into a semiconductor, and presents an exciting opportunity for incorporating organic-based magnets into contemporary data processing systems.   

Tuning Molecule-Mediated Spin Coupling in Bottom-Up-Fabricated Vanadium-Tetracyanoethylene Nanostructures

We have fabricated hybrid magnetic complexes from vanadium atoms and tetracyanoethylene (TCNE) ligands via atomic manipulation with a cryogenic scanning tunneling microscope. Using tunneling spectroscopy we observe spin-polarized molecular orbitals as well as a structure-dependent Kondo resonance. For complexes having two V atoms, the Kondo behavior can be switched on and off by a minute structural change, even as the spin-containing orbitals remain unchanged. This can be explained by variable spin-spin (i.e., V-V) ferromagnetic exchange coupling through the TCNE molecule, as supported by density functional calculations. These findings offer a new route for designing molecule-based magnetic nanostructures with tunable spin-spin exchange coupling.   

Successful fabrication and characterization of V[TCNE]$_x$-based hybrid spin-LED

V(TCNE)$_x$ (x$\sim$2) is a fully spin polarized organic-based magnet with an ordering temperature above room temperature (T$_c$ $>$ 350 K). Chemical vapor deposited (CVD) magnetic V[TCNE]$_x$ films also exhibit semiconductor-like charge transport behavior with a room temperature conductivity of 10$^{-2}$ S/cm and activation energy of $\sim$ 0.5 eV. Electronic transport through V[TCNE]$_x$ leads to spin-polarization of free carriers due to splitting of the $\pi^{\ast}$ band in [TCNE]$^-$ into two subbands (occupied: $\pi^{\ast}$ and unoccupied: $\pi^{\ast} +$ U$_c$ ) with opposite spin polarization, driven by on-site Coulomb repulsion. We have successfully constructed a hybrid III-V/V[TCNE]$_x$ spin light emitting diode (spin-LED) device and investigated its electrical and magnetic properties. We observed strong temperature dependence of the turn on voltage and positive magnetoresistance, indicating charge flow through the V[TCNE]$_x$ layer ($\sim$ 400 nm). Detailed electrical characterization of the hybrid device and fabrication techniques will be presented and implications for the optical detection of electrical spin injection in hybrid organic/inorganic devices will be discussed.   

Optical detection of electrical spin injection in a V(TCNE)$_x$ –based hybrid spin-LED

The integration of the organic-based magnet V(TCNE)$_x$ (x$\sim$2, T$_c$ $>$ 350 K) with inorganic compound semiconductors offers the potential for a new class of hybrid spintronic structures and devices. This work realizes that potential by coupling a GaAs/AlGaAs quantum well light emitting diode (LED) with a V(TCNE)$_x$ spin injector to create a hybrid organic/inorganic spin-LED. In control measurements, optically excited photoluminescence from a V(TCNE)$_x$ coated quantum well show no significant magnetic circular dichroism. In contrast, magneto-transport studies verify the electronic coupling of the magnetization of the V(TCNE)$_x$ to charge flow through the structure and circular polarization of the electroluminescence from a full spin-LED device (2$\%$ at 0.1 T and 60 K) follows the magnetization curve of V(TCNE)$_x$. Together, these results demonstrate optical detection of electrical spin injection across the organic/inorganic interface. This demonstration in turn lays the foundation for a new class of hybrid spintronic structure.   

Spintronic applications of the organic semiconductors

Spintronic applications of organic semiconductor have received growing attentions recently. We demonstrated all main operations of the spintronics, the spin injection/detection/transport/polarizing within/between organic non-magnetic/magnetic semiconductors. The thorough study of the device characteristics unravels the physical mechanisms concerning how the spins are transferred into and through an organic semiconductor [1]. The carrier injection into the organic semiconductor is well described by the phonon assisted field emission. The spin valve effects in our devices, which exhibit a charge transfer regime in the bulk of the organic semiconductor, provide proof of the electrical detection of the spin injection and transport across the organic semiconductor layer [1]. Moreover, the organic-based hybrid magnetic system holds an unique perspective of a highly spin polarized electronic structure, suitable for the spintronic applications [2]. The observed tunneling magnetoresistance using the organic- based magnetic semiconductor introduces a new avenue for the realization of the organic spintronics. [1] Yoo et al., Phys. Rev. B in press [2] Prigodin et al., Adv. Mater. 14 1230 (2002)   

Organic multiferroic tunnel junctions with ferroelectric poly(vinylidene difluoride) barriers

Organic polymers, such as poly(vinylidene difluoride) (PVDF), form high quality ordered layers and exhibit robust ferroelectricity down to a monolayer [1]. This property makes PVDF polymers promising as barriers in multiferroic tunnel junctions (MFTJs) -- devices which exhibit multiple resistance states associated with different magnetization and ferroelectric polarization configurations [2]. In this work we present first-principles calculations of the spin-polarized tunneling conductance of crystalline Co/PVDF/Co(0001) MFTJs. Using the Landauer-B\"{u}ttiker formalism implemented within a plane-wave pseudopotential method we calculate spin-resolved transmission for parallel and antiparallel magnetization of the electrodes. Our calculations predict a negative spin polarization of the tunneling conductance and a sizable tunneling magnetoresistance (TMR) in these junctions. Further efforts are aimed at exploring the tunneling electroresistance (TER) effects in asymmetric MFTJs where a monolayer of Au is deposited at one of the interfaces. Our results indicate that organic ferroelectric materials may open a new promising direction in organic spintronics. [1] A. V. Bune et al, \textit{Nature} \textbf{391}, 874 (1998). [2] J. P. Velev et al, \textit{Nano Lett.} \textbf{9}, 427 (2009).   

Contacting Organic and Organometallic Monolayers with Ferromagnetic Electrodes

A key technical challenge on the way to functional molecular spintronic devices is solving the so-called ``contact problem.'' That is, how to robustly make electrical contact to such devices without damaging the molecular system. Methods that have been shown to have varying degrees of success in solving this problem for charge based devices include deposition of a passivating layer, either thermally (e.g. TiO$_{x}$) or by atomic layer deposition (e.g. Al$_{2}$O$_{3}$), as well as lift-off, float-on electrodes. We present data assessing the utility of these methods for depositing ferromagnetic electrodes onto fatty acid Langmuir-Blodgett monolayers as an eventual means of studying molecular spin transport. The morphology of the deposited electrodes, measured by AFM, is compared with the quality of the underlying monolayer, examined by Brewster angle microscopy, contact angle analysis and AFM. Progress towards fabricating molecular magnetic tunnel junctions with one or more of these techniques will be discussed.