Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session B7: Focus Session: Metalorganic Magnets & Films |
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Sponsoring Units: GMAG Chair: Michael Baker, New York University Room: 106 |
Monday, March 3, 2014 11:15AM - 11:27AM |
B7.00001: Metal / Metal - Free Phthalocyanine Crystalline ``Alloys": Organic Analogues to Diluted Magnetic Semiconductors M. Furis, N. Rawat, L.W. Manning, R. Headrick, S. McGill Magnetic properties of organic crystalline thin films alloys of Phthalocyanines made of a mixture of organo-soluble derivatives of (Co-Pc) and metal-free ($H_{2}$Pc) molecules are investigated. Thin films with metal to metal-free Pc ratios ranging from 1:1 to 1:10 are fabricated using a novel solution processing method that produces macroscopic grains with molecules stacked parallel to each other along the crystalline c-axis. Magnetic Circular Dichroism (MCD) measurements reveal a substantial enhancement of spin-dependent exchange interactions involving the spin polarized electrons localized on the d-orbitals of cobalt and the delocalized $\pi$-electrons, in comparison to the pure Co-Pc films. More importantly, they indicate the presence of a low temperature ferromagnetic state in these films whose characteristics depend on the mixing ratio. We hypothesize this ordered state is caused by an organic analogue of the RKKY mechanism where the delocalized $\pi$-electrons play the role of the conduction and/or valence band electrons that mediate and control the sign of exchange between the localized unpaired d-electron spins. This hypothesis is supported by luminescence results that indicate the $\pi$-electrons wavefunctions extend over at least two neighboring Co spins. [Preview Abstract] |
Monday, March 3, 2014 11:27AM - 11:39AM |
B7.00002: Exchange Mechanisms in Long Range Ordered Thin Film Organic Magnetic Semiconductors N. Rawat, R. Headrick, M. Furis, S. Mcgill, L. Kilanski, R. Waterman Magnetic exchange mechanisms in crystalline thin films of Metal Phthalocyanines (M-Pc) are explored using Magnetic Circular Dichroism (MCD) and SQUID measurements up to 10 T and 2K. Long range ordered thin films of organo-soluble derivatives of Co-Pc and Mn-Pc were fabricated using solution processing technique. In the case of Mn-Pc, our measurements show enhanced hybridization of ligand $\pi$-electronic states with the Mn d-orbitals. MCD active states beyond 1 $\mu$m have been observed for the first time, providing crucial information on the orbital arrangements of MPc's that result in competing (co-existing) long-range superexchange and indirect exchange reminiscent of RKKY. The evolution of Zeeman splitting of specific MCD-active states is very well described by enhanced effective $\pi$-electrons g-factors as large as 100, analogous to diluted magnetic semiconductors (DMS) systems. In Co-Pc MCD data indicates a weaker exchange interaction between delocalized charge carriers and d-like spin-polarized electrons, however SQUID measurements reveal magnetic ordering up to 180K. A comparison between Mn-Pc and Co-Pc and earlier results from the spin 1/2 Cu-Pc and their non-magnetic Zn-Pc counterpart, offers an interesting view on the role of long range order in magnetic interactions. [Preview Abstract] |
Monday, March 3, 2014 11:39AM - 11:51AM |
B7.00003: Charge transport through a 4f spin state in a single molecule magnet Cyrus F. Hirjibehedin, Ben Warner, Philipp Seibt, Michael Waters, Andrew J. Fisher, Joris van Slageren, Fadi El Hallak The coupling between charge and spin in nano-scale systems is of fundamental interest and also key for creating novel devices at this scale. There may be advantages in utilizing magnetism produced by f-shell states, especially in controlled local environments such as molecules. Recently, it has been shown that charge transport through a molecule can access f-shell states despite their localization. Here we show that for charge transport through DyPc$_2$ that is strongly coupled to a copper surface it is possible to directly access the 4f spin. Spatially resolved scanning tunneling spectroscopy shows a variation in the amplitude of a Fano line shape near the Fermi energy, indicative of a Kondo effect due to screening of a localized spin coupled to a metallic continuum. The spin is attributed to the 4f states on Dy rather than the delocalized spin of an electron on the Pc ligands. This work demonstrates that the coupling to the surface can define which spins are present on a molecule as well as whether the spin state can be accessed in transport. [Preview Abstract] |
Monday, March 3, 2014 11:51AM - 12:03PM |
B7.00004: Potential for spin-based information processing in a thin-film molecular semiconductor Marc Warner, Salahud Din, Igor Tupitsyn, Gavin Morley, Marshall Stoneham, Jules Gardener, Zhenlin Wu, Andrew Fisher, Sandrine Heutz, Christopher Kay, Gabriel Aeppli Organic semiconductors are studied intensively for applications in electronics and optics, and even spin-based information technology, or spintronics. Fundamental quantities in spintronics are the population relaxation time ($T_{1})$ and the phase memory time ($T_{2})$: $T_{1}$ measures the lifetime of a classical bit, in this case embodied by a spin oriented either parallel or antiparallel to an external magnetic field, and $T_{2}$ measures the corresponding lifetime of a quantum bit, encoded in the phase of the quantum state. Here we establish that these times are surprisingly long for a common, low-cost and chemically modifiable organic semiconductor, the blue pigment copper phthalocyanine, in easily processed thin-film form of the type used for device fabrication. At 5 K, a temperature reachable using inexpensive closed-cycle refrigerators, $T_{1}$ and $T_{2}$ are respectively 59 ms and 2.6 ms, and at 80 K, which is just above the boiling point of liquid nitrogen, they are respectively 10 ms and 1 ms, demonstrating that the performance of thin-film copper phthalocyanine is superior to that of single-molecule magnets over the same temperature range. [Preview Abstract] |
Monday, March 3, 2014 12:03PM - 12:15PM |
B7.00005: Tunable magnetoresistance driven by magnetically sensitive negative differential resistance in an asymmetrically coupled single molecule junction Ben Warner, Fadi El Hallak, John Sharp, Mats Persson, Andrew J. Fisher, Cyrus F. Hirjibehedin Using scanning tunneling microscopy, we study the effects of interactions between individual magnetic molecules that are separated from an underlying copper surface by a thin-insulating layer of copper nitride (Cu$_2$N). For electrical transport through a junction containing an individual iron phthalocyanine (FePc) molecule on Cu$_2$N, we observe two novel magnetoresistance behaviors that arise from negative differential resistance (NDR) that shifts by unexpectedly large amounts in a magnetic field. Because voltage is dropped asymmetrically in this double barrier junction, the FePc can become transiently charged when its states are aligned with the Fermi energy of the Cu, resulting in the observed NDR effect. Furthermore, the asymmetric coupling magnifies the observed voltage sensitivity of the magnetic field dependence of the NDR - which inherently is on the scale of the Zeeman energy - by almost two orders of magnitude. These findings represent a new basis for making magnetoresistance devices at the single molecule scale. Furthermore, the enhancement of the energy scales created by asymmetric coupling of the junction can be combined with other multi-step tunneling processes to allow for the investigation of other phenomena that normally would be difficult to observe. [Preview Abstract] |
Monday, March 3, 2014 12:15PM - 12:27PM |
B7.00006: Coupling TbPc2 single molecule magnets to antiferromagnetic FeMn layers Corneliu Nistor, Cornelius Krull, Aitor Mugarza, Christian Stamm, Svetlana Klyatskaya, Mario Ruben, Marcio Soares, Sebastian Stepanow, Pietro Gambardella Coupling of single molecule magnets to magnetically ordered (ferromagnetic or antiferromagnetic) layers is a novel research field that has potential applications in molecular-scale spintronic devices. In this study we explore the possibility to magnetically couple TbPc2 molecules to FeMn layers deposited on a Cu (100) substrate. Using X-ray magnetic circular dichroism we demonstrate that, following field cooling, the out-of-plane Tb magnetization loop is vertically shifted and, furthermore, the Tb and Fe magnetization are antiferromagnetically coupled. Additionally, it is found that the Fe magnetization loop is vertically shifted and that this vertical shift depends on the elemental composition of the FeMn layer. The hysteretic behavior of the Tb magnetization together with the horizontal shift of the Tb loop are consistent with the hypothesis that a fraction of the TbPc2 molecules are coupled to the uncompensated Fe spins through a ligand-mediated superexchange mechanism. [Preview Abstract] |
Monday, March 3, 2014 12:27PM - 12:39PM |
B7.00007: Spin transport through one-dimensional transition metal organometallic cluster systems Ivan Stich, Lucia Horvathova, Rene Derian, Lubos Mitas Using very accurate quantum Monte-Carlo methods we have studied vanadium-benzene multidecker clusters, V$_{\mathrm{n}}$Bz$_{\mathrm{n+1}}$, in the range n $=$ 1 - 3. The most important prospective applications of these and related systems are in spintronics as spin filters, which requires them to be half-metal ferromagnets, featuring a semiconducting gap for majority spin electrons and metallic behavior for minority spin electrons. We find that while magnetic structure of these systems is consistent with ferromagnetic coupling, their electronic structure is not consistent with half-metallic behavior as previously assumed, but rather this system is a ferromagnetic insulator with large and broadly similar $\uparrow $-/$\downarrow $-spin gaps implying thus a limited potential of these materials for spintronic applications unless they are further modified or functionalized. [Preview Abstract] |
Monday, March 3, 2014 12:39PM - 12:51PM |
B7.00008: Thin film organic magnet based on vanadium methyl tricyanoethylenecarboxylate Yu Lu, Megan Harberts, Chi-Yueh Kao, Howard Yu, Arthur Epstein, Ezekiel Johnston-Halperin We report a new organic magnetic thin film based on vanadium and methyl tricyanoethylene carboxylate (MeTCEC) prepared by low temperature chemical vapor deposition (CVD). The magnetic ordering temperature, Tc, is above 300K and x-ray spectroscopy reveals the composition of the thin film is V[MeTCEC]$_{\mathrm{2}}$. IR spectroscopy shows both cyano-vandium and carboxylate-vanadium bonding confirming the predicted chemical structure. The temperature dependence of the magnetization reveals spin glass behavior in the thin film below a blocking temperature of 190K. The hysteresis reveals a soft ferrimagnet with coercive field of 10 Oe at 5 K and 20 Oe at 300 K, respectively. Finally transport measurements show semiconducting behavior with an activation energy of 0.56 eV, consistent with the expected bandgap. Taken together, these properties reveal significant potential for a new class of semiconducting organic-based magnetic materials that complement the existing library of M[TCNE] compounds (where M $=$ V, Co, Fe, Mn). [Preview Abstract] |
Monday, March 3, 2014 12:51PM - 1:03PM |
B7.00009: A study of the spin-gaps in the organic conductor Perylene2[Pt(mnt)2] with the application of high magnetic field using an inductive AC susceptibility method Laurel Winter, James Brooks, Pedro Schlottmann, Manuel Almeida, Shermane Benjamin, Claude Bourbonnais To further understand the spin-charge coupling present in the dual-chain organic conductor Per$_2$[Pt(mnt)$_2$] we utilized an inductive ac susceptibility method to study the spin-Peierls (SP) ordered state.$^2$ Besides reaffirming the coexistence of the SP-CDW below 8 K and 20 T, our measurements also showed the emergence of a second spin-gapped phase above 20 T that coincides with the previous observed field-induced insulating phase. Our results provide support for the continued coupling of the charge and spin order parameters even in high magnetic fields and hints at the possibility of further spin and charge gaps above 45 T. $^2$ L.E. Winter, J.S. Brooks, P. Schlottmann, M. Almeida, S. Benjamin, and C, Bourbonnais, $\mathit{Europhys. Lett.}$, $\mathbf{103}$ (2013) 37008. [Preview Abstract] |
Monday, March 3, 2014 1:03PM - 1:15PM |
B7.00010: Spin-dipole moment in low symmetry structures Biplab Sanyal, Sumanta Bhandary, Soumyajyoti Haldar, Olle Eriksson The spin-dipole contribution (T$_{z}$) is usually neglected in x-ray magnetic circular dichroism measurements for bulk systems, as the value is negligible compared to the spin moment. However, in the last few years, it has been demonstrated quite clearly from experiments and theory that T$_{z}$ can acquire relatively large values for systems with low dimensions, e.g., organometallic molecules like Fe porphyrine/phthalocyanine [1] or small inorganic clusters. In some cases, the large T$_{z}$ contribution can be opposite to the spin moment and hence, the effective moment (2S+7T$_{z}$) turns out to be very small [2]. With the aid of first principles density functional calculations, the role of T$_{z}$ will be demonstrated for organometallic molecules and magnetite nanoparticles. The calculated effective moments follow the same trend as experimental measurements.\\[4pt] [1] S. Bhandary {\it et al.}, Phys. Rev. Lett. {\bf 107}, 257202 (2011); S. Bhandary {\it et al.}, Phys. Rev. B {\bf 88}, 024401 (2013). \\[0pt] [2] H. Herper {\it et al.}, Phys. Rev. B {\bf 87}, 174425 (2013). [Preview Abstract] |
Monday, March 3, 2014 1:15PM - 1:27PM |
B7.00011: Novel effects at metal-organic interfaces: Magnetic interactions between high-spin molecules and non-magnetic metals triggered by interface chemistry Amilcar Bedoya, Luis Hueso The adsorption of molecules on metal surfaces presents a rich variety of physical phenomena, which move from the creation of interface dipoles to hybridization and charge-transfer via strong chemisorption. In the strong interaction regime, some metal-molecule systems could even undergo a surface rearrangement and lead to the formation of new magnetically active phases, which could be used as templates for spin-injection or magnetization switching. For this purpose, we study the interaction of novel high-spin quinoline molecules (Tb$_{3}$q$_{9})$ with non-magnetic metallic surfaces. The molecules preserve their structural, chemical and magnetic properties when deposited onto noble metal (Au) and passivated (Si02) surfaces; while the adsorption on reactive metals such as Cu induces a magnetic phase at the interface involving molecular Tb-atoms, as measured via SQUID magnetometry and X-ray magnetic circular dichroism (XMCD). Remarkably, the magnetic ordering persists up to room-temperature for the Tb$_{3}$q$_{9}$/Cu system and is linked to a chemically-triggered change in structure and stoichiometry of the interfacial species. The occurrence of a molecular-driven magnetic phase at otherwise nonmagnetic metal surfaces highlights the importance of interface chemistry to tailor new magnetic interfaces and functional hybrid structures for spintronic applications. [Preview Abstract] |
Monday, March 3, 2014 1:27PM - 1:39PM |
B7.00012: Metal free half metallic graphitic carbon nitride on BN layer Jisang Hong, Arqum Hashmi We have investigated structural, adsorptive, and magnetic properties of metal free graphitic carbon nitride ({\it g}-C$_4$N$_3$) layer on hexagonal BN layer (h-BN) using the optB88-vdW van der Waals density functional theory. The free standing {\it g}-C$_4$N$_3$ layer is known to have (2 $\times$ 2) surface reconstructed structure with 0.3 $\AA$ buckling feature. However, the surface reconstruction disappears on BN layer and the {\it g}-C$_4$N$_3$ layer becomes flat. Interestingly, the {\it g}-C$_4$N$_3$/BN hybridized system has a new lattice constant which differs from that of either BN or {\it g}-C$_4$N$_3$ and this lattice change is responsible for adsorption of {\it g}-C$_4$N$_3$ on BN layer. More surprisingly, we have observed half metallic behavior in {\it g}-C$_4$N$_3$ even on BN layer. We propose that our theoretical prediction can be verified using normal incidence of K-edge X-ray magnetic circular dichroism (XMCD) measurement and also our finding indicates that the {\it g}-C$_4$N$_3$/BN system can be utilized for novel metal free spintronics material. [Preview Abstract] |
Monday, March 3, 2014 1:39PM - 1:51PM |
B7.00013: Ab-initio study of p magnetism in CaN and CaC monolayers on Cu(001) Hadi Akbarzadeh, Maedeh Zahedifar, Zahra Nourbakhsh, S. Javad Hashemifar Ab-initio calculations are performed to study the p ferromagnetic CaC and CaN compounds in the zinc-blend (zb) and rock-salt (rs) structures and their monolayers on Cu(001). It is observed that within the generalized gradient functional both structures of the bulk compounds are half-metal, while the rs structure exhibits higher stability. It is argued the strong interatomic exchange interaction in these systems controls the splitting of the spin resolved bond points while it has no considerable effect on total bond strength. In contrast to the bulk compounds, the CaC and CaN monolayers on Cu(001) generally favor the zb structure and the anion terminated monolayers are more stable than the cation terminated ones. On the other hand, the anion terminated systems are non- or weak magnetic systems while the less stable cation terminated layers exhibit strong magnetization. In the case of CaN monolayer on Cu(001), the nudged elastic band calculations show an activation barrier of 1.18 eV per CaN unit between the higher energy ferromagnetic and the stable nonmagnetic terminations. Therefore, epitaxial growth of a Ca terminated CaN thin film on Cu (001) is likely a practical way to form a novel half-metal/metal junction. [Preview Abstract] |
Monday, March 3, 2014 1:51PM - 2:03PM |
B7.00014: Giant Rashba effect and Dirac points in deep d-orbital noble metal surface states Ryan Requist, Polina Sheverdyaeva, Paolo Moras, Carlo Carbone, Erio Tosatti The chiral spin polarization, band splitting, and topological states generated by Rashba spin-orbit interaction at crystal surfaces and interfaces have received a lot of attention recently. Most studies have focused on $sp$ states near the Fermi energy, which are relevant for transport and have long lifetimes. Far less explored, although in principle stronger, are Rashba effects within d states, including those deep below the Fermi energy. Here we report a joint ARPES/first principles study of ``giant" Rashba effects in the deep $d$ surface states of a 20-layer Ag film grown on Au(111) and a 20-layer Au film grown on Ag(111). Several surface states predicted in [1], some split by $\sim$1 eV, are clearly observed in good overall agreement with first principles calculations. We also find Dirac points at the time-reversal symmetric $M$ point within a large spin-orbit-induced bulk gap, which are visible in both Ag and Au and display all the characteristics of topological surface states, such as chiral spin polarization and robustness to perturbation. Unlike the usually symmetric dispersion at $\Gamma$ point Dirac cones, these $M$ point cones are strongly anisotropic away from the degeneracy point. [1] R. Mazzarello, A. Dal Corso, E. Tosatti, Surf. Sci. 602, 893 (2008). [Preview Abstract] |
Monday, March 3, 2014 2:03PM - 2:15PM |
B7.00015: Ferromagnetic ordering in an insulator by itinerant electrons J.N.B. Rodrigues, Aigu L. Lin, A.H. Castro Neto, S. Adam, Andrew T.S. Wee Motivated by recent experimental work of variable range hopping of electrons between magnetic nanoparticles in oxidized graphene, we consider theoretically an ensemble of randomly oriented classical Heisenberg magnetic moments which are superparamagnetic at room temperature and have negligible magnetostatic coupling. Itinerant electrons hopping through random sites experience a Zeeman coupling with these moments. Using Monte Carlo simulations, we demonstrate that this generates an effective electron-mediated coupling between the Heisenberg moments giving rise to spontaneous magnetization of the sample. We make predictions for the temperature dependence of this magnetization and compare with experimental data. [Preview Abstract] |
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