AVS2009 Session MI-FrM: Molecular/Organic Based Magnetism

In recent years a broad range of magnetic and magnetotransport phenomena have been reported for organic semiconductors. Unpaired spins in the p orbitals of organic systems have been shown to yield new physics and be the basis of potential new technologies of broad interest. Organic-based magnets with magnetic ordering temperatures from a few K [1] to > 400 K [2] have been successfully synthesized. In addition, new magnetic phenomena have been discovered including fractal magnetism, photonically controlled magnetism, nanoscale magnetic bubbles, and fully spin polarized magnetic semiconductors[3]. Further, magnetotransport (MR) in magnetic and nonmagnetic organic semiconductors has revealed a host of heretofore unknown spin-dependent phenomena, including 20% change in resistance at room temperature for application of as little as 100 Oe to nonmagnetic organic semiconductors. The room temperature magnetic semiconductors V[TCNE]_x~2, (TCNE ≡ tetracyanoethylene) [2,3] has many new properties such as fully spin polarized energy bands and magnetism from 0 to 400 K and photonic response. The analogue Fe[TCNE]_x forms monolayer thick spin layers that produce ‘spin bubbles’ upon application of a critical field.

These advances in science have prompted interest in the possibility of technologies based on these new materials. These new potential technologies will be discussed with emphasis on organic-based spintronics including tunneling magnetoresistance (TMR) and giant magnetoresistance (GMR) devices and also use of these materials as sensors in the emerging THz range.

This work was supported in part by NSF, DOE, AFOSR, and OSU Inst. for Mater. Res.

[1] S. Chittipeddi, K. Cromack, J.S. Miller, and A.J. Epstein, “Ferromagnetism in Molecular DecamethylferroceniumTetracyanoethenide (DMeFc)(TCNE)”, Phys. Rev. Lett. 58, 2695 (1987).

[2] J.M. Manriquez, G.T. Yee, R.S. McLean, A.J. Epstein, and J.S. Miller,“A Room Temperature Molecular/Organic-Based Magnet”, Science 252, 1415 (1991).

[3] A.J. Epstein, “Organic-based Magnets: Opportunities in Photoinduced Magnetism, Spintronics, Fractal Magnetism, and Beyond”, Mater. Res. Soc. Bull. 28, 492-499 (2003).

[4 V.N. Prigodin, J.D. Bergeson, D.M. Lincoln, and A.J. Epstein, “Anomalous Room Temperature Magnetoresistance in Organic Semiconductors”, Synth. Met. 156, 757 (2006).

[5] T. Francis, O¨. Mermer, G. Veeraraghavan, and M. Wohlgenannt, “Large Magnetoresistance at RoomTemp. in Semiconducting Polymer Sandwich Devices”,New J. Phys. 6, 185 (2004).

Acknowledgements: This work was supported by the DFG within the GrK 611 and the SFB 668-A5 and by the European Union in the project “SPiDMe”.

Over the last decade there has been a tremendous effort in order to create new kinds of supramolecular organic nanostructures on surfaces, with the prospect of possible catalytic, electronic, optical or magnetic applications. In particular, a lot of attention has been paid to metalorganic coordination networks (MOCNs), with the idea of creating functional metallo-supramolecular arrays on surfaces which combine the properties of their constituent metal ions and ligands.

Following this approach, the chemisorption of small molecules with ending carboxylic acids on metal surfaces has been extensively studied and deprotonation of the acid groups to produce carboxylate groups described [1]. These deprotonated groups can interact strongly with both metal surfaces and metal adatoms (either intentionally deposited or already existing as a 2D background gas that results from the emission of atoms from low coordination sites such as steps and kinks). This metal-to-carboxylate interaction, when properly addressed, leads to the formation of regular patterns of MOCNs. We have deposited oxalic acid, i.e. the smallest possible molecule with two carboxylic groups (C2O4H2), on non magnetic Cu(100) surfaces, both clean and with a small pre-deposited amount of Fe. Scanning Tunnelling Microscopy (STM) shows that moderate annealing of these systems lead to the formation of two different, new MOCNs: a rectangular copper-oxalate network, and a honeycomb iron-oxalate network, where the regularly spaced Fe spins have the smallest distance (5.2 Å) reported up to date, making the Fe-oxalate MOCN a promising system for an in-depth study of their magnetic properties.

[1] S. Stepanov, N. Lin, J. V. Barth, J. Phys.: Condens. Matter 20 (2008) 184002.

M-TCNE molecule-based ferrimagnets demonstrate high magnetic ordering temperatures up to 400 K (M = V) due to strong AFM exchange between d-electrons of the metal and the anion-radical spin of the TCNE ligand. Magnetic and optical properties of the family of molecule-based 2D magnets [M(TCNE)(NCMe)₂] X (M=Fe, Mn, Ni; X=BPh₄, FeCl₄, SbF₆) will be discussed. Classical bonding-sensitive IR spectroscopy has difficulties distinguishing between the bonding and backbonding interactions (possibly mediating the strong superexchange), since their effects on CN bond stretching mode frequencies may cancel each other. In contrast, Raman active n(C=C) modes solely depend on charge transfer to/from the p* antibonding orbital, and thus are only backbonding sensitive. The observed strengthening of the n(C=C) and n(NºC) Raman modes for the compounds with higher T_c suggests the depopulation of the p* orbital and enhanced ligand to metal charge transfer resulting in a hybrid M3d-CN ground state with substantial admixture of ligand electron. The observed pressure-induced strengthening of the n(C=C) and n(NºC) Raman modes is in accord with proposed backbonding model. The correlation between the frequency shift (degree of backbonding) and magnitude of T_c from M(T) is established.