FERROMAGNETISM IN MOLECULAR ...

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[9] Iwamura, H., these proc. [lo] Miller, J. S., Calabresse, J. C., Rommelmann, H.,. Chittipeddi, S. R., Zhang, J. H., Reiff, W. M. and. Epstein, A. J., J. Am. Chem. SocĀ ...
JOURNAL DE PHYSIQUE Colloque C8, Suppl6ment au no 12, Tome 49, decembre 1988

FERROMAGNETISM IN MOLECULAR DECAMETHYLFERROCENIUM TETRACYANDETHANIDE (DMeFc) (TCNE) A. J. Epstein (I), S. Chittipeddi

(I)

and J. S. Miller (2)

(I) Department of Physics, The Ohio State University, Columbus, Ohio 43210-1106, U.S:A. (2) Central Research and Development Department, E.I. de Pont de Nemours and Go, INC, Wizmington, Delaware 19898, U.S.A.

Abstract. - The first molecular ferromagnet, (DMeFc) (TCNE), has a 1-D Heisenberg-like behavior for T > 116 K, with a crossover to 3-D behavior below 16 K. Spontaneous ferromagnetism occurs for T < 4.8 K, as studied by magnetization and neutron diffraction. A generalized Hubbard model has been proposed to account for the ferromagnetic behavior. Transition metal and rare earth based ferromagnets have been known for several millenia and their properties have been an important basis for the development of modern technologies. The development of magnetic materials, the understanding of the origins of ferromagnetic exchange and the influence of the anisotropy in the exchange interactions have been the subjects of intense study in recent years. The possibility of the occurence of ferromagnetism in molecular or organic materials has motivated study of new classes of materials. McConnell had early on suggested local ferromagnetic coupling through Heitler-London spin exchange between. positive spin density on one radical and negative spin density on another [I]. Subsequently it was suggested that molecules with a partly filled doubly degenerate orbital (other than overhalf filled) can have ferromagnetic exchange through the role of a virtual triplet excited state [2-51. A third generalized approach involves the stabilization of ferromagnetic exchange through topological symmetry within covalently bonded networks containing singly occupied degenerate nonbonding molecular orbitals [6-91. We summarize the results of our recent studies of (DMeFc) (TCNE). These experiments are the first confirmation of the existence of a ferromagnetic ground state in a molecular material [lo, 111. (Bulk ferrimagnetism in systems containing lAn2+ (S = 512) and stabilized by either three dimensional ferrimagnetic exchange [12] or dipolar interactions [13] have subsequently been reported.) Temperature T and magnetic field (H) dependent studies of the susceptibility (x) and magnetization (M) of single crystals of (DMeFc) (TCNE) for H parallel and perpendicular to the stacking axis show that the system behaves as a one dimensional ferromagnetic chain with nearly Heisenberg like exchange above 16 K. Below 16 K three-dimensional correlations dominate, resulting in mean field like critical exponents culminating with a transition to the 3-D ordered ferromagnetic state at 4.8 K. The ferromagnetic order has been confirmed by T-dependent powder neutron diffraction. A model for the origin of the ferromagnetic exchange based upon the formation of an excited state triplet has been proposed and can be represented in terms of a degenerate Hubbard model [5, 111.

The (DMeFc) (TCNE) salt as grown from acetonitrile and after loss of solvant has an orthorombic unit cell [lo]. The data discussed here refer to this phase, which consists of stacks of alternating (DM~FC)' and TCNE-', figure 1. Half of the adjacent stacks are approximately in registry and half are approximately out of registry [lo, 14161. The (DM~FC)+' radical ion has five 3d electrons, with two occupying the d: orbital and three in the degenerate d,, and d,z-,2 orbitals resulting in a net cation spin of S = 112. In contrast, the highest occupied orbital of the anion radical is the singly occupied b3,n* orbital (S = 112) . Magnetic susceptibility studies of isomorphous metallocene charge transfer salts with either spinless cations (for example, DM~CO') or anions (for example, C3 (CN),- ) showed the absence of any significant exchange interaction and that spin on both metallocene cation and organic anion were essential for ferromagnetic exchange in this system [15].

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Fig. 1. - A view of the decamethyl/ferrocene (DMeFc) (1) and tetracyanoethylene (TCNE) (2) molecules. At room temperature the magnetic susceptibility measured parallel to the stacking axis, xll, is nearly equal to that calculated for the sum of two indepen~4 2 [ll].For T > 100 K, dent of g r M e F c and X I /(T) can be fit to a mean-field Curie-Weiss behavior. Below 100 K, the experimental XI] is substantially less than the 3-0 mean field prediction. Taking account of the stack-like crystal structure, it is expected that the exchange interaction between chains is much weaker than that within a stack of alternating (DMFC+') and (TCME-') . Using a Pad6 approximation [17] an excellent fit is obtained for the 1-D Heisenberg model

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19888366

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[17-191 for J = 27.4 K. Below 16 K, diverges more rapidly than predict by the 1-D model, varying as x cc (T - TC)-' with Tc=4.8 K and y = 1.2 K [ll], indicative of the onset of a 3-D fluctuation regime. Measurements of x~..alsoagree with calculated values of x after adjustment for a small (< 10") crystal alignment error. Ising coupling is too week to account for XI (T) and it is better fit with a one dimensional Heisenberg model for T > 16 K using JI = 15.8 K. The small JLillustrates that this material is not quite an ideal Heisenberg system [20]. Below 16 K, XI diverges in a manner similar to with nearly identical y, supporting the 3-D nature of the regime above the ferromagnetic transition. Below T,= 4.8 K spontaneous magnetization is observed [ll]. Application of a magnetic field results in hysteresis loops, with the coercive field increasing with decreasing temperature. For HII, the saturation moment, M!"', is equal to that calculated for the sum of aligned donor and acceptor spins [ll], while for HI, only partial hysteresis loops are observed consistent with domain formation [20]. Measurements of the H dependence of MILat Tc yields M cc H'/' with S 4.4, a critical exp.&nentlarger than that expected for mean field theory [21]. Neutron diffraction studies at Brookhaven National Laboratory confirm the ferromagnetic nature of the 4.8 K transition [20]. It is suggested that the origins of the ferromagnetic exchange lies in the presence of an excited state triplet state present due to the occupation of the degenerate d,z-,z and d,, orbitals of the (DM~FC+)by three electrons 15, 14-16]. If the lowest energy virtual excited state is the ( D M ~ F C + ~(TCNE-2, ) configuration, then Hund's rule favors ferromagnetic alignment of spins. A more complete representation of the exchange interaction may use a generalization of the degenerate Hubbard model [3]. Among the terms to be included are S E (the difference in site energy between an electron in the DM~FC' d,~-,Z or d,, orbital and one in the TCNE- bs, orbital), U, (the Coulomb repulsion for two electrons in the bs, orbital of TCNE-2), Udz2-ar2and Ud,, (the Coulomb repulsion for 2 electrons in either the d,z-,z or d,, orbitals of DMeFc respectively), Vd (the Coulomb repulsion between one electron each in the d,, and d,~-,z orbitals of a DMeFc), Va, (the Coulomb repulsion between electrons on adjacent DMeFc d and TCNE n orbitals (dependent or crystallographic direction), and Vdd and V,, for Coulomb repulsion between adjacent DMFc and TCNE molecules (dependent on crystallographic direction) respectively. In addition the nearest neighbor charge transfer integrals need to be parametrized. The competition between ferromagnetic and antiferromagnetic exchange terms determines whether a particular system will be ferromagnetic, antiferromagnetic or metamagnetic in its ground state. Judicious choice of metallocene and/or acceptors allows the chemical

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control of the magnitude and sign of the exchange interaction 151. In sum, extensive study of the first molecular ferromagnet, (DMeFc) (TCNE) shows nearly a 1-D Heisenberg like ferromagnetic behavior for T > 16 K with a crossover to 3-D behavior below 16 K. Spontaneous ferromagnetism occurs for T < 4.8 K. A generalized Hubbard model has been proposed to account for the ferromagnetic behavior. Acknowledgment This work was supported in part by the U.S. Department of Energy, Division of Materials Science, Grant NO. DE-FG02-86ER45271,AOOO. [I] Mc Connell, H. M., J. Phys. Chem. 39 (1963) 1910. [2] Mc Connell, H. M., Proc. Robert A. Welch Found. Chem. Res. 11 (1967) 144. [3] Lyon-Caen, C. and Cyrot, M., J. Phys. C 8 (1975) 2091. [4] Breslow, R., Pure Appl. Chem. 54 (1982) 927. [5] Miller, J. S. and Epstein, A. J., J. Am. Chem. SOC.109 (1987) 3850. [6] Mataga, N., Thwr. Chim. Acta 10 (1968) 372. [7] Ovchinnikov, A. A., Thwr. Chim. Acta 47 (1978) 297. [8] Nasu, K., Phys. Rev. B 33 (1986) 330. [9] Iwamura, H., these proc. [lo] Miller, J. S., Calabresse, J. C., Rommelmann, H., Chittipeddi, S. R., Zhang, J. H., Reiff, W. M. and Epstein, A. J., J. Am. Chem. Soc. 109 (1987) 769. [ll] Chittipeddi, S., Cromack, K. R., Miller, J. S. and Epstein, A. J., Phys. Rev. Lett. 58 (1987) 2695. [12] Journaux, Y., Van Koningsbruggen, P., Lloret , F., Nakatani, K. Pei, Y., Kahn, 0. and Renard, J. P., these proc. [13] Caneschi, A., Gatteschi, D., Renard, J. P., Rey, P. and Sessoli, R., these proc. [14] Miller, J. S., Epstein, A. J. and Reiff, W. M., Accts. Chem. Rev. 21 (1988) 114. [15] Miller, J. S., Epstein, A. J. and Reiff, W. M., Chem. Rev. 88 (1988) 201. [16] Miller, J. S., Epstein, A. J. and &iff, W. M., Science 240 (1988) 40. [17] Baker, Jr., G. A., Rushbrooke, Gr. S. and Gilbert, H. E., Phys. Rev. 135 (1964) A1272. [18] Bonner, J. and Fisher, M. E., Phys. Rev. 135 (1964) A640. [19] Schlottmann, P., Phys. Rev. Lett. 54 (1985) 2131. [20] Chittipeddi, S., Selover, M. A., Shapiro, S. M., Miller, J. S. and Epstein, A. J., to be published. [21] Stanley, H. E., Introduction to Phase Transitions and Critical Phenomena (Oxford Univ. Press, Oxford) 1971.