W. M. Reiff a, M. Feist b, and E. Kemnitz b a Department of Chemistry, Northeastern University, Boston MA, 02115, USA; b Institut für Chemie der Humboldt Universität, Brook-Taylor-Str. 2, D Berlin, Germany, Exotic Dimers Part Two: A High- Spin Ferrous Dimer that Exhibits Genuine Spontaneous Long Range 3D- Magnetic Order in Zero Field and Metamagnetism

Simple dimers with antiferromagnetic intradimer exchange are generally not expected to exhibit spontaneous, long range antiferromagnetic order in zero field owing to their non-magnetic, singlet (S Total = o) ground state. However, some exceptions exist. Thus in the series Cs 3 Cr 2 X 9 (X = Cl, Br, I); the chloride and bromide magnetically order but only in very large externally applied fields, pulsed fields out to ~ 42 T for T ~ 0.5 K (so called field induced magnetic ordering(1)). The iodide does indeed order in zero field at ~7 K as confirmed via inelastic neutron scattering(2). (1)Ajiro, Y; Inagaki, T; Asano, H; Mitamura, H; and Goto, T, J. Magn. Magnet. Mat., 2003, , (2)Leuenberger, B., Güdel, H. and Fischer, P., Phys. Rev. Lett. 1985,55, 2983 The possible interdimer magnetic exchange interactions in such dimers are weak and correspond primarily to close contacts of delocalized metal electron spin density on the halogens as opposed to direct interdimer metal-metal magnetic exchange interactions. Hence, the zero field ordering of the above iodide is not surprising and is fully consistent with its well established largest polarizability in the halogen series. This in turn leads to the greatest spin delocalization to the iodine centers on the periphery of the dimer In this vignette, we present results for a ferrous dimer that acts as a text book example of the situation in which there is clear competition between intra- and interdimer exchange interactions. The latter prevail at low temperature and lead to genuine spontaneous long range 3D-order in zero field as discerned via the Mössbauer Effect. This outcome stems from the hydrogen bonded exchange network of the structure illustrated on the next slide.

EXTENSIVELY HYDROGEN BONDED CHAINS OF DI-CHLORO BRIDGED NEUTRAL FERROUS DIMERS (HEREAFTER 1) BASED ON MER-Fe(II)(H 2 O) 3 CL 3 CHROMOPHORES The neutral structure(3) of 1 is stabilized with accompanying 1,4-dimethylpiperazinium dications and “free”chloride anions not shown for purposes of clarity. [----- = Cl–H HYDROGEN BONDS ] (3)S. I. Troyanov, M. Feist, and E. Kemnitz, Z. Anorg. Allg. Chem., 625, 806 (1999). 1

Best fit (HDVV model) J/k B = ~-0.5K (-0.35cm -1 ) The intradimer antiferromagnetic exchange leads to a set of five S Total states spanning S T = 0, 1, 2, 3, 4, where the separation (2J) of the ground singlet, S T = 0, and first excited state, a triplet, S T =1 for the case of 1 is ~ 1 K.

From 80K to 5K, the magnetic moment of 1 gradually decreases from near the spin only value of high spin Fe 2+ (4.9  /Fe) to~ 3  /Fe, a factor of ~1.6. This behavior is typical of weak intra-dimer antiferromagnetic exchange, J/k B  -0.5K for 1. However, below  5K, 1’s magnetic moment/Fe further decreases rapidly to ~ 0.6  at 1.4K, a factor of 6! 0.6  125 Hz H AC =1 Oe

Magnetic hyperfine splitting of the zero field Mössbauer spectrum of 1 essentially coincident with the inflection point (~ 4.3 K) in  ’ vs T is evident for T  5K. At 1.49 K, the nuclear Zeeman splitting is fully resolved and corresponds to an internal field of 14 T. The long range 3-dimensional magnetic order suggested by these observations is further confirmed by dual phase AC susceptibility (1 Oe, 125 Hz) measurements that indicate that 1 has an uncanted antiferromagnetic ground state, i.e.  = 0. Rapidly Relaxing Paramagnet Saturated Uncanted Antiferromagnet

Definitive evidence for the antiferromagnetic nature of the system’s magnetic ground state comes from the observation of a well defined metamagnetic phase transition at 1.95 T in the (DC) field dependence of the AC susceptibility [4] at 1.97 K, cf: to similar behavior for anhydrous FeCl 2 for which H critical  1T at 4.2 K. One concludes that the inter-dimer (hydrogen bonding based) super-exchange for 1 at T  5 K is of sufficient magnitude as to lead to long range (cooperative 3d) order “before” the moments of individual dimers “die out” with decreasing T owing to their intrinsic (intra-dimer) antiferromagnetic exchange. [4] A. J. Van Duyneveldt, J. Appl. Phys., 53, 8006 (1982).

It seems evident that the Mössbauer Effect clearly provides a facile tool for discerning genuine magnetic order. This is especially so for cases such as the present where the crossover from seemingly simple antiferromagnetcally coupled dimers to long range order is quite subtle in the context of conventional susceptibility methods. In addition, as an experimental technique, it is quite complementary to other zero field techniques, e.g. constant pressure heat capacity and powder neutron diffraction measurements. There is little doubt that it will continue as an invaluable perhaps first choice method for detailed investigations of highly correlated electron spin behavior in iron based systems. Conclusion