Exotic Dimers Part Two: A HighSpin Ferrous Dimer that
Exhibits Genuine Spontaneous
Long Range 3DMagnetic Order in Zero Field
and Metamagnetism
W. M. Reiffa, M. Feistb, and E. Kemnitzb
aDepartment
of Chemistry, Northeastern University, Boston MA, 02115, USA;
E-mail:w.reiff@neu.edu; bInstitut für Chemie der Humboldt Universität,
Brook-Taylor-Str. 2, D-12489 Berlin, Germany, e-mail: feistm@chemie.hu-berlin.de,
erhard.kemnitz@chemie.hu-berlin.de
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 (STotal = o) ground state. However, some exceptions exist.
Thus in the series Cs3Cr2X9 (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, 272-276, 218 219.
(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)(H2O)3CL3 CHROMOPHORES
(3)S. I. Troyanov, M. Feist, and E. Kemnitz, Z. Anorg. Allg. Chem., 625, 806 (1999).
1
[----- = Cl–H HYDROGEN BONDS]
The neutral structure(3) of 1 is stabilized with accompanying 1,4-dimethylpiperazinium
dications and “free”chloride anions not shown for purposes of clarity.
The intradimer antiferromagnetic exchange leads to a set of five S Total states
spanning ST = 0, 1, 2, 3, 4, where the separation (2J) of the ground singlet, ST = 0,
and first excited state, a triplet, ST=1 for the case of 1 is ~ 1 K.
Best fit (HDVV model) J/kB= ~-0.5K (-0.35cm-1)
From 80K to 5K, the magnetic moment of 1 gradually decreases from near the
spin only value of high spin Fe2+ (4.9/Fe) to~ 3/Fe, a factor of ~1.6. This
behavior is typical of weak intra-dimer antiferromagnetic exchange, J/kB  -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!
125 Hz
HAC=1 Oe
0.6
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 FeCl2 for which Hcritical  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).
Conclusion
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.