Polyhedron 20 (2001) 1459 – 1463
Experimental observation of quantum coherence in molecular
magnetic clusters with half-integer spin
F.L. Mettes
a
, G. Aromı ´
b
, F. Luis
c
, M. Evangelisti
a
, G. Christou
d
, D. Hendrickson
e
,
L.J. de Jongh
a,
*
a
Kamerlingh Onnes Laboratory, Leiden Uniersity, PO Box 9506, 2300 RA, Leiden, The Netherlands
b
Leiden Institute of Chemistry, Leiden Uniersity, PO Box 9502, 2300 RA, Leiden, The Netherlands
c
Instituto de Ciencia de Materiales de Arago´n, Uni. de Zaragoza, Facultad de Ciencias, 50009 Zaragoza, Spain
d
Department of Chemistry, Indiana Uniersity, Bloomington, IN 47405 -4001, USA
e
Department of Chemistry, Uniersity of California at San Diego, La Jolla, CA 92093, USA
Received 17 September 2000; accepted 15 December 2000
Abstract
From field-dependent specific heat studies we conclude that magnetic relaxation becomes temperature independent for two Mn
4
compounds, below some temperature T
Q
. At low temperatures, the specific heat gives evidence for the presence of a tunnel
splitting
t
of the ground state S
z
= 9/2 for both compounds. The predicted quenching of
t
for clusters with half-integer spin
apparently does not occur, probably due to the joint action of dipolar and hyperfine fields, in combination with the applied
perpendicular field of sufficient strength. © 2001 Elsevier Science Ltd. All rights reserved.
Keywords: Magnetic relaxation; Manganese; Molecular magnetic clusters; Specific heat
www.elsevier.nl/locate/poly
1. Introduction
We present specific heat data on magnetic clusters of
Mn
4
O
3
X(OAc)
3
(dbm)
3
, with X =Cl and OAc, in the
temperature range T =0.2–6 K and in magnetic fields
up to 6 T. At high temperatures (T 1 K) we find that
our data can be represented by a total spin S =9/2 per
cluster, which is zero-field split by a uniaxial an-
isotropy. For T 1 K, the specific heat shows an up-
turn for both compounds, which can be explained by
introducing dipolar interactions between the clusters
and hyperfine interactions within each cluster. The hy-
perfine contribution from the Mn nuclei can be calcu-
lated exactly [1,2] and, consequently, the average
strength of the dipolar fields can be estimated.
The field dependence of the specific heat at low
temperatures (T 1 K) reveals fascinating magnetic
quantum behavior for the total spin of the clusters. At
low temperatures the magnetic relaxation time of the
clusters can become much longer than the characteristic
time in the experiment, resulting in a freezing or block-
ing of the magnetic moment. We have studied the
magnetic relaxation as a function of temperature and
observed that it becomes temperature independent be-
low some temperature, denoted T
Q
. Clearly, T
Q
will
depend on the strength of the anisotropy. Below T
Q
,
magnetic relaxation can occur through the (tunnel-split)
Fig. 1. Representation of the Mn
4
O
3
X cluster core.
* Corresponding author. Present address: Faculty of Mathematics
and Natural Sciences, Leiden University, PO Box 9506, 2300 RA,
Leiden, The Netherlands. Tel.: +31-71-5275625; fax: +31-71-
5275404.
E-mail address: dejongh@phys.leidenuniv.nl (L.J. de Jongh).
0277-5387/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
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