Proton NMR study in hexanuclear manganese single-molecule magnets
M. Belesi,
1
X. Zong,
1
F. Borsa,
1,2
C. J. Milios,
3
and S. P. Perlepes
3
1
Ames Laboratory-USDOE and Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011, USA
2
Dipartimento di Fisica “A. Volta,” ed Unità CNISM, Università di Pavia, I27100, Pavia, Italy
3
Department of Chemistry, University of Patras, 265 00 Patras, Greece
Received 5 November 2006; published 12 February 2007
We report a detailed proton NMR study, as a function of temperature and external magnetic field, of two
hexanuclear manganese magnetic molecule clusters with chemical formula Mn
6
O
2
O
2
CMe
2
salox
6
EtOH
4
·4EtOH in short Mn
6
acetate and Mn
6
O
2
O
2
CPh
2
salox
6
EtOH
4
·4EtOH henceforth Mn
6
ben-
zoate. Both clusters are characterized by a ferrimagnetic ground state with total spin S
T
=4 and a large uniaxial
anisotropy, which gives rise to an effective energy barrier for the relaxation of the magnetization of the order
of U
eff
28 K. The main characteristics of the
1
H NMR spectra measured between 1.5 K and room tempera-
ture for different fields are explained in terms of the dipolar hyperfine interaction of the proton nuclei with the
adjacent magnetic ions. At low temperatures T 3.5 K, the spectra broaden significantly and become struc-
tured due to the slowing down of the local fluctuating fields at the proton sites, caused by the gradual freezing
of the Mn
3+
moments into the S
T
= 4 collective ground state. The spin dynamics of the exchange coupled
magnetic ions was also probed by proton spin-spin relaxation rate T
2
-1
and spin-lattice relaxation rate T
1
-1
measurements. On decreasing the temperature, a gradual enhancement of both relaxation rates is observed,
followed by a significant decrease of the signal intensity wipe-out effect. The low frequency regime of the
spin fluctuations as probed by T
1
-1
, can be described and analyzed in terms of a single characteristic correlation
frequency
c
T, which is interpreted as the lifetime broadening of the discrete magnetic energy levels due to
spin-phonon interactions.
DOI: 10.1103/PhysRevB.75.064414 PACS numbers: 75.50.Xx, 76.60.-k, 75.75.+a
I. INTRODUCTION
The recent progress in the field of molecular chemistry
has made feasible the synthesis of a large variety of crystal-
line structures built from identical molecular units. Each
such unit contains a relatively small number of magnetic
transition metal ions embedded in a large, nonmagnetic or-
ganic matrix. These ions are strongly coupled via intramo-
lecular super-exchange interactions J of typical order of a
few tens of Kelvin, while being magnetically “shielded”
from ions belonging to adjacent molecular units due to the
presence of the large organic matrices. Hence, the intermo-
lecular magnetic interactions are typically of dipolar origin
and thus, for most cases, can be neglected.
1
Accordingly, the
measurements in bulk crystalline samples reflect the mag-
netic properties of each “single-domain particle.”
The research in the field of molecular magnetism has been
very active with particularly emphasis being given to the
study of molecular clusters with high spin ground states and
large, negative magnetoanisotropy such as the Mn
12
, Fe
8
,
and Fe
4
. The interest in these so-called single molecule
magnets SMM’s, relies on the remarkable manifestation of
purely quantum effects at low T, such as macroscopic quan-
tum tunneling of the magnetization
2–4
and quantum interfer-
ence effects.
5
Of particular experimental and theoretical interest has
also been the study of the electronic spin correlations. One
can follow the evolution of these correlations from the high
temperature regime T J / k
B
, where thermal fluctuations
dominate and the ions behave independently zero dimen-
sional paramagnet, down to intermediate T T J / k
B
,
where correlations have been established by the exchange
interactions, and finally to very low T where each molecular
unit occupies its collective ground state. In addition, and
because of the discreteness of the magnetic energy spectrum,
the effects of the various small interactions of the exchange
coupled spins with the excitations of the thermal bath for
instance phonons are also of great interest. Such small in-
teractions cause a long-time decay of the spin fluctuations,
which in turn can be probed by dynamic NMR measure-
ments. For instance, for the so-called antiferromagnetic
AFM ring systems, it has been found that the characteristic
cut-off frequency of the long-time decay of the electronic
spin correlations decreases by a few orders of magnitude
when decreasing the temperature below J / k
B
, and this is
manifested in measurements of T
1
-1
by an enhancement at
T J / k
B
Refs. 6 and 7. In SMM’s, a similar enhancement
has also been observed in T
1
-1
and T
2
-1
, but the corresponding
analysis had been hindered by a simultaneous gradual loss of
the signal intensity. A detailed analysis of this so called
“wipe-out effect” in the SMM’s can be found in Ref. 8,
which includes some of our data from the two present
SMM’s. Here, we present and analyze the full body of our
data and provide additional information on the static and
dynamic properties of these two SMM’s.
The two hexanuclear molecular clusters of the present
study belong to a class of SMM’s consisting exclusively of
Mn
3+
ions.
9
Except for their different ligands and the fact
that the Mn
6
benzoate comprises two nearly identical hexa-
nuclear molecules per unit cell, these two samples have quite
similar molecular structures. Each molecule consists of two
exchange coupled triangular spin units. The competition be-
tween antiferromagnetic and ferromagnetic exchange inter-
actions stabilizes a ferrimagnetic ground state of collective
PHYSICAL REVIEW B 75, 064414 2007
1098-0121/2007/756/0644147 ©2007 The American Physical Society 064414-1