Muon spin relaxation investigation of tetranuclear iron(III) Fe
4
(OCH
3
)
6
(dpm)
6
molecular cluster
D. Procissi,
1
P. Arosio,
2,
* F. Orsini,
2,3
M. Marinone,
2,3
A. Cornia,
4
and A. Lascialfari
2,3,5
1
CalTech Brain Imaging Center, California Institute of Technology, E. California Boulevard, Pasadena, California 91125, USA
2
Department of Molecular Sciences Applied to Biosystems (DISMAB), Università degli Studi di Milano,
Via Trentacoste 2, I-20134 Milan, Italy
3
S3-CNR-INFM, Via G. Campi 183, I-41100 Modena, Italy
4
Department of Chemistry and INSTM Research Unit, University of Modena and Reggio Emilia,
Via G. Campi 183, I-41100 Modena, Italy
5
Department of Physics “Volta,” University of Pavia, Via Bassi 6, 27100 Pavia, Italy
Received 3 April 2009; revised manuscript received 25 June 2009; published 29 September 2009
We present a study of the spin dynamics of Fe
4
OCH
3
6
dpm
6
single molecule magnet by means of SQUID
magnetization and muon relaxation
+
SR measurements. In longitudinal field
+
SR experiments performed
at magnetic fields H = 200, 1000 Oe, the muon asymmetry Pt could be fitted by means of three components,
the first constant, the second fast relaxing through a quasiexponential decay, and the third, the slowest relaxing,
showing an exponential decay. The slowest muon relaxation rate studied as a function of temperature T
displayed two structures, a broad peak at T 15 / 20 K and a shoulder at T 5 K, both decreasing in amplitude
and displacing toward higher temperatures as the field is increased. To mimic qualitatively the temperature
behavior T at the investigated fields, we used a function expressed as the sum of two Bloembergen-Purcell-
Pound BPP-like laws, reproducing the mechanism of relaxation. The exponential data resulted well fitted by
means of a heuristic model which takes into account two correlation times ' and , related to the ground-state
multiplet barrier ' / k
B
=7.25 K and to the intermultiplet separation / k
B
=86.4 K between S=5 and
S=4.
DOI: 10.1103/PhysRevB.80.094421 PACS numbers: 76.75.+i, 76.60.-k, 75.45.+j
I. INTRODUCTION
The study of magnetic clusters of transition-metal ions
has attracted much interest after the discovery of single mol-
ecule magnet SMM behavior. At low temperatures, SMMs
exhibit magnetic bistability at the level of a single molecule
due to the slow relaxation of the magnetization, which oc-
curs via thermal activation over an anisotropy barrier and, at
very low T, by a quantum tunneling mechanism.
1–3
SMMs
are often clusters of exchange-coupled transition-metal ions
and the height of the magnetic anisotropy barrier is mainly
associated with the total spin value in the ground state and
with the sum of single-ion anisotropies, which in turn are
dependent on the nature of the ligands and their arrangement
around the metal centers. Dipole-dipole interactions and
mixing of states with different total spin can also play an
important role
4
and must usually be taken into account. After
the first observation of magnetic bistability in a Mn
12
cluster
5
considerable efforts have been made to increase the barrier
height and, consequently, the blocking temperature in order
to make real applications viable. In addition, many funda-
mental studies have been devoted to quantum-size effects, as
evidenced by the thermodynamic properties,
6
and to the situ-
ation of near degeneracy of two magnetic levels, where
quantum phenomena such as tunneling can occur. These ef-
fects have been explored in the high-spin molecules Mn
12
and Fe
8
, two systems characterized by a peculiar steplike
magnetic hysteresis cycle, and a very slow relaxation of the
magnetization at low temperature.
1,3,7
Here we present a muon spin-relaxation SR investiga-
tion of a tetraironIII SMM having a formula
Fe
4
OCH
3
6
dpm
6
, in the following Fe
4
where Hdpm
= dipivaloylmethane. Its molecular structure is shown in
Fig. 1. The four iron atoms lie exactly in a plane, the inner Fe
atom being in the center of an isosceles triangle. The mol-
ecule has a twofold symmetry due to the presence of a crys-
tallographic C
2
axis passing through Fe1 and Fe2 for more
details see Ref. 8. Although Fe
4
has a smaller anisotropy
with respect to Mn
12
or Fe
8
and, as a consequence, a lower
FIG. 1. Molecular structure of Fe
4
OCH
3
6
dpm
6
. The four iro-
nIII ions are represented as dark-gray spheres while the oxygen
and carbon atoms are drawn as light gray and medium-gray spheres,
respectively. Hydrogen atoms are omitted for clarity. The arrows
provide the spin configuration in the ground S
T
= 5 state.
PHYSICAL REVIEW B 80, 094421 2009
1098-0121/2009/809/0944216 ©2009 The American Physical Society 094421-1