Neutron diffraction and specific heat studies on the magnetic ordering in the
†Fe
II
„…Fe
II
„…„ox…
2
„Phen…
2
‡
n
molecular magnet
C. J. Ho,
1
J. L. Her,
1
C. P. Sun,
1
C. C. Yang,
2
C. L. Huang,
1
C. C. Chou,
1
Lu-Lin Li,
3
K. J. Lin,
3
W. H. Li,
2
J. W. Lynn,
4
and H. D. Yang
1,
*
1
Department of Physics, Center of Nanoscience and Nanotechnology, National Sun Yat-Sen University, Kaohsiung 804, Taiwan
2
Department of Physics and Center for Neutron Beam Applications, National Central University, Chung-Li 32054, Taiwan
3
Department of Chemistry, Center of Nanoscience and Nanotechnology, National Chung-Hsing University, Taichung 402, Taiwan
4
NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA
Received 31 March 2007; revised manuscript received 5 October 2007; published 19 December 2007
The magnetic characteristics of the molecular magnet Fe
II
Fe
II
ox
2
Phen
2
n
, having chemical for-
mula C
28
H
16
Fe
2
N
4
O
8
for unity, has been studied by magnetization, neutron diffraction, and field-dependent
specific heat-measurements. In the high-temperature regime T T
m
, the one-dimensional Ising chain model
with alternate Landé factors is applied to describe its quasiferrimagnetic behavior as temperature approaches
T
m
. In the low-temperature region T T
m
, the increase of interchain interactions gives rise to long-range
magnetic ordering as indicated by an anomaly in specific heat. Furthermore, an intrinsic antiparallel alignment
of spins with a net ferrimagnetic structure is deduced from neutron diffraction study. The field-dependent
-type anomaly of specific heat indicates that applying a magnetic field raises magnetic ordering temperature.
An additional small anomaly in specific heat is also seen below T
m
, which could be due to the zero-field
splitting caused by the internal crystal field.
DOI: 10.1103/PhysRevB.76.224417 PACS numbers: 75.50.Xx, 61.12.Ld, 65.40.-b
I. INTRODUCTION
The field of molecular magnets is an emerging area and
has received intensive study due to its potential applications
in magnetic devices, such as magnetic memory.
1
The mag-
netic behavior of the molecules that changes with dimension
is an attractive academic topic as the state of the system
whether bulk or nanoscale particles is greatly controlled by
its dimensionality. While the bulk shows spontaneous mag-
netization below critical temperature T
c
, the nanoscale par-
ticles have nearly independent magnetic interactions. Re-
searchers have tried to realize these magnetic properties by
investigating the relationships among chelate molecules,
magnetic centers, and connecting bridges between magnetic
centers. Each of them plays an important role in constructing
magnetic interactions. Due to the interest in these topics, the
categories of research in the molecular magnets are usually
complex and are continually renewed by researchers’
efforts.
2–9
One of the well-known molecular magnets is Mn
12
Ac,
which was first synthesized in 1980.
10
Its ground state com-
posed of spin number S = 10 was confirmed by high magnetic
field and ac susceptometry experiments.
11–13
Through com-
prehensive studies, Mn
12
Ac is considered to be a well-
described candidate for a single-molecule magnet SMM,
which reveals slow relaxation in magnetization and quantum
tunneling phenomena at very low temperatures.
14
Another
type of molecular magnet called single-chain molecular mag-
net SCM, by analogy to SMM, has been investigated since
2001.
15
SCM type molecular magnets are composed of indi-
vidual chains in which their intrachain interactions are 10
4
larger than interchain interactions, making them behave as
magnets.
16
Though long-range ordering in pure one-
dimensional 1D materials only occurs at T =0 K, strong
intrachain and/or interchain interactions form a two-
dimensional 2D or three-dimensional 3D network, ren-
dering the long-range magnetic ordering LRMO possible at
finite temperatures. However, the limitations of producing
SCM for industrial applications seem less than that of SMM.
This is the reason why study on SCM has quickly become
more active.
17
In this work, we focus on the newly synthesized com-
pound Fe
II
Fe
II
ox
2
Phen
2
n
,
18
which includes two
locally distinct Fe
II
ions as the magnetic centers. Every Fe
II
connects with two nitrogen atoms of the phen group while
linking with each other with an oxalate that serves as a
bridge of four oxygen atoms, thus forming a zigzag chain.
The - interactions within the phen groups between
the zigzag chains develop a quasi-2D framework within the
ac plane, and hence, the magnetic properties of
Fe
II
Fe
II
ox
2
Phen
2
n
may be strongly correlated
with the dimensionality. In our previous paper,
18
the
magnetic hysteresis was observed below 8.6 K, and was
associated with the ferromagnetic or ferrimagnetic interac-
tions. However, the negative Curie-Weiss temperature
was derived from the magnetic susceptibility measurement,
indicating the negative exchange interaction among
magnetic Fe
II
ions. It follows that the ground state of
Fe
II
Fe
II
ox
2
Phen
2
n
could be canted-
antiferromagnetic or ferrimagnetic. In order to solve this
problem, in this paper, we have applied an Ising-like model,
assigning Landé factors of Fe
II
ions of different environ-
ments with different magnitudes. Due to this improvement,
we have been able to reproduce the magnetic susceptibility
correctly and also achieve the short-range ferrimagnetism
present in the system.
19
To further explore the nature of the
magnetic ordering of Fe
II
Fe
II
ox
2
Phen
2
n
, includ-
ing the configuration of spins and the dimension of ordering,
the neutron diffraction and magnetic-field-dependent specific
heat measurements have also been performed.
PHYSICAL REVIEW B 76, 224417 2007
1098-0121/2007/7622/2244178 ©2007 The American Physical Society 224417-1