Magnetic ordering in the rutile molecular magnets M
II
† N„CN…
2
‡
2
„ M Ä Ni, Co, Fe, Mn, Ni
0.5
Co
0.5
, and Ni
0.5
Fe
0.5
…
Alexandros Lappas,
1,
* Andrew S. Wills,
2,3
Mark A. Green,
2,3
Kosmas Prassides,
4
and Mohamedally Kurmoo
5
1
Institute of Electronic Structure and Laser, Foundation for Research and Technology—Hellas,
P.O. Box 1527, 711 10 Heraklion Crete, Greece
2
Department of Chemistry, Christopher Ingold Laboratories, University College London,
20 Gordon Street, London, WC1H 0AJ, United Kingdom
3
The Davy-Faraday Research Laboratory, The Royal Institution of Great Britain,
21 Albermarle Street, London, W1S 4BS, United Kingdom
4
School of Chemistry, Physics and Environmental Science, University of Sussex, Brighton BN1 9QJ, United Kingdom
5
Institut de Physique et Chimie des Mate ´riaux de Strasbourg, CNRS-UMR 7504, 23 rue de Loess,
F-67037 Strasbourg Cedex, France
Received 25 September 2002; revised manuscript received 2 December 2002; published 7 April 2003
Rietveld refinement of powder neutron diffraction data, combined with group theory considerations, is used
to determine the magnetic structures of the binary metal dicyanamide, M
II
NCN
2
2
where M =Ni, Co, Fe,
Mn, Ni
0.5
Co
0.5
, and Ni
0.5
Fe
0.5
. Compounds with M =Mn or Fe show a canted antiferromagnetic arrangement
of spin oriented in the ab crystallographic plane, with antiparallel components of the two sublattices along the
a axis and parallel along the b axis. Symmetry considerations forbid an additional moment, whether compen-
sated or not, to be present along the c axis. The compounds with fewer unpaired electrons Co and Ni are
ferromagnets, with all moments oriented along the c axis. The mixed composition of Ni
0.5
Co
0.5
displays the
same collinear ferromagnetic structure as its parent compounds. However, the composition with M
=Ni
0.5
Fe
0.5
, whose parent compounds show different magnetic behavior, does not exhibit long-range magnetic
ordering down to 1.7 K. Magnetostriction was observed for the ferromagnets for which we investigated the
variable temperature powder neutron diffraction. The cobalt-rich compounds show more pronounced effects,
consistent with their increasing magnetocrystalline anisotropy.
DOI: 10.1103/PhysRevB.67.144406 PACS numbers: 75.25.+z, 61.12.-q, 75.10.-b
I. INTRODUCTION
Over the past 20 years magnetic ground states that are
more commonly associated with elements or alloys have
been discovered in a variety of extended lattices involving
molecule-based solids.
1
The observation of properties such
as ferromagnetism and superconductivity in these systems
has created a lot of interest because their high degree of
chemical flexibility allows direct structural control of their
electronic properties.
1,2
The range of accessible organic con-
nectors enables the tailoring of properties for specific appli-
cations such as magnetic and/or photonic devices for infor-
mation storage media. The molecular magnetic systems are
often composed of a number of different chemical constitu-
ents, namely a central transition-metal ion, its coordinating
ligand, charge-balancing ions, and solvent molecules. The
coordinating ligand plays an important role in the connectiv-
ity of the magnetic metal ions, as well as playing a direct
active role in the interaction between the localized magnetic
moments or conduction electrons. The versatility of the
structure and bonding in these systems enables one to syn-
thesize materials exhibiting dual properties, such as super-
conductivity and magnetism, or coupling of properties, e.g.,
optical sensitivity and magnetism.
Magnets based on the Prussian blue family
3
have been
extensively studied as the linear metal-cyanide-metal con-
nectivity gives rise to three-dimensional 3D structures with
high Curie temperatures, for example, T
C
=315 K in
V CrCN
6
0.86
• 2.8H
2
O.
4
While work has concentrated on
varying the oxidation and spin states of the metal centers, it
has also been shown that an effective replacement of the
bridging cyanide ligand, CN
-
, may be afforded by the dicy-
anamide anion, NCN
2
-
.
5–11
The new family of isostruc-
tural compounds with the general formula M
II
NCN
2
2
M
is a transition-metal ion exhibits particularly interesting
magnetic properties.
5–12
They crystallize in a distorted rutile-
like 3D structure in which the connectivity between the met-
als is through (NwCuNuCwN)
-
and NuCuN link-
ages. The structural features of importance for magnetism
are the near-orthogonal arrangement of the octahedral M N
6
units connected through three atoms and the presence of the
electrons.
5,6
Bulk magnetic susceptibility
5–11
and muon-
spin relaxation
12
(
+
SR) measurements of the
M
II
NCN
2
2
series have revealed a plethora of electronic
ground states as a function of transition-metal ion, including
paramagnetism, ferromagnetism, and canted antiferromag-
netism. Application of pressure has different effects on each
of these compounds.
13
An important aspect in this area of
research is the determination of the key structural or elec-
tronic characteristics that control the resulting magnetic
ground states.
In this paper, we employ high-resolution and high-
intensity neutron powder diffraction measurements together
with the implementation of symmetry-allowed models, to de-
termine the magnetic structure of M NCN
2
2
. The proper-
ties of the mixed metal systems, Ni
0.5
Co
0.5
NCN
2
2
and
Ni
0.5
Fe
0.5
NCN
2
2
, are also explored; the former shows
ferromagnetism, analogous to the parent compositions,
PHYSICAL REVIEW B 67, 144406 2003
0163-1829/2003/6714/1444068/$20.00 ©2003 The American Physical Society 67 144406-1