A Three Dimensional Magnetically Frustrated Metal-Organic Framework via Vertices Augmentation of Underlying Net

Abstract

In our efforts to fabricate magnetically frustrated materials, a feasible vertices augmentation method was used succesfully to construct a 4-fold interpenetrating three dimensional metal-organic framework (MOF) with rare eta-c4 topology by linking [Fe3(μ3-O)(μ-O2CCH3)6]+ triangular moieties through the pure anti, anti acetate ligands. Strong antiferromagnetic interactions were found to exist between the neighboring FeIII ions without long-range magnetic ordering above 2 K, indicating strong geometric spin frustration nature of this MOF.

Full text

This journal is ©The Royal Society of Chemistry 201 5 Chem. Commun., 2015, 51, 4627--4630 | 4627
Cite this: Chem. Commun., 2015,
51,4627
A three dimensional magnetically frustrated
metal–organic framework via the vertices
augmentation of underlying net†
Jiong-Peng Zhao,
ab
Song-De Han,
a
Xue Jiang,
a
Jian Xu,
a
Ze Chang
a
and
Xian-He Bu*
a
In our efforts to fabricate magnetically frustrated materials, a feasible
vertices augmentation method was successfully used to construct a
4-fold interpenetrating three dimensional metal–organic framework
(MOF) with rare eta-c4 topology by linking [Fe
3
(l
3
-O)(l-O
2
CCH
3
)
6
]
+
triangular moieties through the pure anti,anti acetate ligands. Strong
antiferromagnetic interactions were found to exist between the neigh-
boring Fe
III
ions without long-range magnetic ordering above 2.2 K,
indicating the strong geometric spin frustration nature of this MOF.
Geometrically spin-frustrated systems have received considerable
attention in the area of magnetism and condensed matter physics,
as they can render the magnetic systems with various exotic proper-
ties,suchasspinice,spinliquids,spinglasses,high-T
c
super-
conductivity, and monopoles in spin,
1–3
which makes the fabrication
of a frustration system an attractive goal in this active field.
4
To date,
most geometrically frustrated systems can be conveniently identified
through topology considerations based on the well-established
structure databases. However, the rational design of novel
geometrically frustrated systems is still a challenge due to the
uncertainty of the assembly process that constrain the feasibility
of arranging the spin-carriers in an artificial manner.
5
In recent years, the low-dimensional spin-frustrated inor-
ganic compounds have been well explored and strategies for
targeted construction have been developed.
5,6
At the same time,
some unique molecule-based frustrated magnetic materials have
also been obtained.
7
Nevertheless, compared with the broadly
explored low-dimensional spin-frustrated lattices, such as triangular
and Kagome
´lattice, the effective creation of three-dimensional (3D)
frustrated systems is still a challenging task for researchers.
2,8
To achieve this goal, new design and construction strategies are
urgently required.
Among the various potential frustrated magnetic materials,
metal–organic frameworks (MOFs) show unique advantages in
structure modulation since it has a desired complicated structure
composed of simple underlying nets, which could be constructed
by following a reverse engineering process.
9
Based on this heuristic
work, we contend that a 3D magnetically frustrated framework
could be built from underlying nets with the vertices as frustration
units. However, the structural match between basic units and the
underlying network structure as well as the proper connections
between spin centers needs to be considered simultaneously to
realize the construction of materials with desired structures
and properties. In the following study, we will corroborate that
this method works well and should be generally applicable for
coordination-based systems (Scheme 1).
Herein, we report the successful construction of a magneti-
cally frustrated MOF, [NH
4
]
2
[Fe
9
(m
3
-O)
3
(m-OAc)
22
(H
2
O)OAc] (1)
(OAc = acetate), with triangular moiety [Fe
3
(m
3
-O)(m-OAc)
6
]
+
cations as
frustrated units
10
and acetate as linkers between the units.‡This
MOF exhibits a 4-fold interpenetrating 3D framework structure with
rare eta-c4 topology. More importantly, strong antiferromagnetic
interactions were observed between the Fe
III
ions without long-range
magnetic ordering above 2.2 K, indicating a strong geometric spin
frustration nature of this MOF as expected.
1was synthesized from the solvothermal reaction of FeCl
3
6H
2
O
in HOAc with urea saturation (Fig. S1–S3, ESI†).§As depicted in
Fig. S4 (ESI†), there are two different types of triangular cluster
Scheme 1 The 3D frustrated framework via the vertices augmentation of
underlying net.
a
TKL of Metal- and Molecule-Based Material Chemistry and Collaborative
Innovation Center of Chemical Science and Engineering (Tianjin), Nankai
University, Tianjin 300071, P. R. China. E-mail: buxh@nankai.edu.cn;
Fax: +86 22-23502458
b
School of Chemistry and Chemical Engineering, Tianjin University of Technology,
Tianjin 300384, P. R. China
†Electronic supplementary information (ESI) available: Additional crystallo-
graphic and magnetic data. CCDC 1042546. For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/c4cc09547b
Received 29th November 2014,
Accepted 30th January 2015
DOI: 10.1039/c4cc09547b
www.rsc.org/chemcomm
ChemComm
COMMUNICATION
Published on 30 January 2015. Downloaded on 12/03/2015 06:07:48.
View Article Online
View Journal
| View Issue

4628 |Chem. Commun., 2015, 51, 4627--4630 This journal is ©The Royal Society of Chemistry 201 5
[Fe
3
(m
3
-O)(m-OAc)
6
]
+
involved in the asymmetric unit of 1(denoted
hereafter as type I and type II triangle for clarity), together with
two acetate ligands in the anti,anti mode, one ammonium cation
as well as one coordinated water. Obviously, type I triangular
cluster is a m
3
-oxo-bridged trinuclear cluster containing three iron
ions (Fe1, Fe2 and Fe3) in equivalent coordination environments.
Each iron ion adopts a distorted-octahedral coordination geome-
try accomplished by the central m
3
-O atom, four equatorial O
centering from four syn,syn acetate anions, and one oxygen atom
belonging to an anti,anti acetate ligand. On the other hand,
the same coordination situation only holds for two iron ions
(Fe4, Fe4
ii
) of the type II triangle, while the remaining Fe5 is
coordinated by the central m
3
-O atom, four equatorial O atoms
from four syn,syn acetate ligands, and one terminal water mole-
cule. Two uncoordinated acetate anions with part occupancy were
also found in the space of the lattice. The Fe–O distances in 1are
all in the range of 1.896(5)–2.073(11) Å (Table S1, ESI†), and the
bond valence model gives 3.029, 3.115, 3.147, 3.082 and 3.028 for
Fe1 to Fe5, respectively. These results indicate that all the iron
ions in 1are trivalent.
11
To further confirm the valence state of Fe
ions in the bulk sample, UV-vis spectrum of 1was investigated
(Fig. S5, ESI†). The intense absorption peak of the bulk sample
appears around 200 nm, while the characteristic bands for the
intervalence charge-transfer in mixed-valence iron(II,III) complexes
are thoroughly absent in the region of 520–800 nm, indicating the
uniform trivalent state of the Fe ions.
12
In 1, the neighboring triangular clusters are bridged by the
two anti,anti acetate ligands but in different manners. For
instance, the acetate ligand containing O3 and O4 serves as a
bridge between different types of triangles (i.e., those associated
with Fe3 and Fe4 ions, respectively), while the one having O13
and O14 links only two type I clusters through the coordination
bonds Fe1–O14 and Fe2–O13
i
. In this way, each type I triangular
cluster is coupled to three of its neighbors, including two type I
and one type II triangles, and the type II cluster bridges only two
non-adjacent type I triangles. Following this linkage pattern, a
single helix was generated by linking the type I triangles (Fig. 1a),
and the type II cluster serves as a bridge linking six neighboring
single helices (Fig. 1b). Consequently, this will lead to the growth
of a double helix chain structure, as presented in Fig. 1c. It is
worth noticing that all the double helices in 1are of the same
handedness, which is the opposite to that of the single helices.
This feature plays a key role in the formation of the interpene-
trating 3D net structure, presented in Fig. 2a, as it allows the
linkage between different types of helices (Fig. 2b). It is amazing
that there are 43-membered and 20-membered ring channels
present in the current framework, along the cand [110] (or [100])
direction (Fig. S6, ESI†). While the channels (defined by rings)
containing 8-, 10-, and 12-membered rings are often found in
zeotype inorganic structures, the complex with extra-large pores
(412-membered rings) has been sporadic.
13
The presence of
20-membered and 43-membered ring structures in 1indicate
that the proposed vertices augmentation strategy would be an
effective way to obtain the structure containing large ring
channels. In the context of the topological structure, the type
I triangle is a 3-connected node, upon which an eta net
framework can be built by following the pattern illustrated in
Fig. 2b.
14
It is interesting to compare the structure of 1and the
"star" lattice constructed by the same triangular entity.
10b
In the
"star" lattice the isolated triangular building blocks serve as
templates to make all of the triangular moieties in the "star" net
coplanar. In 1the whole eta framework is an anionic net, where
each involved ammonium cation acts as a charge-balancer, and
the neighbor triangular moieties bridged by anti,anti acetates
have angles from 68.51to 115.01. All these ammonium cations
are stabilized by the hydrogen bonds contributed by the oxygen
atoms of the carboxylate ligands from two individual eta
nets (Fig. S7, ESI†). Based on the aforementioned linkage, a
4-fold interpenetrating eta-c4 net can then be conveniently
established, as demonstrated in Fig. 2c. It should be noted
that the interpenetrating structure is very scarce in the presence
of pure short-bridging ligands, which implies the key role of
long-bridging ligands in the formation of interpenetrating
structures. In this study, despite the use of short acetate ligands,
the bridge moiety consisting of one type II triangle and two
Fig. 1 (a) The single helix in 1. (b) The linkage of the two types of helix
structure. (c) The double helix structure in 1.
Fig. 2 (a) The predigested 3D iron triangular net of 1. The [Fe
3
(m
3
-O)(m-OAC)
6
]
+
clusters simplified as triangles and the acetate anions linking the triangles as
green sticks. (b) The eta net with the triangles as single nodes showing the
left-/right-handed helices. (c) View of the 4-fold interpenetrating eta net.
(d) Spacefilling view of the 4-fold interpenetrating framework in 1.
Communication ChemComm
Published on 30 January 2015. Downloaded on 12/03/2015 06:07:48.
View Article Online

This journal is ©The Royal Society of Chemistry 201 5 Chem. Commun., 2015, 51, 4627--4630 | 4629
coordinated anti,anti acetate ligands could be considered as
long-bridging metalloligands, which contributes significantly
to the formation of an interpenetrating 3D framework. Further-
more, though this MOF is highly interpenetrated, it reveals a
relatively free volume of 20.4% per cell as evaluated by PLATON
analysis (Fig. 2d),
15
but shows poor performance on gas adsorp-
tion (i.e., 16.82 cm
3
g
1
for CO
2
at 273 K and 1 bar). This might
be attributed to the considerable relaxation of the interpene-
trated framework after the removal of guest molecules, which
reduced its porosity.
16
On the other hand, the structure of 1promised a potential to
exhibit frustrated magnetic properties. Neglecting the magnetic
interactions between the four interpenetrating nets leads 1to
be a 3D magnetic net of {3;20;21}
2
{3;20
2
}
6
{3}, where each Fe
III
ion
(except Fe5) interacts with three neighbors. Further ignoring
Fe5 ions in 1result in a new magnetic net of {20}{3;20
2
}
3
with a
40-number metal ions ring, where every two adjacent Fe3 ions on
the same ring are linked to each other through two antiferro-
magnetically coupled Fe4 ions. Since the magnetic interaction
between Fe3 and Fe4 ions is also antiferromagnetic, the neigh-
boring Fe3 ions should be considered to be antiferromagneti-
cally coupled to the net. Based on the aforementioned points, an
ideal 3D frustrated magnetic net can be established by Fe1, Fe2,
and Fe3 with point symbol of {3;16
2
}(Fig.S8,ESI†).
14
The magnetic properties of 1were further investigated to
confirm the frustrated nature of this MOF. Magnetic dc suscepti-
bility of 1was measured on the crystallized powder samples in
an applied dc field of 0.1 T. As shown in Fig. 3, the result reveals
the temperature dependence of the inverse molar magnetic
susceptibility of 1, ranging from 2 to 300 K. According to the
inset, the value of w
M
Tat 300 K is only 11.57 cm
3
mol
1
K, which
is much smaller than that reported in the spin-only system.
Moreover, the value of w
M
Tat 300 K is 39.36 cm
3
mol
1
K when
considering nine fully spin-decoupled Fe
III
ions with a magnetic
spin of S=5/2.
17
This discrepancy should be attributed to the
significant antiferromagnetic couplings between the three high-
spin Fe
III
ions linked through the oxo bridges. Furthermore, the
plot of 1/w
M
vs. T in the range of 90–300 K can be well fitted to the
Curie–Weiss law with the Curie constant C= 32.64 cm
3
mol
1
K
and the Weiss temperature y=551.21 K. Thus, the effective
magnetic moment m
eff
is 5.46 m
B
per Fe
III
ion, which is much
smaller than the one expected for the high spin d
5
Fe
III
ion
(5.92 m
B
per Fe
III
ion). The prominent reduction of m
eff
and the
relatively large negative value for yboth support the presence of
strong antiferromagnetic interactions in 1.
It is notable that there is a "rebound" behavior of the magnetic
dc susceptibility of 1:thew
M
Tvalue decreases continuously with
decreasing temperature to reach a minimum of 0.43 cm
3
mol
1
K
at 2.4 K, but then increases with further cooling. In fact, the
increase of w
M
Tbelow 2.4 K is more evident at low external
magnetic fields. A reliable explanation is that the odd number
of metal centers prevents the total cancellation of the antifer-
romagnetically coupled spins and/or spin-canting induced by
the Dzyaloshinsky–Moriya interactions. Furthermore, there is a
little difference between the zero-field-cooled (ZFC) and field-
cooled (FC) magnetic susceptibilities at very low temperatures
(Fig. S9, ESI†), suggesting that the irreversible temperature (T
N
)
between the ZFC and FC for 1is comparatively low and very
close to 2.2 K, at which both small coercive force and remnant
magnetization can be found (Fig. S10, ESI†). In light of the
aforementioned results, it can be concluded that the strong spin
frustration is indeed present in 1due to the trigonal arrangement
of spins, which would lead to a dramatic suppression of the long-
range magnetic ordering at a considerable temperature range.
According to the prediction by the quantum many-body Heisenberg
interaction theory, the geometric lattice-induced magnetic frustra-
tion can only be found in the system with half-odd-integer spins
but otherwise in integer spin or mixed-spin complexes.
10,19
This
is because the half-odd-integer spins of 1couldyieldtheground-
state lattice-induced frustration that give rise to an extremely low
antiferromagnetic phase transition temperature.
An alternative way to evaluate the magnetic spin frustration
is also given here by introducing the so-called frustration index
of f=|y|/T
N
, with f410 being an indication of strong spin
frustration effects.
18
From the above yvalue and the T
N
value of 2.2 K,
a value of 250 for fis obtained. On comparing the magnetism of 1
with the "star" complex [Fe
3
(m
3
-O)(m-OAc)
6
(H
2
O)
3
][Fe
3
(m
3
-O)(m-OAc)
7.5
]
2

7H
2
O,
10b
we found that although the Weiss constant 551 K in 1was
larger than 581 K in the "star" net, but the coercive force, remnant
magnetization and T
N
was lower than that in the 2D complex, which
led to a larger fvalue of 250 in 1and indicated that a stronger spin
frustration effect could be gained in a 3D framework.
In conclusion, an unprecedented 4-fold interpenetrating 3D
framework was successfully constructed from the iron triangular
clusters, with strong frustrated antiferromagnetic interactions
achieved due to the triangular arrangement of spins. Thus, this
study provides a promising and practical way to construct 3D
magnetically frustrated MOFs, which is to augment the vertices
of a simple underlying net with frustrated units. We believe this
method might be generally applied to other coordination-driven
systems. Moreover, it also sheds light on the investigation in the
field of condensed matter physics, since the interpenetrating
framework is not common for pure inorganic compounds but is
indeed feasible in the area of molecular materials.
Fig. 3 The temperature dependence of the inverse molar magnetic
susceptibility of 1obtained in an applied dc field of 0.1 T; the solid line
represents the behavior according to the Curie–Weiss law. Inset: The w
M
T
vs. T plots of 1.
ChemComm Communication
Published on 30 January 2015. Downloaded on 12/03/2015 06:07:48.
View Article Online

4630 |Chem. Commun., 2015, 51, 4627--4630 This journal is ©The Royal Society of Chemistry 201 5
This study was supported by the 973 Program of China (Grants
2014CB845600) and the NSF of China (Grants 21290171, 21471112
and 21421001) and MOE Innovation Team (IRT13022) of China.
Notes and references
‡Crystal data of 1,C
46
H
79
Fe
9
N
2
O
50
:M= 1962.76, trigonal, space group P3
2
21,
a=b= 18.927(3) Å, c= 24.693(5) Å, Z=3,r=1.276gcm
3
. Of 52 932 total
reflections collected, R
1
= 0.0518, wR
2
= 0.1474 and GOF = 1.089, Flack(x)=
0.137(8). CCDC 1042546.
§Synthesis of 1: a mixture of FeCl
3
6H
2
O (5 mmol) and acetic acid
solution 10 mL saturated with urea was sealed in a teflon-lined stainless
steel vessel, heated at 140 1C for 2 days under autogenous pressure, and
then cooled to room temperature. Red crystals of 1were harvested in
about B30% yield based on FeCl
3
6H
2
O.
1 L. Balents, Nature, 2010, 464, 199.
2 S. T. Bramwell and M. J. P. Gingras, Science, 2001, 294, 1495.
3(a)J.Struck,C.O
¨lschla
¨ger,R.L.Targat,P.Soltan-Panahi,A.Eckardt,
M.Lewenstein,P.WindpassingerandK.Sengstock,Science, 2011, 333, 996;
(b)L.Pauling,J. Am. Chem. Soc., 1935, 57, 2680; (c) G. H. Wannier, Phys.
Rev., 1950, 79, 357; (d)N.P.OngandR.J.Cava,Science, 2004, 305, 52.
4(a)N.Choudhury,L.Walizer,S.LisenkovandL.Bellaiche,Nature, 2011,
470,513;(b)T.Fennell,P.P.Deen,R.Wildes,K.Schmalzl,
D. Prabhakaran, A. T. Boothroyd, R. J. Aldus, D. F. McMorrow and
S. T. Bramwel, Science, 2009, 326, 415; (c)R.MoessnerandA.P.Ramirez,
Phys. Today, 2006, 59,24;(d)M.J.Harris,Nature, 1999, 399, 311;
(e)M.Mostovoy,Nat. Mater., 2008, 7, 269; ( f) C. Castelnovo,
R.MoessnerandS.L.Sondhi,Nature, 2008, 451, 42.
5(a) R. Becker, M. Johnsson, R. K. Kremer, H. Klauss and P. Lemmens,
J. Am. Chem. Soc., 2006, 128, 15469; (b)R.Gautier,K.Oka,T.Kihara,
N. Kumar, A. Sundaresan, M. Tokunaga, M. Azuma and K. R.
Poeppelmeier, J. Am. Chem. Soc., 2013, 135, 19268; (c)J.L.Atwood,Nat.
Mater., 2002, 1,91.
6(a) D. E. Freedman, T. H. Han, A. Prodi, P. Mu
¨ller, Q.-Z. Huang,
Y.-S. Chen, S. M. Webb, Y. S. Lee, T. M. McQueen and D. G. Nocera,
J. Am. Chem. Soc., 2010, 132, 16185; (b) M. P. Shores, E. A. Nytko,
B. M. Bartlett and D. G. Nocera, J. Am. Chem. Soc., 2005, 127, 13462;
(c) S. Chu, T. M. McQueen, R. Chisnell, D. E. Freedman, P. Mu
¨ller,
Y. S. Lee and D. G. Nocera, J. Am. Chem. Soc., 2010, 132, 5570.
7(a) E. A. Nytko, J. S. Helton, P. Mu
¨ller and D. G. Nocera, J. Am.
Chem. Soc., 2008, 130, 2922; (b) S. A. Baudron, P. Batail, C. Coulon,
R. Cle
´rac, E. Canadell, V. Laukhin, R. Melzi, P. Wzietek, D. Je
´rome,
P. Auban-Senzier and S. Ravy, J. Am. Chem. Soc.,2005,127, 11785;
(c) E.-Q. Gao, N. Liu, A.-L. Cheng and S. Gao, Chem. Commun., 2007,
2470; (d)X.-Y.Wang,L.Wang,Z.-M.WangandS.Gao,J. Am. Chem. Soc.,
2006, 128, 674; (e)J.L.Manson,E.RessoucheandJ.S.Miller,Inorg.
Chem., 2000, 39, 1135; ( f)X.-Y.WangandS.C.Sevov,Chem. Mater.,
2007, 19, 3763.
8(a) W. Ouellette, M. H. Yu, C. J. O’Connor, D. Hagrman and
J. Zubieta, Angew. Chem., Int. Ed., 2006, 45, 3497; (b) X.-M. Zhang,
T. Jiang, H.-S. Wu and M.-H. Zeng, Inorg. Chem., 2009, 48, 4536;
(c) T. Sarkar, V. Pralong, V. Caignaert and B. Raveau, Chem. Mater.,
2010, 22, 2885; (d) B. Moulton, J. Lu, R. Hajndl, S. Hariharan and
M. J. Zaworotko, Angew. Chem., Int. Ed., 2002, 41, 2821.
9 M. O’Keeffe and O. M. Yaghi, Chem. Rev., 2012, 112, 675.
10 (a) P. Albore
´s and E. Rentschler, Inorg. Chem., 2008, 47, 7960;
(b) Y.-Z. Zheng, M.-L. Tong, W. Xue, W.-X. Zhang, X.-M. Chen,
F. Grandjean and G. J. Long, Angew. Chem., Int. Ed., 2007, 46, 6076
and reference cited therein.
11 (a) I. D. Brown and D. Altermatt, Acta Crystallogr., Sect. B: Struct. Sci.,
1985, 41, 244; (b) N. E. Brese and M. O’Keeffe, Acta Crystallogr.,
Sect. B: Struct. Sci., 1991, 47, 192; (c) I. D. Brown, R. J. Gillespie,
K. R. Morgan, Z. Tun and P. K. Ummat, Inorg. Chem., 1984, 23, 4506.
12 (a) D. Lee, C. Krebs, B. H. Huynh, M. P. Hendrich and S. J. Lippard,
J. Am. Chem. Soc., 2000, 122, 5000; (b) D. Lee, J. L. DuBois, B. Pierce,
B. Hedman, K. O. Hodgson, M. P. Hendrich and S. J. Lippard, Inorg.
Chem., 2002, 41, 3172; (c) K. S. Hagen, S. G. Naik, B. H. Huynh,
A. Masello and G. Christou, J. Am. Chem. Soc., 2009, 131, 7516.
13 (a) L. Wei, Q. Wei, Z.-E. Lin, Q. Meng, H. He, B.-F. Yang and
G.-Y. Yang, Angew. Chem., Int. Ed., 2014, 53, 7188; (b) H.-Y. Lin,
C.-Y.Chin,H.-L.Huang,W.-Y.Huang,M.-J.Sie,L.-H.Huang,Y.-H.Lee,
C.-H.Lin,K.-H.Lii,X.BuandS.-L.Wang,Science, 2013, 339,811and
reference cited therein.
14 (a) Reticular Chemistry Structure Resource (RCSR), http://rcsr.anu.
edu.au/; (b) Euclidean Patterns in Non-Euclidean Tilings (EPINET),
http://epinet.anu.edu.au/; (c) V. A. Blatov and A. P. Shevchenko,
TOPOS 4.0, Samara State University, Russia.
15 A. L. Spek, PLATON, A Multipurpose Crystallographic Tool, Untrecht
University, Utrecht, The Netherlands, 1999.
16 Y.-X. Tan, Y.-P. He and J. Zhang, Chem. Mater., 2012, 24, 4711.
17 (a) R. D. Cannon and R. P. White, Prog. Inorg. Chem., 1988, 36, 95;
(b) R. D. Cannon, U. A. Jayasooriya, R. Wu, S. K. Koske, J. A. Stride,
O. F. Nielsen, R. P. White, G. J. Kearley and D. Summerfield, J. Am.
Chem. Soc., 1994, 116, 11869; (c) W. Hibbs, P. J. Koningsbruggen,
A. M. Arif, W. W. Shum and J. S. Miller, Inorg. Chem., 2003, 42, 5645.
18 (a) S. T. Bramwell, S. G. Carling, C. J. Harding, K. D. M. Harris, B. M.
Kariuki, L. Nixon and I. P. Parkin, J. Phys.: Condens. Matter, 1996,
8, L123; (b) H. Serrano-Gonza
´lez, S. T. Bramwell, K. D. M. Harris,
B. M. Kariuki, L. Nixon, I. P. Parkin and C. Ritter, J. Appl. Phys.,
1998, 83, 6314.
19 J.N.Behera,A.Sundaresan,S.K.PatiandC.N.R.Rao,ChemPhysChem,
2007, 8, 217.
Communication ChemComm
Published on 30 January 2015. Downloaded on 12/03/2015 06:07:48.
View Article Online