ARTICLE IN PRESS
Effect of doping in BaNi2V2O8, a two-dimensional
honeycomb antiferromagnet
R. Nath, J. Das, A.V. Mahajan
Department of Physics, Indian Institute of Technology, Bombay, Mumbai-400076, India
Abstract
We report the temperature dependence of susceptibility wðTÞ of the two-dimensional (2D) honeycomb
antiferromagnetic (AF) BaðNi1x Cux Þ2 V2 O8 for x ¼ 0:0; 0:05 and 0.1. The undoped BaNi2V2O8 is known to exhibit
long-range-order (LRO) below T N 50 K with J=kB ¼ 48 K. We find that doping Cu (S ¼ 1=2) at Ni (S ¼ 1) site results
in a magnetic dilution effect which suppresses the three-dimensional (3D) ordering temperature (T N ). Our results are
compared with those of nonmagnetic Mg-doped BaNi2V2O8.
PACS: 75.40.Cx
Keywords: 2D XY antiferromagnet; Dilution effect
1. Introduction
There is currently an enormous interest in quasione-dimensional (1D) and quasi-two-dimensional
(2D) magnetic systems due to a host of exotic
features that they exhibit. For instance, S ¼ 12 and
S ¼ 1 antiferromagnetic (AF) chains have novel
ground states such as Spin-Peierls [1] and Haldane
gap [2] states. On the other hand 2D AF systems
are interesting due to their intimate connection
with parent compounds of high-T c cuprate super-
conductors [3] and various ground state properties
such as spin-flop [4] and Kosterlitz–Thouless
transitions [5]. A study of impurity induced effects
in such systems constitutes a powerful probe of the
ground state and the excitations from the ground
state. So far not many compounds have been
reported where the dilution effect has been
extensively studied [6–8].
BaNi2V2O8 is an S ¼ 1 2D AF system belonging
to the space group R-3 with lattice constants a ¼
( and c ¼ 22:33 A
( [9]. This compound is
5:0375 A
known as a 2D XY model system. It consists of
honeycomb layers of edge-sharing NiO6 octahedra. These magnetic V4þ O4 layers are separated by
ARTICLE IN PRESS
73
nonmagnetic V5þ O4 tetrahedra and Ba2þ ions.
The magnetic properties are expected from the AF
interactions of Ni2þ 2O2Ni2þ within the plane.
Magnetic properties of undoped and Mg-doped
BaNi2V2O8 have been investigated by Rogado
et al. [9]. It undergoes magnetic long-rangeordering (LRO) at T N 50 K which has been
confirmed from specific heat and neutron diffraction measurements. Apart from this, Mg doping at
the Ni2þ site results in a decrease of the ordering
temperature with doping content. Herein we
examine the effect of doping Cu (S ¼ 12) at Ni
(S ¼ 1) site in BaNi2V2O8 and compare it with the
effect of nonmagnetic Mg doping.
The susceptibility wðTÞ (¼ M=H) of our polycrystalline single phase BaNi2V2O8 (Fig. 1) exhibits a broad peak at around 125 K, indicative of
short-range order. At low temperatures, a Curielike upturn is observed. This is likely due to the
presence of a small amount of natural (intrinsic)
defects and extrinsic paramagnetic impurities. We
fit our high-temperature (150–300 K) data to the
following high temperature series expansion for an
S ¼ 1 2D honeycomb antiferromagnet given by
Rushbrook and Wood [10]:
wAFM ðTÞ ¼
N A m2B g2
3kB T
½SðS þ 1Þð1 þ Ax þ Bx2 þ Cx3
þ Dx4 þ Ex5 þ Fx6 Þ1 ,
ð1Þ
2. Experimental details
All the peaks in the X-ray diffraction patterns of
undoped and doped BaNi2V2O8 could be indexed
based on a Rhombohedral unit cell. The lattice
constants for the undoped BaNi2V2O8 were
( and c ¼
determined to be a ¼ 5:032ð2Þ A
(
22:352ð7Þ A, which are in agreement with Ref. [9].
No significant variation of lattice constants were
seen for the Cu-doped samples.
undoped
Ba(Ni1-xCux)2V2O8
3.0
5% Cu
10% Cu
TN
2.5
2.0
dχ/dT ( 10-6cm3/K mole Ni)
3. Results and discussion
where x ¼ jJj=kB T, J=kB is the exchange interaction between Ni2þ ions, kB is the Boltzmann
constant, N A is the Avogardro number, mB is the
Bohr magneton and g is the Landé g factor. The
value of the numerical coefficients A, B, C, D, E
and F are given in Ref. [10]. From the fit we
obtained J=kB ’ ð50 2Þ K and g ¼ 2:3 for the
undoped sample, in agreement with the published
results [9].
As reported by Rogado et al., we observed a
change of slope in wðTÞ at T50 K. A clear
χ (10-3cm3/mole Ni)
Polycrystalline samples of BaðNi1x Cux Þ2 V2 O8
(x ¼ 0:0; 0:05; 0:1) were prepared by solid state
reaction technique using BaCO3 (99.997%), NiO
(99.99%), CuO (99.97%) and V2O5 (99%) as
starting materials. The stoichiometric mixtures
were fired at 950 1C in air for three days with
two intermediate grindings and the powder samples were pressed into pellets and reacted at
1000 1C for 18 h. Single phase samples were
confirmed from X-ray powder diffraction which
was performed with a Panalytical Xpert-Pro
powder diffractometer. Lattice parameters were
obtained using a least-square fitting procedure.
The magnetization (M) data were measured
between 2 and 300 K in an applied field H ¼
5 kG using Quantum Design SQUID magnetometer.
18
9
0
-9
20
40
60
80
T(K)
0
100
200
300
T (K)
Fig. 1. Temperature dependence of magnetic susceptibility wðTÞ
of polycrystalline BaðNi1x Cux Þ2 V2 O8 for ð0pxp0:1Þ measured in an applied field of 5 kG. The inset shows the dw=dT
around T N and the arrow marks are the inflection points.
ARTICLE IN PRESS
74
50
40
TN (K)
signature of 3D ordering was, however, observed
by Rogado et al. in neutron diffraction [9]. We
have investigated the effect of doping S ¼ 12 Cu2þ
ions into the hexagonal Ni–O layer. The wðTÞ data
for polycrystalline BaðNi1x Cux Þ2 V2 O8 with x ¼
0:0; 0:05; 0:1 are shown in Fig. 1. In a similar
fashion as has been done for undoped sample, we
fitted the high temperature data of doped samples
to Eq. (1) and the J=kB value was found to be
nearly unchanged. As seen in the figure, Cu doping
has three effects: (i) an increase of the low
temperature Curie-like upturn, (ii) appearance of
a distinct feature at the LRO temperature (T N )
and (iii) a decrease of T N with increasing Cu
content.
wðTÞ at low temperature increases with Cu
content, indicative of the creation of free spins in
the honeycomb lattice through the introduction of
S ¼ 12 ions at S ¼ 1 Ni sites. Rogado et al. have
estimated the Curie term for their Mg-doped
samples by fitting the high-temperature wðTÞ data
to C=T þ wAFM . However, in our Cu-doped
samples, the Curie contribution is insignificant at
high-T (150–300 K) where, wðTÞ for 5% and 10%
Cu run almost parallel to the undoped one. So, we
are unable to extract the Curie terms with the
above procedure. Since the J=kB value remains
unchanged after doping, we assumed that the
wAFM is also same for all the samples. Therefore
we subtracted the wðTÞ of undoped sample from
the doped ones and fitted them to w0 þ C=T in the
temperature range of 70–300 K, which gives the
Curie constant of about 0.015 cm3 K/mole for 10%
Cu doping. This is almost equal to the value
reported by Rogado et al. for their 3% Mg-doped
sample. A more interesting observation is that the
Cu doping results in the appearance of a more
distinct feature at T N as shown in the inset of
Fig. 1. In the undoped system, 2D order appears
below T125 K. As the temperature decreases
further, the system locks into LRO at about
T50 K. However, by then spins in all the planes
are already well-ordered and no significant feature
is seen in magnetization data. But with dilution,
the spins become disordered and the uniform field
couples the spins randomly to the order parameter. The onset of ordering now shows up as a
more prominent feature in wðTÞ. It can also be
30
20
Mg doped ( Ref. 9)
Cu doped ( this work)
10
0
0
4
8
12
16
x (%)
Fig. 2. T N vs. x is shown for BaðNi1x Cux Þ2 V2 O8 samples and
Mg-doped BaNi2V2O8 samples reported in Ref. [9].
observed from Fig. 1 that this LRO peak shifts to
lower temperature with increasing Cu content.
This suggests that the dilution effect increases as
free spins are introduced by substituting Cu2þ ions
into Ni sites. This leads to a weakening of the
Ni–Ni AF interactions. The T N for our Cu-doped
samples were taken as the point of inflection from
the dw=dT plot (inset of Fig. 1). In Fig. 2, T N vs. x
is plotted for our Cu-doped samples and Mgdoped data taken from Ref. [9] are also shown. As
T N decreases faster with Mg doping than Cu, this
implies that the effect of Cu is weaker than the
effect Mg. This is because, Mg corresponds to a
spinless impurity and when substituted at the Ni2þ
site, one expects an uncompensated S ¼ 1 magnetic moment. However, Cu2þ ðS ¼ 12Þ substitution
might give rise to an uncompensated magnetic
moment corresponding to only S ¼ 12 thereby
having a weaker dilution effect.
4. Conclusion
In summary, we have investigated the effect of
doping S ¼ 12 impurity in BaNi2V2O8, a S ¼ 1 2D
XY antiferromagnet. It results in an increase of the
low-temperature susceptibility and suppresses T N .
ARTICLE IN PRESS
75
The decrease of T N was found to be linear with
increasing Cu content. The dilution effect on T N of
magnetic Cu2þ is found to be approximately half
the effect of nonmagnetic Mg2þ at the Ni2þ site.
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