Magnetic and optical properties of Fe doped crednerite CuMnO2


RSC Advances

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Magnetic and op
aDepartment of Physics, Indian Institute of

Varanasi-221005, India. E-mail: schatterji.a
bSchool of Material Science & Technology,

Hindu University), Varanasi 221005, India
cDepartment of Physics, Banaras Hindu Uni
dSolid State Physics Division, Bhabha Atomic
eIndus Synchrotrons Utilization Division,

Technology, Indore 452013, India
fCSIR-National Physical laboratory, Dr. K.S.
gDepartment of CMP & MS Tata Institute of

India

Cite this: RSC Adv., 2015, 5, 83504

Received 7th July 2015
Accepted 28th September 2015

DOI: 10.1039/c5ra13305j

www.rsc.org/advances

83504 | RSC Adv., 2015, 5, 83504–835
tical properties of Fe doped
crednerite CuMnO2
Kaushal K. Shukla,a P. Shahi,a Gopal S.,b A. Kumar,a A. K. Ghosh,c Ripandeep Singh,d

Neetika Sharma,d A. Das,d A. K. Sinha,e Amish G. Joshi,f A. K. Nigamg

and Sandip Chatterjee*a

A geometrically frustrated magnetic CuMnO2 system has been investigated because of its rich magnetic

properties. Neutron diffraction, synchrotron X-ray, magnetic, X-ray photoemission spectroscopy (XPS)

and UV-Visible spectroscopy measurements have been carried out on CuMnO2 and 5% Fe doped

CuMnO2 samples. Fe doping reduces the distortion. Moreover, Fe doping induces the ferromagnetic

coupling between ab planes. The value of magnetization is increased with Fe doping but coercivity is

decreased. These might be due to the direct Mn–Mn exchange and Mn–O–Cu–O–Mn super–super

exchange interactions. The UV-Vis data indicate the appearance of new energy bands in these

compounds. The XPS study indicates that Fe is in the 3+ state.
Introduction

Geometrically frustrated magnetic systems have attracted much
attention due to their interesting magnetic properties.1–3 In these
systems, spin ordering is suppressed by competing exchange
interactions well below the Curie temperature. ABO2-type
triangular-lattice antiferromagnets in which not all the interac-
tions are minimized simultaneously, are popular examples of
geometrically frustrated systems.4–7 Crednerite CuMnO2 is an
ABO2-type triangular-lattice antiferromagnet. The crystal struc-
ture of CuMnO2 contains of isosceles-triangular lattices of Mn. It
has a monoclinic structure with C2/m space group; it is distorted
from the hexagonal delafossite structure because of Jahn–Teller
effect of Mn3+ ions that have crystal-eld splitting in eg orbitals
due to d4 electronic conguration. Moreover, different directions
in the triangular ab plane become inequivalent: for each Mn, two
short and four long Mn–Mn distances exist. It is observed that
the exchange interaction at short bonds is stronger, which, with
the uniaxial magnetic anisotropy of Mn3+ (spins are oriented
predominantly along the long Mn–O bonds), leaves the system
frustrated. This degeneracy is lied below magnetic transition:
Technology (Banaras Hindu University),

pp@iitbhu.ac.in

Indian Institute of Technology, (Banaras

versity, Varanasi 221005, India

Research Centre, Mumbai 400085, India

Raja Ramanna Centre for Advanced

Krishnan Road, New Delhi 110 012, India

Fundamental Research, Mumbai, 400005,

11
at T < TN ¼ 65 K and the structure changes from monoclinic to
triclinic (space group C1) due to magnetostriction.8 Recently,
Ushakov et al. have shown that the presence of ferro-orbital
ordering in Cu1+xMn1�xO2 plays a very important role in deter-
mining the exchange constants and the magnetic structure.9

Furthermore, in nonstoichiometric crednerite with a small
excess of copper (Cu1.04Mn0.96O2), the in plane magnetic ordering
remains practically the same as for pure CuMnO2, the interlayer
exchange coupling changes from antiferromagnetic in CuMnO2
to ferromagnetic in Cu1.04Mn0.96O2 and vice versa.

10 Recent
neutron diffraction studies have revealed the magnetic structure
of CuMnO2 and the crystal structure deformation associated with
the magnetic ordering.8,11 The magnetic structure below TN ¼
65 K is the collinear one with the magnetic propagation vector
k1 ¼ (1/2$1/2$1/2). In addition to k1, they observed the other
group of the magnetic Bragg reections which are assigned by k2
¼ (1/2$1/2$0). While the intensity for k1 is dominant, that for k2 is
speculated to be caused by a small amount of impurity of
Cu1+xMn1�xO2.

8 Trari et al.12 reported the magnetic susceptibility
of Cu1+xMn1�xO2 (with x ¼ 0–0.2), suggesting that the magnetic
susceptibility is highly sensitive to the atomic disorders.
However, the minor fraction for k2 has not been investigated thus
far. The structural phase transition also occurs below TN from the
monoclinic C2/m to the triclinic C�1 in CuMnO2.

8,11 The degen-
eracy in the exchange interaction paths between base sites and
apex sites in isosceles triangular lattice, J2, is lied by the
distortion. It is evident from these previous studies that inter-
layer coupling plays a signicant role in the long range magnetic
ordering in CuMnO2, and that this can be characterised using
neutron diffraction. There is some dispute, however, over the
source of the k2 ordering as to whether it is intrinsic to the
structure or a result of some impurity. In this study, we have thus
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Fig. 1 Rietveld refinement of synchrotron X-ray powder diffraction
data of CuMn0.95Fe0.05O2 at room temperature.

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doped Fe into the Mn site in CuMnO2 so as to examine the effects
on the inter-layer coupling and on this ordering.

Experimental

The CuMn1�xFexO2 (with x ¼ 0.0 and 0.05) samples were
prepared by solid state reaction in an evacuated quartz tube.
Powders CuO, MnO and Fe2O3 were mixed in appropriate ratio
and pressed into pellets. The pellets were then placed in an
alumina crucible, sealed in quartz tube under high vacuum
(�10�6 mbar) and heated at 950 �C for 12 h. Powder XRD data
were recorded using ADXRD beam line (on bending magnet
port BL-12) of the Indus-2 (2.5 GeV, 100 mA) synchrotron radi-
ation (SR) source at Raja Ramanna Centre for Advanced Tech-
nology (RRCAT), Indore, India. The diffraction data were
collected on a image plate (mar 345) detector. The diffraction
images were integrated using FIT2D program. Wave length and
sample to detector distance were accurately calibrated using
XRD pattern of LaB6 NIST standard. Neutron powder diffraction
(NPD) patterns were recorded on the PD2 diffractometer (l ¼
1.2443 Å) at Bhabha Atomic Research Centre, Mumbai, India.
The observed XRD and neutron powder diffraction patterns
were analyzed by Rietveld method using the Fullprof-2K so-
ware package. The basis vector for the magnetic renement was
determined using BASIREPS program. Magnetic measurements
were carried out using MPMS, SQUID (Quantum Design)
magnetometer with the bulk samples. Data were collected
during warm up cycle. The absorption spectra were measured in
the range of 200–800 nm using UV-Vis spectrometer. X-ray
Photoelectron Spectroscopy (XPS) experiments were per-
formed using Omicron Nanotechnology UHV system equipped
with a twin anode Mg/Al X-ray source (DAR400), a mono-
chromatic source (XM 1000) and a hemispherical electron
energy analyzer (EA125). All the XPS measurements were per-
formed inside the analysis chamber under average base vacuum
of 8.1 � 10�10 Torr using monochromatized AlKa at 15 kV and
300 watt. The total energy resolution, estimated from the width
of the Fermi energy, was about 250 meV for monochromatic
AlKa line with photon energy 1486.7 eV. During photoemission
studies, small specimen charging was observed which was later
calibrated by assigning the C1s signal at 284.6 eV. Resistivity
measurements have been performed by four probe method.

Results and discussions
Structural characterization

The results of the renements using Synchrotron X-ray
diffraction data and neutron diffraction data are shown in
Fig. 1 and 2 (room temperature), and Tables 1 and 2, respec-
tively. These show good agreement with previously reported
data for the crednerite structure of CuMnO2.

13,14 The cell volume
of CuMn0.95Fe0.05O2 (92.1062 Å

3) is slightly larger than that of
CuMnO2 (91.9023 Å

3); it corresponds mainly to a decrease of a,
an increase of the b angle, and a small increase of b and c. The
changes in the b angle, a and b parameters will have effects on
the exchange interactions between ab planes and in the basal
plane, respectively. In the same way, the dilution on the Mn site,
This journal is © The Royal Society of Chemistry 2015
because of the small substitution of Fe for Mn, induces a
smaller (Mn/Fe–O) average distance 2.0446(3) Å compared to
2.0451(5) Å for CuMnO2 and a smaller Jahn–Teller distortion of
the MnO6 octahedra (by comparing the d ¼ d(Mn–O)apical/
d(Mn–O)equatorial). The irregularity in the triangular Mn
lattice in the basal plane also increases very slightly with the
shortest Mn–Mn distance (2.8877(3) Å) which is slightly longer
than in CuMnO2 (2.8876(8) Å) and also the two longest
distances (3.1532(1) Å) are larger than in CuMnO2 (3.1531(5) Å).
At room temperature, the Cu–O distances also slightly vary by
the substitution, close to 1.8392(5) Å in CuMnO2 and to
1.8470(1) Å in CuMn0.95Fe0.05O2.

We have also measured the neutron diffraction at 6 K for
both CuMnO2 and CuMn0.95Fe0.05O2. In Fig. 3 we have shown
the neutron diffraction pattern of CuMn0.95Fe0.05O2. On
lowering the sample temperature superlattice reections in
both these compounds are observed indicating the antiferro-
magnetic nature of these compounds. On lowering of temper-
ature we do not observe the splitting of the (220) reection (the
splitting of which indicates the transition from monoclinic to
triclinic structure).15 A marginal improvement in the t is
obtained in the triclinic phase but we have analysed the
diffraction in the monoclinic structure in C2/m space group at
6 K. The cell parameters of Fe doped CuMnO2 (a ¼ 5.5582(4) Å,
b ¼ 2.8850(2) Å, c ¼ 5.8986(4) Å, b ¼ 104.230(7)), signicantly
differ from those of CuMnO2 (a ¼ 5.5675(5) Å, b ¼ 2.8759(2) Å,
c ¼ 5.8811(5) Å, b ¼ 104.058(1)). Compared to RT, the difference
in the cell volume is more at low temperature (LT) in Fe doped
CuMnO2. In this low temperature structure, the oxygen atoms
occupy a general symmetry lattice site (x, y, z) and the oxygen
position at low temperature also varies with Fe doping. Dealing
with the triangular Mn-array in the (a, b) plane, the Mn–Mn
shortest edge of the triangle is slightly elongated (from 2.8759 Å
along [010] to 2.8850 Å along [110]). In the MnO6 octahedron at
low temperature, the two long apical Mn–O distances are
2.2574 Å, while the four equatorial distances are (1.9278 Å).
While, for CuMnO2 in the MnO6 octahedron at low temperature,
the apical Mn–O distances are 2.2618 Å, and the equatorial
distances are 1.9277 Å indicating that Fe doping decreases the
distortion in MnO6 octahedra. For nonstoichiometric
Cu1.04Mn0.96O2 sample also the distortion in MnO6 octahedra is
decreased.10 Therefore, no change in the chemical structure is
RSC Adv., 2015, 5, 83504–83511 | 83505

http://dx.doi.org/10.1039/c5ra13305j


Fig. 2 Rietveld refinement of neutron powder diffraction data of CuMn0.95Fe0.05O2 at room temperature.

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observed on Fe doping, although the magnetic structure is
found to be different on Fe doping. The structural parameters
obtained from the analysis are summarized in Table 1. In the
Table 1 Rietveld refinement of room temperature synchrotron XRD of
CuMnO2 and CuMn0.95Fe0.05O2

Sample/parameters CuMnO2, 300 K CuMn0.95Fe0.05O2, 300 K

Space group C2/m C2/m

Cell parameters
a (Å) 5.6063(3) 5.6064(3)
b (Å) 2.8876(8) 2.8877(3)
c (Å) 5.8990(3) 5.8990(3)
Cell volume (Å)3 92.6774 92.6823
a (deg) 90 90
b (deg) 103.965(3) 103.961(3)
g (deg) 90 90

Atomic positions
O(4i)x 0.4073 0.39978
y 0 0
z 0.17890 0.17789

Occupancy
Cu 1 1
Mn 1 0.95
Fe 0.05
Mn–Mn 2.8876(8) � 2 2.8877(3) � 2

3.1531(5) � 4 3.1532(1) � 4
Cu–O 1.8392(5) � 2 1.8470(1) � 2
Bragg R factor 4.139 2.886
RF 2.601 2.101
RP 9.70 8.07
Rwp 10.09 9.72
X2 2.44 1.98

83506 | RSC Adv., 2015, 5, 83504–83511
case of CuMnO2 the superlattice reections were indexed using
the two propagation vectors k1 ¼ (1/2$1/2$1/2) and k2 ¼
(1/2$1/2$0). The intensities corresponding to k2 were very weak
in agreement with Damay et al.,8 but not absent as reported
previously in this compound.16 The magnetic structure
described by k1 consists of antiferromagnetic chains in the (a, b)
plane coupled antiferromagnetically along the c-axis. The
components of the moment are 3.2 mB and 1.9 mB along a and
c-axes corresponding to 3.3 mB. For k2 vector the magnetic
Table 2 Rietveld refinement of the neutron powder diffraction data
for CuMnO2 and CuMn0.95Fe0.05O2 at 6 K

Sample/parameters CuMnO2, 6 K
CuMn0.95Fe0.05O2,
6 K

Space group C2/m C2/m

Cell parameter
a (Å) 5.5675(5) 5.5582(4)
b (Å) 2.8759(2) 2.8850(2)
c (Å) 5.8811(5) 5.8986(4)
Cell volume (Å)3 91.3454 91.6842
a (deg) 90 90
b (deg) 104.058 104.230(7)
g (deg) 90 90

Atomic positions
O(4i)x 0.4081(5) 0.4087(5)
y 0 0
z 0.1802(4) 0.1790(4)
Mn–O0 2.2618(�2) 2.2574(�2)
Mn–O00 1.9277(�4) 1.9278(�4)
Mn–Mn 3.1332(�4) 3.1312(�4)

2.8759(�2) 2.8850(�2)

This journal is © The Royal Society of Chemistry 2015

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Fig. 3 Rietveld refinement of neutron powder diffraction data of CuMn0.95Fe0.05O2 at 6 K.

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moment is 0.9(3) mB and oriented along c-axis. The total
moment is lower than the expected moment of 4 mB for Mn

3+ in
high spin state. Fe is found to substitute at the Mn site. It results
in a large increase in the intensity of (1/2$1/2$0) reection
corresponding to k2. The moment oriented along a and c are 2.4
mB and 2.0 mB, respectively leading to a total moment of 2.7 mB.
The moment corresponding to k1 vector is 1.8 mB. A net increase
in the moment (4.5 mB) is observed as expected for a mixture of
Mn3+ (4 mB) and Fe

3+ (5 mB), which is nearly equal to the satu-
ration value of the Cu(Mn, Fe)O2 sample. The appearance of k2
propagation vector indicates the ferromagnetic coupling
between ab planes.
Optical properties

We have also studied the electronic structure of CuMnO2 and
CuMn0.95Fe0.05O2 using X-ray photoemission spectroscopy
(XPS). The purpose of this study was to investigate any role of
the electronic structure on the magnetic properties of CuMnO2.
The XPS core level spectra of Cu2p, Mn2p and Fe2p and O1s are
shown in Fig. 4(a–d). Fig. 4(a) shows high resolution spectra of
Cu2p core level. Two clear distinct states of Cu(2p3/2) at 952 eV
and Cu(2p1/2) at 932 eV are separated by 19.75 eV and 19.9 eV for
CuMnO2 and CuMn0.95Fe0.05O2, respectively. Fig. 4(b) shows
core level spectra of Mn2p. Two separate states of Mn(2p3/2) and
Mn(2p1/2) observed at 641.6 eV and 652.95 eV for CuMnO2 and
641.3 eV and 652.9 eV for CuMn0.95Fe0.05O2, respectively.
Fig. 4(c) exhibit Fe2p core level spectra of CuMn0.95Fe0.05O2
compound which shows the spin–orbit splitting of the Fe2p
level, manifested as Fe2p3/2 and Fe2p1/2. The difference between
these two Fe peaks is 13.5 eV, which conrms the presence of
This journal is © The Royal Society of Chemistry 2015
Fe3+ state. These observed doubly states are due to the spin–
orbit coupling. Slight shiing in the states with Fe doping
occurs due to change in interaction energy between Cu and
transition metal ion. These data clearly suggest that Cu is in +1
state and both Mn and Fe are in +3 state. Fig. 4(d) shows the
spectra of oxygen which has two peak structures. Two peaks
marked as X observed at 529.8 eV and 529.6 eV and Y at 531.55
eV and 531.1 eV for CuMnO2 and CuMn0.95Fe0.05O2, respec-
tively. The rst peak marked as X, is characteristic peak of “O2�”
ions of the lattice oxygen, while peak Y denotes O(1s) lateral
structure. This lateral peak corresponds to the ionizations of
weakly adsorbed species17 and also the ionizations of oxygen
ions with particular coordinates, more specically integrated in
the subsurface. This suggests that the existence, in the
subsurface of oxygen ions that bear lower electron density than
the “O2�” ions. Normally, these oxide ions can be described as
“O�” species or excess oxygen. When the density of lattice
oxygen varies, the area ratio of these two peaks i.e. X and Y also
changes. Valence level spectra of CuMnO2 and CuMn0.95Fe0.05-
O2 samples are shown in Fig. 5. Four features (A, B, C, D) can be
identied in the experimental spectra of both the samples.
Feature A relate to state of dominant Cu3d atomic character
while B and C relate to the hybridization with Cu3d and Mn3d
to O2p.18 Feature D is a tail like structure near the Fermi level,
EF. The electronic states near the Fermi level are mostly
responsible for the electronic properties. A comparison of the
valence band spectra for the two samples reveals that the
density of states is negligibly small but nite at EF which is clear
from the inset of Fig. 5. The density of states slightly increases
with Fe doping. Therefore, conductivity increases slightly with
Fe doping. The XRD and neutron diffraction data also support
RSC Adv., 2015, 5, 83504–83511 | 83507

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Fig. 5 Valance-band XPS spectra of CuMnO2 and CuMn0.95Fe0.05O2
[(C) blue dots for CuMnO2 and red dots (C) for CuMn0.95Fe0.05O2].

Fig. 4 XPS core level spectra of (a) Cu2p (b) Mn2p (c) Fe2p and (d) O1s [(C) blue dots for CuMnO2 and red dots (C) for CuMn0.95Fe0.05O2].

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this. It is observed from Table 1 that on Fe doping Cu–O bond
length increases and Mn–O bond length decreases. This will
lead to the increase of bandwidth and as a consequence band
gap will decrease.18 Moreover, it has been observed (Table 1)
that on Fe doping the c axis is elongated leading to weaker
Cu3d–O2p hybridization. In the valence band spectra (Fig. 5)
also it is found that in CuMn0.95Fe0.05O2 the B and C features are
reduced with respect to the A feature in accordance with the
XRD and neutron diffraction data (Table 1). It is worthwhile to
mention that the valence band spectrum of CuMnO2 differs
from that of CuCrO2 (ref. 19) in respect of non-existence of the
shoulder at the upper part of the valence band indicating that
Mn3d is located away from the Fermi level. The most interesting
feature is the shi of Fermi level towards lower binding energy
for both CuMnO2 and CuMn0.95Fe0.05O2 unlike CuCrO2.

18,19

Similar behavior is observed in K doped SrCu2O2.
20 This shi is

related to low activation energy of CuMnO2. Furthermore, with
Fe doping it is observed that shi in Fermi energy is slightly
larger than that in undoped sample. This is close to the further
change in activation energy. Therefore, the photo emission
spectroscopy measurements conrm the movement of Fermi
level towards the valence band edge on Fe doping which is
accompanied by corresponding shis in core level binding
energies. Similar kind of movement is observed in CuCrO2 with
Mg doping.20 In the present investigation the Cu2p3/2 peak
shis with Fe doping from 932.45 eV to 932.2 eV.

The absorption curves of CuMnO2 and Fe doped CuMnO2 are
shown in Fig. 6. As the photon energy increases the absorption
83508 | RSC Adv., 2015, 5, 83504–83511
intensity increases and attains a maximum. For the undoped
sample the maximum occurs at E ¼ 4.2 eV whereas for the Fe
doped sample it increases to 4.5 eV. The peak can be assigned to
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http://dx.doi.org/10.1039/c5ra13305j


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an excitonic excitation from Cu3d + O2p to Cu3d + 4s.19,20 For
the Fe doped sample the peak intensity becomes negligibly
small, the reason of which is not yet clear. It deserves further
study. Moreover, the optical band gaps result from the rela-
tionship between the optical absorption coefficient and the
photon energy (hn) can be expressed as:

(ahn)
2 ¼ A(hn � Eg)

where A is a constant and Eg is the direct optical band gap of the
material. The inset of Fig. 6 shows the optical band gap of the
undoped and Fe doped CuMnO2. It is observed from the gure
that both samples have two energy band gaps. The band gaps of
the crednerite CuMnO2 (4.60 eV and 4.90 eV) and CuMn0.95-
Fe0.05O2 (4.60 eV and 4.79 eV) are close to each other. Therefore,
for both the samples an additional energy level exists near the
valence band edge, as also revealed by valence band spectra
(Fig. 5). The difference between the new band and the conduc-
tion band is 4.60 eV for both the samples. On the other hand, the
new band resides 0.30 eV and 0.19 eV above the top of the valence
band respectively for CuMnO2 and CuMn0.95Fe0.05O2. Similar,
new band is observed in CuGa0.8Cr0.2O2 lm.

21 Appearance of
new band might be the reason of the shiing of Fermi level
towards the valence band edge observed in valence band spectra
(Fig. 5). Moreover, observed band gaps of these systems come in
the range of wide band gap semiconductors and these values are
even more than the band gap of ZnO (3.37 eV) and GaN
(3.44 eV).22–24 Additionally, these systems show much better
magnetic ordering and magnetic moment than any known
diluted magnetic semiconductors, which might prove signicant
in the application of these materials as magnetic
semiconductors.
Electrical properties

In order to understand the intrinsic characteristic, we have also
studied the temperature dependence of resistivity for both
CuMnO2 and CuMn0.95Fe0.05O2 compounds. The exponential
decrease in resistivity with the increase in temperature indi-
cates the semiconducting nature of both the samples. The plot
of ln r versus 1000/T (Fig. 7) shows that thermally activated band
Fig. 6 Absorption spectra of CuMnO2 and CuMn0.95Fe0.05O2. Inset:
optical band gap from UV-Visible spectroscopic measurement of
CuMnO2 and CuMn0.95Fe0.05O2.

This journal is © The Royal Society of Chemistry 2015
conduction is the dominant mechanism at high-temperature
region. The thermally activated resistivity at high-temperature
region follows the Arrhenius law

r(T) ¼ r0 exp[Ea/kBT] (1)

where kB is the Boltzmann's constant and Ea is the activation
energy. The deviation from the linear t indicates that thermal
activation mechanism is not valid at lower temperature region.
The variable-range-hopping (VRH) conduction of polarons has
been found to dominate in this temperature region. The
conduction mechanism due to the variable range hopping of
polaron at low temperature can be described by the Mott's
equation25

r(T) ¼ r0 exp[T0/T]1/4 (2)

where r0 and T0 are constants and are given by r0 ¼ {[8pakBT/
N(EF)]

1/2}/(3e2nph) and T0 ¼ 18a3/[kN(EF)] where nph (�1013 s�1) is
the phonon frequency at Debye temperature, N(EF) is the
density of localized electron states at the Fermi level, and a is
the inverse localization length. For CuMnO2 two slopes are
observed which can be tted with the eqn (1). The estimated
activation energy (Ea) from the two slopes are 0.29 eV and
0.96 eV. The estimated activation energy (Ea) for CuMn0.95-
Fe0.05O2 using Arrhenius law is 0.90 eV. Moreover, we did not get
proper tting with the eqn (2).
Magnetic properties

Fig. 8 shows the magnetization of CuMnO2 as a function of
magnetic eld. At 5 K, a clear hysteresis with a coercive eld of
2.46 kOe is observed. The magnetization increases almost
linearly with magnetic eld aer closing of the hysteresis. The
magnetic properties of CuMnO2 are controversial.

26,27 Our data
match with the data in ref. 19. The data indicate the presence of
the dominating AFM order with some FM ordering. The AFM
Fig. 7 Variation of resistivity versus temperature for CuMn1�xFexO2
(where x ¼ 0, 0.05). Inset variation of ln r vs. 1000/T for (x ¼ 0 and
0.05) samples.

RSC Adv., 2015, 5, 83504–83511 | 83509

http://dx.doi.org/10.1039/c5ra13305j


Fig. 9 Magnetization curve M(H) of CuMn0.95Fe0.05O2 at 5 K.

Fig. 8 Magnetization curve M(H) of CuMnO2 at 5 K.

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ordering at 50 K has also been reported recently by Kurakawa
et al.28 The increase of high eld magnetization along with the
appearance of Hc indicates the emergence of an uncompen-
sated moment. When Fe is doped (Fig. 9), the magnetization is
increased but the coercivity is decreased. With increase of the
magnetic eld the ferromagnetic correlation precipitates and
the antiferromagnetic correlation is increased which is clear
from the M(H) curve. The change in magnetic behavior with Fe
doping can be explained in terms of magnetic exchanges, direct
Mn–Mn interactions may be considered as dominant in-plane
but indirect exchanges (via oxygen) could also play a role, all
Mn–O–Mn angles being close to 90�. The super–super exchange,
along Mn–O–Cu–O–Mn path ways, may also play the role for the
3D magnetic ordering. In fact super–super exchange via
diamagnetic cation is quite common, as is observed in different
oxides.11 The difference in magnetic behavior with Fe doping
might be due to the presence of k2 ¼ (1/2$1/2$0) vector which
has been observed from neutron diffraction measurement. As
has been mentioned that the appearance of k2 ¼ (1/2$1/2$0)
vector is the indication of ferromagnetic coupling between ab
planes implying that Fe doping induces this ferromagnetic
coupling. Similar behavior is observed in the non-
stoichiometric Cu1.04Mn0.96O2.

10
Conclusion

Neutron diffraction, synchrotron X-ray diffraction, XPS,
magnetic and UV-Visible spectroscopic measurements have
been investigated on CuMnO2 and 5% Fe doped CuMnO2
samples, with assumption that these measurements have
83510 | RSC Adv., 2015, 5, 83504–83511
complementary information on structural and magnetic
behaviour of the samples. On Fe doping, the apical Mn–O
distances decrease while the equatorial distances slightly
increase, reducing the distortion in MnO6 octahedra. Moreover,
when Fe is doped along with k1 ¼ (1/2$1/2$1/2) the magnetic
peaks can also be indexed with the propagation vector k2 ¼
(1/2$1/2$0) indicating the appearance of ferromagnetic
coupling between ab planes. Value of magnetization is
increased with Fe doping but coercivity is decreased. These
might be due to direct Mn–Mn exchange and Mn–O–Cu–O–Mn
super–super exchange interactions. The UV-Vis data showed the
increase in one of the two energy gaps, on Fe doping, indicating
the usefulness of these materials as wide band gap magnetic
semiconductors.

Acknowledgements

S.C. is grateful to BRNS, DAE, India (Grant No.: 2013/37P/43/
BRNS) for providing nancial support. P.S. is grateful to CSIR,
India for providing nancial support. Authors are also grateful
to D. Budhikot for his help in magnetization measurement.

References

1 T. Kimura, T. Goto, H. Shintani, K. Ishizaka, T. Arima and
Y. Tokura, Magnetic control of ferroelectric polarization,
Nature, 2003, 426, 55–58.

2 N. Hur, S. Park, P. A. Sharma, J. S. Ahn, S. Guha and
S. W. Cheong, Electric polarization reversal and memory in
a multiferroic material induced by magnetic elds, Nature,
2004, 429, 392–395.

3 T. Lottermoser, T. Lonkai, U. Amann, D. Hohlwein,
J. Ihringer and M. Fiebig, Magnetic phase control by an
electric eld, Nature, 2004, 430, 541–544.

4 A. J. W. Reitsma, L. F. Feiner and A. M. Oles, Orbital and spin
physics in LiNiO2 and NaNiO2, New J. Phys., 2005, 7, 121.

5 F. Ye, J. A. Fernandez-Baca, R. S. Fishman, Y. Ren, H. J. Kang,
Y. Qiu and T. Kimura, Magnetic Interactions in the
Geometrically Frustrated Triangular Lattice Antiferromagnet
CuFeO2, Phys. Rev. Lett., 2007, 99, 157201.

6 D. J. Singh, Magnetic and electronic properties of LiMnO2,
Phys. Rev. B: Condens. Matter Mater. Phys., 1997, 55, 309–312.

7 T. Jia, G. Guo Zhang and Z. Zeng, Orbitally relieved magnetic
frustration in NaVO2, Phys. Rev. B: Condens. Matter Mater.
Phys., 2009, 80, 045103.

8 F. Damay, M. Poienar, C. Martin, A. Maignan, J. Rodriguez-
Carvajal, G. Andre and J. P. Doumerc, Spin-lattice coupling
induced phase transition in the S ¼ 2 frustrated
antiferromagnet CuMnO2, Phys. Rev. B: Condens. Matter
Mater. Phys., 2009, 80, 094410.

9 A. V. Ushakov, S. V. Streltsov and D. I. Khomskii, Orbital
structure and magnetic ordering in stoichiometric and
doped crednerite CuMnO2, Phys. Rev. B: Condens. Matter
Mater. Phys., 2009, 80, 094410.

10 M. Poienar, C. Vecchini, G. Andre, A. Daoud-Aladine,
I. Margiolaki, A. Maignan, A. Lappas, L. Chapon,
M. Hervieu, F. Damay and C. Martin, Substitution Effect
This journal is © The Royal Society of Chemistry 2015

http://dx.doi.org/10.1039/c5ra13305j


Paper RSC Advances

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sh

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ow
nl

oa
de

d 
by

 U
ni

ve
rs

it
y 

of
 L

et
hb

ri
dg

e 
on

 0
9/

10
/2

01
5 

22
:5

5:
46

. 
View Article Online
on the Interplane Coupling in Crednerite: the
Cu1.04Mn0.96O2 Case, Chem. Mater., 2011, 23, 85–94.

11 C. Vecchini, M. Poienar, F. Damay, O. Adamopoulos,
A. Daoud-Aladine, A. Lappas, J. M. Perez-Mato,
L. C. Chapon and C. Martin, Magnetoelastic coupling in
the frustrated antiferromagnetic triangular lattice
CuMnO2, Phys. Rev. B: Condens. Matter Mater. Phys., 2010,
82, 094404.

12 M. Trari, J. Topfer, P. Dordor, J. C. Grenier, M. Pouchard and
J. P. Doumerc, Preparation and physical properties of the
solid solutions Cu1+xMn1�xO2 (0 # x # 0.2), J. Solid State
Chem., 2005, 178, 2751–2758.

13 J. Topfer, M. Trari, P. Garavcreau, J. P. Chaminade and
J. P. Z. Doumerc, Kristallograya, 1995, 210, 184.

14 I. D. Kandrashev, Sov. Phys. Crystallogr., 1959, 3, 703.
15 V. Ovidiu Garlea, A. T. Savici and R. Jin, Tuning the magnetic

ground state of a triangular lattice system Cu(Mn1�xCux)O2,
Phys. Rev. B: Condens. Matter Mater. Phys., 2011, 83, 172407.

16 Y. Shirako, H. Satsukawa, X. X. Wang, J. J. Li, Y. F. Guo,
M. Arai, K. Yamaura, M. Yoshida, H. Kojitani,
T. Katsumata, Y. Inaguma, K. Hiraki, T. Takahashi and
M. Akaogi, Integer spin-chain antiferromagnetism of the
4d oxide CaRuO3 with post-perovskite structure, Phys. Rev.
B: Condens. Matter Mater. Phys., 2011, 83, 17441.

17 G. Gusmano, G. Montesperelli, E. Traversa and G. Mattogno,
Microstructure and electrical properties of MgAl2O4 thin
lms for humidity sensing, J. Am. Ceram. Soc., 1993, 76,
743–750.

18 D. Shin, J. S. Foord, D. J. Payne, T. Arnold, D. J. Aston,
R. G. Egdell, K. G. Godinho, D. O. Scanlon, B. J. Morgan,
G. W. Watson, E. Mugnier, C. Yaicle, A. Rougier,
L. Colakerol, P. A. Glans, L. F. J. Piper and K. E. Smith,
Comparative study of bandwidths in copper delafossites
from x-ray emission spectroscopy, Phys. Rev. B: Condens.
Matter Mater. Phys., 2009, 80, 233105.

19 T. Arnold, D. J. Payne, A. Bourlange, J. P. Hu, R. G. Egdell,
L. F. J. Piper, L. Colakerol, A. D. Masi, P. A. Glans,
This journal is © The Royal Society of Chemistry 2015
T. Learmonth, K. E. Smith, D. O. Scanlon, A. Walsh,
B. J. Morgan and G. W. Watson, X-ray spectroscopic study
of the electronic structure of CuCrO2, Phys. Rev. B:
Condens. Matter Mater. Phys., 2009, 79, 075102.

20 C. C. B. Lynch, R. G. Egdell and D. S. L. Law, High resolution
X-ray photoemission of SrCu2O2, Chem. Phys. Lett., 2005,
401, 223–226.

21 M. Han, K. Jiang, J. Zhang, W. Yu, Y. Li, Z. Hu and J. Chu,
Structural, electronic band transition and optoelectronic
properties of delafossite CuGa1�xCrxO2 (0 # x # 1) solid
solution lms grown by the sol–gel method, J. Mater.
Chem., 2012, 22, 18463–18470.

22 A. Mang, K. Reimann and S. Rubenacke, Band gaps, crystal-
eld splitting, spin–orbit coupling, and exciton binding
energies in ZnO under hydrostatic pressure, Solid State
Commun., 1995, 94, 251–254.

23 O. Madelung, Semiconductor basic data 2nd review edition,
Springer, Berlin, 1996.

24 S. Kumar, S. Chatterjee, K. K. Chattopadhyay and
A. K. Ghosh, Sol–gel derived ZnO: Mn nanocrystals: study
of structural, Raman and optical properties, J. Phys. Chem.
C, 2012, 116, 16700–16708.

25 N. F. Mott, Conduction in glasses containing transition
metal ions, J. Non-Cryst. Solids, 1968, 1, 1–17.

26 J. P. Doumerc, M. Trari, J. Topfer, L. Fournes, J. C. Greinier,
M. Pouchard and P. Hagenmuller, Magnetic properties of
the crednerite CuMnO2, Eur. J. Solid State Inorg. Chem.,
1994, 31, 705–715.

27 Y. Bessekhouad, Y. Gabes, A. Bouguelia and M. Trari, The
physical and photo electrochemical characterization of the
crednerite CuMnO2, J. Mater. Sci., 2007, 42, 6469–6476.

28 A. Kurakawa, T. Yanoh, S. Yano and Y. Ichiyanagi,
Preparation and Magnetic Properties of Multiferroic
CuMnO2 Nanoparticles, J. Nanosci. Nanotechnol., 2014, 14,
2553–2556.
RSC Adv., 2015, 5, 83504–83511 | 83511

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	Magnetic and optical properties of Fe doped crednerite CuMnO2
	Magnetic and optical properties of Fe doped crednerite CuMnO2
	Magnetic and optical properties of Fe doped crednerite CuMnO2
	Magnetic and optical properties of Fe doped crednerite CuMnO2
	Magnetic and optical properties of Fe doped crednerite CuMnO2
	Magnetic and optical properties of Fe doped crednerite CuMnO2
	Magnetic and optical properties of Fe doped crednerite CuMnO2
	Magnetic and optical properties of Fe doped crednerite CuMnO2

	Magnetic and optical properties of Fe doped crednerite CuMnO2
	Magnetic and optical properties of Fe doped crednerite CuMnO2