IEEE TRANSACTIONS ON MAGNETICS, VOL. 50, NO. 4, APRIL 2014
2900104
Influence of the Change in Oxidation Number of Mn on
Magnetic Properties of BaTi1−x Mn x O3
T. L. Phan1 , T. D. Thanh1,2, T. A. Ho1 , P. D. Thang3, and S. C. Yu1
1 Department
of Physics, Chungbuk National University, Cheongju 361-763, South Korea
of Materials Science, Vietnam Academy of Science and Technology, Hanoi, Vietnam
3 Department of Engineering Physics and Nanotechnology, University of Engineering and Technology,
Vietnam National University, Hanoi 10000, Vietnam
2 Institute
We prepared BaTi1−x Mn x O3 samples (x = 0 and 0.05) by standard solid-state reaction at 700 °C and 900 °C in an Ar
ambient to change oxidation number of Mn dopants. The fabricated samples were then studied structural characterization and
electronic structures by means of X-ray diffraction and absorption, and Raman scattering and electron-spin-resonance spectrometers.
We found oxidation numbers 2+ and 3+ of Mn ions coexisting in BaTi1−x Mn x O3 with x = 0.05, where the Mn2+ /Mn3+ ratio is
about one for the sample annealed at 700 °C. These Mn ions prefer locating at the Ti site in the tetragonal BaTiO3 host lattice. In
particular, there is a change in the oxidation number of Mn2+ → Mn3+ when the annealing temperature changes from 700 °C to
900 °C. Their magnetic properties are accordingly changed. Meanwhile, annealing pure BaTiO3 in an Ar ambient at 700 °C and
900 °C does not lead to ferromagnetic (FM) order; they are almost diamagnetic. With the obtained results, we believe that FM
order in Mn-doped BaTiO3 annealed in an Ar ambient is associated with exchange interactions of Mn2+ ions mediated by oxygen
vacancies rather than associated with Mn3+ ions.
Index Terms— BaTiO3 -based multiferroics, local structure, magnetic properties.
I. I NTRODUCTION
I
N RECENT years, BaTiO3 -based multiferroics [defined
as materials exhibiting simultaneously either two of ferroelectric, ferromagnetic (FM), and ferroelastic properties]
have attracted much interest of the solid-state physics community because doping a transition metal (TM, with TM
= Mn, Co, Fe, Cr, or Ni, etc.) can lead to FM order at
room temperature [1]–[8]. This makes TM-doped BaTiO3
compounds become promising candidates for applications
of multifunctional electronic and spintronic devices. Among
these, BaTi1−x Mn x O3 materials have been of special interest
because it is realized that the Mn doping can give a large
value of magnetic moment [1], [6], [7], and enhances the
positive temperature coefficient of resistance [9]. The application range of BaTiO3 -based materials in electronic and spintronic devices is thus extended to high-speed nonvolatile and
holographic memories, capacitors with tunable capacitance,
actuators, dielectric resonators, electromagnetic-interference
filters, and so forth [10], [11].
Previous studies revealed that the magnetic properties of
BaTi1−x Mn x O3 depend on many factors, such as Mn-doping
concentration (x), fabrication conditions (annealing temperature, pressure, and ambient), and sample types (i.e., bulk
single-crystal and polycrystalline samples, thin films or nanostructures) [3]–[5], [7], [8], [12]–[14]. For bulk ceramic samples, it has been found that magnetic changes as varying
x are related directly to the crystalline fraction of tetragonal and hexagonal phases, the strength of double-exchange
Manuscript received August 2, 2013; revised October 7, 2013; accepted
October 24, 2013. Date of current version April 4, 2014. Corresponding
author: S. C. Yu (e-mail: [email protected]).
Color versions of one or more of the figures in this paper are available
online at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TMAG.2013.2288293
interactions between Mn3+ and Mn4+ ions, and the concentration of Mn3+,4+ ions [1]. Their ferromagnetism at
low temperatures (<50 K) is usually attributed to secondary
manganese oxides [7]. For single-phase BaTi1−x Mn x O3 samples, FM interactions between these Mn ions can exist in
both tetragonal and hexagonal structures [1], [7]. Carefully
considering the experimental results reported in [13], [15],
and [16], one can see an oxidation-number change of Mn
dopants in BaTi1−x Mn x O3 depending on sample fabrication
and processing conditions. A similar result was also found in
Mn-doped Ba0.998 La0.002 TiO3 in [17]. However, it appears that
a detailed study of how an oxidation-number change influences
the magnetic properties of BaTi1−x Mn x O3 materials has not
been thoroughly carried out yet. Thus, the nature of magnetism
observed in this material system is still a topical issue needed
to be clarified. In an attempt to gain more insight into these
problems, we prepared BaTi1−x Mn x O3 ceramic samples in an
Ar ambient at 700 °C and 900 °C. We expect that the annealing
in the Ar ambient at different temperatures changes the oxidation state of Mn dopants in BaTi1−x Mn x O3 . This influences
their electronic structures and structural characterization, and
magnetic properties.
II. E XPERIMENTAL D ETAILS
Polycrystalline BaTi1−x Mn x O3 samples with x = 0 and
0.05 were prepared by conventional solid-state reaction. Highpurity precursors in powder of BaCO3 , rutile-TiO2 , and/or
MnCO3 were combined with nominally stoichiometric quantities, and mixed well using an agate mortar and pestle. These
mixtures were then calcined at 500 °C for 24 h. After several
times of remixing and calcination under the same conditions,
each mixture (i.e., x = 0 or 0.05) was divided into two parts,
and then pressed into pellets and annealed at 1000 °C in air
for 24 h. After the preparation, the pellet with x = 0.05
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2900104
IEEE TRANSACTIONS ON MAGNETICS, VOL. 50, NO. 4, APRIL 2014
Fig. 1.
XRD patterns of polycrystalline BaTi1−x Mnx O3 samples with
x = 0 and 0.05 annealed in an Ar ambient at 700 °C and 900 °C. Asterisks
shown in the XRD pattern of x = 0.05 annealed at 700 °C reveal the presence
of hexagonal BaTiO3 with a small crystalline volume fraction.
exhibits the paramagnetic behavior at room temperature, which
is related to Mn4+ ions [18], while the pellet with x = 0 is diamagnetic. These two pellets were then divided into two parts,
and in turn annealed in an Ar ambient at 700 °C and 900 °C for
6 h. After annealing, their crystal structure was checked by an
X-ray diffractometer (Bruker AXS, D8 Discover) and Raman
scattering (RS) spectroscopy (LABRAM 1B, Jobin Yvon). The
local geometric and electronic structures were studied using
X-ray absorption fine structure (XAFS) technique in the
Pohang Accelerator Laboratory (South Korea), where XAFS
spectra in the transmission configuration were measured for
both the Mn and Ti K edges corresponding to E 0 = 6539
and 4966 eV, respectively. For reference, XAFS spectra of
two oxides MnO and Mn2 O3 corresponding to Mn2+ and
Mn3+ ions, respectively, were also recorded. Magnetic studies were performed on a superconducting quantum interference device, and electron-spin-resonance (ESR) spectroscopy
(JEOL-TE300) operated at an X-band frequency of 9.4 GHz.
All the above investigations were executed at room temperature.
III. R ESULTS AND D ISCUSSION
Fig. 1 shows X-ray diffraction (XRD) patterns of
BaTi1−x Mn x O3 (x = 0 and 0.05) annealed at 700 °C and
900 °C as using an X-ray radiation source of Cu–K α with λ
= 1.5406 Å. It appears that excepting the sample with x =
0.05 annealed at 700 °C having a small crystalline fraction
of hexagonal BaTiO3 -related secondary phase (space group:
P63/mmc, see XRD peaks denoted by asterisks), all the
samples with Miller-indexed peaks exhibit a single phase
in the tetragonal BaTiO3 structure (space group: P4 mm).
The lattice parameters a = b ≈ 3.998 Å, and c ≈ 4.030 Å
of tetragonal BaTi1−x Mn x O3 with x = 0 are slightly changed
Fig. 2. RS spectra of BaTi1−x Mnx O3 (x = 0 and 0.05) annealed at 700 °C
and 900 °C under an excitation wavelength of 488 nm. The asterisk is an
indication of a secondary phase of hexagonal BaTiO3 in addition to the main
tetragonal phase.
with the presence of 5% Mn (i.e., the sample with x = 0.05).
Meanwhile, the lattice parameters of hexagonal
BaTi1−x Mn x O3 with x = 0.05 annealed at 700 °C are
a = b ≈ 5.725 Å, and c ≈ 13.879 Å. These values are in
good agreement with those determined from BaTiO3 -based
materials [1], [2], [13], revealing good crystalline quality of
our samples.
Their structural characterization was also studied by means
of an RS spectrometer. For the undoped samples (x = 0)
annealed at 700 °C and 900 °C, their RS spectrum exhibits
typical vibration modes peaked at ∼265, 306, 520, and
716 cm−1 , see Fig. 2, which are characteristic of the tetragonal BaTiO3 structure. It has been suggested that 265 cm−1
is related to A1 (TO), 306 cm−1 is E(TO+LO) and/or B1
associated with a tetragonal-cubic phase transition, 520 cm−1
is E(TO) and/or A1 (TO), and 716 cm−1 can be due to
A1 (LO) and/or E(LO) [19]. These modes persist when tetragonal BaTiO3 is doped with 5% Mn (see the RS spectra of
x = 0.05 annealed at 700 °C and 900 °C shown in Fig. 2).
However, their peak position is shifted slightly toward lower
frequencies due to an incorporation of Mn [1]. Concurrently,
there is an additional mode peaked at about 630 cm−1 for
both the samples, which is characteristic of hexagonal BaTiO3 .
This result proves RS spectroscopy more sensitive than the
XRD technique. Although above XRD and RS studies revealed
an incorporation of Mn dopants into BaTiO3 lattices, their
oxidation state is difficult to identify.
To identify the oxidation state of Mn dopants, we have
based on the XAFS technique [20]. It is known that an XAFS
recorded from an absorbing atom includes three characteristic
regions: the pre-edge, X-ray absorption near edge structure
PHAN et al.: INFLUENCE OF THE CHANGE IN OXIDATION NUMBER OF Mn
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Fig. 3. XAFS spectra of BaTi1−x Mnx O3 (x = 0.05) recorded for the Mn
K edge. Inset: Fourier-transformed EXAFS spectra at the Mn and Ti K edges
for a typical sample annealed at 700 °C.
Fig. 4. ESR spectra of BaTi1−x Mnx O3 (x = 0.05) annealed at 700 °C and
900 °C. No ESR signal is recorded for the samples with x = 0.
(XANES), and extended XAFS (EXAFS). Among these, the
pre-edge region is related to transitions (with a low probability)
from core electrons to some bound states. The XANES region
gives information on valence states of the selected atom.
Different oxidation states result in a chemical shift in the
absorption edge. For the EXAFS region, it includes the sum
of all outgoing and incoming waves, and dependent on the
immediate environment surrounding the absorbing atom. The
EXAFS analysis thus gives important information related
to the bond distances, geometric structure, and coordination
number of neighbor atoms. In Fig. 3, it shows XAFS spectra
recorded for the Mn K edge of BaTi1−x Mn x O3 samples with
x = 0.05. Considering the XANES region, it comes to our
attention that the absorption edge of the sample annealed at
700 °C is located in between the edges of MnO and Mn2 O3 .
Annealing at 900 °C (the sample annealed at 900 °C) leads to
the shift of this edge toward the high-energy region of Mn2 O3 .
If more attention is given to Fourier transformed XAFS
spectra, the inset of Fig. 3, one can see the bond distances of
RMn−O and RTi−O obtained from both Mn and Ti K edges are
comparable with each other. Such the features reflect a coexistence of Mn2+ and Mn3+ ions in BaTi1−x Mn x O3 , where the
concentration ratio of Mn2+ /Mn3+ is about one for the sample
annealed at 700 °C. These Mn ions prefer locating at the Ti site
in the tetragonal BaTiO3 host lattice. Particularly, there is an
oxidation-state change of Mn2+ → Mn3+ when the annealing
temperature changes from 700 °C to 900 °C. For the Ti K
edge (not shown), we did not observe its shift as varying the
Mn concentration and the annealing temperature. This proves
that the oxidation-state change of Mn in BaTi1−x Mn x O3
(x = 0.05) is be related to an oxygen vacancy (VO ) due to
the annealing in an Ar ambient.
The coexistence of Mn2+ and Mn3+ ions in BaTi1−x Mn x O3
(x = 0.05) can be further confirmed using ESR spectroscopy,
a sensitive tool to probe dopants with unpaired electrons [21].
As shown in Fig. 4, the ESR spectra of the samples consist of Mn2+ hyperfine lines (due to allowed transitions of
−1/2 ↔ +1/2), and a broadening line in the Lorentzian shape
associated with dipole–dipole interactions of Mn2+ –Mn2+ ,
Mn3+ –Mn3+ , and/or Mn3+ –Mn3+ exchange pairs [22], [23].
The annealing at 900 °C reduces the intensity of the hyperfine
lines because of the concentration decrease of Mn2+ ions
caused by the oxidation-state change of Mn2+ → Mn3+ as
mentioned. Here, the resonance takes place at the magnetic
field Hr ≈ 3440 Oe, corresponding to a effective Lande value
of g ≈ 1.98 (obtained from a simple relation g = hν/μ B Hr ,
where μ B is the Bohr magneton [21]). A small deviation
from the standard value g = 2.0023 (for unpaired electrons) is assigned to spin–spin, spin–lattice, and/or spin–orbit
interactions between Mn2+ and Mn3+ ions. It is emphasized that earlier studies on BaTi1−x Mn x O3 also revealed
the oxidation-state change of Mn2+ → Mn4+ as increasing
the Mn-doping concentration and annealing temperature in
air [16], [22]. The annealing in a reducing atmosphere introduces more Mn2+ ions, and the presence Mn3+ ions favors
the formation of hexagonal BaTiO3 [13]. However, a detailed
study about the influence of these changes on the magnetic
properties of BaTi1−x Mn x O3 has not been performed.
In this paper, to study the impact of Mn2+ and Mn3+
ions on the magnetic properties of BaTi1−x Mn x O3 samples,
we measured magnetic-field dependences of magnetization at
room temperature. The data graphed in Fig. 5 show that the
Mn-doped samples exhibit weak FM order, with saturation
magnetization values Ms ≈ 1.5 × 10−3 and 0.5 × 10−3 emu/g
for the samples annealed at 700 °C and 900 °C, respectively.
An increase in the annealing temperature reduces FM order.
For the sample annealed at 700 °C, its field coercivity (Hc ) is
∼120 Oe. Different from the Mn-doped samples, pure BaTiO3
samples (x = 0) are almost diamagnetic, see the inset of
Fig. 5. This is understandable because Ti4+ (3d 0 ) in BaTiO3
is a nonmagnetic ion that its oxidation state is unchanged
by the annealing. We herein believe that ferromagnetism in
BaTi1−x Mn x O3 annealed in an Ar ambient is due to exchange
interactions Mn2+ ions in the tetragonal structure mediated by
VO defects. This judgment is based on the fact that more Mn3+
ions in the sample annealed at 900 °C favor the formation
2900104
IEEE TRANSACTIONS ON MAGNETICS, VOL. 50, NO. 4, APRIL 2014
Fig. 5. Room-temperature hysteresis loops for BaTi1−x Mnx O3 with x = 0
(the inset) and x = 0.05 (main figure) annealed at 700 °C and 900 °C.
of the hexagonal phase [13], and thus reduces FM order.
Furthermore, a coexistence of Mn2+,3+ ions, and their magnetic interactions leads to room-temperature paramagnetism as
studied for the case of Mn-doped ZnO samples annealed in air
at 1100 °C [20]. In general, high FM order is usually obtained
for BaTi1−x Mn x O3 samples containing simultaneously both
Mn3+ and Mn4+ ions with a suitable ratio, as reported in [1].
These results are considered as an important database that it
can be consulted to fabricate FM Mn-doped BaTiO3 materials.
IV. C ONCLUSION
We fabricated ceramic samples BaTi1−x Mn x O3 with x = 0
and 0.05 at 700 °C and 900 °C in an Ar ambient for 6 h. Structural analyses based on XRD patterns and RS spectra revealed
the formation of a secondary phase of hexagonal BaTiO3 in
addition to the main phase of tetragonal BaTiO3 . There is
the coexistence of Mn2+ and Mn3+ ions in BaTi1−x Mn x O3
(x = 0.05) that the Mn2+ /Mn3+ ratio is about one for the sample annealed at 700 °C. The annealing at higher temperatures
introduces more Mn3+ concentration due to the oxidation-state
change Mn2+ → Mn3+ . These Mn ions are favorable to locate
at the Ti site of tetragonal BaTiO3 , as confirmed by the XAFS
study. While pure BaTiO3 samples are diamagnetic, the Mndoped samples exhibit room-temperature FM order. However,
annealing a high temperature of 900 °C reduces FM order
because of the additional presence of Mn3+ ions. Such the
results prove that FM order in Mn-doped BaTiO3 annealed
in an Ar ambient is associated with exchange interactions of
Mn2+ ions mediated by VO defects. With the obtained results,
we believe that the coexistence of VO and Mn2+ , and/or Mn3+
and Mn4+ at suitable ratio is expected to achieve high FM
order in BaTi1−x Mn x O3 ceramic samples.
ACKNOWLEDGMENT
This research was supported by the Converging Research
Center Program through the Ministry of Science, ICT and
Future Planning, Korea (2013K000405).
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