Search for ferromagnetism in manganese-stabilized zirconia

Article abstract of DOI:10.1002/pssa.201026304

Magnetic properties of Mn-stabilized cubic zirconia (ZrO₂) powder samples were investigated to verify the recent theoretical predictions of ferromagnetism in transition-metal-doped ZrO₂. It was found that 5% Mn-doped cubic ZrO_2<

Full text of DOI:10.1002/pssa.201026304

Phys. Status Solidi A 208, No. 1, 172–179 (2011) / DOI 10.1002/pssa.201026304
a
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applications and materials science
Search for ferromagnetism in
manganese-stabilized zirconia
M. Chandra Dimri*, H. Kooskora, J. Pahapill, E. Joon, I. Heinmaa, J. Subbi, and R. Stern
National Institute of Chemical Physics and Biophysics, Tallinn-12618, Estonia
Received 15 June 2010, revised 14 September 2010, accepted 19 September 2010
Published online 18 October 2010
Keywords antiferromagnetism, ferromagnetism, Mn doping, oxygen vacancies, spintronics, X-ray diffraction, zirconia
* Corresponding author: e-mail mukeshdimri@yahoo.com, Phone: þ372 6398370, Fax: þ372 670 3662
Magnetic properties of Mn-stabilized cubic zirconia (ZrO2) Antiferromagnetic interactions were observed to be enhanced
powder samples were investigated to verify the recent for higher Mn doping. Even though we could not produce the
theoretical predictions of ferromagnetism in transition-metal- room-temperature long-range ferromagnetism in our samples,
doped ZrO2. It was found that 5% Mn-doped cubic ZrO2 we discuss the observed effects of annealing on structural and
samples, when heat treated in argon and air environments, magnetic properties and their sensitivity to moisture.
exhibit tiny hysteresis loops at room temperature (300 K).
ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1 Introduction Dilute magnetic semiconductors
There are very few magnetic studies on ZrO2, though
(DMSs) and dilute magnetic dielectrics (DMDs) have room-temperature ferromagnetism has been observed and
attracted considerable interest in recent years due to their reported in transition-metal-doped TiO2, SnO2, and CeO2 [6,
potential spintronics applications [1, 2]. A key factor for 11, 12]. Mn-stabilized zirconia samples has been prepared
application of these materials in spintronic devices is to have and carefully studied by researchers in the past for fuel-cell
their Curie temperature much above room temperature. With applications, catalysts, sensors, etc., but those studies
this goal a remarkable amount of research has been done to concentrated on improvement of conductivity and ionic
understand the nature of magnetism in transition-metal- properties and neglected magnetic properties. There was a
doped oxide systems, such as ZnO, TiO2, HfO2, CeO2, SnO2, magnetic study on Mn-stabilized zirconia [13] and recently
and In2O3 [3–8]. However, most of these experimental two experimental studies on magnetic measurements of Zr1–
studies have not managed to make firm claims about the real xMnxO2 and Zr1–xFexO2 nanoparticles by Yu et al. [14] and
origin of ferromagnetism and often are lacking in Clavel et al. [15] revealing no signs of ferromagnetism in
reproducibility.
these samples. We have prepared Mn-stabilized zirconia
While looking for more reliable solutions, in the recent bulk powder samples by a chemical solution technique and
past there were a few theoretical predictions for room- studied their magnetic properties for the experimental
temperature ferromagnetism with high Curie temperature in verification of Ostanin et al. [9]. For understanding the role
Mn-stabilized cubic zirconia [9, 10]. One of these groups, of oxygen vacancies, as-prepared powders was heat treated
Ostanin et al. [9], predicted on basis of ab initio calculations in various gaseous environments. In the present paper, we
that cubic Zr1–xMnxO2 can be ferromagnetic for Mn report the structural and magnetic properties of Mn-
concentration higher than 5% (Tc > 500 K for x ¼ 0.25), stabilized zirconia with different compositions.
with applicable spintronic properties. They suggested that
the Curie temperature will increase with Mn concentration 2 Experimental procedure Zr1–xMnxO2 (x ¼ 0,
and decrease with an increase in oxygen vacancies. ZrO2 is a 0.05, 0.10, 0.20, and 0.30) powders were prepared by a
well-known high dielectric insulator oxide compatible with chemical citrate route. Zirconyl oxychloride
semiconductor (Si and Si–Ge) technology, and the existence (ZrOCl2 ꢀ 8H2O) and MnO2 were used as starting materials.
of magnetism in this material would create a new direction A solution was prepared in deionized water for the zirconium
for potential spintronic devices.
salt, whereas MnO2 was dissolved in nitric acid and
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Phys. Status Solidi A 208, No. 1 (2011)
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peroxide. Citric acid was added in 1:1 proportion for every
cation. The resulting solution after mixing was first heated at
60 8C for 2 h and then at 80 8C till evaporation of solvents.
Such prepared powders, were calcined at 500, 650, or 800 8C
for 5 h in air. To see the effects of environments other than
air, the as-prepared powder samples were also calcined in
argon, hydrogen and oxygen environments at 500 8C/5 h. Nb
doping (Zr0.8Mn0.1Nb0.1O2) was additionally done in one
sample by heating it at 1500 8C/3 h. Thermogravimetric
analysis (TGA) was performed in air or H2 environments to
investigate the various cationic oxidation states. A powder
X-ray diffraction technique was used for structural identi-
fication, and a vibrating sample magnetometer (PPMS by
Quantum Design) was used for magnetic measurements in a
wide temperature range (10–350 K). Scanning electron
micrographs (SEM) for microstructure and EDS measure-
ments for stoichiometry were also done for some of the
representative samples.
(a)
3 Results and discussion Figures 1a and b show
XRD patterns for powders calcined at 500 and 650 8C/5 h,
whereas Fig. 2A shows X-ray diffractogram for powders
calcined at 800 8C in air for 5 h. No extra peaks for metallic
Mn or oxides of manganese (MnxOy), were detected in the
X-ray diffractograms, upto 20% of Mn substitution in
zirconia. The solubility limit of Mn in ZrO2 was found to be
14 wt.% [16] and 27 at.% by a coprecipitation method [14],
whereas in our case Mn2O3 phase segregates for samples
with 30 at.% Mn in ZrO2. Powders calcined at 500 and
650 8C exhibit cubic structure, whereas powders calcined at
800 and 1500 8C reflect the mixture of phases including
monoclinic as well as tetragonal phases of ZrO2, with Mn3O4
as an impurity phase. The existence of manganese oxide
phase is verified also in magnetic measurements where the
transition peak around 43 K for Mn3O4 was observed in
the magnetization results (not included in present paper).
Pure ZrO2 has monoclinic structure at room temperature, but
Mn substitution stabilizes the cubic phase, as can be seen
from the X-ray diffractograms.
(b)
Figure 2B shows the X-ray diffraction pattern for the 5%
Mn-stabilized zirconia annealed at 500 8C/5 h in air, argon,
H2, and O2 atmospheres. Powders heated in an argon
environment show poor crystallinity, which can be seen from
the broad peaks of cubic zirconia due to the nanoparticle
nature, whereas powders heated at 500 8C/5 h in air,
hydrogen, and oxygen environments reflect the intense
peaks of cubic zirconia phase. Increase in annealing
temperature on the one hand results in better crystallinity,
but on the other hand introduces mixture of other phases of
Figure 1 (online colour at: www.pss-a.com) XRD patterns for
Zr1–xMnxO2 powder samples, (a) calcined at 500 8C/5 h (i) 5%,
(ii) 10%, (iii) 20%, and (iv) 30% Mn; (b) 650 8C in air/5 h
[(1) 0% Mn (2) 5% Mn].
zirconia along with the desired cubic one. The powders were in lattice parameter, which is understandable due to the lower
3þ
heat treated at lower temperatures to stabilize single-phase ionic radius of Mn2þ (0.81 A) and Mn
˚
cubic zirconia, because at higher temperature it transforms in compared to Zr4þ (0.86 A).
˚
˚
(0.79 A) as
monoclinic and tetragonal phases. Table 1 summarizes the
Figure 3 shows diffractograms for the aged and
different phases formed with varying the concentration as dehydrated samples of Zr0.95Mn0.05O2 powders annealed in
well as the calcination temperatures. The lattice parameters air and argon. It can be seen that the aged and dehydrated
a for the cubic phase samples are also given in Table 1. samples still retain the same cubic crystal structure. The
Increasing the Mn concentration in ZrO2 results in decrease broad peaks for the sample treated in argon were due to the
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M. C. Dimri et al.: Search for ferromagnetism in manganese-stabilized zirconia
(A)
(B)
Figure 2 (online colour at: www.pss-a.com) (A) XRD patterns for Zr1–xMnxO2 powder samples calcined at 800 8C/5 h in air: (i) 5%, (ii)
10%, (iii)20%, and(iv)30%Mn;(B)diffractogramsforZr0.95Mn0.05O2 samplesannealedat500 8C/5 hindifferentenvironments:(i)argon,
(ii) air, (iii) H2, and (iv) O2.
Table 1 Different phases of zirconia (monoclinic – m, tetragonal – t, and cubic – c) identified from XRD on variation in calcination
temperatures and compositions of Zr1–xMnxO2 samples heated in air.
concentration (x in %)
500 8C/5 h
650 8C/5 h
800 8C/5 h
1500 8C/3 h
˚
lattice parameter a (A)
0
5
10
20
30
m þ c þ t
–
m
–
–
–
˚
c
5.090
5.086
5.080
5.028
c (a ¼ 5.10 A)
c þ m
c
c
c þ m
c þ m
–
m þ Mn3O4
–
c þ Mn3O4
c þm þ Mn3O4
c þ m þ Mn3O4
c þ Mn2O3
–
nanosized particles, which is also confirmed by the SEM compared to heating in air. In H2 environments this loss may
micrograph shown in Fig. 5b. be related to creation of more oxygen vacancies than in air.
Figure 4 shows EDS spectra for Zr0.95Mn0.05O2 and The other flat plateau shows conversion of Mn oxides in
Zr0.8Mn0.2O2 samples heated at 500 8C/5 h in air. From these different oxidation states. Mn stabilizes in consecutively
spectra we can see that desired stoichiometry is well lower oxidation states on increase in temperature. From the
maintained in the prepared powders. The SEM micrographs TGA curves, it seems that Mn would be in different valence
for these two samples are shown in Fig. 5. The typical size of states when annealed in different atmospheres. The oxi-
the polycrystalline grains was found to be in the micrometer dation state of Mn is lower in samples annealed in hydrogen
range, whereas only argon-treated samples have particles in at 500 8C, which also affects the magnetic properties.
the nanometer range.
We have studied the magnetic properties for all the
The TG pattern for the Zr1–xMnxO2 powder samples prepared samples, but here we will discuss only those of the
measured in air and H2 environment is shown in Fig. 6. The pure cubic phase samples (i.e., samples heated at a low
first observable weight loss occurred below 200 8C due to temperature of 500 8C/5 h). Hysteresis loops measured at
removal of water. For higher temperatures the more 300 K for 5% Mn-stabilized zirconia samples calcined in
substantial weight loss was found in a H2 environment as different environments (air, H2, and argon) are shown in
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Phys. Status Solidi A 208, No. 1 (2011)
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the subtracted hysteresis curves (inset). The enhanced
ferromagnetism in the Ar-treated sample may be related to
the nanoparticle nature of the powder, as nanopowders have
more surface defects due to the much greater surface to
volume ratio than a bulk sample. We are the first to observe
the room-temperature hysteresis in Mn-stabilized ZrO2
samples, though it shows rather weak ferromagnetism.
Magnetic parameters from the hysteresis loops measured at
300 K (Fig. 7a) for Mn-stabilized zirconia samples are given
in Table 2.
We have also measured the magnetic hysteresis loops for
5% Mn-doped ZrO2 samples, several months after prep-
aration, to see the effects of aging and moisture on magnetic
properties. Figure 7b shows the hysteresis loops measured at
room temperature for the Zr0.95Mn0.05O2 samples (6 months
old) heat treated in air (500 and 800 8C/5 h) as well as in
argon environment (500 8C/5 h). Magnetic properties dimin-
ish, and samples heated at 500 8C are more affected due to
moisture and aging. Tetragonal ZrO2 treated at lower
temperature were also found to be hygroscopic and water
Figure 3 (online colour at: www.pss-a.com) Diffractograms for
Zr0.95Mn0.05O2samplesannealedinairandargonat500 8Cfor5 h.(i) molecules fill the oxygen vacancies [17]. So, low-tempera-
Agedsampleair,(ii)dehydratedsampleair,(iii)agedsampleAr,and
(iv) dehydrated sample Ar.
ture treatment may be one of the reasons for the hygroscopic
nature and finally filling the oxygen vacancies and affecting
the magnetic properties. Figure 8 shows the measured
hysteresis curves for the fresh, aged and dehydrated samples
Fig. 7a, whereas the inset picture shows the curves after of Zr0.95Mn0.05O2 heated in argon and air at 500 8C/5 h. It can
subtracting the paramagnetic signals from the measured M– be seen that the magnetic properties improve, but do not
H curves. It can be seen from the graph that the sample retain the optimal values after dehydration. The values are
calcined in hydrogen environment exhibits almost perfect more influenced in the argon-treated sample due to the higher
diamagnetism (curve ‘‘5’’ in Fig. 7a), after removing the hygroscopic nature of nanopowders, whereas there is not
paramagnetic signal from the measured curve. None of the much change in coercivities for samples treated in air
measured magnetization curves saturates due to this environments, despite some increase in the paramagnetic
paramagnetic contributions in addition to the ferromagnetic signal.
signal. The magnetization is maximal for the sample
M–H curves measured at 300 K for powders calcined in
calcined in an argon environment, as is clearly seen from air at 500 8C/5 h, with different Mn concentrations are shown
(a)
(b)
1
Zr
Spectrum 57
Zr
O
O
Mn
Mn
Mn
Zr
Mn
Zr
Zr
Cl
Zr
Cl
Cl
Mn
Cl
Na
1
Cl
Zr
Zr
Mn
Cl
Mn
6.5
Mn
6.5
0.5
1.5
2
2.5
3
3.5
4
4.5
5
5.5
6
7
7.5
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
5.5
6
7
Full Scale 2748 cts Cursor: 2.774 (91 cts)
keV
Full Scale 1743 cts Cursor: 3.668 (29 cts)
keV
Figure 4 (online colour at: www.pss-a.com) EDS spectra for Zr0.95Mn0.05O2 (a) and Zr0.8Mn0.2O2 (b) samples heated at 500 8C/5 h in air.
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M. C. Dimri et al.: Search for ferromagnetism in manganese-stabilized zirconia
Figure 5 SEM micrographs for Zr0.95Mn0.05O2 samples annealed for 500 8C/5 h in air (a) and in argon (b).
in Fig. 9a. For the lower percentage of Mn (5 and 10%, shown
(a)
in the inset), the curves are ‘‘S’’ shaped, but for the higher Mn
concentration (20 and 30% Mn) these curves have a higher
paramagnetic signal (straight line). From Fig. 9a it can be
seen that there is an increase in magnetization on increased
Mn content, though it does not help in stabilization of
ferromagnetism, because above a certain concentration limit
antiferromagnetic interactions dominate. Figure 9b shows
the susceptibility dependence on temperature for different
compositions of Mn-stabilized ZrO2 samples heated at
500 8C/5 h in air. Extrapolated linear fits on the inverse
susceptibility curves reflect the antiferromagnetic character
with negative Curie–Weiss temperatures for higher Mn
concentration, whereas it is positive for 5% Mn-doped ZrO2
samples treated in air and argon (values are given in Table 3).
(b)
Figure 7 (online colour at: www.pss-a.com) Hysteresis loops at
300 K for the Zr0.95Mn0.05O2 samples heated at 500 8C/5 h in air,
argon, and H2 environments (a) as measured, and in inset: after
Figure 6 (online colour at: www.pss-a.com) TG curves for the subtracting the paramagnetic straight line, (b) subtracted hysteresis
Zr0.8Mn0.2O2 sample heated at 500 8C/5 h in air and H2 environ- curves for Zr0.95Mn0.05O2 samples (measured 6 months after prep-
ments.
aration).
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Phys. Status Solidi A 208, No. 1 (2011)
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Table 2 Coercivities and saturation magnetization from M–H curves for Mn-stabilized zirconia samples, heat treated at different
temperatures in air and argon.
air
argon, 5% Mn,
500 8C/5 h
5% Mn, 500 8C/5 h
5% Mn, 650 8C/5 h
5% Mn, 800 8C/5 h
10% Mn, 500 8C/5 h
coercivity (Oe)
sat. magn. (emu/gm)
100
0.010
120
0.007
120
0.005
110
0.014
60
0.038
(a)
(b)
Figure 8 (onlinecolourat:www.pss-a.com)HysteresisloopsforagedanddehydratedsamplesofZr0.95Mn0.05O2samplesheatedat500 8C/
5 h in argon (a) and air (b).
The temperature dependence of susceptibility for the
Most of the reports on transition-metal-doped oxides
prepared samples was also studied and it was observed that (TiO2, CeO2, or HfO2), relate the origin of magnetism to the
all curves show typical Curie-type behavior. We present here existence of oxygen vacancies, but only for certain
results for a representative sample having 20% Mn-doped concentrations above the critical threshold. In our case
ZrO2. The temperature dependence of susceptibility and samples heat treated in H2 environments, are expected to
inverse susceptibility of Zr0.8Mn0.2O2 samples are shown in have more oxygen vacancies and should demonstrate good
Figs. 10a and b, heated in air and H2 environments at 500 8C. ferromagnetic properties. Surprisingly, we observe only
1/x versus T graphs are almost linear in the high-temperature enhanced paramagnetic behavior on the M–H curve (Fig. 7a)
region and follow the well-known Curie–Weiss behavior. as compared to samples heat treated in air and argon. This
Curie–Weiss temperatures, derived from the extrapolated contradiction may be because this sample exceeds the
linear fits to susceptibility curves are given in Table 3. The concentration limit of oxygen vacancies required for
negative values of the Curie–Weiss temperature suggest ferromagnetism ordering as suggested by Coey et al. [18].
the antiferromagnetic character of interactions in these The improved ferromagnetism in the Ar-treated sample may
samples.
be due to the more surface defects arising from the nanosize
Table 3 Curie temperatures obtained from extrapolated linear fits of inverse susceptibility curves for Mn-stabilized zirconia samples,
heated in different ambients.
environment
air (8C)
argon
500 8C/5 h
hydrogen
500 8C/5 h
oxygen
500 8C/5 h
Nb substituted in
air (1500 8C/3 h)
500/5 h
650/5 h
800/5 h
1500/3 h
Q/K
20% Mn
5% Mn
–75
20
–75
–
–250
–
–300
–
–25
25
–30
–
–20
–25
0
–
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M. C. Dimri et al.: Search for ferromagnetism in manganese-stabilized zirconia
of the particles, as compared to the other samples treated in
air that have particles in the micrometer range. Also, the
ability of zirconia to exist in different polymorphs compli-
cates the control over the oxygen vacancies and magnetic
properties. Yu et al. [14] have explained the absence of
ferromagnetism due to an excess of oxygen vacancies in their
studies on Zr1–xMnxO2 samples, whereas Clavel et al. [15]
tried doping below 5% of Mn in zirconia, and related the
paramagnetic behavior to low Mn concentration and lower
oxidation states of Mn (Mnþ2 or Mnþ3), and suggested that
the higher oxidation state (Mnþ4) would be more likely to
give the ferromagnetic interactions. We tried to compensate
the oxygen vacancies by substituting pentavalent niobium
ions, but it (Nb5þ) has problems in going into zirconia lattice
sites at lower temperatures. So, the sample was treated at
1500 8C/3 h for better solubility of Nb in ZrO2, but this
heat treatment results in transformation to monoclinic (m)
phase of zirconia. Susceptibility measurements for the
Zr0.8Mn0.1Nb0.1O2 sample heat treated at 1500 8C in air also
shows paramagnetic behavior, and does not solve our
problem of stabilizing long-range ferromagnetism upto
room temperature in Mn-stabilized zirconia.
(a)
0.4
0.2
0.0
-0.2
-0.4
(b)
4 Conclusions Mn-stabilized cubic zirconia samples
were prepared at low temperatures by a chemical method and
their structural and magnetic properties were investigated.
Heating in air above 650 8C results in transformation to
monoclinic zirconia and MnxOy, in addition to cubic phase.
To see the effects of oxygen vacancies, samples were heat
treated in various gas atmospheres. There were no signs of
room temperature long-range ferromagnetism in any of the
samples from the magnetization and susceptibility measure-
ments. The 5% Mn-doped samples heat treated in argon and
Figure 9 (onlinecolourat:www.pss-a.com)(a)M–Hcurvesmeas-
ured at 300 K, for Zr1–xMnxO2 samples calcined at 500 8C/5 h in air
(subtracted curves are shown in the inset) (b) temperature depend-
ence of inverse susceptibility for Mn-stabilized zirconia samples
heated at 500 8C/5 h in air (susceptibility variation in inset).
(a)
(b)
Figure 10 (onlinecolourat:www.pss-a.com)SusceptibilitycurvesforZr0.8Mn0.2O2 samplesheattreatedat500 8C/5 hin(a)airand(b)H2
environments.
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Phys. Status Solidi A 208, No. 1 (2011)
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air show small coercivity as well as tiny hysteresis, even at
room temperature. Enhanced ferromagnetism in the Ar-
treated sample can be related to increased surface defects due
to the nanosize of the powder. It is difficult to control defects
and oxygen vacancies in polycrystalline powders precisely at
lower temperatures. In our view, it would be better to control
process parameters and vacancies in thin-film form where
one could stabilize some ferromagnetic character of the
zirconia samples. So, our experiments do not contradict fully
the theoretical prediction of ferromagnetism in Mn-stabil-
ized cubic zirconia bulk samples, but point to still existing
experimental challenges to achieve the high temperature
ferromagnetism in Mn-doped zirconia samples. Low-
temperature treatment of powders results in hygroscopic
nature in some of the samples, which diminishes the
magnetic parameters.
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Acknowledgements This work was financially supported
by the Estonian Science Foundation (grant numbers: JD53, MJD 65,
ETF 6852 and ETF 8440), and Estonian targeted financial grants
SF0690029s09 and SF0690034s09.
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