University of Groningen
Dielectric properties of (Bi0.9La0.1)(2)NiMnO6 thin films
Langenberg, E.; Fina, I.; Ventura, J.; Noheda, Beatriz; Varela, M.; Fontcuberta, J.
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Physical Review. B: Condensed Matter and Materials Physics
DOI:
10.1103/PhysRevB.86.085108
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Langenberg, E., Fina, I., Ventura, J., Noheda, B., Varela, M., & Fontcuberta, J. (2012). Dielectric properties
of (Bi0.9La0.1)(2)NiMnO6 thin films: Determining the intrinsic electric and magnetoelectric response.
Physical Review. B: Condensed Matter and Materials Physics
,
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(8), 085108-1-085108-7. [085108].
https://doi.org/10.1103/PhysRevB.86.085108
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PHYSICAL REVIEW B 86, 085108 (2012)
Dielectric properties of (Bi
0.9
La
0.1
)
2
NiMnO
6
thin films: Determining the intrinsic electric and
magnetoelectric response
E. Langenberg,
1,3,*
I. Fina,
2
J. Ventura,
1
B. Noheda,
4
M. Varela,
1
and J. Fontcuberta
2
1
Departament de F
´
ısica Aplicada i
`
Optica, Universitat de Barcelona, Mart
´
ı i Franqu
`
es 1, 08028 Barcelona, Spain
2
Institut de Ci
`
encia de Materials de Barcelona, CSIC, Campus de la UAB, 08193 Bellaterra, Spain
3
Instituto de Nanociencia de Arag
´
on, Universidad de Zaragoza, Mariano Esquillor, 50018 Zaragoza, Spain
4
Zernike Institute for Advanced Materials, University of Groningen, Groningen 9747AG, the Netherlands
(Received 23 May 2012; published 8 August 2012)
We have used temperature-dependent impedance spectroscopy to study the dielectric response of thin films
of ferromagnetic and ferroelectric (Bi
0.9
La
0.1
)
2
NiMnO
6
oxide. This technique has allowed us to disentangle its
intrinsic dielectric response and extract the dielectric permittivity of ∼220, which is in close agreement with bulk
values but significantly smaller than early reported values for similar thin films. The permittivity is found to be
temperature independent in the vicinity of the ferromagnetic transition temperature and independent of magnetic
field. We have shown that the measured magnetocapacitance arises from the magnetoresistance of the films, thus
indicating a negligible magnetoelectric coupling in these double perovskites, probably due to the different energy
scales and mechanisms of ferroelectric and magnetic order.
DOI: 10.1103/PhysRevB.86.085108 PACS number(s): 75.85.+t, 77.22.−d, 73.40.−c, 73.61.−r
I. INTRODUCTION
The possibility of controlling the magnetization or polariza-
tion by an electric or magnetic field, respectively, in materials
displaying ferroelectric and magnetic order in the same phase,
so-called multiferroic materials, has triggered a great amount
of research.
1
The coupling between these two ferroic orders
not only is interesting in terms of fundamental research but also
could have great technological impact in some applications,
i.e., spintronics. For this latter purpose, ferromagnetic multi-
ferroics would have greater advantages over antiferromagnetic
multiferroics, because the net magnetization could allow easier
control of the magnetic state and subsequently its polar state
in the presence of large magnetoelectric coupling. However,
the antiferromagnetic order prevails in multiferroic perovskite
oxides; thus, research should focus on identification of new
ferromagnetic ferroelectrics.
The scarcity of ferromagnetic multiferroics might be over-
come by designing oxides with double-perovskite structure,
A
2
BB
O
6
, because the combination of cations with different
electronic configurations at the B-site enables engineering
superexchange ferromagnetic paths by appropriately choosing
the B–B
cations.
2
In the case of Bi
2
NiMnO
6
(BNMO) and
La
2
NiMnO
6
(LNMO), both compounds are ferromagnetic
below T
FM
≈ 140 and 280 K, respectively,
3,4
but only the
former is ferroelectric (T
FE
≈ 485 K).
3
The magnetoelectric
coupling might be favored in the solid solution BNMO–
LNMO due to the approach of the two transition temperatures
(T
FM
, T
FE
),
5
because La-doping in BNMO diminishes the
ferroelectric response while it increases the magnetic Curie
temperature.
6
It has been shown that in bulk form,
6
up to 20%
of La-doping, the structure of (Bi
1−x
La
x
)
2
NiMnO
6
remains
noncentrosymmetric monoclinic C2, for which ferroelectric
order would be permitted. Recently, we have shown that
(Bi
0.9
La
0.1
)
2
NiMnO
6
(BLNMO) thin films are ferroelectric,
having a T
FE
≈ 450
7
, and ferromagnetic below 100 K,
8
which
bears out the multiferroic character of the material.
Directly stating the coupling between the two ferroic prop-
erties entails measuring the effect of the electric field on the
magnetization or, equivalently, the effect of the magnetic field
on the electric polarization. Yet many multiferroic materials
tend to be poor insulators, preventing a sufficient electric
field from being applied
9,10
and thus hampering experimental
determination of the magnetoelectric coupling. An alternative
route to investigating the magnetoelectric character consists of
studying the effect on the dielectric permittivity ε of changes
of the magnetic state of the magnetic layer—either by applying
a magnetic field, the so-called magnetocapacitance, or by
searching for variations of ε in the temperature dependence
ε(T ) in the vicinity of the magnetic transition temperature.
11
However, determining the intrinsic dielectric permittivity,
and therefore the real dielectric response of multiferroics, can
be, by itself, an arduous task. First, it is well known that
extrinsic contributions, such as parasite capacitances formed
at the interface between the dielectric film and the electrodes
or at the grain boundaries in ceramic samples, often account
for the apparent colossal dielectric constants reported for many
dielectric materials.
12–17
Second, the leaky behavior of most
multiferroic materials may give rise to an apparent large
magnetodielectric response, when in reality it might not be
the permittivity but the resistivity of the dielectric material
that is changing either on applying a magnetic field or with
temperature.
9
Whereas the former contribution can in principle
be handled by appropriate experimental methods, the role of
the latter, which can be of the highest relevance in double-
perovskite thin films due to the multivalent configuration of
B-cations, is still to be addressed.
These difficulties become apparent when comparing di-
electric data reported for double-perovskite thin films. For
instance, it has been reported that LNMO films show tem-
perature (T ) dependence and frequency (ν) dependence of
the dielectric permittivity ε(T,ν),
18
which was attributed
to temperature-dependent electric dipole relaxation. Similar
ε(T,ν) features have been reported for BNMO films,
19
where
a modest value (ε ≈ 50) was reported at low temperature and
an unexpected sharp increase takes place at 150 K (ε ≈ 450).
In contrast, in bulk BNMO, dielectric permittivity was found
085108-1
1098-0121/2012/86(8)/085108(7) ©2012 American Physical Society
E. LANGENBERG et al. PHYSICAL REVIEW B 86, 085108 (2012)
to be roughly constant (ε ≈ 200) for temperatures well below
T
FE
.
3
Here, we report on a quantitative study of the apparent
temperature-dependent dielectric relaxation of BLNMO thin
films, which we prove is driven by the temperature depen-
dence of the resistivity instead of the dielectric relaxation
of electric dipoles. Impedance spectroscopy methods
10,12,20
have been used to disentangle extrinsic contributions to
the measured permittivity and to extract the temperature
dependence of the intrinsic dielectric and resistive properties of
(Bi
0.9
La
0.1
)
2
NiMnO
6
films. Moreover, we show that the mag-
netocapacitance of (Bi
0.9
La
0.1
)
2
NiMnO
6
films is likely due to
extrinsic effects, suggesting a weak intrinsic magnetoelectric
coupling in (Bi,La)
2
NiMnO
6
compounds.
II. EXPERIMENTAL DETAILS
We grew 100-nm-thick BLNMO thin films by pulsed
laser deposition on conducting 0.5% Nb-doped SrTiO
3
(001)
(Nb:STO) substrates. The growth conditions to obtain single-
phase samples are reported elsewhere.
21
Sputtered Pt top
electrodes were deposited through a shadow mask. Dielectric
measurements were performed in a probe station, using a top-
to-top electrode configuration
22
in the 10–300 K temperature
range. An impedance analyzer was used with an applied
alternating current (ac) of 50 mV of amplitude operating be-
tween 40 Hz and 1 MHz. Magnetocapacitance measurements,
varying the magnetic field up to 9 T, were performed in a
physical properties measurement system.
III. RESULTS AND DISCUSSION
A. Complex dielectric constant and ac conductivity:
Qualitative analysis
Fig. 1 depicts the temperature dependence of the dielectric
permittivity at different frequencies, assuming that the mea-
sured capacitance C is only due to the dielectric response of
the BLNMO film: C = ε
ε
0
A/(2d),
22
where A and d are the
area of the electrode and the thickness of the film, respectively,
FIG. 1. (Color online) Temperature dependence of the dielectric
permittivity of Pt/BLNMO/Nb:STO capacitors measured at different
frequencies.
FIG. 2. (Color online) Frequency dependence of (a) the effective
capacitance and (b) the loss tangent of Pt/BLNMO/Nb:STO capaci-
tors measured at different temperatures. The dashed line signals the
frequency at which magnetocapacitance measurements are done.
and ε
0
and ε
are the vacuum permittivity and the real part of
the complex dielectric constant, respectively. The dielectric
permittivity ε
increases as temperature rises, with a visible
steplike behavior. The steplike feature moves toward higher
temperatures on increasing frequency; a similar observation
was reported for LNMO thin films.
18
The reported dielectric
permittivity of BNMO thin films measured at 100 kHz and at
1 MHz of Refs. 19 and 23 shows a clear resemblance to data in
Fig. 1 for 100 kHz and 1 MHz, respectively. Nonetheless,
ε
at high temperatures is clearly frequency dependent,
and it strongly reduces when frequency is increased. This
suggests that extrinsic effects are contributing to the dielectric
response,
12,13,15–17
thus claiming a new regard for data and
related conclusions.
To assess the frequency response, the complex representa-
tion of the dielectric constant ε
∗
= ε
− iε
is used. Fig. 2
depicts the frequency dependence of the effective capaci-
tance C = ε
ε
0
A/(2d) and the loss tangent tan δ = ε
/ε
of
Pt/BLNMO/Nb:STO capacitors at different temperatures. The
measured capacitance [Fig. 2(a)] shows the existence of two
frequency regimes, where C(ν)—and thus the permittivity
ε
(ν)—is rather constant, separated by a steplike region
accompanied by a peak of tan δ and thus the imaginary part
085108-2
DIELECTRIC PROPERTIES OF (Bi
0.9
La
0.1
) ... PHYSICAL REVIEW B 86, 085108 (2012)
FIG. 3. (Color online) Frequency dependence of σ
at different
temperatures. The dashed line indicates the ideal behavior, σ
=
σ
dc
+ σ
0
ω
α
(see text). The inset shows the temperature dependence
of the resistivity obtained by extrapolation to zero frequency of σ
from the main panel (solid squares) and by impedance spectroscopy
(open squares). Solid lines are visual guides.
of the dielectric constant ε
[Fig. 2(b)]. Both the steplike
region in C
(ν) and the peak of tan δ (ν) shift toward higher
frequencies on increasing temperature. At first sight, this
behavior could be attributed to thermally activated Debye-like
dielectric relaxation dominating the frequency dependence
of ε
(ν). The low-frequency dielectric permittivity, obtained
assuming that the dielectric response is due to a unique
capacitor ε
= C · (2d/ε
0
A), is anomalously high (ε
≈ 1000),
as shown in Fig. 1, and strongly dependent on temperature. In
contrast, at higher frequency, the capacitance is about one order
of magnitude smaller and tends to be weakly dependent on
frequency, thus signaling a more plausible intrinsic character.
The corresponding permittivity is found to be in the range of
that reported for BNMO bulk materials.
3
In the high-frequency range, we can also observe a small but
perceptible ε
(ν) dependence [Fig. 2(a)], which is at odds with
the response of an ideal dielectric, but it is commonly observed
in most dielectrics and attributed to the frequency-dependent
ac conductivity σ
ac
. This can be better seen in Fig. 3, where
we plot the frequency dependence of the real part (σ
)ofthe
complex conductivity (σ
∗
= iωε
0
ε
∗
, where ω stands for the
angular frequency 2πν) at various temperatures (80, 110, and
140 K). σ
should be the sum of the frequency-independent
direct current (dc) conductivity σ
dc
, which is always present
due to the leakage of the dielectric, and a frequency-dependent
term σ
ac
, which typically shows a power-law dependence on
frequency (universal dielectric response contribution): σ
ac
=
σ
0
ω
α
, α 1.
13,24
The σ
(ν) log–log data in Fig. 3 show that
at high frequency, there is a powerlike σ
ac
(ν) contribution
superimposed to σ
dc
term responsible for the flattening of
σ
(ν) at intermediate frequencies. The extrapolation of σ
(ν)
from the plateau toward zero frequency (dashed lines in Fig. 3)
allows us to estimate the σ
dc
of the material or, equivalently,
the resistivity at any temperature. Some illustrative values
are shown in Fig. 3 (inset; solid squares), where a rough
exponential increase of resistance when lowering temperature
can be appreciated. When further lowering ν, conductivity is
steeply reduced, deviating from the ideal behavior marked in
dashed lines. As shown by the data in Fig. 3,thisdropisalso
temperature dependent and shifts toward higher frequencies
as temperature rises. Not surprisingly, it coincides with large
enhancement of the dielectric constant shown in Fig. 2(a) and
the peak of ε
[Fig. 2(b)].
Next we show that both the low-frequency region and the
steplike region are not intrinsic properties of the BLNMO film
but rather result from the contribution of interface effects. To
obtain more quantitative insight into the dielectric response of
the sample, to investigate the number of electrical responses
present, and to determine the intrinsic properties of the
film, complex impedance (Z
∗
= Z
+ iZ
) spectroscopy was
performed as described in the following.
B. Impedance spectroscopy: Quantitative analysis
The impedance of the dielectric film can be represented by
two circuit elements connected in parallel: one resistive, R,
accounting for the leakage of the material, and one capacitive,
C, accounting for the dielectric character. Moreover, to account
for the nonideal dielectric response attributed to the frequency-
dependent ac conductivity (Fig. 3), C is commonly replaced
by a constant phase element (CPE).
10,25–27
The impedance of
this R-CPE circuit is given by
Z
∗
R−CPE
=
R
1 + RQ(iω)
α
(1)
where Q and α (α 1, being α = 1 for an ideal ca-
pacitor) denote the amplitude and the phase of the CPE,
respectively.
10,25–28
Capacitance values, C, can be obtained
according to the relationship C = (Q · R)
(1/α)
/R.
28
A useful representation of impedance data consists of
plotting the negative imaginary term of the complex impedance
−Z
versus the real term Z
. In this complex impedance
plane, the impedance of a dielectric film should depict a
semicircle of radius R/2 with a maximum at a frequency
in which the condition ω
max
= 1/RC is fulfilled, C being
the capacitance of an ideal capacitor.
10,12,20,25
In R-CPE
elements, the semicircle is slightly depressed, depending on
how α deviates from the unity.
10
In Fig. 4(a), we show the
illustrative −Z
–Z
plot of the impedance measured at 110
K. Data signal the existence of two incomplete semicircles
(marked with a dashed line as a visual guide) at high and low
frequency, respectively. The existence of these circles indicates
that a unique R-CPE element is not sufficient to describe the
impedance data, confirming that the dielectric response shown
in Sec. III A is not due to the intrinsic dielectric properties
of BLNMO thin films alone. Hence, another element should
be considered when accounting for the extrinsic contribution,
i.e., the resistance of connecting circuit and the interface
capacitance, as indicated in the circuit model shown in
the inset of Fig. 4(a). Consequently, the impedance of the
Pt/BLNMO/Nb:STO system (film + extrinsic contribution)
is given by
Z
∗
(R − CPE,R
ext
− C
ext
) =
R
1 + R · Q · (iω)
α
+
R
ext
1 + iω · R
ext
· C
ext
(2)
where the first and the second terms correspond to the film and
the extrinsic contributions, respectively.
085108-3
E. LANGENBERG et al. PHYSICAL REVIEW B 86, 085108 (2012)
FIG. 4. (Color online) (a) Impedance complex plane (−Z
–Z
plots) data at 110 K. The visual guide of the dashed line indicates the
two semicircles at high and low frequency. The inset sketch shows the equivalent circuit, describing the different electrical responses present
in Pt/BLNMO/Nb:STO capacitors (see text for symbols). (b) −Z
–Z
data plots at different temperatures. (c) Experimental data (open
symbols) and fitting (solid line) using the model of Eq. (2) of some illustrative temperatures. The inset zooms in on the high-frequency region.
(d) Temperature dependence of the CPE exponent.
It can be appreciated in Fig. 4(a) that the radius of the
low-frequency semicircle is much larger than that of the
high-frequency one; this reflects that the low-frequency contri-
bution is more resistive than the high-frequency contribution.
Inspection of the impedance data at different temperatures,
showninFig.4(b), indicates that the high-frequency semicircle
significantly decreases on increasing temperature, evidencing
that the resistivity of the high-frequency contribution decreases
as temperature rises, according to data of the conductivity
shown in Fig. 3.
Equation (2) was used to fit impedance data to validate
the proposed model. In Fig. 4(c), we show some illustrative
results of the fits to −Z
–Z
data at different temperatures.
As shown, model (solid lines) and data match reasonably,
which is extended for the rest of the temperatures (not
shown) up to 170 K, corroborated by the low goodness-of-fit
indicator χ
2
which, in all temperature ranges, is ∼10
−3
.
Above this temperature, the low-frequency contribution nearly
dominates the experimentally available frequency range (up to
1 MHz), and accurate extraction of the intrinsic properties
(high-frequency contribution) was unfeasible.
Thus, the dielectric behavior shown in Sec. III A can be
explained by the formation of a capacitive layer at the interface
and by the temperature dependence of the resistivity of the core
of the film. The band bending caused by the difference between
the work function of the metal and the electronic affinity of the
dielectric gives rise to a charge depletion/accumulation region
at the interface,
9,10,13,15
which forms a relatively thin layer,
behaving as a highly resistive barrier [modeled as a capacitor
C
ext
and a low conductance R
−1
ext
connected in parallel, as
illustrated by the circuit model sketched in Fig. 4(a)]. At
high frequency, charge carriers have no time to follow the
alternating electric field, and the measured capacitance is the
film and the interface capacitances in series. However, at low
frequencies, charge carriers do respond to the electric field in
the low resistive part, i.e., the core of the film [modeled as R in
the inset of Fig. 4(a)], forming an electric current. This entails
that the drop of the electric field mostly takes place at the
interface barrier, yielding the apparent high dielectric constant
shown in Fig. 2(a) because of the apparent reduction of the
dielectric thickness (measured capacitance is proportional to
1/d).
9
On increasing temperature, the film resistivity decreases
085108-4
