Magnetic and transport properties of charge ordered La₀.₅Ca₀.₅Mn₀.₃ and La₀.₄Ca₀.₆Mn₀.₃ films
The magnetic and the transport properties of La₀.₅Ca₀.₅Mn₀.₃ and La₀.₄Ca₀.₆Mn₀.₃ films with different thickness, prepared by rf-magnetron sputtering by using the so-called «soft» (or powder) target on LaAlO₃ substrate, have been investigated. The electron-diffraction and the high-resolution elect...
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irk-123456789-1201262017-06-12T03:03:14Z Magnetic and transport properties of charge ordered La₀.₅Ca₀.₅Mn₀.₃ and La₀.₄Ca₀.₆Mn₀.₃ films Prokhorov, V.G. Komashko, V.A. Kaminsky, G.G. Lee, Y.P. Park, S.Y. Hyun, Y.H. Svetchnikov, V.L. Kim, K.W. Rhee, J.Y. Низкотемпеpатуpный магнетизм The magnetic and the transport properties of La₀.₅Ca₀.₅Mn₀.₃ and La₀.₄Ca₀.₆Mn₀.₃ films with different thickness, prepared by rf-magnetron sputtering by using the so-called «soft» (or powder) target on LaAlO₃ substrate, have been investigated. The electron-diffraction and the high-resolution electron microscopy (HREM) studies show that the charge-ordered phase is observed at room temperature for all films. Both the paramagnetic-to-ferromagnetic transition at TC ≈ 250 K upon cooling and the appearance of an antiferromagnetic (AFM) phase at TN ≧ 140 K were observed in the La₀.₅Ca₀.₅Mn₀.₃ films, while the La₀.₄Ca₀.₆Mn₀.₃ films exhibited the AFM transition only at the same temperature, excepting the small ferromagnetic (FM) response from a «dead» layer. It was shown that the volume fraction of the FM phase in the La₀.₅Ca₀.₅Mn₀.₃ film did not exceed of and the FM phase coexisted with the AFM one at low temperature. All films manifest an exponential temperature dependence of resistance without evidence of the metal-insulator transition. This is explained by the scarcity of the FM phase for the formation of infinite percolating cluster and by an existence of the charge-ordered phase. The field-dependent magnetoresistance at low temperature is described in terms of the spin-assisted polaron-hopping model. 2006 Article Magnetic and transport properties of charge ordered La₀.₅Ca₀.₅Mn₀.₃ and La₀.₄Ca₀.₆Mn₀.₃ films / V.G. Prokhorov, V.A. Komashko, G.G. Kaminsky, Y.P. Lee, S.Y. Park, Y.H. Hyun, V.L. Svetchnikov, K.W. Kim, J.Y. Rhee // Физика низких температур. — 2006. — Т. 32, № 2. — С. 176-183. — Бібліогр.: 39 назв. — англ. 0132-6414 PACS: 75.70.-i, 75.47.-m, 71.30.+h http://dspace.nbuv.gov.ua/handle/123456789/120126 en Физика низких температур Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
| institution |
Digital Library of Periodicals of National Academy of Sciences of Ukraine |
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| language |
English |
| topic |
Низкотемпеpатуpный магнетизм Низкотемпеpатуpный магнетизм |
| spellingShingle |
Низкотемпеpатуpный магнетизм Низкотемпеpатуpный магнетизм Prokhorov, V.G. Komashko, V.A. Kaminsky, G.G. Lee, Y.P. Park, S.Y. Hyun, Y.H. Svetchnikov, V.L. Kim, K.W. Rhee, J.Y. Magnetic and transport properties of charge ordered La₀.₅Ca₀.₅Mn₀.₃ and La₀.₄Ca₀.₆Mn₀.₃ films Физика низких температур |
| description |
The magnetic and the transport properties of La₀.₅Ca₀.₅Mn₀.₃ and La₀.₄Ca₀.₆Mn₀.₃ films with
different thickness, prepared by rf-magnetron sputtering by using the so-called «soft» (or powder)
target on LaAlO₃ substrate, have been investigated. The electron-diffraction and the high-resolution
electron microscopy (HREM) studies show that the charge-ordered phase is observed at room
temperature for all films. Both the paramagnetic-to-ferromagnetic transition at TC ≈ 250 K upon
cooling and the appearance of an antiferromagnetic (AFM) phase at TN ≧ 140 K were observed in
the La₀.₅Ca₀.₅Mn₀.₃ films, while the La₀.₄Ca₀.₆Mn₀.₃ films exhibited the AFM transition only at
the same temperature, excepting the small ferromagnetic (FM) response from a «dead» layer. It
was shown that the volume fraction of the FM phase in the La₀.₅Ca₀.₅Mn₀.₃ film did not exceed of
and the FM phase coexisted with the AFM one at low temperature. All films manifest an exponential
temperature dependence of resistance without evidence of the metal-insulator transition. This
is explained by the scarcity of the FM phase for the formation of infinite percolating cluster and
by an existence of the charge-ordered phase. The field-dependent magnetoresistance at low temperature
is described in terms of the spin-assisted polaron-hopping model. |
| format |
Article |
| author |
Prokhorov, V.G. Komashko, V.A. Kaminsky, G.G. Lee, Y.P. Park, S.Y. Hyun, Y.H. Svetchnikov, V.L. Kim, K.W. Rhee, J.Y. |
| author_facet |
Prokhorov, V.G. Komashko, V.A. Kaminsky, G.G. Lee, Y.P. Park, S.Y. Hyun, Y.H. Svetchnikov, V.L. Kim, K.W. Rhee, J.Y. |
| author_sort |
Prokhorov, V.G. |
| title |
Magnetic and transport properties of charge ordered La₀.₅Ca₀.₅Mn₀.₃ and La₀.₄Ca₀.₆Mn₀.₃ films |
| title_short |
Magnetic and transport properties of charge ordered La₀.₅Ca₀.₅Mn₀.₃ and La₀.₄Ca₀.₆Mn₀.₃ films |
| title_full |
Magnetic and transport properties of charge ordered La₀.₅Ca₀.₅Mn₀.₃ and La₀.₄Ca₀.₆Mn₀.₃ films |
| title_fullStr |
Magnetic and transport properties of charge ordered La₀.₅Ca₀.₅Mn₀.₃ and La₀.₄Ca₀.₆Mn₀.₃ films |
| title_full_unstemmed |
Magnetic and transport properties of charge ordered La₀.₅Ca₀.₅Mn₀.₃ and La₀.₄Ca₀.₆Mn₀.₃ films |
| title_sort |
magnetic and transport properties of charge ordered la₀.₅ca₀.₅mn₀.₃ and la₀.₄ca₀.₆mn₀.₃ films |
| publisher |
Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
| publishDate |
2006 |
| topic_facet |
Низкотемпеpатуpный магнетизм |
| url |
http://dspace.nbuv.gov.ua/handle/123456789/120126 |
| citation_txt |
Magnetic and transport properties of charge ordered La₀.₅Ca₀.₅Mn₀.₃ and La₀.₄Ca₀.₆Mn₀.₃ films / V.G. Prokhorov, V.A. Komashko, G.G. Kaminsky, Y.P. Lee, S.Y. Park, Y.H. Hyun, V.L. Svetchnikov, K.W. Kim, J.Y. Rhee // Физика низких температур. — 2006. — Т. 32, № 2. — С. 176-183. — Бібліогр.: 39 назв. — англ. |
| series |
Физика низких температур |
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| fulltext |
Fizika Nizkikh Temperatur, 2006, v. 32, No. 2, p. 176–183
Magnetic and transport properties of charge ordered
La0.5Ca0.5MnO3 and La0.4Ca0.6MnO3 films
V.G. Prokhorov, V.A. Komashko, and G.G. Kaminsky
Institute of Metal Physics, National Academy of Sciences of Ukraine
36 Vernadskogo Ave., Kiev, 03142, Ukraine
E-mail: pvg@imp.kiev.ua
Y.P. Lee, S.Y. Park, and Y.H. Hyun
Quantum Photonic Science Research Center and Department of Physics
Hanyang University, Seoul 133-791, Korea
V.L. Svetchnikov
National Center for HREM, TU Delft 2628AL, The Netherlands
K.W. Kim
Department of Physics, Sunmoon University, Asan, Choongnam 336-840, Korea
J.Y. Rhee
BK21 Physics Research Division and Institute of Basic Science, Sungkyunkwan University
Suwon 440-746, Korea
Received July 15, 2005
The magnetic and the transport properties of La0.5Ca0.5MnO3 and La0.4Ca0.6MnO3 films with
different thickness, prepared by rf-magnetron sputtering by using the so-called «soft» (or powder)
target on LaAlO3 substrate, have been investigated. The electron-diffraction and the high-resolu-
tion electron microscopy (HREM) studies show that the charge-ordered phase is observed at room
temperature for all films. Both the paramagnetic-to-ferromagnetic transition at TC � 250 K upon
cooling and the appearance of an antiferromagnetic (AFM) phase at TN � 140 K were observed in
the La0.5Ca0.5MnO3 films, while the La0.4Ca0.6MnO3 films exhibited the AFM transition only at
the same temperature, excepting the small ferromagnetic (FM) response from a «dead» layer. It
was shown that the volume fraction of the FM phase in the La0.5Ca0.5MnO3 film did not exceed of
and the FM phase coexisted with the AFM one at low temperature. All films manifest an exponen-
tial temperature dependence of resistance without evidence of the metal-insulator transition. This
is explained by the scarcity of the FM phase for the formation of infinite percolating cluster and
by an existence of the charge-ordered phase. The field-dependent magnetoresistance at low temper-
ature is described in terms of the spin-assisted polaron-hopping model.
PACS: 75.70.-i, 75.47.-m, 71.30.+h
Keywords: colossal magnetoresistance, manganite films, charge ordering, polaronic transport
1. Introduction
The hole-doped perovskite manganites L1–xAxMnO3,
where L and A are a trivalent lanthanide and a divalent
alkaline-earth ions, respectively, have attracted con-
siderable attention due to their interesting fundamen-
tal science and potential for applications [1]. It has
been shown that the spin, charge, and lattice are
strongly coupled with each other in these compounds
and may be considered as strongly correlated systems
[2]. A complex interplay of charge, lattice, spin, and
© V.G. Prokhorov, V.A. Komashko, G.G. Kaminsky, Y.P. Lee, S.Y. Park, Y.H. Hyun, V.L. Svetchnikov, K.W. Kim, and J.Y. Rhee, 2005
orbital degrees of freedom in these systems leads to the
complicated phase diagram of L1–xAxMnO3 and is not
completely understood. As the composition x changes,
they show a variety of phenomena, such as ferromag-
netic (FM), antiferromagnetic (AFM), charge (CO)
and orbital (OO) ordering. La1–xCaxMnO3 is a typi-
cal system which demonstrates the change of the ground
state from FM metal to CO AFM insulator, when the
Ca doping level crosses 0.5 [3]. It was shown that, upon
lowering the temperature, the La0.5Ca0.5MnO3 com-
pound first underwent a paramagnetic (PM)-to-FM
phase transition at TC � 225 K and then become the
CO AFM phase at TCO � TN � 225 K [4]. In the letter
case the transition is attributed to the well-known
charge-exchange-type AFM ordering, which has been
proposed long ago by Goodenough [5], and observed
by Wollan and Koehler in this compound [6]. The
low-temperature phase with rich Mn4+ hole doping
(x > 0.5) is AFM and insulating [7–10]. Nevertheless,
numerous contradictory experimental data on the
magnetic and transport properties for this composition
can be found in the literature. There are, for example,
the observation of a ferromagnetic residual magnetiza-
tion [3,11–13] and a metallic-like behavior of the
resistivity [14–16] of La0.5Ca0.5MnO3 below TCO.
These discrepancies were explained by the phase-sepa-
ration effect and by the coexistence of the different
magnetic and electronic phases in the wide tempera-
ture range, which were recently observed experi-
mentally [4,17–21]. However, the origin of these
inhomogeneities in the magnetic and the electronic
states is not clear. There are some trends to explan this
feature in the framework of the electronic-phase-sepa-
ration scenario proposed for manganites [22], but it is
more probable that such a behavior can be mainly
attributed to the structural inhomogeneities of the
samples.
The situation is significantly complicated by the
fact that the magnetic and the transport properties of
manganites are strongly dependent upon the cation
size, the lattice strain, and the microstructure. It is be-
lieved that the cooperative Jahn–Teller effect plays an
important role in the formation of the ground state of
colossal-magnetoresistance materials [23,24] and the
insulating CO state can be controlled by a long-range
strain [25]. Recently evidence was presented for the
appearance of the CO insulating phase in
La1–xCaxMnO3 films at low temperatures with x � 0.3
[26–28], while the films with 0.52 demonstrated only
the FM metallic ground state through the whole tem-
perature range [29]. In both cases the disagreement
with the experimental data for bulk materials was ex-
plained by the lattice strains, which are accumulated
during the deposition of thin film. Consequently, the
magnetic and electronic phase diagrams for
La1–xCaxMnO3 thin films can be significantly differ-
ent from that for the bulk and, therefore, it is desir-
able to peform an additional experimental study. In
this paper we report some peculiar results for
La0.5Ca0.5MnO3 and La0.4Ca0.6MnO3 films.
2. Experimental techniques
All films were prepared by rf-magnetron sputtering
by using the so-called «soft» (or powder) target [30].
The total pressure in chamber was 5�10–2 Torr with a
gas mixture of Ar and O2 (3 : 1). The substrate was a
pseudocubic LaAlO3 (001) single crystal with a lat-
tice parameter � 0.379 nm. The substrate temperature
during the deposition was 750 °C. Under these condi-
tions were deposited the La0.5Ca0.5MnO3 (LCM05)
and the La0.4Ca0.6MnO3 (LCM06) films with diffe-
rent thicknesses: � 30 nm and � 100 nm. The �–2�
x-ray diffraction (XRD) patterns were obtained by
using a Rigaku diffractometer with Cu K
�
radiation.
The lattice parameters evaluated directly from the
XRD data were plotted against cos sin2 � �/ . From the
intercept of the extrapolated straight line to
cos sin2 0� �/ � , a more precise lattice parameter was
obtained. The high-resolution electronmicroscopy
(HREM) studies were carried out by using a Philips
CM300UT-FEG microscope with a field emission gun
operated at 300 kV. The resolution of the microscope
was of the order of 0.12 nm. The cross-sectional speci-
mens were prepared by the standard techniques using
mechanical polishing followed by ion-beam milling at
a grazing incidence. All microstructure measurements
were carried out at room temperature. The resistance
measurements were performed by using the four-probe
method in a temperature range of 4.2–300 K and in a
magnetic field up to 5 T. The geometrical dimensions
of samples were specified by photolithographic tech-
nique: width was 1.0 mm, distance between potential
contacts was 6 mm, and length was 10 nm. The value
of a dc transport current was 10 �A. The cur-
rent-source-mode was used for the recording of resis-
tance. The applied magnetic field was directed paral-
lel to the film surface and perpendicular to the current
flow. The in-plane field-cooled (FC) and the
zero-field-cooled (ZFC) magnetization curves under
an applied magnetic field of 100 Oe and the magneti-
zation hysteresis loops were taken with a Quantum
Design SQUID magnetometer [31].
3. Microstructure and evidence of charge
ordering
Figure 1 presents the (002) and the (004) Bragg
peaks for the investigated films. The LCM05 films dis-
Magnetic and transport properties of charge ordered La0.5Ca0.5MnO3 and La0.4Ca0.6MnO3 films
Fizika Nizkikh Temperatur, 2006, v. 32, No. 2 177
play the well-defined peaks for d � 30 nm (1) and
100 nm (2), which correspond to the out-of-plane lat-
tice parameter c of 0.3812 nm and 0.3810 nm, respec-
tively, for the cubic symmetry. The obtained results
agree well with the published data for both film and
bulk compound [10,20,32]. The Bragg peaks of the
LCM06 films are located very close to those of the
substrate (peaks 3 and 4), corresponding to c � 0.38
nm for both film thicknesses, and are coincident with
that for bulk [10,32].
The cross-sectional low-magnification HREM
image and electron diffraction (ED) pattern of the
LCM05 film (d � 100 nm) are displayed in Fig. 2,a.
The HREM image exhibits sharp, flat and well de-
fined interface between the substrate and the film (in-
dicated by the black arrow) while the very thin (� 1
nm) intermediate (or «dead») layer on the film side is
also present. The ED pattern can be indexed as the
well-known orthorhombic crystal structure with a small
orthorhombic distortion: a b ap� � 2 and c ap� 2 ,
where ap is the lattice parameter of the simple pe-
rovskite structure. Figures 2,b and 2,c show the
high-magnification HREM images of the film and the
«dead» layer, respectively. Insets are the correspond-
ing fast-Fourier-transform (FFT) patterns. It is seen
that, in the first case, FFT produces a rectangular pat-
tern of the spots, which is typical for a regular crystal
lattice, while in the second one the FFT pattern displays
smeared spots and slightly ring-like traces, typical for a
randomly oriented mosaic microstructure. Analysis of
ED and HREM data for the LCM05 film reveals that
the estimated lattice parameter ap � 0.382 nm is almost
identical to that obtained by the XRD data. At the
same time, well-defined superlattice spots are evident
for the (1/2, 0, 0) or (0, 1/2, 0) positions in addi-
tional to the fundamental Bragg reflections, which are
indicated by the white arrows in the insets of Figs. 2,a
and 2,b. Similar superlattice spots in ED pattern were
observed in La0.5Ca0.5MnO3 at T = 95 K and treated
as the appearance of a charge ordering of Mn4+ and
Mn3+ ions [33]. Therefore, one can conclude that the
CO state occurs in the LCM05 film above the room
temperature.
Figure 3 displays results of the same HREM study
carried out for the LCM06 film with d = 100 nm. The
«dead» layer of this film is much larger than that in
LCM05 and reaches 10 nm. The ED pattern, displayed
in the inset, also exhibits the superlattice spots similar
to the LCM05 films, which evidently indicates the ex-
istence of the CO state at room temperature. An anal-
ysis of the high-magnification HREM images reveals
that the LCM06 films have an orthorhombic crystal
structure with ap� 0.38 nm. Figures 3,a and 3,b dis-
play the microstructures of the film and the «dead»
layer, respectively. Similar to the case of LCM05,
FFT produces a rectangular pattern of well defined
spots in the former (with slight traces of additional
sub erlattice spots), caused by the formation of a per-
fect crystalline structure, while they are smeared in
178 Fizika Nizkikh Temperatur, 2006, v. 32, No. 2
V.G. Prokhorov et al.
46 47 48
10
15
L
A
O
3 4,
1
2 a(002)
104 106 108 110
5
10
15
L
A
O
3 4,
1
2
b(004)
2�, deg
5
In
te
n
s
it
y
1
0
3
c
p
s
,
Fig. 1. (002) (a) and (004) (b) Bragg peaks: 30- (1) and
100-nm (2) thicknesses for LCM05; and 30- (3) and
100-nm (4) for LCM06. LAO indicates the Bragg peacks
for the substrate.
a
b c
15 nm
4 n m 4 n m
LCM05
LAO
(110)(110)
Fig. 2. Low-magnification cross-sectional HREM image
for LCM05 (100 nm). Inset displays the electron diffrac-
tion pattern (a). High-magnification HREM images of the
middle (b) and the bottom (close to the «dead» layer) ar-
eas of the film (c). Insets are FFT patterns of the corre-
sponding images.
the bright halos and slightly split in the latter. There-
fore, we can conclude that the «dead» layer has a
nanocrystalline mosaic structure.
4. Experimental results
4.1 Magnetic properties
Figures 4,a and 4,b present the in-plane FC and
ZFC temperature dependencies of the magnetization,
M(T), for the LCM05 (a) and LCM06 (b) film with
d � 30 nm (1) and 100 nm (2), respectively, measured
under an applied magnetic field of 100 Oe. Both sam-
ples of LCM05 demonstrate the PM–to–FM magnetic
transition at TC � 250 K and an appearance of the
AFM phase at TN � 140 K. These temperatures were
obtained from the analysis of the first derivative of
ZFC M(T)–versus–T curves, which are displayed in
the inset in Fig. 4,a. Both the FM and the AFM tran-
sitions are well defined for the thinner film, while the
onset of the AFM ordering is barely discernible for the
thick one, owing to the presence of larger volume of
the FM phase. Moreover, the inset also displays a
two-peaks behavior of dMZFC/dT–versus–T in the
range of the FM transition, indicating the presence of
two Curie points, which are slightly separated from
each other. The two-peak behavior can be explained
by the existence of inhomogeneous microstructures in
this film near the substrate, revealed by the HREM
study. One can suggest that with decreasing tempera-
ture the FM phase appears first inside of the film and
then into the «dead» layer.
An analysis of the dMZFC/dT–versus–T curves for
LCM06, displayed in the inset in Fig. 4,b, reveals that
both films with different thicknesses manifest the
AFM transition at TN � 140 K only. At the same time,
the large difference between ZFC and FC M(T)
curves above TN suggests the presence of a small vol-
ume of the FM phase.
Figures 5,a and 5,b show the in-plane magnetic
hysteresis loops, M(T), at 10 K for the LCM05 (a)
and the LCM06 (b) films with (1) d � 30 nm and (2)
100 nm, respectively. Although the LCM05 films
show the FM transition at TC � 250 K [see Fig. 4,a], it
is not clear whether the volume of the FM phase re-
mains the same below TN . For comparison, the M(T)
dependence for the La0.7Ca0.3MnO3 film [curve 3 in
Fig. 5,a], deposited on the LaAlO3 substrate with the
thickness of 100 nm is shown. It is seen that the satu-
rated magnetic moment of the La0.7Ca0.3MnO3 film is
� 3.2 �B/Mn, which is close to the theoretical value
of an average effective magnetic moment for a fully
FM sample (�eff � 3.5 �B/Mn). On the other hand,
the saturated magnetic moment for both the LCM05
Magnetic and transport properties of charge ordered La0.5Ca0.5MnO3 and La0.4Ca0.6MnO3 films
Fizika Nizkikh Temperatur, 2006, v. 32, No. 2 179
a
b c
25 n m
4 n m 4 n m
LCM06
LAO
(110)
Fig. 3. Low-magnification cross-sectional HREM image
for LCM06 (100 nm). Inset displays the electron diffrac-
tion pattern (a). High-magnification HREM images of the
film (b) and the «dead» layer (c). Insets are FFT patterns
of the corresponding images.
0.1
0.2
0 100 200 300
0.02
0.04
0.06
100 200
–5
0
100 200
–2
0
2
2
1
a
2
1
b
T, K
2
1
TCTN
T, K
2
1
TN
T, K
M
,
/M
n
�
B
d
M
/d
T
, a
.u
.
Z
FC
d
M
/d
T
, a
.u
.
Z
FC
Fig. 4. Field-cooled (solid) and zero-field-cooled (open)
magnetization curves for the LCM05 (a) and the LCM06
(b) films of 30- (1) and 100-nm (2) thicknesses under an
in-plane magnetic field of 100 Oe. Insets display the first
derivatives of corresponding ZFC M(T) dependencies.
Arrows indicate the onset of FM and AFM transition. Tri-
angles indicate the two-stage FM transition in the LCM05
films.
films is 0.95 �B/Mn. Taking into account the fact
that whole volume of the La0.7Ca0.3MnO3 film be-
comes ferromagnetic at low temperature, one can con-
clude that � 30% of the volume of the LCM05 film be-
longs to the FM phase, that is almost coincident with
the published results for the bulk [21]. Therefore, the
LCM05 films exhibit the coexistence of the FM and
the AFM phases at low temperature and can be treated
as a magnetically inhomogeneous systems. Figure 5,b
shows that the LCM06 films demonstrate almost
anhysteretic magnetization loops, as it is represented
by the inset (A), with the coercive field Hc � 100 Oe,
while the LCM05 films have Hc � 600 Oe. This fact
suggests that we deal with a superparamagnetic
(SPM) rather than a real FM state in these films. It is
reasonable to assume that such type of magnetic state
can occur in the «dead» layer, which has nano-
crystalline disordered microstructures. On the other
hand, the M(T) dependence measured under applied
magnetic field of 2 kOe manifests the FM response
around 250 K [see inset (B) in Fig. 5,b].
4.2. Transport properties
Figure 6 shows the temperature dependences of re-
sistance, R(T), for the LCM05 (1, 2) and the LCM06
(3, 4) films with (open symbols) and without (solid
symbols) an applied magnetic field of 5 T. The experi-
mental curves reveal an exponential R(T) behavior in
the whole temperature range, which is cleary shown as
the linear behavior in the ln (R/T)–versus–T–1 plots
(see the inset of Fig. 6), and is almost insensitive to
the applied magnetic field. This is typical characteris-
tics of the bulk sample with the same composition.
Therefore, we did not find the evidence of the MI
transition in the films.
Figure 7 displays the magnetic-field dependence of
the negative magnetoresistance (MR) for the LCM05
(a) and the LCM06 (b) films at T = 10 K. The MR
value is defined by 100% � [R(H) – R(0)]/R(H)
where R(H) and R(0) are the resistances with and
without the magnetic field, respectively. It is seen
that MR does not exceed of 5% in the applied mag-
netic field of 2 T.
180 Fizika Nizkikh Temperatur, 2006, v. 32, No. 2
V.G. Prokhorov et al.
–3
–2
–1
0
1
2
3
–10 –5 0 5 10
–5
0
5
a
T = 10 K
3
1 2,
1 2,
b
H, T
–1 0 1
–1
0
1 A
H, kOe
150 250
0
5
B
TC
T, K
M
,
/M
n
�
B
M
, 1
0
e
m
u
–
5
M
, 1
0
/M
n
–
2
B
M
, 1
0
e
m
u
–
5
Fig. 5. In-plane hysteresis loops of LCM05 (a) and
LCM06 (b) at 10 K, respectively, for 30 (solid symbols)
and 100 nm (open symbols) thickness. Curve (3) in (a)
corresponds to the La0.7Ca0.3MnO3 film. Inset A in (b)
exhibits the low-field hysteresis loop of the 100-nm-thick-
ness LCM06 film in detail. Inset B in (b) displays the
M(T) dependence at magnetic field of 2 kOe (open sym-
bols) and the corresponding first derivative (solid line)
for the same film. Lines are guides to the eyes.
100 200 300
10
2
10
3
10
4
10
5
10
6
4 6 8
2
4
6
8
2
4
3
1
T, K
T
N
4
3
2
1
T , 10 K–1 –3 –1
R
,
�
ln
(R
/T
),
K
–
1
Fig. 6. Temperature dependence of the resistance for
LCM05 of 100- (1) and 30- (2) and for LCM06 of 100-
(3) and 30-nm (4) thicknesses without (solid symbols)
and with (open symbols) an applied magnetic field of 5 T.
Inset displays the corresponding ln(R/T)–versus–T–1
plots. Solid lines are fitting curves for the thermally acti-
vated conductivity approximation. Arrow indicates tem-
perature of the AFM transition.
5. Discussion
5.1. Influence of «dead» layer on magnetic
properties
Let us consider the magnetic properties of the inves-
tigated samples. The LCM05 films manifest the M(T)
dependence, which agrees well with a magnetic phase
diagram for this composition [3,4,8,9,32]. Both FM
and the AFM magnetic transitions are observed with
decreasing temperature at TC � 250 K and TN � 140 K,
respectively. The observed two-stage behavior of the
FM ordering is explained by the presence of the thin
«dead» layer with disordered microstructure. In con-
trast, the LCM06 films exhibit not only the AFM
transition at TN � 140 K, which is typical for this com-
position, but also an additional magnetic transition at
a higher temperature. Inset B in Fig. 5,b shows the re-
markable increase in M(T) at � 250 K under an ap-
plied magnetic field of 2 kOe, while the similar pecu-
liarity is not observed on the ZFC M(T) curves at low
applied magnetic field of 100 Oe. This indicates that,
though the long-range spontaneous magnetization
does not appear in the film, the small-size random-
ly-oriented FM clusters are present at T � 250 K. On
the other hand, Fig. 5,a displays that the directly
measured value of the saturated magnetic moment,
Ms , is the same for both films, in spite of the almost
three times difference in their thickness: 30 and 100
nm for curves 1 and 2, respectively. Consequently, the
volume of the FM phase does not depend on the film
thickness and can be associated with the presence of
the nanocrystalline 10-nm-thickness «dead» layer in
both films. This is further confirmed by the compari-
son with the Ms value for the fully ferromagnetic
La0.7Ca0.3MnO3 (� 9.2�10–4 emu) and the LCM06
film (� 7.5�10–5 emu) with the equal thickness and
area. The estimated volume fraction of the FM phase
in LCM06 (8) is almost coincident with the relative
volume of «dead» layer (10), verified by the HREM
study.
5.2. Spin-assisted polaron transport in AFM
CO state
Now let us consider the transport properties of the
films. Figure 6 exhibits that all the films, independent
of composition and thickness, manifest an exponential
R(T) dependence in the whole investigated tempera-
ture range. Therefore, we did not find the evidence of
the MI transition in our films, while some publica-
tions claim its presence at these compositions
[14–16,29,34,35]. Because the volume of the FM
phase in the LCM05 film (� 30%) is smaller than the
threshold value (� 40%) of a percolation for 3-dimen-
sional systems [36], the MI transition can not be ex-
pected in this sample [22]. A similar argument can be
applied to the LCM06 films, in which the FM ordered
phase is located in «dead» layer only and occupies the
negligible fraction of the film volume. At the same
time, we did not observe any peculiarities on the R(T)
curves, which could be treated as the formation of a
CO state. The ln (R/T)–versus–T–1 plots repre-
sented by the inset in Fig. 6 manifest the straight lines
for all films in the whole temperature range, even near
the AFM transition. The absence of CO transition sug-
gests us that it may occur at higher temperature. This
agrees with the ED data which indicate the existence of
the CO state in the film already at room temperature.
Therefore, one can suggest that the thermally activated
polaronic transport is dominating in the films at low tem-
perature because the charge ordering and the antiparallel
spin orientation of neighboring ions lead to the blocking
of a nonadiabatic through-the-barrier tunneling of carri-
ers. It is supported by an excellent agreement between
the experimental R(T) dependencies and the theoretical
curves obtained on the basis of thermally activated con-
ductivity model, R T R T T /TA( ) exp(� 0 ), where TA is
the activation energy in unit of temperature (indicated
by solid lines in the inset of Fig. 6). The activation en-
ergy turn out to be TA � 1350 K for all films. According
to Appel [37] the hopping probability of insulator with
a short-range magnetic ordering should be modified by a
multiplicative term ( )1 22 2� M /M /s . In this case the
negative magnetoresistance ratio, represented by Fig. 7,
Magnetic and transport properties of charge ordered La0.5Ca0.5MnO3 and La0.4Ca0.6MnO3 films
Fizika Nizkikh Temperatur, 2006, v. 32, No. 2 181
–5
–4
–3
–2
–1
0
La Ca MnO0.5 0.5 3
a
–2 –1 0 1 2
–5
–4
–3
–2
–1
0 b
H, T
La Ca MnO0.4 0.6 3
M
R
,%
Fig. 7. Magnetic-field dependence of MR ratio for LCM05
(a) and LCM06 (b) of 100 (solid symbols) and 30 nm
(open symbols) thickness at 10 K. Solid lines are theoreti-
cal curves obtained in the framework of a spin-assisted
polaron hopping approach.
can be expressed by MR(%) � M /Ms
2 2 and its mag-
netic-field dependence should be duplicates the squared
reduced value of the magnetic moment. The solid lines
in Fig. 7 are the theoretical curves representing the
squared Langevin function, L a a /a( ) coth( )� 1 ,
where a H/kB� � and � is effective magnetic moment of
the magnetic ion [38]. The � value for each composition
of the film can be estimated from the expression:
� � � � �g xS S x S S1 1 2 21 1 1( ) ( ) ( ),
where x is the Ca concentration, S /1 3 2� and S2 2�
are the spin values of Mn4+ and Mn3+ ions, respec-
tively, and g � 2 is the Land� factor. The obtained
values � � 4.4 and 4.3 �B for LCM05 and LCM06,
respectively, were used in the theoretical curves. It is
seen that the theoretical curves, obtained without any
fitting parameters, almost coincide with experimental
data, especially for thick films. Therefore, although
this approximation is more suitable for an analysis of
the charge transport in the paramagnetic insulating
state, it can also be successfully used for the descrip-
tion of the magnetotransport properties in AFM insu-
lating state.
The appearance of CO state at high temperature in
the films is probably triggered by a ferroelastic phase
transition in LAO substrate which occurs at T = 544 °C
[39]. The elastic stresses generated in the film during
the orthorhombic-to-rhombohedral structural trans-
formation of the LAO substrate stimulate the transi-
tion of the film in more stable state at higher tempera-
ture than that is predicted by the equilibrium phase
diagram.
6. Conclusions
The La1–xCaxMnO3 films with x � 0 5. (LCM05)
and 0.6 (LCM06) were prepared by rf-magnetron
sputtering using the «soft» (or powder) target. The
LCM05 film demonstrates the PM-to-FM magnetic
transition at TC � 250 K and the appearance of an
AFM phase at TN � 140 K, while the LCM06 one
manifests the AFM transition only at the same temper-
ature and the presence of a small volume (8%) of FM
phase in the «dead» layer. It was shown that volume
fraction of the FM phase in LCM05 reaches of 30% at
10 K and the FM phase coexists with the AFM phase
at low temperature. All films manifest an exponential
R(T) dependence in the whole temperature range
without the evidence of the MI transition. It is ex-
plained by the scarcity of FM phase for the formation
of an infinite percolating cluster and by the appear-
ance of CO state at T � 300 K, according to the ED
data. The field-dependent magnetoresistance at low
temperature can be described on the basis of the
spin-assisted polaron hopping approach.
This work was supported by the KOSEF through the
Quantum Photonic Science Research Center, by Korea
Research Foundation Grant (KRF-2001-015-DS0015),
and by MOST, Korea.
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Magnetic and transport properties of charge ordered La0.5Ca0.5MnO3 and La0.4Ca0.6MnO3 films
Fizika Nizkikh Temperatur, 2006, v. 32, No. 2 183
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