Magnetic and electronic phase separation driven by structural clustering in La₀.₇ (Ca₁₋ySry)₀.₃ MnO₃ thin films
The structural, magnetic and transport properties of La₀.₇ (Ca₁₋ySry)₀.₃ MnO₃ films deposited on LaAlO₃ (001) single-crystalline substrate by rf-magnetron sputtering using «soft» (or powder) targets are investigated. It was found that at 0.3 ≤ y ≤0.5 both rhombohedral (R c3 ) and orthorhombic (Pn...
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Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України
2006
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| Cite this: | Magnetic and electronic phase separation driven by structural clustering in La₀.₇ (Ca₁₋ySry)₀.₃ MnO₃ thin films / V.G. Prokhorov, V.A. Komashko, G.G. Kaminsky, Y.P. Lee, Y.H. Hyun, K.K. Yu, J.S. Park, V.L. Svetchnikov // Физика низких температур. — 2006. — Т. 32, № 7. — С. 853–860. — Бібліогр.: 34 назв. — англ. |
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Digital Library of Periodicals of National Academy of Sciences of Ukraine| _version_ | 1859974170018840576 |
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| author | Prokhorov, V.G. Komashko, V.A. Kaminsky, G.G. Lee, Y.P. Hyun, Y.H. Yu, K.K. Park, J.S. Svetchnikov, V.L. |
| author_facet | Prokhorov, V.G. Komashko, V.A. Kaminsky, G.G. Lee, Y.P. Hyun, Y.H. Yu, K.K. Park, J.S. Svetchnikov, V.L. |
| citation_txt | Magnetic and electronic phase separation driven by structural clustering in La₀.₇ (Ca₁₋ySry)₀.₃ MnO₃ thin films / V.G. Prokhorov, V.A. Komashko, G.G. Kaminsky, Y.P. Lee, Y.H. Hyun, K.K. Yu, J.S. Park, V.L. Svetchnikov // Физика низких температур. — 2006. — Т. 32, № 7. — С. 853–860. — Бібліогр.: 34 назв. — англ. |
| collection | DSpace DC |
| container_title | Физика низких температур |
| description | The structural, magnetic and transport properties of La₀.₇ (Ca₁₋ySry)₀.₃ MnO₃ films deposited
on LaAlO₃ (001) single-crystalline substrate by rf-magnetron sputtering using «soft» (or powder)
targets are investigated. It was found that at 0.3 ≤ y ≤0.5 both rhombohedral (R c3 ) and
orthorhombic (Pnma) crystal phases are coexistent at room temperature, forming a nanoclustered
microstructure. The clustered films manifest the two-stage magnetic and electronic transition,
which are typical for the phase-separated systems. It was shown that for 0.5 ≥ y ≥ 0 the nonlinear
(almost parabolic) field-dependent magnetoresistance is typical at room temperature while for
0.65 ≤y ≤1.0 its transform to the linear behavior. The magnetotransport properties of the films
are explained within the framework of a field-dependent activation energy model. The magnetic
phase diagram for the La₀.₇ (Ca₁₋ySry)₀.₃ MnO₃ thin-film system is presented.
|
| first_indexed | 2025-12-07T16:22:49Z |
| format | Article |
| fulltext |
Fizika Nizkikh Temperatur, 2006, v. 32, No. 7, p. 853–860
Magnetic and electronic phase separation driven by
structural clustering in La0 7. (Ca1�y Sr y )0 3. MnO3 thin films
V.G. Prokhorov, V.A. Komashko, and G.G. Kaminsky
Institute of Metal Physics of the National Academy of Sciences of Ukraine, Kiev 03142, Ukraine
E-mail: pvg@imp.kiev.ua
Y.P. Lee, Y.H. Hyun, K.K. Yu, and J.S. Park
q-PSI and Department of Physics, Hanyang University, Seoul 133-791, Korea
V.L. Svetchnikov
National Center for HREM, TU Delft 2628AL, The Netherlands
Received October 3, 2005, revised December 1, 2005
The structural, magnetic and transport properties of La07. (Ca1�ySry)03. MnO3 films deposited
on LaAlO3 (001) single-crystalline substrate by rf-magnetron sputtering using «soft» (or powder)
targets are investigated. It was found that at 0.3 � �y 0.5 both rhombohedral (R c3 ) and
orthorhombic (Pnma) crystal phases are coexistent at room temperature, forming a nanoclustered
microstructure. The clustered films manifest the two-stage magnetic and electronic transition,
which are typical for the phase-separated systems. It was shown that for 0.5 � �y 0 the nonlinear
(almost parabolic) field-dependent magnetoresistance is typical at room temperature while for
0.65 � �y 1.0 its transform to the linear behavior. The magnetotransport properties of the films
are explained within the framework of a field-dependent activation energy model. The magnetic
phase diagram for the La07. (Ca1�ySry)03. MnO3 thin-film system is presented.
PACS: 75.70.–i, 75.47.–m, 71.30.+h
Keywords: structural, magnetic and transport properties; magnetic phase diagram; crystal phases.
1. Introduction
The discovery of a huge negative magnetoresistive
effect in doped manganite perovskites with the gen-
eral formula R A1�x xMnO3, where R is a rare-earth
cation and A is alkali or alkaline earth cation [1],
called «colossal magnetoresistance» (CMR), at-
tracted interest to these compounds [2–5]. The trans-
port and magnetic properties of doped manganites can
be understood within the framework of «double ex-
change» (DE) model which considers the magnetic
coupling between Mn3� and Mn4� that results from
the motion of an itinerant electron between two par-
tially filled d shells with strong on-site Hund’s cou-
pling [6–8], and taking into account Jahn–Teller
spin- and charge–lattice interactions [9,10]. At the
same time, because the hole-doped perovskite manga-
nites belong to the strongly correlated systems, they
manifest a tendency toward a phase separation, typi-
cally involving the ferromagnetic (FM) metallic and
charge-ordered insulating domains [11]. Such type of
the phase-separated state usually occurs in a tempera-
ture range below the Curie point (TC). On the other
hand, the nanoscale structural correlations were ob-
served recently in the La065. (Ca0 45. Sr055. )035. MnO3
single crystal at the temperature more above TC [12].
The main reason of this effect is the trend of
La (Ca07 1. �ySr y)03. MnO3 to the structural transition
from the rhombohedral (R c3 ) to orthorhombic
(Pnma) crystal lattice with decreasing temperature
near y � 0.5 [13–17]. Moreover, this transition is con-
trolled by an external magnetic field, indicating a
small difference between the ground state energies for
these crystal symmetries [15]. At the same time, the
overwhelming majority of these data were observed on
© V.G. Prokhorov, V.A. Komashko, G.G. Kaminsky, Y.P. Lee, Y.H. Hyun, K.K. Yu, J.S. Park, and V.L. Svetchnikov, 2006
the bulk materials which undergo the structural tran-
sition under equilibrium thermodynamic conditions.
It is reasonable to suggest that a strong influence of
the substrate on the crystal lattice of deposited film
should be result in the structural modification of this
compound.
In this paper we report on the magnetotransport prop-
erties of the as-deposited La07. (Ca1�ySry)03. MnO3
films at fixed the Mn3�/Mn4� ratio for varying size
of the dopant atoms. It was shown that the
0 3 0 5. .� �y films manifest the multiple micro-
structure which contains the nanoscale clusters of
both the orthorhombic and the rhombohedral crystal
lattice at room temperature. We find that films with
0.5 � �y 0 demonstrate a nonlinear (almost parabolic)
behavior of the magnetoresistance (MR) versus an ap-
plied magnetic field while for 0.65 � �y 1.0 the
MR(H) has almost a linear one at room temperature.
This effect is described within the framework of
a field-affected activation energy approximation, tak-
ing into account a competition between the spin-de-
pendent trapping of charges and the Weiss-magnetiza-
tion contribution. Using experimental data, the
magnetic phase diagram is constructed for the
La (Ca Sr ) MnO0.7 0.3 31�y y thin-film system.
2. Experimental Techniques
The films were prepared by on-axis rf-magnetron
sputtering using the so-called «soft» (or powder) tar-
get [18]. The substrate was a LaAlO3 (LAO) single
crystal with an out-of-plane lattice parameter
c � 0 379. nm for a pseudocubic symmetry. The total
pressure in chamber was 4�10 �2 Torr with a gas mix-
ture of Ar and O2 (2 : 1). The substrate temperature
during deposition was 750° C. Under these conditions
La07. (Ca1�ySr y)03. MnO3 films with y = 1.0, 0.9,
0.8, 0.65, 0.5, 0.3, and 0 were prepared. The thickness
of all the films was d � 100 nm. The �–2� x-ray
diffraction (XRD) patterns were obtained using
a Rigaku diffractometer with Cu-K
�
radiation.
High-resolution electron-microscopy (HREM) studies
were carried out using a Philips CM300UT-FEG mi-
croscope with a field emission gun operated at 300 kV.
The point resolution of the microscope was of the or-
der of 0.12 nm. The cross-sectional specimens were
prepared by the standard techniques using mechanical
polishing followed by ion-beam milling at a grazing
incidence. All microstructure studies were carried out
at a room temperature. The resistance measurements
were performed by using the four-probe method in a
temperature range of 4.2–300 K and a magnetic field
up to 5 T. The in-plane field-cooled (FC) and the
zero-field-cooled (ZFC) magnetization curves in a
field of 100 Oe and the magnetization hysteresis loops
at 10 K were taken with a Quantum Design SQUID
magnetometer.
3. Microstructures of the films
The analysis of the �–2� XRD scans (not shown)
manifests that the deposition results in highly c-ori-
ented films without the traces of parasitic phases. Fi-
gure 1 shows, in detail, the (004) Bragg peak for all
La07. (Ca1�ySr y)03. MnO3 films. It is seen that the
substitution of Sr by Ca shifts, from the beginning,
the Bragg peak to high-angle side while at y � 0.5 the
opposite tendency is observed. Therefore, the
out-of-plane lattice parameter nonmonotonically de-
pend on y in contrast to the expected behavior for
bulk. It is clear that reduction in Sr doping should be
lead to a monotonic decrease in the lattice parameter
because the Ca ion radius is smaller than that of the Sr
one. The observed nonmonotonic behavior of c can be
explained by the more expressive tendency of the
La07. Ca03. MnO3 film to accumulation of the lattice
strain, during epitaxial growth, than that observed for
the La07. Sr03. MnO3 one [19–21].
For illustration of this fact Fig. 2 exhibits the
high-magnification cross-sectional HREM images of
the La07. Ca03. MnO3 (a) and the La07. Sr03. MnO3 (b)
films for regions including the interface between film
and substrate (denoted by white dashed line). Insets
(A) are the corresponding fast Fourier transform
(FFT) of these HREM images. It is seen that the FFT
854 Fizika Nizkikh Temperatur, 2006, v. 32, No. 7
V.G. Prokhorov et al.
102 104 106
1
2
3
4
(004)
0
0.3
0.5
0.65
0.8
0.9
y = 1.0
In
te
n
s
it
y
a
rb
.
u
n
it
s
� �–2 , degree
,
Fig. 1. XRD patterns in the vicinity of the (004) Bragg
peak for the La07. (Ca1�ySry)03. MnO3 films.
pattern of the La07. Sr03. MnO3/LAO interface re-
veals elongated and slightly-split spots in both c (nor-
mal to the interface) and a (along the interface) direc-
tions (indicated by white arrows). This is an evidence
for a semicoherent (or weaklycoherent) lattice cou-
pling between the film and the substrate. In contrast
to that FFT for the La07. Ca03. MnO3/LAO interface
produces a rectangular pattern with a well-defined spot
splitting in the out-of-plane direction only, manifesting
a nearly coherent interface between the film and the
substrate. It is confirmed by the moire patterns (or in-
verse Fourier transforms [22]), which are represented
by the insets (B) in Figs. 2,a,b. It is seen that the edge
misfit dislocation occurs in the La07. Sr03. MnO3 film
while it is not observed in the La07. Ca03. MnO3 one.
The measurement of various interspot spacings on the
high-magnification HREM image allows us to obtain
the average values of lattice parameters. Analysis
reveals that both films have a tetragonal distortion
of the crystal lattice c/a � 101. and 1.02 for
La07. Sr03. MnO3 and La07. Ca03. MnO3, respectively,
where a is an in-plane lattice parameter. Therefore, the
observed increasing in the out-of-plane lattice parame-
ter in the Ca-rich (y � 0.5) films is provided by an in-
tensive tetragonal distortion owing to formation of a
coherent interface between the film and the substrate.
Figure 3 shows the high-magnification cross-sectional
HREM images of La07. (Ca1�ySr y)03. MnO3 for y =
= 0.5 (a) and 0.65 (b), respectively. The y = 0.5 film
contains areas with more or less different crystal struc-
ture, the boundaries of which are marked by white
dashed lines. It is coincident with the FFT pattern
which exhibits that the main spots are elongated in
the out-of-plane direction, confirming a variety of the
lattice parameters. Moreover, the additional slight
spots (correspondent to the doubled lattice parame-
ter) are distinguishable on the FFT pattern (denoted
by white arrow) in zones with a violent atomic orde-
ring. Similar zone in the real space is indicated by
white arrow in Fig. 3,a. Figure 3,c shows the Fourier
filtration of the original HREM image, which allows
us to distinguish the areas with different atomic order-
ing more accurately. These areas, corresponding to
dark and bright regions in Fig. 3,c, have a size from
few a nanometers to tens of nanometers. The measure-
ment of a large number of interdot spacings and angles
between dot rows allow us to obtain the average
lattice parameters for both crystalline phases. Upon
analysis, one can conclude that the more-ordered
zones manifest a rhombohedral crystal structure with
aR � 0.5484 nm and �R � 60.35° while, the more-dis-
Magnetic and electronic phase separation driven by structural clustering in La07. (Ca1�ySr y)03. MnO3
Fizika Nizkikh Temperatur, 2006, v. 32, No. 7 855
LSMO
LAO
c
A
B
a
b
A
c
LCMO
LAO
B
Fig. 2. High-magnefication cross-sectional HREM images
for the La07. Sr03. MnO3 (a) and La07. Ca03. MnO3 (b)
films. Dashed lines indicate the film/substrate interface.
Insets (A) are corresponding FFT. Insets (B) show corre-
sponding moire patterns.
a b
c d
4 nm 4 nm
Fig. 3. (a) and (b) cross-sectional HREM images for the
La07. (Ca1�ySry)03. MnO3 film with y = 0.5 and 0.65, re-
spectively. Insets are the FFT of corresponding HREM
images. (c) and (d) Fourier filtration of the (a) and (b)
original HREM images. Dashed lines in (a) indicate re-
gions with different crystal structure.
ordered ones have an orthorhombic crystal lattice with
tetragonal ratio of c/a � 1.02 and a � 0.3812 nm.
Therefore, the La07. (Ca1�ySr y)03. MnO3 film at y =
= 0.5 can be treated as a composite object which in-
volves two kinds of the nanoscale clusters with a dif-
ferent crystal lattice. The observed contradiction with
the XRD data, which manifest only a single unsplit
Bragg peak, can be explained by the similarity of the
lattice parameters (for a pseudocubic symmetry) for
the both crystal structures and the detection limit of
the x-ray diffraction with Cu-K
�
radiation. However,
it should be noted that structural clustering was ob-
served recently by a powder XRD measurement near
the (022, 220) reflection in the corresponding bulk
compound [17]. For comparison, Fig. 3,b shows that
the film at y = 0.65 manifests an almost uniform crys-
tal lattice. Inset in Fig. 3,b displays that FFT pro-
duces a rectangular pattern of circular and non-elon-
gated spots, and Fig. 3,d represents the more monoto-
nous contrast of the Fourier filtration than that
observed for y = 0.5. Analysis reveals that this film
has mainly a rhombohedral crystal structure with
aR � 0.5524 nm and �R � 60.46°, which are coinci-
dent with the bulk [15].
4. Magnetic and transport properties
Figure 4 shows the in-plane FC (solid symbols) and
ZFC (open symbols) temperature-dependent magneti-
zation curves, M(T), for y = 1.0, 0.8, 0.65, 0.5, 0.3,
and 0. The applied magnetic field was H = 100 Oe.
It is seen that an increase in the substitution of Sr by
Ca leads to a gradual decrease of TÑ, except for the
y = 0.5 and 0.3 films. For these films M(T) manifests
behavior typical for a two-phase magnetic system. A
first transition occurs at TÑ1 � 260 K and second one
at TÑ2 � 130 K, as a slight change in the slope of the
FC curve. It worthy of note that similar two-step
M(T) behavior was observed recently in a single crys-
tal at y = 0.45 and explained by the occurrence of a
structural transition at temperature below TÑ [16].
However, according to the microstructural analysis
for y = 0.5, in our case we are dealing with a composite
film which involves two kinds of nanoscale clusters
with a different crystal lattice. Therefore, it is reason-
able to suggest that the two-stage M(T) dependence is
a simple superposition of two separated FM transi-
tions in clusters with the rhombohedral and
orthorhombic structure. At the same time, the y = 0.3
film also demonstrates the two-step M(T) behavior
and can be treated as a crystal-phase-separated system
as well, though the FM response at TC1 is significantly
suppressed in comparison with y = 0.5.
Figure 5 shows the temperature-dependent resis-
tance, R(T), for the same films without (solid sym-
bols) and with (open symbols) an applied magnetic
field of 5 T. The magnetic field was directed parallel
to the film surface and at right angle to the transport
856 Fizika Nizkikh Temperatur, 2006, v. 32, No. 7
V.G. Prokhorov et al.
100 200 300
0
1
2
3
M
a
rb
.
u
n
it
s
T, K
y = 1.0
0.8
0.65
0.5
0
0.3
TC2
TC1
0
,
Fig. 4. Temperature dependence of the FC (solid symbols)
and ZFC (open symbols) magnetizations for the
La (Ca Sr ) MnO0.7 0.3 31�y y films. Arrows indicate the Curie
point for the magnetic phase-separated films.
0 100 200 300
10
0
101
102
103
104
100 200 300
50
1.0
0.65
0.5
0.3
y = 0
T, K
T P1
T P2
T, K
y = 0
0.5
1.0
R
,
M
R
,%
Fig. 5. Temperature dependence of the resistance for the
La07. (Ca1�ySry)03. MnO3 films without (solid symbols)
and with (open symbols) an applied magnetic field of 5 T.
Arrows indicate the two-stage MI transition in the y = 0.5
and 0.3 films.
current. It is seen that the y � 0 film demonstrates a
typical CMR R(T) behavior with the well-defined
metal–insulator (MI) transition at TP � 200 K,
while the y � 1.0 one manifests only a change in the
slope on the R(T) dependence at T � 300 K. It is
known that the La07. Sr03. MnO3 compound, owing to
a large one-electron bandwidth, does not undergo a
real MI transition near Curie point and retain a
metal-like state in the paramagnetic phase up to high
temperatures [5,15]. On the other hand, the y � 0.3 and
0.5 films demonstrate two-peak behavior of R(T), which
can be treated as two MI transitions at TP1 � 210 K and
TP2 � 130 K (indicated by arrows), which are go-
verned by the appearance of FM ordering in the
rhombohedral and orthorhombic clusters, respec-
tively. It is confirmed by the similar two-peak temper-
ature behavior of MR for y � 0.5, which is shown in
the inset. Moreover, these peaks occur at temperatures
close to TP1 and TP2 on the R(T) curve. Here the MR
value is defined by 100% � [R(0) – R(H)]/R(0),
where R(0) and R(H) are the resistances without and
with a magnetic field of 5 T, respectively.
Figure 6 displays the magnetic-field dependence of
MR at room temperature for the y � 0.3, 0.5, 0.8, and
0.9 films. In this case the MR value is defined by
100% [� R(H) – R(0)]/R(0). It is seen that a
magnetoresistive effect is enhanced with increasing Sr
doping, and the MR(H) dependence is changed from
almost parabolic (y � 0.3 and 0.5) to close to linear
(y � 0.8 and 0.9). A similar variation of the MR( )H
behavior at T TC� has already been observed in these
lanthanides and explained by a transition from the in-
sulating to metal-like state with increasing of Sr con-
centration [23–26]. However, in these publications
the opposite dependence of a magnetoresistive effect
on the Sr doping was observed [15,26].
5. Discussion
Figure 7 exhibits a magnetic phase diagram for the
La07. (Ca1�ySr y)03. MnO3 films deposited on the
LAO substrate. The triangular symbols display a tem-
perature TP of the MI transition. It is seen that the de-
crease of Sr doping, in general, leads to decrease of
TÑ, which is coincident with published results for
bulk [15] and thin films deposited on NdGaO3 [26].
Because the FM ordering is governed by the transfer
interaction of an eg -orbital carrier between the neigh-
boring Mn sites, which should be determined mainly
by Mn–O bond length and Mn–O–Mn angle [23], the
final result for TC can be approximately written
as [27,28] T W /dC � � cos ,. Mn–O
35 where W is the
bandwidth, is the tilt angle in the plane of the bond,
and dMn–O is the Mn–O bond length. Consequently,
the modification of a crystal lattice from rhombohedral
to orthorhombic owing to a substitution of Sr for Ca,
which usually is accompanied by the decreasing tilt an-
gle, must lead to theW narrowing and the TC decreas-
ing. It is believable that the parent La07. Sr03. MnO3
compound has a rhombohedral crystal structure with
165°, while La07. Ca03. MnO3 has a orthorhombic
one with
155°. However, in contrast to bulk which
shows the structural R c Pnma3 � transition at a cer-
tain y concentration [15], our films manifest the suffi-
Magnetic and electronic phase separation driven by structural clustering in La07. (Ca1�ySr y)03. MnO3
Fizika Nizkikh Temperatur, 2006, v. 32, No. 7 857
–20 –10 0 10 20
–12
–10
–8
–6
–4
–2
0
–20 –10 0 10 20
–10
–5
0
y = 0.3
0.9
300 K
0.5
H, kOe
0.3
0.9
0.8M
R
,%
M
R
,%
H, kOe
Fig. 6. Magnetic-field dependence of MR ratio for the
La07. (Ca1�y Sry)03. MnO3 films measured at room tempe-
rature. Inset displays the experimental (solid lines) and
theoretical (dashed lines) MR(H) dependence for the y =
= 0.3 and 0.9 films.
0 0.2 0.4 0.6 0.8 1.0
150
200
250
300
350
T C
,
T P
K
Sr concentration
,
O
rt
h
o
-
rh
o
m
b
ic
R
h
o
m
b
o
-
h
e
d
ra
l
Fig. 7. Magnetic phase diagram for the La07. (Ca1�ySry)
03. MnO3 thin-film system. Solid squares and open trian-
gles correspond to the Curie point and the temperature of
the MI transition. The concentration range of the struc-
tural phase-separated state is crosshatched.
ciently broad concentration range (0.3 � �y 0.5)
where both crystalline phases are coexistent. It was
declared above, the observed multiple clustering in
these films probably is governed by a nonuniform dis-
tribution of the lattice strain. Consequently, for the
0.3 � �y 0.5 films, it is reasonable to suggest that
with temperature decreasing the FM phase appears at
first in the rhombohedral clusters and then in the
orthorhombic regions of the film, providing the
two-stage M(T) behavior. The small magnetization
raising at TÑ1 for the y = 0.3 film (see curve 6 in
Fig. 4) can be explained by the decrease in an amount
of clusters with the rhombohedral structure with the
increasing Ca doping. Analysis of Figs. 4 and 5 dis-
plays that the magnetic and electronic transition tem-
peratures (T TC P2 2� � 130 K) are almost coincident
for the orthorhombic regions of the y = 0.3 and 0.5
films which testify to the typical MI transition re-
sulted from the DE mechanism [5]. At the same time a
significant discrepancy between the FM and MI tran-
sition temperatures is observed for the rhombohedral
clusters: TC1 � 260 K against TP1 � 180 and 210 K for
y = 0.3 and 0.5, respectively. This divergence can be
explained by a percolating mechanism of the MI tran-
sition, which is very often observed in the inhomo-
geneous manganites independently of the physical na-
ture of this inhomogeneity [11].
Let us consider a field-dependent magnetoresis-
tance behavior of the films at room temperature.
According to the phase diagram the films with large Sr
doping (y � 0.65) manifest an onset of the FM transi-
tion at T � 300 K. Therefore, MR(H) should be obey
the DE mechanism of the charge transport, which
predicts a square dependence on magnetization:
MR(H) = C(M/Ms )
2 [2,29]. Here Ms is the satura-
tion magnetization andC is a nearly field- and tempera-
ture-independent constant. On the other hand, the films
with lager Ca doping (y � 0.5) have the Curie point be-
low a room temperature and at T TC� demonstrate the
thermally activated polaronic transport of carriers (see
Fig. 5), expressed by R T H R T E /k TA B( , ) exp( )� 0 .
Here R0 is the constant, which is inversely propor-
tional to the polaron hopping frequency, EA is the acti-
vation energy, and kB is the Boltzmann constant. Con-
sequently, the y � 0.5 films can be treated as
paramagnetic insulators at room temperature. The
MR(H) behavior of such materials is explained as a
rule by two approaches. The first of them is based on
the idea that the hopping probability of the insulator
with a short-range magnetic ordering should be modi-
fied by a multiplicative term ( )1 22 2
M /M /s [30].
In this case the negative magnetoresistance ratio also
has the square dependence on a magnetization with
C � 1 and expressed by MR(H) = (M/Ms )
2. The se-
cond approach assumes that the trapping of a charge
(treated as a ferromagnetic polaron) can be minimized
by the transition of the paramagnetic neighborhood
from random disorder to spin alignment due to an ap-
plied magnetic field [25,31]. In this case the activation
energy has to change in the presence of a magnetic field,
E EA A ij� �0 1( cos )� , where EA
0 is the field-inde-
pendent activation energy and � ij is the angle between
the i and j ion spins. Taking into account that for the
uncorrelated spins cos cos ( ) ,� �ij i sM/M� �
2 2
MR( ) exp[ ( ) ]H M/Ms� �� 2 , where � � E /k TA B
0 .
Let us first analyze the field-dependent MR of the
low Sr-doped films. Because we assume that these
films turn out to be in a paramagnetic state, the mag-
netization can be expressed by the Brillouin function
B g H/k TS B B( )� , where g = 2 is the Land� factor and
�B is the Bohr magneton. The average spin for this
composition is S S S S S�
0 3 1 0 7 11 1 2 2. ( ) . ( ) =
= 2.3076, where S1 = 3/2 and S2 = 2 are the spin val-
ues of Mn4� and Mn3� ions, respectively. The prior
analysis of the R T( ) dependence for y = 0 at T T TC P� ,
allow us to estimate the field-independent activation
energy as EA
0 = 1500 K in the temperature units, and
therefore, � � 5 at T = 300 K. The inset in Fig. 6 dis-
plays the experimental (solid line) and both theoreti-
cal (dashed lines) MR(H) curves for y � 0.3. For the
better agreement between theory and experimental
data we used S NSeff � as the fitting parameter. Here
Seff is treated as an effective polaron spin which con-
sists of N single ion ones [25]. It is seen that both the
considered approaches with the same degree of accu-
racy describe MR(H) behavior for y � 0.3. Moreover,
the exploitable fitting parameters are very close to
each other, N = 3.5 and 2.35 for the Appel [30] and
the field-dependent activation energy [25,31] models,
respectively.
Unfortunately the MR(H) dependence for the y =
= 0.9 film can not be adequately described in the
framework of DE model, which also predicts the para-
bolic MR(H) dependence instead of the near linear
one, manifested by the experiment [29]. The main
reason for that is probably the inhomogeneous
(phase-separated) magnetic state of the high Sr-doped
(y � 0.65) films at room temperature. It is confirmed
by the anhysteretic MR(H) behavior in a low-field
range, which is contradicted with that for the similar
films being in the completely FM state [32]. This hys-
teresis becomes apparent in the form of two splitted
peaks located near a coercive field and is provided by
the hysteretic behavior of magnetization loop for
ferromagnet [33]. Therefore, one can conclude that
our y � 0.65 films at room temperature represent the
magnetic phase-separated system, containing the FM
and paramagnetic (PM) clusters. As we discussed
858 Fizika Nizkikh Temperatur, 2006, v. 32, No. 7
V.G. Prokhorov et al.
before, the formation of this multiphase magnetic
state also can be affected by a nonuniform distribution
of the lattice strain. The description of a magnetoresis-
tance for a such type of the magnetic phase-separated
system can be understood in the framework of the ap-
proach presented by Wagner et al. [34]. According to
this phenomenological model, the activation barrier
(they consider the Mott hopping mechanism of con-
ductivity [31], but it is not fundamental point in our
case) includes two main field-affected terms. First of
them is similar to considered above for the PM state
and reveals E B g H/k TA S B B�
2 ( )� , while the second
one (so-called the Weiss magnetization contribution)
becomes dominant in the FM state and manifests a lin-
ear magnetic-field dependence of the activation energy
E B g H/kA S B B� ( )� . The experimental MR(H)
dependence for y = 0.9 (see the inset in Fig. 6)
was approximated by an expression MR( )H �
� �exp[ ( )]� M/Ms with the number of the single-ion
spins N as a fitting parameter and � � E /k TA B
0 = 5
again. It is seen that the theoretical curve is practically
coincident with the experimental data at N = 1.2.
Therefore, the magnetotransport properties of the
La07. (Ca1�ySr y)03. MnO3 films at room temperature
can be excellently described within the framework of
a field-affected activation energy approximation, tak-
ing into account a competition between the spin-de-
pendent trapping of charges in PM state and the
Weiss-magnetization contribution in FM one.
Conclusions
We have studied the magnetic and transport prop-
erties of La07. (Ca1�ySr y)03. MnO3 films as-deposited
on LaAlO3 (001) single crystal substrates.
Microstructure analysis reveals that the films at
0 3. � �y 0.5 are phase-separated into nanoscale clus-
ters with the orthorhombic and rhombohedral crystal
structure at room temperature. The observed cluster-
ing accompanied by the two-stage magnetic and elec-
tronic transition in the films. It was shown that for
the 0.5 � �y 0 films a nonlinear (almost parabolic)
MR(H) dependence is typical, while for the
0 65 10. .� �y films a linear MR(H) behavior is ob-
served at 300 K. The magnetotransport properties of
the La07. (Ca1�ySr y)03. MnO3 films at room tempera-
ture can be explained on the base of a field-dependent
activation energy model, considering simultaneously
the spin-dependent trapping of charges in PM state
and the Weiss-magnetization contribution at the
FM ordering. Using the experimental data, the mag-
netic phase diagram was constructed for the
La (Ca Sr ) MnO0.7 0.3 31�y y thin-film system.
This work was supported by the KOSEF through
the Quantum Photonic Science Research Center.
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|
| id | nasplib_isofts_kiev_ua-123456789-120220 |
| institution | Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| issn | 0132-6414 |
| language | English |
| last_indexed | 2025-12-07T16:22:49Z |
| publishDate | 2006 |
| publisher | Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
| record_format | dspace |
| spelling | Prokhorov, V.G. Komashko, V.A. Kaminsky, G.G. Lee, Y.P. Hyun, Y.H. Yu, K.K. Park, J.S. Svetchnikov, V.L. 2017-06-11T13:05:27Z 2017-06-11T13:05:27Z 2006 Magnetic and electronic phase separation driven by structural clustering in La₀.₇ (Ca₁₋ySry)₀.₃ MnO₃ thin films / V.G. Prokhorov, V.A. Komashko, G.G. Kaminsky, Y.P. Lee, Y.H. Hyun, K.K. Yu, J.S. Park, V.L. Svetchnikov // Физика низких температур. — 2006. — Т. 32, № 7. — С. 853–860. — Бібліогр.: 34 назв. — англ. 0132-6414 PACS: 75.70.–i, 75.47.–m, 71.30.+h https://nasplib.isofts.kiev.ua/handle/123456789/120220 The structural, magnetic and transport properties of La₀.₇ (Ca₁₋ySry)₀.₃ MnO₃ films deposited on LaAlO₃ (001) single-crystalline substrate by rf-magnetron sputtering using «soft» (or powder) targets are investigated. It was found that at 0.3 ≤ y ≤0.5 both rhombohedral (R c3 ) and orthorhombic (Pnma) crystal phases are coexistent at room temperature, forming a nanoclustered microstructure. The clustered films manifest the two-stage magnetic and electronic transition, which are typical for the phase-separated systems. It was shown that for 0.5 ≥ y ≥ 0 the nonlinear (almost parabolic) field-dependent magnetoresistance is typical at room temperature while for 0.65 ≤y ≤1.0 its transform to the linear behavior. The magnetotransport properties of the films are explained within the framework of a field-dependent activation energy model. The magnetic phase diagram for the La₀.₇ (Ca₁₋ySry)₀.₃ MnO₃ thin-film system is presented. This work was supported by the KOSEF through the Quantum Photonic Science Research Center. en Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України Физика низких температур Низкотемпеpатуpный магнетизм Magnetic and electronic phase separation driven by structural clustering in La₀.₇ (Ca₁₋ySry)₀.₃ MnO₃ thin films Article published earlier |
| spellingShingle | Magnetic and electronic phase separation driven by structural clustering in La₀.₇ (Ca₁₋ySry)₀.₃ MnO₃ thin films Prokhorov, V.G. Komashko, V.A. Kaminsky, G.G. Lee, Y.P. Hyun, Y.H. Yu, K.K. Park, J.S. Svetchnikov, V.L. Низкотемпеpатуpный магнетизм |
| title | Magnetic and electronic phase separation driven by structural clustering in La₀.₇ (Ca₁₋ySry)₀.₃ MnO₃ thin films |
| title_full | Magnetic and electronic phase separation driven by structural clustering in La₀.₇ (Ca₁₋ySry)₀.₃ MnO₃ thin films |
| title_fullStr | Magnetic and electronic phase separation driven by structural clustering in La₀.₇ (Ca₁₋ySry)₀.₃ MnO₃ thin films |
| title_full_unstemmed | Magnetic and electronic phase separation driven by structural clustering in La₀.₇ (Ca₁₋ySry)₀.₃ MnO₃ thin films |
| title_short | Magnetic and electronic phase separation driven by structural clustering in La₀.₇ (Ca₁₋ySry)₀.₃ MnO₃ thin films |
| title_sort | magnetic and electronic phase separation driven by structural clustering in la₀.₇ (ca₁₋ysry)₀.₃ mno₃ thin films |
| topic | Низкотемпеpатуpный магнетизм |
| topic_facet | Низкотемпеpатуpный магнетизм |
| url | https://nasplib.isofts.kiev.ua/handle/123456789/120220 |
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