Anomalous transport effects for the metal and insulator doped La 0.833K0.167MnO3 systems

Article abstract of DOI:10.1016/j.jmmm.2012.02.117

The La_0.833K_0.167MnO₃:Ag₂O and the La_0.833K_0.167MnO₃:SrTiO₃ samples are fabricated by the solgel method. The microstructure, magnetic and transportation properties have been systematically studied. X-ray diffraction patterns show that the La_0.833K_0.167MnO ₃:Ag₂O (abbreviated as LKMO/Ag) sample is a two-phase composite and consists of a magnetic La_0.833K_0.167MnO ₃ (abbreviated as LKMO) perovskite phase and a nonmagnetic Ag metal phase, while the structure of the La_0.833K_0.167MnO ₃:SrTiO₃ (abbreviated as LKMO/STO) sample is a homogeneous solid solution phase. Comparing with the pure LKMO sample, the room temperature magnetoresistance (MR) effect for the LKMO/Ag sample is enhanced significantly due to the addition of Ag metal. The MR ratio increases from ～25% for the pure LKMO sample to 65% for the LKMO/Ag sample under a higher field of 5.5 T at 300 K. For the LKMO/STO sample, however, the room temperature MR effect is weakened dramatically and is almost close to zero due to the addition of SrTiO₃ insulator. In the low temperature regime below the Curie temperature, the MR behaviors are different from that of the room temperature; that is, the MR effect is decreased for the LKMO/Ag sample and increased for the LKMO/STO sample with temperature decrease. In fact, the low-field (μ₀H=0.5 T) MR decreases from 32% to 5% for the LKMO/Ag sample, while increasing from 0.07% to 25% for the LKMO/STO sample with decreasing temperature from 300 to 4 K. The relative change between the intrinsic and the extrinsic MR, and varied roles of the spin-polarized-tunneling and the spin-dependent scattering mechanisms in different temperature regimes are employed to interpret the anomalous transport behaviors.

Full text of DOI:10.1016/j.jmmm.2012.02.117

Journal of Magnetism and Magnetic Materials 324 (2012) 2183–2187
Contents lists available at SciVerse ScienceDirect
Journal of Magnetism and Magnetic Materials
journal homepage: www.elsevier.com/locate/jmmm
Anomalous transport effects for the metal and insulator doped
La0.833K0.167MnO3 systems
Wu Jian n
College of Sciences, Hohai University, Nanjing 210098, China
a r t i c l e i n f o
a b s t r a c t
Article history:
The La0.833K0.167MnO3:Ag2O and the La0.833K0.167MnO3:SrTiO3 samples are fabricated by the sol–gel
method. The microstructure, magnetic and transportation properties have been systematically studied.
X-ray diffraction patterns show that the La0.833K0.167MnO3:Ag2O (abbreviated as LKMO/Ag) sample is a
two-phase composite and consists of a magnetic La0.833K0.167MnO3 (abbreviated as LKMO) perovskite
phase and a nonmagnetic Ag metal phase, while the structure of the La0.833K0.167MnO3:SrTiO3
(abbreviated as LKMO/STO) sample is a homogeneous solid solution phase. Comparing with the pure
LKMO sample, the room temperature magnetoresistance (MR) effect for the LKMO/Ag sample is
enhanced significantly due to the addition of Ag metal. The MR ratio increases from ꢀ25% for the pure
LKMO sample to 65% for the LKMO/Ag sample under a higher field of 5.5 T at 300 K. For the LKMO/STO
sample, however, the room temperature MR effect is weakened dramatically and is almost close to zero
due to the addition of SrTiO3 insulator. In the low temperature regime below the Curie temperature, the
MR behaviors are different from that of the room temperature; that is, the MR effect is decreased for the
LKMO/Ag sample and increased for the LKMO/STO sample with temperature decrease. In fact, the low-
Received 23 November 2011
Received in revised form
5 February 2012
Available online 3 March 2012
Keywords:
Magnetoresistance
Spin-polarized tunneling
Spin-dependent scattering
field (
m0H¼0.5 T) MR decreases from 32% to 5% for the LKMO/Ag sample, while increasing from 0.07% to
25% for the LKMO/STO sample with decreasing temperature from 300 to 4 K. The relative change
between the intrinsic and the extrinsic MR, and varied roles of the spin-polarized-tunneling and the
spin-dependent scattering mechanisms in different temperature regimes are employed to interpret the
anomalous transport behaviors.
& 2012 Elsevier B.V. All rights reserved.
1. Introduction
temperature below TC. The low field MR effect has drawn wide-
spread attention because not only it has application prospect in
The perovskite-type compounds have been extensively inves-
tigated due to the colossal magnetoresistance (CMR) effect and a
variety of transport and magnetic properties that sensitively
practical spintronic devices, but also the physical mechanism is
still to be more investigated. In previous literatures, several models
have been proposed for their theoretical explanation, and some of
them are discussed controversially. Hwang et al. [9] propose a
model based on spin-polarized tunneling between ferromagnetic
grains through an insulator. Gupta et al. [10] report that the
switching of magnetic domains in the grains and disorder-induced
canting of Mn spins in the grain boundary region play an important
role in low field MR. Ziese et al. [11] suggest a description of the
transport characteristic of the grain boundary based on tunneling
via magnetically ordered states in the barrier. Guinea et al. [12]
propose that probably tunneling via paramagnetic impurity states
in the grain boundary barrier is also an important factor.
Srinitiwarawong et al. [13] point out that the suppression of
magnetic frustration contributes to the MR effect. Pin et al. [14]
show the variation of the electronic spin polarization and the
inelastic intergrain tunneling induced by the collective excitations
of local spins at the grain boundaries are simultaneously respon-
sible for the rapid decay of the low field MR ratio with increasing
temperature.
depend on the doping ions and contents [1–4]. For perovskite
3þ
manganites R1ꢁxAx(Mn
Mn4x þ)O3, in general, the mechanism of
1ꢁx
the magnetoresistance (MR) effects is divided into two classes,
namely the intrinsic and extrinsic MR [5,6]. The former is referred
to intragrain MR, which has a maximum near the Curie tempera-
ture (TC), and is well explained by the combination of the double
exchange model [7] and the electron–phonon interaction [8]. The
latter is intergrain MR observed over a wider temperature range
below TC and is characteristic of a large low field MR. As found by
Hwang [9] and Gupta [10], the manganite single crystals or
epitaxy-grown films do not exhibit MR effect, but the manganite
polycrystalline ceramics or films exhibit apparent MR effect of
ꢀ20% in the magnetic field of several hundred Gauss at the low
n Tel.: þ86 25 13951604368; fax: þ86 25 83786672.
E-mail address: jwucnwl@yahoo.com
0304-8853/$ - see front matter & 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmmm.2012.02.117

2184
W. Jian / Journal of Magnetism and Magnetic Materials 324 (2012) 2183–2187
In order to get a clearer picture of the mechanism of the grain
boundary MR effect, we produce the La0.833K0.167MnO3:SrTiO3
(abbreviated to LKMO/STO) and the La0.833K0.167MnO3:Ag2O
(abbreviated to LKMO/Ag) systems. The Ag2O oxide is decom-
posed at 300–400 1C, and the Ag metal will be mixed with
La0.833K0.167MnO3 (abbreviated to LKMO), leading to a metal
doped system. The Ag metal does not interact with the perovskite
phase, which has been reported by other research groups [15–17],
and is known to segregate among the grain boundaries. The
SrTiO3 insulator, however, is easily interacted with the LKMO
system due to the fact that they are both the similar perovskite
structures. When the Ag metal and the SrTiO3 insulator are doped
into the perovskite manganites respectively, they result in a great
change of the barriers of grain boundaries among the LKMO
manganites. The SrTiO3 insulator will increase the potential of the
barriers, while the Ag metal contrarily decreasing it.
2. Experimental
Fig. 1. XRD patterns for LKMO, LKMO/Ag and LKMO/STO.
Polycrystalline LKMO powders are prepared by the sol–gel
method. An appropriate amount of citric acid (as a complex agent)
and glycol (as a dispersing agent) are added into a solution which is
previously obtained by dissolving the high-purity oxides La2O3,
K2CO3 and Mn(NO3)2 in dilute nitric acid. The gel is decomposed
slowly at 200 1C in air. The precursor is decarbonated at 700 1C for
6 h, and the powder obtained is ground, pelletized, and sintered
at 1100 1C for 10 h to form stable LKMO phase. For the LKMO/Ag
sample, the Ag2O powders are added directly according to the
appropriate ratio to the LKMO powders (The molar ratio of Ag2O:
LKMO is 0.25:1). Then the mixtures are ground carefully, pressed
into disks with a radius of 10 mm and a thickness of 5–6 mm and
sintered at 1200 1C for 10 h. In order to get the LKMO/STO sample,
the LKMO powders are put into SrTiO3 precursive solution that is
prepared by the sol–gel method, and mixed together for about 3 h.
The mixtures are filtered to removal the redundant SrTiO3 pre-
cursive solution (The molar ratio of SrTiO3: LKMO is 0.13:1, which
can be calculated from the weight loss of precursive solution.).
After this, the mixtures are dried in the furnace, pressed into disks
with a radius of 10 mm and a thickness of 5–6 mm and sintered at
1200 1C for 10 h.
single peak and indicated by the inversed triangle symbol (.) in
Fig. 1. The lattice parameters, refined by the Rietveld method, are
a¼0.5467(3) nm and c¼1.3349(8) nm for the LKMO sample and
slight less than that of the LKMO/STO sample with a¼0.5490(2) nm
and c¼1.3391(3) nm. This phenomenon can be attributed to the
following two reasons. On one hand, we know that LKMO crystal-
lites are enclosed by the precursive solution of SrTiO3, which can be
seen from the producing process of the LKMO/STO sample. When
they are dried and sintered, the thin films of SrTiO3 formed on the
surface of LKMO crystallites make potassium atoms be evaporated
difficultly during the sintering process. The Kþ ion in /AS site has
larger radius (0.133 nm) than La3þ (0.122 nm), therefore, resulting
in the LKMO/STO sample having larger lattice constants than the
LKMO sample in which some potassium atoms are evaporated. On
the other hand, the effect of substitution of Ti4þ for Mn4þ in the
La0.7Sr0.3Mn1ꢁxTixO3 system has been researched by Kallel et al.
[18]. They find that the Ti4þ ions can enter into the lattice of
perovskite and form homogeneous solid solutions after sintering at
1200 1C. As for our LKMO/STO sample, the Sr2þ and Ti4þ ions will
also enter into the LKMO perovskite structure, diffuse into grains of
LKMO along the interface gradually, and replace the Mn4þ ions in
the /BS sites, forming a homogeneous solid solution phase in the
sintering process with temperature as high as 1200 1C. The radius of
Ti4þ (0.064 nm) is larger than Mn4þ (0.052 nm), so the LKMO/STO
sample has larger lattice constants than the LKMO sample.
The structure of the sample is characterized by x-ray diffrac-
tion (XRD) using Cu K radiation and measured at room tempera-
a
ture. Resistivity is measured by the standard four-probe
technique. The MR ratio is defined as
D
r0
rH r0
ꢁ
rH
¼
,
ð1Þ
r0
The non-magnetic Ti4þ ions that enter into the lattice of the
LKMO sample and replace the Mn ions in the /BS sites reduce the
exchange interaction between Mn3þ and Mn4þ, herewith TC,
which can be proved by the ac susceptibility versus temperature
curves in Fig. 2. The TC that is defined as the inflexion point of the
where
r
0 and
rH are the resistivities in the zero and applied field H,
respectively. Resistivity is measured by a standard dc four-probe
technique with the electrical current parallel to the magnetic field in
a physical property measurement system (PPMS, quantum design).
The magnetic measurements are carried out with a vibration sample
magnetometer (Lakeshore Cryotronics, Inc.).
w
–T curve are 277, 287 and 142 K for the LKMO, LKMO/Ag, and
LKMO/STO samples, respectively. We can see that the TC of the
LKMO/Ag sample is slightly increased in comparison with the
pure LKMO sample. The similar result has also been reported by
Joly et al. [19], and is attributed to Ag2O releasing oxygen. The
LKMO is oxygenated by the additional oxygen incorporating in
the structure, which increases Mn4þ content and reducing the
Mn3þ/Mn4þ ratio. The TC of the LKMO/STO sample, however, is
dramatically lower than that of the pure LKMO sample. In
consideration of the fact that the doped SrTiO3 insulator has the
similar structure with the LKMO matrix and is easy of entering
into the surface structure of the LKMO grains, we believe that the
final LKMO/STO sample is a system with the inner pure LKMO
grains wrapped by a LKMO/STO surface layer. The magnetic
3. Results
The XRD patterns for the LKMO, LKMO/Ag and LKMO/STO
samples at 300 K are shown in Fig. 1, respectively. From the figure,
we can see that the LKMO and LKMO/STO samples are homoge-
neous rhombohedral perovskite structures (space group R3c) which
show double peaks and indicated by the diamond symbol (~) in
Fig. 1. The LKMO/Ag sample is inhomogeneous system containing an
Ag metal phase which is indicated by the circle symbol (
orthorhombic perovskite phase (space group Pbnm) which shows
ꢂ
) and an

W. Jian / Journal of Magnetism and Magnetic Materials 324 (2012) 2183–2187
2185
curves. The MR of the LKMO sample is 25%–40% in the whole
temperature range. For the LKMO/Ag sample, the MR effect is the
3þ
same as that of the common perovskite of R1ꢁxAx(Mn
1ꢁx
Mn4x þ)O3, which has a peak with a value of 65% near TC. The MR
effect of the LKMO/STO sample is completely different from the
LKMO/Ag sample’s behavior, which increases from 0% to 65% with
temperature decreasing from 300 to 4.2 K.
The normalized resistivities as a function of magnetic field at
300 and 4.2 K are shown in Figs. 5 and 6. As shown in Fig. 5, the
magnitude of the MR for the LKMO/STO sample is almost negligible
at 300 K, however, it is more than 32% for the LKMO/Ag sample and
is higher than that of the pure LKMO sample (about 7%) under the
magnetic field 0.5 T. It can be seen from Fig. 6 that the LKMO/STO
sample shows a sharp drop of the resistivity at 4.2 K under low
fields (o0.1 T), followed by a linear decrease of resistivity with
further increase of the field. The magnitude of the MR for the
LKMO/STO sample is about 25% at 4.2 K, and is slightly higher than
that of the pure LKMO sample (about 23%) under the magnetic field
0.5 T. For the LKMO/Ag sample, the MR effect is dramatically
reduced down to 5% at 4.2 K under the magnetic field of 0.5 T.
Fig. 2. Temperature dependence of the ac magnetic susceptibility for LKMO/STO,
LKMO/Ag and LKMO.
Fig. 4. MR ratios of LKMO, LKMO/Ag and LKMO/STO versus temperature.
Fig. 3. Resistivities (
m0H¼0 and 5.5 T) of LKMO, LKMO/Ag and LKMO/STO versus
temperature.
coupling between the pure LKMO grains is now weakened greatly
by the wrapped folium, resulting in the decrease of TC.
The resistivities as a function of temperature at
m0H¼0 and
5.5 T are shown in Fig. 3. The zero-field resistivities for the LKMO
and the LKMO/Ag samples show an insulator–metal transition
with decreasing temperature, and the resistivity of the LKMO/STO
sample shows the semiconductor behavior over the whole tem-
perature range. In comparison with the pure LKMO sample, the
amplitude of the resistivity of the LKMO/Ag sample decreases by a
factor of 10 and increases by six orders of magnitude for the
LKMO/STO sample. The reduction of resistivity of the LKMO/Ag
sample is due to the fact that Ag melts at 962 1C and Ag atoms or
clusters may distribute among the grain boundaries during the
sintering, which provide another pathway for current flow. For
the LKMO/STO sample, some Ti4þ ions enter into the lattice of
LKMO and replace of the Mn4þ ions in /BS sites in the grain
surface, forming a wrapped insulator folium and resulting in the
rapid increases of resistivity with decreasing temperature.
Under a magnetic field of
and gives a large MR ratio. Fig. 4 shows MR versus temperature
m0H¼5.5 T, the resistivity reduces
Fig. 5. Normalized resistivities of LKMO, LKMO/Ag and LKMO/STO versus magnetic
field at 300 K.

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W. Jian / Journal of Magnetism and Magnetic Materials 324 (2012) 2183–2187
which is exactly a typical feature of spin-polarized tunneling in
granular ferromagnets [20,21]. The second microscopic mechan-
ism for the extrinsic MR effect in polycrystalline films is the spin-
dependent scattering. The concept of spin-dependent scattering is
originally introduced by Zhang and Levy [22] in heterogeneous
granular films with a composition A1ꢁxBx (A¼Co, Fe; B¼Ag, Cu),
in which the spin-dependent scattering within the granules and at
interfaces between the magnetic granules and the non-magnetic
matrix are employed to interpret the giant magnetoresistance
(GMR) effect. Li et al. [10] investigate the magnetotransport
properties of polycrystalline La0.67Ca0.33MnO3 and La0.67Sr0.33
MnO3 films. Based on the typical features in MR–H curves,
r–T
behaviors and the I–V curves of these films, they adopt the point of
view of the spin-dependent scattering. They consider that the
large low-field MR is due to the spin-dependent scattering of
polarized electrons at the grain boundaries and serve as pinning
centers for the domain walls.
For our LKMO sample, we have reason to suppose that the
transport mechanism is concerned with the spin-polarized tun-
neling on the basis of the similarity in preparation procedure for
the La0.67Sr0.33MnO3 ceramic samples by Hwang et al. [9]. For the
LKMO/Ag sample the Ag metal phase exists at grain boundaries of
the perovskite phase and consists of many fine Ag granules
distributed around the grains, leading to the formation of inter-
faces between the perovskite grains and the Ag granules. In
essence these ferromagnet/Ag interfaces are similar to those
between the ferromagnetic Co granules and the Ag matrix in
Co–Ag granular films. Therefore, it is expected that the spin-
dependent scattering will also play an important role in the
LKMO/Ag sample with spin-polarized electrons pass through the
interfaces. Because Ag metal is a good conductor, the existence of
Ag granules at grain boundaries provides another conduction
pathway for conduction electrons, leading to weakening of spin-
polarized tunneling effect. At the same time the spin-dependent
scattering effect becomes enhanced. According to Hwang et al. [9]
the spin-polarized tunneling model presupposes the existence of
an insulating, nonmagnetic barrier. As mentioned above, we
know that the final LKMO/STO sample is a system with the inner
pure LKMO grains wrapped by a LKMO/STO surface layer. The
magnetic coupling between the pure LKMO grains is now wea-
kened greatly by the wrapped folium. In result, the spin-polarized
tunneling effect will enhanced and the spin-dependent scattering
effect will decrease for the LKMO/STO sample.
Fig. 6. Normalized resistivities of LKMO, LKMO/Ag and LKMO/STO versus magnetic
field at 4.2 K.
Fig. 7. Magnetization curves of LKMO, LKMO/Ag and LKMO/STO at 4.2 K.
The magnetization curves of these samples at 4.2 K are shown
in Fig. 7. The magnetizations for all three samples reach 495% of
their saturation values with the magnetic field more than 0.3 T.
In other words, the misaligned spins are dramatically reduced
when temperature is low.
In general, for our LKMO, the LKMO/Ag and the LKMO/STO
systems, we employ two mechanics, one is the intrinsic resistivity
rin and the other is the extrinsic resistivity
transport behaviors. The extrinsic resistivity
r
ex, to explain the
rex, as mentioned
above, contains two contributions from spin-polarized tunneling
and the spin-dependent scattering. The intrinsic resistivity is
referred to intragrain resistance, which has a maximum near TC,
and is well elucidated by the combination of the double exchange
model and the electron–phonon interaction. Therefore, the resis-
tivity of the sample can be simply written as
4. Discussion
In order to explain the extrinsic MR effect in polycrystalline
ceramics, two main microscopic mechanisms have been suggested
by now. The first one is proposed by Hwang et al. [9]. They
investigate differences in the MR–H and M–H behavior at various
temperatures from 5 to 280 K for the La0.67Sr0.33MnO3 polycrystal-
line ceramics and a single crystal with the same composition. They
find that the low-field MR is small in the single crystal, but is large
in polycrystalline ceramics. As for the disordered spin between
grains is absented in single crystal, therefore, the authors suggest
that the large low-field MR in polycrystalline ceramics is arising
from spin-polarized tunneling in misaligned grains through an
insulating, nonmagnetic barrier. Furthermore, they prove their
argument with the fact that the magnitude of the sharp initial
r0
¼
rex
þ
rin
¼
rsds
þ
rspt
þ
rin
,
ð2Þ
where rsds and rspt are the spin-dependent scattering and spin-
polarized tunneling resistivity, respectively. It is known [9] that
the spin-polarized tunneling effect is very small at high tempera-
ture, that is to say, the MR near TC is mainly dominated by the
intrinsic MR and the spin-dependent scattering effect. The MR
ratio can be expressed as
D
r0
rH
D
rsds
þ
Drin
ꢀ
:
ð3Þ
r0
The metal Ag exists near the grain boundaries and serves as
drop in the
r–H curve varies with temperature as [aþb/(Tþc)],
scattering centers, leading to the magnetic spin disorder at the

W. Jian / Journal of Magnetism and Magnetic Materials 324 (2012) 2183–2187
2187
boundaries, therewith, enhancing the spin-dependent scattering
effect for the LKMO/Ag sample, as shown in Fig. 4 and Fig. 5. For
LKMO perovskite lattice. For the LKMO/STO sample, the Sr2þ and
Ti4þ ions enter into the LKMO perovskite structure and form a
homogeneous solid solution. The MR value more than 64% under
the high field of 5.5 T and more than 25% under a lower field of
0.5 T are obtained for the LKMO/Ag sample at 300 K and for the
LKMO/STO sample at 4 K, respectively. They are much larger than
that of the single phase compound of LKMO (6% at 0.5 T and 35%
at 5.5 T). The addition of Ag metal and SrTiO3 insulator cause
different transport behaviors in different temperature regimes.
Addition of Ag metal in the LKMO system enhances the MR effect
at room temperature (300 K), while decreases the MR effect in the
low temperature region (4.2–250 K); on the contrary, addition of
SrTiO3 insulator causes an enhanced MR effect at low tempera-
ture and a weakened MR effect at high temperature. These results
are qualitatively explained by the consideration of the relative
contribution of intrinsic MR effect and the extrinsic MR effect of
spin-polarized tunneling and spin-dependent scattering.
the LKMO/STO sample, the resistivity r0 of the LKMO/STO sample
(23
O
m) near TC is larger than that of LKMO (2.55 ꢃ 10ꢁ4
O m),
hence the MR ratio is reduced. Furthermore, Drin of LKMO/STO is
smaller than that of LKMO because Ti4þ ions substitute for Mn4þ
ions in the LKMO lattices and weaken the double-exchange effect.
Therefore, the MR of the LKMO/STO sample near TC is decreased
and the MR peak disappears, as shown in Fig. 4.
In the low temperature regime (o100 K, especially), the
intrinsic MR, according to the double exchange mechanism, is
small. As shown in Fig. 7, all three samples are in ferromagnetic
phases, the spin moment are aligned paralleled and misaligned
spins are dramatically reduced, resulting in the decrease of the
spin dependent scattering effect. What’s more, the magnetiza-
tions for all three samples tend to their saturation values with the
magnetic field more than 0.3 T. It means that the reduced spin-
dependent scattering MR is close to their saturation under low
magnetic field of 0.3 T. From the MR of the LKMO/Ag sample
under low magnetic field at 4.2 K, as shown in Fig. 6, we can
deduce that the saturated spin-dependent scattering MR is very
small at low temperature. For simplification, we omit the small
saturated spin-dependent scattering effect when we discuss the
MR at low temperature. Note that the MRs of the LKMO and the
LKMO/STO samples are more than 40% below temperature 100 K
under the high magnetic field of 5.5 T (Fig. 4). Excluding the
intrinsic MR and spin dependent scattering effect, the major
mechanism governing the MR in the system below TC is attributed
to the spin-polarized tunneling effect.
Acknowledgment
This work is supported by the Fundamental Research Funds for
the Central Universities under Grant no. 2010B09114.
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For the LKMO/Ag sample, the metal Ag located near the GBs
can reduce the height and the width of the energy barriers with
another pathway for current flow. This can be seen from the fact
that the residual r0 values at 4 K (Fig. 3) are 9.6 ꢃ 10ꢁ5
O m for
the LKMO sample and 2 ꢃ 10ꢁ6 O m for the LKMO/Ag sample.
Therefore, the existence of metal Ag phase reduces the spin-
polarized tunneling effect. For the LKMO/STO sample, the Sr2þ
and Ti4þ ions enter into the magnetic phase of LKMO gradually
and increase the height and the width of the energy barriers
between crystallites. The residual resistivity r0 values is
7.2 ꢃ 102
O m at 4 K for the LKMO/STO sample. The spin-polarized
tunneling effect therefore is enhanced. From Fig. 4, we can see
that the MR under the field of 5.5 T increases from 40% for the
LKMO sample to 65% for the LKMO/STO sample at 4 K.
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5. Conclusions
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In summary, we have investigated the phase structure and the
MR effects in bulk polycrystalline LKMO, LKMO/Ag and LKMO/STO
samples. We find that the LKMO/Ag sample is a two-phase
composite consisting of a ferromagnetic LKMO perovskite phase
and a nonmagnetic Ag metal phase. The Ag metal cannot enter the
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