Peculiarities of the volume magnetostriction in La₁₋xSrxMnO₃ in the Curie point region
The parallel λ∥ and perpendicular λ⊥ magnetostriction (with respect to the applied magnetic field) and the thermal expansion Δl/l are studied on La1−xSrxMnO₃ single crystals with x=0.1, 0.15, and 0.3. For the conducting sample with x=0.3 and the semiconducting sample with x=0.15 the volume magnetost...
Saved in:
| Published in: | Физика низких температур |
|---|---|
| Date: | 2001 |
| Main Authors: | , , , |
| Format: | Article |
| Language: | English |
| Published: |
Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України
2001
|
| Subjects: | |
| Online Access: | https://nasplib.isofts.kiev.ua/handle/123456789/130017 |
| Tags: |
Add Tag
No Tags, Be the first to tag this record!
|
| Journal Title: | Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| Cite this: | Peculiarities of the volume magnetostriction in La₁₋xSrxMnO₃ in the Curie point region / L.I. Koroleva, A.I. Abramovich, R.V. Demin, A.V. Michurin // Физика низких температур. — 2001. — Т. 27, № 4. — С. 398-402. — Бібліогр.: 15 назв. — англ. |
Institution
Digital Library of Periodicals of National Academy of Sciences of Ukraine| id |
nasplib_isofts_kiev_ua-123456789-130017 |
|---|---|
| record_format |
dspace |
| spelling |
Koroleva, L.I. Abramovich, A.I. Demin, R.V. Michurin, A.V. 2018-02-04T16:44:17Z 2018-02-04T16:44:17Z 2001 Peculiarities of the volume magnetostriction in La₁₋xSrxMnO₃ in the Curie point region / L.I. Koroleva, A.I. Abramovich, R.V. Demin, A.V. Michurin // Физика низких температур. — 2001. — Т. 27, № 4. — С. 398-402. — Бібліогр.: 15 назв. — англ. 0132-6414 PACS: 75.80.+q, 75.30.Kz, 75.40.-s, 74.72.-h https://nasplib.isofts.kiev.ua/handle/123456789/130017 The parallel λ∥ and perpendicular λ⊥ magnetostriction (with respect to the applied magnetic field) and the thermal expansion Δl/l are studied on La1−xSrxMnO₃ single crystals with x=0.1, 0.15, and 0.3. For the conducting sample with x=0.3 and the semiconducting sample with x=0.15 the volume magnetostriction (ω=λ∥+2λ⊥) is negative and the |ω|(T) curves go through a maximum at the Curie point TC. At T>TC its Δl/l temperature dependence is stronger than linear. For the semiconducting sample with x=0.1, ω is negative at T<TC and |ω|→0 at T∼TC. Its Δl/l is linear at T<=TC. The behavior of ω and Δl/l are explained by a magnetic two-phase state, due to strong s−d exchange. This paper was supported by grants from INTAS, #97-0253, NATO #HTECH LG 972942, and the Russian Basic Investigations Foundation #00-15- 96695, #00-02-17810. We are grateful by A.M. Balbashov for preparation of the samples and their analysis. en Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України Физика низких температур Специальный выпуск Низкотемпеpатуpная магнитостpикция магнетиков и свеpхпpоводников Под редакцией В. В. Еременко и В. А. Сиренко Peculiarities of the volume magnetostriction in La₁₋xSrxMnO₃ in the Curie point region Article published earlier |
| institution |
Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| collection |
DSpace DC |
| title |
Peculiarities of the volume magnetostriction in La₁₋xSrxMnO₃ in the Curie point region |
| spellingShingle |
Peculiarities of the volume magnetostriction in La₁₋xSrxMnO₃ in the Curie point region Koroleva, L.I. Abramovich, A.I. Demin, R.V. Michurin, A.V. Специальный выпуск Низкотемпеpатуpная магнитостpикция магнетиков и свеpхпpоводников Под редакцией В. В. Еременко и В. А. Сиренко |
| title_short |
Peculiarities of the volume magnetostriction in La₁₋xSrxMnO₃ in the Curie point region |
| title_full |
Peculiarities of the volume magnetostriction in La₁₋xSrxMnO₃ in the Curie point region |
| title_fullStr |
Peculiarities of the volume magnetostriction in La₁₋xSrxMnO₃ in the Curie point region |
| title_full_unstemmed |
Peculiarities of the volume magnetostriction in La₁₋xSrxMnO₃ in the Curie point region |
| title_sort |
peculiarities of the volume magnetostriction in la₁₋xsrxmno₃ in the curie point region |
| author |
Koroleva, L.I. Abramovich, A.I. Demin, R.V. Michurin, A.V. |
| author_facet |
Koroleva, L.I. Abramovich, A.I. Demin, R.V. Michurin, A.V. |
| topic |
Специальный выпуск Низкотемпеpатуpная магнитостpикция магнетиков и свеpхпpоводников Под редакцией В. В. Еременко и В. А. Сиренко |
| topic_facet |
Специальный выпуск Низкотемпеpатуpная магнитостpикция магнетиков и свеpхпpоводников Под редакцией В. В. Еременко и В. А. Сиренко |
| publishDate |
2001 |
| language |
English |
| container_title |
Физика низких температур |
| publisher |
Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
| format |
Article |
| description |
The parallel λ∥ and perpendicular λ⊥ magnetostriction (with respect to the applied magnetic field) and the thermal expansion Δl/l are studied on La1−xSrxMnO₃ single crystals with x=0.1, 0.15, and 0.3. For the conducting sample with x=0.3 and the semiconducting sample with x=0.15 the volume magnetostriction (ω=λ∥+2λ⊥) is negative and the |ω|(T) curves go through a maximum at the Curie point TC. At T>TC its Δl/l temperature dependence is stronger than linear. For the semiconducting sample with x=0.1, ω is negative at T<TC and |ω|→0 at T∼TC. Its Δl/l is linear at T<=TC. The behavior of ω and Δl/l are explained by a magnetic two-phase state, due to strong s−d exchange.
|
| issn |
0132-6414 |
| url |
https://nasplib.isofts.kiev.ua/handle/123456789/130017 |
| citation_txt |
Peculiarities of the volume magnetostriction in La₁₋xSrxMnO₃ in the Curie point region / L.I. Koroleva, A.I. Abramovich, R.V. Demin, A.V. Michurin // Физика низких температур. — 2001. — Т. 27, № 4. — С. 398-402. — Бібліогр.: 15 назв. — англ. |
| work_keys_str_mv |
AT korolevali peculiaritiesofthevolumemagnetostrictioninla1xsrxmno3inthecuriepointregion AT abramovichai peculiaritiesofthevolumemagnetostrictioninla1xsrxmno3inthecuriepointregion AT deminrv peculiaritiesofthevolumemagnetostrictioninla1xsrxmno3inthecuriepointregion AT michurinav peculiaritiesofthevolumemagnetostrictioninla1xsrxmno3inthecuriepointregion |
| first_indexed |
2025-11-27T08:36:32Z |
| last_indexed |
2025-11-27T08:36:32Z |
| _version_ |
1850806214676447232 |
| fulltext |
Fizika Nizkikh Temperatur, 2001, v. 27, No. 4, p. 398–402 Kor oleva L. I., Ab ra movich A . I., Demin R. V., a nd Mich urin A . V.Peculiar it ie s o f the volum e mag net ostr ic tion in La1– xSr xMnO3 in th e Cu rie point r eg io nKoro le va L. I. , Ab ram ovich A. I. , De min R. V., an d Michu rin A. V .Pe culiarities of t he volume m ag neto strict io n in La 1–xSrxMnO3 in the Cur ie point r egion
Peculiarities of the volume magnetostriction in
La1–xSrxMnO3 in the Curie point region
L. I. Koroleva, A. I. Abramovich, R. V. Demin, and A. V. Michurin
M. V. Lomonosov Moscow State University, Vorobyevy Gory, Moscow 119899, Russia
E-mail: koroleva@ofef343.phys.msu.su
Received September 25, 2000
The parallel λ|| and perpendicular λ⊥ magnetostriction with respect to the applied magnetic field and
the thermal expansion ∆l/l are studied on La1−xSrxMnO3 single crystals with x = 0.1, 0.15, and 0.3. For
the conducting sample with x = 0.3 and the semiconducting sample with x = 0.15 the volume magneto-
striction (ω = λ||
+ 2λ⊥) is negative and the |ω|(T) curves go through a maximum at the Curie point T
C
.
At T > TC its ∆l/l temperature dependence is stronger than the linear one. For the semiconducting
sample with x = 0.1 ω is negative at T < TC and |ω| → 0 at T ∼ TC . Its ∆l/l is linear at T ≥ TC . The
behavior of ω and ∆l/l are explained by a magnetic two-phase state, due to strong s–d exchange.
PACS: 75.80.+q, 75.30.Kz, 75.40.–s, 74.72.–h
Introduction
At present the perovskite Mn oxides R1−xAxMnO3
(R = La, Pr, Y, Nd, Sm and other rare-earth ele-
ments; A = Ca, Sr and Ba) are intensively studied.
These investigations are described in a great number
of papers and reviews [1–4]. The interest in these
materials is associated with a colossal magnetore-
sistance near room temperature which has been
observed at certain doping levels. Unfortunately,
there is no common point of view on the physical
processes leading to colossal magnetoresistance in
manganites. Attempts have been made to relate this
observed colossal magnetoresistance to the Zener
double exchange, to the polaron effect caused by
a very strong electron–phonon coupling stemming
from a Jahn–Teller splitting of the Mn3+ ions, and
to the charge ordering. However, the calculations
performed in Refs. 5, 6 have shown that double
exchange alone cannot account for the very large
resistivity of the T > TC phase or for the sharp drop
in resistivity just below TC . In addition, the calcu-
lated resistivity has a too weak doping dependence
and incorrect behavior for T > TC or in a field.
Millis and co-workers [6,7] proposed to combine
the physics of dynamic Jahn–Teller and double-ex-
change effects to explain the anomalies of the elec-
trical resistivity ρ and colossal magnetoresistance in
these compounds. However, this hypothesis cannot
explain the fact that the temperatures of the metal–
semiconductor transition and of the region of maxi-
mum colossal magnetoresistance are focused in the
immediate vicinity of the Curie point.
In this paper we propose another mechanism for
the explanation of anomalies of the electrical resis-
tivity, colossal magnetoresistance, volume magne-
tostriction, and thermal expansion of La1−xSrxMnO3
compounds in the TC region. Namely, we believe
that the magnetic phase separation that is charac-
teristic for magnetic semiconductors [8] is respon-
sible for these features.
Experimental procedure
We studied the parallel λ|| and perpendicular λ⊥
magnetostriction with respect to the applied mag-
netic field, the thermal expansion ∆l/l, the mag-
netization, and the paramagnetic susceptibility of
La1−xSrxMnO3 single crystals.
Single crystals were grown by the method of
floating-zone method by Balbashov and have a
rhombohedral structure (the sample with x = 0.3)
or an orthorhombic structure (the samples with
x = 0.1 and 0.15). The magnetization measure-
ments, carried out with the aid of a vibrating
magnetometer, showed that the magnetization
reaches saturation at a magnetic field H < 0.2 T.
The Curie points were determined by the Belov–
Arrot method and practically coincide with the
published data [9]. The temperature dependence of
© L. I. Koroleva, A. I. Abramovich, R. V. Demin, and A. V. Michurin, 2001
the paramagnetic susceptibility, measured by the
Faraday balance method, is described by the Curie–
Weiss law. The strain gauge technique was used for
study of the magnetostriction and thermal expan-
sion. The magnetostriction was measured in a dc
magnetic field up to H ≤ 1 T and for the sample
with x = 0.15 up to H = 12 T in the laboratory of
M. R. Ibarra (University of Zaragoza, Spain). The
accuracy of the ∆l/l measurements was better than
4⋅10−6.
Results and discussion
By way of example, Fig. 1 shows the isotherms
of the parallel and perpendicular magnetostriction
with respect to the applied magnetic field at some
selected temperatures for the sample with x = 0.3.
From the experimental λ||(H) and λ⊥(H) curves the
isotherms of the anisotropic magnetostriction λt =
= λ|| − λ⊥ and volume magnetostriction ω = λ|| + 2λ⊥
were constructed. Their temperature dependence for
the sample with x = 0.3 at H = 0.9 T is shown in
Fig. 2. On increase in temperature the anisotropic
magnetostriction decreases continuously to zero in
the TC region, as can be seen in Fig. 2. The λt(T)
curves, measured in magnetic fields above 0.2 T,
practically coincide with the λt(T) curve shown in
Fig. 2. It should be pointed out that the λt value is
large in the low-temperature region, e.g., λt ≅ 10−4
at 90 K (H = 1 T) and λt ≅ 10−3 at 4.2 K (H = 3 T).
As illustrated in Fig. 2, the volume magnetostric-
tion is positive at T < 280 K; however, it becomes
negative at T > 280 K and its magnitude reaches a
maximum in the vicinity of TC = 371 K. On further
heating |ω| vanishes rapidly. The ω(T) dependence in
some selected magnetic field is shown in the insert
in Fig. 2. As will be seen from Fig. 3, the behavior
of ω for the sample with x = 0.15 is rather like the
one for the sample with x = 0.3. For the sample
with x = 0.15 the anisotropic magnetostriction λt is
positive and its value decreases continuously to zero
in the Curie point region, too (TC = 268 K). The
temperature dependence of the thermal expansion
∆l/l in the TC region is stronger than linear for the
samples with x = 0.15 and 0.3. This is apparent
from Fig. 4, which shows the ∆l/l(T) dependence
for the sample with x = 0.3. It is well known that
this dependence is nearly linear for dia- and para-
magnetic systems.
Fig. 1. The isotherms of the perpendicular (curves 1–4) and
parallel (curves 5–7) magnetostriction λ at different tempera-
tures T, K: 361 (1), 97 (2), 349 (3), 375 (4), 300 (5), 188 (6),
and 96 (7) for the single crystal La0.7Sr0.3MnO3 .
Fig. 2. Temperature dependence of the anisotropic magneto-
striction λt and volume magnetostriction ω in the magnetic field
of 0.9 T for the single crystal La0.7Sr0.3MnO3 . Insert: tempera-
ture dependence of ω in the TC region in some selected magnetic
fields for this sample.
Fig. 3. Temperature dependence of the volume magnetostriction
ω in some selected magnetic fields for the single crystal
La0.85Sr0.15MnO3 .
Peculiarities of the volume magnetostriction in La1–xSrxMnO3 in the Curie point region
Fizika Nizkikh Temperatur, 2001, v. 27, No. 4 399
The positive anisotropic magnetostriction λt of
the sample with x = 0.1 decreases continuously to
zero in the TC region (TC = 162 K), as in the
samples with x = 0.15 and 0.3. However the beha-
vior of volume magnetostrictriction ω of the sample
with x = 0.1 differs from that of the samples with
x = 0.15 and 0.3. Figure 5 shows the temperature
dependence of ω at some selected magnetic fields for
a La0.9Sr0.1MnO3 single crystal. Notice that the
volume magnetostriction of the sample with x = 0.1
is negative. With increasing temperature the values
of |ω| decrease continuously to zero in the TC
region, as can be seen in Fig. 5. As mentioned
above, the ω(T) curves have a minimum in the TC
region for the single crystals La0.7Sr0.3MnO3 (Fig. 2)
and La0.85Sr0.15MnO3 (Fig. 3). Figure 5 shows that
the ω(T) curves have no such minimum for the
sample with x = 0.1. Their temperature dependence
of the thermal expansion ∆l/l is nearly linear at
T ≥ TC as well as at T ≤ TC , and no surplus ther-
mal expansion occurs at T ≥ TC for this sample.
According to data [9,10] for these compositions,
the electrical resistivity ρ of the metallic type is
observed at T < TC , and ρ increases abruptly in the
TC region for the sample with x = 0.3; for the
samples with x = 0.1 and 0.15 a semiconducting
type of conductivity is observed. The transition
from the semiconducting to the metallic type
of conductivity takes place at x ∼ 0.17 in the
La1−xSrxMnO3 system.
Ibarra et al. [11] observed a similar behavior of
the volume magnetostriction and thermal expansion
of a La0.4Y0.07Ca0.33MnO3 ceramic sample. In their
opinion, the anomalies of the thermal expansion and
the volume magnetostriction are due to the forma-
tion of a small polaron at T ≥ TC . As discussed in
the Introduction, this hypothesis fails to explain the
fact that the polaron formation occurs only in the
vicinity of the Curie point.
Anomalies of the volume magnetostriction, ther-
mal expansion, and electrical resistivity, listed
above, may be attributed to the existence of a
magnetic two-phase state (MTPS) in this crystal
[8]. As is well known, in magnetic semiconductors
the charge carrier energy is minimal when the total
ordering in the crystal is ferromagnetic. However,
in nongenerate antiferromagnetic semiconductors
the carrier concentration is small, so they are not
able to modify the state of the entire crystal. None-
theless, these electrons may cause local changes in
the magnetic ordering, creating ferromagnetic mic-
roregions, which provide a gain in the s–d exchange
energy, and stabilize them by autolocalization in-
side them. At a not-too-high density of the charge
carriers an insulating MTPS is realized in the crys-
tal: ferromagnetic small droplets, in which the
charge carriers are localized, are embedded in the
insulating host. On increase in the carrier density,
ferromagnetic droplets begin to make contact with
each other. Thus, percolation of the electron liquid
occurs and another MTPS is formed: the insulating
antiferromagnetic microregions are embedded in a
conducting ferromagnetic host. This is a conducting
MTPS. Yanase and Kasuya showed [12] that inside
a ferromagnetic part of crystal the lattice constants
are reduced. The reason is that in a ferromagnetic
part of crystal the spacing between an impurity ion
and its nearest magnetic ion is shortened to screen
the new charge distribution and to lower the energy
of the ferromagnetic part of crystal by increasing
the overlap between the valence electron shells of
the impurity and the d shells of the nearest mag-
netic ions.
La0.7Sr0.3MnO3 is a heavily doped antiferromag-
netic semiconductor LaMnO3 , in which a conduct-
ing MTPS is realized. The MTPS is destroyed at
T ≥ TC and so an extra contribution in ∆l/l arises.
An applied magnetic field induces magnetization
Fig. 4. Temperature dependence of the thermal expansion ∆l/l
for the single crystal La0.7Sr0.3MnO3 .
Fig. 5. Temperature dependence of the volume magnetostriction
in some selected magnetic fields for the single crystal
La0.9Sr0.1MnO3 .
L. I. Koroleva, A. I. Abramovich, R. V. Demin, and A. V. Michurin
400 Fizika Nizkikh Temperatur, 2001, v. 27, No. 4
near impurities at T > TC , since its action is en-
hanced by the s–d exchange. One produces MTPS
and the lattice compression inherent in it. The sharp
increase in the negative volume magnetostriction at
the TC region (Fig. 2) can be explained by this
effect. However, the above-mentioned process of
MTPS restoration by a field takes place only in a
limited temperature interval at T ≥ TC . Because of
this the ω(T) curves have a sharp minimum in the
TC region, and |ω| quickly falls upon further in-
crease in temperature. The MTPS in the sample
with x = 0.3 is confirmed by the fact that the value
of its spontaneous magnetization at 4.2 K is less
than the value expected in the case of a total
ferromagnetic ordering. Namely, the former is equal
to 95% of the latter (our data agree with those
found in Refs. 9, 10) and a value of 84% is obtained
by neutron experiments [13]. This indicates that the
ratio between the volumes of the ferromagnetic and
antiferromagnetic portions of the crystal is
∼ 90/10. In this case the TC value is determined by
the ferromagnetic portion of the crystal only. It is
well known that the paramagnetic Curie point Θ is
determined by the sum of the exchange interactions
realized in the crystal. The contribution from the
antiferromagnetic microregions to the total ex-
change lowers the Θ value and, therefore, TC = 371 K
exceeds Θ = 364 K in the sample with x = 0.3 (in
ferromagnetic ordering TC ≤ Θ is normally ob-
served).
It should be remarked that the volume magneto-
striction of the sample with x = 0.1 is negative (Fig. 5).
By this we mean that the sample shrinks in an
applied magnetic field. It is known [9] that this
sample is a p-type semiconductor. There is a small
maximum of the electrical resistivity ρ and a nega-
tive colossal magnetoresistance in the TC region for
this crystal [9,10]. Thus ρ and the magnetoresist-
ance anomalies are attributable to an insulating
MTPS [8]. If an insulating MTPS is present in this
sample, the negative ω denotes that the radii of the
ferromagnetic droplets are increased by a magnetic
field. This is characteristic of an insulating MTPS
[8]. At the same time, the ferromagnetic phase in an
insulating MTPS sample occupies as little as a few
per cent of the sample volume [8]. Therefore the
volume of the ferromagnetic part is small in the
sample with x = 0.1, and the anomalies of ω and
∆l/l are not detected at the TC region. In the
conducting MTPS sample with x = 0.3 the ferro-
magnetic phase occupies ∼ 90% of the sample volu-
me, and the destruction of MTPS in the TC region
may have a marked effect on ω and ∆l/l. The
sample with x = 0.15 is situated on the boundary
between the semiconducting and metallic states [9].
Therefore the volume of its ferromagnetic phase is
larger than in the sample with x = 0.1, and the
anomalies of ω and ∆l/l at T ∼ TC are observed in
this sample.
In connection with the aforesaid we may be make
the following supposition. It is well known that the
crystal volume per manganese ion is higher in the
orthorhombic than in the rhombohedral structure of
this system. Recently it has been found that in the
semiconducting compound with x = 0.17 of the
system considered, a transition from the orthorhom-
bic to the rhombohedral phase occurs in an applied
magnetic field at T ≤ TC [14,15]. This can be ex-
plained by the increase of the volume of the ferro-
magnetic phase in an applied magnetic field, accom-
panied by the lattice compression.
It is known that the compounds of this system
with x ≤ 0.17 have the orthorhombic structure and
the semiconductive type of conductivity, while the
compounds with 0.175 ≤ x ≤ 0.6 have the rhom-
bohedral structure and the metallic type of conduc-
tivity [9,10]. On this basis it is reasonable to expect
that the transition from the semiconductive ortho-
rhombic phase to the metallic rhombohedral phase
in this system is caused by the transition from the
insulating MTPS to the conducting MTPS, which is
accompanied by lattice compression of the ferro-
magnetic phase, occupied the nearly all volume of
crystal.
Summary
It has been found that for La1−xSrxMnO3 single
crystals with x = 0.15 and 0.3 the volume magneto-
striction ω is negative, the |ω|(T) curves go through
a maximum at the Curie point TC , and the tem-
perature dependence of the thermal expansion
∆l/l(T) at T > TC is stronger than linear. For the
sample with x = 0.1 ω is negative at T < TC ,
|ω| → 0 at T ∼ TC , and ∆l/l(T) is linear at T ≥ TC .
The behavior of ω, ∆l/l, ρ, and the colossal magne-
toresistance are explained by a magnetic two-phase
state, due to strong s–d exchange.
In antiferromagnetic semiconductors the charge
carriers self-trap near impurities and produce ferro-
magnetic microregions because of energy gain in
respect to the s–d exchange [8]. At a not-too-high
density of the charge carriers an insulating MTPS is
realized in the crystal: ferromagnetic small droplets,
in which the charge carriers are localized, are em-
bedded in an insulating host. On increase in the
carrier density, percolation of the electron liquid
occurs and a conducting MTPS is formed: the
insulating antiferromafnetic microregions are em-
Peculiarities of the volume magnetostriction in La1–xSrxMnO3 in the Curie point region
Fizika Nizkikh Temperatur, 2001, v. 27, No. 4 401
bedded in a conducting ferromagnetic host. Yanase
and Kasuya showed [12] that inside a ferromagnetic
part of the crystal the lattice constants are reduced.
La1−xSrxMnO3 is a doped antiferromagnetic se-
miconductor LaMnO3 . Let us suppose that a con-
ducting MTPS is realized in the sample with x = 0.3.
MTPS in this case is destroyed at T ≥ TC , and thus
an extra contribution to ∆l/l arises. An applied
magnetic field induces magnetization near impu-
rities at T > TC , since its action is enhanced by the
s–d exchange. A field produces MTPS and the
corresponding lattice compression. It follows that
ω is negative in the TC region and that a minimum
on the ω(T) curves is observed at this region.
The sharp increase in the electrical resistivity in
the TC region is characteristic of a conducting
MTPS [8]. There are two mechanisms through
which the impurity–magnetic interaction influences
the resistance: the scattering of charge carriers,
which reduces their mobility; the formation of band
tails, consisting of the localized states. The decrease
of the mobility of the charge carriers and their
partial localization in band tails are most prominent
in the TC region. Imposition of a magnetic field on
the sample increases the charge carrier mobility and
excites the charge carriers from the band tails; this
is the cause of the colossal magnetoresistance.
If an insulating MTPS is present in the sample
with x = 0.1, the negative ω indicates that the
ferromagnetic droplet radii increase with applied
magnetic field; this is typical for an insulating
MTPS [8]. But the ferromagnetic phase in an insu-
lating MTPS sample occupies only a few percent of
the sample volume [8]. Therefore the volume of
ferromagnetic part is small in the sample with x =
= 0.1, so that the anomalies of ω and ∆l/l are not
detected near TC . There is a small maximum of ρ
and a colossal magnetoresistance in the TC region
for this semiconducting crystal [9,10]. Thus ρ and
the magnetoresistance anomalies can be attributed
to an insulating MTPS, too [8]. One can explain
the colossal magnetoresistance by the increase of
the ferromagnetic droplet radii in the magnetic
field, which facilitates electron tunneling between
ferromagnetic droplets. Moreover the magnetic mo-
ments of the ferromagnetic droplets are aligned
along with the external field, and that also facili-
tates the tunneling. Ultimately the field tends to
destroy the ferromagnetic droplets. Thus the mag-
netic field increases the electron energy inside the
droplets and in doing so it facilitates their transi-
tion to a delocalized state.
The sample with x = 0.15 has conductivity of the
semiconducting type, but it is situated near the
boundary between the semiconducting and metallic
states [9]. Therefore the volume of its ferromagnetic
phase is larger than in the sample with x = 0.1, and
the anomalies of ω and ∆l/l at T ∼ TC are observed
in this sample. The anomalies of ρ and the colossal
magnetoresistance in the TC region for the sample
with x = 0.15 are explained as well as for the
sample with x = 0.1.
Acknowledgments
This paper was supported by grants from INTAS,
#97-0253, NATO #HTECH LG 972942, and the
Russian Basic Investigations Foundation #00-15-
96695, #00-02-17810. We are grateful by A.M.
Balbashov for preparation of the samples and their
analysis.
1. E. L. Nagaev, Sov. Phys. Usp. 166, 833 (1996).
2. A. P. Ramirez, J. Phys.: Condens. Matter 9, 7 (1997).
3. Y. Tokura and Y. Tomioka, J. Magn. Magn. Mater. 200, 1
(1999).
4. A. Moreo, S. Yunoki, and E. Dagotto, Cond-mat/9901057
8 Jan (1999).
5. A. J. Millis, P. B. Littlewood, and B. I. Shraiman, Phys.
Rev. Lett. 74, 5144 (1995).
6. A. J. Millis, B. I. Shraiman, and R. Mueller, Phys. Rev.
Lett. 77, 175 (1996).
7. A. J. Millis, Phys. Rev. B53, 8434 (1996).
8. E. L. Nagaev, Physics of Magnetic Semiconductors, Mir,
Moscow (1983).
9. A. Urushibara, Y. Moritomo, T. Arima, A. Asamutsu,
G. Kido, and Y. Tokura, Phys. Rev. B51, 14103 (1995).
10. H. Y. Hwang, S.-W. Cheong, N. P. Ong, and B. Battlogg,
Phys. Rev. Lett. 77, 2041 (1996).
11. M. R. Ibarra, P. A. Algarabel, C. Marquina, J. Blasco, and
J. Garcia, Phys. Rev. Lett. 75, 3541 (1995).
12. A. Yanase and T. Kasuya, J. Phys. Soc. Jpn. 25, 1025
(1968).
13. P. G. Radaelli, G. Iannone, M. Marezio, H. Y. Hwang,
S.-W. Cheong, J. D. Jorgensen, and D. N. Argyriou, Phys.
Rev. B56, 8265 (1997).
14. A. Asamutsu, Y. Moritomo, Y. Tomioka, T. Arima, and
Y. Tokura, Nature 373, 407 (1995).
15. K. V. Kamenev, G. J. McIntyre, D. McK Paul, M. R. Lees,
and G. Balakrishnan, Phys. Rev. B57, R6775 (1998).
L. I. Koroleva, A. I. Abramovich, R. V. Demin, and A. V. Michurin
402 Fizika Nizkikh Temperatur, 2001, v. 27, No. 4
|