The temperature dependence of the EPR line intensity and resistivity evidence a spin–lattice interaction in the paramagnetic region near the Curie point.. Some authors [4, 5] attributed
Trang 1phys stat sol (b) 241, No 7, 1482 – 1485 (2004) / DOI 10.1002/pssb.200304688
Adiabatic polaron transport in La0.9Pb0.1MnO3 manganites
A N Ulyanov1, 2, H D Quang1, M Vasundhara3, C Nguyen4, and S C Yu*, 1
1
Department of Physics, Chungbuk National University, Cheongju, 361-763, Korea
2
Donetsk Physico-Technical Institute of National Academy of Sciences, 83114 Donetsk, Ukraine
3
Department of Physics, Indian Institute of Technology, Kharagpur-721 302, India
4
Center for Material Science, National University of Hanoi, 334 Nguyen Trai, Hanoi, Vietnam
Received 15 November 2003, revised 1 April 2004, accepted 2 April 2004
Published online 18 May 2004
PACS 71.38.+c, 76.30.–v, 77.80.Bh
A study of single-phase La0.9Pb0.1MnO3 compositions is presented The temperature dependence of the EPR line intensity and resistivity evidence a spin–lattice interaction in the paramagnetic region near the Curie point In the high temperature region a spin–spin interaction is dominant
© 2004 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim
1 Introduction Perovskite-like lanthanum manganites attract a renewed interest due to large
mag-netoresistance (MR) observed near the ferromagnetic ordering temperature [1] The MR effect was initially explained by the “double exchange” model [2] According to Zener, electrons move between two partially filled 3d shells of Mn3+ and Mn4+ ions with strong on-site Hund’s coupling The dou- ble-exchange model, revealing the connections between the electrical and magnetic subsystems of lan-thanum manganites, was complemented by the electron–phonon interaction caused by the Jahn–Teller splitting of external d-level of manganese [3] However, the nature of colossal magnetoresistance (CMR)
is not yet clear enough despite a large number of publications on this problem The difficulties consist
in a close connection between structure, electronic and magnetic properties of perovskite-type mangan-ites Resonance methods can give useful information about the internal dynamics of these materials,
especially near the Curie temperature, TC, where they show the highest value of the CMR effect Elec-tron paramagnetic resonance (EPR) has been studied in a variety of lanthanum manganites Some authors [4, 5] attributed the temperature dependence of the EPR linewidth to a spin–phonon interaction (one-phonon relaxation process) and explained it by spin–lattice relaxation of exchange coupled
Mn3+
–Mn4+
spin system under the condition of a strong relaxation bottleneck [6, 7] Other authors [8 – 10] explain the temperature dependence of ESR linewidth by the spin–spin (exchange) interaction
A careful analysis [11] of both mechanisms, describing the temperature dependence of EPR
linewidth, based on the measurement of the longitudinal electron–spin relaxation time T1 and
trans-verse relaxation time T2 did not clarify this question The temperature dependence of the ESR line
inten-sity I(T) is also not so clear According to Refs [1, 12] I(T) decreased exponentially with increasing
temperature and was inversely proportional [5] to temperature To clarify the situation we carried out the EPR and resistivity measurements of La0.9Pb0.1MnO3 manganite to study the nature of interaction near the Curie point
2 Sample preparation and experimental technique The La0.9Pb0.1MnO3 (LPMO) composition was prepared by ceramic technology and were single phase according to X-ray diffraction analysis The EPR study was performed at 9.2 GHz with the Jeol JES-TE300 ESR Spectrometer In the paramagnetic region
* Corresponding author: e-mail: scyu@chungbuk.ac.kr, Phone: +82-43-271-8146, Fax: +82-43-275-6416
Trang 2phys stat sol (b) 241, No 7, (2004) / www.pss-b.com 1483
the EPR spectra were single Lorentzian lines with g ≈ 2 and that value was temperature independent
Resistance measurements were performed using the four-probe technique Magnetization was measured
using a vibrating sample magnetometer
3 Results and discussion The Curie temperature of the sample was determined by the extrapolation of
magnetization to zero value in its temperature dependence and equals to 230 K
Figure 1 shows the temperature dependence of resistivity, ρ It can be fitted well using the
nearest-neighbor hopping model of small polarons [13]
B ( )T Texp (E k T σ/ )
with an activation energy Eσ = 0.16 eV (kB is the Boltzmann constant)
The EPR line intensity, I(T), was determined by double integration of the experimental derivative
absorption curve The temperature dependences of EPR line intensity I(T) and inverse line intensity,
1/I(T), for LPMO composition are shown in Fig 1 Let us consider some expressions, describing the
CMR material parameters, to fit the I(T) dependence The EPR linewidth obeys
dc
1 /( )
according to the spin–spin interaction model [8] (χdc is the static susceptibility) At the same time [7],
( )
and, according to the adiabatic polaron hopping model [13] (see Eq (1)), σ(T) ∝ (1/T) exp (–Eσ/kBT)
(σ is the conductivity) Taking into account also that EPR line intensity [8]
dc ( )
one can find
( ) exp ( / )
Equation (5) gives a good fit for the I (T) data obtained for the La0.9Pb0.1MnO3 compositions It
permit-ted to deduce a value of the activation energy E a (= 0.11 eV) The value of the activation energy obtained
from the resistivity (Eσ) and EPR (Ea) measurements is consistent with the ones reported in Ref [8, 12]
The less value of activation energy deduced from the I (T) dependence than that from the ρ(T)
depend-ence can be caused by the decrease of activation energy in magnetic field, where the EPR resonance was
observed By extrapolating the linear (high temperature part) of the 1/I (T) dependence to zero value it is
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
Temperature (K)
0 1 2 3 4 5 6 7 8
I(T)
ρ(T)
La
0.9Pb
0.1MnO
3
Θ
Fig 1 Resistivity vs temperature (solid triangular) and
fitting curve (solid) according to Eq (1) and temperature
dependencies of EPR line intensity, I (T) (closed squares); inverse intensity 1 /I (T) (open cycles); and
fitting curves according to exponential decay (Eq (5)) (solid line), and for inverse intensity (dot line)
Trang 31484 A N Ulyanov et al.: Adiabatic polaron transport in La0.9Pb0.1MnO3
possible to obtain the Curie–Weiss temperature, Θ (≈278 K) As one can see from Fig 1 the Curie–
Weiss law, I (T) ∝ 1/(T–Θ), does not describe properly the temperature dependence of the EPR line intensity in a wide temperature range around TC The exponential decay and Curie–Weiss law coincide at high temperatures, indicating a decrease of spin–lattice interaction and the dominance of the spin–spin one
The deviation of the temperature dependence of the EPR line intensity from the Curie–Weiss law near
TC was explained by the formation of spin clusters, when TC is approached from above [12, 14] The authors of works [8, 9] explained the deviation of the static susceptibility from the Curie–Weiss law in the frame of the constant coupling approximation model At the same time, polaron formation across the ferromagnetic–paramagnetic phase transition was observed, which was reported by the study on the
La1–xAxMnO3 (A = Ca, Pb) perovskite system by the extended X-ray absorption fine structure (EXAFS)
spectroscopy [15] It seems, that deviation of experimental I (T) data from Curie–Weiss law observed can
be caused by the spin–lattice interaction in the vicinity of TC and are in agreement also with the predic-tion of the adiabatic polaron hopping model [13] From this point of view, the exponential dependence of the EPR line intensity on temperature reflects the formation of polarons near the Curie point Those pola-rons are centered by the Mn3+ and Mn4+ ions and mediated by the double (through the O2– ion) activated hopping of the electrons
Let us comment on validity of Eq (5) Assumptions (1) and (3) were made to deduce it It is known [13] that the resistivity in the paramagnetic region can be described by the nearest-neighbor hopping of small (Holstein) polarons (Eq (1)) Mott’s variable-range hopping expression σ(T) ∝ exp (–T0/T )1/4 is appropriate to describe the experimental data if the carriers are localized by the random potential
fluctua-tions (T0 is related to the localized length) The temperature dependence (1) in mixed-valence manganites was observed, except the present work, for examples, in works [16 – 18] And, about the validity of
Eq (3) Except in [7] where the authors observed the proportionality between EPR linewidth, ∆H, and
conductivity σ(T) ∝ (1/T) exp (–Eσ/kBT) taken from [16], we know the only works [8, 9] where the EPR
linewidth in La0.7Ca0.3MnO3 sample was measured in wide temperature range (up to 1000 K) We replot-ted that data in the scale ln (∆H–T) on 1/T (see insert in Fig 2) and, as one can see, the obtained
depend-ence is very close to the linear one that confirms the validity of expression (3) again So, it can be con-cluded that the Eqs (1) and (3) as well as the deduced Eq (5) are valid under the predicted determined conditions
Acknowledgements The Research at Chungbuk National University was supported by the Korea Science and
Engineering Foundation through the Research Center for Advanced Magnetic Materials at Chungnam National University A N Ulyanov was supported by Korea Institute of Science and Technology Evaluation and Planning (KISTEP) during this work
11
12
13
14
1/T (1/K)
[8,12]
Fig 2 Natural logarithm of the EPR linewidth ∆H–T dependence
on reciprocal temperature from Ref [8, 12] The solid line is a
linear fit for the experimental data
Trang 4phys stat sol (b) 241, No 7, (2004) / www.pss-b.com 1485
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