The Curie temperature Tc increases from 235 K for x ¼ 0:12310 K for x ¼ 0:2 and is almost constant about 360 K for xX0:3: The field-cooled and zero-field-cooled thermomagnetic curves measu
Trang 1Structure, magnetic, magnetocaloric and magnetoresistance
Nguyen Chaua,*, Hoang Nam Nhata, Nguyen Hoang Luonga,
Dang Le Minhb, Nguyen Duc Thoa, Nguyen Ngoc Chaua
a Center for Materials Science, National University of Hanoi, 334 Nguyen Trai, Hanoi, Viet Nam
b Department of Solid State Physics, National University of Hanoi, 334 Nguyen Trai, Hanoi, Viet Nam
Abstract
La1xPbxMnO3(x ¼ 0:1; 0.2, 0.3, 0.4, and 0.5) perovskites were prepared by a solid-state reaction Except for x ¼ 0:5 (cubic) and x ¼ 0:4 (rhombohedral), the structure of the other compositions was pseudo-rhombohedral with P1 symmetry The particle size of the grains is depending on the Pb content of the samples The Curie temperature Tc
increases from 235 K for x ¼ 0:12310 K for x ¼ 0:2 and is almost constant (about 360 K) for xX0:3: The field-cooled and zero-field-cooled thermomagnetic curves measured at low field show a split below a so-called irreversibility temperature Tr; which is somewhat smaller than Tcexcept for x ¼ 0:1; where it is 270 K From a series of magnetic isotherms the magnetic entropy changes DSðT Þ were determined for a field step of 500 Oe The maximum value of
DSmax increases with increasing x till x ¼ 0:3 and then decreases with further increasing x: The conductivity of perovskites is metallic at low temperatures and semiconducting at high temperatures Magnetoresistance measurements have been performed
r2002 Elsevier Science B.V All rights reserved
Keywords: Perovskite structure; Magnetic oxides; Magnetocaloric effect
1 Introduction
The Ln1xAxBO3 perovskites (Ln=rare earth,
A=alkaline earth element, B=Mn or Co) are
attracting considerable interest because they
ex-hibit interesting physical effects and have potential
for applications due to the complex relationship
between crystal structure, electrical, magnetic and
thermal properties Colossal magnetoresistance
in manganese perovskites [1] and large
magneto-caloric effects around the Curie temperature in
La1xSrxCoO3(LSCO) [2]have been found The metal–insulator transition [3], an anomaly in the thermal expansion and hysteresis of the resistance around the Curie temperature[4]are interpreted as first-order transitions [4–6] The magnetocaloric effect has not only been studied in cobaltites but also in manganites
The manganites also have potentials as solid electrolytes, catalysts, sensors and novel electronic materials Their rich electronic phase diagrams reflect the fine balance of interactions, which determine the electronic ground state These compounds represent in microcosm, the interplay
*Corresponding author Tel.: 858-9496; fax:
+84-4-858-9496.
E-mail address: chau@cms.edu.vn (N Chau).
0921-4526/03/$ - see front matter r 2002 Elsevier Science B.V All rights reserved.
PII: S 0 9 2 1 - 4 5 2 6 ( 0 2 ) 0 1 7 5 9 - 3
Trang 2of experiment, theory and application, which is at
the heart of solid-state physics [7]
Most research on manganites and cobaltites is
concerned with alkaline-earth elements like Sr, Ba
or Ca or combinations of these Young et al [8]
have studied the crystal structure and magnetic
properties of La0.7Pb0.3Mn1xCoxO3 (0pxp1)
perovskites For x ¼ 0; these authors found a
rhombohedral structure and also a rather large
magnetoresistance effect, about 50% below 200 K
and about 25% around 300 K Troyanchuk et al
[9] have observed that the La1xPbxMnO3
per-ovskites (x ¼ 0:420:6) have a rhombohedral
(slightly distorted) cubic structure Huang et al
[10] have studied the crystal structure and the
magnetic scaling behaviour of La1xPbxMnO3
perovskites (x ¼ 0:020:5) and have shown that
all the samples crystallize in the rhombohedral
structure Moreover, the substitution of Pb+2ions
on La+3 sites induces a mixed-valence state of
Mn3+/Mn4+ and enhances the magnetic
transi-tion temperature in this system In this work, we
report on our study of structure, magnetic, electric
and magnetocaloric properties of La1xPbxMnO3
manganite
2 Experimental
La1xPbxMnO3 samples (x ¼ 0:1; 0.2, 0.3, 0.4,
and 0.5) were prepared using a conventional
powder solid-state reaction method
Stoichio-metric amounts of 3N purity La2O3, PbO and
MnCO3powders were homogeneously mixed and
completely ground Then, the mixed samples were
presintered at 9001C for 15 h The heated samples
were cooled to room temperature, reground to fine
particles, and pressed into pellets and sintered at
9201C for 15 h All high-temperature treatments
were performed at ambient atmosphere with a
programmed heating and cooling rate of 501C–
1501C/h
The structure of the samples was examined in a
Brucker D 5005 X-ray diffractometer The
micro-structure and chemical composition were studied
in a 5410 LV Jeol scanning electron microscope
(SEM), which includes an energy dispersion
spectrometer (EDS) Thermal phase transitions
were determined by differential scanning calori-metry (DSC) and thermogravimetric analysis (TGA) using TA Instruments Apparatus SDT
2960, with a heating rate of 201C/min Magnetic measurements were performed in a vibrating sample magnetometer (VSM) DMS 880 in mag-netic fields up to 13.5 kOe
3 Results and discussion Since Pb is an element having a low melting temperature and easily evaporates, presintering and sintering were performed at not too high temperatures
Fig 1shows SEM photographs of some repre-sentative samples In the samples with small Pb content (x ¼ 0:1—Fig 1a, x ¼ 0:3—Fig 1b) the crystallites are homogeneous and small (about 0.3–0.4 mm) For a larger amount of Pb (x ¼ 0:5), the size of the crystallites increases up to 0.5– 0.7 mm and there are several particles with sizes up
to l mm (Fig 1c) This allows us to suggest that in the sample with x ¼ 0:5 the crystalline particles develop easily due to existence of local liquid phases
Fig 2 presents the EDS spectrum of
La0.7Pb0.3MnO3 One can see that there are no strange elements and the sample composition is similar to the nominal one, i.e there has been no evaporation of Pb So, the sintering temperature was not too high
Fig 3 presents the X-ray diffraction (XRD) patterns of the investigated samples We can see that all five samples are of single phase
For La1xPbxMnO3 (x ¼ 0:120:5) the refine-ment was successful for pseudo-cubic lattices with symmetries decreasing from cubic (x ¼ 0:5) to rhombohedral (x ¼ 0:4) and triclinic (x ¼ 0:3; 0.2, 0.1) For x ¼ 0:3 (where the symmetry changes from rhombohedral to triclinic), the metal-atom positions must be fixed while the oxygen positions were refined; for x ¼ 0:2; 0.1(P1) all atomic positions including the metal sites could be refined The existence of a cubic cell for x ¼ 0:5 is very similar to the results obtained for La1xSrxMnO3 [11] as well as for La1xSrxCoO3 [12,13] The variation of unit-cell volume is DV =V ¼ 1:0% i.e
N Chau et al / Physica B 327 (2003) 270–278 271
Trang 3Fig 1 SEM photographs of the surface of La 1x Pb x MnO 3 : (a) x ¼ 0:1; (b) x ¼ 0:3; (c) x ¼ 0:5:
Fig 2 Energy dispersion spectrum of La Pb MnO Fig 3 XRD patterns of La Pb MnO perovskites.
Trang 4a max–min variation=0.6 (A3 The largest volume
occurs for x ¼ 0:2; 0.3 and the smallest volume for
x ¼ 0:5: The lattice constants vary from 3.877
(x ¼ 0:5) to 3.895 (A (x ¼ 0:3), i.e 0.5% Table 1
shows the bond lengths and bond angles for Mn–
O and Mn–O–Mn Note that in triclinic symmetry (x ¼ 0:1; 0.2, and 0.3) there are three independent oxygen positions O1, O2, O3, so the values are given for each case individually The average Mn– O–Mn bond angle increases clearly with increasing
Pb2+content and reaches a maximum for x ¼ 0:4 and 0:5 (1801), whereas the bond Mn–O lengths decrease continuously to a minimum for x ¼ 0:5 (1.939 (A), resulting in the most tight bonding between Mn and O atoms
The La1xPbxMnO3 compounds have been investigated in Refs [10,14] with hexagonal cells
of space group R3c (for all x ¼ 0:020:5) To
Table 1
Bond lengths ( ( A) and bond angles (1) for La 1x Pb x MnO 3
x to Mn–O Mn–O–Mn x Mn–O Mn–O–Mn
0.1O 1 1.962, 1.965 164.3 1.964, 1.971 162.6
O 2 1.972, 1.979 158.7 0.3 1.960, 1.966 164.5
O 3 1.961, 1.966 162.3 1.937, 1.974 169.7
0.2 O 1 1.979, 1.984 158.5 0.4 1.943 180.0
O 2 1.967, 1.979 161.0
O 3 1.956, 1.956 166.9 0.5 1.939 180.0
Fig 4 DSC curves for La Pb MnO (a), DSC and TGA curves for the raw material La O (b).
N Chau et al / Physica B 327 (2003) 270–278 273
Trang 5compare these results with the present ones, the
pseudo-cubic lattice should be transformed by the
transformation matrix [(1,1,1)(0,1,1)(2,2,2)]
Except for x ¼ 0:1and 0.2, where small angular
deformations are seen in our samples (o0.21), all
other cases differ only in the cell constants The
cells reported in the literature are all of the same
symmetry and appear to be smaller, except for x ¼
0:4: On the average, these cells are 0.5% smaller
i.e 0.3 (A3/unit cell Because of the lower sensitivity
to angular deformation of the hexagonal cell, the
development of Mn–O–Mn angles was not seen in
Ref.[14]as clearly as in our samples
Fig 4 shows the results of thermal analysis for
La1xPbxMnO3 samples (Fig 4a) as well as DSC
and TGA curves for the starting material La2O3
(Fig 4b) All DSC curves show one broad
endothermal peak around 801C and one sharp
endothermal peak around 3951C It is suggested that the first peak is due to evaporation of water out of the samples and that the second one corresponds to the decomposition of La2O3 nH2O into intermediate phases The fuzzy endothermal peak around 8601C can be considered to corre-spond to the start of the reaction in which the perovskite structure is formed
For all samples, zero-field-cooled (ZFC) and field-cooled (FC) magnetization measurements were performed in a magnetic field of 20 Oe
Fig 5 shows that the FC and ZFC curves for
x ¼ 0:1; 0.3, and 0.5 are separated from each other
at low temperatures The ferromagnetic transition temperature, TC was determined from these thermomagnetic curves and is presented in
Table 2 The Curie temperature increases with increasing Pb content from x ¼ 0:1 (T ¼ 235 K)
Fig 5 Thermomagnetic curves of the La 1x Pb x MnO 3 perovskites in a magnetic field of 20 Oe (a) x ¼ 0:1; (b) x ¼ 0:3; (c) x ¼ 0:5:
Trang 6to x ¼ 0:4 (TC¼ 360 K) and after that TC
becomes somewhat lower in the sample with
x ¼ 0:5: Clearly, substitution of Pb2+ for La3+
induces a mixed-valent state of Mn3+/Mn4+and
enhances the ferromagnetic transition
tempera-ture The dependence of TC on x in the system
La1xPbxMnO3 is in good qualitative agreement with results for La1xSrxCoO3[12]
In low applied field, the ZFC and FC magne-tization curves are splitted at temperatures below a so-called irreversibility temperature, Tr (oTC) (Fig 5) Note that in sample with x ¼ 0:1 (Fig 5a) due to the low sintering temperature, obviously the sample is in large homogeneity The magnitude of the splitting and the temperature Trdecrease with increasing external field In addition, the low field ZFC of the MðT Þ curves clearly show a cusp at a so-called spin freezing (or spin–glass transition) temperature Tg: As the strength of external magnetic field increase, Tg also shifts to a lower temperature and the cusp in the ZFC MðT Þ is smeared out to broad maximum These phenom-ena are identifying features of a spin-glass or cluster-glass state [15,16] The maximum will be the result of the competition between (local)
Table 2
Magnetic-transition temperatures of La 1x Pb x MnO 3 T C :
ferromagnetic transition temperature; T r : irreversibility
tem-perature; T g : spin–glass transition temperature
MR (%) 0.40 3.40 5.26 5.60 4.20
Fig 6 Isothermal magnetization curves for La Pb MnO (a); DSðT Þ for La Pb MnO (b); and DSðT Þ for La Pb MnO (c).
N Chau et al / Physica B 327 (2003) 270–278 275
Trang 7anisotropy (decreasing with increasing
tempera-ture, so allowing an increasing magnetization) and
eventually the decrease of the magnetic order,
when Curie temperature is approached InTable 2,
also the values of Tr and Tg are presented The
composition dependence of Trand Tghas a similar
tendency as that of TC:
MðHÞ isotherms have been measured for all
La1xPbxMnO3 samples, at various temperatures
in a narrow temperature interval around the Curie
temperature, in magnetic fields up to 13.5 kOe
The entropy change resulting from the spin
ordering, induced by the applied magnetic field,
can be obtained according to the thermodynamic
relation [17]:
DSðT ; HÞ ¼ SðT ; 0Þ SðT; HÞ
¼
Z H max
0
fqMðT ; HÞ=qT gHdH: ð1Þ
Here, SðT; 0Þ and SðT ; HÞ represent the entropy without and with applied magnetic field, respec-tively From the set of isothermal MðHÞ curves we have evaluated the entropy change, DSðT Þ; for the field change from 0 to 13.5 kOe, as a function of
DSðT Þmax was evaluated Examples are given in
Figs 6a–c The maximum magnetic-entropy change, DSðTÞmax is found to be 0.65, 1.30, 1.53, 0.87, 0.81J/kg K for x ¼ 0:1; 0.2, 0.3, 0.4, and 0.5, respectively, so is maximal for x ¼ 0:3: These materials can be considered as good magnetic refrigerant materials operating at temperatures ranging from below to above room temperature For checking the ferromagnetic state of the samples, their hysteresis loops were measured at room temperature in a maximum field of 700 Oe
Fig 7presents hysteresis loops for x ¼ 0:2 and 0.5 The sample with x ¼ 0:1is paramagnetic at room temperature The magnetic parameters derived from these loops are shown in Table 2 The magnetization jumps from a low value at x ¼ 0:2
to a maximum value at x ¼ 0:3; and then decreases
at further increase of x: The coercivity has a similar concentration dependence This result is different from that in Ref.[18], where the authors showed that HC is almost independent of the substitution of Ag for La As usual for soft-magnetic materials, the coercivity of perovskites depends on temperature As an example, Fig 8
shows this dependence for x ¼ 0:3: We suggest
Fig 7 Hysteresis loops of La 1x Pb x MnO 3 at room
tempera-ture (a) x ¼ 0:2; (b) x ¼ 0:5:
Fig 8 Temperature dependence of the coercivity of
La 0.7 Pb 0.3 MnO 3
Trang 8that the temperature dependence of the
magneto-crystalline anisotropy has a similar behaviour
Examination of the electrical properties of
the perovskites shows that the conductivity is
metallic at low temperatures and they are semi-conducting at high temperatures Fig 9 presents the temperature dependence of the resistance of
La0.6Pb0.4MnO3as an example This dependence is
in agreement with results in other systems of perovskites[1,7]
We have determined the magnetoresistance (MR) of all samples Fig 10 presents the results
at room temperature for x ¼ 0:2; 0.3, and 0.5 The
MR is determined as the ratio ½RðHÞ Rð0Þ =Rð0Þ; where RðHÞ and Rð0Þ correspond to the resistance
of the sample with and without applied magnetic field, respectively The MRðHÞ curve for x ¼ 0:2 (Fig 10a) is of second order in H: It should be noted that for this composition TC is close to the measuring temperature The rupture point
in the MRðHÞ curves for the samples with
x ¼ 0:3 and 0.5 (Fig 10b and c) is expected to be related to irreversible displacement of domain walls in the magnetizing process Table 2 shows the MR values of all samples studied at the maximum applied magnetic field of 10 kOe We
Fig 9 Temperature dependence of the resistance of
La 0.6 Pb 0.4 MnO 3
Fig 10 Magnetoresistance of perovskites La Pb MnO at room temperature: (a) x ¼ 0:2; (b) x ¼ 0:3; (c) x ¼ 0:5:
N Chau et al / Physica B 327 (2003) 270–278 277
Trang 9see that MR reaches a maximum value of 5.6% at
room temperature for La0.6Pb0.4MnO3 It is well
known that MRðT Þ normally reaches a maximum
at a temperature close to the Curie temperature
The study of MR at different temperatures is in
progress
4 Conclusions
1 Single phase La1xPbxMnO3(0:1pxp0:5)
per-ovskites were prepared The symmetry
de-creases from cubic (x ¼ 0:5) to rhombohedral
(x ¼ 0:4) and triclinic (x ¼ 0:3; 0.2, 0.1)
2 The Curie temperature increases from 235 K for
x ¼ 0:12310 K for x ¼ 0:2 and then remains
almost constant with further increasing x:
3 The coercivity of perovskites at room
tempera-ture depends on the Pb content in the samples
4 The studied compounds may be considered as
magnetic refrigerant materials operating at
temperatures ranging from below to above
room temperature
5 The MR value of La0.6Pb0.4MnO3 reaches
5.60% at room temperature in a field of
10 kOe This is the highest room-temperature
value of the samples investigated
Acknowledgements
This work was supported by the Natural Science
Council of Vietnam and by the National Basic
Research Program No KT 420101
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