Journal of Magnetism and Magnetic Materials 304 2006 e325–e327Structure, magnetic, magnetocaloric and magnetoresistance properties D.T.. Variation of doping content leads to alternating
Trang 1Journal of Magnetism and Magnetic Materials 304 (2006) e325–e327
Structure, magnetic, magnetocaloric and magnetoresistance properties
D.T Hanh, N Chau , N.H Luong, N.D Tho
Center for Materials Science, College of Science, Vietnam National University, Hanoi, 334 Nguyen Trai Street, Hanoi, Vietnam
Available online 28 February 2006
Abstract
In our previous work, we have studied structure and properties of La1xPbxMnO3perovskites Variation of doping content leads to alternating structure and magnetic properties of materials In this paper, the investigation of structure, magnetic, magnetocaloric and magnetoresistance properties of family Pr1xPbxMnO3(x ¼ 0.1–0.5) is presented The grain size of samples increases with Pb content The FC and ZFC thermomagnetic curves measured at low field and low temperatures exhibit the spin-glass-like behavior The magnetic entropy changes, |DSm(T)|, were determined and showed belong to GMCE The resistance measurements indicated that first two samples exhibited semiconducting conductivity in the whole measured temperature range, whereas in the rest of samples there is insulator–metallic transition on R(T) curves Magnetoresistance measurements have also been performed
r2006 Elsevier B.V All rights reserved
PACS: 75.47.Lx; 75.30.Sg; 75.47.Gk
Keywords: Manganites; Magnetocaloric effect; Magnetoresistance
Most recent papers aim to study the interplay between
structure, magnetic and transport properties in
perovskite-type manganites Ln1xAxMnO3 (where Ln ¼ rare earth,
A ¼ alkaline element) Magnetic and structural properties
(as functions of ionic radius at A-site, A-site disorder,
applied magnetic field, doping content and temperature)
have revealed many interesting effects such as
magnetore-sistance, charge ordering, magnetocaloric, spin glass
behavior, etc [1–3] It was pointed out that the mother
compound LnMnO3 is an antiferromagnetic insulator,
accounts of its A-type magnetic structure, where
ferro-magnetically aligned layers are coupled
antiferromagneti-cally In this case, magnetic and transport properties are
related to the Jahn–Teller distortion among Mn3+ ions
When trivalent rare earth ions (La, Pr, Nd, Sm, etc.) are
partly substituted by divalent ions, there is the
mixed-valent state of Mn3+ and Mn4+ ions and that results in
potential charge carriers for metallic conductivity
Recently, several authors have paid attention in hole-doped manganites with A ¼ Pb [4–6] In our lab, overall investigation of properties of La1xPbxMnO3(x ¼ 0.1–0.5) has been performed [7] The results showed that the symmetry decreases from cubic ðx ¼ 0:5Þ over rhombohe-dral ðx ¼ 0:4Þ to triclinic ðx ¼ 0:3; 0:2; 0:1Þ, moreover 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
Manganites Pr1xPbxMnO3(x ¼ 0.1–0.5) were prepared
by conventional powder solid-state reaction technique The sintering temperature for all compositions was 1000 1C The microstructure was studied by a scanning electron microscope (SEM) 5410 LV Jeol Structure of samples was examined in a Bruker D5005 X-ray diffractometer The magnetic measurements were performed on a vibrating sample magnetometer (VSM) DMS 880 in magnetic field
up to 13.5 kOe
The SEM pictures indicated that crystallites of the samples are quite homogeneous and their size increased from around 0.5 mm ðx ¼ 0:1Þ to around 1.0 mm ðx ¼ 0:5Þ The X-ray diffraction patterns showed that all studied
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doi:10.1016/j.jmmm.2006.02.045
Corresponding author Tel.: +84 4 5582216; fax: +84 4 8589496.
E-mail address: chau@cms.edu.vn (N Chau).
Trang 2samples are of single phase with orthorhombic structure.
Significant increase of lattice parameter a and volume of
unit cell relates to the increase of Pb2+content (with large
ionic radius) substituted for Pr3+(Table 1)
For all samples, zero-field-cooled (ZFC) and field-cooled
(FC) magnetization curves were measured in magnetic field
of 20 Oe From Fig 1we can see that they are separated
from each other at below irreversibility temperature Trand
there is a cusp in ZFC curves at a so-called freezing or
spin-glass-like transition temperature, Tf These phenomena are
specific features of spin-glass- or cluster-glass-like state
The FM–PM transition temperature, TC (see Table 2),
increases with increasing Pb content, from 152 K ðx ¼ 0:1Þ
to 256 K ðx ¼ 0:4Þ and then becomes a little lower in the
sample x ¼ 0:5 The doping content dependence of Tfhas
the same tendency (Table 2) Clearly, the substitution of
Pb2+ for Pr3+ induces a mixed-valent state of Mn3+/
Mn4+ions and enhances the FM–PM transition tempera-ture due to double exchange interaction Moreover, with large average ionic radius at A-site cation, /rAS, the internal pressure increases yielding decrease the buckling of the MnO6octahedra and then also enhancing TC
To determine magnetic entropy change, DSm, as a function of temperature, M(H) isotherms have been measured for all samples at various temperatures around respective TC and in magnetic field up to 13.5 kOe Based
on Maxwell’s thermodynamic relations, DSm can be calculated by the formula
DSmðT; HÞ ¼ SðT ; 0Þ SðT; HÞ
¼
Z H max
0 fqMðT ; HÞ=qT gHdH, where SðT ; 0Þ and SðT; HÞ are the entropy without and with applied magnetic field, respectively[8]
Fig 2presents the dependence of |DSm| on temperature for studied samples The maximum magnetic entropy change, |DSm|max, of the samples is quite large, the lowest value is of 2.58 J/kg K for sample with x ¼ 0:2 and it reaches highest value of 3.91 J/kg K for sample
Pr0.9Pb0.1MnO3 Our materials could be considered as giant magnetocaloric materials In the same magnetic field change (DH ¼ 13.5 kOe), the values of |DSm|max here are quite higher than those investigated in our previous reports for LaSr(Mn,Co)O3[9,10], La1xPbxMnO3[7]and a little higher than that of La0.7Sr0.3MnO3with small substitution
of Ni for Mn[11] There should be attribution to magnetic spin of Pr3+ions instead of nonmagnetic La3+ions The temperature dependence of resistance of studied samples was determined by standard four-probe method The results indicated that while the two first compositions (x ¼ 0:1 and 0.2) exhibited semiconducting conductivity in the whole measured temperature range, the rest composi-tions (x ¼ 0:3, 0.4, and 0.5) established the metallic conductivity in the ferromagnetic state and semiconducting
Table 1
Lattice parameters of Pr 1x Pb x MnO 3 perovskites (error of 70.1%)
Sample a (A˚) b (A˚) c (A˚) V (A˚3)
100 125 150 175 200 225 250 275 300 325 350
0
1
2
3
4
5
FC
FC
FC
ZFC
ZFC
ZFC
ZFC
H = 20 Oe FC
Temperature (K)
Pr0.9Pb0.1MnO3
Pr0.8Pb0.2MnO3
Pr0.7Pb0.3MnO3
Pr0.6Pb0.4MnO3
Pr0.5Pb0.5MnO3
Fig 1 Thermomagnetic field-cooled (FC) and zero-field-cooled (ZFC)
curves of samples measured at 20 Oe.
Table 2
Composition dependence of some parameters of Pr 1x Pb x MnO 3
manga-nites (error of 1%)
Sample orA 4(A˚) T C (K) T f (K) |DS m | max (J/kg K)
100 125 150 175 200 225 250 275 300 0.0
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
Sm
Temperature (K)
Pr0.9Pb0.1MnO3
Pr0.8Pb0.2MnO3
Pr0.7Pb0.3MnO3
Pr0.6Pb0.4MnO3
Pr0.5Pb0.5MnO3
Fig 2 Magnetic entropy change versus temperature, |DS m (T)|, of studied samples.
Trang 3conductivity in paramagnetic state Fig 3 displays
con-ducting behavior for samples with x ¼ 0:2 and 0.5 We
suppose that only at high Pb content substituted for Pr
ðxX0:3Þ, the double exchange is still strong enough to
favor metallic conductivity Magnetoresistance MR as a
function of magnetic field, defined as ½rHFr0=r0, where
r0 is the zero field resistance and rH is the resistance in
13 kOe, was measured and the highest MR has reached
20% for sample x ¼ 0:5 and this value belongs to CMR
The authors acknowledge the financial support from the Vietnam National Fundamental Research Program (Project 421004)
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0
50
100
150
200
250
300
350
400
1.0 1.5 2.0 2.5 3.0 3.5 4.0
4.5
Pr0.5Pb0.5MnO3
Temperature (K)
Pr0.8Pb0.2MnO3
Fig 3 Resistance versus temperature for the samples x ¼ 0:2 and 0.5.