Journal of Magnetism and Magnetic Materials 272–276 2004 1292–1294Large positive entropy change in several charge-ordering perovskites N.. At the charge-ordering temperature the magnetic
Trang 1Journal of Magnetism and Magnetic Materials 272–276 (2004) 1292–1294
Large positive entropy change in several
charge-ordering perovskites
N Chaua,*, D.H Cuonga, N.D Thoa, H.N Nhata, N.H Luonga, B T Congb
a
Center for Materials Science, Hanoi Vietnam National University, 334 Nguyen Trai, Hanoi 844, Viet Nam
b
Department of Solid State Physics, Vietnam National University, Hanoi, 334 Nguyen Trai, Hanoi, Viet Nam
Abstract
The Nd0.5Sr0.5Mn1xCuxO3 (x ¼ 0:00; 0.02; 0.10) and Nd0.25Pr0.25Sr0.5MnO3 perovskites have been prepared by solid reaction technology The manganites are of single phase with orthorhombic structure The FC and ZFC measurements show an evident existence of charge-ordering effect at low temperatures At the charge-ordering temperature the magnetic entropy change, DS; is positive and the jDSmaxj is larger than that determined at ferromagnetic-paramagnetic phase transition temperature
r2003 Elsevier B.V All rights reserved
PACS: 75.47.Lx Manganites
Keywords: Magnetocaloric effect; Magnetic oxides; Perovskite structure; Magnetization; Charge ordering
Charge-ordered phases of rare earth manganites are
novel effect arising from interactions between the charge
carriers and phonons, giving rise to the localization of
carriers at specific sites in the lattice below a certain
temperature Charge-ordering (CO) compounds have a
large variation of resistivity and magnetization as a
function of temperature and magnetic field[1–3] In this
work we report our study of magnetocaloric effect in
several charge-ordering perovskites
The four compositions:
No: 1: Nd0:5Sr0:5MnO3;
No: 2: Nd0:5Sr0:5Mn0:98Cu0:02O3;
No: 3: Nd0:5Sr0:5Mn0:9Cu0:1O3;
No:4: Nd0:25Pr0:25Sr0:5MnO3
are prepared by solid reaction technique
Fig 1shows the X-ray diffraction patterns of studied samples The analysis showed that the crystal structure
of the samples is orthorhombic
The Rietveld refinement was done for determination
of bond angle and bond lengthin investigated samples Compared to the structure of Nd0.5Sr0.5MnO3published
in Ref.[4], our sample No 1 showed a good coincidence
Fig 2 shows the thermomagnetic field-cooled (FC) and zero-field-cooled (ZFC) magnetization curves mea-sured at 20 and 50 Oe for all samples studied We can see that the existence of charge-ordering effect is clearly evident in sample Nos 1, 2 and 4 (Fig 2a,b and d) but a larger amount of Cu subtituted for Mn destroys CO (Fig 2c) Two magnetic transitions are clear from these curves: the paramagnetic (PM) to ferromagnetic (FM) at
TC¼ 265 K for the sample Nos 1 and 4, TC¼ 230 K for the sample No 2 and TC¼ 260 K for sample No 3 as well as FM-AFM transition TCO¼ 175 K for sample No
1, 170 K for sample Nos 2 and 4 Note that the small amounts of Cu substituted for Mn lead to the strong reduction of magnetization and TC(Fig 2b and c) The entropy change resulting from the spin ordering induced by the applied magnetic field can be obtained
*Corresponding author Tel./fax: +84-4-8589496.
E-mail address: chau@cms.edu.vn (N Chau).
0304-8853/$ - see front matter r 2003 Elsevier B.V All rights reserved.
doi:10.1016/j.jmmm.2003.12.074
Trang 2according to the thermodynamic relation[5]:
DSðT ; HÞ ¼
Z H max
0
The evaluation of DSðT Þ was presented in Ref.[6]
Fig 3shows the magnetic entropy changes, DS; over
large temperature interval for the sample Nos 1 and 4
From these curves we can see that a sharp positive peak
in DSðT Þ at T is occurred together with the second
negative peak in DSðT Þ around TC and jDSj at TCO is nearly two times larger than that determined around TC: The doping of Cu in sample Nos 2 and 3 not only leads
to the decreasing of magnetization and Curie tempera-ture of the samples but also strongly reduces jDSmaxj by nearly one order of magnitude Note that CO transition
is of first order in nature[7], whereas at TCthere is the second-order transition
We suggest that when Cu is partially substituted for
Mn, the Mn3+/Mn4+ratio decreases, because Cu ions are in the state of Cu3+or even Cu2+ This factor leads
to weakening the double exchange in both sample Nos 2 and 3 That is the main reason of the decrease of magnetization, Curie temperature as well as jDSmaxj:
In summary we conclude that the four compositions
Nd0.5Sr0.5Mn1xCuxO3 (x ¼ 0:00; 0.02, and 0.10) and
Nd0.25Pr0.25Sr0.5MnO3 were prepared withsingle phase and exhibited orthorhombic structure The FC and ZFC thermomagnetic measurements show the evident existence of charge-ordering effect in sam-ples Nd0.5Sr0.5MnO3, Nd0.5Sr0.5Mn0.98Cu0.02O3, and
Nd0.25Pr0.25Sr0.5MnO3 The maximum value of magnetic entropy change, DSmax measured at first-order phase transition is positive and absolute value of DSmax is about two times larger than that measured at TC: In the sample with x ¼ 0:1 the charge ordering is destroyed, moreover magnetization is strongly reduced
References [1] G Xiao, G.Q Gong, C.L Canedy, E.J McNiff Jr.,
A Gupta, J Appl Phys 81 (1997) 5324.
[2] Y Moritomo, Phys Rev B 60 (1999) 10374.
[3] J L !opez, P.N Lisboa-Filho, O.F Delima, F.M Aranjo-Moreira, J Magn Magn Mater 242–245 (2002) 683.
2-Theta
Fig 1 X-ray diffraction patterns of studied samples.
ZFC
H=20 Oe
(a)
ZFC FC
(b)
FC ZFC
0
0.5
1.5
0.5
1.5
0.05
0.15
0.02
0.06
ZFC FC
H = 50 Oe (c)
H=20 Oe
H = 2 0 O e
Temperature (K)
Fig 2 Thermomagnetic field-cooled (FC) and
zero-field-cooled (ZFC) curves for the samples: (a) No 1, (b) No 2, (c)
No 3 and (d) No 4.
1T 0.5T
Tem perature (K) -1
0 1 -1 0 1
2 (a)
1.35T 1T 0.5T
Fig 3 The magnetic-entropy change as a function of temperature for the samples No 1 (a) and No 4 (b).
Trang 3[4] V Caignaert, F Millange, M Hervieu, E Suard,
B Raveau, Solid State Commun 99 (1996) 173.
[5] A.M Tishin, J Magn Magn Mater 184 (1998)
62.
[6] N Chau, H.N Nhat, N.H Luong, D.L Minh, N.D Tho,
N.N Chau, Physica B 327 (2003) 270.
[7] H Kuwahara, Y Tomioka, A Asamitsu, Y Morytomo,
Y Tokura, Science 270 (1995) 961;
H Kuwahara, Y Tokura, in: C.N.R Rao, B Raveau (Eds.), Colossal Magnetoresistance, Charge Ordering and Related Properties of Manganese Oxides, World Scientific Pub., Singapore, 1998, p 217.