The field-cooled FC and zero-field-cooled ZFC curves showed that samples with xX0.25 exhibit the spin glass-like state at low field and low temperatures, whereas, in the samples with xo0.25
Trang 1Journal of Magnetism and Magnetic Materials 303 (2006) e402–e405
Spin glass-like state, charge ordering, phase diagram and positive
N Chaua, , N.D Thoa, N.H Luonga, B.H Giangb, B.T Congb
a Center for Materials Science, University of Science, Vietnam National University, Hanoi, 334 Nguyen Trai Street, Hanoi, Vietnam
b Department of Physics, University of Science, Vietnam National University, Hanoi, 334 Nguyen Trai Street, Hanoi, Vietnam
Available online 17 February 2006
Abstract
The mixed rare earth manganites Nd0.5xPrxSr0.5MnO3(x ¼ 0.1–0.5) have been prepared using solid state reaction technique All samples are of single phase with orthorhombic structure The microstructure of the samples was determined by SEM The field-cooled
(FC) and zero-field-cooled (ZFC) curves showed that samples with xX0.25 exhibit the spin glass-like state at low field and low
temperatures, whereas, in the samples with xo0.25, there is the charge ordering (coexisting with FM–AFM transition) established at low temperatures The Curie temperature of the samples increases with increasing Pr content due to increase oforA4 Interesting feature is that at the FM–AFM transition region, the magnetic entropy change has positive value, in contrary to that at FM–PM transition region The electrical property of the samples from 10 K to room temperature is examined in detail
r2006 Elsevier B.V All rights reserved
PACS: 75.47.Lx; 75.30.Kz; 75.30.Sg
Keywords: Manganites; Charge ordering; Magnetocaloric effect
The study of Ln0:5A00:5MnO3 manganites (Ln ¼ rare
earth, A0¼alkaline element) has brought out a novel
effect: charge-ordering (CO) effect [1,2] There is the
ferromagnetic (FM) interaction due to the double exchange
(DE) interaction among the carriers and the
antiferromag-netic (AFM) interaction caused by the super exchange (SE)
interaction which depends on combination of Ln3+ and
A0 2+
cations Previous works have pointed out that the
CO state associated with insulating and AFM behaviors
is strongly affected by the average radius of cations
Ln3+ and A0 2+
or of A site, orA4 [3–5] In compound
Nd0.5Sr0.5MnO3 with a middle orA4( ¼ 1.236 A˚), the
ferromagnetic metallic (FMM) state (TC¼250 K)
trans-forms to the AFM CO state on cooling to 150 K
Manganite Pr0.5Ca0.5MnO3 with small orA4 (p1.17 A˚)
does not exhibit the FMM state at any temperature
and CO occurs in the paramagnetic (PM) state Two types
of charge ordering can be distinguished in manganites
based on the dependence on magnetic field of the CO
state [6,7] We have reported for the first time on the large positive magnetic entropy change in several
CO perovskites Nd0.5Sr0.5Mn1xCuxO3 (x ¼ 0:00, 0.02) and Nd0.25Pr0.25Sr0.5MnO3 [8,9] In this work, we report our study on spin glass-like state, charge ordering,
Nd0.5xPrxSr0.5MnO3 (x ¼ 0:1, 0.2, 0.3, 0.4 and 0.5) perovskites
The five compositions above were prepared by the solid state reaction technique The microstructure was studied in
5410 LV Jeol scanning electron microscope (SEM) The SEM pictures showed that the samples are homogeneous The grain size decreases from nearly 0.5 mm (x ¼ 0:1 —
Fig 1a) to around 0.25 mm (x ¼ 0:2 —Fig 1b) and around 0.15 mm in sample with x ¼ 0:5 Substitution of Pr for Nd leads to refinement of particles To classify the structure symmetry in perovskites we use a geometrical index defined
as t ¼ ðrAþrOÞ= ffiffiffi
2
p
ðrMnþrOÞ (where rA, rO and rMn are the ionic radius at A, O, and Mn site, respectively).Table 1
presents the value of orA4 and tolerant factor t of the studied samples Obviously while Nd is partly substituted
by Pr,orMn4 is constant, orA4 and t increase due to
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0304-8853/$ - see front matter r 2006 Elsevier B.V All rights reserved.
doi:10.1016/j.jmmm.2006.01.062
Corresponding author Tel./fax: +84 4 8589496.
E-mail address: chau@cms.edu.vn (N Chau).
Trang 2larger ionic radius of Pr3+ion The value of t which is in
the range of 0.954–0.957 corresponds to the stable
perovskite structure[10]
The structure of the samples was examined by Bruker
X-ray Diffractometer D5005 and showed that all samples
are of single phase with orthorhombic structure The lattice
parameter a is slightly decreased with increasing x, whereas
the parameters b and c as well as the volume of unit cell are
continuously enhanced with increasing x When Nd is partly substituted by Pr, as mentioned above, orA4 increases due to larger ionic radius of Pr3+, leading to increase of internal pressure or volume of unit cell The field-cooled (FC) and zero-field-cooled (ZFC) magnetization measurements were carried out in the applied field of 20 Oe by using vibrating sample magnet-ometer (VSM) DMS 880 From Fig 2we can see that in sample with x ¼ 0:5, FC and ZFC curves separate each other at low temperatures The temperature at which FC and ZFC curves begin to split is called irreversibility temperature, Tr(TroTC) The low field ZFC curve clearly shows a cusp at a so-called freezing (or spin-glass transition) temperature, Tg These phenomena are the typical features, which belong to the spin glass-like state behavior[11] The samples with x ¼ 0:3 and 0.4 exhibit the same behavior With low doping content of Pr (x ¼ 0:1 and 0.2), there are two magnetic transitions from FC and ZFC curves: the PM to FM transition at TCand FM to AFM transition at Ne´el temperature (TN—in this case coincides with TCO) (Fig 3) The TC of sample with x ¼ 0:1 is less than that of sample with x ¼ 0:2, however TCOof sample with x ¼ 0:2 is less than that of sample with x ¼ 0:1 The values of TC and TCO depend on the average radius of
A cations TC enhances with larger orA4 while TCO decreases (seeTable 1)
Based on the present results as well as of the previous work [9], phase diagram is composed and displayed in
Fig 4 TCof samples increases continuously with increas-ing Pr content due to increase in orA4 We suppose CO and AFM states exist in samples with x ¼ 0:020:25 When
Pr is substituted for Nd at higher amount (x40:25), the CO state at low temperatures is vanished by competi-tion between DE and SE interaccompeti-tion This result suggests that there is considerable mismatch effect located at A-site cations [12,13] Accordingly, for Ln A MnO
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Fig 1 SEM pictures of samples: (a) x ¼ 0; 1, (b) x ¼ 0:2.
Table 1
Some parameters of the studied samples Nd 0.5x Pr x Sr 0.5 MnO 3
Sample or A 4 (A˚) t T C (K) T CO (K)
or A 4: average ionic radius at site A; t: tolerant factor, T C : Curie
temperature, T CO : Charge-ordering transition temperature.
Fig 2 FC and ZFC thermomagnetic curves of sample with x ¼ 0:5.
N Chau et al / Journal of Magnetism and Magnetic Materials 303 (2006) e402–e405 e403
Trang 3perovskites, the drastic changes in properties are seen over
the small range of orA4, from 1.13 to 1.24 A˚ We can
conclude that the value oforA41.241 A˚ is defines a limit
at which magnetic and conducting properties drastically
change in our studied samples Fig 4 shows that TC
increases quite sharply when orA4 is greater than
1.241 A˚ This feature is in full agreement with the remark
in Ref.[6]for Ln0.5A0
0,.5MnO3perovskites
For all studied samples the magnetic entropy change as a
function of temperature, DSm(T), was evaluated andFig 5
displays DSm(T) for the sample with x ¼ 0:2 It is clear that
besides the negative peak around TC we can see a sharp
positive peak at TCO The existence of the positive peak of
DSm originates from the increase of magnetic entropy by
applied magnetic field when material makes a transition
from AFM to FM state in heating ðqM=qT 40Þ We could observe the interesting behavior that the material should be cooled by magnetizing at this AFM–FM transition Note that CO transition is the first-order transition while FM–PM transition is the second-order one For the rest samples with x ¼ 0:320:5, there is only a sharp peak of
DSm(T) around TC Fig 6shows that behavior in sample with x ¼ 0:3, for instance The magnetic entropy change at
TCOhas been also studied by other groups Sande et al.[14]
have reported that there is a large magnetocaloric effect in manganites with CO transition They found that the magnitude of DSm(T) at the first-order transition is around three times larger than that obtained at the second-order transition for sample Nd0.5Sr0.5MnO3 Szewczyk et al.[15]
have measured the giant magnetocaloric effect in manga-nites La1xSrxMnO3 (x ¼ 0:13, 0.16) with CO transition but they did not study the effect at T
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Fig 3 The existence of two magnetic transitions in FC and ZFC
thermomagnetic curves for sample with x ¼ 0:2.
Fig 4 The phase diagram of system Nd 0.5x Pr x Sr 0.5 MnO 3
Fig 5 The magnetic entropy change as a function of temperature for samples with x ¼ 0:2.
Fig 6 The magnetic entropy change as a function of temperature for sample with x ¼ 0:3.
N Chau et al / Journal of Magnetism and Magnetic Materials 303 (2006) e402–e405 e404
Trang 4In conclusion, perovskites Nd0.5xPrxSr0.5MnO3 (x ¼
0:120:5) were prepared with single-phase and
orthorhom-bic structure Two samples with x ¼ 0:1 and 0.2 exhibited
CO transition (coincides with FM–AFM transition) at low
temperatures Whereas, in the rest samples, there is spin
glass-like state at low temperatures and low field The
magnetic entropy change around CO transition has
positive value, contrary to that around FM–PM transition
Acknowledgments
The authors acknowledge the financial support from the
Vietnam National Fundamental Research Program
(Pro-ject 421004)
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