DSpace at VNU: On the magnetic and magnetocaloric properties of perovskite La1-xSrxCoO3

The crystal structure of the samples is rhombohedral with x ¼ 0:2020:45 and is cubic with x ¼ 0:50: Magnetization, Curie temperature Tc and maximum magnetic-entropy change DSmax dependin

On the magnetic and magnetocaloric properties of perovskite

Nguyen Hoang Luonga,*, Nguyen Chaua, Phan Manh Huonga, Dang Le Minhb,

Nguyen Ngoc Chaua, Bach Thanh Congb, M Kurisuc

a Faculty of Physics, 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

c School of Materials Science, Japan Advanced Institute of Science and Technology, Tatsunokuchi, Ishikawa 923-1292, Japan

Abstract

Structural, magnetic and magnetocaloric studies on La1
xSrxCoO3 (0.20pxp0.50) were reported The crystal structure of the samples is rhombohedral with x ¼ 0:2020:45 and is cubic with x ¼ 0:50: Magnetization, Curie temperature (Tc) and maximum magnetic-entropy change (DSmax) depending on the Sr content in the samples are discussed La0.55Sr0.45CoO3exhibits the largest value for DSmax and Tc among the compounds investigated r 2002 Elsevier Science B.V All rights reserved

Keywords: Magnetization; Magnetocaloric effect; Magnetic oxides; Perovskite structure

The perovskite compounds La1
xSrxCoO3 are

be-lieved to exhibit spin glass (SG)-like properties at low

temperatures for xo0:20 and cluster glass (CG)-like

properties at higher Sr concentraion [1] In high Sr

concentration range, x ¼ 0:50 is a typical concentration

corresponding to CG behavior [2] Wang and Zhang [3]

suggested that the double-exchange interaction played

an important role in the ferromagnetic metallic

con-ductivity of La1
xSrxCoO3 Takahashi et al [4] have

studied the electronic structures of La1
xSrxCoO3and

showed that La1
xSrxCoO3becomes metallic near x ¼

0:20 because of the ferromagnetic coupling between O

2p and Co 3d electrons Ibarra et al [5] have found a

huge anisotropic magnetostriction in La1
xSrxCoO3

(x ¼ 0:30; 0.50) Recently, Chaudhary et al [6] have

found moderately large magnetocaloric effect in

La1
xSrxCoO3(0.05pxp0.40) with maximum entropy

change in La0.6Sr0.4CoO3among samples studied In the

present work, we report on our study of magnetic and

magnetocaloric properties of La1
xSrxCoO3 with

0.20pxp0.50

Polycrystalline La1
xSrxCoO3samples with x ¼ 0:20; 0.25, 0.30, 0.35, 0.40, 0.45, and 0.50 were prepared by the conventional solid-state reaction technique from a stoichiometric mixture of La2O3, SrCO3 and CoO at pre-sintering temperature of 10001C for 15 h After regrinding and pressing for parallelepipeds, the samples were again heat treated to 9501C for 12 h and then to 10501C for 10 h The structure of the samples was examined by Bruker X-ray diffractometer D5005 The thermal stability associated with crystallization and melting was determined by differential scanning calori-metry (DSC) and thermogravimetric analysis (TGA) using TA Instruments apparatus SDT 2960 with a heating rate of 201C/min The magnetization measure-ments were performed with an SQUID magnetometer in the temperature range between 1.7 and 300 K and in the magnetic fields up to 70 kOe as well as with vibrating sample magnetometer (VSM) DMS 880 in the fields up

to 13.5 kOe

X-ray diffraction analysis revealed that all the samples are of single phase The crystal structure of the samples

is rhombohedral with x ¼ 0:2020:45 and is cubic with

x ¼ 0:50: These results are in good agreement with those obtained by Ibarra et al [5], who reported that the

*Corresponding author Fax: 84-4-858-94-96.

E-mail address: luong@cms.edu.vn (N.H Luong).

0304-8853/02/$ - see front matter r 2002 Elsevier Science B.V All rights reserved.

PII: S 0 3 0 4 - 8 8 5 3 ( 0 1 ) 0 1 0 1 6 - 2

crystal structures of x ¼ 0:30 and 0.50 are rhombohedral

and cubic, respectively

The DSC curves for all the samples show three sharp

endothermic peaks The DSC curves for the samples

with x ¼ 0:25 and 0.45 are displayed in Fig 1, as

examples, together with DSC and TGA curves for

starting material La2O3 It is suggested that, the first two

endothermic peaks are due to the decomposition of

La2O3into intermediate oxide phases It is interesting to

note that the third endothermic peak of both samples

around 931–9321C corresponds to the sharp

endother-mic peaks in the DSC curves for the starting materials

CoO and SrCO3 (not shown in the figure) This third

peak can be considered as corresponding to the reaction

forming a perovskite structure Probably because of this

reason, several research groups have sintered La1
xSr

x-CoO3 at temperatures lower than those for La1
xSrx

MnO3[5,6]

The zero-field-cooled (ZFC) and field-cooled (FC)

magnetization measurements were performed for all the

samples investigated Representative results obtained for

x ¼ 0:30 and 0.50 at H=100 Oe are displayed in Fig 2

The Curie temperature, Tc; of the samples determined

from these measurements is presented in Table 1 It can

be seen from Table 1 that Tc slightly increases with Sr

content up to x ¼ 0:45 then becomes somewhat lower in

the sample with x ¼ 0:50: These results differ from those

obtained by Chaudhary et al [6] who reported that the

Curie temperature is almost independent of Sr

concen-tration The results of ZFC and FC magnetization

measurements obtained for the samples with xp0:40

revealed the divergence of the ZFC and FC

measure-ments at a temperature Tir marking the appearence of

irreversible magnetic behavior (see Fig 2) We observe

the divergence of ZFC and FC measurements in the

samples with highest Sr contents in this study (x ¼ 0:45

and 0.50) in the whole temperature range apart from the

paramagnetic region

Fig 1 DSC curves for La 1
x Sr x CoO 3 with x ¼ 0:25 and 0.45.

Included are DSC and TGA curves for La O

Fig 2 Temperature dependence of magnetization for

La 1
x Sr x CoO 3 with x ¼ 0:30 and 0.50 Open squares and open circles represent the field-cooled (FC) and zero-field-cooled (ZFC) curves, respectively.

Table 1 Curie temperature, Tc; and maximum magnetic-entropy change, DSmax; as a function of the Sr concentration in

La 1
x Sr x CoO 3

Fig 3 shows the magnetization curves at 5 K for all

La1
xSrxCoO3samples As can be seen from this figure,

magnetization clearly increases with Sr content from

x ¼ 0:20 up to x ¼ 0:45 then drops rather dramatically

in x ¼ 0:50: We note that, as discussed earlier, the

crystal structure is rhombohedral in the samples with

x ¼ 0:2020:45; whereas it is cubic in the sample with

x ¼ 0:50: We suggest that, with increasing Sr content

from 0.20 up to 0.45, the ferromagnetic exchange

interaction between Co3+ and Co4+ is enhanced,

causing the increase of magnetization of the compounds

However, with higher Sr concentration x ¼ 0:50;

corre-sponding to higher Co4+ concentration, lower-spin

states of Co3+and Co4+occur, leading to the decrease

of magnetization

The isothermal M(H) curves for all La1
xSrxCoO3

samples have been measured at various temperatures

with small temperature intervals around the Curie

temperature in magnetic fields up to 13.5 kOe The

entropy change resulting from the spordering,

in-duced by the applied magnetic field, can be obtained

according to the thermodynamic relation [7]

DSðT ; HÞ ¼

Z H max

From the isothermal M(H) curves we evaluated the

entropy change, DS; as a function of temperature for all

the samples Results for La0.55Sr0.45CoO3are shown in

Fig 4, as example As is evident from Fig 4, the sample

exhibits a maximum in DS near Tc: Similar behavior was

observed for other samples investigated The maximum

magnetic-entropy change, DSmax; has been derived for

all La1
xSrxCoO3samples and presented in Table 1 As one can see from Table 1, the sample with x ¼ 0:45 displays the largest value of 1.04 J/kg K for DSmaxat an applied magnetic field of 13.5 kOe With higher Sr concentration, i.e x ¼ 0:50; DSmax decreases to a value

of 0.87 J/kg K Our results for Sr concentrations up to

x ¼ 0:4 basically agree with those reported by Chaudh-ary et al [6] However, the Sr content studied in Ref [6]

is limited only to x ¼ 0:4: This could be the reason that authors of Ref [6] could not observe the largest value for DSmax in x ¼ 0:45: Thus, La0.55Sr0.45CoO3 can be considered as an active magnetic refrigerant (AMR) material near 255 K

The authors are grateful to the Vietnam National Program for Natural Sciences for financial support and the VNU project QGTD-00-01 for a partial support

References

[1] M Itoh, I Natori, S Kubota, K Motoya, J Phys Soc Jpn 63 (1994) 1486.

[2] D.N.H Nam, K Jonason, P Nordblad, N.V Khiem, N.X Phuc, Phys Rev B 59 (1999) 4189.

[3] Z.L Wang, J Zhang, Phys Rev B 54 (1996) 1153 [4] H Takahashi, F Munakata, M Yamanaka, Phys Rev B

57 (1998) 15211.

[5] M.R Ibarra, R Mahendiran, C Marquina, B Garcia-Landa, J Blasco, Phys Rev B 57 (1998) R3217.

[6] S Chaudhary, V.S Kumar, S.B Roy, P Chaddah, S.R Krishnakumar, V.G Sathe, A Kumar, D.D Sarma, J Magn Magn Mater 202 (1999) 47.

[7] A.M Tishin, J Magn Magn Mater 184 (1998) 62.

Fig 3 Magnetization curves at 5 K for La 1
x Sr x CoO 3

Fig 4 The magnetic-entropy change as a function of tempera-ture for La 0.55 Sr 0.45 CoO 3