*Corresponding author. Tel.: #82-31-330-4362; fax: #82-31-333-1696.E-mail address: bwlee@san.hufs.ac.kr (B.W. Lee).

Journal of Magnetism and Magnetic Materials 226}230 (2001) 803}805

Phase transitions in La���

Ce�MnO

�(x"0.2, 0.3, 0.4)

B.W. Lee��*, K.Y. Seo�, Y.J. Kim�, H. Han�, H.H. Lee�, J.C. Han�,S.Y. Park�, C.S. Kim�

�Department of Physics, Hankuk University of Foreign Studies, Yongin, Kyungki, 449-791, South Korea�Department of Physics, Kookmin University, Seoul, 136-702, South Korea

Abstract

The phase transitions in La���

Ce�MnO

�(LCeMO; x"0.2, 0.3, 0.4) have been studied by using magnetization M,

resistivity �, speci"c heat C, and photoacoustic measurements. The substitution of La by Ce in LaMnO�induces

a metal}insulator transition accompanied by the occurrence of ferromagnetic ordering. No observable thermal hysteresisat transition temperatures implies that the phase transitions in LCeMO can be regarded as a second-order phasetransition with no latent heat at transition temperature. � 2001 Elsevier Science B.V. All rights reserved.

Keywords: Colossal magnetoresistance; Phase transitions*second-order; Resistivity*temperature dependent; Photoacoustic e!ect

The magnetic and electric properties of manganitesLa

���A

�MnO

�(LAMO; A"divalent cation) strongly

depend on the doping concentration x. In particular, ina range of doping, 0.2)x)0.4, LAMO exhibits a para-magnetic (PM) to ferromagnetic (FM) phase transitionaccompanied by a simultaneous insulator to metaltransition [1]. Similar to divalent ion doped LAMO, Cedoped La

���Ce

���MnO

�compounds exhibit the

metal}insulator (M}I) transition accompanied by fer-romagnetism and the colossal magnetoresistance (CMR)e!ect [2}4]. If Ce ions exist in a mixed-valent state witha valence between 3 and 4 as in the electron-dopedsuperconductors [5], then Mn ions are expected to beMn��/Mn�� instead of Mn��/Mn�, and the excesselectrons provided by Ce doping are responsible for theM}I transition and ferromagnetism. This raises the possi-bility of CMR occurring in system with a mixed-valentstate of Mn�� and Mn��.According to Mandal and Das [2], the �(¹) curve of

oxygen overdoped La���Ce

���MnO

�sample shows

double peaks with a small hysteresis around the peakregion. This implies that the phase transition can be

regarded as a "rst-order phase transition in the vicinity ofthe double peaks. However, we found that there is noobservable thermal hysteresis in �(¹), and the FM order-ing, determined from C(¹) and photoacoustic (PA)measurements, arises at the same temperature in bothcases with decreasing and increasing temperatures.Polycrystalline samples were prepared by standard

conventional solid-state reaction. The as-prepared sam-ples were annealed at 10503C in oxygen for 5 h. PowderX-ray di!raction studies showed similar patterns as inRef. [2]. Resistivity was measured by a standard four-probe technique, and magnetization was performed ona vibrating sample magnetometer. Speci"c heat was mea-sured by a di!erential scanning calorimeter, and PAmeasurements were done using a gas-microphone PA cellconstructed in the form of a Helmholtz resonator [6].Fig. 1 shows M(¹) data for LCeMO with x"0.2, 0.3,

and 0.4 in an applied "eld of 0.5T. A sharp increase inM(¹) implies the occurrence of magnetic ordering whichcorresponds to a PM to FM transition. The FMtransition temperature ¹

�, de"ned as the temperature of

the in#ection point of M(¹) curve, is 249K for x"0.2,260K for x"0.3, and 249K for x"0.4, respectively.The change of¹

�is less sensitive to doping concentration

x than that observed in La���

Ca�MnO

�system where

¹�is 176, 249, and 261K for x"0.2, 0.3, and 0.4,

respectively [6].

0304-8853/01/$ - see front matter � 2001 Elsevier Science B.V. All rights reserved.PII: S 0 3 0 4 - 8 8 5 3 ( 0 0 ) 0 0 8 6 1 - 1

Fig. 1. M(¹) for La���

Ce�MnO

�(x"0.2, 0.3, 0.4). Inset: C(¹)

for La��Ce

���MnO

�.

Fig. 2. Normalized resistivity �(¹)/�(300K) forLa

���Ce

�MnO

�(x"0.2, 0.3, 0.4). Temperatures correspond-

ing to the ¹�are indicated by arrows.

Fig. 3. Temperature dependence of the photoacoustic signalamplitude for La

���Ce

�MnO

�(x"0.2, 0.3, 0.4).

The inset of Fig. 1 displays C(¹) data for LCeMOwithx"0.2. The anomaly in C(¹) is observed, which hasa peak in the neighborhood of ¹

�. The maximum tem-

perature in the speci"c-heat anomaly is 249, 261, and249K for x"0.2, 0.3, and 0.4, respectively, This is ingood agreement with the ¹

�estimated from the M(¹)

measurement, indicating that the anomaly is due to mag-netic ordering associated with the magnetic phasetransition, which causes the variation in the internalenergy. Fig. 2 presents the normalized resistivity data forLCeMO, measured cooling and warming. The temper-ature corresponding to the ¹

�is indicated by the arrow

in �(¹) data. The �(¹) curve exhibits a peak superim-posed on a more rounded peak below ¹

�. This is very

similar to the previously reported results [2,3]. The peaktemperature is 256, 265, and 256K for x"0.2, 0.3, and0.4, respectively. As shown in Fig. 2, there is no observeddi!erence in �(¹) between cooling and warming, imply-ing that the transition is of second order. According toMandal and Das [2], the �(¹) curve of oxygen over-doped La

���Ce

���MnO

�sample shows a small hysteresis

around the peak region. This disagreement is probablydue to the oxygen excess, which cannot occupy inter-stitial positions in the lattice but results in equal amountsof La(Ce) andMn vacancies [7]. As in oxygen overdopedLCeMo, the hysteresis has been observed in high-pres-sure oxygenated La

�CuO

�� crystal, indicating that theprocess is weakly of "rst order [8]. The resistivity isa!ected by the presence of an external "eld. For x"0.3,magnetoresistance ratio, de"ned as [�(0)!�(H) ]/�(0)where �(0) is zero-"eld resistivity and �(H) is the resistiv-ity at 0.7 T, reaches a value of 17% at temperaturenear ¹

�.

Since PA signal is dependent on the thermal propertiesof samples, PA e!ect can be used as a simple techniquefor the characterization of phase transition in solid sam-ples. Fig. 3 shows the amplitude of PA signal vs. temper-ature data for LCeMO, measured cooling and warming.The data are normalized to the value at 300K, and

properly shifted for convenient viewing. The anomalouschange of PA signal in amplitude occurs at ¹

�. The

transition width, which is de"ned as the temperaturedi!erence between the sudden changes of PA signal, is8K for x"0.2, 8K for x"0.3, and 10K for x"0.4,respectively. It is found that changes of PA signal arise atthe same temperature in both cases with decreasing andincreasing temperatures. This implies that phasetransitions in LCeMO can be regarded as a second-orderphase transition with no latent heat at ¹

�, which is

consistent with the result of �(¹) measurements.

This work was "nancially supported by KOSEF(97-07-02-04-01-5) and BK21 Project.

References

[1] P. Schi!er, A.P. Ramirez, W. Bao, S.W. Cheong, Phys. Rev.Lett. 75 (1995) 3336.

804 B.W. Lee et al. / Journal of Magnetism and Magnetic Materials 226}230 (2001) 803}805

[2] P. Mandal, S. Das, Phys. Rev. B 56 (1997) 15073.[3] J.R. Gebhardt, S. Roy, N. Ali, J. Appl. Phys. 85 (1999)

5390.[4] S. Das, P. Mandal, Z. Phys. B 104 (1997) 7.[5] J.T. Markert et al., Physica C 158 (1989) 178.

[6] Y.J. Kim, B.W. Lee, C.S. Kim, IEEE Trans. Magn. 35 (1999)2874.

[7] J.A.M. Van Roosmalen, E.H.P. Cordfunke, J. Solid StateChem. 110 (1994) 106.

[8] M.F. Hundley et al., Phys. Rev. B 41 (1990) 4062.

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