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Journal of Magnetism and Magnetic Materials 281 (2004) 261–266

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doi:10.1016

Inhomogeneous magnetic behavior of Pr0.7Ca0.3CoO3 andNd0.7Ca0.3CoO3

Asish K. Kundua, E.V. Sampathkumaranb, K.V. Gopalakrishnanb, C.N.R. Raoa,*a Chemistry and Physics of Materials Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur P.O.,

Bangalore-560064, Indiab Department of Condensed Matter Physics and Materials Science, Tata Institute of Fundamental Research, Mumbai-400005, India

Received 18 February 2004; received in revised form 10 April 2004

Available online 26 May 2004

Abstract

Unlike La0.7Ca0.3CoO2.97, Pr0.7Ca0.3CoO3 and Nd0.7Ca0.3CoO2.95 do not show distinct ferromagnetic transitions, but

instead they exhibit very low magnetic moments down to 50 K. A detail study of magnetic properties of Pr0.7Ca0.3CoO3

and Nd0.7Ca0.3CoO2.95 shows that the materials are inhomogeneous, exhibiting properties similar to those of frustrated

magnetic systems. In both these cobaltates, small ferromagnetic clusters seem to be present in an antiferromagnetic

host.

r 2004 Elsevier B.V. All rights reserved.

PACS: 72.80.Ga; 72.15.E; 74.25.Ha; 75.50.Lk

Keywords: Transition-metal oxides; Rare-earth compounds; Magnetic Properties; Spin-glasses

1. Introduction

Investigations of rare-earth manganates of thegeneral formula Ln1xAxMnO3 (Ln=rare earth,A=alkaline earth) have revealed remarkableaspects of these materials which include colossalmagnetoresistance (CMR), charge ordering (CO),orbital ordering as well as electronic phaseseparation [1–4]. Properties of these materialsare crucially controlled by the average radius of

onding author. Tel.: +91-80-23623075; fax: +91-

0.

address: cnrrao@jncasr.ac.in (C.N.R. Rao).

$ - see front matter r 2004 Elsevier B.V. All rights reserve

/j.jmmm.2004.04.112

the A-site cation /rAS [3–7]. Thus, whileLa0.7Ca0.3MnO3 (/rAS ¼ 1:205 (A) shows insula-tor to metal transition and ferromagnetism, withmetallicity associated with ferromagnetism at lowtemperatures [3,4,8], Pr0.7Ca0.3MnO3 with a smal-ler /rASð1:179 (A) shows no ferromagnetism orinsulator to metal transition. Instead, the lattermanganate exhibits charge ordering, orbital order-ing and electronic phase separation [3,4,9]. Wewere interested in exploring whether the analogouscobaltates of general formula Ln1xAxCoO3

(Ln=rare earth, A=alkaline earth) exhibit similarfeatures. In this cobaltate system, the x ¼ 0:3composition is ferromagnetic and metallic when

d.

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A.K. Kundu et al. / Journal of Magnetism and Magnetic Materials 281 (2004) 261–266262

Ln=La and A=Sr or Ca [10–15] and also whenLn=Pr and A=Sr [15–17]. A cluster-glass beha-vior has also been suggested at low temperatures inthese materials [11], but this has not been entirelyestablished [14]. We considered it important toinvestigate the properties of Pr0.7Ca0.3CoO3 andNd0.7Ca0.3CoO3, with smaller /rAS values of1.179 and 1.168 (A, respectively, to examine howtheir properties vary from those of La0.7Ca0.3CoO3

(/rAS ¼ 1:354 (A).

2. Experimental procedure

Polycrystalline samples of Ln0.7Ca0.3CoO3d

(Ln=La, Pr, Nd) were prepared by the conven-tional ceramic method. Stoichiometric mixtures ofthe respective rare-earth oxides, alkaline earthcarbonates and Co3O4 were weighed in desiredproportions and milled for few hours withpropanol. After the mixed powders were dried,they were calcined in air at 950C followed byheating at 1000C and 1100C for 12 h each in air.The powders thus obtained were pelletized and thepellets sintered at 1200C for 12 h in air. Toimprove the oxygen stoichiometry, the sampleswere annealed in an oxygen atmosphere at a lowertemperature (p900C). The oxygen stoichiometrywas determined by iodometric titrations, the errorin the determination being 70.02. The oxygenstoichiometry in the Ln0.7Ca0.3CoO3d (Ln=La,Pr, Nd) thus obtained were 2.97, 3.00 and 2.95,respectively, in the La, Pr and Nd derivatives.

The phase purity of the samples was establishedby recording the X-ray diffraction patterns in the2y range of 10–80 with a Seiferts 3000 TTdiffractometer, employing Cu-Ka radiation. Theunit cell parameters of Ln0.7Ca0.3CoO3d

Table 1

Crystal structure data of Ln0.7Ca0.3CoO3d (Ln=La, Pr, Nd)

Composition /rAS ( (A) Space group

La0.7Ca0.3CoO2.97 1.354 R%3C

Pr0.7Ca0.3CoO3.00 1.179 Pnma

Nd0.7Ca0.3CoO2.95 1.168 Pnma

(Ln=La, Pr, Nd) are listed in Table 1 along withthe weighted average radius /rAS: The /rASvalues were calculated using the Shannon radii for12-coordination in the case of rhombohedralcobaltates, and for 9-coordination in the case ofthe orthorhombic ones. The uncertainties in theunit cell parameters are 70.004 (A. Magnetizationmeasurements were made with a vibrating samplemagnetometer (Lakeshore 7300) and with aSQUID magnetometer (Quantum Design). Elec-trical resistivity (r) measurements were carried outby the four-probe method with silver epoxy as theelectrodes in the 300–20 K temperature ranges.

3. Results and discussion

Preliminary measurements (at 1 kOe) of the DCmagnetic susceptibilities of Ln0.7Ca0.3CoO3d withLn=La, Pr, Nd showed that while La0.7Ca0.3-CoO2.97 clearly exhibits a ferromagnetic transition(TcB175 K), Pr0.7Ca0.3CoO3 and Nd0.7Ca0.3-CoO2.95 do not show distinct ferromagnetictransitions down to 50 K (Fig. 1a). There is aslight increase in the susceptibility of Pr0.7Ca0.3-CoO3 around 75 K, but this is not due to a genuineferromagnetic transition. The magnetic behaviorof a single crystal of Pr0.7Ca0.3CoO3 is similar tothat of the polycrystalline sample (see inset of Fig.1a). On the basis of the /rAS values, theferromagnetic Tc values of Pr0.7Ca0.3CoO3 andNd0.7Ca0.3CoO3 would be expected to be wellabove 100 K. Electrical resistivities of these cobal-tates are also much higher (Fig. 1b). The largedrop in the magnetic moment at low temperaturesin the Pr and Nd derivatives is noteworthy. Inorder to understand the nature of these materials,we have investigated the magnetic properties of

Lattice parameters ( (A) V ( (A3)

a b c

5.3906 — — 111.60

5.3577 7.5774 5.3436 216.94

5.3460 7.5638 5.3287 215.47

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0.0

0.2

0.4

0.6

SinglecrystalPolycrystal

75 150 225 3000

100

200

T(K)

100 200 300

0

1

2

3

χ-1(m

ol /e

mu)

χ(em

u /m

ol)

Ln = Nd

Ln = Pr

Ln= La H = 1 kOe

Ln0.7Ca0.3CoO3 Pr0.7Ca0.3CoO3

χ(em

u /m

ol)

T(K)

0 100 200 300

10-3

10-2

10-1

Ln= Nd

Ln= La

Ln= Pr

T(K)

ρ(Ω

.cm

)

(b)

(a)

Fig. 1. Temperature dependence of (a) the magnetic suscept-

ibility, w; (H ¼ 1000 Oe) and (b) the electrical resistivity, r; of

Ln0.7Ca0.3CoO3d (Ln=La, Pr or Nd). The inset in (a) shows

the magnetic susceptibility, w; and inverse magnetic suscept-

ibility, w1; of Pr0.7Ca0.3CoO3 for single crystal and polycrystal-

line samples.

0 20 40 60 80 100 120

2

4

6

8

10

ZFC

FCPr0.7Ca0.3CoO3

H = 100 Oe

χ (e

mu/

mol

)

T(K)

Fig. 2. Temperature dependence of magnetic susceptibility, w;of Pr0.7Ca0.3CoO3 (H ¼ 100 Oe). Solid and dotted lines

represent ZFC and FC data, respectively.

0 50 100 150 200 250

0.0

0.1

0.2

0.3

χ-1(m

ol /e

mu)

Pr0.7Ca0.3CoO3

H = 5 kOe

χ(em

u/m

ol)

T(K)

0 80 160 240

0

50

100 150< T < 250

θp = -30 K

µeff = 4.68 /f.u.

T(K)

Fig. 3. Temperature dependence of magnetic susceptibility, w;(H ¼ 5000 Oe) of Pr0.7Ca0.3CoO3. The inset shows the tem-

perature dependence of inverse magnetic susceptibility, w1;(H ¼ 5000 Oe).

A.K. Kundu et al. / Journal of Magnetism and Magnetic Materials 281 (2004) 261–266 263

Pr0.7Ca0.3CoO3 in detail, down to low tempera-tures.

In Fig. 2, we show the temperature variation ofthe DC magnetic susceptibility of Pr0.7Ca0.3CoO3

in the zero-field-cooled (ZFC) and field-cooled(FC) states (H ¼ 100 Oe). There is considerabledivergence in the ZFC and FC behavior just as inmagnetically frustrated systems [11]. The datashow two broad transitions around 60 and 30 K.Measurements carried out at 5 kOe, however, donot reveal the two peaks (Fig. 3), suggesting thatthe intermediate temperature range M2H beha-vior of this material is rather complex at low fields.The data in Fig. 3 suggest that magnetic orderingsets in around 75 K with the susceptibility going

through a broad maximum around 15 K. Inversemagnetic susceptibility data, shown in the inset ofFig. 3, yield a Curie temperature (yp) of 30 K.The high temperature linear region of the inversesusceptibility data gives a magnetic moment of4.7 mB. The shape of w2T plot below 75 K is rathercomplex, not typical of normal ferromagnets. Itappears as though there is a spread of magnetictransition temperatures due to local environmentaleffects.

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0 20 40 60 80 100 120

0.2

0.4

0.6

0.8

60 K

Pr0.7Ca0.3CoO3

100 K

50 K

25 K

T= 5K

M (

µ B/f.

u.)

H (kOe)

Fig. 4. High-field magnetization curve of Pr0.7Ca0.3CoO3 at

low temperatures.

-10 -5 5 10-0.4

-0.2

0.0

0.2

0.4

60 K

Pr0.7Ca0.3CoO3

50 K 25 K 5K

M (

µ B/f.

u.)

H (kOe)0

Fig. 5. Low-field magnetic hysteresis of Pr0.7Ca0.3CoO3 at low

temperatures.

Fig. 6. AC-magnetic susceptibility data of Pr0.7Ca0.3CoO3 at

1 Oe.

A.K. Kundu et al. / Journal of Magnetism and Magnetic Materials 281 (2004) 261–266264

In Fig. 4, we show the M2H behavior ofPr0.7Ca0.3CoO3. The behavior is rather complexespecially in the temperature range of 25–60 K.The plots remain nonlinear up to 120 kOe even at5 K. The behavior is unlike of ferromagnets and issomewhat comparable to that of frustrated sys-tems. Extrapolation of the M2H data in the high-field region to zero field gives a saturation momentof around 0.4 mB. The small value of the momenton cobalt in the apparently ferromagnetic state,compared with the value in the paramagnetic state,indicates itinerant ferromagnetism, which ispossible because the material is conducting. FromFig. 5, we see that there is hysteresis at 5 K even atlow fields, suggesting a ferromagnet-like behavior.The width of the hysteresis loop decreasesmarkedly with increasing temperature. The aboveresults reveal that ferromagnetic and antiferro-magnetic interactions coexist at low temperatures,with the small conducting ferromagnetic domainsor clusters giving rise to a small magnetic moment.

AC susceptibility measurements (Fig. 6) showthat the low-temperature transition has a fre-quency dependence of about 1.3 K, as thefrequency is increased from 1.3 to 1330 Hz. The60 K peak, however, shows little shift (Fig. 6). Theposition of the low-temperature peak in the ACsusceptibility data at 1.3 Hz, for which the field ofmeasurement is 1 Oe, occurs at 37.4 K, and shiftsto lower temperatures at higher fields. Thus, for

H ¼ 100 and 5000 Oe, the peak occurs at 31.5 and12.7 K, respectively. Because of the inhomoge-neous nature, it is difficult to clearly assign onetemperature for the bulk transition in this cobal-tate, although the first transition clearly occursaround 60 K. While we have compared the

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0 15 30 45

0.0

0.5

1.0

1.5

2.0

ZFC

FCNd0.7Ca0.3CoO3

H = 100 Oe

χ(em

u/m

ol)

T(K)

Fig. 7. Temperature dependence of the magnetic susceptibility,

w; of Nd0.7Ca0.3CoO2.95 (H ¼ 100 Oe). Solid and dotted lines

represent ZFC and FC data, respectively.

-60 -30 0 30 60

-1.0

-0.5

0.0

0.5

1.0

T= 5 K

T= 2 K

M (

µ B/f.

u.)

H(kOe)

0 100 200 300

0.00

0.05

0.10

0.15

0.20

H = 5 kOe

Nd0.7Ca0.3CoO3

χ(em

u /m

ol)

T(K)

Fig. 8. Temperature dependence of magnetic susceptibility, w;of Nd0.7Ca0.3CoO2.95 (H ¼ 5000 Oe). The inset shows low-

temperatures hysteresis.

A.K. Kundu et al. / Journal of Magnetism and Magnetic Materials 281 (2004) 261–266 265

inhomogeneous nature of Pr0.7Ca0.3CoO3 at lowtemperatures to that of cluster or spin-glasses [11],isothermal remnant magnetization measurementsin the 5–60 K range rule out that the material isactually a glass. Thus, isothermal remnant magne-tization is time-independent and does not decaylogarithmically or exponentially. We, therefore,conclude that the behavior of Pr0.7Ca0.3CoO3

represents a special case of electronic phaseseparation.

We have carried out studies on polycrystallineNd0.7Ca0.3CoO2.95 as well. This sample also showsdivergence in the ZFC and FC behavior atH ¼ 100 Oe (Fig. 7), but the divergence is notmarked as much as in Pr0.7Ca0.3CoO3. The ZFCdata seem to suggest two close transitions between0 and 20 K. The DC susceptibility data at highfields (H ¼ 5 kOe) show one distinct transitionaround 20 K (Fig. 8). The inverse magneticsusceptibility data yield a yp value 170 K. TheM2H behavior of this cobaltate is also nonlinearjust as Pr0.7Ca0.3CoO3. The material shows narrowhysteresis below 5 K and below, at high fields (seeinset Fig. 8).

The electronic phase separation and associatedmagnetic properties of Pr0.7Ca0.3CoO3 andNd0.7Ca0.3CoO2.95 arise because of the smallaverage size of the A-site cations. In these twocobaltates, the average radius (for orthorhombicstructure) is less than 1.18 (A, which is the critical

value only above which long-range ferromagnet-ism manifests itself [18]. It is known that increasein size disorder and decrease in size favor phaseseparation.

4. Conclusions

Pr0.7Ca0.3CoO3 does not show a sharp ferro-magnetic transition down to 50 K. There is largedivergence between the DC magnetic susceptibilityof the ZFC and FC sample. The magnetization isnonlinear with field. AC susceptibility data showevidence for a magnetic transition around 60 Kand a frequency-dependent transition at lowtemperatures. Isothermal remnant magnetizationmeasurements, however, reveal that the cobaltateis not a spin-glass. Properties of Nd0.7Ca0.3CoO2.95

are not unlike those of the Pr analogue. Thesevarious features indicate that these cobaltates aremagnetically inhomogeneous, with small ferro-magnetic clusters or domains being present in anantiferromagnetic matrix.

Acknowledgements

The authors would like to thank BRNS (DAE),India for support of this research. A.K.K wants to

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thank University Grants Commission, India forfellowship award.

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