Magneto-transport and specific heat behavior of Cd-doped

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Journal of Magnetism and Magnetic Materials 285 (2005) 130–137

Magneto-transport and speciﬁc heat behavior of Cd-doped La

_0.5

Ca

_0.5

MnO

₃

K.F. Wang

^a,b

, Q. Xiao

^a,b

, H. Yu

^a,b

, M. Zeng

^a,b

, M.F. Zhang

^a,b

, J.-M. Liu

^a,b,

aLaboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, China

bInternational Center for Materials Physics, Chinese Academy of Sciences, Shenyang, China

Received 2 June 2004 Available online 9 August 2004

Abstract

The temperature-dependent magneto-transport properties and the speciﬁc heat of La_0.5(Ca_0.5xCd_x)MnO₃ (x¼0:020:5) are measured for understanding the electronic phase separation behaviors in manganites. It is surprising to observe that Cd-doping enhances the ferromagnetic metallic state and suppresses the insulating state. Two insulator–metal transitions in La_0.5(Ca_0.3Cd_0.2)MnO₃are observed, one occurs at temperatureT_P1in accordance with the observed paramagnetic–ferromagnetic transition pointTCand the other appears at a lower temperatureTP2. The magnetoresistance of La_0.5(Ca_0.3Cd_0.2)MnO₃as a function of temperature shows a sharp peak aroundT_C, and remains roughly unchanged over a wide temperature range belowTP1. It is argued that these fantastic properties are attributed to the coexistence of ferromagnetic metallic state and ferromagnetic insulating state under an antiferromagnetic background due to the Cd-doping effect.

PACS:75. 47. Lx; 75. 47. Gk; 71. 30. +h

Keywords:Manganites; Colossal magnetoresistance; Electronic phase separation

1. Introduction

Colossal magnetoresistance (CMR), an unu- sually large change of resistivity following applica-

tion of a magnetic ﬁeld H, observed in certain materials, has been extensively investigated in pervoskite manganites with a general formula RE1xAExMnO3 (RE: trivalent rare-earth ions and AE: divalent alkaline-earth ions) due to their exotic electrical and magnetic properties [1–4].

From a fundamental point of view, the central challenge is to identify the mechanisms responsible for CMR. The earlier works revealed that the

www.elsevier.com/locate/jmmm

doi:10.1016/j.jmmm.2004.07.026

Corresponding author. Laboratory of Solid State Micro- structures, Nanjing University, Nanjing 210093, China. Tel./

fax: +86-25-8359-7060.

E-mail address:[email protected] (J.-M. Liu).

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CMR effect was attributed to the double exchange (DE) [5], i.e. the hopping of egelectrons between spin-aligned Mn³⁺ and Mn⁴⁺ ions through the oxygen ions, which favors ferromagnetic metallic (FMM) state. However, it is now well accepted that the electronic phase diagram of CMR manganites is very complex. The DE model alone is insufﬁcient to explain the CMR effect and related magneto-transport properties [6–8]. The recent works allow us to conclude that given a temperature T and ﬁeld H the electronic and magnetic ground state of manganites can be inhomogeneous due to the coexistence of FMM phase and FM insulating phase (FMI) or antiferromagnetic insulating (AFI) phase, originating from the electronic phase separation[9–11]. As the carrier concentration varies, various ordering states and phase transitions can be observed. The preference of one ordering state rather than others or coexistence of two or more states depends on the doping, pressure (external or chemical), T andH. Consequently, the system property can be very different.

One case of the above-mentioned phase separation is the coexistence of FMI and FMM phases under an AF background [11]. The coexistence of multi-ground states is interesting due to the fact that these states compete energetically, giving rise to fascinating transport properties. One of the well-studied systems is La1xCaxMnO3where AF charge-ordering (CO) state may exist for 0.5pxo0.9[12,13]. The charge ordering transition of doped manganites usually accompanies an antiferromagnetic transition with the so-called CE-type structure. The FM ordering occurs below 200 K and the AFI state does not appear above 100 K. It indicates that La1xCaxMnO3

(x¼0:5) is still dominated by FMM state but with an AF background unlessTis extremely low. A doping at A- or B-site would be very signiﬁcant in modulating the energy levels of these ordered states, thus generating fascinating transport properties. In fact, the doping effect on La1xCaxMnO3 by various elements has been widely studied only recently[14–17]. Element Cd is divalent and its ionic radius is comparable with Ca²⁺. We found that La0.5Cd0.5MnO3

is dominated by an FMI state although at very

low T the AF background becomes signiﬁcant too. Therefore, one may simply argue that an FMI state rather than an FMM state would be preferred if Cd-doping into La0.5Ca0.5MnO3is made. In fact, a substitution of Ca sites by Cd in La0.7Ca0.3MnO3 was investigated earlier [18]

and this system does not prefer the CO state unless T is extremely low. The main effect is a progressive decrease both in spontaneous magnetization M and in Curie temperature TC. It was concluded that Cd-doping progressively suppresses the DE interaction, thus weakening the FMM behavior. Nevertheless, note that M of La0.5Cd0.5MnO3 is several times bigger than that of La0.5Ca0.5MnO3. The Cd-doping may enhance the local magnetic moment in La0.5(Ca0.5xCdx)MnO3, which on other hand favors the FMM state. Thus, some interesting phenomena different from these once observed in La0.7Ca0.3MnO3 can be expected, which offers a complementary understanding of the magnetic and electronic properties associated with competition between the FMI state and FMM state (DE state) under an AF background.

In this paper, we shall study the effect of Cd- doping on the magneto-transport behaviors in La0.5(Ca0.5xCdx)MnO3.

2. Experimental details

Bulk ceramic La0.5(Ca0.5xCdx)MnO3

(x¼0:020:2) were prepared by the conventional solid-state reaction in air. We also prepared La0.5Cd0.5MnO3for reference. Stoichiometric pro- portions of high-purity La2O3, CaCO3, MnCO3

and CdO powders were mixed and ground, then ﬁred at 10001C for 24 h. After grounding, the powders were pelleted and sintered in air at 12001C for 24 h. The crystallinity of the samples was examined by X-ray diffraction (XRD) with Cu Ka radiation. The magnetic measurements were carried out using a vibrating sample magnetometer (VSM) in a cooling run down to 4 K under H=500 G. The magneto-transport measurements were performed by a standard four-probe method using a Quantum Design physical property measurement system (PPMS,T=2–400 K,H=0–7 T).

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The measurement of the speciﬁc heat was carried out by the same PPMS. The X-ray photoemission spectroscopies (XPS) were used to study the actual oxidation state of Mn in our samples.

3. Results and discussion

In Fig. 1 the XRD spectra for different x are presented. At room temperature the reflections of all samples can be accurately indexed in an orthorhombic space group (Pnma) except La0.5Cd0.5MnO3, which instead shows a rhombo- hedralðR3cÞ space groups (as shown by the arrows in Fig. 1) [18]. It is concluded that Cd-doping at Ca sites causes no identified lattice effects and the samples remain single-phase in terms of lattice configuration.

In general, by Cd-doping into the sample, one sees an overall decrease of resistivity r(0) with increasing doping levelx. r(0) and r(H=7T) as a function of T for several samples is presented in Fig. 2(a)–2(c) and that for La0.5Cd0.5MnO3 is in Fig. 2(d)for a comparison. The magnetoresistance

ratio MR=(r(0)–r(7T))/r(7T) is also shown in Fig. 2. InFig. 3, we presentMas a function ofT, where the FM transition pointTCis deﬁned at the maximum slope9dM/dT9_max. An overall comparison of the data clearly demonstrates the signiﬁ- cant effect of Cd-doping on the magneto-transport behaviors. For x¼0 (Fig. 2(a)), rð0Þ increases with decreasing T and reaches the maximal at T=TP–160 K, close toTC as reported earlier [12], indicating the insulator–metal (I–M) transition at T_P. As expected, forH40,r(H) is suppressed and

20 30 40 50 60 70 80

L a_0.5C d_xC a_0.5-xMn O₃

x= 0. 5

x= 0. 2

x=0.1

x=0.0

Intensity (a.u.)

2θ(degree)

Fig. 1. X-ray diffraction spectra of polycrystalline La0.5(Cdx-

Ca0.5x)MnO3samples withx¼0;0:1;0:2;0:5:

0 50 100 150 200

0 5 10 15

0.0 0.1 0.2

0 50 100 150 200 250 300 10⁰

10² 10⁴ 10⁶

0 4 8 12

0 1 2

0 2 4

(a) x=0 ^{H=0 T}

H=7 T

x=0.1 ^{H=0 T}

H=7 T

x=0.2 ^{H=0 T}

H=7 T

(Ω.cm)

x=0.5 H=0 T

H=7 T

T (K)

MR

(b)

(c)

(d)

Fig. 2. Resistivity as a function of T under zero-ﬁeld and H=7 T for La0.5(CdxCa0.5x)MnO3 samples below room temperature; and temperature dependence of magnetoresistance (MR) ratio underH=7 T ﬁeld for the samples.

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TPshifts to a higher value. The MR ratio exhibits a sharp peak nearTP. Here, it should be noted that the transport behaviors for x¼0 sample is well reproduced but not exactly the same as reported previously [12], probably due to the difference in the sintering processing and microstructures.

The sample with x¼0:1 has similar transport behaviors (TP=230 K). But it is surprising to ﬁnd that the doping does not suppress but improves the system conductivity by almost one order of magnitude. Furthermore, in comparison with x¼0; not onlyTCandTPare higher but also M below TC is 3 times bigger. These observed effects are further strengthened in the x¼0:2 sample, but we turn to the more interesting effects.

At H=0, besides the expected I–M transition (TP1245 K), close to TC and related to a CMR component, another broad resistivity peak is observed at a lower T (TP2180 K). As H40,

both TP1 and TP2 shift towards higher values.

What should be mentioned here is that the MR ratio for this sample not only shows a peak around T–TP1, but also exhibits a platform as a function of T over 230–20 K although a weak and broad peak near TP2 can be identiﬁed. That is to say, over this range the MR ratio remains roughly constant, which is useful from the point of view of practical applications in sensing and reading.

Similarly, the FM transition occurs at a higher T than that forx¼0:1:

We also look at La0.5Cd0.5MnO3. Bothr(0) and r(H) as a function of T shows an insulating behavior over the whole T-range covered here, although a weak resistivity peak almost immersed by the rapidly varying resistivity with T at TP150 K can be identified. This weak peak is located exactly at the FM transition point TC150 K. Correspondingly, a weak MR ratio peak at T–TC is observed too. These results indicate that La0.5Cd0.5MnO3 prefers the FMI state at ToTC. However, the immersed resistivity peak at TC indicates that the double-exchange coupling makes a sense too, and the FMM state exists simultaneously. At ToTC, the AF background becomes significant, confirmed by the slowly decreasing Mwith decreasingT.

Fig. 4shows the fitting of the high-temperature (T4y_D=2;y_D being the Debye temperature) resistivity data under zero-field with the small- polaron-hopping (SPH) model of Mott and Davis [21], i.e. r=T ¼r_a expðEP=kBTÞ; where r_a¼ ½kBvphNe²R²Cð1CÞexpð2aRÞ;kB is the Boltzmann constant, Nis the number of ion sites per unit volume (obtained from density data),Ris the average intersite spacing given by R¼ ð1=NÞ¹⁼³; C is the fraction of sites occupied by a polaron, a is the electron wave function decay constant, vph is the optical phonon frequency (estimated from the relation hv_ph¼k_ByD ). Plot- ting the resistivity curves as lnðr=TÞversus 1/T, the activation energies EP has been estimated for all the samples both in the presence and absence of a magnetic field. The values of EPare listed in Table 1. The theoretically predicted decreasing activation energy due to breaking of the CO–AF networks is verified from the decreasingEPvalues.

For La0.5Cd0.5MnO3, the observed FMI phase 0

1 2 3 4 5

0 1 2 3

0 50 100 150 200 250 300 0

1 2 3 4 5

0 20 40 60 80 0 20 40 60 80

0 40 80 120 x=0.1

x=0.2

M (emu/g)

x=0.5

T (K)

1/M (a.u.)

(a)

(b)

(c)

Fig. 3. Temperature dependence of the DC magnetization, and reciprocal magnetization (1/M) as a function of temperatureT.

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may arise mainly due to the superexchange between Mn ions, and the resistivity increased obviously. And hence the EP value increased.

Application of a magnetic ﬁeld decreases the charge localization, accordingly,E_Pdecreases.

The above experiments tell us that La0.5Ca0.5

MnO3 prefers FM order below 200 K and AF order below 100 K, suggesting the FMM as ground state under the AF background. The Cd- doping leads to shifting ofTCfrom 160 K (x¼0) to 250 K (x¼0:2), i.e. the FMM state is preferred at a higherT asxis larger. In spite of this effect, the pointTN100 K at whichMbegins to fall with

decreasing T does not change much with x, and we deﬁne it as the AF-ordering point. This AF-ordering is weak in relation to the FMM ordering since the overall metallic behavior below TC is observed. It can be roughly concluded that as TNoToTC;the dominant ground state is the FMM state. As T4TC, the paramagnetic insulating state is preferred, and as ToTN, the FMM state is still the ground state but with the AF-ordering background.

It is well known that the metal–insulator transition near TCresults from the percolation of FMM transition. As we know, the double exchange between Mn³⁺and Mn⁴⁺mediates the ferromagnetism and metallic conduction. The Cd substitution of Ca sites decreases the cationic radii and results in the decrease of the tolerance factor, and thus distorts the cubic perovskite cell and gradually suppresses the double exchange interaction. Ultimately, those Mn ions adjacent to those Cd occupied sites no longer effectively participate in the DE sequences. Along this line, it would be expected that the conduction of Cd-doped systems is worse than zero-doping system, and the FM transition would occur at a lower temperature thanT_Cforx¼0:However, these predictions are in controversial to our experimental observations.

Although La0.5Cd0.5MnO3 prefers FMI state, its magnetization is several times bigger than La0.5Ca0.5MnO3. One may argue that a partial substitution of Ca sites by Cd may enhance the local FM-ordering around Cd ions. This will enhance the spin-ordering tendency of the local sites. Thus, a decrease of resistivity and shifting of both TC and TP towards higher value are observed. In Fig. 3, the reciprocal magnetization (1/M) as a function of T is also shown. The nonlinear behavior of 1/M as function of T (deviation from Curie–Weiss law) in the paramagnetic state for x¼0:1 and 0.2 indicates the existence of magnetic clusters in the high-Trange.

It supports our argument that the Cd-doping prefers the local FM-ordering state at high-T range.

Another possible case is that the doping Cd ions entered at the Mn sites instead of entering at the Ca sites. In order to ascertain the sites at which the doping Cd entered, we studied the actual oxidation

3. 15 3.50 3.85 4.20

-8 -7 -6 -5 -4 -3

ln(/T)

1000/T(K ^-1)

x= 0.0 x= 0.1 x= 0.2 x= 0.5

Fig. 4. Variation of ln(r/T) of the samples as a function of inverse temperature. The units of randT areOcm and K, respectively.

Table 1

g(mJ/mol K²) EP(meV)

B=0.0 T B=7.0 T

x¼0:0 3.48 201.83 182.32

x¼0:1 — 167.33 118.81

x¼0:2 13.71 114.36 20.31

x¼0:5 7.53 181.50 160.78

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state of Mn ions in our samples by X-ray photoemission spectroscopy (XPS). Fig. 5 gives the X-ray photoemission spectroscopes of the Mn 2p electron orbital energy level in two samples withx¼0:0 and 0.2. We can see that there are two peaks at two binding energies, which correspond to the Mn 2p1/2and 2p3/2electron orbital energy levels, respectively. Furthermore, the binding energies at the peaks, and the proportion between the relative intensities of two peaks do not vary evidently with the increasing of doping Cd. So we conclude that the doping Cd entered the Ca sites and the valence states of Mn did not vary with the Cd-doping.

Due to partial substitution of larger Ca ions by smaller Cd ions, Mn–O–Mn bond bending increase with a decrease of average A site ionic radius/rAS. This substitution causes a suppression of the Mn–O–Mn 1801superexchange interaction which leads to antiferromagnetic ordering.

So the Cd ions distorted the cubic perovskite cells and frustrate the long-range orbital order associated with the AFM insulating state. With the suppression of long-range orbital ordering, a short-range FMI state shows up, as we stated

above. In Cd-doped samples, it randomly occupies the Ca site in the lattice, which suppresses the antiferromagnetic ordering. This, together with the competitive antiferromagnetic superexchange and the double exchange interaction, leads to the formation of ferromagnetic insulated clusters in the FM matrix. Therefore, the abnormal magneto- transport property is basically the result of electronic phase separation.

One possible origin of the double-peaks in the r–Tcurve may be the effect of grain boundaries in polycrystalline samples. Li et al.[22]and Hwang et al. [23] demonstrated the presence of two MR components in polycrystalline samples—one near TC from the regular CMR mechanism inside the grains, and the other most important at low-T from magnetic scattering or spin-polarized inter- grain tunneling at grain boundaries. However, there is only one MR peak in our sample x¼0:2:

And the effect of grain boundaries present only in polycrystalline samples under low magnetic ﬁeld (several hundred or thousand Oe). In our experiments, the effect of grain boundaries must be invalid under a high ﬁeld of 7 T. So the double peak ofr–Tcurve should not be a result of grain boundaries but an intrinsic property inside the grains.

To explain the double-peaks in ther–Tcurve at x¼0:2;the above phase separation picture can be adopted too. It was reported that Cu-doped La0.7Ca0.3MnO3 at Ca site and Ru-doped Pr0.5Ca0.5MnO3 at Mn site exhibit the double- peaks[19–20]. They are ascribed to the double FM transitions. However, only one FM transition is observed in our Cd-doped systems, may the observed double-peaks here be induced by other mechanisms? One does not need to explain the occurrence of the high-T peak in the r–T curve.

For the weak but broad peak at low-T, the coexisted FMM clusters as matrix where the double exchange is dominant and FMI regions centered around Cd ions may play their roles over different T-ranges. AsToTC, the FMI phase still exist in sample. In fact, over the low-Trange the r–T curves do not show the pure metallic behavior. A lowering of T will lead to the percolating process of FMI–FMM transition for the Cd-rich regions, and hence result in a broad

6500 7000 7500 8000 8500 9000 9500 10000

645 650 655 660 665 670 9000

10000 11000 12000 13000 14000

Relative Intensity (a. u.)

x=0.2 x=0.0

Binding Energy (eV)

Fig. 5. The X-ray photoemission spectroscopes of the Mn 2p electron orbital energy level in two samples with x¼0:0 and 0.2.

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I–M transition. Note here again thatTP2(x¼0:2) is just slightly higher thanTC(x¼0:5), as shown in Fig. 3. An external magnetic ﬁeld forces the FMI phase transformed to the FMM phase, so thatTP2shifts to a higherT. The spins of Mn ions in the FMI phase are ordered and aligned gradually, leading to the signiﬁcant MR at low-T range. In fact, for La0.5Cd0.5MnO3, the weak resistivity peak under H=0 comes from this percolating process, as we stated above.

In order to better understand the phase transitions of our samples, the measurements of speciﬁc heat of three typical samples withx¼0:0;0.2 and 0.5 were performed, as shown inFig. 6. We can see the transitional temperatures of all samples are consistent with the values got by resistivity and

magnetization measurements. We focused on the lower temperature range, where it is easier to take into account the phonon contributions. The inset in Fig. 5 shows the specific heat of La0.5(Ca0.5xCdx)MnO3 (x¼0:0;0:2;0:5) plotted as C/T versus T² respectively. This denotation displays phonoic contributions (pT³) to the specific heat as a linear increase. The values of the T linear slope (g) could be determined from the well-defined intercepts of C/Tversus T²plots which show straight lines as T-0. By the extrapolations of the linear part of the specific heat in La0.5(Ca0.5xCdx)MnO3 at temperatures between 20–35 K, we estimate the g for La0.5(Ca0.5xCdx)MnO3(x¼0:0;0.2, 0.5) respectively, as listed in Table 1. The renormalized density of states at the Fermi level N(EF) can be calculated using the formulation g=(p²/3)kB2

N(EF). Evidently, from the g values we can see N(EF) in La0.5(Ca0.5xCdx)MnO3 increases with decreasing resistivity asx increasing, but shows a critical decrease as the end x¼0:5 insulator is approached. These results are consistent with the results of the ﬁtting of the high-temperature resistivity data with the small-polaron-hopping model as we stated above.

We admit that the above explanation is some- how preliminary and qualitative. More direct experimental evidence is required for checking this conceptual picture. For example, an experimental evidence for the magnetic polarons at T4TCand between TP2oToTP1 would be very helpful for conﬁrming this phase separation, by utilizing the r–Tmeasurement over extremely low-Trange and T-variable Raman spectroscopy. These works are going on.

4. Conclusion

In conclusion, we have investigated the magneto- transport properties and the behavior of the speciﬁc heat of polycrystalline La0.5(Ca0.5xCdx)MnO3

samples (x¼0:020:5). Cd-doping in the Ca site of the La0.5Ca0.5MnO3 drives the system from the low-conductivity regime to the higher-conductivity regime except the end x¼0:5 insulator. It is surprising to ﬁnd that the Cd-doping favors the 0

0 6 12 18 24

4 8 12 16 10

20 30 40 50

Temperature (K) CP (J/mol-K)

x=0.0 x=0.2 x=0.5 T ²/100 (K ²)

CP / T (mJ/mol-K2 )

100 200 300

Fig. 6. The curves of the speciﬁc heat of three typical samples with x¼0:0; 0.2 and 0.5. Inset shows the speciﬁc heat of La0.5(Ca0.5xCdx)MnO3 bulk materials (x¼0:0; 0.2, 0.5) plotted asC/TversusT².

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FM ordering and enhances the I–M transition to a higher temperature. The activation energy decreases and the electronic speciﬁc heat coefﬁcientg(accordingly, the renormalized density of states at the Fermi level,N(EF)) increases due to increase of Cd content except the endx¼0:5 insulator. We have observed a double-peaked resistivity-temperature curve for the x¼0:2 sample although only single FM transition is probed. For this sample, a roughly constant magnetoresistance ratio is recorded over a wide temperature range. The coexistence of FMM phase and FMI phase under an AF-ordering background, due to the electronic phase separation, is argued to be responsible for the observed complicated magneto-transport property for the La0.5(CdxCa0.5x)MnO3systems.

Acknowledgements

The authors acknowledge the ﬁnancial support of this work from the National Key Project for Basic Researches of China, the National Natural Science Foundation of China through the innova- tive group project and normal projects (50332020, 10021001), and LSSMS of Nanjing University.

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