phys. stat. sol. (a) 172, 451 (1999)

Subject classification: 72.60.+g; 72.80.Ga; 75.30.Vn; S10.15

Insulator±Metal Transition and Colossal Magnetoresistancein Granular Layered Perovskites La2ÿ2xCa1�2xMn2O7

Ting Yu, Wenhai Song, Kaiyou Wang, Shouguo Wang, and Yuping Sun

Institute of Solid State Physics, Academia Sinica, Hefei 230031,People's Republic of China

(Received September 23, 1998; in revised form December 8, 1998)

The electric and magnetic properties in granular layered perovskites La2ÿ2xCa1�2xMn2O7

�0:2 � x � 0:4� have been investigated. It is found that the temperature dependence of resistivitycan be well described by a phenomenological formula rH�T� � 1=sH�T� � 1=�a� bTÿ2:45

�g exp �ÿT0=T�1=4�, where the fitting parameters a; b; g and T0 vary as the doping concentrationx changes. The results of resistivity and magnetization measurements have clearly shown that thereis a large deviation in Tr and TC, possibly originating from the interaction of the anisotropic ex-change effect and the grain size effect.

1. Introduction

The discovery of colossal magnetoresistance (CMR) effect in the perovskite-type com-pounds R1ÿxAxMnO3 (R being rare-earth ions and A divalent ions) has attracted greatattention with regard to scientific study and application to magnetic storage and sensortechnology [1 to 4]. In addition to the CMR effect, systematic studies on R1ÿxAxMnO3

perovskite-type compounds indicate other noteworthy properties as follows: 1. in anappropriate doping range (0.2 to 0.5), those compounds exhibit a paramagnetic to fer-romagnetic transition accompanied by an insulator±metal transition; 2. the temperaturefor the maximum magnetoresistance correlates well with the peak resistivity tempera-ture and the ferromagnetic transition temperature TC. The double exchange (DE) theo-ry first proposed by Zener [5], which considers the transfer of an electron betweenneighboring Mn3� and Mn4� ions through the Mn±O±Mn path, is now widely acceptedto qualitatively interpret the origin of the CMR effect. However, it was argued byMillis et al. [6] that the double exchange alone is incompatible with many aspects of theresistivity, and the polaron effects due to a very strong electron±phonon coupling froma Jahn-Teller splitting of the Mn3� ion was proposed as a necessary extension. Directexperimental evidence for Jahn-Teller distortions has become available [7]. However,the exact mechanism of the CMR effect remains unclear, and the goal of understandingand applying the CMR in the perovskites is still challenging.

Recently, layered variants of the perovskite structure have been explored with theexpectation that they will provide new insights and interesting physics in their ownright due to unique anisotropy and dimensionality effects. For example, studies onLa2ÿ2xCa1�2xMn2O7 �x � 0:25� polycrystalline bulk samples [8], La1:2Sr1:8Mn2O7 singlecrystals [9], La2ÿ2xCa1�2xMn2O7 �x � 0:3� thin films [10] have shown that these manga-nese oxides of A1�nBnO3n�1 �n � 2� type all exhibit colossal magnetoresistance ef-fects. So far, however, there have been few systematic investigations of polycrystalline

Ting Yu et al.: Insulator±Metal Transition and Colossal Magnetoresistance 451

samples with small grain sizes. In this paper, we present the observation of electricaland magnetic properties in low-temperature sintered samples of layered perovskitesLa2ÿ2xCa1�2xMn2O7 �0:2 � x � 0:4, in steps of Dx � 0:05�.

2. Experimental

The granular samples were prepared with the sol±gel method. This method presentsthe advantage of using low-temperature synthesis and results in not only getting smallergrain sizes but also producing high purity and homogeneous samples. La2O3, CaCO3,Mn(NO3)2 were mixed stoichiometrically with nitric acid and urea in aqueous solution.The solution was evaporated to gel, then it was preheated to 250 �C to remove theremaining organic of the gel, finally the resultant powder was sintered at 1000 �C in theair for 24 h, followed by furnace-cooling. The crystal structure and the phase purity ofthe samples were examined by X-ray diffraction with CuKa radiation. The electricalresistance and magnetoresistance were measured as a function of temperature by astandard four-probe technique. The electrical contacts were made with silver paste. TheMR ratio here is defined as MR � ÿ�RH ÿ R0�=R0 � 100% (where RH and R0 are theresistances in an applied magnetic field and zero field, respectively). Magnetization wasmeasured on a vibrating sample magnetometer. All measurements were carried outusing a personal computer through an IEEE-488 data bus.

3. Results and Discussion

X-ray diffraction indicates that all of the samples are single phase ones. Fig. 1 shows theX-ray diffraction pattern for a selected sample of La2ÿ2xCa1�2xMn2O7 �x � 0:4�. All thediffraction peaks are indexed with the Sr3Ti2O7-type tetragonal perovskite (I4/mmm)

452 Ting Yu, Wenhai Song, Kaiyou Wang, Shouguo Wang, and Yuping Sun

Fig. 1. X-ray diffraction pattern of the La2ÿ2xCa1�2xMn2O7 �x � 0:4� sample. In the inset we showthe width of the (110) peak

and show that the sample has a high degree of phase purity. The X-ray line widthsprovide us to estimate the average grain sizes through the classical Scherrer formula-tion D � kl=B cos 2q, where D is the mean diameter of the grain, k is a constant(shape factor �0.9), B is the width of the half-maximum of the diffraction peaks, and lis the wavelength of the X-rays. The obtained D values of the samples are about 18 to26 nm, as seen in Table 1.

The resistivity of La2ÿ2xCa1�2xMn2O7 in zero field as a function of temperature isshown in Fig. 2. For the whole doping range, the r T curves exhibit a peak with insula-tor behavior above and metallic behavior below the peak temperature. Hereafter, wedefine the peak temperature in zero field as Tr. With increments in the nominal holeconcentration x from 0.2 up to 0.35, Tr increases from 121 K �x � 0:2� to 172 K�x � 0:35�. For x � 0:4, we observed a slight reduction of Tr to 141 K. These insulator±metal transitions, which depend on the doping concentration x, are similar to those ofthe conventional manganites such as La1ÿxCaxMnO3. It can be seen very clearly thatthere is a large deviation between the insulator±metal transition temperature Tr and

Insulator±Metal Transition and Colossal Magnetoresistance in La2ÿ2xCa1�2xMn2O7 453

Ta b l e 1Summary of physical properties of La2ÿ2xCa1�2xMn2O7. Tr; TC and D mean the peaktemperature in resistivity under zero field, the magnetic transition temperature, themean diameter of the grain, respectively

nominalcomposition x

Tr

(K)TC

(K)D (XRD)(nm)

0.2 121 203 180.25 152 205 200.3 165 209 190.35 172 209 260.4 141 257 21

Fig. 2. The resistivity of La2ÿ2xCa1�2xMn2O7 samples under zero field as a function of temperature

454 Ting Yu, Wenhai Song, Kaiyou Wang, Shouguo Wang, and Yuping Sun

Fig. 3. a) Temperature dependence of resistivity for La2ÿ2xCa1�2xMn2O7 samples where the tempera-ture is below Tr. The solid lines are fits to the equation r�T� � r0 �AT2:45; b) ln �r� versus Tÿ1=4 forLa2ÿ2xCa1�2xMn2O7 samples where temperature is above Tr. The solid lines are provided only as aguide to the eyes; c) temperature dependence of resistivity for La2ÿ2xCa1�2xMn2O7 samples. Thesolid lines are fits to the phenomenological expression rH�T� � 1=sH�T� � 1=�a� bTÿ2:45

�g exp �ÿT0=T�1=4�. The resistivities of these samples are all measured under an applied field of0.8 T

the magnetic transition temperature TC. The origin of the deviation will be discussedlater.

To understand the transport properties of La2ÿ2xCa1�2xMn2O7 in an applied magneticfield more clearly, we tested several different models previously proposed to explainthe conduction mechanism of perovskite manganites above and below Tr. In the low-temperature regime T � Tr, it seems to be governed by the electron scattering process.r�T� is fitted well by the equation r�T� � r0 �AT2:45, as shown by the solid lines inFig. 3a. Here r0 is the resistivity due to the domain boundaries and other temperature-independent scattering mechanism, and AT2:45 represents a combination of electron±electron, electron±phonon, and electron±magnon scattering. This result is similar to theLa1ÿxKxMnO3 thin films, as reported by Chen and Lozanne [11]. Based on a calcula-tion of electron±magnon scattering, Kubo and Ohata [12] suggested that the tempera-ture dependence of r varies as T9=2, but this has never been seen in our experiments.So a combination of terms in T2 (from electron±electron scattering) and T9=2 would fitthe data adequately. On the other hand, in the high-temperature regime T � Tr, one canobserve a perfect linearity of ln �r�T�� � �T0=T�1=4 for the experimental data shown inFig. 3b. r�T� is fitted well by the model ln �r�T�� � Tÿ1=4. It seems that the variablehopping mechanism [13] is responsible for the transport properties of La2ÿ2xCa1�2xMn2O7

at high temperature. Because the conductivity is the reciprocal resistivity, in the low-temperature range the conduction mechanism can be described as s�T� � a� bTÿ2:45,while in the high-temperature range s�T� � g exp �ÿT0=T�1=4. Finally, taking simulta-neously the electron-scattering and the variable-hopping transport properties into ac-count, we give a universal phenomenological description of the temperature depen-dence of the resistivity:

rH�T� �1

sH�T� �1

a� bTÿ2:45 � g exp �ÿT0=T�1=4:

We fit the experimental data by selecting the proper parameters a; b; g and T0. Thephysical meaning of the parameter a can be understood as the conductivity due to thedomain boundaries and other temperature-independent scattering mechanisms, bTÿ2:45

represents a combination of electron±electron, electron±phonon, electron±magnon scat-tering, while g represents the electrical conductivity by the variable hopping of carriers,T0 is a physical parameter which is related to the localization length. So the physicalimage of the conduction mechanism can be simply interpreted as follows: It is so smallthat the quantity of the exponential term in the low-temperature range T � Tr can be

Insulator±Metal Transition and Colossal Magnetoresistance in La2ÿ2xCa1�2xMn2O7 455

Ta b l e 2The fitting parameters a; b; g and T0 obtained from the phenomenological equation

rH�T� �1

sH�T� �1

a� bTÿ2:45 � g exp �ÿT0=T�1=4

x a �Wÿ1 cmÿ1� b (K2:45=W cm) g �Wÿ1 cmÿ1� T0 (K)

0.2 6:280� 10ÿ3 349.15 5:060� 109 9:618� 107

0.25 6:772� 10ÿ3 472.75 1:342� 1013 3:197� 108

0.3 4:485� 10ÿ2 8530.42 5:415� 1016 6:427� 108

0.35 3:986� 10ÿ2 5208.61 5:065� 1018 1:023� 109

0.4 2:433� 10ÿ1 8163.05 1:119� 1014 3:066� 108

ignored, so the combination of electron±electron, electron±phonon, electron±magnonscattering dominates the conduction mechanism; in the high-temperature range T � Tr

the polynomial expression decreases with increasing temperature, and the conductionmechanism would be governed by the variable-range hopping mechanism; in the vici-nity of Tr the influence to the conduction of these two mechanisms is very contiguous

that we should consider the combina-tion of these two mechanisms. Thefitting curves are plotted in Fig. 3c,and their fitting parameters are sum-marized in Table 2. As shown inFig. 3c, the experimental data arewell fitted by the theoretical formula;the probable error is not larger than�3%. However, there is still a littledifference between the experimentaldata and the fitting data in the vici-nity of Tr. It demonstrates that inthis temperature regime the fittingformula is so cursory that it cannotdescribe the transport property per-fectly with the simple superpositionof the two conduction mechanisms.Spin fluctuations may affect the elec-tron scattering process in the vicinityof Tr. From Table 2, we can see thatthe general changing trend of T0 issimilar to that of Tr. The localizationlength hxi can be deduced from thevalues of T0 using the relation [14]kBT0 � 1:5e2=�khxi�, where e is theelectronic charge and k is the dielec-tric constant. From this equation wecan find that hxi is inversely propor-tional to T0. When T0 increases, itcosts more energy for the carrierhopping with the decrease of the lo-calization length hxi, so it makes Tr

shift to higher temperature. There-

456 Ting Yu, Wenhai Song, Kaiyou Wang, Shouguo Wang, and Yuping Sun

Fig. 4. Temperature dependence of the MRratio ÿDR=R0 for the La2ÿ2xCa1�2xMn2O7

samples

fore, we can qualitatively interpret how the value of Tr changes. The influence of ap-plied magnetic field is to suppress the randomness in spin orientation. It leads to alignthe spins parallel, makes the carriers transfer easier and consequently r decreases.

The temperature dependence of the MR ratio ÿDR=R0 (at m0H � 0:8 T) ofLa2ÿ2xCa1�2xMn2O7 is shown in Fig. 4. These samples show negative MR by as much as29% at low temperature. It can be seen that the MR effect of these samples exhibits acomplicated temperature dependence over a wide temperature range. The MR beha-vior can be summarized into two aspects: 1. The MR has a trend to increase withdecreasing temperature. The low-temperature magnetoresistance can reach more than25% at 70 K for all samples, 2. at temperature below Tr, we can see that these samplesexhibit a relatively smooth MR effect in this temperature range. That is to say, themagnetoresistance for this kind of sample is insensitive to the temperature in a certaintemperature range. This property may be beneficial to the technological application.

The magnetic properties of these granular samples are shown in Fig. 5. The magneti-zation was measured in the warming run with a field of 0.05 T after cooling down to100 K in a zero field. The magnetic transition temperature TC was defined as the tem-perature at the maximum slope jdM=dTjmax on the M T curve, which is much higherthan Tr. As x increases, TC increases from 203 K �x � 0:2� to 257 K �x � 0:4�, as seenin Table 1. Comparing the TC and Tr values, we can see that there is no coincidencebetween the insulator±metal transition and the magnetic transition, as commonly ob-served for the traditional ABO3-type ferromagnets. We think there are two effects act-ing in common that cause the large deviation between Tr and TC. First, the large devia-tion in Tr and TC can be interpreted by the intrinsic effect ±± the anisotropic exchangeinteraction. As reported by Asano et al. [8], the electrical properties of the samples aredominated by the weak out-plane exchange interaction, the magnetic transitions of thematerials are governed by the strong in-plane exchange interaction. The structures of

Insulator±Metal Transition and Colossal Magnetoresistance in La2ÿ2xCa1�2xMn2O7 457

Fig. 5. Temperature dependence of the magnetization M in the field of 0.05 T. The magnetizationwas measured in the warming run after cooling down to 100 K in a zero field using a vibratingsample magnetometer

our samples indicate they have a two-dimensional Mn±O network. The double perovs-kite layers are interleaved with La(Ca)O layers and the Mn±O±Mn bonds in the c-axisdirection are separated from one another by the La(Ca)O layers. Thus, the Mn±Mn ex-change interaction takes place between the series of Mn±O±Mn bonds in the in-planedirection, which is similar to that of the cubic perovskite La1ÿxCaxMnO3 system. TheMn±Mn exchange interaction in the out-of-plane direction results from the series ofalternating Mn±O±Mn and Mn±O±O±Mn bonds. Finally, the anisotropic carrier trans-port and exchange interactions make Tr and TC divide. On the other hand, the extrin-sic effect, the grain size effect can result in a large deviation in Tr and TC too. Mahen-diran et al. [15] have discussed the role of grain sizes in La0:7Sr0:3MnO3. They obtainedthree La0:7Sr0:3MnO3 samples with �40% Mn4� and possessing different particle sizesby different heat treatment. The particle sizes of the three samples are 0.25, 0.4, and3.5 mm, respectively. Interestingly, these samples show similar ferromagnetic transitiontemperatures of 360, 370, and 390 K, but the Tr values are 190, 200, and 370 K, respec-tively. According to the report, there are ferromagnetic clusters (consisting mostly ofMn4� and Mn3�) within a grain at T � Tr, and an Mn3� rich region between the grainsthat will have predominantly antiferromagnetic superexchange interaction. As the tem-perature decreases, the magnetization increases. More and more neighboring spins getaligned in the ferromagnetic clusters leading to the increase of the cluster size. Even-tually it leads to a drop of resistance. The observed insulator±metal transition woulddepend on the relative contributions from the insulating regions and the metallic ferro-magnetic clusters. As the grain sizes of our samples are much smaller than that of thebulk samples, the relative contribution of the insulating region increases and leads tothe decrease of Tr. However, since TC is not being affected to a large extent by grainsizes, so Tr and TC are separated.

4. Conclusion

In conclusion, with using the sol±gel method, we have prepared and investigated thetransport and magnetic properties of layered granular perovskite La2ÿ2xCa1�2xMn2O7

series with various doping concentrations x. In the temperature range 80 K < T < 240 K,the temperature dependence of resistivity in an applied magnetic field �m0H � 0:8 T)can be well described by a phenomenological expression. We have observed a largedeviation between the insulator±metal transition temperature and the magnetic transi-tion temperature. The deviation can be interpreted by the interaction of the grain sizeeffect and the exchange interaction of the two-dimensional Mn±O networks. Neutronmeasurements are essential for a deeper understanding of the crystallographic and mag-netic structures of the layered pervoskite manganites.

Acknowledgement The research work was supported by the Fundamental Bureau andthe Laboratory of Internal Friction and Defects in Solids, Academia Sinica.

References

[1] R. von Helmolt, J. Wecker, B. Holzapfel, L. Schultz, and K. Samwer, Phys. Rev. Lett. 71,2331 (1993).

[2] A. Urushibara, Y. Moritomo, T. Arima, A. Asamitsu, G. Kido, and Y. Tokura, Phys. Rev.B 51, 14103 (1995).

458 Ting Yu, Wenhai Song, Kaiyou Wang, Shouguo Wang, and Yuping Sun

[3] Guo-Qing Gong, Ch. Canedy, Gang Xiao, J. Z. Sun, A. Gupta, and W. J. Gallagher, Appl.Phys. Lett. 67, 1783 (1995).

[4] T. Yotsuya, Jpn. J. Appl. Phys. 35, L23 (1996).[5] C. Zener, Phys. Rev. 82, 403 (1951).[6] A. J. Millis, P. B. Littlewood, and B. I. Shraiman, Phys. Rev. Lett. 74, 5144 (1995).[7] P. G. Radaelli, M. Marezio, H. Y. Hwang, S.-W. Cheong, and B. Batlogg, Phys. Rev. B 54,

8992, (1996).[8] H. Asano, J. Hayakawa, and M. Matsui, Appl. Phys. Lett. 68, 3638 (1996).[9] Y. Moritomo, A. Asamitsu, H. Kuwahara, and Y. Tokura, Nature (London) 380, 141 (1996).

[10] H. Asano, J. Hayakawa, and M. Matsui, Jpn. J. Appl. Phys. Lett. 36, L104 (1997).[11] Chun-Che Chen and A. de Lozanne, Appl. Phys. Lett. 71, 1424 (1997).[12] K. Kubo and N. Ohata, J. Phys. Soc. Jpn. 33, 21 (1972).[13] N. F. Mott, Adv. Phys. 21, 785 (1972).[14] B. I. Shklovskii and A. L. Efros, Electronic Properties of Doped Semiconductors, Springer-

Verlag, Berlin 1984.[15] R. Mahendiran, R. Mahesh, A. K. Raychauhuri, and C. N. Rao, Solid State Commun. 99, 149

(1996).

Insulator±Metal Transition and Colossal Magnetoresistance in La2ÿ2xCa1�2xMn2O7 459