Pr0.67Ca0.33MnO3

Magnetic field induced insulator-metal transition in nanocrystalline Pr0.67Ca0.33MnO3 compounds: Evidence of large temperature co-efficient of resistance

Kalipada Das and I. Das

Citation: Journal of Applied Physics 117, 175103 (2015); doi: 10.1063/1.4919823

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Magnetic field induced insulator-metal transition in nanocrystalline Pr 0.67Ca 0.33MnO 3compounds:Evidence of large temperature co-efficient of resistance

Kalipada Das a)and I.Das

CMP Division,Saha Institute of Nuclear Physics,1/AF Bidhannagar,Kolkata 700064,India

(Received 16March 2015;accepted 25April 2015;published online 5May 2015)

We report the electronic transport,magneto-transport,and magnetic properties of nanocrystalline Pr 0.67Ca 0.33MnO 3compound.A magnetic ?eld induced insulator—metal transition appears for the external magnetic ?eld higher than 50kOe.We have obtained large value of the temperature coef?-cient of resistance (TCR)along with magnetoresistance and ?eld coef?cient of resistance (FCR).The value of TCR is 135%/K at 48K.The calculated magnetoresistance is about à9.8?107%for 70kOe and maximum FCR is about 320%/kOe around 75K.Due to the application of the external magnetic ?eld,charge ordered state of the compound is destabilized leading to such large values of TCR and https://www.wendangku.net/doc/5217167550.html,rge values of TCR and FCR along with the large magnetoresistance exhibited by

the material is interesting from the application point of view.V

C 2015AIP Publishing LLC .[https://www.wendangku.net/doc/5217167550.html,/10.1063/1.4919823]

I.INTRODUCTION

The physical properties of doped perovskite manganites have been extensively studied due to their different func-tional properties.1–9As,for example,two crucial parameters for bolometric applications are temperature coef?cient of re-sistance (TCR)and ?eld coef?cient of resistance (FCR)which are found to be large in manganites.4,10–13The TCR and FCR of a material depend on the rate of change of resis-tances with respect to temperature and external magnetic ?eld,respectively.Generally,the undoped manganites (RMnO 3,R is trivalent ion)show insulating properties.In contrast to that,insulator to metal transition takes place in the doped manganites,with general formula:R 1àx B x MnO 3,where “B”is bivalent ions (Ca 2t,Ba 2t,Sr 2t,etc.).The insulator-metal transition frequently appears at low doping range (x <0.5)and the resistivity rapidly drops with the low-ering of temperature stabilizing a metallic ground state.On the other hand,the charge ordering (CO),another intriguing properties of doped manganites,appears for doping level of x $0.5.CO is a real space ordering of the Mn 3tand Mn 4tions occurring below a certain temperature (T CO ).It is occa-sionally accompanied by an antiferromagnetic transition.Below T CO ,resistivity of the compounds sharply increases with the lowering of temperature giving rise to an insulating ground state at the low temperature.The charge ordered insulating state can be destabilized by the application of external magnetic ?eld stabilizing a metallic state (magnetic ?eld induced metamagnetic transition).

Although there are many studies regarding charge order-ing in bulk single crystalline as well as polycrystalline man-ganites,a little attention has been paid to understand the modi?cation of this phenomenon in the cases of nanoparticles.

In this context,some experimental studies reveal the non-existence of CO transition for different charge ordered

manganites in the form of nanoparticles.14,15However,there are also reports of observation of CO in nanocrystalline manganites.16–18

Dong et al.have addressed this issue in their theoretical works.According to their analysis,surface effect plays the dominant role in determining magnetic and transport proper-ties of the systems in nanometer scale range.19,20They have proposed a core-shell type model in nanometer length scale,where a charge ordered antiferromagnetic core is developed inside a short-ranged ferromagnetic shell (surface).With the reduction of the particle size,the volume of ferromagnetic shell gradually increases in expense of antiferromagnetic core.In that sense,charge ordered state can be signi?cantly weakened in nanoparticles as observed in experiments.17,18Weakening of charge ordered state can also reduce the value of required ?eld for destabilizing CO state.This can give rise to a magnetoresistance with large TCR and FCR in relatively low ?eld which is interesting from functional prospective.

The overall objective of the present study is to under-stand and explore the functional magnetic and transport properties of charge ordered manganite nanoparticles.In this regard,we have carried out a detailed experimental study on Pr 0.67Ca 0.33MnO 3(PCMO)in its nanocrystalline form (aver-age particle size $46nm).Among the doped manganites,Pr 1àx Ca x MnO 3are well studied in their bulk form.These compounds with doping x $0.3–0.5exhibit charge ordering.Depending on single electron band-width determined by the doping concentration,T CO and the value of magnetic ?eld required for melting of charge ordered state vary.Biswas and Das and Rawat et al.have reported occurring of charge ordering in these system even in nanocrystalline form with the average particle size greater than 40nm.16,21These previ-ous studies motivate us to select the sample of particular composition and also the particle size for our study.We have observed large value of TCR in our samples along with large magnetoresistance and FCR.The large values of TCR and

a)

Electronic mail:kalipada.das@saha.ac.in

0021-8979/2015/117(17)/175103/4/$30.00V

C 2015AIP Publishing LLC 117,175103-1

JOURNAL OF APPLIED PHYSICS 117,175103

(2015)

FCR are important?gures of merit for bolometric application.

II.SAMPLE PREPARATION AND CHARACTERIZATION The nanocrystalline PCMO compounds were prepared by sol-gel route.The details about the sample preparation tech-nique are given in Ref.21.For the sample preparation,starting materials were Pr6O11,CaCO3,and MnO2.Appropriate amounts of pre-heated high pure(99.99%)oxides are converted to their nitrates.Then,nitrates were mixed up and required amount of citric acid was added.The mixture was heated at 80–90 C in a water bath until the gel was formed.At the end of the sol-gel process,the decomposed gel was annealed at 900 C for4h to get the nanocrystalline compound.

At the room temperature,x-ray diffraction measurements were carried out by using Rigaku-TTRAX-III diffractometer (wave length,k?1.54A?).For further characterization of the prepared sample,Transmission Electron Microscopy(TEM) measurements were performed.Magneto-transport properties were studied using in conventional four probe longitudinal geometry.To measure the magnetic properties,a supercon-ducting quantum interference device system(SQUID-VSM) (quantum design)was employed.

III.EXPERIMENTAL RESULTS AND DISCUSSION Room temperature x-ray diffraction study indicates the single phase nature of the nanocrystalline PCMO compound. The details about the crystal structure and lattice parameters were reported in Ref.22.The particle size of the nanocrys-talline compound was estimated from the x-ray line width broadening(using Scherrer’s formula).According to Scherer’s formula,the particle size(d)of nanocrystalline compound is given by the following equation:

d?

K k

b cos h

:(1)

Here,K$0.9(constant)and k?1.54A?(wave length of x-ray).To take into account the instrumental broadening, effective full width at half maxima(FWHM),b,is calculated using the relation23

b?Bàb2

B

;(2)

where“B”and“b”are FWHM of a peak of the nanocrystal-line sample at a particular angle of diffraction and FWHM of the corresponding peak of the bulk form of that sample (measured in the same instrument).From X-ray diffraction study,the calculated average particle size is$46nm.

The average particle size is also measured from the TEM images,which agrees quite well with the estimated particle size from x-ray diffraction measurement.One repre-sentative TEM image with particle sizes distribution is given in Fig.1.

According to the phase diagram of the bulk Pr1àx Ca x MnO3,Pr0.67Ca0.33MnO3compounds undergo from para-magnetic to charge ordered state below T CO$230K.24The antiferromagnetic transition appears with further lowering of the temperature at T N?150K.In case of our nanoparticles, we have measured the magnetization as a function of temper-ature in the presence of0.1kOe external magnetic?eld in zero?eld cooled(ZFC)and?eld cooled(FC)protocols.The signatures of charge ordering transition at T CO?225K and antiferromagnetic transition below T N?150K were clearly visible in M vs.T curve.These transitions are indicated by arrows in Fig.2.In addition to the CO and antiferromag-netic ordering,a transition from antiferromagnetic state to a canted antiferromagnetic state(CAF)occurs for the bulk sample of similar compositions below T?100K.We have also observed sudden increase of magnetization at low tem-perature(below100K)in M(T)curve which is an indication FIG.1.Transmission electron microscopy image of the nanoparticles of Pr0.67Ca0.33MnO3compound.Inset shows the particle sizes distribution. FIG.2.Magnetization as a function of temperature in ZFC and FC protocols in the presence of H?0.1kOe external magnetic?eld.Inset indicates the magnetization as a function of the magnetic?eld at T?5K.The virgin curve represented by“1”and sharp increment of magnetization at H$60kOe indi-cate the?eld induced destabilization of the antiferromagnetic charge ordered state.

of the stabilization of CAF in our nanoparticles.This transi-tion is indicated by arrow.With further lowering the temper-ature,another magnetic transition occurs below 30K (represented by T SR in Fig.2).Such a transition at low tem-perature is believed to be due to a spin reorientation transi-tion,which is a characteristic of bulk system as well.25The magnetic ?eld dependent magnetization (M(H))at the low temperature (T ?5K)is shown in the inset of Fig.2.Before recording the M(H)data,samples were cooled down from T ?300K (paramagnetic)in the absence of any external magnetic ?eld (ZFC process)to exclude any magnetic his-tory of the previous measurements.Our measurements reveal that the magnetization is sharply increases at H $60kOe,which is an indication of the ?eld induced melting of the charge ordered state of nanocrystalline PCMO compound.Once the transformation of antiferromagnetic to ferromag-netic state occurs via magnetic ?eld induced melting of the charge ordered state,the ferromagnetic nature remains for further ?eld cycling.

The charge ordering is further probed through a sharp increase in resistivity below T CO $220K which is shown in Fig.3.Below T CO ,the insulating nature of the compound was found at low temperature up to the presence of magnetic ?eld:H $45kOe.However,the application of magnetic ?eld of 50kOe leads to a ?eld induced insulator to metal transition around T $50K.The insulator–metal transition temperature was shifted towards the high temperature region for higher values of external magnetic ?eld.In nanomateri-als,generally grain boundaries play a signi?cant role in determination of their magneto-transport properties.Crystalline defects,dislocations,and oxygen nonstoichiome-try can present in grain boundary region constituting barrier.The electron conduction through this barrier can be enor-mously enhanced in the presence of low magnetic ?eld giv-ing rise to low ?eld magnetoresistance (LFMR)in nanoparticles.26It should be worthy to mention that we did not ?nd any signi?cant LFMR in this case.We have observed large negative magnetoresistance in high magnetic ?eld due to the melting of charge ordered state.

The sharpness of the insulator–metal transition is an im-portant factor determining different potential applications.

The sharpness of such a transition can be quanti?ed by de?n-ing a parameter called as TCR.TCR of a material is calcu-lated as TCR ?1q d q

dT .We have obtained the large value of the TCR ($140%/K)in the presence of the 50kOe external magnetic ?eld for our PCMO nano material,around its insu-lator to metal transition (shown in Fig.4).The obtained value is almost 7times of the value obtained for different bulk samples.10,27It is worth mentioning that the observed values of TCR are fairly large even in 70kOe (38%/K)and 80kOe (26%/K)external magnetic ?eld.In addition to that,present sample also exhibits extremely large magnetoresist-ance eMR T?MR ?R eH TàR e0T

at the low temperature region.The values of MR are à9.8?107%and à1.1?108%for 70and 80kOe magnetic ?elds.The variation of negative MR as a function of temperature is plotted in the inset of Fig.4.

Another parameter of practical application is FCR,which

is de?ned as FCR ?1R eH TdR

dH .To calculate the FCR,we have measured resistance as a function of external magnetic ?eld at ?xed temperatures (inset of Fig.5,enlarge portion,indicating the ?eld induced destabilization of charge ordered state).

The

FIG.3.Temperature dependent resistivity at different magnetic ?elds of PCMO nanocrystalline compound.In the presence of H !50kOe magnetic ?eld,insulator–metal transition was observed at the low temperature region.FIG.4.Temperature coef?cient of resistance as a function of temperature for different magnetic ?eld values.Inset of the ?gure indicates extremely large value of the magnetoresistance of the PCMO nanocrystalline compound.

FIG.5.Field coef?cient of resistance as a function of the magnetic ?eld at different temperatures.Inset represents the enlarged region of the magnetic ?eld dependent resistances at several ?xed temperatures.

calculated FCR as a function of external magnetic?eld(shown in Fig.5)indicates that the FCR is reasonably large for the nanocrystalline PCMO compound($320%/kOe around75K). The observation of large TCR and FCR along with negative magnetoresistance of the material is very interesting from application point of view.As mentioned earlier,FCR and TCR are important?gures of merits for bolometric application. IV.SUMMARY

To summarize,we have studied the transport,magneto-transport,and magnetic properties of nanocrystalline Pr0.67Ca0.33MnO3compound.A magnetic?eld induced insu-lator to metal transition with large TCR has been observed for the system.In addition to this,the obtained magnetore-sistance and FCR are also signi?cantly large suggesting the possibility of using it in multiple applications,e.g.,magneto-resistive devices,bolometry,etc. ACKNOWLEDGMENTS

Kalipada Das would like to acknowledge CSIR-India for the fellowship.Pallab Bag and R.Rawat,UGC-DAE Consortium for Scienti?c Research,Indore,are acknowledged for resistivity/magnetoresistance measurements.

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