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Physica B 403 (2008) 1582–1584

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On the nature of the low temperature insulating stateof ferromagnetic and charge ordered manganites

Himanshu Jaina,�, A.K. Raychaudhurib,1

aDepartment of Physics, Indian Institute of Science, Bangalore 560 012, IndiabS. N. Bose National Centre for Basic Sciences, Salt Lake, Kolkata 700 098, India

Abstract

Based on electroresistance (ER) measurements founded on a current induced resistivity switching (CIRS) phenomena, we establish the

presence of a ‘colossal’ ER in the low temperature ferromagnetic insulating (FMI) phase exhibited by certain hole doped manganites.

Notably, concomitant with the build-up of ER, is a sharp drop in the magnetoresistance (MR). This intelligibly demonstrates an effective

decoupling of the mechanisms underlying ER and MR in the FMI phase. ER (CIRS) and MR were measured on single crystals of two

widely different FMI manganites: La0:82Ca0:18MnO3 and Nd0:7Pb0:3MnO3. The samples have Curie temperatures, TC � 165 and 150K,

and the FMI state is realized for temperatures, Tt100 and 130K, respectively. The ER, arising from a strong nonlinear dependence of

resistivity (r) on current density ðjÞ, attains a value ’ 100% in the FMI state. The severity of the nonlinear behavior of resistivity at high

current densities is progressively enhanced with decreasing temperature. The MR, however, collapses ðo20%Þ even in magnetic field,

H ¼ 14T. Comparison with magnetotransport data on charge ordered insulating (COI) manganites reveal discernible differences in

response to applied current and magnetic field. This is credible proof that the nature of the insulating state, in the FMI and COI phases,

is different.

r 2007 Elsevier B.V. All rights reserved.

PACS: 75.47.Lx; 75.47.Gk

Keywords: Rare earth manganites; Colossal electroresistance

Electronic transport in colossal magnetoresistive (CMR)hole doped rare earth manganites L1�xAxMnO3 (L � Nd,La, Pr; A � Pb, Ca, Sr, Ba; xt0:3) is an issue of currentinterest [1]. More recently, a ‘colossal’ electroresistance(CER) phemomena has been observed in these materials,wherein applied electric fields (in a field effect (FE)configuration [2]) or applied current (in a 4-probeconfiguration [3,4]) cause a significant change in theresistivity ðrÞ of the sample. In the absence of anycomprehensive theoretical understanding of CMR andCER, it would be useful to ascertain experimentally, theinter-relationship of these effects. With this aim, we focuson one very clearly defined phase of these materials: the

front matter r 2007 Elsevier B.V. All rights reserved.

ysb.2007.10.195

ng author. Tel.: +918023608653; fax: +918023602602.

ss: himanshu@physics.iisc.ernet.in (H. Jain).

: Department of Physics, Indian Institute of Science,

12, India.

ferromagnetic insulating (FMI) by investigating two widelydifferent (single crystal) compositions, La0:82Ca0:18MnO3

(LCMO18) and Nd0:7Pb0:3MnO3 (NPMO30).The CIRS measurements to determine ER were performed

using pulsed excitation current density in 4–probe geometry.Details of technique including precautions against Jouleheating artefacts are discussed elsewhere [4].In Fig. 1 we show the resistivities ðrÞ of LCMO18 and

NPMO30 as a function of temperature ðTÞ measuredin absence of magnetic field ðHÞ and at current density,j ¼ jlow � 9:5� 10�4 A=cm2 and � 6:7� 10�4 A=cm2,respectively. At these values of j, the r are independentof the measuring current density. The samples undergoconcomitant paramagnetic–ferromagnetic (Curie) andinsulator–metal transitions at TC � 165 and � 150K,respectively. A second metal–insulator transition occursat TFMI � 120 and � 130K, respectively, signalling theonset of the FMI phase in which the r increases rapidly

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107

106

105

104

103

102

101

100

� (Ω

cm

)

0 50 100 150 200 250 300

T (K)

100

80

60

40

20

0

ER

%

Fig. 1. Resistivity vs temperature (r–T) data at low bias current density ðjlowÞ and in magnetic field, H ¼ 0 and 14T, and the r–T data taken at high

current density ðjhighÞ in H ¼ 0T. Electroresistance (ER) vs T is shown. For To100K, the ER! 100%, while the magnetoresistance, MR! 0%.

5x10-2

4x10-2

3x10-2

2x10-2

1x10-2

0

j th (

A/c

m2)

0 20 40 60 80 100 120 140 160 180

T (K)

Fig. 2. Threshold current density ðjthÞ vs temperature ðTÞ. Note

pronounced anomalies at T ¼ 78K (LCMO18) and T ¼ 38K

(NPMO30). Left inset: Typical resistivity vs current density (r–j) data.

The jth are indicated by arrows. Right inset: Typical current density vs

electric field (j–E) characteristics. The absence of hyteresis is noteworthy.

H. Jain, A.K. Raychaudhuri / Physica B 403 (2008) 1582–1584 1583

following a variable range hopping law. Fig. 1 alsoshows r measured at high j, jhigh � 9:5� 10�1 and� 6:7� 10�1 A=cm2, respectively. The significant decreasein r due to the (high) applied j is evident, and at the lowestT measured, rðjhighÞ=rðjlowÞo10�2and o5� 10�5, respec-tively. Also plotted in Fig. 1 is r measured at jlow inH ¼ 14T. It is clear that there is substantial magnetore-sistance (MR) in the temperature range 100–250 and65–225K, respectively attaining its highest value of � 80%close to the respective TC’s, which decreases sharply withthe onset of the FMI state. These two observationsconsidered together, make evident that, while the magneticfield has a negligible effect, the current density induces asubstantial (2–4 orders of magnitude) depression of r. InFig. 1 the ER% ¼ 100� ðrðjlowÞ � rðjhighÞÞ=rðjlowÞ is alsoshown. It is clear that, in the FMI phase, as MR! 0% theER! 100%. This experiment, thus, demonstrates aneffective decoupling of mechanisms of ER and MR in theFMI state. This result is distinct from previous results oncharge ordered insulating (COI) compositions [5,6] whichcan be destabilized, to a more conducting state, by bothcurrent and magnetic field.

Measurements of current density vs electric field (j–E)characteristics (plotted in Fig. 2 (right inset)) providefurther proof distinguishing the FMI and COI states. Thenonlinear nature of the transport is evident, and NPMO30for To25K, even exhibits negative differential resistivityðdE=djo0Þ [4]. The absence of hysteresis in the j–E curvesof the present FMI samples is noteworthy, because a COIstate is characterized by a strong hysteresis in the j–E

characteristics [5], which is often taken as a signature ofelectronic phase separation, its magnitude being deter-mined by the potential barrier seperating the phases. Basedon this observation of non-hysteretic j–E characteristics in

the FMI state, we suggest that, whatever may be theunderlying natures of the phases comprising the phase-separated state, the potential barrier between them is lowenough to have precluded the observation of any sub-stantial kinetic effects in the time window of presentmeasurements.In Fig. 2 (left inset) we show representative data of

variation of r as a function of j. The existence of athreshold current density ðjthÞ seperating two power-lawregimes of r–j dependence is apparent. Such data sets then,may be fitted to the relation, r ¼ anjm

n where the n ¼ 1 and2 denote the regimes jojth and j4jth, respectively. Thethus obtained exponents m1 and m2, and the coefficients a1

ARTICLE IN PRESSH. Jain, A.K. Raychaudhuri / Physica B 403 (2008) 1582–15841584

and a2 have been presented elsewhere [3,4]. The obtainedjth are plotted in Fig. 2. The CER is related to the strongnonlinear component of conductivity that appears abovejth. It is evident, that while jth has a shallow T dependenceabove T ¼ 78 and T ¼ 38K, respectively, there is a largejump at, followed by a rapid drop below these tempera-tures. In fact, all parameters—a1, a2, m1, m2—exhibit asharp anomaly at this temperature. It is suspected that thisanomaly may be caused by a phase transition type ofphenomena setting in at these temperatures. It appears thatCER is a manifestation of the charge current enhancing thetransfer integral between the eg electrons of the neighbor-ing atoms, thereby enhancing the bandwidth and thusreducing the gap in the density of states, which in turnleads to a decrease of the resistivity [7].

To conclude, decoupling of ER and MR in the FMIstate, and essential differences in response (to current and

magnetic field) of the FMI and COI state of manganiteshas been experimentally demonstrated.

H.J. thanks CSIR (India) for a fellowship. A.K.R.thanks DST (India) for a sponsored project. Part of thiswork was performed at UGC-DAE CSR, Indore.

References

[1] Y. Tokura, Rep. Progr. Phys. 69 (2006) 797.

[2] T. Wu, et al., Phys. Rev. Lett. 86 (2001) 5998.

[3] H. Jain, et al., Appl. Phys. Lett. 89 (2006) 152116.

[4] H. Jain, et al., Phys. Rev. B 76 (2007) 104408.

[5] A. Guha, et al., Phys. Rev. B 62 (2000) 5320.

[6] A. Asamitsu, et al., Nature 388 (1997) 50.

[7] L. Berger, J. Appl. Phys. 89 (2001) 5521.