DSpace at VNU: Effects of dilution on magnetic and transport properties of La(0.7)Ca(0.3)Mn(1-x)M(x)(')O(3)

For the R-site substitution, magnetic and transport properties have been found to be quite systematically influenced by the GdFeO3-type lattice distortion and lattice disorder associated

Effects of dilution on magnetic and transport properties of La0.7Ca0.3Mn1−xMx⬘ O3

D N H Nam,*N V Dai, T D Thanh, L T C Tuong, L V Hong, and N X Phuc

Institute of Materials Science, VAST, 18 Hoang-Quoc-Viet, Hanoi, Vietnam

H S Hong and N V Khien

College of Technology, Hanoi National University, Hanoi, Vietnam and Institute of Materials Science, VAST, 18 Hoang-Quoc-Viet, Hanoi, Vietnam

共Received 9 April 2008; revised manuscript received 17 May 2008; published 13 June 2008兲 Magnetic and transport properties of La0.7Ca0.3Mn1−x M x⬘O3共M⬘= Al, Ti兲 are studied The dilution of the Mn

network results in a weakening of the ferromagnetism, a deterioration of the metallic conductivity, and a strong

enhancement of the magnetoresistance Although T c linearly decreases with x in the low substitution ranges for

both M⬘series, the scaling behavior T c 共n p兲 previously observed for La0.7Sr0.3Mn1−x M x⬘O3关D N H Nam et al.,

Phys Rev B 73, 184430 共2006兲兴 is no longer obtained Extrapolations of the T c 共n p 兲 linear curves to T c

= 0 K give n p values much smaller than one These results suggest that, according to a molecular-field

approximation, antiferromagnetic superexchange between Mn ions is significant in La0.7Ca0.3MnO3, in contrast

to what was observed in La0.7Sr0.3MnO3 Additionally, the structural data of the Al-substituted samples suggest

that variations of the e g -electron bandwidth W cannot explain the decrease in T c in magnetically diluted

manganites

I INTRODUCTION

Mixed-valance manganites 共R,A兲MnO3 共where R is the

rare-earth and A is the alkaline earth elements兲 have been

intensively studied for more than a decade since the

discovery of the colossal magnetoresistance 共CMR兲

phenomenon.1 3This class of materials is attractive to

scien-tists due to not only their potential for practical applications

but also their rich and intriguing fundamental physics The

Mn3+/Mn4+mixed valence leads to a coexistence of

compet-ing interactions among the Mn ions While antiferromagnetic

共AF兲 and insulatorlike behaviors are found in the systems

dominated by Mn3+-O2−-Mn3+and Mn4+-O2−-Mn4+

superex-change 共SE兲 interactions, ferromagnetic 共FM兲 and metallic

behaviors realized in the systems with Mn3+-O2−-Mn4+

double-exchange 共DE兲 couplings4are pertinently introduced

by doping A2+ ions at the R site Along with the mixed

in-teraction character, lattice distortions共such as of Jahn–Teller

and GdFeO3 types兲 and phase segregation phenomena also

play important roles governing the physical properties of the

materials Depending on the compositions and external

con-ditions, the properties of manganites can be ranged from

dis-ordered to dis-ordered, from antiferromagnetic to ferromagnetic,

and from insulating to metallic.5 , 6

On the search for compositions with novel properties,

re-sults for both R- and Mn-site substitutions have been quite

well documented in the literature For the R-site substitution,

magnetic and transport properties have been found to be

quite systematically influenced by the GdFeO3-type lattice

distortion and lattice disorder associated with the size and

size mismatch of the R-site cations.7 9 However, doping at

the Mn-site would create a much more complicated scenario

Since double exchange has been found to exist so far only

between Mn ions, any substitution at the Mn site would lead

to a weakening of the DE ferromagnetism and therefore a

deterioration of the metal-type conduction If the substituting

ion carries a net spin moment, magnetic interactions between

Mn ions and the substituent are inevitable The exchange interactions among Mn ions would also vary with doping, as

a result of the structural modification due to the discrepancy

in ionic size In addition, the Mn3+/Mn4+ ionic ratio would

be changed adapting to the valence and concentration of the substituting ions These effects therefore effectively hinder systematic studies of the Mn-site substitution in manganites

In a recent study,10 it has been found that selective sub-stitution of Mn by nonmagnetic elements could reveal impor-tant information on the interactions among Mn ions in a manganite The authors proposed that the dilution of the Mn lattice leads to a decrease in the molecular field acting on Mn ions and therefore suppresses the ferromagnetism In the low substitution ranges, La0.7Sr0.3Mn1−x M x⬘O3 共M⬘= Al, Ti兲

ex-hibits a linear scaling behavior between T cand the dilution

concentration n p of M⬘ions共n p = x/0.7 for M⬘= Al and x/0.3

for M⬘= Ti兲 Since T c has the tendency to reduce to zero at

n p= 1, based on a molecular-field approximation, the authors suggested that the DE mechanism is totally dominant

in La0.7Sr0.3MnO3, possibly playing a role behind the

fact that the system has the highest T c among perovskite manganites known to date In the present paper, we report our study on the magnetic and transport properties of

La0.7Ca0.3Mn1−x M x⬘O3 共M⬘= Al, Ti兲 compounds Results from previous studies on similar systems were reported and can be referenced in a number of publications.11 – 17Our study is focused mainly on the dilution effects

in La0.7Ca0.3Mn1−x M x⬘O3 in comparison with

La0.7Sr0.3Mn1−x M x⬘O3 The dilution of the Mn network results

in a strong suppression of the ferromagnetism, a deteriora-tion of the conductivity, and a huge development of the

mag-netoresistance The T c 共n p兲 curves obtained for the two series

of our samples, according to the molecular-field model, sug-gest a significant contribution of antiferromagnetic superex-change in La0.7Ca0.3MnO3, in contrast to La0.7Sr0.3MnO3

II EXPERIMENT

All the La0.7Ca0.3Mn1−x M x⬘O3 共denoted as LCMAx for

M⬘= Al and LCMTx for M⬘= Ti兲 samples were prepared

from pure 共ⱖ99.99 %兲 raw powders of La2O3, CaCO3,

MnO2, Al2O3, and TiO2using a conventional solid state

re-action method The powders with appropriate amounts were

thoroughly ground, mixed, pelletized, and then calcined in

several processing steps with increasing temperatures from

900 to 1200 ° C and intermediate grindings The products

were then sintered at 1300 ° C for 48 h in an ambient

atmo-sphere, followed by a very slow cooling process from the

sintering to room temperature with an annealing step of

700 ° C for 5 h Room-temperature x-ray diffraction patterns

共measured by a SIEMENS-D5000 with Cu K␣ radiation兲,

showed that all of the samples were single phase with

per-ovskite orthorhombic 共space group pnma兲 structures;

struc-tural data were calculated using Rietveld refinements Redox

titration experiments 共using K2Cr2O7 titrant and

C24H20BaN2O6S2 colorimetric indicator兲 showed almost no

oxygen deficiencies or excesses 共␦ⱕ0.006兲 in all the

samples Magnetic and four-probe resistance and/or

magne-toresistance measurements were carried out in a Quantum

Design PPMS6000 system

III RESULTS AND DISCUSSION

In several previous publications, there were concerns that

Al3+could substitute for Mn4+, resulting in an oxygen

under-stoichiometry in some Al-doped manganites.17 – 19In the case

of Ti doping, an oxygen overstoichiometry might be realized

if Mn3+is replaced with Ti4+ If the oxygen concentration is

kept stoichiometric, the substitution of Al and Ti for Mn is

therefore expected to be selective, implying that Al3+would

only substitute for Mn3+ and Ti4+ for Mn4+.10 , 13 This is

in-deed supported in the present La0.7Ca0.3Mn1−x M x⬘O3samples

by the fact that there is almost no deficient or excessive

oxygen as determined from the redox titration experiments

Moreover, the plots in Fig 1show that the unit cell volume

derived from x-ray diffraction data monotonically decreases

with increasing Al concentration, while it increases with Ti

substitution These structural variations give a further

con-vincing evidence for the selective substitutions of Al3+ and

Ti4+ for Mn3+ and Mn4+, respectively, considering that the ionic radius of Al3+ 共0.535 Å兲 is smaller than that of Mn3+

共0.645 Å兲 and Ti4+ 共0.605 Å兲 is larger than Mn4+

共0.530 Å兲.20 Measurements of temperature dependent magnetization,

M 共T兲, are carried out for all the samples with both

zero-field-cooling and field-zero-field-cooling protocols Figure2共a兲presents the

M 共T兲 curves of the LCMA x series A sharp ferromagnetic-paramagnetic 共FM-PM兲 phase transition is observed for all the samples in the low doping region The parent compound,

La0.7Ca0.3MnO3, shows an FM-PM transition temperature T c

of ⬃247 K With increasing x, the ferromagnetism is

se-verely suppressed that is indicated by a drastic decrease in

T c The shape of the MZFC共T兲 and MFC共T兲 curves of the x

ⱖ0.15 samples suggests that, although an FM-PM transition

is still observed for x = 0.15, they are no longer pure

ferro-magnets but spin-glass-like systems of short-range

ferromag-netic correlations The T c vs x data extracted from the M共T兲

curves for the ferromagnetic samples are plotted in the inset

of Fig 5and will be discussed later in detail

As can be seen in Fig 2共b兲, the conductivity is also strongly suppressed by the Al substitution Although a metal-insulator共MI兲 transition is observed for all the ferromagnetic

共xⱕ0.1兲 compounds near the transition temperature T c, the resistivity increases in both FM and PM states The drop in

224

226

228

230

232

234

LCMAx

LCMTx

3 )

x

(A

FIG 1.共Color online兲 Variations of the unit cell volume with Al

and Ti concentrations This result demonstrates an additional

evi-dence for the selective substitutions of Al3+and Ti4+for Mn3+and

Mn4+, respectively

10 -2

10 -1

100

10 1

10 2

103

104

10 5

0.15 0.2

0.1 0.075 0.05 0.025

x=0

H=0

b

0 1000 2000 3000 4000 5000

T (K)

x=0

0.025 0.05 0.075

0.1 0.15

0.2

H=60 kG

c

0 2 4 6 8 10 12

La0.7Ca0.3Mn1-xAlxO3

0.0 025 05 075 0.1 15 0.2

a

H=100 G

FIG 2 共Color online兲 LCMAx:共a兲 MZFC共T兲 and MFC共T兲 curves measured in H = 100 G, 共b兲 ␳共T兲 curves in zero field, and 共c兲

−MR共T兲 curves in H=60 kG.

resistivity at T c and the metallic conducting behavior in the

FM state are consistent with the DE mechanism, where a

parallel alignment of the magnetic moments favors the

trans-fer of e gelectrons between Mn3+ and Mn4+ions Since Al3+

does not participate in double exchange, it blocks all the

conducting paths at the sites it occupies The metallic

con-ducting behavior is thus not expected in the highly

substi-tuted samples, where the long-range FM order collapses into

short-range FM correlations; the system consists of

conduct-ing FM regions separated by insulatconduct-ing non-FM boundaries

Such behavior can be seen in the x = 0.15 and 0.2 samples as

the resistance with decreasing temperature monotonically

in-creases up to the limit of our measurement system 共⬃2

⫻106 ⍀兲 showing no observable MI transition The

resis-tance of these samples is still unmeasurably high, down to

temperatures as low as of 2 K and even in an applied field of

60 kG The magnetoresistance ratios, defined as −MR共%兲

=共R0− R H 兲⫻100/R H , measured in an applied field H

= 60 kG for all the LCMAxcompounds are presented in Fig

2共c兲 As expected, all the low doped samples show a peak of

−MR near the FM-PM phase transition that is typical of the

CMR effect Although a maximum of −MR cannot be

mea-sured for the highly doped samples, x = 0.15 and 0.2, a clear

development of the CMR effect with increasing Al

concen-tration is observed At the phase transition, the −MR value

increases from⬃400% for x=0 to ⬃4200% for x=0.1 The

x = 0.15 sample shows a monotonous increase in −MR with

lowering temperature that reaches up to⬃4700% at ⬃73 K.

A similar improvement of the CMR effect was previously

observed in La0.67Sr0.33Mn1−xAlxO3 by Turilli et al.11 The

authors proposed a phenomenological model to

quantita-tively explain the relationship between the magnetic and

transport characteristics Later, Blasco et al.17 reported a

huge development of magnetoresistance 共−MR=107% in H

= 12 T兲 in La2 /3Ca1 /3Mn1−xAlxO3⫾␦ and attributed the

in-crease in −MR to the effect of disorder in the insulating

regions, considering the fact that their samples were not

ho-mogeneous In our work, all the ferromagnetic samples 共x

ⱖ0.1兲 exhibit an MI transition and a maximum of −MR at

temperatures very close to the FM-PM phase transitions

Thus, the increase in −MR should not be attributed to the

magnetic field effect on the insulating regions but directly to

the FM ordering according to the DE mechanism The partial

substitution of Al for Mn not only locally blocks conducting

paths where Al occupies but also globally weaken the FM

order of the Mn network; both lead to an increase in R0 The

later case is directly associated with the decrease in T c and

the enhancement of −MR at T c in the low doped samples

For highly doped ones共xⱖ0.15兲, the increase in −MR with

lowering temperature is due to the expansion of

ferromag-netic regions, which is favored at low temperatures under

high magnetic field If the Al concentration is high enough to

prevent the ferromagnetic regions from percolating

through-out the sample, both resistivity and magnetoresistance would

increase monotonically with lowering temperature However,

too high Al concentration would also suppress the growth of

ferromagnetic regions and therefore reduce the

magnetoresis-tance effect

Figure 3 presents the magnetization, transport, and

mag-netotransport characterizations of the LCMTx compounds

Qualitatively, the Ti substitution causes similar effects on the magnetic and transport properties as those caused by the Al

substitution The substitution suppresses T c, increases resis-tivity in both PM and FM states and strongly enhances the magnetoresistance effect A sharp FM-PM phase transition

with a well-defined T cis observed for all the samples at low substitution concentrations Although a PM-FM phase

tran-sition is still observed for x = 0.075, the insulating behavior

关Fig 3共b兲兴 suggests that this compound is no longer a pure ferromagnet, but a system consisting of conducting FM re-gions separated by insulating non-FM boundaries The

high-est substituted sample, x = 0.1, shows the behavior of a spin-glass-type insulating system While all the FM samples, x

ⱕ0.05, exhibit an MI transition concomitant with a

maxi-mum of −MR at T c, those with insulating behavior demon-strate a monotonous increase in −MR with lowering tem-perature 关see the inset of Fig 3共b兲兴 The −MR value for x

= 0.075 reaches ⬃9000% at 50 K in an external field of 60

kG

There have been several explanations for the reduction of

T c in diluted ferromagnetic manganites, where Mn is par-tially substituted by a trivalent or quadrivalent nonmagnetic element such as Al or Ti One explanation is simply that this

is due to the suppression of long-range FM order of the

lo-calized t2g spins by local breakdown of the exchange cou-plings where the substitution occurs.21 , 22Hu et al.23assumed

a demolition of the DE Mn3+-O2−-Mn4+ bonds and a lower

hole-carrier concentration caused by Ti substitution Kallel et

al.24suggested that the presence of Ti favors the SE interac-tion and suppresses the DE mechanism Some authors

attrib-uted the decrease in T c to the variation of structural

param-10-3

10-1

101

103

105

T (K)

x=0

0.025 0.05

0.075

0.1

H=0

b

0 2500 5000 7500

1 10 4

0 100 200 300

T (K)

x=0

.025

0.05 075

0.1

H=60 kG

0 2 4 6 8 10 12

La0.7Ca0.3Mn1-xTixO3

H=100 G

0.0 025 05 075 0.1

FIG 3 共Color online兲 LCMTx:共a兲 MZFC共T兲 and MFC共T兲 curves measured in H = 100 G,共b兲␳共T兲 curves in zero field, and the inset

of共b兲 shows −MR共T兲 curves in H=60 kG.

eters such as the average Mn-O bond length d具Mn-O典 and

Mn-O-Mn bond angle ␪具Mn-O-Mn典.16 , 25 By calculating the

e g -electron bandwidth W using an empirical formula, W

⬀cos关1

2共␲−␪具Mn-O-Mn典兲兴/d具Mn-O典3.5

, Kim et al.25observed a

nar-rowing of W in La0.7Sr0.3Mn1−xTixO3 and related it to the

decrease in T c Although W共x兲 was found to vary in a manner

that seems to be consistent with T c 共x兲 in R 1−x A xMnO3,26

ex-planations for the decrease in T crelying solely on the

varia-tion of W共x兲 may not be relevant in Mn-site substituted

man-ganites, especially when Mn is substituted by Al.10 To

present the evidence for the inconsistent variation between

W 共x兲 and T c 共x兲, we plot in Fig.4the W共x兲 curves calculated

for the present La0.7Ca0.3Mn1−xAlxO3 samples and also for

the La0.7Sr0.3Mn1−xAlxO3 samples used in Ref 10 The

re-sults clearly show, for both the sample series, a widening of

the bandwidth with x, in contrast to the decrease in T c共see

Fig 5 and the results in Ref 10兲 These results

unambigu-ously indicate that the e g-electron bandwidth alone does not

account for the suppression of ferromagnetism and

conduc-tivity in magnetically diluted manganites

Nam et al.10 suggested that the variation of T cin diluted

ferromagnetic manganites can be explained in terms of a

molecular-field system with mixed interactions; T c can be

written as

T c=2S共S + 1兲 3k B 兺␣n z J␣,

where k B is the Boltzmann’s constant, S is the average spin

per magnetic ion, and z␣ is the number of ions involved in

the interaction ␣with exchange coupling constant J␣ If the

system is only slightly diluted 共so that any change in J␣ is

not significant兲 by a nonmagnetic element with concentration

n p , a linear dependence of T cis expected following the

rela-tion z␣= z␣0共1−n p 兲, where z␣0is the z␣value of the undiluted

system In mixed-valence manganites, while the

Mn3+-O2−-Mn4+ coupling 共denoted as DE兲 is ferromagnetic

according to the DE mechanism, the Mn3+-O2−-Mn3+ 共SE1兲

and Mn4+-O2−-Mn4+ 共SE2兲 couplings are superexchange

an-tiferromagnetic A selective substitution of Al3+共or Ti4+兲 for

Mn3+ 共or Mn4+兲 causes both zDE and zSE1 共or zSE2兲 to vary

proportionally with 1 − n p while zSE2 共or zSE1兲 is left intact The study on La0.7Sr0.3Mn1−x M x⬘O3 has found that共Ref 10兲

in the undiluted La0.7Sr0.3MnO3 and slightly diluted

com-pounds, both JSE1 and JSE2 are basically negligible, leading

to a linear T c 共n p 兲 dependence that crosses T c = 0 K at n p= 1

scaled for both M⬘= Al and Ti Although the largest W

ob-tained for La0.7Sr0.3MnO3 would be one main reason for its

highest T c ever found among manganites,26 the absence of antiferromagnetic superexchange interactions might

contrib-ute to maximizing the T c value On the other hand, if the antiferromagnetic superexchange interactions in the

undi-luted system are significant, T c will be reduced and the

T c 共n p 兲 linear dependence should intersect T c = 0 K at a n p

value smaller than 1 This seems to be qualitatively consis-tent with the results obtained for the present LCMAx and LCMTxsystems presented in Fig.5, where the Tc 共n p兲 curves are linear at low substitution concentrations and extrapolate

to T c = 0 K at n p= 0.27 for LCMAx and 0.35 for LCMTx Due to the presence of antiferromagnetic exchange, the scal-ing behavior observed for La0.7Sr0.3Mn1−x M x⬘O3 is no longer relevant for La0.7Ca0.3Mn1−x M x⬘O3 Using the data in Fig 5

and adopting JSE1= −0.58 meV derived from neutron scatter-ing measurements on LaMnO3,27 we obtained JSE2=

−4.34 meV and JDE= 5.92 meV These values may not be

very accurate since JSE1 may change from LaMnO3 to

La0.7Ca0.3MnO3 because of the difference in crystal struc-tures, but essentially indicate the presence of significant an-tiferromagnetic interactions that coexist and compete with the DE FM one in La0.7Ca0.3MnO3 and the slightly diluted compounds

IV CONCLUSION

Selective dilution of the Mn network in

La0.7Ca0.3Mn1−x M x⬘O3共M⬘= Al, Ti兲 suppresses the ferromag-netism and metallic conduction but strongly enhances the

magnetoresistance In contrast to the scaling behavior T c 共n p兲 previously reported for La0.7Sr0.3Mn1−x M x⬘O3, extrapolations

of the T c 共n p 兲 linear curves to T c = 0 K give different n p val-ues and both much smaller than 1 These results indicate that according to a molecular-field approximation,

antiferromag-0 50 100 150 200 250

T c

0.27 0.35

100 150 200 250

Tc

x

FIG 5 共Color online兲 The effect of selective dilution on T cof

La0.7Ca0.3Mn1−x M x⬘O3 at low dilution concentrations, n p 共n p

= x/0.7 for LCMAx and x/0.3 for LCMTx兲 The inset shows the

original T c 共x兲 data.

0.093

0.094

0.095

0.096

0.097

La0.7Sr0.3Mn1-xAlxO3

x

FIG 4 共Color online兲 Variations of the conduction electron

bandwidth W with Al and Ti concentrations The monotonic

in-crease in W 共x兲 is contradictory to the reduction in T c 共x兲 in both

La0.7Ca0.3Mn1−xAlxO3and La0.7Sr0.3Mn1−xAlxO3systems

netic superexchange among Mn ions is significant in

La0.7Ca0.3MnO3 Our results suggest that the variations of

e g -electron bandwidth W alone cannot explain the decrease

in T c of Mn-site substituted manganites Qualitatively, this

simple selective dilution technique can be quite effective in

probing the presence of competing interactions in mixed

sys-tems such as manganites

ACKNOWLEDGMENTS

This work has been performed using the facilities of the State Key Laboratories 共IMS, VAST兲 The authors thank N

N Toan and D T A Thu for their help in redox titration experiments

*daonhnam@yahoo.com

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