GIANT MAGNETORESISTIVE MANGANITES DOPED

WITH In : AN ESR STUDY

O.Raita, D.Gavre, X.Filip, Al.Darabont1 , M.N.Grecu

2, L.V.Giurgiu

National Institute for Research&Development of Isotopic and Molecular Technologies

P.O.Box 700, 3400 Cluj-Napoca, Romania

1Faculty of Physics, Babes-Bolyai University, 3400 Cluj-Napoca, Romania

2National Institute for Material Physics,

P.O.Box MG-7, 76900 Bucharest, Romania

Abstract

The effects of the substitution for Mn with In in the giant magnetoresistive

manganites La2/3 Ca1/3 Mn1-x InxO3 ( x < 0.05 ) have been studied by ESR. The temperature

and x dependence of the resonance linewidth and integral intensity of the ESR signal have

been investigated and disscused in terms of the bottlenecked spin relaxation and small

polaron hopping models.In the paramagnetic regime, the exchange coupling integral, J,

between Mn3+

and Mn4+

ions shows a decrease with increasing x.It could arises from

weakening of the double exchange interaction by In – doping. It was also found a strong

variation of the polaron activation energy with the doping level.

1. INTRODUCTION

The manganese perovskites are well known compounds for their giant ( or

colossal ) magnetoresistive ( CMR ) properties. They are mixed valent compounds

52

containing Mn3+

and Mn4+

ions. Coexistence of ferromagnetic and metallic

conduction in these materials has been explainded in terms of double exchange

mechanism ( DE ) [ 1 ]. However ,DE alone does not explain the CMR and an

additional mechanism based on the polaronic effects was included

[ 2 ].Here a strong electron – phonon coupling is expected because the electronic

ground state of Mn3+

ions is degenerate giving rise to the Jahn – Teller ( JT ) effect.

The basic magnetic and structural properties of CMR compounds R1-y Ay

Mn O3 ( R- rare earth; A – Ca,Ba,Sr ) were widely studied in the past.Despite the

exhaustive study of the effects of the rare-earth replacement in these manganites,

the influence of the substitution at Mn sites with diamagnetic elements is much less

investigated [ 3,4 ].

ESR spectroscopy is a powerfull tool to study the complex magnetic state

in CMR compounds [ 5,6,7,8 ]. The linewidth could be considered to be caused by

spin – lattice relaxation of the exchange coupled Mn4+

ions via Mn3+

JT – ions to

the lattice under the condition of a strong relaxation bottleneck [6] .The energy

transfer in the paramagnetic regime is provided by the activated hopping of small

polarons in a connection with the jumps of eg charge carriers from Mn3+

ions to the

Mn4+

ions [ 7 ].

We report here an ESR investigation on the effects of the replacement of

Mn by In in the archetypal giant magnetoresistive La2/3 Ca1/3 Mn O3 . In3+

- ion was

chosen because it has no magnetic moment and its atomic radius is larger than the

Mn one.Due to this difference in the ionic size , a strong effect of the substitution

on the ESR data is expected.The results could be compared with the already

reported ones for Al substitution [ 9, 10 ].

2. EXPERIMENTAL

Polycrystalline La2/3 Ca1/3 Mn1-x Inx O3 ( x < 0.05 ) were prepared with the

conventional solid-state reaction method.The Mn4+

content was determined by

53

redox titration and the transmision temperature TC from the susceptibility curves (

Tabel 1).

The samples with x > 0.03 are nominally stoichiometric and in order to

preserve the charge equilibrium the substitution of In for Mn leads to an oxidation

from Mn3+

to Mn4+

.

The lineshape of the powdered samples was found to be Lorentzian over

the investigated temperature range and for all x. In order to evaluate the ESR

parameters the derivative spectra were fitted with a Lorentzian lineshape with fit

parameters being the half- width at half- height , δH1/2 of the corresponding

absorption line and the resonance field, H0

3. RESULTS AND DISCUSSION

The temperature dependences of the ESR linewidth δH1/2 are shown in

Fig.1 for samples with various In concentration x. ( Fig.1 ) .The linewidth goes

through a minimum at T min � 1.1 TC and the temperature Tmin shows a decrease

with increasing x in accordance with TC variation [10]. The effect of In substitution

is to progressively broaden the resonance line and the linewidth at fixed

temeprature shows an increase with increasing x ( Fig.2 ) .It could arises from the

weakening of the DE interaction by In doping which reduces the exchange field at

the Mn sites and consequently an increase of the effective dipolar broadening

would be expected.

We have found that at high temperatures, the ESR integral intensity, IESR ,

for La2/3 Ca1/3 Mn1-x Inx O3 samples follows a ferromagnetic Curie –Weiss ( CW )

temperature dependence

θ−

=T

C)T(I

ESR (1)

arising from the ferromagnetic coupling of the Mn4+

and Mn3+

subsystems. At

lower temperatures in the paramagnetic regime there is a deviation from the CW

54

law. In Fig. 2 we show a plot of 1/ I ESR for two selected In doped samples. From

the linear behaviour for T � 1.9 TC and the extrapolation to lower temperatures, the

CW temperatures θ could be obtained ( Tabel 1 ).

I ESR is proportional to the static magnetic susceptibility of the ions

responsable for the signal ( I ES R ≈ χ tot ) .In what follows we consider the ESR

response of the system to contain three disting components: Mn4+

ions ( 3d3 with S

= 3/2 ) ; Mn3+

ions ( 3d4 with S = 2 ) and the lattice.

In the bottlenecked spin relaxation approach [7 ],

σχ+χ=

sI

ESR (2)

where s

χ and σ

χ are the renormalized static susceptibilities of the Mn4+

and

Mn3+

spin systems, respectively

0

s

02

0

0

ss 1

1

χχλ−

χλ+χ=χ

σ

σ , 0

s

02

0

s0

1

1

χχλ−

χλ+χ=χ

σ

σσ (3)

Here 0

sχ and

0

σχ are the bare ( without exchange ) susceptibilities of the

Mn4+

and Mn3+

ions.The parameter λ is given by

0

2

Bs

B

ggN

kJz

µµ=λ

σ

(4)

where z = 6 is the number of nearest neighbours, N is the total number of Mn

spins per cm3 , g s ( g σ ) is the g factor of Mn

4+ ( Mn

3+ ) ions and J is the exchange

coupling integral for the exchange interaction between Mn4+

and Mn3+

spins.

We can get an estimate of J by fitting Eq.(2) and (3) to the I ESR * T data

(Fig 4) .The multiplication of IESR by T eliminates the intrinsec temperature

dependence of the ESR signal caused by the Boltzman population of the Zeeman

levels involved. We assumed that the bare susceptibility of Mn4+

ions follows a

Curie law T/CS

0

s=χ while that of Mn

3+ is given by Curie – Weiss law,

55

)T/(C0 θ−=χσσ

, where θ = - 220 K is taken to be the same as for undoped

LaMnO3 parent compound [11].

The results can be found in Table 2 .For x = 0 sample the evaluated J

value is of the same order of magnitude as the values obtained for La0.8 Ca0.2 Mn16

O3+y ( J = 78 K) [ 6 ], La0.8Ca0.2 Mn18

O3+y ( J = 71 K) [ 6 ] and Pr0.6Ca0.4 MnO3 ( J

= 154 K ) [ 12 ] from ESR measurements.

As one can see, the effect of In doping is to perturbe DE coupling between

Mn4+

and Mn3+

ions, causing a decrease in J ( Fig. 5 ) . It could arises from the

bending of the Mn - O – Mn in the ( a-b ) plane which is expected to slow down

the carrier hopping and weaken the DE [5,13]. The diluation of J in In - substituted

samples is higher than the corresponding one for Al substitution [ 9,10 ].

In the following we discuss the temperature dependence of the ESR

linewidth (Fig.1). It is well established that in the paramagnetic regime of

manganites , the conductivity σ is dominated by the adiabatic hopping motion of

small polarons with the temperature dependence of the form

)Tk/E(expT/1)T(Bσ

−∝σ [ 14 ]. If one assumes that the polaronic charge

carriers also exists in In – substituted manganites, than the temperature dependent

part of the ESR linewidth should arises froma thermally activated process

involving the adiabatic hopping of small polarons. The solid line in Fig.6

represents the best fit to the data using the expression [ 7 ]

)Tk/E(expATH)T(HBa

1

02/1−+δ=δ −

(5)

From such fits we obtained the polaron activation energy Ea and the

residual linewidth δ H0 . The data are also summerized in Table 2.In Fig.7 we

demonstrate that Ea decreases with increasing doping level x ( i.e. the decreasing

Mn3+

content ).

56

Recent investigations on La 1-y Cay MnO3 and La MnO3+δ have shown that

the polaron binding energy Eb ( and consequently Ea ) increases linearly with the

Mn3+

content [ 15 ].Here was argued that the polaronic binding energy is

proportional to the Mn – O distorsion and there is a linear relation between Mn – O

distorsion and the Mn3+

content. Our finding on La 2/3Ca1/3 Mn1-x Inx O3 is in

agreement to that.

4. CONCLUSIONS

The effects of In substitution in La 2/3Ca1/3 Mn O3 perovskites have been

investigated by ESR. The temperature dependence of the integral intensity of ESR

signal could be explained by a model in which a bottlenecked spin relaxation of

the entire exchange coupled Mn3+

- Mn4+

spins system is considered. The

weakening of the double – exchange interaction by In - doping reduces the

exchange field at Mn sites which result in a decrease of the exchange coupling

integral J between Mn spins.Considering that the relaxation path is provided by

thermaly activated hopping of small polarons ,the activation energy Ea could be

evaluated from the temperature dependence analysis of the ESR linewidth. A

decrease of Ea with increasing In - doping level was found.

REFERENCES

[1] C. Zener, Phys. Rev. B 82, 403 (1951)

[2] A.J .Millis, P.B. Zitlewood and B.I. Shraiman, Phys Rev. Lett. 74 ,5144 (1995)

[3] J. Blasco, J.García, J.M.deTeresa , M.R. Ibarra, J.Perez, P.A. Algrabel , C.

Marquina and C.Ritter , Phys. Rev. B55, 8905 (1997)

[4] M.C.Sánchez ,J.Blasco, J.García, J.Stankiowicz, J.M. de Teresa and

M.R.Ibarra , J. Solid State Chem. 138 , 226, (1998)

57

[5] X.Filip, M.N.Grecu, L.V.Giurgiu , Al.Darabont ,O.Raita and J.Blasco Studia,

Univ. Babes – Bolyai , Physica, Special Issue 2 , 355 (2001)

[6] A.Shengelaya, Gou-meng Zhao, H.Keller and K.A.Muller , Phys Rev. Lett.,

77, 5296 (1996)

[7] A.Shengelaya, G.Zhao, H.Keller, K.A.Muller and B.I. Kachelaev, Phys. Rev.

B 61, 5888 (2000)

[8] V.A. Atsarkin, V.V. Demidov, G.A.Vasneva and K.Conder Phys. Rev. B 63,

92405 (2000)

[9] L.V. Giurgiu, M.N.Grecu,X.Filip, Al. Darabont ,C. Filip, R.Ordean,

G.D.Popescu, Proc. 30-th Congress AMPERE , Lisbon, Ed. A.F.

Martins, Universidade Nova de Lisboa, Lisbon, Portugal, p.51 (2000)

[10] L.V.Giurgiu, M.N.Grecu, X. Filip, A.Darabont, E.Indrea, I.G.Deac, C.Filip,

R.Ordean, G.D.Popescu, E.Surducan, J.Blasco and G.Papavassiliou, J.

Optoelectronics and Adv.Mat., 2 , 375 ( 2000)

[11] M.T.Causa, G.Alejandro, R.Zysler, F.Prado, A. Carneiro and M.Tovar,

J.Magn. Magn. Mater . 196-197 , 506 ( 1999)

[12] R. Gupta, J.P.Joshi, S.V.Bhat, A.K.Sood and C.N.R. Rao , J.Phys : Condens.

Matter 12, 6919 ( 2000)

[13] L.V.Giurgiu, Plenary Lecture, International Workshop on Advanced Materials

and Structures, Timisoara, Romania ( 2002)

[14] D.C. Worledge, G. Jeffrey Smyder, M.R. Beasley and T.H. Geballe, J.Apll.

Phys.80, 5158 ( 1996)

[15] J.M.de Teresa, K.Dorr, K.H.Muller, L.Schultz and R.I. Chakalova Phys. Rev.

B 58, R5928 (1998)

58

Tabel 1 Percentage of Mn4+

, critical temperatures TC, Curie – Weiss temperature θ

and the ratio TC / θ for La2/3 Ca1/3 Mn1-x Inx O3

x

Mn4+

( % )

T C

( K ) θ

( K )

T C / θ

0.00 30 263 339 0.78

In

0.01

0.03

0.05

31

32

28

235

190

( 65 )

294

220

161

0.80

0.86

0.41

Table 2 Exchange coupling integral J between Mn spins, polaron activation

energy Ea and the residual linewidth δH 0 in the paramagnetic regime of La

2/3Ca1/3Mn1-xInxO3 manganites for different x.

Compound J

( K )

E a

( meV)

δ H0

( G )

La 2/3Ca1/3MnO3 116 120 24

La 2/3Ca1/3Mn1-xInxO3

x = 0.01

x = 0.03

x = 0.05

85

71

50

106

97

84

77

138

298

59

Fig. 1.

Fig. 2.

60

Fig. 3.

Fig. 4.

61

Fig. 5.

Fig. 6.

62

Fig. 7.

FIGURE CAPTIONS

Fig. 1 Temperature dependence of the linewidth δ H ½ for La2/3 Ca1/3 Mn1-x Inx O3

manganites

Fig. 2 Dependence of the ESR linewidth on concentration x wtih an implicit

parameter of temperature. The lines are only guides to the eye

Fig.3 1 / I ESR as function of temperature for La2/3 Ca1/3 Mn1-x Inx O3 ( x = 0, 0.03)

samples. The solid lines represent the best fits to the Eq. (1 ) at high temperatures (

T ≥ 1.5 TC)

Fig.4 Temperature depenence of I ESR * T for La2/3 Ca1/3 Mn1-x Inx O3 ( x = 0; 0.03)

samples. The solid lines represent the best fit to Eq(2) in the temperature range

1.5TC ≤ T ≤ 500 K

63

Fig. 5 Doping dependence of the exchange coupling integral J between Mn spins in

La 2/3 Ca1/3 Mn1-x Inx O3 manganites .

Fig.6 Temperature dependence of the ESR linewidth δ H ½ for La 2/3Ca1/3Mn1-

xInxO3 fitted with Eq.(5)

Fig.7 Polaron activation energy Ea as a function of doping level x in La 2/3 Ca1/3

Mn1-x InxO3