Giant magnetoresistance in bulk (La34Tb14)23Ca13MnO3

Solid State Communications, Vol. 96, No. 9. pp. 627-630, 1995 Elsevier Science Ltd Printed in Great Britain 0038-1098#5 $9.50+.00 0038-1098(95)00533-1

GIANT MAGNETORESISTANCE IN BULK (La3/4Tb&$al~Mn03 J.M. De Teresa, J. Blasco, M.R. Ibarra, J. Garcfa, C. Marquina, P. Algarabel and A. de1 Moral. Departamento de Ffsica de la Materia Condensada e Xnstituto de Ciencia de Ma&ales de Arag6n. Universidad de Earagoxa-CSIC, 50009- Zaragoxa, Spain.

(Received 12 July 1995; accepted 3 August 1995 by F. Yndurain)

Magnetoresistance (MR) as large as 70080% at H=12T has been observed in bulk (La3/4Tb&&alflnO3 at Tc-lOOK, the temperature at which the compound becomes ferromagnetic. This unusual effect is accompanied by magnetovolume effects above Tc which vanish rapidly below Tc. This fact along with the shape of the isotherms above and below Tc suggest two different mechanisms for the giant MR above and below Tc. Keywords: A. magnetically ordered systems, D. electronic transport, E. strain

1. Introduction

cubic perovskite-like

structure

where Mn ions are

separated by O-2 ions. In LaMnQ Materials exihibiting giant magnetoresistance

the Mn ions are

trivalent but the addition of B+2 ions causes equal number

(GMR) are desirable for many technoIogical applications

of Mnd ions to appear. They found that for some Mn4

and consequently research on this field is becoming vast.

concentrations the compounds become ferromagnetic and

Recently LaxBl_xMnO3

Zener proposed

(B=Ca,Sr,Ba,Pb)

compounds

the double-exchange

mechanism

to

have been found to show negative GMRl-6. MR values

explain the ferromagnetic interaction between Mn+3 and

greater than lOOBOO%were reported in La-Ca-Mn-0

films2 but MR values of bulk samples appear to be far

Mn4 ions through the Oe2 ion.@. The behaviour of the resistivity of these compounds is intriguing. Above Tc the

from such figures. The value for the MR is defined here as

behaviour is semiconductor-like and below Tc is metallic-

MR(%)=1OOx(p(0)-p(H))/p(H)).

like. The overall result is a p vs T curve with a maximum

A recent

polycrystalline ~.tjy&u7Ca()j$vhlO3 as large

as

polycrystalline

10000%.

on

showed MR values

In a systematic

(Tbr.,Lax)~Cal&vlnO3

work

thin

study

at around Tc. Conduction by magnetic polarons (based on

of

Mott’s ideas”)

we have found

has been

proposed

as being

the

for the x=0.75 compound the highest MR reported so far

mechanism responsibIe for the behaviour of resistivity above T&10. When magnetic polarons (holes which

in bulk samples: 70000% at H=12T. The mechanism

become localised, polarising a small region around them)

responsible

compounds is not completely clear but some ideas have

form. the conduction takes place via thermal hopping which would explain the rapid increase of p above Tc.

been put forward.

Very recent magnetoeIastic results support the idea of the

for such GMR values

in this sort of

existence of localised charges (polarons) below a determined temperature, Tp, which bear a local distortion

The basic magnetic and structural properties of LaxBl_,Mn03 were studied a long time ago by Jonker and Van Santen7l. They found small distortions from the

of the lattice*z. This fact brings about an anomalous connibution to the thermal expansion over the anharmonic 627

628

GIANT MAGNETORESISTANCE

Vol. 96. No. 9

phonon contribution in the temperature range T&TsTp.

done with silver paste on bar-shaped samples and a 220

The role of an external magnetic field in this region is to

Keithley current source was used to apply wnstant D.C.

inhibit the formation of polarons causing the observed

current. The magnetic field was applied parallel to the

negative MR. Below Tc the onset of ferromagnetism

curmnt. Linear thermal expansion (magnetostriction) was

causes the cross-over to the metallic regime. Ju et al.3

measured

using the strain-gauge

have proposed the double-exchange mechanism to explain

magnetostriction

the transfer of electrons through the material and the drop

perpendicular

technique.

results the strain parallel (111) and

(XI) to the applied field was measured.

of resistiviy. In this region they explained the GMR taking

Volume magnetostriction

into account the effect of domain-walls at low fleds and

tion are straightfowatdly calculated as

spin-alignment at high fields.

ht=xll- hl

field (up to 12T) on the transport and

magnetoelastic

properties

Tbl/4h/3CatnMn03,

of the compound

(La314

which shows the largest GMR

and anisotropic magnetostricco=Xl1+2hl and

respectively. Magnetization

In this paper we aim at studying the effect of the magnetic

For the

and

initial

magnetic

susceptibility (%A& under applied fields up to 5T were carried

out

with

a quantum

interference

device

magnetometer (SQUID).

reported on bulk samples so far.

2. Experimental details The (Lay4l%t/4)&al~Mn03

3.Results and discussion sample was

As ferromagnetism

seems to be a necessary

prepared using a gel precursor in order to obtain well-

condition for the appearance of GMR in these manganite

mixed reagents. Stochiometric amounts of La205, Tb407,

compounds, we firstly performed the magnetic charac-

CaCG3 and MnC05 with nominal purities higher than

terisation

99.9%, were dissolved in concentrated nitrid acid msulting

temperatures. In Egure 1 we show the results of X&C. .

of the compound

at different

fields

and

in a light solution. Afterwards, citric acid and ethylene glycol were added in a ratio of 4g citric acid to lml

The inset of figure 1 shows the spontaneous magnetization (Ms) vs temperature as obtained from Arrott’s plot from

ethylene glycol and lg metal nitrates. The solution was

where Tc=l03K is obtained. As X&C, at zero tleld gives

heated and the excess nitrid acid and water wen boiled off

us information about the spin correlation and it increases

giving a brown gel. The gel was heated to give a blackbrown powder. This precursor was calcined at 1173K overnight. The remaining black powder was cold pressed to 4Kbar and sintered

at 1273K for 3 hours with

intermediate grindings. Finally the pellet was sin&ted at 1573K for 8 hours resulting in a hard black ceramic material. The sample was analyzed by means of X-ray powder diffraction

resulting

in a single-phase

with a

perovskite-like structure. The X-ray diffraction pattern can be indexed

in the Pbnm spatial group showing

following

lattice

parameters:

a=0.5442( 1)

the nm,

b=O.5456( 1) nm and &).76994(g) nm.

1 A superconducting coil was used to produce steady magnetic

fields

0

up to 12T. Both resistance

50

100

150 T (K)

200

250

300

(magnetoresistance) and thermal expansion (magnetostrlction) were simultaneously

measured in two different

pieces of the same sample in order to correlate both

Figure 1. XA.C. as a function of temperature. The inset

properties. Resistance (magnetoresistance-) was measured

shows the results of the Arrott’s plot (see text). Lines are

with the standard four points technique; the contacts were

visual guides.

629

GIANT MAGNETORESISTANCE

Vol. 96, No. 9

abruptly with temperature reaching values of 3284 Qxcm

104

at around Tc. We also show the values of nsistance under E

different magnetic fields (1T and 12T). Two facts are

103

remarkable. Firstly the resistance is lower as the field is 2 x

102

.z > ‘S .z? 2

10’

higher and secondly

the temperature

at which the

maximum of resistivity takes place moves upwards with increasing fields (96K at H=O, 102K at H=l and 140K at H=l2T). The inset of figure 2 displays the huge values of MR at 12T. The MR ratio reaches the maximum value of 70000% at T= 90K.

100

50

0

100

150

200

250

300

T (K)

In figure 3 we have selected the isotherms at T=75K, 95K and 115K. At T=95K the curve shows hysteresis at low fields. Once the high field state has been

Figure

2. Resistance

under 0, 1T and 12T applied

magnetic fields. The inset shows the MR(%) ratio at 12T. The line is a visual guide.

reached, if this field is supressed the sample returns to another state with lower isotherms below (T=75K) inset of figure 4) suggests the GMR above and below

resistance. The shape of the and above (T=llSK) Tc (see two different mechanisms for Tc as observed in other related

with an extra contribution over the Curie-Weiss law from

compound&.

200K down it seems to indicate the presence of short

curvature. Above Tc a negative curvature appears to be

range magnetic order in this large range of temperatures

present at low fields changing to positive at high fields

above Tc. The decrease of XA.c. below &OK is a signal

through an inflexion point.

of the decreasing

mobility

of the domain

walls for

decreasing temperatures.

In order

In figure 2 we show the resistivity measured as a function

of temperature

under different

magnetic fields. At zero field the resistivity

9

0 4

Below Tc the curves show a positive

steady changes

to get better

insight

the

mechanisms

investigated

the change of length in the sample with

temperature and field. isotherms

(75K,

in both regions

into

underlying

we have

Figure 4 shows three selected

95K and

115K) of the volume

35

30 x - 25 E U 20

0

2

4

6

8

10

1;

H (T) Figure 3. Isotherm of MR at 95K increasing (0) and decreasing (0) the magnetic field. The inset shows the

Figure 4. Isotherms of o=hl+ 2x1 at 75K, 95K and 115K.

isotherms at 75K and 115K increasing the magnetic field. Lines are visual guides.

The inset shows the volume distortion at 12T obtained from LTE (+) and w (0). Lines are visual guides.

630

GIANT MAGNETORESISTANCE

Vol. 96, No. 9

magnetostrlction. The change of the shape of the curves below and above Tc is also visible. The anisotropic

follows: the mechanism which produces the increasing

magnetostriction

temperature is quenched applying magnetic field or with

(not shown here) was found to be the

typical of a ferromagnetic compound: zero above Tc and small below Tc (typically

in these compounds

the onset of ferromagnetism

when lowering

the

at Tc. Lower Tc implies

The inset of

larger resistivity and consequently a larger MR ratio at Tc.

figure 4 shows the difference in the relative volume of the

Above Tc, the conduction is considered to take place via

sample

hopping of polarons and the effect of the magnetic field is

under

aforementioned

0 and

Xt=SOxl@).

resistivity

12T obtained

isotherms

through

and through

the

the thermal

to destroy the polarons giving rise to negative GMR and

expansion under such fields (AV/V=3xAL/L=3x(L(OT)-

volume

L(lZT))/L(OT)). This difference becomes substantial at around Tp=ZOOK and disappears below Tc when the long

proposed by Ju et al. to explain the GMR is in agreement

range magnetic order between Mn+3 and Mn+4 ions

alignment seem to be responsible for the magnetization

appears.

and MR properties. The shape of the MR isotherms above and below Tc and the magnetoelastic effect above Tc From all the results

obtained

crystalline (La&Ibl&&al&in03

for poly-

we can say that

magnetostriction.

Below Tc the mechanism

with the experimental facts. Domain-walls effect and spin-

strongly support different mechanisms for the GMR below and above Tc.

substitution of 25% of La by Tb in the polycrystalline LayjCalnMnO3

compound

brings

about

To sum up, in a systematic study of the series

dramatic

changes in the magnetic and transport properties of this

(Tbl_xLa&&al~MnO3

material.

crystalline x20.75 compound a GMR ratio (70000% at

Tc changes from =265 K for the undoped

we have found for the poly-

compound to =103K for the doped one and the associated

12T) at the temperature marking the cross-over from

peak in MR reaches values of =70000%. The main effect

semiconductor-like to metallic-like behaviour. Magnetostriction effects above Tc should be taken into account

of Tb must be to weaken the double-exchange interaction between Mn+3 and Mn+4 lowering the value of Tc. The experimental facts seem to indicate that the MR effect at Tc is larger as Tc is lowe&*3. It can be explained as

to clarify the GMR mechanism. The GMR mechanism below Tc appears to be different from the mechanism above Tc. Low Tc seems necessary to get high GMR values.

References

1) R. von Helmolt, J. Wecker, B. Holxapfel, L. Schultz

6) S. Jin, H. M. O’Bryan, T. H. Tiefel, M. McCormack

and K. Samwer. Phys. Rev. Len. 71,2331(1993).

and W. W. Rhodes, Appl. Phys. Lett. 66,382 (1995)

2) M. MC Cormack, S. Jin, T. H. Tiefel, R. M. Fleming,

7) G. H. Jonker and J. H. Van Santen, Physica 16,337

Julia M. Phillips and R. Ramesh, Appl. Phys. Len. 64,

(1950) 8) G. H. Jonker, Physica 22,707 (1956)

3045 (1994) 3) H. L. Ju, J. Gopalakrishnan,

J. L. Peng, Qi Li, G. C.

9) C. Zoner, Phys.Rev. 82,403 (1952)

Xiong, T. Venkatesan, and R. L. Greene, Phys. Rev. B 51,

10) R. M. Kusters, J. Singleton, D. A. Keen, R. McGreevy

6143 (1995)

and W. Hayes, Physica B 155,362 (1989)

4) R. Mahesh, R. Mahendiran, A. K. Raychaudhuri and C.

11) N. F. Mott, Adv. Phys. 21,785 (1972)

N. R. Rao, J. Solid State Chem. 114,297 (1995)

12) M. R. Ibarra et al. (to be published)

5) J. 2. Liu, I. C. Chang, S. Irons, P. Klavins, R. N.

13) V. Caignaert, A. Maignan, and B. Raveau, Solid State

Shelton, K. Song and S. R. Wasserman, Appl. Phys. Lea.

Commun. 95.357 (1995)

66.3218 (1995)