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Magnetoresistance in (La1−xCax) MnO3 films
Journal of Magnetism and Magnetic Materials 140-144 (1995) 2049-2050
Journal of magnetism and magnetic materials
ELSEVIER
Magnetoresistance in ( Lal_x Ca x) MnO3 films J.F. Lawler *, J.M.D. Coey Physics Department, Trinity College, Dublin 2, Ireland Abstract
Thin films of (La I _xCax)MnO3 with 0 < x < 1 have been prepared by pulsed laser deposition. Films with x = 0.3 and 0.5 which are in the composition range for ferromagnetism show an increased resistance and a giant magnetoresistance effect around the Curie temperature. In the case of x = 0.3, the resistance falls from 50 m l ) cm at Tc to 400 i.tl) cm as T ~ 0 K. There is also a large magnetoresistance effect around the Curie point ( A p / p o = 80% in 5 T), which persists for x = 0.5 to low temperatures. These results are interpreted in terms of the magnetic polaron transport in the imperfectly ordered ferromagnetic material.
1. Introduction
The mixed valence perovskite manganites (La3+_x M~ +)MnO 3 (M = Ca, Ba, Sr) were first studied by Jonker and van Santen in 1950 [1]. They exhibit ferromagnetic ordering for compositions around x = 0.3. The ferromagnetic behaviour was explained by Zener [2] with his theory of double exchange in 1951. Resistivity measurements on these materials showed decreases below the Curie temperature and they also exhibit a large negative magnetoresistance effect in this region, which was explained in terms of spin disorder scattering by de Gennes and Friedel [3]. There are recent reports of tunnelling-like behaviour when manganite films are used as barriers in sandwich heterostructures with cuprate superconductors [5,6] and the transport properties have been re-examined in terms of modern ideas on giant magnetoresistance [4]. Here we report on the resistive and magnetoresistive properties of thin films of (La3+_xMn2x+)MnO 3 produced by pulsed laser deposition.
cycle helium refrigerator. Fields of 0.4 T could be applied using an electromagnet. Higher field measurements (6 T) were made in a superconducting solenoid. X-ray diffraction showed the films to be single phase, with highly oriented growth on the substrate [6]. 3. Results and discussion
The resistivity of the films for the range of compositions are shown in Fig. 1. Films at either end of the composition range (x = 0.0, 0.1, 0.7 and 1.0) show semiconductor-like behaviour with decreasing temperature. For these compositions the material is antiferromagnetic and conduction is by variable range hopping. The initial activation energies for the x = 0.0 and 1.0 films are E a = 0.13 and 0.10 eV respectively. The two intermediate values (x = 0.3 and 0.5) show an initial increase in resistivity followed by a drop to almost metallic-like conductivity at lower temperatures. The improved conductivity is associated with the ferromagnetic ordering in the compound
2. Experimental 4
The films were deposited using a KrF (248 nm) excimer laser. The films were formed in-situ from sintered ceramic targets with compositions x = 0.0, 0.1, 0.3, 0.5, 0.7 and 1.0. The substrates were single crystal MgO (100), deposition took place at 720°C with a target substrate distance of 4.5 cm and a background oxygen pressure of 20 Pa. Films were typically 200 nm thick; the resistivity was measured by the van der Pauw [7] method in a closed
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1000/T (K) * Corresponding author. Fax: +353-1-6711759.
Fig. 1. Resistivity of thin films of the (La I xCax)MnO3 system as a function of temperature.
0304-8853/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved
SSDI 0 3 0 4 - 8 8 5 3 ( 9 4 ) 0 1 2 1 5 - 6
J.F. Lawler, J.M.D. Coey/Journal of Magnetism and Magnetic Materials 140-144 (1995) 2049-2050
2050
3o
.
.
.
x 0 3 .
25
~-
2o
~.a.°
lslO
x =0.5=.
N
5 0
,,,1 . . . . I . . . . I . . ~ 50
100
150
200
Temperature
250
300
(K)
Fig. 2. Magneto-resistance as a function of temperature for x = 0.3 and x = 0.5 for a field of 0.4 T.
below the Curie point. The lowest resistivities were recorded for the x = 0.3 sample ( p = 400 Ix~ cm at 10
K). The compositions which show a reduction in resistivity below their Curie point also exhibit a large negative magnetoresistance. Results as a function of temperature in a fixed field of 0.4 T are shown in Fig. 2. The x = 0.3 film only shows the GMR effect around the Curie temperature but with x = 0.5 film it persists and increases in magnitude with decreasing temperature. The magnetoresistance as a function of field is shown for the two films in Fig. 3. For x = 0.3 the measurement is made at 225 K, where the effect is greatest, and for x = 0.5 it is measured at 4.2 K. In both cases the magnitude of the GMR reaches - 80% at 5 - 6 T. The curve for the x = 0.5 film shows an unusual increase in resistance on reversing the field followed by a rapid unstable reduction before resuming a steady monotonic reduction. The behaviour is attributed to a complex magnetisation process associated with the magnetic hysteresis at 4.2 K in the film. These results all show that the ferromagnetism and the electrical conductivity are intimately related and that the conduction electrons are strongly involved in the magnetic ordering process. 120
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The giant magnetoresistance effects originate from a modification of the magnetic structure on the scale of the mean free path of the charge carriers. For intermediate values of x there are competing ferromagnetic double exchange and antiferromagnetic superexchange interaction in the film. An electron moving through the spin lattice in a narrow conduction band carries with it a local magnetic polarisation, leading to partial localisation and the formation of a magnetic polaron [8]. Around and above the Curie temperature the conduction is by variable range hopping of the magnetic polarons and gives rise to the negative temperature coefficient of resistance. The decrease in resistance below the Curie temperature is due to the tendency towards ferromagnetic long range order which is promoted by the conduction electrons. At low temperature in the x = 0.3 film the electrons are essentially delocalised and the polarons can no longer be identified because all the ions are ordered ferromagnetically. Application of a magnetic field near Tc increases the ferromagnetic polarisation and thus produces a large magnetoresistance effect. The magnetic order in the x = 0.5 film is not fully ferromagnetic at any temperature. The magnetisation is only 0.5/xB/Mn compared to the value of 3.5/xB/Mn expected for collinear ferromagnetic order [1]. An applied field can modify the magnetic structure so the film shows a GMR effect extending from the vicinity of Tc to low temperatures.
4. Conclusions
Thin films of (Lal_xCax)MnO 3 in the ferromagnetic composition range have remarkably large negative magnetoresistance ( = - 8 0 % in 5 T) which is associated with transport by magnetic polarons in an imperfectly ordered ferromagnetic matrix. The Curie temperature of these mixed valence manganites can be altered to be above or below room temperature by appropriate choice of divalent cation, so there may be scope for applying the effect in thin film devices.
I ' ' '
References 100
~
so
a.o 6o
~"
40 20
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i
i
i
I
l
.4.0
l
,
l
,
.2.0
1,
I
, . I I , . ,
o.o 2.0 Field (T)
I
4.0
11
6.0
Fig. 3. Magneto-resistance as a function of field for the two compositions x = 0.3 (225 K) and x = 0.5 (4.2 K).
[1] G.H. Jonker and J.H. van Santen, Physica 16 (1950) 337. [2] C. Zener, Phys. Rev. 81 (1951) 403. [3] P.G. de Gennes and J. Friedel, J. Phys. Chem. Sol. 4 (1958) 71. [4] R. yon Helmholt, J. Wecker, B. Hopzapfei, L. Schultz and K. Samwer, Phys. Rev. Lett. 71 (1993) 2331. [5] M. Kasai, Y. Kanke, T. Ohno and Y. Kozono, J. Appl. Phys. 72 (1992) 534. [6] J.F. Lawler, Ph.D. thesis, University of Dublin (1994). [7] L.J. van der Pauw, Philips Res. Rep. 13 (1958) 1. [8] G. Matsumoto, J. Phys. Soc. Jpn. 29 (1970) 606.
Journal of magnetism and magnetic materials
ELSEVIER
Magnetoresistance in ( Lal_x Ca x) MnO3 films J.F. Lawler *, J.M.D. Coey Physics Department, Trinity College, Dublin 2, Ireland Abstract
Thin films of (La I _xCax)MnO3 with 0 < x < 1 have been prepared by pulsed laser deposition. Films with x = 0.3 and 0.5 which are in the composition range for ferromagnetism show an increased resistance and a giant magnetoresistance effect around the Curie temperature. In the case of x = 0.3, the resistance falls from 50 m l ) cm at Tc to 400 i.tl) cm as T ~ 0 K. There is also a large magnetoresistance effect around the Curie point ( A p / p o = 80% in 5 T), which persists for x = 0.5 to low temperatures. These results are interpreted in terms of the magnetic polaron transport in the imperfectly ordered ferromagnetic material.
1. Introduction
The mixed valence perovskite manganites (La3+_x M~ +)MnO 3 (M = Ca, Ba, Sr) were first studied by Jonker and van Santen in 1950 [1]. They exhibit ferromagnetic ordering for compositions around x = 0.3. The ferromagnetic behaviour was explained by Zener [2] with his theory of double exchange in 1951. Resistivity measurements on these materials showed decreases below the Curie temperature and they also exhibit a large negative magnetoresistance effect in this region, which was explained in terms of spin disorder scattering by de Gennes and Friedel [3]. There are recent reports of tunnelling-like behaviour when manganite films are used as barriers in sandwich heterostructures with cuprate superconductors [5,6] and the transport properties have been re-examined in terms of modern ideas on giant magnetoresistance [4]. Here we report on the resistive and magnetoresistive properties of thin films of (La3+_xMn2x+)MnO 3 produced by pulsed laser deposition.
cycle helium refrigerator. Fields of 0.4 T could be applied using an electromagnet. Higher field measurements (6 T) were made in a superconducting solenoid. X-ray diffraction showed the films to be single phase, with highly oriented growth on the substrate [6]. 3. Results and discussion
The resistivity of the films for the range of compositions are shown in Fig. 1. Films at either end of the composition range (x = 0.0, 0.1, 0.7 and 1.0) show semiconductor-like behaviour with decreasing temperature. For these compositions the material is antiferromagnetic and conduction is by variable range hopping. The initial activation energies for the x = 0.0 and 1.0 films are E a = 0.13 and 0.10 eV respectively. The two intermediate values (x = 0.3 and 0.5) show an initial increase in resistivity followed by a drop to almost metallic-like conductivity at lower temperatures. The improved conductivity is associated with the ferromagnetic ordering in the compound
2. Experimental 4
The films were deposited using a KrF (248 nm) excimer laser. The films were formed in-situ from sintered ceramic targets with compositions x = 0.0, 0.1, 0.3, 0.5, 0.7 and 1.0. The substrates were single crystal MgO (100), deposition took place at 720°C with a target substrate distance of 4.5 cm and a background oxygen pressure of 20 Pa. Films were typically 200 nm thick; the resistivity was measured by the van der Pauw [7] method in a closed
3
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.
,
I
•
'
'
L
I
'
'
"
I
'
'
•
I
'
'
'
x=O.O =
_
1 "~.
0
.~
-3
F~"
.,"-
x =o.s x=0.3
-
-4
*
3
•
I
5
,
,
,
I
7
*
,
,
i
.
9
•
,
i
I1
,
*
,
13
1000/T (K) * Corresponding author. Fax: +353-1-6711759.
Fig. 1. Resistivity of thin films of the (La I xCax)MnO3 system as a function of temperature.
0304-8853/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved
SSDI 0 3 0 4 - 8 8 5 3 ( 9 4 ) 0 1 2 1 5 - 6
J.F. Lawler, J.M.D. Coey/Journal of Magnetism and Magnetic Materials 140-144 (1995) 2049-2050
2050
3o
.
.
.
x 0 3 .
25
~-
2o
~.a.°
lslO
x =0.5=.
N
5 0
,,,1 . . . . I . . . . I . . ~ 50
100
150
200
Temperature
250
300
(K)
Fig. 2. Magneto-resistance as a function of temperature for x = 0.3 and x = 0.5 for a field of 0.4 T.
below the Curie point. The lowest resistivities were recorded for the x = 0.3 sample ( p = 400 Ix~ cm at 10
K). The compositions which show a reduction in resistivity below their Curie point also exhibit a large negative magnetoresistance. Results as a function of temperature in a fixed field of 0.4 T are shown in Fig. 2. The x = 0.3 film only shows the GMR effect around the Curie temperature but with x = 0.5 film it persists and increases in magnitude with decreasing temperature. The magnetoresistance as a function of field is shown for the two films in Fig. 3. For x = 0.3 the measurement is made at 225 K, where the effect is greatest, and for x = 0.5 it is measured at 4.2 K. In both cases the magnitude of the GMR reaches - 80% at 5 - 6 T. The curve for the x = 0.5 film shows an unusual increase in resistance on reversing the field followed by a rapid unstable reduction before resuming a steady monotonic reduction. The behaviour is attributed to a complex magnetisation process associated with the magnetic hysteresis at 4.2 K in the film. These results all show that the ferromagnetism and the electrical conductivity are intimately related and that the conduction electrons are strongly involved in the magnetic ordering process. 120
•
,
u
,
'
'
l
'
'
'
I ' ' '
I ' ' '
The giant magnetoresistance effects originate from a modification of the magnetic structure on the scale of the mean free path of the charge carriers. For intermediate values of x there are competing ferromagnetic double exchange and antiferromagnetic superexchange interaction in the film. An electron moving through the spin lattice in a narrow conduction band carries with it a local magnetic polarisation, leading to partial localisation and the formation of a magnetic polaron [8]. Around and above the Curie temperature the conduction is by variable range hopping of the magnetic polarons and gives rise to the negative temperature coefficient of resistance. The decrease in resistance below the Curie temperature is due to the tendency towards ferromagnetic long range order which is promoted by the conduction electrons. At low temperature in the x = 0.3 film the electrons are essentially delocalised and the polarons can no longer be identified because all the ions are ordered ferromagnetically. Application of a magnetic field near Tc increases the ferromagnetic polarisation and thus produces a large magnetoresistance effect. The magnetic order in the x = 0.5 film is not fully ferromagnetic at any temperature. The magnetisation is only 0.5/xB/Mn compared to the value of 3.5/xB/Mn expected for collinear ferromagnetic order [1]. An applied field can modify the magnetic structure so the film shows a GMR effect extending from the vicinity of Tc to low temperatures.
4. Conclusions
Thin films of (Lal_xCax)MnO 3 in the ferromagnetic composition range have remarkably large negative magnetoresistance ( = - 8 0 % in 5 T) which is associated with transport by magnetic polarons in an imperfectly ordered ferromagnetic matrix. The Curie temperature of these mixed valence manganites can be altered to be above or below room temperature by appropriate choice of divalent cation, so there may be scope for applying the effect in thin film devices.
I ' ' '
References 100
~
so
a.o 6o
~"
40 20
0 -6.o
i
i
i
I
l
.4.0
l
,
l
,
.2.0
1,
I
, . I I , . ,
o.o 2.0 Field (T)
I
4.0
11
6.0
Fig. 3. Magneto-resistance as a function of field for the two compositions x = 0.3 (225 K) and x = 0.5 (4.2 K).
[1] G.H. Jonker and J.H. van Santen, Physica 16 (1950) 337. [2] C. Zener, Phys. Rev. 81 (1951) 403. [3] P.G. de Gennes and J. Friedel, J. Phys. Chem. Sol. 4 (1958) 71. [4] R. yon Helmholt, J. Wecker, B. Hopzapfei, L. Schultz and K. Samwer, Phys. Rev. Lett. 71 (1993) 2331. [5] M. Kasai, Y. Kanke, T. Ohno and Y. Kozono, J. Appl. Phys. 72 (1992) 534. [6] J.F. Lawler, Ph.D. thesis, University of Dublin (1994). [7] L.J. van der Pauw, Philips Res. Rep. 13 (1958) 1. [8] G. Matsumoto, J. Phys. Soc. Jpn. 29 (1970) 606.