- Email: [email protected]
Giant magnetoresistance in ceramic perovskites LaLCaMnO (L = Y,Gd)
Journal of Magnetism and Magnetic Materials 157/158 (1996) 260-261
~ 4 journalof ~ 4 magneti and sm magnetic materials
N ELSEVIER
Giant magnetoresistance in ceramic perovskites L a - L - C a - M n O (L = Y,ad) A. Seffar, J. Fontcuberta, B. Martlnez *, J.L. Garcla-Mufioz, S. Pifiol, X. Obradors Institut de Cibncia de Materials de Barcelona - CSIC, Bellaterra 08193, Spain Abstract
Giant negative magnetoresintance response has been observed in ferromagnetic La 1-x A xMnO3 (A = Ca 2+ ) perovskites sintered ceramic materials near room temperature. We have also observed that the magnetoresistance response of these materials increases, while the temperature at which the maximum of the resistance appears decreases, by substituting some amount of La by Y. Keywords: Giant magnetoresistance; Perovskites; Resistivity
After the discovery of high Tc oxides, physics of strongly correlated electron systems has been revisited. Of especial interest are the effects of strong correlation among free charge carriers and their interaction with the antiferromagnetic background. One of the fascinating groups of materials in which strong correlation effects lie at the heart of its anomalous physical properties are manganese oxides with perovskite-type structure La1_xAxMnO3 (A: Sr 2+, Ca 2+, Ba2+). The giant magnetoresistive (MR) response of some L a - A - M n - O thin films [1,2] has been proved to occur also in bulk samples of this family [3]. Not only the control and understanding of this giant change in resistance is pursued but also new technological applications appear in the near horizon. We present here the data obtained on compounds of fixed doping (Ca 2+ ) concentration: Lao. 7_ x L xCa0.3 MnO 3, where L is a rare-earth trivalent cation (Y, Dy, Yb, Gd). Substitutions ranging from X = 0 to x = 0.20 have been used, allowing us to explore the effect of cationic mean size variations [4]. Ceramic samples were prepared by the solid state reaction of precursor oxides, following the method described in [4]. The as-prepared materials were characterized by X-Ray diffraction and were found to be a single phase with a rhombohedrically distorted (R3c) perovskite structure. Magnetoresistivity measurements have been performed under fields up to 5.5 T by conventional four-probes method with the current parallel to the applied field. Magnetic properties have been measured by using a QD SQUID magnetometer up to 5.5 T.
* Corresponding author. [email protected].
Fax:
÷34-3-580-5729;
email:
Fig. 1 shows the field dependence of the resistivity of Lao.7_xYxCa0.3MnO 3 for x = 0 and 0.15 at temperatures near to the respective maximum of the resistive peak occurring approximately at the ferromagnetic ordering temperature T M. The magnetoresistance A R ( H ) / R = [ R m a x ( H ) - R ( l O K, 5 T)]/R(10 K, 5 T) is temperature dependent and reaches its maximum at temperatures close to TM decreasing above and below this temperature. For temperatures T>> T M the resistivity is in the range of 0.1-0.3 g2 cm and almost insensitive to the rare-earth substitution. The value of A R ( T M ) / R = [R(T M) - R ( 3 0 0 K)]/R(300 K) is about 2 × 104% for L = Y 0 . l s . This magnitude should be compared with a much lower variation in resistivity A R ( T M ) / R = 2 × 102% observed for
100 L=Y A x=O a
x=O.15
10
0.1 -60000 -40000 -20000
0
2 0 0 0 0 40000 60000
H(Oe)
Fig. 1. Field dependence of the resistivity of the samples Lao.7Cao.3MnO3 and Lao.55Yo15Cao.3MnO3 near to the corresponding TM temperatures (see Fig. 2).
0304-8853/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. SSD10304-8853(95)01245-1
A. Seffaret al. / Journal of Magnetism and MagneticMaterials 157/158 (1996) 260-261 x = 0. At temperatures well below T M, a metallic-like behaviour is almost recovered. One of the most remarkable results is illustrated in Fig. 2, where we have depicted the temperature dependence of the normalized resistivity R / R ( T = 300 K) for Lao.7Cao.3MnO3 and Lao.55Yo.15Ca0.3MnO 3. As shown is this figure, the amplitudes of the zero-field resistivity peak and the ferromagnetic ordering temperature TM are closely related. W h e n T M decreases A R(T)/R increases and both are controlled by the mean ionic radii ( r A ) . The amplitude of the maximum depends on the temperature T M and on another parameter, p(TM,O), which is characteristic of the sample. At temperatures above T M and up to room temperature, p(T) can be described by: p(T)=p~ exp(To/T) 1/4, with To increasing monotonously with decreasing ( r A > [5]. The observed temperature dependence of the resistivity is believed to be related to the reduced carrier mobility when approaching the ferromagnetic ordering temperature. Moreover, M(T) and p(T) curves above the peak clearly correlate (see Fig. 3), thus unequivocally signalling the intimate connection between both phenomena. The same behavior is observed for the resistivity measurements under magnetic field. In that case, To is found to decrease with the applied magnetic field. W e recall here that the physics behind the variable range hopping (VRH) is the temperature dependence of the carrier mobility when conduction take place by hopping between localized states. Within this model the To parameter is related to the spatial extension (1) of the localized states: 1 = To 1/3 [5]. Our data show that when reducing ( r A), the localization length and thus the carrier mobility, decreases. The abrupt increase in resistivity when reducing the temperature and while the ferromagnetic order is established appears to be related to the dynamics of charge carriers and their interaction with the surrounding spin bath. According to de Gennes [6], the doping hole can i000
L=y lOO
•
x=0.15
a
x=0
e~ lO ii
261
0.9 0.8 0.7 0.6 0.5 0.4 0.3
, 0
0.2
0.4
0.6
0.8
1
Fig. 3. Normalized reduced resistivity p(T,H)/p o versus reduced magnetization for the sample La0.yCa0.3MnO 3. The full line corresponds to the fitting by using the A(M/Ms) z law.
induce ferromagnetic coupling between neighbouring ionic spins, thus giving rise to some sort of a ferromagnetic polaron. Within this scenario, the localization length extracted from our transport data can be viewed as the size of this ferromagnetic cloud surrounding trapped holes. Notice that the reduction in the polaron size reflects a weakening of the magnetic coupling and thus a lowering of the Curie temperature should be anticipated. The field acts just opposite: when H increases, the field-induced parallel alignment of the ionic spins adds to the intrinsic ferromagnetic interactions mediated by the mobile charge carriers, the localization becomes weaker and thus l is enhanced. W e have found that two main reasons lead to the higher magnetoresistance of samples having smaller ( r A ) : (a) a pronounced increase in resistivity at T = TM when TM is lowered; and (b) their reduced carrier mobility (i.e. higher To values). In Fig. 3 we plot the field dependence of the resistivity of La0.7Ca0.3MnO 3 at various temperatures in terms of the magnetization. The reduced variables P/Po and M / M s are used where M s is the saturation magnetization and P0 = p(T,O) is the zero-field resistivity. The field dependence of the resistivity p(T,H) follows a quadratic -~AMZ(T,H) law in agreement with recent theoretical models for the G M R [7]. References
[1] [2] [3] [4]
0.1 0
50
100
150
200
250
300
T(K)
Fig. 2. Normalized resistivity as a funtion of temperature for the samples Lao.7Cao.3MnO 3 and Lao.ssYo.lsCao.3MnO 3 at zero field.
R. von Helmolt et al., Phys. Rev. Lett. 71 (1993) 2331. Jin et al., Science 264 (1994) 413. R. Mahesh et al., J. Solid State Chem. 114 (1995) 297/ J. Fontcuberta, B. Martfnez, A. Seffar, S. Pifiol, J.L. GarclaMufioz and X. Obradors, Phys. Rev. Lett. 76 (1996) 1122. [5] N.F. Mott and E.A. Davis, in Electronic Processes in Noncrystalline Materials (Clarendon Press, Oxford, 1971). [6] P.G. de Gennes, Phys. Rev. 118 (1960) 141. [7] N. Furukawa, J. Phys. Soc. Jpn. 63 (1994) 3214.
~ 4 journalof ~ 4 magneti and sm magnetic materials
N ELSEVIER
Giant magnetoresistance in ceramic perovskites L a - L - C a - M n O (L = Y,ad) A. Seffar, J. Fontcuberta, B. Martlnez *, J.L. Garcla-Mufioz, S. Pifiol, X. Obradors Institut de Cibncia de Materials de Barcelona - CSIC, Bellaterra 08193, Spain Abstract
Giant negative magnetoresintance response has been observed in ferromagnetic La 1-x A xMnO3 (A = Ca 2+ ) perovskites sintered ceramic materials near room temperature. We have also observed that the magnetoresistance response of these materials increases, while the temperature at which the maximum of the resistance appears decreases, by substituting some amount of La by Y. Keywords: Giant magnetoresistance; Perovskites; Resistivity
After the discovery of high Tc oxides, physics of strongly correlated electron systems has been revisited. Of especial interest are the effects of strong correlation among free charge carriers and their interaction with the antiferromagnetic background. One of the fascinating groups of materials in which strong correlation effects lie at the heart of its anomalous physical properties are manganese oxides with perovskite-type structure La1_xAxMnO3 (A: Sr 2+, Ca 2+, Ba2+). The giant magnetoresistive (MR) response of some L a - A - M n - O thin films [1,2] has been proved to occur also in bulk samples of this family [3]. Not only the control and understanding of this giant change in resistance is pursued but also new technological applications appear in the near horizon. We present here the data obtained on compounds of fixed doping (Ca 2+ ) concentration: Lao. 7_ x L xCa0.3 MnO 3, where L is a rare-earth trivalent cation (Y, Dy, Yb, Gd). Substitutions ranging from X = 0 to x = 0.20 have been used, allowing us to explore the effect of cationic mean size variations [4]. Ceramic samples were prepared by the solid state reaction of precursor oxides, following the method described in [4]. The as-prepared materials were characterized by X-Ray diffraction and were found to be a single phase with a rhombohedrically distorted (R3c) perovskite structure. Magnetoresistivity measurements have been performed under fields up to 5.5 T by conventional four-probes method with the current parallel to the applied field. Magnetic properties have been measured by using a QD SQUID magnetometer up to 5.5 T.
* Corresponding author. [email protected].
Fax:
÷34-3-580-5729;
email:
Fig. 1 shows the field dependence of the resistivity of Lao.7_xYxCa0.3MnO 3 for x = 0 and 0.15 at temperatures near to the respective maximum of the resistive peak occurring approximately at the ferromagnetic ordering temperature T M. The magnetoresistance A R ( H ) / R = [ R m a x ( H ) - R ( l O K, 5 T)]/R(10 K, 5 T) is temperature dependent and reaches its maximum at temperatures close to TM decreasing above and below this temperature. For temperatures T>> T M the resistivity is in the range of 0.1-0.3 g2 cm and almost insensitive to the rare-earth substitution. The value of A R ( T M ) / R = [R(T M) - R ( 3 0 0 K)]/R(300 K) is about 2 × 104% for L = Y 0 . l s . This magnitude should be compared with a much lower variation in resistivity A R ( T M ) / R = 2 × 102% observed for
100 L=Y A x=O a
x=O.15
10
0.1 -60000 -40000 -20000
0
2 0 0 0 0 40000 60000
H(Oe)
Fig. 1. Field dependence of the resistivity of the samples Lao.7Cao.3MnO3 and Lao.55Yo15Cao.3MnO3 near to the corresponding TM temperatures (see Fig. 2).
0304-8853/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. SSD10304-8853(95)01245-1
A. Seffaret al. / Journal of Magnetism and MagneticMaterials 157/158 (1996) 260-261 x = 0. At temperatures well below T M, a metallic-like behaviour is almost recovered. One of the most remarkable results is illustrated in Fig. 2, where we have depicted the temperature dependence of the normalized resistivity R / R ( T = 300 K) for Lao.7Cao.3MnO3 and Lao.55Yo.15Ca0.3MnO 3. As shown is this figure, the amplitudes of the zero-field resistivity peak and the ferromagnetic ordering temperature TM are closely related. W h e n T M decreases A R(T)/R increases and both are controlled by the mean ionic radii ( r A ) . The amplitude of the maximum depends on the temperature T M and on another parameter, p(TM,O), which is characteristic of the sample. At temperatures above T M and up to room temperature, p(T) can be described by: p(T)=p~ exp(To/T) 1/4, with To increasing monotonously with decreasing ( r A > [5]. The observed temperature dependence of the resistivity is believed to be related to the reduced carrier mobility when approaching the ferromagnetic ordering temperature. Moreover, M(T) and p(T) curves above the peak clearly correlate (see Fig. 3), thus unequivocally signalling the intimate connection between both phenomena. The same behavior is observed for the resistivity measurements under magnetic field. In that case, To is found to decrease with the applied magnetic field. W e recall here that the physics behind the variable range hopping (VRH) is the temperature dependence of the carrier mobility when conduction take place by hopping between localized states. Within this model the To parameter is related to the spatial extension (1) of the localized states: 1 = To 1/3 [5]. Our data show that when reducing ( r A), the localization length and thus the carrier mobility, decreases. The abrupt increase in resistivity when reducing the temperature and while the ferromagnetic order is established appears to be related to the dynamics of charge carriers and their interaction with the surrounding spin bath. According to de Gennes [6], the doping hole can i000
L=y lOO
•
x=0.15
a
x=0
e~ lO ii
261
0.9 0.8 0.7 0.6 0.5 0.4 0.3
, 0
0.2
0.4
0.6
0.8
1
Fig. 3. Normalized reduced resistivity p(T,H)/p o versus reduced magnetization for the sample La0.yCa0.3MnO 3. The full line corresponds to the fitting by using the A(M/Ms) z law.
induce ferromagnetic coupling between neighbouring ionic spins, thus giving rise to some sort of a ferromagnetic polaron. Within this scenario, the localization length extracted from our transport data can be viewed as the size of this ferromagnetic cloud surrounding trapped holes. Notice that the reduction in the polaron size reflects a weakening of the magnetic coupling and thus a lowering of the Curie temperature should be anticipated. The field acts just opposite: when H increases, the field-induced parallel alignment of the ionic spins adds to the intrinsic ferromagnetic interactions mediated by the mobile charge carriers, the localization becomes weaker and thus l is enhanced. W e have found that two main reasons lead to the higher magnetoresistance of samples having smaller ( r A ) : (a) a pronounced increase in resistivity at T = TM when TM is lowered; and (b) their reduced carrier mobility (i.e. higher To values). In Fig. 3 we plot the field dependence of the resistivity of La0.7Ca0.3MnO 3 at various temperatures in terms of the magnetization. The reduced variables P/Po and M / M s are used where M s is the saturation magnetization and P0 = p(T,O) is the zero-field resistivity. The field dependence of the resistivity p(T,H) follows a quadratic -~AMZ(T,H) law in agreement with recent theoretical models for the G M R [7]. References
[1] [2] [3] [4]
0.1 0
50
100
150
200
250
300
T(K)
Fig. 2. Normalized resistivity as a funtion of temperature for the samples Lao.7Cao.3MnO 3 and Lao.ssYo.lsCao.3MnO 3 at zero field.
R. von Helmolt et al., Phys. Rev. Lett. 71 (1993) 2331. Jin et al., Science 264 (1994) 413. R. Mahesh et al., J. Solid State Chem. 114 (1995) 297/ J. Fontcuberta, B. Martfnez, A. Seffar, S. Pifiol, J.L. GarclaMufioz and X. Obradors, Phys. Rev. Lett. 76 (1996) 1122. [5] N.F. Mott and E.A. Davis, in Electronic Processes in Noncrystalline Materials (Clarendon Press, Oxford, 1971). [6] P.G. de Gennes, Phys. Rev. 118 (1960) 141. [7] N. Furukawa, J. Phys. Soc. Jpn. 63 (1994) 3214.