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Observation of low field magnetoresistance in the layered manganite Sr1.6Sm1.4Mn2O7
PERGAMON
Solid State Communications 112 (1999) 61–65 www.elsevier.com/locate/ssc
Observation of low field magnetoresistance in the layered manganite Sr1.6Sm1.4Mn2O7 N.H. Hur a,*, E.O. Chi b, Y.U. Kwon b, J. Yu c, J.-T. Kim d, Y.K. Park d, J.C. Park d a
Center for CMR Materials, Korea Research Institute of Standards and Science, Yusong, P.O. Box 102, Taejon 305-600, South Korea b Department of Chemistry, Sungkyunkwan University, Suwon 440-746, South Korea c Department of Physics, Sogang University, Seoul 121-742, South Korea d Korea Research Institute of Standards and Science, Yusong, P.O. Box 102, Taejon 305-600, South Korea Received 19 May 1999; accepted 27 June 1999 by H. Akai
Abstract The structural, transport, and magnetic properties of the naturally layered manganese oxide, Sr1.6Sm1.4Mn2O7 (Sm-327), which consists of two [MnO2] bi-layers intertwined with a rock-salt type layer of [(Sr,Sm)2O2] have been investigated. Both a sharp metal–insulator (M–I) transition and an antiferromagnetic transition were found to occur simultaneously at about 118 K. Even in the absence of ferromagnetic ordering the resistance is suppressed remarkably upon applying a magnetic field. These results suggest that the M–I transition and colossal magnetoresistance behavior observed in Sm-327 are not driven mainly by the double exchange mechanism but ascribed rather to the canted antiferromagnetic spin configurations of [MnO2] bi-layers, which eventually leads to the field-induced ferromagnetic ordering. q 1999 Elsevier Science Ltd. All rights reserved. Keywords: A. Magnetically ordered materials
Ternary manganese oxides have drawn considerable attention recently because of large changes in electrical resistance upon applying a magnetic field, which is known as the colossal magnetoresistance (CMR) effect [1–5]. The CMR phenomena in these systems are generally understood in terms of the double exchange mechanism which involves 3 1 the ferromagnetic coupling between Mn 31(t2g eg ) and 3 Mn 41(t2g ) spins [6–8]. However, one of the important unresolved issues is whether ferromagnetism based on the double exchange interaction is a prerequisite for exhibiting CMR accompanied by a metal–insulator (M–I) transition. Many studies revealed that other interactions such as charge–lattice and spin–lattice should be considered to elucidate the CMR phenomena [9–11]. Here we report unusual magnetic and transport properties of the layered manganese oxide Sr1.6Sm1.4Mn2O7 (Sm-327) which consists of the bi-layers of [MnO2] planes intertwined with a rocksalt type layer [(Sr,Sm)2O2]. The most intriguing properties * Corresponding author. Tel.: 1 82-42-868-5233; fax: 1 82-42868-5475. E-mail address: [email protected] (N.H. Hur)
of Sm-327 is the simultaneous occurrence of the sharp M–I transition and the antiferromagnetic transition near 118 K, which provides clear evidence for the importance of antiferromagnetic interaction in the CMR materials. Moreover, Sm-327 shows a generic spin-valve type tunneling MR in the absence of a fully ordered ferromagnetic state. These results are in contrast to the characteristics of other perovskite-type manganites, implying that the mechanism associated with the M–I transition and CMR behavior of Sm-327 is somewhat different from that related to the double exchange interaction. Polycrystalline samples of Sr1.6Sm1.4Mn2O7 were prepared from stoichiometric mixtures of SrCO3, Mn2O3, and Sm2O3. The samples were reacted at about 14508C in air for 80 h with several intermediate grindings until pure Sm-327 was obtained. Phase purity was established using a Rigaku RAD X-ray powder diffractometer equipped with Cu Ka radiation. Rietveld refinement was performed with the program GSAS [12]. The magnetic moment was measured by a SQUID magnetometer (Quantum Design). The measurements were taken in both zero-field-cooled and field-cooled conditions. Magnetoresistance was also
0038-1098/99/$ - see front matter q 1999 Elsevier Science Ltd. All rights reserved. PII: S0038-109 8(99)00315-4
62
N.H. Hur et al. / Solid State Communications 112 (1999) 61–65
Fig. 1. X-ray powder diffraction patterns of Sr1.6Sm1.4Mn2O7.
measured with a standard four probe method, applying magnetic fields of up to 5 T. Successful preparation of a single phase of Sm-327 was achieved by carefully controlling the reaction temperature and cooling rate. Fig. 1 displays the X-ray powder diffraction patterns of Sm-327. All the peaks are well matched with the expected values for the Ruddlesden–Popper phase (Sr,Ln)3Mn2O7 (Ln: lanthanide) [13–16]. The Rietveld refinement using this simple model yielded a reasonable value of agreement between the observed and calculated diffraction profiles. The detailed structural data are summarized in Table 1 and a schematic structure of Sm-327 projected along [010] is shown in Fig. 2. A noticeable feature in the refined data is that about 90% of the smaller Sm 31 ions predominantly go into the rock-salt position while most of the larger Sr 21 ions are located within the perovskite blocks. This trend in the partial ordering of the ions is consistent with previous results on
Sr22x Ln11x Mn2 O7 in which smaller lanthanide ions also prefer to occupy the nine-coordinated rock-salt sites [17]. Another interesting feature is the large distortion of the MnO6 octahedra. The four-equatorial Mn–O(3) bond length ˚ , whereas two apical bonds are elonis equal to 1.9111(3) A ˚ that correspond to Mn– gated to 1.962(3) and 2.140(10) A O(1) and Mn–O(2), respectively. The Jahn–Teller distortion (D) coordinate which can be denoted as D
Table 1 Structural parameters of Sr1.6Sm1.4Mn2O7 determined from Rietveld analysis of X-ray powder diffraction data at ambient temperature Rwp (space group: I4/mmm, a 3:8197 1 c 20:2027 4 A; 9:81%; Rp 7:32%; x2 3:02). Sr/Sm(1) and Sr/Sm(2) correspond to the atoms of the perovskite block and the rock-salt layer in Fig. 2, respectively Atom
x
y
z
˚ 2) B (A
Occup. (%)
Sr/Sm(1) Sr/Sm(2) Mn O(1) O(2) O(3)
0 0 0 0 0 0
0 0 0 0 0 0.5
0.5 0.3168(1) 0.0971(1) 0 0.2030(7) 0.1005(4)
0.048(1) 0.045(1) 0.042(1) 0.055(5) 0.101(5) 0.059(2)
88/12 35/65 100 100 100 100
Fig. 2. The schematic structure of Sr1.6Sm1.4Mn2O7 projected along [010]. The MnO6 octahedra are connected with a solid line. Solid squares represent Mn atoms. The open and dotted circles represent (Sr,Sm) atoms at the perovskite and rock-salt sites, respectively.
N.H. Hur et al. / Solid State Communications 112 (1999) 61–65
63
Fig. 3. The field-cooled (empty mark) and zero-field-cooled (filled mark) magnetization as a function of temperature for Sr1.6Sm1.4Mn2O7 measured in fields of 0.05 T (top panel). Temperature dependence of the resistivity of Sr1.6Sm1.4Mn2O7 for various magnetic fields (bottom panel).
Fig. 4. Field dependence of the normalized resistance for Sr1.6Sm1.4Mn2O7 at various temperatures.
d Mn–Oapical =d Mn–Oequatorial is about 1.07, where d(Mn– Oapical) has taken the average distance of two apical bonds [18]. This value is quite large compared with those of other CMR oxides which are less than 1.03 [17,19]. Such a large distortion of the MnO6 octahedra apparently affects ferromagnetic ordering and charge delocalization which are very sensitive to lattice change, which allows antiferromagnetic ordering and the accompanying canted state in low temperature regime. The temperature dependence of resistivity under various magnetic fields for Sm-327 is shown in the bottom panel of Fig. 3. Its zero-field resistivity displays a sharp maximum at 118 K. Below and above this temperature it exhibits a metallic and semiconducting behavior, respectively. Dramatic reductions in resistivity are observed upon application of magnetic fields. The MR ratio defined as r 0 2 r H=r H is about 2.368% at 118 K and 3 T, which is higher than those of other isostructural analogs Sr1.6Ln1.4Mn2O7 [20,21]. A striking feature is that the resistance is remarkably suppressed by a magnetic field even in the absence of ferromagnetic ordering. Presumably, the magnetic Sm 31 ions in the intervened layer [(Sr,Sm)2O2] mediate the tunneling of spin polarized electrons between the [MnO2] bi-layers rather than disrupt. The temperature dependence of magnetization for Sm327 shown in the top panel of Fig. 3 exhibits complicated features. Above 118 K it displays a typical paramagnetic
64
N.H. Hur et al. / Solid State Communications 112 (1999) 61–65
behavior, where the magnetic data in the range of 120 , T , 300 are well fitted to a Curie–Weiss law with the Curie temperature of 214.7 K. The transition at 118 K corresponds to the antiferromagnetic ordering of the ferromagnetically ordered [MnO2] bi-layer blocks which is accompanied by a weak ferromagnetic component. This eventually leads to the canting of the antiferromagnetic spin moments. This spin canting may thus arise from the competition between ferromagnetic double exchange and antiferromagnetic superexchange [22,23]. Here, the magnetic Sm 31 ions in the rock-salt layer, presumably coupled to the Mn spins of the [MnO2] bi-layers through the sf exchange interactions, are considered to play a significant role to induce the canted antiferromagnetic moment. The magnetic hysteresis observed below 118 K also suggests that the canted spin state is present in the low temperature metallic regime. The CMR behavior of Sm327 is thus not directly driven by the double exchange ferromagnetic ordering but rather associated with the presence of antiferromagnetism. Fig. 4 shows the field dependence of the normalized resistivity, r H=r 0; at various temperatures for Sm-327. A sharp drop of r H=r 0 reaches its maximum at 108 K which is close to the onset of antiferromagnetic transition. The different trends of the MR curves are observed in the change of both temperature and magnetic field. At the temperatures below 108 K, the r H=r 0 curves display a sharp linear decrease as the magnetic field is increased but their decreasing rate gets slower as the temperature is lowered. This behavior can be understood in terms of tunneling of spin polarized electrons between the antiferromagnetically ordered [MnO2] bi-layers [24]. When an external field is applied to the spins, presumably, along the direction of the canted spins, the parallel spin components in the adjacent bi-layers are developed proportional to the applied field. This leads to the increase of conductivity. In contrast to the sharp linear drop of the normalized resistivity below 108 K, the field dependence above this temperature is not so significant for the low fields, which is largely due to the absence of the antiferromagnetic ordering between the bi-layers in this regime. Instead, short range magnetic fluctuations within the [MnO2] bi-layers are considered to be a dominant factor for the MR above 108 K. The implication of the normalized resistivity data at various temperatures is that large negative MR of Sm-327 below 108 K is mainly ascribed to spin polarized tunneling while the MR above the temperature is largely associated with the field-induced suppression of spin fluctuations. The tunneling behavior of Sm-327, as proposed here, is quite analogous to that found in artificially layered films of the spin valve type. The spin valve is composed of two ferromagnetic layers which are intervened by a nonmagnetic conducting spacer [25]. Although there are some discrepancies between two systems, the rock salt type [(Sr,Sm)2O2] unit and two [MnO2] bi-layer blocks in the structure of Sm-327 can be viewed as a spacer and two
ferromagnetic layers in the multi-layer film, respectively, as illustrated at the right-hand side of Fig. 2. Since 90% of the Sm atoms occupy the rock-salt site, it is reasonable to consider that the spins of a spacing layer [(Sr,Sm)2O2] and two [MnO2] bi-layers are mostly involved in the magnetic interaction. Here, the spins of the [MnO2] bi-layers are antiferromagnetically coupled but slightly canted out of the layers by a small angle due to lattice distortion, which are susceptible to align in one direction by a small external field. It is noteworthy to mention that the intervening Sm atom at the rock-salt layer is crucial for exhibiting CMR found in Sm-327 although the exact role is not clear at this stage due to the difficulty to obtain the neutron diffraction data of the Sm-containing sample. An important feature we address is that the Sm atom turns out to be a magnetically mediating and field-sensitive element despite of its own paramagnetic characteristic. In summary, we have found the M–I transition and low field MR in the layered manganite Sm-327 in the absence of ferromagnetic ordering. The MR observed in Sm-327 is primarily due to the interlayer tunneling of spin polarized electrons. The tunneling process appears to be accelerated by the presence of the antiferromagnetically layered stacking of the [MnO2] bi-layers, which can easily switch to ferromagnetic configuration even in low field, implying that antiferromagneism plays a key role in driving the M–I transition and accompanying low field MR in Sm-327. This result also provides new insight into the tunnel junction device where a spacer is mostly composed of non-magnetic elements thus far, opening up a possibility to make new multi-layer films that contains a magnetic spacing layer. Acknowledgements We thank B.J. Suh and K.S. Moon for helpful discussions and are grateful to the Ministry of Science and Technology and KOSEF for financial support. This work was also supported, in part, by the Creative Research Initiative Program.
References [1] P.M. Levy, Science 256 (1992) 972. [2] R. von Helmolt, J. Wecher, B. Holzapfel, L. Schultz, K. Samwer, Phys. Rev. Lett. 71 (1993) 2331. [3] Y. Tokura, A. Urushibara, Y. Moritomo, T. Arima, A. Asamitsu, G. Kido, N. Furukawa, J. Phys. Soc. Jpn 63 (1994) 3931. [4] S. Jin, T.H. Trefel, M. McCormack, R.A. Fastnacht, R. Ramesh, L.H. Chen, Science 264 (1994) 413. [5] C.N.R. Rao, A.K. Cheetham, Science 272 (1996) 369. [6] C. Zener, Phys. Rev. 82 (1951) 403. [7] J.B. Goodenough, Phys. Rev. 100 (1955) 564. [8] P.W. Anderson, H. Hasegawa, Phys. Rev. 100 (1955) 675.
N.H. Hur et al. / Solid State Communications 112 (1999) 61–65 [9] A.J. Millis, P.B. Littlewood, B.I. Shraiman, Phys. Rev. Lett. 74 (1995) 5144. [10] J.M. De Teresa, M.R. Ibara, P.A. Algarabel, C. Ritter, C. Marquina, J. Blasco, J. Garcia, A. del Moral, Z. Arnold, Nature 386 (1997) 256. [11] K.H. Kim, J.Y. Gu, H.S. Choi, G.W. Park, T.W. Noh, Phys. Rev. Lett. 77 (1996) 1877. [12] A.C. Larson, R.B. von Dreele, General Structural Analysis Systems, Los Alamos National Laboratory, 1994. [13] H. Asano, J. Hayakawa, M. Matsui, Appl. Phys. Lett. 68 (1996) 3638. [14] Y. Morotomo, A. Asamitsu, H. Kuwahara, Y. Tokura, Nature 380 (1996) 141. [15] D.N. Argyriou, J.F. Mitchell, J.E. Goodeneough, O. Chmaissem, S. Short, J.D. Jorgensen, Phys. Rev. Lett. 78 (1997) 1568. [16] D. Louca, G.H. Kwei, J.F. Mitchell, Phys. Rev. Lett. 80 (1998) 3811. [17] P.D. Battle, M.A. Green, N.S. Laskey, J.E. Millburn, L.
[18] [19] [20] [21] [22] [23]
[24] [25]
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Murphy, M.J. Rosseinsky, S.P. Sullivan, J.F. Vente, Chem. Mater. 9 (1997) 552. V. Caignart, E. Suard, A. Maignan, Ch. Simon, B. Raveau, J. Magn. Magn. Mater. 153 (1996) L260. H.Y. Hwang, S.W. Cheong, P.G. Radeli, M. Marezio, B. Batlogg, Phys. Rev. Lett. 75 (1995) 914. N.H. Hur, J.-T. Kim, K.H. Yoo, Y.K. Park, J.C. Park, E.O. Chi, Y.U. Kwon, Phys. Rev. B 57 (1998) 10740. T. Kimura, Y. Tomika, H. Kuwahara, A. Asamitsu, M. Tamura, Y. Tokura, Science 274 (1996) 1698. D.I. Golosov, M.R. Norman, K. Levin, K. J, Appl. Phys. 83 (1998) 7360. S. Rosenkrantz, R. Osborn, J.F. Mitchell, L. Vasiliu-Doloc, J.W. Lynn, S.K. Sinha, D.N. Argyriou, J. Appl. Phys. 83 (1998) 7348. T.G. Perring, G. Aeppli, T. Kimura, Y. Tokura, M.A. Adams, Phys. Rev. B 58 (1998) R14693. S. Gider, B.-U. Runge, A.C. Marley, S.S.P. Parkin, Science 281 (1998) 797.
Solid State Communications 112 (1999) 61–65 www.elsevier.com/locate/ssc
Observation of low field magnetoresistance in the layered manganite Sr1.6Sm1.4Mn2O7 N.H. Hur a,*, E.O. Chi b, Y.U. Kwon b, J. Yu c, J.-T. Kim d, Y.K. Park d, J.C. Park d a
Center for CMR Materials, Korea Research Institute of Standards and Science, Yusong, P.O. Box 102, Taejon 305-600, South Korea b Department of Chemistry, Sungkyunkwan University, Suwon 440-746, South Korea c Department of Physics, Sogang University, Seoul 121-742, South Korea d Korea Research Institute of Standards and Science, Yusong, P.O. Box 102, Taejon 305-600, South Korea Received 19 May 1999; accepted 27 June 1999 by H. Akai
Abstract The structural, transport, and magnetic properties of the naturally layered manganese oxide, Sr1.6Sm1.4Mn2O7 (Sm-327), which consists of two [MnO2] bi-layers intertwined with a rock-salt type layer of [(Sr,Sm)2O2] have been investigated. Both a sharp metal–insulator (M–I) transition and an antiferromagnetic transition were found to occur simultaneously at about 118 K. Even in the absence of ferromagnetic ordering the resistance is suppressed remarkably upon applying a magnetic field. These results suggest that the M–I transition and colossal magnetoresistance behavior observed in Sm-327 are not driven mainly by the double exchange mechanism but ascribed rather to the canted antiferromagnetic spin configurations of [MnO2] bi-layers, which eventually leads to the field-induced ferromagnetic ordering. q 1999 Elsevier Science Ltd. All rights reserved. Keywords: A. Magnetically ordered materials
Ternary manganese oxides have drawn considerable attention recently because of large changes in electrical resistance upon applying a magnetic field, which is known as the colossal magnetoresistance (CMR) effect [1–5]. The CMR phenomena in these systems are generally understood in terms of the double exchange mechanism which involves 3 1 the ferromagnetic coupling between Mn 31(t2g eg ) and 3 Mn 41(t2g ) spins [6–8]. However, one of the important unresolved issues is whether ferromagnetism based on the double exchange interaction is a prerequisite for exhibiting CMR accompanied by a metal–insulator (M–I) transition. Many studies revealed that other interactions such as charge–lattice and spin–lattice should be considered to elucidate the CMR phenomena [9–11]. Here we report unusual magnetic and transport properties of the layered manganese oxide Sr1.6Sm1.4Mn2O7 (Sm-327) which consists of the bi-layers of [MnO2] planes intertwined with a rocksalt type layer [(Sr,Sm)2O2]. The most intriguing properties * Corresponding author. Tel.: 1 82-42-868-5233; fax: 1 82-42868-5475. E-mail address: [email protected] (N.H. Hur)
of Sm-327 is the simultaneous occurrence of the sharp M–I transition and the antiferromagnetic transition near 118 K, which provides clear evidence for the importance of antiferromagnetic interaction in the CMR materials. Moreover, Sm-327 shows a generic spin-valve type tunneling MR in the absence of a fully ordered ferromagnetic state. These results are in contrast to the characteristics of other perovskite-type manganites, implying that the mechanism associated with the M–I transition and CMR behavior of Sm-327 is somewhat different from that related to the double exchange interaction. Polycrystalline samples of Sr1.6Sm1.4Mn2O7 were prepared from stoichiometric mixtures of SrCO3, Mn2O3, and Sm2O3. The samples were reacted at about 14508C in air for 80 h with several intermediate grindings until pure Sm-327 was obtained. Phase purity was established using a Rigaku RAD X-ray powder diffractometer equipped with Cu Ka radiation. Rietveld refinement was performed with the program GSAS [12]. The magnetic moment was measured by a SQUID magnetometer (Quantum Design). The measurements were taken in both zero-field-cooled and field-cooled conditions. Magnetoresistance was also
0038-1098/99/$ - see front matter q 1999 Elsevier Science Ltd. All rights reserved. PII: S0038-109 8(99)00315-4
62
N.H. Hur et al. / Solid State Communications 112 (1999) 61–65
Fig. 1. X-ray powder diffraction patterns of Sr1.6Sm1.4Mn2O7.
measured with a standard four probe method, applying magnetic fields of up to 5 T. Successful preparation of a single phase of Sm-327 was achieved by carefully controlling the reaction temperature and cooling rate. Fig. 1 displays the X-ray powder diffraction patterns of Sm-327. All the peaks are well matched with the expected values for the Ruddlesden–Popper phase (Sr,Ln)3Mn2O7 (Ln: lanthanide) [13–16]. The Rietveld refinement using this simple model yielded a reasonable value of agreement between the observed and calculated diffraction profiles. The detailed structural data are summarized in Table 1 and a schematic structure of Sm-327 projected along [010] is shown in Fig. 2. A noticeable feature in the refined data is that about 90% of the smaller Sm 31 ions predominantly go into the rock-salt position while most of the larger Sr 21 ions are located within the perovskite blocks. This trend in the partial ordering of the ions is consistent with previous results on
Sr22x Ln11x Mn2 O7 in which smaller lanthanide ions also prefer to occupy the nine-coordinated rock-salt sites [17]. Another interesting feature is the large distortion of the MnO6 octahedra. The four-equatorial Mn–O(3) bond length ˚ , whereas two apical bonds are elonis equal to 1.9111(3) A ˚ that correspond to Mn– gated to 1.962(3) and 2.140(10) A O(1) and Mn–O(2), respectively. The Jahn–Teller distortion (D) coordinate which can be denoted as D
Table 1 Structural parameters of Sr1.6Sm1.4Mn2O7 determined from Rietveld analysis of X-ray powder diffraction data at ambient temperature Rwp (space group: I4/mmm, a 3:8197 1 c 20:2027 4 A; 9:81%; Rp 7:32%; x2 3:02). Sr/Sm(1) and Sr/Sm(2) correspond to the atoms of the perovskite block and the rock-salt layer in Fig. 2, respectively Atom
x
y
z
˚ 2) B (A
Occup. (%)
Sr/Sm(1) Sr/Sm(2) Mn O(1) O(2) O(3)
0 0 0 0 0 0
0 0 0 0 0 0.5
0.5 0.3168(1) 0.0971(1) 0 0.2030(7) 0.1005(4)
0.048(1) 0.045(1) 0.042(1) 0.055(5) 0.101(5) 0.059(2)
88/12 35/65 100 100 100 100
Fig. 2. The schematic structure of Sr1.6Sm1.4Mn2O7 projected along [010]. The MnO6 octahedra are connected with a solid line. Solid squares represent Mn atoms. The open and dotted circles represent (Sr,Sm) atoms at the perovskite and rock-salt sites, respectively.
N.H. Hur et al. / Solid State Communications 112 (1999) 61–65
63
Fig. 3. The field-cooled (empty mark) and zero-field-cooled (filled mark) magnetization as a function of temperature for Sr1.6Sm1.4Mn2O7 measured in fields of 0.05 T (top panel). Temperature dependence of the resistivity of Sr1.6Sm1.4Mn2O7 for various magnetic fields (bottom panel).
Fig. 4. Field dependence of the normalized resistance for Sr1.6Sm1.4Mn2O7 at various temperatures.
d Mn–Oapical =d Mn–Oequatorial is about 1.07, where d(Mn– Oapical) has taken the average distance of two apical bonds [18]. This value is quite large compared with those of other CMR oxides which are less than 1.03 [17,19]. Such a large distortion of the MnO6 octahedra apparently affects ferromagnetic ordering and charge delocalization which are very sensitive to lattice change, which allows antiferromagnetic ordering and the accompanying canted state in low temperature regime. The temperature dependence of resistivity under various magnetic fields for Sm-327 is shown in the bottom panel of Fig. 3. Its zero-field resistivity displays a sharp maximum at 118 K. Below and above this temperature it exhibits a metallic and semiconducting behavior, respectively. Dramatic reductions in resistivity are observed upon application of magnetic fields. The MR ratio defined as r 0 2 r H=r H is about 2.368% at 118 K and 3 T, which is higher than those of other isostructural analogs Sr1.6Ln1.4Mn2O7 [20,21]. A striking feature is that the resistance is remarkably suppressed by a magnetic field even in the absence of ferromagnetic ordering. Presumably, the magnetic Sm 31 ions in the intervened layer [(Sr,Sm)2O2] mediate the tunneling of spin polarized electrons between the [MnO2] bi-layers rather than disrupt. The temperature dependence of magnetization for Sm327 shown in the top panel of Fig. 3 exhibits complicated features. Above 118 K it displays a typical paramagnetic
64
N.H. Hur et al. / Solid State Communications 112 (1999) 61–65
behavior, where the magnetic data in the range of 120 , T , 300 are well fitted to a Curie–Weiss law with the Curie temperature of 214.7 K. The transition at 118 K corresponds to the antiferromagnetic ordering of the ferromagnetically ordered [MnO2] bi-layer blocks which is accompanied by a weak ferromagnetic component. This eventually leads to the canting of the antiferromagnetic spin moments. This spin canting may thus arise from the competition between ferromagnetic double exchange and antiferromagnetic superexchange [22,23]. Here, the magnetic Sm 31 ions in the rock-salt layer, presumably coupled to the Mn spins of the [MnO2] bi-layers through the sf exchange interactions, are considered to play a significant role to induce the canted antiferromagnetic moment. The magnetic hysteresis observed below 118 K also suggests that the canted spin state is present in the low temperature metallic regime. The CMR behavior of Sm327 is thus not directly driven by the double exchange ferromagnetic ordering but rather associated with the presence of antiferromagnetism. Fig. 4 shows the field dependence of the normalized resistivity, r H=r 0; at various temperatures for Sm-327. A sharp drop of r H=r 0 reaches its maximum at 108 K which is close to the onset of antiferromagnetic transition. The different trends of the MR curves are observed in the change of both temperature and magnetic field. At the temperatures below 108 K, the r H=r 0 curves display a sharp linear decrease as the magnetic field is increased but their decreasing rate gets slower as the temperature is lowered. This behavior can be understood in terms of tunneling of spin polarized electrons between the antiferromagnetically ordered [MnO2] bi-layers [24]. When an external field is applied to the spins, presumably, along the direction of the canted spins, the parallel spin components in the adjacent bi-layers are developed proportional to the applied field. This leads to the increase of conductivity. In contrast to the sharp linear drop of the normalized resistivity below 108 K, the field dependence above this temperature is not so significant for the low fields, which is largely due to the absence of the antiferromagnetic ordering between the bi-layers in this regime. Instead, short range magnetic fluctuations within the [MnO2] bi-layers are considered to be a dominant factor for the MR above 108 K. The implication of the normalized resistivity data at various temperatures is that large negative MR of Sm-327 below 108 K is mainly ascribed to spin polarized tunneling while the MR above the temperature is largely associated with the field-induced suppression of spin fluctuations. The tunneling behavior of Sm-327, as proposed here, is quite analogous to that found in artificially layered films of the spin valve type. The spin valve is composed of two ferromagnetic layers which are intervened by a nonmagnetic conducting spacer [25]. Although there are some discrepancies between two systems, the rock salt type [(Sr,Sm)2O2] unit and two [MnO2] bi-layer blocks in the structure of Sm-327 can be viewed as a spacer and two
ferromagnetic layers in the multi-layer film, respectively, as illustrated at the right-hand side of Fig. 2. Since 90% of the Sm atoms occupy the rock-salt site, it is reasonable to consider that the spins of a spacing layer [(Sr,Sm)2O2] and two [MnO2] bi-layers are mostly involved in the magnetic interaction. Here, the spins of the [MnO2] bi-layers are antiferromagnetically coupled but slightly canted out of the layers by a small angle due to lattice distortion, which are susceptible to align in one direction by a small external field. It is noteworthy to mention that the intervening Sm atom at the rock-salt layer is crucial for exhibiting CMR found in Sm-327 although the exact role is not clear at this stage due to the difficulty to obtain the neutron diffraction data of the Sm-containing sample. An important feature we address is that the Sm atom turns out to be a magnetically mediating and field-sensitive element despite of its own paramagnetic characteristic. In summary, we have found the M–I transition and low field MR in the layered manganite Sm-327 in the absence of ferromagnetic ordering. The MR observed in Sm-327 is primarily due to the interlayer tunneling of spin polarized electrons. The tunneling process appears to be accelerated by the presence of the antiferromagnetically layered stacking of the [MnO2] bi-layers, which can easily switch to ferromagnetic configuration even in low field, implying that antiferromagneism plays a key role in driving the M–I transition and accompanying low field MR in Sm-327. This result also provides new insight into the tunnel junction device where a spacer is mostly composed of non-magnetic elements thus far, opening up a possibility to make new multi-layer films that contains a magnetic spacing layer. Acknowledgements We thank B.J. Suh and K.S. Moon for helpful discussions and are grateful to the Ministry of Science and Technology and KOSEF for financial support. This work was also supported, in part, by the Creative Research Initiative Program.
References [1] P.M. Levy, Science 256 (1992) 972. [2] R. von Helmolt, J. Wecher, B. Holzapfel, L. Schultz, K. Samwer, Phys. Rev. Lett. 71 (1993) 2331. [3] Y. Tokura, A. Urushibara, Y. Moritomo, T. Arima, A. Asamitsu, G. Kido, N. Furukawa, J. Phys. Soc. Jpn 63 (1994) 3931. [4] S. Jin, T.H. Trefel, M. McCormack, R.A. Fastnacht, R. Ramesh, L.H. Chen, Science 264 (1994) 413. [5] C.N.R. Rao, A.K. Cheetham, Science 272 (1996) 369. [6] C. Zener, Phys. Rev. 82 (1951) 403. [7] J.B. Goodenough, Phys. Rev. 100 (1955) 564. [8] P.W. Anderson, H. Hasegawa, Phys. Rev. 100 (1955) 675.
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