Large room-temperature magnetoresistance and phase separation in La 1x Na x MnO 3 with 0.1x0.3 S. L. Ye, W. H. Song, J. M. Dai, K. Y. Wang, S. G. Wang, J. J. Du, Y. P. Sun, J. Fang, J. L. Chen, and B. J. Gao Citation: Journal of Applied Physics 90, 2943 (2001); doi: 10.1063/1.1396823 View online: http://dx.doi.org/10.1063/1.1396823 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/90/6?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Magnetoelastic effects of ( La 1x Gd x ) 0.7 Ca 0.3 MnO 3 (x=0,0.05,0.2,0.3 ,and 0.4) compound J. Appl. Phys. 93, 1142 (2003); 10.1063/1.1531213 Mössbauer spectroscopy and magnetoresistivity of 57 Fe substituted Mn in La 0.7x Y x Ca 0.3 MnO 3 manganites J. Appl. Phys. 91, 7932 (2002); 10.1063/1.1446130 Observation of heat flow at transition temperature in La 1x Ca x MnO 3+ Oxides J. Appl. Phys. 90, 4583 (2001); 10.1063/1.1406551 Double-exchange ferromagnetism and magnetoresistance in LaMn 1x Cu x O 3 (x0.3) Appl. Phys. Lett. 77, 2734 (2000); 10.1063/1.1320021 Magnetostriction study of structural and magnetic transitions in La 1x Sr x MnO 3 (0.1 J. Appl. Phys. 87, 3011 (2000); 10.1063/1.372292
[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 159.226.35.188 On: Wed, 23 Apr 2014 07:03:34
JOURNAL OF APPLIED PHYSICS
VOLUME 90, NUMBER 6
15 SEPTEMBER 2001
Large room-temperature magnetoresistance and phase separation in La1À x Nax MnO3 with 0.1Ï x Ï0.3 S. L. Ye,a) W. H. Song,a) J. M. Dai,a) K. Y. Wang,a) S. G. Wang,a) and J. J. Dua) Laboratory of Internal Friction and Defects in Solids, Institute of Solid State Physics, Academia Sinica, Hefei 230031, People’s Republic of China
Y. P. Sunb) Laboratory of Internal Friction and Defects in Solids, Institute of Solid State Physics, Academia Sinica, Hefei 230031, People’s Republic of China
J. Fang, J. L. Chen, and B. J. Gao National High Magnetic Field Laboratory, Hefei 230031, People’s Republic of China
共Received 29 January 2001; accepted for publication 29 June 2001兲 The structural, magnetic, and electronic properties of the polycrystalline La1⫺x Nax MnO3 共x ⫽0.10, 0.15, 0.20, and 0.30兲 are investigated. The result of the Rietveld refinement of x-ray powder diffraction shows that these compounds crystallize in a rhombohedrally distorted structure with ¯ C. The magnetic measurement shows that Curie temperature T C of the studied space group R3 samples is near or above room temperature. The temperature dependence of resistivity shows that all samples undergo a sharp transition accompanying a paramagnetic to ferromagnetic with the decrease of temperature, however, for x⭓0.15 samples, double transition peaks with a single ferromagnetic transition is observed. In the meanwhile, a large room-temperature magnetoresistance with low applied magnetic field is observed. The co-existing ferromagnetic metallic phases and ferromagnetic insulating 共FMI兲 phases induced by the electronic inhomogeneity as well as the additional FMI phases caused by the presence of vacancies at the A sites, are presented to account for the transport properties and large magnetoresistance in these compounds. © 2001 American Institute of Physics. 关DOI: 10.1063/1.1396823兴
I. INTRODUCTION
Since the discovery of colossal magnetoresistance 共CMR兲 in perovskite manganese oxides R1⫺x Ax MnO3 共where R is a trivalent rare-earth element and A is a divalent metal element such as Ca, Sr, Ba, or Pb兲, much theoretical and experimental work has been done to investigate the physical mechanism of CMR effect because of its interesting physical properties and application potential.1–7 It has long been thought that the spin structure and electronic transport properties of R1⫺x Ax MnO3 are correlated via the doubleexchange 共DE兲 mechanism,8 i.e., the hopping of e g electrons between Mn3⫹ and Mn4⫹ ions mediated by oxygen anions. The effective transfer integral of e g electrons depends on the relative angle of the localized spins of manganese ions. However, Millis et al.9–11 argued that the DE mechanism alone could not quantitatively account for some features of CMR effect in R1⫺x Ax MnO3, such as the magnitude of the resistivity and the magnitude of CMR. A strong Jahn–Teller electron–phonon coupling arising from the deformation of the Mn3⫹O6 octahedra due to the Jahn–Teller 共JT兲 effect should play an important role. The lattice distortion may not only influence the effective transfer integral of e g but also a兲
Also at: National High Magnetic Field Laboratory, Hefei 230031, People’s Republic of China. b兲 Also at: Structure Research Laboratory, University of Science and Technology of China, Hefei 230026, People’s Republic of China; electronic mail:
[email protected]
result in complicated magnetic structure of the compounds. Recently, phenomenon of the phase separation 共PS兲 in perovskite-type doped manganates has stimulated scientists’ interest since it may be a powerful candidate for the origin of the insulator–metal 共I–M兲 behavior as well as the CMR effect. Uehara et al.12 found that the CMR effect in low-T C systems could be explained by percolative transport through the ferromagnetic domain. They presented transmission electron microscopy images of La5/8⫺y Pry Ca3/8MnO3, which showed the mixture of the charge ordering phase and ferromagnetic phase at low temperatures. Especially, the CMR effect is enhanced near the phase boundary between the ferromagnetic metallic 共FMM兲 and the charge-ordered insulating phases, where the PS effect is also enhanced. Since then, many experimental and theoretical studies have been carried out on the PS problem. So far, most of studies have focused on the divalent alkaline–earth–metal doping in R1⫺x Ax MnO3 compounds. In contrast, there are few reports on the study of monovalent alkali–metal–doped samples.13–16 Due to the difference in valence, alkaline–earth–metal doping and alkali–metal doping in LaMnO3 can result in remarkably different consequences. Therefore, the study of alkali–metal doping will offer a complementary understanding on the structure and electronic transport properties of the doped LaMnO3 system, which is significant for achieving a complete understanding of CMR effects in the distorted perovskite manganates. In the present work, structural, electronic transport, and mag-
0021-8979/2001/90(6)/2943/6/$18.00 2943 © 2001 American Institute of Physics [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 159.226.35.188 On: Wed, 23 Apr 2014 07:03:34
2944
Ye et al.
J. Appl. Phys., Vol. 90, No. 6, 15 September 2001
netic properties of Na-doped LaMnO3 compounds are investigated. We find that Na doping can lead to high Curie temperature T C and large CMR effect near room temperature. The high T C of these compounds can be understood by their structural feature and the increase of Na-doped level. Based on the PS mechanism, the large CMR effect is discussed by the percolative transport through the ferromagnetic domains which are the mixture of the ferromagnetic insulating 共FMI兲 phase and FMM phase in Na-doped compounds. II. EXPERIMENT
La1⫺x Nax MnO3 共x⫽0.10, 0.15, 0.20, and 0.30兲 bulk samples were synthesized by a conventional solid state reaction method. Well mixed stoichiometric mixtures of high purity La2O3, Na2CO3, and MnCO3 powders were calcined at 1000 °C for 24 h. The powder obtained was ground, pelletized, and sintered at 1200 °C for 48 h with an intermediate regrinding, then furnace cooled to room temperature. X-ray powder diffraction 共XRD兲 was performed using a Philips PW 1700 x-ray diffractometer with a Cu K ␣ radiation. The magnetization of the samples was measured using a Quantum Design MPMS superconducting quantum interference device magnetometer. The resistance was measured by a standard four-probe method from 4.2 to 350 K. The magnetoresistance 共MR兲 is defined as MR⫽⌬ / 0 ⫽„( H ⫺ 0 )/ 0 …* 100%, where 0 and H is the resistivity at zero field and applied magnetic fields, respectively. III. RESULTS AND DISCUSSION
XRD at room temperature shows that the prepared samples are single phase. XRD patterns of the samples can ¯ C, be indexed by rhombohedral lattice with space group R3 the structure parameters are refined by a standard Rietveld technique.17 Figures 1共a兲 and 1共b兲 show the experimental and calculated XRD patterns for x⫽0.10 and x⫽0.30 samples, respectively. The structural parameters are listed in Table I. It can be seen that the structural parameters vary regularly. It is well known that one of possible origins of the lattice distortion of perovskite-based structures is the deformation of the Mn3⫹O6 octahedra originating from the JT effect that is inherent to the high-spin (S⫽2) Mn3⫹ ions with double degeneracy of the e g orbital. Obviously, this kind of distortion is directly related to the concentration of Mn3⫹ ions. Another possible origin of the lattice distortion is the average ionic radius of the A-site element, which is governed by the tolerance factor t (t⫽(r A ⫹r O )/&(r B ⫹r O ) 共where r i 共i⫽A, B, or O兲 represents the average ionic size of each element兲. As t is close to 1, the cubic perovskite structure is realized. As r A decreases, t also does, the lattice structure transforms to the rhombohedral (0.96⬍t⬍1) and then to the orthorhombic structure (t⬍0.96), in which the bending of B–O–B bond and the deviating of the bond angle from 180° increase. Such structural transition have been confirmed by Sr- and Badoping system,18,19 which show that structural properties are governed by the ionic sizes while physical properties are controlled by both the ionic sizes and the hole doping level. For La1-x Nax MnO3 samples, the ionic size of Na 关 r(Na) ⫽1.390 Å 兴 is slightly larger than that of La 关 r(La)
FIG. 1. XRD patterns of the compound La1⫺x Nax MnO3, 共a兲 x⫽0.10 and 共b兲 x⫽0.30 are shown. Crosses indicate the experimental data and the calculated profile is the continuous line overlying them. The lowest curve shows the difference between experimental and calculated patterns. The vertical bars indicate the expected reflection positions.
⫽1.360 Å 兴 , 20 which results in the tolerance factor t is larger than 0.96 as showed in Table I. The Rietveld fitting result indicates the structure of these compounds is rhombohedral, which is consistent with the structure expected. Based on the structural results of Na-doping and Sr共Ba兲-doping systems, it can be concluded that the structure symmetry of doped perovskite manganese oxides is affected by the size of doped ions. The temperature dependence of the magnetization of La1⫺x Nax MnO3 with 0.1⭐x⭐0.3 is shown in Fig. 2. The field-cooled 共FC兲 magnetization shows that all samples undergo a sharp paramagnetic to ferromagnetic transition. The inset of Fig. 2 reveals that Curie temperature T C 共defined as the inflection point on the M (T) curve兲 is near or above the room temperature and increases with the increase of the Nadoping level. We argue it reflects the correlation between ferromagnetism and JT distortion mediated by spin–charge– lattice coupling because the lattice distortion may not only influence the effective transfer integral of e g electrons but also play an important role in magnetic property of the compound. Before discussing the magnetic property, it is useful to define the coherent distortion parameter JT . 21 TABLE I. Room-temperature structural parameters and tolerance factors of La1⫺x Nax MnO3 are shown. Parameters a/共Å兲 b/共Å兲 c/共Å兲 Mn–O/共Å兲 Mn–O–Mn共°兲 t
x⫽0.10 5.525 5.525 13.354 1.965 163.32 0.966
x⫽0.15 5.497 5.497 13.343 1.960 163.33 0.972
x⫽0.20 5.494 5.494 13.334 1.959 163.34 0.978
x⫽0.30 5.490 5.490 13.327 1.956 163.35 0.991
[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 159.226.35.188 On: Wed, 23 Apr 2014 07:03:34
Ye et al.
J. Appl. Phys., Vol. 90, No. 6, 15 September 2001
FIG. 2. The temperature dependence of magnetization in La1⫺x Nax MnO3 (x⫽0.10, 0.15, 0.20, and 0.30兲 measured in a magnetic field of 100 G is shown. FC curves are shown. The inset is the variation of T c with x.
JT⫽
冑兺 1 3
i
关共 Mn⫺O兲 i ⫺ 具 Mn⫺O典 兴 2
共1兲
where 共Mn–O兲i stands for the three independent Mn–O bond lengths and 具 Mn–O典 is the average Mn–O bond ¯ C) length. In our studied samples, the rhombohedral (R3 structure is observed. Since all three Mn–O distances are ¯ C phase, the coherent JT distortion parameter equal in the R3 is zero from Eq 共1兲. That is to say, a static coherent JT distortion is absent in the studied samples. As we know, a static coherent JT distortion of the Mn3⫹O6 octahedra can provide an additional carrier localization.18 Therefore, the absence of a static coherent JT distortion is favorable for the hopping of e g electrons and leads to enhanced DE interaction, which gives rise to high Curie temperature T C of all studied samples. In addition, Table I shows the Mn–O–Mn bond angle increases and the Mn–O bond length decreases monotonously with the increase of Na-doped level. Because a shorter Mn–O distance and the larger Mn–O–Mn bonding angle lead to the increase of the bandwidth and the mobility of e g electrons, DE interaction magnetism is enhanced and the T C is increased in terms of DE interaction theory. Therefore, all these features indicate that high Curie temperature induced by Na doping results from the enhanced DE interaction due to the weakened JT distortion, which is mediated by the spin–charge–lattice coupling. The magnetization as a function of the applied magnetic field at 5 K is showed in Fig. 3. At 5 K, the magnetization reaches saturation at about 1 T and keeps constant up to 5 T. The saturation magnetization S decreases with the increase of Na-doped level as shown in the inset of Fig. 3. For all samples, the S value deviates from the theoretical value (4⫺2x) B , assuming that the magnetic moment of the Mn3⫹ is 4 B and that of Mn4⫹ is 3 B . Here, x is the Na-doped level and B is Bohr magnetron. The difference originates from the existence of nonmagnetic randomness induced by the presence of vacancies at the A sites that modifies the net magnetic moment per Mn atom as discussed later. Figure 4共a兲 shows the temperature dependence of the resistivity of the La1⫺x Nax MnO3 共x⫽0.10, 0.15, 0.20, and
2945
FIG. 3. Field dependence of the magnetization in La1⫺x Nax MnO3 共x ⫽0.10, 0.15, 0.20, and 0.30兲 at 5 K is shown. Inset is the variation of the saturation magnetization, S with x, ST is the theoretical value, and SM is the experimental value.
0.30兲 samples at zero magnetic field. All samples undergo a sharp transition at T p1 accompanying a paraferromagnetic transition. However, the samples with x⭓0.15 exhibit another broad transition at lower temperature T p2 , which shifts to lower temperatures with the increase of Na-doped level from T P2 ⫽238 K of x⫽0.15 to T P2 ⫽217 K of x⫽0.20, and T P2 ⫽213 K of x⫽0.30. According to the DE model, in the homogenous ferromagnetic state, the electrons are mobile and the ferromagnetic state and metallic state should accompany and facilitate each other. In contrast, the transport property of the studied samples with x⭓0.15 does not fully show the metallic state below T C , and the resistivity increases with the increase of Na-doped level while T C increases. Considering the possibility of the phase separation as the origin of the I–M behavior as well as the CMR effect, this unusual phenomenon seems to be related to the phase separation induced by inhomogeneity. Taking into account the fact that there is a large difference in valence between La3⫹ and Na1⫹ ions, the random distribution of La3⫹ and Na1⫹ ions in the A sites will cause the inhomogeneity in the background potential experienced by the e g electrons as they move through the crystal, leading to some regions with lower potential in which the e g electrons can be trapped.22–24 That is to say, the distribution of the e g electrons is inhomogeneous. On the other hand, when Na1⫹ ion is substituted for the La3⫹ ion in LaMnO3, two Mn3⫹ ions need to be oxidized to Mn4⫹ ions leading to the formation of the rich Mn4⫹ region and rich Mn3⫹ region, which also favors electronic inhomogeneity and induces possible phase or domain separation.25 These phases could be antiferromagnetic insulating 共AFI兲 phases, FMI phases, and FMM phases, respectively. Therefore, when the system is transferred from the paramagnetic to ferromagnetic phase with the decrease of temperatures, the transport property of the x⭓0.15 samples does not fully show the metallic state because the FMM phases are disconnected for the existence of the FMI phases among them. With further decrease of temperature below T C , the FMI region decreases and an apparent I–M transition is observed when the metallic phases are connected in a percolative manner. That is the lower broad transition peak in our studied samples. In addi-
[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 159.226.35.188 On: Wed, 23 Apr 2014 07:03:34
2946
Ye et al.
J. Appl. Phys., Vol. 90, No. 6, 15 September 2001
TABLE II. The fitting parameter T 0 of La1⫺x Nax MnO3 共x⫽0.15, 0.20, and 0.30兲 in the temperature region above T P1 is shown. Fitting parameter
x⫽0.15
x⫽0.20
x⫽0.30
T 0 共K兲 (T⬎T p1 )
9.198⫻106
2.973⫻106
2.095⫻106
the nominal Na-doping level increases, the additional FMI phase increases and leads to the increase of the total FMI phase which can be evidenced by the increase of the resistivity at low temperatures and the shift to a lower temperature of T p2 as shown in Fig. 4共a兲. In order to understand the electronic transport behavior of the studied samples more clearly, the resistivity data is fitted by Mott’s threedimensional variable range hopping expression27
冋冉 冊 册
共 T 兲 ⫽ 0 exp
FIG. 4. 共a兲 The temperature dependence of the resistivity in zero field for the samples La1⫺x Nax MnO3 共x⫽0.10, 0.15, 0.20, and 0.30兲, 共b兲 Plot of ln() against 1/T 1/4 of the resistivity data above T P1 and 共c兲 Plot of the normalized (T)/ (4.2 K) are shown.
tion, it is difficult for the synthesis of stoichiometric sodium substituted lanthanum manganate due to the easy evaporation of sodium26 in the temperature range of 1000 °C– 1300 °C at which lanthanum manganates are generally prepared. Based on this consideration, the Na-doped sample exists vacancies at the A site, which causes the saturation magnetization S value to deviate from the theoretical value (4⫺2x) B as shown in inset of Fig. 3. Moreover, it can be speculated that there are more vacancies at A sites with the increase of the Na doped level because the deviation of the saturation magnetization S value increases gradually. The developed vacancies trap mobile electrons, leading to partial FMM phases transformed to some additional FMI phases. Therefore, when
T0 T
1/4
,
共2兲
in the temperature region of above T P1 as shown in Fig. 4共b兲. T 0 is a characteristic temperature which is related to the density of states in the vicinity of the Fermi energy N(E F ) and the localization length , i.e., k B T 0 ⬇21/关 3 N(E F ) 兴 . From Fig. 4共b兲, it can be seen that Eq. 共2兲 gives a good fit for the temperature region of T⬎T P1 . The fitting results show that T 0 decreases, which means that the localization length increases with the increase of Na-doped level in this temperature region for x⭓0.15 as shown in Table II. Above T P1 which corresponds with T C , T 0 is mainly associated with the effect of JT distortion because an incoherent JT is large in this temperature region. The increase of the localization length shows that the DE interaction increases with the increase of the Na-doping level due to the weakening of JT distortion effect. In the low-temperature regime where the transport properties fully show the metallic state, it seems to be governed by the electron scattering process, (T) is well fit by the empirical equation ⫽ ⬘ ⫹AT n 共n⫽2 and 2.5兲. Here ⬘ is the resistivity due to the domain boundaries and other temperature-independent scattering mechanism.28 The fitting results show that for the (T) with x⫽0.10, the equation with n⫽2.5 fits the data well, which represents a combination effect of electron–electron, electron–phonon, and electron–magnon scattering. However, for the (T) with x ⭓0.15, the equation with n⫽2 fits the data better, which shows that the resistivity is mainly attributed to the electron– electron scattering. The fitting results indicate that the increase of Na-doped level significantly decreases the electron–phonon interaction, which favors the moving of the charge carriers as seen from Fig. 4共c兲. However, because the vacancies at A sites lead to an additional localization of e g electrons, the total resistivity of the samples increases with the Na-doped level due to the increase of ⬘ increases. Figure 5 is the temperature dependence of the resistivity for the sample with x⫽0.15 at different magnetic fields. It shows that T p2 moves toward higher temperatures and T p1 gradually disappears with the increase of magnetic fields. At H⫽7 T, T p2 shifts to a high temperature for about 20 K. The (T) behavior shown in Fig. 5 is thought to originate from
[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 159.226.35.188 On: Wed, 23 Apr 2014 07:03:34
Ye et al.
J. Appl. Phys., Vol. 90, No. 6, 15 September 2001
FIG. 5. The temperature dependence of the resistivity at different applied fields 共H⫽0, 1, 5, and 7 T兲 for the sample La0.85Na0.15MnO3 is shown.
the reduction of the insulating phase due to the increase of the applied field, which consequently results in the decrease of resistivity. That is to say, the localization of e g electrons decreases with the increase of the magnetic field. The delocalization of e g electrons seems to be mainly associated with the reduction of the FMI phase caused by applied magnetic fields. The temperature dependence of the MR of x⫽0.10 and x⫽0.15 is presented in Fig. 6. It can be seen that the MR of the sample with x⫽0.15 is larger than that of the sample with x⫽0.10 at low applied fields. As we all know, larger MR is usually associated with low T C in lanthanum compounds. For divalent Ba-, Sr-, and Pb-doped lanthanum manganates, although T c is above 300 K, their MR is smaller at low fields. One possible reason for the large MR in the studied sample with x⫽0.15 seems to be ascribed to the FMI phases induced by the vacancies at the A site, because the low applied magnetic field has little effect on the AFI phases. Indeed, a larger MR at 220 K is observed in La1⫺x MnO3⫺ ␦ thin film.29 Thus, the additional FMI phase can be transformed to the FMM phase at low applied fields and results in a higher MR at low applied magnetic fields. The magnetic field dependence of MR at a fixed temperature of 4.2 K is shown in Fig. 7 for studied samples. At T⫽4.2 K, a sharp variation of ⬃20% in MR is observed at
2947
FIG. 7. The magnetic field dependence of MR at the low temperature of 4.2 K for samples La1⫺x Nax MnO3 共x⫽0.1, 0.15, 0.20, and 0.30兲 is shown.
low fields below 0.5 T for all the samples. The pronounced change in resistance over this field range at low temperatures is associated with a field-induced reduction in the scattering from the domain walls at the grain boundaries of the polycrystalline samples. This interpretation is confirmed by the absence of such effects in epitaxial films and single crystals.30,31 As it can be seen in Fig. 7, MR varies rapidly throughout the region in which the domains undergo rotation. The same behavior was investigated and interpreted as spin-polarized tunneling, by Hwang et al.30 in a similar polycrystalline sample and by Li et al.31 in polycrystalline films. IV. CONCLUSION
In summary, the magnetic, transport, and structural properties of the polycrystalline La1⫺x Nax MnO3 共x⫽0.10, 0.15, 0.20, and 0.30兲 are investigated. The result indicates that the structure of these compounds is all rhombohedral with a larger tolerance factor t, which proves that the structure is affected by the ionic sizes. The Curie temperature T C is near or above room temperature. We argue the magnetic properties are close related to the competition between ferromagnetism and JT distortion that is mediated by spin–charge– lattice coupling. For x⭓0.15 samples, double transition with a single ferromagnetic transition is observed. In the meanwhile, near the Curie temperature T C a large roomtemperature MR is observed. Considering the role of PS in the CMR effect, the co-existing FMI and FMM phases induced by the electronic inhomogeneity as well as the additional FMI phase caused by the vacancies at the A sites, is presented to account for the transport properties and large MR in these compounds. To achieve an overall understanding of the CMR effects in the distorted perovskite manganates, further investigations of the perovskite manganates with alkali–metal doping are necessary. ACKNOWLEDGMENTS
This work was supported by the Fundamental Bureau, Chinese Academy of Sciences, and the National Natural Science Foundation of China, under the Contract No. NSF10074066.
FIG. 6. The temperature dependence of MR for samples with x⫽0.1 and 0.15 at 0.8 T is shown. [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:
159.226.35.188 On: Wed, 23 Apr 2014 07:03:34
2948 1
Ye et al.
J. Appl. Phys., Vol. 90, No. 6, 15 September 2001
R. von Helmolt, J. Wecker, B. Holzapfel, L. Schultz, and K. Samwer, Phys. Rev. Lett. 71, 2331 共1993兲. 2 S. Jin, T. H. Tiefel, M. McCormack, R. A. Fastnacht, R. Ramesh, and L. H. Chen, Science 264, 413 共1994兲. 3 G. Q. Gong, C. Canedy, G. Xiao, J. Z. Sun, A. Gupta, and W. J. Gallagher, Appl. Phys. Lett. 67, 1783 共1995兲. 4 G. C. Xiong, Q. Li, H. L. Ju, S. N. Mao, L. Senapati, X. X. Xi, R. L. Greene, and T. Venkatesan, Appl. Phys. Lett. 66, 1427 共1995兲. 5 A. Asamitsu, Y. Moritomo, Y. Tomioka, T. Arima, and Y. Tokura, Nature 共London兲 373, 407 共1995兲. 6 V. Caignaert, A. Maignan, and B. Raveau, Solid State Commun. 95, 357 共1995兲. 7 J. M. D. Coey, M. Viret, and S. von Molnar, Adv. Phys. 48, 167 共1999兲. 8 C. Zener, Phys. Rev. 82, 403 共1951兲. 9 A. J. Millis, P. B. Littlewood, and B. I. Shraiman, Phys. Rev. Lett. 74, 5144 共1995兲. 10 A. J. Millis, B. I. Shraiman, and R. Mueller, Phys. Rev. Lett. 77, 175 共1996兲. 11 A. J. Millis, Phys. Rev. B 53, 8434 共1996兲. 12 M. Uehara, S. Mori, C. H. Chen, and S.-W. Cheong, Nature 共London兲 399, 560 共1999兲. 13 M. Itoh, T. Shimura, J. D. Yu, T. Hayashi, and Y. Inaguma, Phys. Rev. B 52, 12522 共1995兲. 14 T. Shimura, T. Hayashi, Y. Inaguma, and M. Itoh, J. Solid State Chem. 124, 250 共1996兲. 15 X. L. Wang, S. J. Kennedy, H. K. Liu, and S. X. Dou, J. Appl. Phys. 83, 7177 共1998兲. 16 G. H. Rao, J. R. Sun, K. Ba¨rner, and N. Hamad, J. Phys.: Condens. Matter 11, 1523 共1999兲.
D. B. Wiles and R. A. Young, J. Appl. Crystallogr. 14, 149 共1981兲. P. G. Radaelli, M. Marezio, H. Y. Hwang, S.-W. Cheong, and B. Batlogg, Phys. Rev. B 54, 8992 共1996兲. 19 R. Mallik, E. S. Reddy, P. L. Paulose, S. Majumdar, and E. V. Sampathkumaran, J. Phys.: Condens. Matter 11, 4179 共1999兲. 20 R. D. Shannon, Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor. Gen. Crystallogr. 32, 751 共1976兲. 21 X. Xiong, B. Dabrowski, O. Chmaissem, Z. Bukowski, S. Kolesnik, R. Dybzinski, C. W. Kimball, and J. D. Jorgensen, Phys. Rev. B 60, 10186 共1999兲. 22 J. M. D. Coey, M. Viret, and L. Ranno, Phys. Rev. Lett. 75, 3910 共1995兲. 23 J. R. Sun, G. H. Rao, and J. K. Liang, Appl. Phys. Lett. 70, 1900 共1997兲. 24 G. H. Rao, J. R. Sun, J. K. Liang, and W. Y. Zhou, Phys. Rev. B 55, 3742 共1997兲. 25 S. L. Ye, W. H. Song, J. M. Dai, S. G. Wang, K. Y. Wang, C. L. Yuan, and Y. P. Sun, J. Appl. Phys. 88, 5915 共2000兲. 26 M. Sahana, R. N. Singh, C. Shivakumara, N. Y. Vassanthacharya, M. S. Hegde, S. Subramanian, V. Prasad, and S. V. Subramanyam, Appl. Phys. Lett. 70, 2909 共1997兲. 27 N. F. Mott and E. A. Davis 共unpublished兲. 28 J. M. De Teresa, M. R. Ibarra, J. Blasco, J. Garcia, C. Marquina, P. A. Algarabel, Z. Arnold, K. Kamenev, C. Ritter, and R. Von Helmolt, Phys. Rev. B 54, 1187 共1996兲. 29 S. S. Manoharan, D. Kumar, M. S. Hegde, K. M. Satyalakshmi, V. Prasad, and S. V. Subramanyam, J. Solid State Chem. 117, 420 共1995兲. 30 H. Y. Hwang, S.-W. Cheong, N. P. Ong, and B. Batlogg, Phys. Rev. Lett. 77, 2041 共1996兲. 31 X. W. Li, A. Gupta, Gang Xiao, and G. Q. Xiao, Appl. Phys. Lett. 71, 1124 共1997兲. 17 18
[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 159.226.35.188 On: Wed, 23 Apr 2014 07:03:34
