research papers
High-resolution photoemission spectroscopy for the layered antiferromagnetic (La1−zNdz)0.46Sr0.54MnO3†
aDepartment of Crystalline Materials Science, Nagoya University, Nagoya 464-8603, Japan, bInstitute for Solid State Physics, University of Tokyo, Kashiwa 277-8581, Japan, cDepartment of Physics, Tohoku University, Sendai 980-8578, Japan, dJapan Synchrotron Radiation Research Institute, SPring-8, Hyogo 679-5148, Japan, eJapan Atomic Energy Research Institute, SPring-8, Mikazuki, Hyogo 679-5148, Japan, and fCenter for Integrated Research in Science and Engineering, Nagoya University, Nagoya 464-8603, Japan
*Correspondence e-mail: takeuchi@nuap.nagoya-u.ac.jp
High-resolution HeI photoemission spectroscopy (UPS), Mn 2p–3d resonant photoemission spectroscopy (RPES) and Mn 2p have been performed to investigate the electronic structure and its effect on the electrical resistivity in (La1−zNdz)0.46Sr0.54MnO3 (z = 0, 0,2, 0.6 and 1.0). It was found that in the UPS and RPES spectra the Fermi edge persisted over the temperature range 15 ≤ T ≤ 340 K regardless of the magnetic structure or the composition of the samples. The at the [N(EF)] in the samples where 0 ≤ z ≤ 0.6 was increased drastically at the Curie temperature (TC) with decreasing temperature, but essentially kept unchanged across the Néel temperature (TN). A fairly large reduction at TC and a small increase at TN in the electrical resistivity with decreasing temperature are found to be well accounted for in terms of the temperature dependence of N(EF). The presence of a finite N(EF) in the insulating Nd0.46Sr0.54MnO3 was also found. Thus the origin of the insulating behavior in this sample can be regarded as the Anderson localization associated with the small and the chemical disorder between Nd and Sr.
Keywords: manganite; metal–insulator transition; colossal magnetoresistance; RPES; UPS; XAS.
1. Introduction
The cubic perovskite manganites are known to be characterized by the variety of peculiar electron transport properties, such as the colossal magnetoresistance (Jin et al., 1994) associated with the magnetic-field-induced metal–insulator (MI) transition. Mn atoms in the perovskite structure provide not only itinerant electrons but also localized magnetic moments. The influence of the localized magnetic moments on the itinerant electrons is so strong that the electron transport properties of the perovskite manganites vary drastically depending on the variation in arrangement of the localized magnetic moments; the metallic conduction is in general observed for both the ferromagnetic phase and so-called A-type layered antiferromagnetic phase, while insulating behaviors were reported for the paramagnetic and other antiferromagnetic phases (Tokura et al., 1994; Urushibara et al., 1995; Akimoto et al., 1998; Moritomo et al., 1998).
The ferromagnetic phase with metallic behavior transforms into the insulating phase accompanied by the magnetic ; de Gennes, 1960). Recently, however, it was pointed out that the DE interaction alone cannot fully account for the between the ferromagnetic metal and the paramagnetic insulator (Millis et al., 1995, 1996; Saitoh et al., 2000). In the framework of the DE theory, the relation between the metallic conductivity and the magnetic ordering is explained as follows. The hopping probability of the itinerant electrons from one site to another is formulated by t = t0 cos(θ/2), where t0 and θ indicate the bare hopping probability and the angle between two localized magnetic moments in the nearest-neighbor atomic sites, respectively. If these localized magnetic moments are ferromagnetically ordered, the hopping probability shows the maximum value with θ = 0°, while it is fairly reduced in the array of disordered magnetic moments and reaches zero in the array of antiferromagnetically ordered magnetic moments with θ = 180°. The reduced hopping probability leads to a reduction in the Fermi velocity. However, no increase in the electrical resistivity can be caused by it as long as a single band is concerned, because a simultaneously introduced increase in the near EF compensates the increase of the resistivity due to the reduced Fermi velocity. Thus, within the DE model, the resistivity variation cannot appear except when the between the antiferromagnetic phase and other phases occurs.
with varying temperature or external magnetic field. This peculiar MI transition had been believed to be explained in terms of the double-exchange (DE) interaction (Anderson & Hasegawa, 1955The A-type layered antiferromagnetic phase has an ordered magnetic structure in which localized magnetic moments are ferromagnetically ordered in planes and those planes stack together antiferromagnetically. Akimoto et al. (1998) reported the temperature dependence of the electrical resistivity for La0.46Sr0.54MnO3, that possesses the paramagnetic phase above TC = 300 K, the ferromagnetic phase at TN ≤ T ≤ TC and the A-type antiferromagnetic phase below TN = 200 K. Here, TC and TN indicate the Curie and Néel temperatures, respectively. In their report the electrical resistivity of this sample was decreased by about 40–50% of its magnitude at TC and slightly increased at TN with decreasing temperature. Obviously, these behaviors cannot be understood in terms of the DE interaction. Thus in order to investigate the factors causing these behaviors in the electrical resistivity, the determination of the electronic structure of the paramagnetic state, the ferromagnetic state and the A-type antiferromagnetic state is strongly required. It should be noted here that although many experiments with photoemission spectroscopy have already been reported for the ferromagnetic and paramagnetic phases (Saitoh et al., 1995, 1997; Park et al., 1996; Sarma et al., 1996; Chainani et al., 1993, 1997; Sekiyama et al., 1999), that for the A-type antiferromagnetic phase has so far not been reported.
In this paper we report the results of high-energy-resolution photoemission spectroscopy measurements for the (La1−zNdz)0.46Sr0.54MnO3 (z = 0.0, 0.2, 0.6 and 1.0) cubic perovskite manganites. These samples, except for z = 1.0, possess two different magnetic transitions with varying temperature; the paramagnetic phase exists above TC = 230–300 K, the ferromagnetic phase at TN ≤ T ≤ TC and the A-type antiferromagnetic phase below TN = 200–230 K (Akimoto et al., 1998). Only Nd0.46Sr0.54MnO3 possesses a direct transition from the paramagnetic phase into the A-type antiferromagnetic phase at TN = 225 K without being in the ferromagnetic phase. The electronic structure of the A-type antiferromagnetic phase was investigated in comparison with that of the paramagnetic and ferromagnetic phases. The relation between the measured at the [N(EF)] and the electrical resistivity in each compound will be discussed in detail.
2. Experimental
High-quality (La1−zNdz)0.46Sr0.54MnO3 (z = 0.0, 0.2, 0.6 and 1.0) single crystals were grown by the floating-zone method. Details of the sample preparation have been reported elsewhere (Akimoto et al., 1998; Moritomo et al., 1998). The bulk-sensitive Mn 2p–3d resonant photoemission spectroscopy (RPES) measurements (Sekiyama et al., 2000) with incident photon energies of hυ = 639–641 eV were performed at beamline BL25SU at the third-generation 8 GeV synchrotron radiation facility SPring-8, Hyogo, Japan. The energy resolution of the measurements was about 80–120 meV, which was determined as the energy width of the intensity drop from 90% to 10% at the Fermi edge of the reference gold electrically contacting with the samples. The HeI photoemission spectroscopy (UPS) measurements with an energy resolution of 50 meV were also performed for (La1−zNdz)0.46Sr0.54MnO3 (z = 0.0, 0.2, 0.6 and 1.0). All UPS and RPES measurements were carried out at temperatures between 15 K and 340 K. The sample surfaces were scraped with a diamond file just before the data accumulation at each temperature under a base pressure better than 10–11 torr. We intentionally finished each measurement within 60 min after the surface scraping to avoid the serious surface degradation reported by Saitoh et al. (1997). Mn 2p soft measurements were performed at BL19B at the Photon Factory, KEK, Japan. We used a bulk-sensitive (∼2000 Å in depth) method for the present Mn 2p measurement.
3. Results and discussions
The electronic structure of these cubic perovskite manganites has been frequently studied by means of photoemission spectroscopy measurements (Saitoh et al., 1995, 1997; Park et al., 1996; Sarma et al., 1996; Chainani et al., 1993, 1997; Sekiyama et al., 1999). The near EF in these reports was fairly small. Sometimes it was reported to be finite in the ferromagnetic metallic phases and to become essentially zero in the insulating phases. The small intensity near EF prevented us from evaluating the gap width and the at EF. In order to overcome this difficulty, we employed bulk-sensitive and high-energy-resolution Mn 2p–3d RPES measurements, by which the associated with the Mn 3d electron is so enhanced that the Fermi edges are expected to be clearly observed in the metallic states.
Fig. 1(a) shows photoemission spectra of La0.46Sr0.54MnO3 taken with incident photon energies across the Mn 2p All spectra are normalized by the It can be confirmed that the intensity of the valence-band spectra increases by more than ten times when the Mn 2p–3d resonance takes place. The enhanced intensity due to the Mn 2p–3d resonance allowed us to easily observe the near EF. We employed hυ = 640 eV as the incident photon energy for the RPES measurements to avoid the effect of the undesirable auger process. Even with this incident energy the spectrum intensity is enhanced by more than five times than that of the off-resonant spectrum measured with hυ = 637 eV. The high-resolution Mn 2p RPES spectra near EF for (La0.8Nd0.2)0.46Sr0.54MnO3 in the paramagnetic phase at 300 K, the ferromagnetic phase at 250 K and the A-type layered antiferromagnetic phase at 150 K are shown in Fig. 1(b). The clearly observed intensity at EF in all spectra indicates that (La0.8Nd0.2)0.46Sr0.54MnO3 must be metallic regardless of the magnetic status involved. It should be noted that the spectrum intensity at EF of the ferromagnetic phase and the A-type antiferromagnetic phase obviously shows a much higher value than in the paramagnetic phase. The electrical resistivity of (La1−zNdz)0.46Sr0.54MnO3 (0.0 ≤ z ≤ 0.4) reported by Akimoto et al. (1998) decreases by about 50% in its magnitude when the paramagnetic phase transforms into the ferromagnetic phase at TC but it is almost kept constant across TN. This variation in the electrical resistivity must be caused by the variation in N(EF) observed in the present RPES measurement.
In order to gain further insight into the electronic structure near EF we employed the HeI UPS measurement with a higher energy resolution of 50 meV. The observed spectra for (La1−zNdz)0.46Sr0.54MnO3 (z = 0.0, 0.2, 0.6 and 1.0) at different temperatures are shown in Figs. 2(a)–2(d). Notably, all UPS spectra possess a steep Fermi edge, which shows much clearer structure than that reported for the ferromagnetic phase in the cubic perovskite manganite (Sarma et al., 1996, 1997; Sekiyama et al., 1999). The easiness in observing the Fermi edge with UPS measurements for the present samples most likely originates from the large O 2p component along with Mn 3d in the near EF.
The UPS spectra of (La1−zNdz)0.46Sr0.54MnO3 with z = 0.0, 0.2 and 0.6 possess a fairly large enhancement in intensity near EF at TC and less obvious variation across TN with decreasing temperature. The same tendency has already been mentioned above in the RPES measurements for z = 0.2. Therefore it is certain that N(EF) associated with both the Mn 3d and O 2p electrons increases when the from the paramagnetic phase to the ferromagnetic phase takes place, and that this variation in N(EF) leads to the reduction in the magnitude of the electrical resistivity.
When the electrical resistivity of metallic phases is discussed in terms of the Boltzmann transport equation, we should consider not only the electronic structure but also the scattering event, that is closely related to the ordering of the local magnetic moments. It is very important to know that a reduction in the electrical resistivity of about 10% in its magnitude occurs at TN with decreasing temperature for Nd0.46Sr0.54MnO3 while its UPS spectrum was almost kept unchanged across TN. The reduction in the electrical resistivity with almost constant N(EF) must be attributed to a reduction in the scattering probability due to the ordering of the local magnetic moments. Thus we conclude that the resistivity drop at TC for the (La1−zNdz)0.46Sr0.54MnO3 is introduced by the enhanced N(EF) and the reduced scattering event, which cause reduction in the electrical resistivity of about 40–50% and less than 10% in its magnitude.
The electrical resistivity of Nd0.46Sr0.54MnO3 was reported to diverge at low temperature (Akimoto et al., 1998). In sharp contrast with the divergence of the electrical resistivity, the UPS spectrum of this sample shows a clear Fermi edge at 15 K. Thus we can safely assign the insulating behavior in this sample to being caused by the Anderson localization, which is brought about by the small coupled with the disorders introduced by the mixture of Nd and Sr. The other samples with z ≤ 0.6 do not show the Anderson localization, most likely because the relatively large below TC weakens the localization tendency of the conduction electrons.
The increase of the EF for the samples with z ≤ 0.6 and its absence for z = 1.0 were also confirmed in the conduction-band structure. The Mn 2p spectra of (La1−zNdz)0.46Sr0.54MnO3 with z = 0.0 and 1.0 measured at room temperature and liquid-nitrogen temperature are shown in Fig. 3. A significant enhancement in N(EF) with decreasing temperature could be observed only for La0.46Sr0.54MnO3, while the intensity at EF in Nd0.46Sr0.54MnO3 was kept almost unchanged. These behaviors are consistent with the present RPES and UPS spectra. Unfortunately, since we were not able to control the temperature in the measurement, the exact temperature where the significant enhancement in intensity at EF occurs was unable to be determined. However, we believe that it took place at TC with decreasing temperature.
in the vicinity ofLet us now introduce the electronic structure commonly adopted for the cubic perovskite manganites to understand the origin of the variation in the t2g, the antibonding t2g, the non-bonding O 2p, the bonding eg and the antibonding eg bands. Band structures calculated by LSDA and LSDA+U methods (Pickett & Singh, 1996; Madvedeva et al., 2001) seem roughly consistent with this configuration. The bonding and antibonding bands are constructed as a consequence of the between Mn 3d and O 2p electrons. The resulting electronic structure is schematically drawn in Fig. 4. Note here that each band can involve only one electron per unit structure rather than two because of the strong on-site Coulomb interaction U and/or the on-site exchange interaction J, which were reported typically to be 4–8 eV and 0.6–0.9 eV, respectively (Madvedeva et al., 2001; Mizokawa & Fujimori, 1995).
A simple cluster-level configuration suggests that the is composed of the bondingWe observed a large change in the electronic structure near EF across TC for the samples with z ≤ 0.6. This strongly indicates that a fairly large variation takes place in the antibonding eg band. If this modification in the antibonding eg band is brought about by the structure distortion about the MnO6 octahedron, the corresponding bonding eg band should also be greatly modified at the same temperature. This is in sharp contrast to the case under the influence of the DE interaction limited only upon the antibonding eg band. Thus we measured valence-band spectra for these compounds and investigated the variation in the valence-band structure below and above TC. The temperature dependence of the UPS valence-band spectra for (La1−zNdz)0.46Sr0.54MnO3 samples with z = 0.2 and 1.0 are shown in Fig. 5. Indeed, a large modification in the valence-band structure of the sample with z = 0.2 occurred at TC in the energy range 4–6 eV where the bonding eg band most likely exists. Thus we conclude that the structure distortion occurs in the (La1−zNdz)0.46Sr0.54MnO3 samples with 0.0 ≤ z ≤ 0.6 at TC with decreasing temperature, and that it is responsible for the large enhancement in N(EF) resulting in the reduction of the electrical resistivity. In sharp contrast to the large change in the spectral shape of the sample with z = 0.2 at 4–6 eV, the spectrum shape of the sample with z = 1.0 showed less significant temperature dependence. This result agrees well with the relatively small variation in the shape of the antibonding eg band near EF for this sample, as shown in Figs. 2(d) and 3(b). Therefore the structure distortion in the sample of z = 1.0 is expected to be much smaller than that in the samples with z ≤ 0.6.
Two possible structure distortions, that can lead to a variation in their electronic structure, are known to exist for the perovskite manganites. The first one is the Jahn–Teller-type (JT-type) distortion brought about inside the MnO6 octahedron. When the degree of the JT-type distortion changes, a complicated modification would be introduced in the valence-band structure, because both the relative energy of the bands and the ratio of Mn 3d components to O 2p components in each band are drastically changed with it. The second is the GdFeO3-type distortion that contributes to the change in the one-electron bandwidth with altering Mn—O—Mn angle of the neighboring MnO6 octahedra. These two structure distortions cooperatively or competitively contribute to the variation in the electronic structure at TC. We speculate that the JT-type distortion rather than the GdFeO3-type distortion dominantly affects the electron transport properties of the present samples because the significant changes in the whole electronic structure were observed in the photoemission spectra with varying temperature.
The structure parameters of Nd0.45Sr0.55MnO3 below and above TN have already been reported (Kawano et al., 1997), but the structure analysis on (La1−zNdz)0.46Sr0.54MnO3 with 0.0 ≤ z ≤ 0.6 has not been reported. Once those data are obtained, the relation between the electronic structure and the atomic structure in the cubic perovskite manganites can be quantitatively and systematically discussed. These discussions will lead to a better understanding of the electron transport behavior in these perovskite manganites.
4. Conclusion
High-resolution HeI UPS, Mn 2p–3d RPES and Mn 2p measurements have been performed for (La1−zNdz)0.46Sr0.54MnO3 (z = 0.0, 0.2, 0.6 and 1.0), which possess phase transitions from the paramagnetic phase through the ferromagnetic phase into the A-type antiferromagnetic phase with decreasing temperature. Temperature dependence of the electrical resistivity is discussed in terms of the observed photoemission spectra. We found that the Fermi edge in (La1−zNdz)0.46Sr0.54MnO3 (0.0 ≤ z ≤ 1.0) persisted over the temperature range 15 ≤ T ≤ 340 K. The temperature dependence of the electrical resistivity of these compounds was well accounted for by considering the combined effect of the varying and scattering events associated with the ordering in the local magnetic moments. The former effect, which is related to the structure distortion, leads to about 50% variation at the maximum in the electrical resistivity. The latter is so weak that only less than 10% variation in the resistivity can be induced by it. The insulating behavior in Nd0.46Sr0.54MnO3 at low temperature is attributed to the Anderson localization, most likely caused by the small at EF coupled with the chemical disorder between Nd and Sr.
Footnotes
†Presented at the `International Workshop on High-Resolution Photoemission Spectroscopy of Correlated Electron Systems' held at Osaka, Japan, in January 2002.
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