J. Phys. Chem. B 2000, 104, 1447-1453
1447
Magnetic and Transport Properties of LaMn0.8Na0.2O3 Newell R. Washburn* and Angelica M. Stacy Department of Chemistry, UniVersity of California, Berkeley and Lawrence Berkeley National Laboratory, Berkeley, California 94720
Alan M. Portis Department of Physics, UniVersity of California, Berkeley and Lawrence Berkeley National Laboratory, Berkeley, California 94720 ReceiVed: August 18, 1999; In Final Form: December 13, 1999
Structural, magnetic, and electrical properties are reported for the Na analogue of the doped manganites, La1-xAxMnO3 (0.2 < x < 0.5, A ) Ca, Sr, Ba, Pb), prepared by precipitation from molten NaOH. Elemental analysis and diffraction data are consistent with the composition LaMn0.8Na0.2O3, indicating that Na substitutes almost exclusively onto the Mn sites of the perovskite structure. The magnetic and transport properties of this material have been investigated and it was found that, despite the substitution of a magnetically and electrically inert cation on the Mn site, LaMn0.8Na0.2O3 displays many of the same properties as the manganites doped with divalent cations on the La site. The main property differences are attributed to the strong structural and magnetic disorder that Na induces in LaMn0.8Na0.2O3.
Introduction The doped manganites, La1-xAxMnO3 (0.2 < x < 0.5, A ) Ca, Sr, Ba, Pb), have been of interest following reports of their unusual magneto-transport properties.1 Manganites with doping concentrations within a certain narrow range exhibit what is called colossal magnetoresistance (CMR).2 Magnetoresistance ratios are defined here as [F(H) - F(0)]/F(0), with F(H) the resistivity in a magnetic field H. The large, negatiVe magnetoresistance ratios that are observed for the doped manganites have been attributed to a magnetic-field-induced insulator-metal transition. The magnetic and transport properties of the manganites are closely related to their perovskite structure. The ideal oxide perovskite structure, ABO3, is shown in Figure 1. The metal atoms on the M sites at the corners of the unit cell are coordinated by six oxygen atoms in an octahedral array. Each atom on an M site is linked to an adjacent M atom through one bridging oxygen atom with an M-O-M bond angle of 180°. The M atoms are typically transition metals that bond covalently to the oxygen atoms. The large degree of orbital overlap between atoms on the M sites and the oxygens may result in magnetic interactions and electron delocalization. Metal atoms on the A site at the center of a unit cell are coordinated by 12 oxygen atoms. These atoms are larger than the atoms on the M site and are typically electropositive metal ions with weaker interactions with oxygen. The atoms on the A site balance the overall charge and fill space in the center of the unit cell but are not directly involved in collective electronic behavior. Since the atoms on the A site are not usually large enough to fill the center of the unit cell, the lattice distorts to fill space and optimize bonding interactions. This distortion * To whom correspondence should be addressed. Present address: Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455. E-mail:
[email protected] Figure 1. Ideal oxide perovskite unit cell, ABO3, where A is a large electropositive cation that occupies the center of the unit cell and is coordinated by 12 oxygen atoms and M is a smaller cation that occupies the corners of the unit cell and is octahedrally coordinated by oxygen. Adjacent M cations are connected by a single bridging oxygen with an ideal M-O-M bond angle of 180°.
lowers the symmetry of the unit cell from cubic to orthorhombic, rhombohedral, or monoclinic. There are associated reductions in the M-O-M bond angle from 180° to 150-165°, depending on the magnitude of the distortion. For reasons discussed below, the M-O-M bond angle is thought to be a key variable in determining the properties of these materials. Thus, the average ionic volume on the A site is critical to the properties of the manganites. The average charge of ions occupying the A site is also of key importance. It is possible to vary the composition continuously from LaMnO3 with Mn in the formal oxidation state of
10.1021/jp9929059 CCC: $19.00 © 2000 American Chemical Society Published on Web 01/28/2000
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Figure 2. Schematic representation of the double exchange process in mixed-valent manganese oxides where electron magnetic moments are represented by arrows. In double exchange, the lone electron in the Mn3+ eg orbital is transferred to the bridging oxygen p orbital, which drives a spin-aligned electron from the oxygen p orbital to the empty Mn4+ eg orbital.
+3 to CaMnO3, where all Mn ions are formally in the +4 oxidation state. The magnetic properties of LaMnO3 and CaMnO3, the end members of the La1-xCaxMnO3 system, are well understood and may be explained on the basis of superexchange theory.3 Both end members undergo transitions from high-temperature paramagnetic states to antiferromagnetically ordered states near 110 K.4 A consequence of the superexchange interaction between neighboring Mn ions is that both materials become insulating on cooling through their respective magnetic transitions. The magnetic and transport properties vary systematically with doping parameter x as the average volume and charge of the metal on the A site is varied in the La1-xCaxMnO3 series of phases. At compositions near La0.67Ca0.33MnO3 there is a transition from a paramagnetic, semiconducting state at high temperatures to a ferromagnetic, metallic state at low temperatures. Transition temperatures in the mixed-valent manganites are much higher (ranging from 260 to 380 K) than those observed in the end members of the series.4 The magnetic and transport properties of solid solutions near the composition La0.67Ca0.33MnO3 are closely related and can be discussed in terms of local interactions between Mn ions. The magnetic moments of the Mn ions are drawn canted in Figure 2 to emphasize fluctuations in their orientation. Electron transfer is favored when the Mn moments are aligned and adjacent ions are strongly coupled. Parallel alignment is increased by applying a magnetic field or lowering the temperature. Coupling between Mn ions is determined by the overlap of the Mn eg orbitals with the oxygen ligands. Maximum coupling between Mn ions occurs when the Mn-O-Mn bond angle is 180°. This geometrical parameter is largely controlled by the effective ionic radius of the cations on the A sitesif the ions occupying this site are too small, the lattice undergoes a distortion from cubic symmetry, as described above. Distortions lower the overlap between Mn orbitals and oxygen ligands and reduce the electron transfer integral. Thus, it is possible to modify the electron transfer integral by varying the average ionic radius of the A site cation. Because of its larger ionic radius, Sr fills the center of the unit cell more effectively and the MnO-Mn bond angle is closer to the ideal value of 180°. The replacement of Ca by Sr to form La0.67Sr0.33MnO3 leads to an increase in transition temperature of over 100 K. The ferromagnetic and metallic behavior observed in the manganites is believed to be the result of double exchange.5 Mn eg electrons are delocalized at low temperatures or in high magnetic fields. The driving force in double exchange is the reduction in electron kinetic energy through delocalization. Because this process induces parallel alignment of the Mn moments, it also leads to ferromagnetism. The insulator-metal transition in the manganites is thought to be an Anderson transition, mediated by disorder in the material.6,7 The effect of disorder on the conduction properties of these materials may be understood as follows.8 At high
Washburn et al. temperatures, there exists a wide range of Mn eg electronic energy levels, the result of large fluctuations in Mn-O-Mn bond angles and Mn-O bond lengths. Further, the magnetic moments of the Mn ions fluctuate and these fluctuations inhibit the transfer of electrons between Mn ions. As the temperature is lowered, the fluctuations decrease and neighboring Mn eg orbitals are more likely to have similar energy levels, leading to delocalization of the eg electron between these orbitals. Adjacent Mn ions are said to be “in resonance” when the energies of the eg orbitals are close enough to permit electron delocalization between the sites. A metallic state is realized when a chain of Mn orbitals, extending the length of the material, moves into resonance in this way. This process is aided by the application of a magnetic field, which reduces magnetic disorder. Thus, the ground state of these materials is metallic, as would be expected from a system with a partially filled d band. As the temperature is increased, fluctuations destroy the metallic state and the material becomes semiconducting. The largest negative magnetoresistance ratios observed in the doped manganites are found at their respective insulator-metal transition temperatures. At the transition temperature, while the resistivity has reached a maximum, the application of a magnetic field may still induce a transition to a ferromagnetic, metallic state. Indeed, relative values in resistivity F(0)/F(H) as large as 5 orders of magnitude have been observed in fields of 70 kOe. This observation has stimulated a great deal of interest in the physics and engineering communities, in part because of the possibility of using this material for read-heads in the detection of magnetically encoded information. While the effects of doping divalent cations into the LaMnO3 lattice have been studied in depth, less is known about the effects of monovalent cation substitution. The best work is on materials grown in fluxes: alkali-metal-doped materials made using solidstate techniques however do not show the expected correlation between composition and properties. Gubkin et al.9 grew single crystals of La0.9Na0.1MnO3 from a La2O3-Mn2O3-Na2ONaF-V2O5 flux at 1000 °C. The properties of this material are very similar to those of manganites with 20% alkaline earth substituted for La. In this paper, we report crystal growth in hydroxide flux at substantially lower temperatures, offering the possibility of producing phases that are thermodynamically unstable at higher temperatures. The synthesis of crystalline powders of Na-doped LaMnO3, grown at 400 °C from molten NaOH, is described in the experimental section. Experiment and Analysis Synthesis. The synthesis of Na-doped LaMnO3 begins with 10.0 g of NaOH (Fischer, 98%) in a silver crucible placed in a Pyrex reactor and heated to 400 °C at a rate of 100 °C/h. The specially designed reactor allows the temperature directly over the crucible to be monitored and the atmosphere inside the reactor to be controlled. Compressed air is bubbled through room-temperature water and passed over the crucible. The melt is allowed to equilibrate under the atmosphere of moist flowing air for 12 h, after which 0.50 g of La(OH)3 (99.9%, Aldrich) and 0.20 g of Mn2O3 (99.9%, Aldrich) (both dried at 120 °C overnight) are added to the hot melt. After the mixture has been heated at 400 °C in a moist atmosphere for 4 h, the dark blue melt is decanted, leaving a black powder on the bottom of the crucible. The crucible is allowed to cool to room temperature and the dark, crystalline product is isolated by washing away the residual melt with water. Elemental Analysis. The composition of the crystalline product was analyzed by wavelength dispersive X-ray fluores-
Magnetic and Transport Properties of LaMn0.8Na0.2O3 cence analysis using a five spectrometer Cameca SX-51 electron-beam microprobe. The intensities of the X-ray emissions from the product were compared with published standards. X-ray Diffraction. High-resolution (λ ) 0.790 01 Å) synchrotron X-ray powder diffraction data were collected for the sample on the X7A beamline of the National Synchrotron Light Source at Brookhaven National Laboratory. The high signalto-noise ratio and resolution of the X7A diffractometer allow identification of sample inhomogeneities and impurity phases that could not be detected using a conventional X-ray or neutron diffractometer. Room-temperature data were collected on 0.2 g of sample loaded in a glass capillary with a step size of 0.01° and a step time of 1 s. A least-squares fit of the diffraction pattern was performed using the General Structural Analysis System (GSAS). Magnetic Susceptibility. Magnetization data were obtained with a Quantum Design SQUID magnetometer. Samples were loaded into a Kel-F (poly(chlorotrifluoroethylene)) container. Temperature measurements of susceptibility were made at 100 Oe and 10 kOe. The signal of the empty container was subtracted from the experimental data. For the low-field measurements, the magnitude of the field was verified using a HgCo(SCN)4 standard. Microwave Conductivity. Conductivity measurements were made by loading a quartz tube with 0.150 g of sample and placing it at the center of a rectangular EPR cavity. The TE103 mode of the cavity was excited using a Hewlett-Packard 8920 vector network analyzer. The network analyzer generates microwaves at constant output power (0.1 W for this experiment) and at discrete frequencies in a given range and records the magnitude and phase of the reflected signal at each frequency. Microwaves propagate from the network analyzer to the cavity via a coaxial cable coupled to a waveguide that excites the cavity through an adjustable iris. The iris is set so that the cavity with sample is nearly impedance-matched to the rest of the microwave circuit at 5 K. The temperature of the sample was controlled to within (1 K by manually adjusting the flow of liquid He through a transfer line connected to the sample holder. The magnetic field dependence of the sample conductivity was investigated with a solenoidal electromagnet that generates fields up to 10 kOe. A full description of the analysis of microwave conductivity will be published elsewhere. Briefly, the quality factor Q and resonance frequency of the filled cavity as well as the power reflection coefficient are determined at each temperature and applied magnetic field. The intrinsic Q and frequency-shift of the sample are calculated from these quantities. Finally, the sample κF product is obtained with κ the relative permittivity of the sample and F the microwave resistivity. For isolated particles, F is the single-particle, barrier-free conductivity. Results Synthesis of Na-doped LaMnO3 was performed by precipitating crystalline powders from molten NaOH.10 Precipitation of crystalline metal oxides from molten hydroxides has been shown to be an effective way of generating alkali-metal-doped materials.11 This technique often leads to highly crystalline, homogeneous alkali-metal-containing products. In contrast, conventional solid-state synthesis techniques require higher temperatures, often leading to alkali metal evaporation and low-quality products.12 Equilibration of the melt and predrying of the reactants overnight is necessary to reach and maintain equilibrium
J. Phys. Chem. B, Vol. 104, No. 7, 2000 1449
Figure 3. Powder X-ray diffraction pattern of LaMn0.8Na0.2O3. The main peaks in the pattern may be indexed on the basis of a cubic unit cell with lattice spacing 3.906 Å.
conditions throughout the reaction. Shorter equilibration times or addition of reactants containing high levels of residual moisture leads to multiphase products, as evidenced by synchotron X-ray diffraction results.13 The deep blue color of the NaOH solution after the addition of Mn2O3 suggests the presence of MnO43-. The existence of this highly oxidized Mn species is supported by Raman spectroscopy.14 The mechanism by which the Mn is reduced from the formal oxidation state +5 to a mixed valent +3/+4 oxidation state in the product that precipitates is not known. However, it appears that by changing the reaction conditions (e.g., temperature, partial pressure of water over the melt, other alkali metals in solution, etc.), it is possible to vary the formal oxidation state of Mn in the product. Although the composition changes are too small (less than 4% Mn3+) to be quantified with powder X-ray diffraction, it is possible to correlate changes in the measured Curie constants with changes in composition.13 Wavelength dispersive X-ray fluorescence analyses indicate that the composition of the product is La1.00(5)Mn0.80(2)Na0.20(2)O3.0(1), referred to as LaMn0.8Na0.2O3. This composition is consistent with Na substituting for Mn. At a doping level of 20%, the formal Mn oxidation state is Mn3.5+ (50% Mn3+ and 50% Mn4+). The synchrotron X-ray diffraction pattern of LaMn0.8Na0.2O3 is shown in Figure 3. The peaks are sharp and intense, indicating that the product is crystalline. The low baseline levels are consistent with low levels of residual amorphous materials from the melt. The most intense peaks may be indexed to a perovskite structure-type with a pseudocubic lattice parameter of 3.906 Å. The structure of the material is assigned to space group Pnma, a common space group for the doped manganites.15 Several peaks with low intensity were also observed. Some of these peaks are attributed to La(OH)3 and Ag impurities; others may be indexed on the basis of an expanded perovskite unit cell with orthorhombic symmetry. A least-squares fit of the diffraction pattern was performed using the General Structural Analysis System (GSAS).16 The result is in agreement with the space group assignment and composition obtained previously. Because most of the electron density is associated with La, the X-ray diffraction pattern is dominated by scattering from this ion. The diffraction pattern appears to be that of an almost perfectly cubic material (with no supercell peaks or splittings in the main peaks), indicating that La atoms form a nearly cubic array. Because the scattering from La is much greater than that from Na, Mn, and O, the refinement is somewhat insensitive to the exact positions and
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Figure 4. Molar magnetic susceptibility vs temperature for LaMn0.8Na0.2O3 in a field of 10 kOe. The susceptibility slowly increases as the temperature is decreased, but the magnetization does not appear to saturate, even at the lowest temperatures.
site occupancies for these atoms. However, the best results were obtained for compositions close to LaMn0.8Na0.2O3. All attempts to place Na on the La site resulted in a worse or even divergent fit to the data. Further evidence for Na on Mn sites comes from comparing the pseudocubic lattice parameter of LaMn0.8Na0.2O3 (3.906 Å) with that of LaMnO3 (3.844 Å).17 While an increase in the lattice parameter is expected for substitution of Na onto the Mn site, a decrease is expected for substitution onto the La site. An estimate of the unit cell size using the ionic radii of Shannon18 for a material with Na substituted onto an octahedral site yields a lattice parameter close to that observed. Thus, we conclude that Na dopes almost exclusively onto Mn sites. This is in contrast with Na-doped manganites prepared at much higher temperatures.9 The magnetic susceptibility of a sample of LaMn0.8Na0.2O3 as a function of temperature in a field of 10.0 kOe is shown in Figure 4. The susceptibility displays non-Curie-Weiss-law behavior as the temperature is lowered below 150 K. The susceptibility increases continuously but does not appear to reach a plateau as would a classic ferromagnet. The low-field susceptibility provides further evidence that the transition in LaMn0.8Na0.2O3 is not that of a classic ferromagnetic. The reciprocal susceptibility, a more sensitive indicator of magnetic behavior, is shown in Figure 5 as a function of temperature in fields of 100 Oe and 10.0 kOe. Extrapolation of the linear portion of the high-temperature data to zero reciprocal susceptibility gives a Curie-Weiss temperature of 135.0 K. However, the low-field susceptibility deviates from the high-field susceptibility near 300 K. The onset of field dependence in the magnetic susceptibility at 300 K marks the critical temperature for the magnetic transition. Plots of resistivity vs temperature in fields of 0 and 10.0 kOe are shown in Figure 6. As the temperature is lowered from 295 to 100 K, the resistivity increases, consistent with charge transport through thermally activated processes and a negative thermal coefficient of resistivity. No magnetic field dependence is observed in this temperature range. As the temperature is decreased below 100 K, the resistivity decreases. This decrease is consistent with the delocalization of carriers: the resistivity is determined by carrier-scattering, which decreases as the temperature is lowered. The application of a 10 kOe field decreases the resistivity below 100 K by reducing magnetic disorder. Negative magnetoresistance of around 10% is observed at the lowest temperatures.
Washburn et al.
Figure 5. Reciprocal susceptibility of LaMn0.8Na0.2O3 as a function of temperature in fields of 100 Oe and 10 kOe. The Curie-Weiss temperature, taken to be the temperature at which the high temperature reciprocal susceptibility extrapolates to zero reciprocal susceptibility, is 135 K. This is considerably lower than Tc equal to 300 K, the temperature at which the onset of field dependence in the susceptibility is observed.
Figure 6. Microwave resistivity vs temperature for LaMn0.8Na0.2O3 in fields of 0 and 10 kOe. The resistivity decreases monotonically as a function of temperature. A sudden decrease in the temperature dependence of the resistivity is observed in both fields near 150 K is observed, consistent with the occurrence of an insulator-metal transition. The resistivity in 10 kOe field is roughly 10% lower than in zero field, indicating the field quenches magnetic disorder which increases carrier mobilities.
Discussion Structural Characterization. From the refinement of the X-ray diffraction data as well as elemental analysis data, it is concluded that Na substitutes almost exclusively for Mn. This result is unexpected for a number of reasons. The ionic volume of 6-coordinate Na in oxide materials is more than twice that of Mn,18 and this substitution likely induces large strains in the lattice in order to fit this larger cation in the octahedral site. These strains could be manifested in a wide range of M-O bond lengths and M-O-M bond angles, where M is the M site cation, Mn or Na. The powder diffraction pattern gives average quantities and the actual material is probably much less ordered than the data of Figure 3 suggest. Further, it is surprising that Na, which exhibits nearly pure ionic bonding with oxygen, would substitute for Mn, which has very strong covalent interactions with oxygen as evidenced by the rich magnetic and electrical properties of the manganites. While it is not possible to explain definitively why LaMnO3 precipitated from molten NaOH incorporates Na on the Mn sites,
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Figure 7. Magnetization of a thin film sample of La0.67Ca0.33MnO3 vs temperature in a field of 50 Oe (after Snyder et al.20). A transition to a ferromagnetically ordered state occurs at 270 K.
there are reasons why this site might be preferred. The oxygen activity in molten hydroxides is very high and phases with metal ions in high formal oxidation states are often obtained.11 For the same amount of Na doping, the formal oxidation state of the Mn is higher if Na substitutes for Mn than it is if Na substitutes for La. Furthermore, although the ionic volume of Na is closer to that of La, the presence of a low-valent ion on the 12-coordinate La site might lead to a larger increase in the Coulomb energy of the lattice relative to substitution of the Na ions onto the 6-coordinate Mn site. The random placement of the large, inert Na+ cation on the perovskite M site likely leads to strong structural disorder in the material. Since the properties of the doped manganites depend sensitively on disorder, the magnetic and transport data need to be discussed from this point of view. Magnetic Characterization. The magnetization of a thin film of La0.67Ca0.33MnO3 as a function of temperature in a field of 50 Oe is shown in Figure 7.20 The material undergoes a transition from a paramagnetic state to a ferromagnetically ordered state at Tc ) 270 K. Neutron diffraction data indicate only ferromagnetic scattering at this doping level,19 but as the concentration of Mn4+ in the lattice is varied away from 33%, evidence of antiferromagnetic interactions is obtained.18 A similar transition is observed in La0.67Sr0.33MnO3 with a critical temperature of 376 K.21 The large increase in Tc is thought to result from the larger Sr2+ cation filling the center of the unit cell more effectively with a resulting Mn-O-Mn bond angle of 163°, as compared with 156° in La0.67Ca0.33MnO3. The difference between the critical temperature Tc and the Curie-Weiss temperature θ has been interpreted as evidence of competing ferromagnetic and antiferromagnetic interactions in the doped manganites.10 Evidence for mixed ferromagnetic and antiferromagnetic interactions was obtained in the neutron diffraction studies of Wollan and Koehler19 for La0.5Ca0.5MnO3, which has the same average Mn formal oxidation state of +3.5. The data for LaMn0.8Na0.2O3 were modeled with the magnetic sublattices displaying a ferromagnetic intrasublattice coupling parameter and an antiferromagnetic intersublattice coupling parameter µ. The mean field expressions for the exchange field BE on the A and B sublattices due to the magnetizations MA and MB are
BE,A ) µ0MA - µµ0MB ) (T/C)µ0MA BE,B ) µ0MB - µµ0MA ) (T/C)µ0MB
(1)
where C/T is the Curie susceptibility and µ0 is the Bohr magneton.
Figure 8. Resistivity of a thin film sample of La0.67Ca0.33MnO3 vs temperature in fields of 0 and 70 kOe (after Snyder et al.20). In zero applied field a transition to a metallic state occurs at 270 K, but application of a 70 kOe field increases the insulator-metal transition temperature by nearly 50 K.
These equations may be solved to yield expressions for the Curie-Weiss and critical temperatures:
Θ ) ( - µ)C
Tc ) ( + µ)C
(2)
The higher of the two temperatures Tc represents the condensation energy of the magnetically ordered phase while the lower temperature represents the competition between ferromagnetic and antiferromagnetic interactions on each sublattice. Using the data of Figure 5, values of C ) 215 K and µC ) 85 K are calculated for the ferromagnetic and antiferromagnetic coupling strengths, respectively. The ferromagnetic coupling parameter is more than twice that of the antiferromagnetic coupling parameter µ. The mean field description of this material is certainly an approximation but it was found that it could account for the linear dependence of the magnetization on applied field and gave a value of µC based on the moment extrapolated to zero field that agreed with that obtained from the susceptibility data.10 Thus, this approach appears to be a reasonable one in estimating the relative strengths of the competing magnetic interactions in LaMn0.8Na0.2O3. Microwave Conductivity. LaMnO3 and CaMnO3 become insulating at their respective magnetic transition temperatures.4 This is a result of the antiferromagnetic spin configuration that forms when superexchange is the mechanism for magnetic ordering. However, materials containing between 20 and 50% Mn4+ display transitions to metallic states at temperatures slightly below the ferromagnetic transition temperature. As an example, the resistivity of La0.67Ca0.33MnO3 as a function of temperature in fields of 0 Oe and 70 kOe is shown in Figure 8.20 As the temperature is lowered to 270 K in zero applied field, the resistivity increases, consistent with semiconductor behavior and thermally activated transport. Just below 270 K, the resistivity in zero field drops sharply. The resistivity continues to decrease as the temperature is lowered to 5 K, consistent with metallic behavior. Carriers are delocalized with resistivity determined by the scattering of carriers by lattice vibrational modes (phonons) and by disorder in both the lattice (through defects) and in the magnetic moments of the Mn ions. High-Temperature ConductiVity. The carriers in the high temperature regime are thought to be small-polarons, localized electrons that induce distortion in the surrounding lattice and effectively drag this distortion with them as they move. This
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Washburn et al. the magnetization was developed by Hundley et al.25 in their investigation of La0.7Ca0.3MnO3:
F(H,T) ) const + F0 exp[-M(H,T)/M0]
Figure 9. (a) Fit of the high-temperature microwave resistivity of LaMn0.8Na0.2O3 to the small polaron form of the resistivity, κF ∼ exp(-EF/kBT)/T. The activation energy is calculated to be 18.1 meV, considerably smaller than that observed in thin film samples of LCMO. (b) Fit of the low-temperature microwave resistivity of LaMn0.8Na0.2O3 to the functional dependence on the sample magnetization noted by Hundley et al.,25 F ∼ exp(-M/M0).
drag leads to a temperature-dependent resistivity of the form22,23
F)
( )
c(1 - c)e2T0 EF exp paT kBT
(3)
where a is the lattice-spacing, T is the temperature, c is the concentration of electrons and (1 - c) is the concentration of holes, kBT0 is the energy of an optical phonon, and EF is an activation energy. From the change in sign of the temperature dependence of the resistivity, it appears that LaMn0.8Na0.2O3 undergoes an insulator-metal transition near 100 K. Above 100 K, the temperature dependence of the resistivity has the same smallpolaron form as for the alkaline-earth-doped manganites, given by eq 3. Plotting the natural logarithm of the directly measured quantity κFT vs 1/T yields from the slope an activation energy of 18.1 meV, as shown in Figure 9a. (Inclusion of the relative permittivity κ adds merely a constant.) This activation energy is significantly lower than the value of 121 meV, measured by Jaime et al.24 on thin films of La2/3Ca1/3MnO3. An increased activation energy at low frequency may arise from grain boundaries. As mentioned above, grain boundaries are not expected to increase the resistivity measured at microwave frequencies because they may carry displacement current. Low-Temperature ConductiVity. An empirical relationship between the resistivity in the ferromagnetic-metallic state and
(4)
where F0 is a constant preexponential factor, M(H,T) is the magnetization of the sample in field H at temperature T, and M0 is the saturation magnetization. This relationship, even though phenomenological, is observed to hold quite generally. The resistivity in a field of 50 kOe displays the same qualitative behavior as in zero field although the insulatormetal transition occurs at a significantly higher temperature, around 320 K. The paramagnetic-ferromagnetic transition temperature also shows a similar increase in the presence of a strong magnetic field. Furthermore, the resistivity of the low temperature state is 7% lower than in the absence of a magnetic field. The nature of the ferromagnetic-metallic state was explained by Zener5 in his theory of double exchange. The doped manganites such as La0.67Ca0.33MnO3 are mixed valence materials, with manganese formally assuming oxidation states of +3 (high spin 3d,4 a strong Jahn-Teller ion) and +4 (3d3) in roughly octahedral coordination by oxygen atoms. Zener envisioned the following situation: the Mn electrons occupying t2g orbitals remain fixed while the lone eg electron is free to move from Mn3+ to Mn4+ provided the orbital energy levels are close and the magnetic moments of the Mn ions are aligned. Double exchange is the transfer of the Mn3+ dx2-y2 electron to the bridging O2-, which drives a spin-aligned electron from the O2- to the empty Mn4+ dx2-y2 orbital. The resistivity of LaMn0.8Na0.2O3 below 100 K appears to be a function of the magnetization of the sample and follows the functional relationship of eq 4 as noted by Hundley et al. A plot of the permittivity-resistivity product κF vs exp(-M/M0) is well fit by a straight line, as shown in Figure 9b. (Again, we choose to use the directly measured quantity κF, which produces only a change of scale.) This fit indicates that magnetic disorder limits the conductivity of the sample, as appears to be the case in the alkaline-earth-doped manganites. At the lowest temperatures, application of a 10 kOe field increases the conductivity by 10%, roughly twice the increase observed for the alkaline-earth-doped manganites. However, the magnetoresistance ratio at the transition is very small, of the order of 1%, which is much less than observed for the CMR materials. This difference is attributed to the strong magnetic and structural disorder induced by the presence of high concentrations of Na on the Mn sites. From modeling the magnetic data, it was concluded that at 50% Mn3+ and 50% Mn4+ there exist strong competing magnetic interactions. This is manifested as magnetic disorder that serves to scatter carriers in the metallic state. Application of a 10 kOe field reduces the disorder and leads to an increase in conductivity. The fact that this increase appears to be larger than in critically doped CMR materials suggests that the magnetic disorder in LaMn0.8Na0.2O3 is larger than in the CMR materials. The reduced magnetoresistance ratio observed at the metalinsulator transition is attributed to strong disorder with LaMn0.8Na0.2O3 inhomogeneous over short distances. This disorder broadens the insulator-metal transition, reducing the magnetoresistance observed at the transition. Finally, the insulator-metal transition is observed well below both the critical temperature (300 K) and the Curie-Weiss temperature (135 K). This is also attributed to the strong disorder present in LaMn0.8Na0.2O3. Delocalization of electrons at an Anderson transition requires a reduction in disorder. In the
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alkaline-earth-doped manganites, ferromagnetic ordering removes virtually all magnetic disorder and the insulator-metal transition occurs just below Tc.7 In LaMn0.8Na0.2O3, the “magnetically ordered” state is still highly disordered due to competing magnetic interactions and random substitution of Na ions on the Mn site. The temperature must be lowered well below Tc in order to freeze out fluctuations in the magnetic moments and allow the carriers to delocalize.
magnetoresistance of the order of 10% is observed in the metallic state, roughly twice that observed in the CMR materials. This observation is also an indication that magnetic disorder is present in the metallic state of the Na-doped manganites. Although the same principles developed for understanding the alkaline-earth-doped manganites appear to apply to LaMn0.8Na0.2O3, structural and magnetic disorder crucially affect the properties of the Na-doped analogue.
Conclusions
Acknowledgment. This work was supported by the National Science Foundation under Grant No. 9102492, and by the Director, Office of Energy Research, Office of Basic Energy Sciences, Materials Science Division of the U.S. Department of Energy. The authors thank Mr. John Donovan of the UC Berkeley Department of Geology and Geophysics for collecting the wavelength dispersive X-ray fluorescence data, Dr. Patrick Woodward for collecting the synchrotron data, and the HewlettPackard Corp. for their generous loan of the vector network analyzer. N.R.W. thanks Dr. Jeff Yarger, Dr. M. P. Klein, and the members of the Klein research group for assistance with the microwave conductivity measurements.
Precipitation of Na-doped LaMnO3 from molten NaOH leads to material with Na substituted for Mn, not for La as expected. The product is crystalline but appears to be structurally disordered around the Mn site due to the substitution of the large Na ion. The product also appears to be magnetically disordered, due both to the random substitution of Na in the lattice as well as the high doping levels, which formally results in a material with 50% Mn3+ and 50% Mn4+. It was noted in the original neutron diffraction studies of the manganites19 that materials with this composition display competing ferromagnetic and antiferromagnetic interactions; the magnetic data reported here suggest that this is the case for LaMn0.8Na0.2O3 as well. The difference between the critical temperature Tc and the Curie-Weiss temperature Θ suggests that competing magnetic interactions are in fact present in the material. A two-sublattice mean-field model of the magnetic interactions in LaMn0.8Na0.2O3 leads to the conclusion that intrasublattice ferromagnetic interactions are more than twice as strong as intersublattice antiferromagnetic interactions. Microwave resistivity studies show that LaMn0.8Na0.2O3 particles display many of the same transport phenomena as the alkaline-earth-doped manganites. The temperature dependence in the insulating regime is consistent with carriers of polaronic character. The activation energy of the polaronic microwave conductivity here is less than one-sixth the activation energy of the polaronic dc conductivity in the alkaline-earth-doped manganites. A plausible source of the difference is the presence of grain boundaries, which increase the activation energy at dc but not at microwave frequencies where activation-free displacement currents readily cross grain boundaries. It appears that LaMn0.8Na0.2O3 undergoes an insulator-metal transition near 100 K, although the transition is much broader than that observed in the alkaline-earth-doped manganites. The breadth of the transition may be attributed to randomness in Na doping, which creates inhomogeneity on a microscopic scale and leads to a broad range of local transition temperatures. While the existence of strong disorder and competing magnetic interactions in LaMn0.8Na0.2O3 suggests the material might undergo a transition to a spin glass state26 at low temperatures, the mobility of the Mn eg electrons appears to inhibit such a transition. The low-temperature conductivity scales with the magnetization of the sample, following the same functional form as observed by Hundley et al. for La0.7Ca0.3MnO3. It appears that magnetic disorder limits the conductivity in LaMn0.8Na0.2O3 just as it does in the alkaline-earth-doped manganites. Negative
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