The electronic configuration, ionic radii, Mn-O bond length and Mn-O-Mn bond angle at room temperature, and TC for La0.7Sr0.3Mn1-xMxO3 (x=0.1, M = Ti, Zr, Zn, Cr, Fe, Co, Ni, and Cu).
Abstract
In this chapter, in order to understand the structural related magnetic and transport properties of B site substituted perovskites La0.7Sr0.3MnO3 (LSMO), we have systematically investigated the effects of replacing some of the Mn with nonmagnetic elements Ti, Zr, Cu, Al, Zn and magnetic elements Co, Ni, Cr, Fe. The structural, magnetic and electrical phase transitions and transport properties of these compounds were investigated by neutron diffraction, magnetization and electric resistivity measurements.
Keywords
- Pervoskite
- crystal structure
- magnetic structure
- neutron diffraction
- magnetic properties
1. Introduction
Perovskite oxides have been an interesting research area for scientists due to their promising physical properties including colossal magnetoresistance (CMR), superconductivity, multiferroelectricity, metal-insulator transition (MIT), charge/orbital ordering, etc. Among various perovskite oxides, the manganite is a representative one with fascinating physical properties [37]. The manganite materials such as La1-xSrxMnO3, Nd1-xSrxMnO3, and Pr1-xCaxMnO3 exhibit rich phase diagram involving spin-charge-orbital ordering, canted antiferromagnetic, antiferromagnetic/ferromagnetic ordering, and electronic phase separation [14, 17, 27, 39]. In addition to the abundant magnetic behavior, the MIT often occurs coincidently with structural or magnetic transition [17]. Pure LaMnO3 is an A-type antiferromagnetic insulator. La3+ can be partially substituted by a divalent cation such as Sr2+ or Ca2+, and La1-x(Sr,Ca)xMnO3 can become a metallic and ferromagnetic material. The Mn ions are in Mn3+ and Mn4+ states, which both have a local spin (S=3/2) from their
In the past decades, the A-site doped manganites A1-xDxMnO3 have been extensively studied with various attractive properties [14, 17, 27, 29, 33, 39]. In contrast, the B-site doped manganites have not been well studied. The substitution for the Mn (B site) has shown dramatic effect on the magnetic and transport properties of the perovskites [1, 2, 5, 6, 36, 38]. Generally, the B-site doping with 3d ions would destroy the ferromagnetic ordering of the Mn network, leading to the changes in the magnetic and electrical properties of manganites. The reentrant spin glass behavior has been observed in the Cr-doped A-type antiferromagnetic La0.46Sr0.54Mn1-xCrxO3 due to the competing interaction between the FM and the A-type AFM coupling. The charge-orbital ordered Nd0.5Ca0.5Mn1-xCrxO3 is a relaxor ferromagnet [26]. The Fe-doped La1-xCaxMnO3 has gone through the localization-delocalization transition as the increase of the dopant concentration [1, 34]. Two ferromagnetic phases appeared in the LaMn0.5Ni0.5O3 sample, which is critically related to the preparation process [18].
Therefore, in this chapter, the samples of La0.7Sr0.3Mn1-xTxO3 (T= Ti, Zr, Cu, Co and Cr) were prepared, and the effects of substitution on Mn were studied using neutron diffraction(ND), X-ray photoelectron spectra (XPS), magnetic and electric resistivity measurements. The relationship between structure and physical properties are explained by the competition between the DE and super exchange interactions, bandwidth W, bond angle, bond length and the frustration of spins and the change of valence states.
2. Experiments
Samples of La0.7Sr0.3Mn1-xTxO3 (T=Ti, Zr, Cu, Co, Cr) were synthesized using the standard solid-state reaction method, starting with the high purity La2O3, MnO2, TiO2, CuO, Cr2O3, Co3O4 and SrCO3 powders. Appropriate amounts of these powders were weighed and mixed according to the desired stoichiometry for each sample, then sintered in air for one day at 800℃, and cooled naturally to room temperature as the raw material. The raw materials were ground and sintered again in air for one day at 1350℃ with a room-air quench. The reacted powders were ground and cold pressed into disks with the thickness of ~2 mm under a pressure of ~10 MPa. These disks were sintered in air for one more day at 1350℃ and cooled naturally to room temperature. X-ray diffraction of the powders was performed at room temperature using with Cu-Kα radiation. Powder neutron diffraction experiments were performed at the University of Missouri-Columbia Research Reactor (MURR, λ = 1.4875Å) and high resolution powder diffractometer at HZB Germany using neutrons of wavelength (λ = 1.79821Å). The patterns were collected at the temperature range from 5K to 300K. Refinement of the XRD and ND data were carried out using the FULLPROF program. Magnetic measurements were conducted with a SQUID magnetometer (MPMS, Quantum design). The zero-field cooling (ZFC) and field cooling (FC) magnetization curves were measured under applied magnetic field of 50Oe. Magnetoresistance data were collected using a physical properties measurement system (PPMS, Quantum design) with a standard four-point probe method.
3. Results and discussion
3.1. Ti-substituted perovskites, La0.7Sr0.3Mn1-xTixO3 [22]
Ti-substituted perovskites La0.7Sr0.3Mn1-xTixO3, with 0 ≤ x ≤ 0.20 were studied using XRD, ND, magnetizatic and magnetoresistance (MR) measurements [22]. Typical ND patterns of the La0.7Sr0.3Mn1-xTixO3 (x=0.05, 0.1 and 0.2) were shown in Fig. 1. All samples show a rhombohedral structure (space group
Fig. 3 plots the average Mn-O bond length and Mn-O-Mn bond angle of La0.7Sr0.3Mn1-xTixO3 obtained from the ND refinement at 10 K and RT. At 10 K, the Mn-O bond length increases up to
The temperature dependent resistivities for La0.7Sr0.3Mn1-xTixO3 compounds (x = 0.0(a), 0.05(b), 0.10(c), and 0.15(d)) under magnetic fields H = 0, 1, 3, and 5 T were plotted in Fig. 4.
As a comparison, the Curie temperatures
where
It is obvious that a field-induced shift of the resistivity maximum occurs for x> 0.05 samples. The MR ratio increases with the Ti content x, and reaches to about 70% for La0.7Sr0.3Mn0.8Ti0.2O3, which is related to the weaker magnetic interaction between Mn-Mn ions. The separation of TC and the resistivity maximum temperature Tρ,max becomes wider as Ti content increases due to the weak coupling between the magnetic ordering and the resistivity as compared with La0.7Sr0.3MnO3.
3.2. Zr-substituted perovskites La0.7Sr0.3Mn1-xZrxO3[23]
We have tried to synthesize the La0.7Sr0.3Mn1-xZrxO3 compounds with different Zr contents. However, it was found that solubility limit of Zr is about x ~ 0.10, due to the large size (0.72 Å) of Zr4+. Fig. 5 is the ND patterns of La0.7Sr0.3Mn1-xZrxO3. It reveals that Zr goes only to the Mn-site. A single phase of La0.7Sr0.3Mn1-xZrxO3 was obtained for x≤0.1, which exhibits a rhombohedral structure from 10 K to RT. An impurity La2Zr2O7 phase was found for x>0.1 samples. The refined lattice parameters
Fig. 6 plots the average Mn-O bond length,Mn-O-Mn bond angle and bandwidth W obtained from the refined ND patterns at RT and 10K. Similar to those of the Ti substituted samples, the Mn-O bond length increases, while Mn-O-Mn bond angle and band width W decrease with the increase of Zr content. The decrease of the bandwidth W will reduce the overlap between the O-2p and the Mn-3d orbitals, which will reduce the exchange interactions between Mn-Mn in this system. This is confirmed by the results that the reduction in magnetic moments and the Curie temperature with increased Zr content. A metallic-like behavior was observed for the La0.7Sr0.3Mn1-xZrxO3 at low temperature. The contribution from the two-magnon scattering in resistivity becomes larger with increasing Zr content.
3.3. Cu-substituted perovskites La0.7Sr0.3Mn1-xCuxO3 [24, 25]
La0.7Sr0.3Mn1-xCuxO3 samples with 0
Fig. 7 plots the Mn-O bond length and Mn-O-Mn bond angle obtained from the ND data. The changes of the Mn-O bond length vs. Cu content show similar trend corresponding to the changes of lattice parameters at 10 K and RT. The average bond angle increases and reaches to a maximum at x = 0.10, then slightly decreases for x > 0.1 at 10K an RT. The bandwidth
X-ray photoelectron spectra (XPS) was used to determine the Cu valence states in these compounds. Fig. 8 plots the mole percent of different Cu ions in the Mn sites obtained by fitting the XPS data. The Two kinds of Cu ions (Cu3+ and Cu2+) were observed when x > 0.1, unlike the other metal substituted systems. The binding energies of both Cu2+ and Cu3+ states shift to the lower BE energy region, which suggests a strong hybridization between the Cu-
Fig. 9 plots the Curie temperatures for the Cu substituted samples. The
The temperature dependence of resistivity under various applied fields was measured using PPMS and shown in Fig. 10. With increasing Cu content, the resistivity of the compound increases, while the resistivity decreases with increasing magnetic field. This is ascribed to a reduction of the Mn3+/Mn4+ ratio to account for the DE interaction and a reduction in the number of hopping electrons and hopping sites by Cu substitution. The resistivity shows a metal-like behavior with decreasing temperature when x is less than 0.10 samples. A MIT occurs for the x ≥ 0.15 samples (Fig. 10). A resistivity peak corresponding to the magnetic transition is present. The suppression of the resistivity by the applied magnetic field occurs over the entire temperature range for all samples. The highest MR ratio of about 80% was obtained for x = 0.15 sample, which might result from the co-existence of Cu3+/ Cu2+ and the dilution effect of Cu-doping on the double exchange interaction [25].
3.4. Co-substituted La0.7Sr0.3Mn1-xCoxO3
Typical ND patterns for the La0.7Sr0.3Mn1-xCoxO3 samples at RT are shown in Figure 11. It is obvious that the peak intensity of (012) decreases and those of (110) and (104) increase with the increase of the cobalt content x, which is due to the different scattering lengths of Mn and Co ions. Since these peaks are related to magnetic scattering, the changes correspond to the decrease of the magnetic contribution. A Rietveld refinement for all the polycrystalline samples was carried out to understand the detailed crystal and magnetic properties. Figure 12 displays the refined lattice parameters for the La0.7Sr0.3Mn1-xCoxO3 samples at RT. It shows that the lattice parameters and unit cell volume decrease with increasing the Co content due to the fact that the radius of Co ions (0.55 Å for Co3+ and 0.40 Å for Co4+) is smaller than that of Mn ions (0.65 Å for Mn3+ and 0.53 Å for Mn4+). Generally speaking, the DE interaction strength among Mn3+ and Mn4+ can be estimated using the transfer integral, t∝cos (θ/2) and thus strongly depends on the Mn-O-Mn bond angle. The changes in θ value also have strong influence on the effective bandwidth
The temperature dependent ND patterns of La0.7Sr0.3Mn1-xCoxO3 (x=0.4, 1.0) samples are shown in Figure 13(a). The representative Bragg reflections of neutron diffraction prior to and with the addition of magnetic phase are shown in Figure 13(b). The misfits indicate the magnetic contributions. It is obvious that the (012) reflection has both nuclear and magnetic intensities and the (104), (110) reflections show little magnetic intensity for x=0.4. However, for x=1.0 sample, the (012) peak has magnetic intensity only and (104), (110) has both nuclear and magnetic intensities. There is almost no change of the intensity for x=0.5 and x=0.6 samples whether to add magnetic phase or not. The intensity of the magnetic peak (012) for LSCO decreases with the increase of temperature until vanishes finally, which is similar to LSMO-Co0.4 (i.e. the magnetic peaks (104), (110)).
The Curie temperature (TC), the coercivity (iHC), the magnetization and the resistivity of La0.7Sr0.3Mn1-xCoxO3 are shown in Figure 1. The critical Co-doping contents for the onset/disappearance of the glassy behavior are x=0.3 and 0.8. The iHC and resistivity show a maximum value, while the TC and magnetization show a minimum value at the critical Co-doping point (x=0.3, 0.8). The ferromagnetic ordered La0.7Sr0.3MnO3 gradually turns into disordered glassy system by the B site Co-doping, which is attributed to the break of the double exchange interaction between Mn-Mn ions and random substitution of the Mn ions. At the intermediate Co-doping region, the ferromagnetic ordered clusters embed in the antiferromagnetic ordering cluster matrix, forming the superparamagnetic-like free spin and reentrant spin glassy states. The resistivity increases due to the break of the double exchange interaction and the phase separation. And the destruction of the long range exchange coupling leads to the decrease of the Curie temperature. The sparsely alignment of the cluster makes the decrease of the magnetization
The typical hysteresis loops at various temperatures for La0.7Sr0.3Mn0.5Co0.5O3 were plotted in figure 15. This is a representative one for the intermediate Co-doping samples (0.2<x<0.8), which is consistent with the simple cluster model (where the Co3+-Co4+ or Mn3+-Mn4+ ferromagnetic clusters exist in the Co3+-Co3+(Mn3+) or Co4+-Co4+(Mn4+) antiferromagnetic cluster). It should be noted that the hysteresis loop at 5K shows a jump at the vicinity of 0 T. The jump disappears when the temperature is just above the Curie temperature. This suggests that the observed phenomenon is related to the competition between the ferromagnetic double exchange interaction and the antiferromagnetic superexchange interaction. The iHC is are significantly enhanced due to the freezing and pinning of the domain wall. The hysteresis loops do not show saturation under a magnetic field of 5T, which is consistent with the cluster glass behaviour. It is proposed that under a high magnetic field, the ferromagnetic clusters and partial antiferromagnetic clusters are forced to align along the direction of magnetic field. But the magnetization could not be saturated on account of the existence of antiferromagnetic clusters, so partial soft ferromagnetic clusters with small coercivity are demagnetized easily around H=0. As a result, a sudden drop of the magnetization has occurred, giving rise to a jump in the hysteresis curve.
3.5. Cr-doped La0.7Sr0.3Mn1-xCrxO3 [7, 8, 9]
The La0.7Sr0.3Mn1-xCrxO3 (0<x<0.6) have been prepared and influence of the Cr3+ substitution for Mn3+ was investigated [7, 8, 9]. Figure 14 is the ND patterns of La0.7Sr0.3Mn1-xCrxO3 (0<x<0.5) at 10 K. The magnetic contributions to the (012) and (110)+(104) peaks are evident. For x<0.2, the samples are simple ferromagnetic with magnetic moments decreasing with increasing Cr content. For 0.2≤x≤0.4, the (104) + (110) reflections become weaker, but two new peaks ((003) + (011)), inconsistent with a simple ferromagnetic solution emerge. The ((003) + (011)) peak is purely magnetic, while the (113) peak has nuclear and magnetic components. For x=0.5, the (104) + (110) reflections are present but weak, while the (003) + (011) reflections are now dominant. A single, homogeneous, long-range magnetically ordered state with compositionally-dependent charge ordering was proposed to fit the ND patterns [8, 9]. The magnetic structures are related to the competition between Mn-Mn, Mn-Cr and Cr-Cr interactions (double-exchange and superexchange). The metal to semi-metal and semi-metal to insulator transitions can be quantitatively described as due to the localization effect of superexchange. The presence of charge ordered states above the M-I transition concentration (x) arises from the favourable energetics of Mn4+-O-Cr3+ superexchange bonds relative to Mn3+-O-Cr3+ bonds.
At low temperature with small x, the net ferromagnetic behavior of the system is due to the large quantities of Mn3+-O-Mn4+ ferromagnetic double exchanges taking place while the system is being driven towards a layered ferromagnetic structure by the antiferromagnetic Cr3+-O-Mn4+ superexchanges. In the intermediate region (0.2<x<0.4) charge ordering creates a layered structure and the antiferromagnetic Cr3+-O-Mn4+ superexchange continues to drive the system towards an antiferromagnetic state. As x>0.4, the antiferromagnetic Cr3+-O-Cr3+ and Cr3+-O-Mn4+ superexchange mechanisms become dominate, with charge order persisting, producing a ferrimagnetic structure in lieu of an antiferromagnetic one.
Figure 17 displays the refined lattice parameters, unit cell volume, bond length
4. Conclusion
The B-site substituted LSMOs can be divided into following two groups, (1) those made with the replacement of Mn by other 3d transition metal ions and (2) those made with the replacement of Mn by non-magnetic, closed shell, metal ions such as Ti, Zr. The ionic radii of the substituted elements, the Mn-O-Mn bond angles, the Mn-O bond length, the calculated bandwidths W, and the corresponding TC’s are given in Table I. It should be pointed out that neutron diffraction scattering lengths of the 3d elements are sufficiently different, and uniquely, the scattering length of Mn is negative. This allows relatively small amounts of other elements substituted into the manganites to be accurately located in the unit cell structure by employing neutron diffraction.
|
|
|
Mn-O bond length (Å) |
|
TC (K) (x=0.1) |
Ti4+ | [Ar] | 0.605 | 1.9621 | 165.142 | 231 |
Zr4+ | [Kr] | 0.72 | 1.9566 | 166.031 | 318 |
Zn2+ | [Ar] | 0.74 | 1.9538 | 166.41 | 340 |
Cu2+/Cu3+ | [Ar]3d9/3d8 | 0.73/0.54 | 1.9507 | 166.733 | 300 |
Fe3+/Fe4+ | [Ar]3d5/3d4 | 0.645/0.585 | 1.9568 | 166.179 | 310 |
Cr3+/Cr4+ | [Ar]3d3/3d2 | 0.615/0.55 | 1.9507 | 166.896 | 327 |
Co3+/Co4+ | [Ar]3d6/3d5 | 0.61/0.53 | 1.9515 | 166.591 | 324 |
Ni2+/Ni3+ | [Ar]3d8/3d7 | 0.69/0.6 | 1.9506 | 166.986 | 289 |
Mn3+/Mn4+ | [Ar]3d4/3d3 | 0.64/0.53 | 1.9451 | 166.03 | 370 |
4.1. Substitution the Mn-site by 3d-transition metals (TM); Cr, Fe, Co, Ni, Cu
For the TM-substituted LSMOs, they show the same crystal structure with space group
4.2. Substitution of the Mn-site by closed shell ions: Ti, Zr or Zn
There are several advantages in using closed shell ions to investigate metal substituted-LSMOs. First, the closed shell ions normally do not affect the magnetic interactions between the Mn ions due to their having no magnetic moment. Second, they have inert gas configurations, and therefore do not contribute to the electron charge density. But there still remains the possibility of secondary effects such as a disturbance of the magnetic ordering and a redistribution of electron charge density by a large ionic size mismatch at the B-site, as with, e.g. Zr4+. In general a decrease in TC and MS with increasing substitution is observed for the closed shell ion-substituted LSMOs. This is attributed to the dilution of the magnetic ions and the weakening of the ferromagnetic DE interaction between them. Substitution with Ti is selective because Ti4+ substitutes for Mn4+. It would be expected as well that Zr4+ or Zn2+ would substitute for Mn4+. However the severe ionic size mismatch of Zr4+and Zn2+ may not allow the substitution of Mn4+ by Zr4+ or Zn2+. Therefore mixed-valent Mn ions in LSMO are selectively diluted by partial substitution of Mn by these ions. LSMO has the highest TC when the ratio of Mn3+/Mn4+ has an optimal value of 7/3. Upon substitution of the Mn ions with closed shell ions, the ratio of Mn3+/Mn4+ deviates from the optimal value of the parent compound. For Ti and Zr substitutions, the Mn3+/Mn4+ ratio increases with increasing Ti and Zr content. It should be noted that Zn2+ replaces Mn3+ which has a larger magnetic moment than Mn4+, and the difference of the magnetic moments in the Mn-sites of the Zn-substituted LSMO and Ti- or Zr-substituted LSMO is only 0.1μB per Mn-site. In this case, the competition between DE and SE interactions is a more important control factor for predicting TC and MS. Therefore, substitution onto the Mn-site with Zn2+ produces more DE couplings and less SE couplings. In turn one would expect the Zn-substituted LSMO to have a larger MS and a higher TC than with Ti or Zr substitution, which produces less DE couplings and more SE couplings.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (Grant Nos 51371009, 50971003 and 51171001), the National Basic Research Program of China (No 2010CB833104, MOST of China), the National High Technology Research and Development Program of China (No 2011AA03A403).
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