Relation between charge ordering and local lattice disorder in manganites studied by EXAFS

We report extended X-ray absorption fine structure measurements at the Mn K absorption edge and in a wide temperature range on La_0.25Ca_0.75MnO₃ samples with a charge ordering transition temperature, T_CO of the Mn³⁺ and Mn⁴⁺ ions around 225 K. The mean Mn-O distances do not show major changes at the transition; however, our results show the presence of an anomalous local lattice disorder and distortion below T_CO, possibly determined by the breaking of the symmetries of the high temperature state, that is the splitting of the Mn-O distances.     

Structural, transport, and magnetic properties in the Ti-doped manganites LaMn1−xTixO3 (0≤x≤0.2)

The magnetism and transport properties of the samples LaMn_1−xTi_xO₃ (0≤x≤0.2) were investigated. All samples show a rhombohedral structure () at room temperature. The sample with x=0 undergoes the paramagnetic-ferromagnetic (PM-FM) transition accompanied by an insulator-metal (I-M) transition due to the oxygen excess. The doped samples show ferromagnetism and cluster behavior at low temperatures. Though no I-M transition associated with the PM-FM transition appears, the magnetoresistance (MR) effect was observed especially at low temperatures under the applied fields of 0.5 T. Due to the fact that the oxygen content in the Ti-doped samples is nearly stochiometry (3.01) and the Hall resistivity at room temperature is negative, the ferromagnetism in LaMn_1−xTi_xO₃ (0.05≤x≤0.2) is believed to be consistent with the Mn²⁺-O-Mn³⁺ double exchange (DE) mechanism. These results suggest that DE can be obtained by direct Mn-site doping.     

Structure, magnetic, and transport properties of the Co-doped manganites La0.9Te0.1Mn1−xCoxO3 (0≤x≤0.25)

The effect of Co doping at Mn-site on the structural, magnetic and electrical transport properties in electron-doped manganties La_0.9Te_0.1Mn_1−xCoxO₃ (0≤x≤0.25) has been investigated. The room temperature structural transition from rhombohedra to orthorhombic (Pbnm) symmetry is found in these samples with x≥0.20 by the Rietveld refinement of X-ray powder diffraction patterns. All samples undergo the paramagnetic-ferromagnetic (PM-FM) phase transition. The Curie temperature T_C of these samples decreases and the transition becomes broader with increasing Co-doping level. The magnetization magnitude of Co-doping samples increases at low temperatures with increasing Co-doping level for x≤0.15 and decreases with increasing Co-doping content further. The metal-insulator (M-I) transitions observed in the sample with x=0 are completely suppressed with Co doping, and the resistivity displays semiconducting behavior within the measured temperature region for these samples with x>0. All results are discussed according to the changes of the structure parameters and magnetic exchange interaction caused by Co-doping. In addition, the different effects between the Co doping and Cu doping in the Mn site for the electron-doped manganites are also discussed.     

Charge order melting and magnetic transition in Nd0.5(1+x)Ca0.5(1−x)Mn(1−x)CrxO3 system

The magnetic property of double doped manganite Nd_0.5(1+x)Ca_0.5(1−x)Mn_(1−x)Cr_xO₃ with a fixed ratio of Mn³⁺:Mn⁴⁺=1:1 has been investigated. For the undoped sample, it undergoes one transition from charge disordering to charge ordering (CO) associated with paramagnetic (PM)-antiferromagnetic (AFM) phase transition at T<250 K. The long range AFM ordering seems to form at 35 K, rather than previously reported 150 K. At low temperature, an asymmetrical M-H hysteresis loop occurs due to weak AFM coupling. For the doped samples, the substitution of Cr³⁺ for Mn³⁺ ions causes the increase of magnetization and the rise of T_c. As the Cr³⁺ concentration increases, the CO domain gradually becomes smaller and the CO melting process emerges. At low temperature, the FM superexchange interaction between Mn³⁺ and Cr³⁺ ions causes a magnetic upturn, namely, the second FM phase transition.     

Effects of correlated disorder on the magneto‐transport in colossal magnetoresistance manganites

Monte‐Carlo simulations predict that a local correlated disorder is responsible for many of the novel transport and magnetic properties of colossal magnetoresistance (CMR) materials such as manganites. One important prediction of these models is that the resistivity at the metal–insulator transition (MIT) in manganites depends strongly on the correlated quenched disorder. However, experimental confirmation has been challenging since it is difficult to control the amount of disorder in these compounds. We carried out experiments on Sm_0.55Sr_0.45MnO₃, a prototypical CMR manganite with a sharp MIT, whereby the oxygen‐related disorder is systematically enhanced by low temperature thermal activation. We observe dramatic changes in the temperature dependence of resistivity at the MIT as the amount of quenched disorder is increased, occurring in a manner that is in agreement with theoretical predictions.

     

Jahn-Teller transition and electron-phonon interaction in Cr-doped manganites Sr0.9Ce0.1Mn1−yCryO3

Cr-doped manganites Sr_0.9Ce_0.1Mn_1−yCr_yO₃ (y=0, 0.05, and 0.10) have been systematically investigated by X-ray, magnetic, transport, and elastic properties measurements. For parent compound Sr_0.9Ce_0.1MnO₃, it undergoes a metal-insulator (M-I) transition at 318 K, which is suggested to originate from a first-order structural transition accompanied by Jahn-Teller (JT) transition. With increasing Cr doping content, the JT transition temperature decreases. The Cr doping suppresses the antiferromagnetic (AFM) state and makes the system spin-glass (SG) behavior at low temperatures. In the vicinity of JT transition temperatures, the softening of Young's modulus originating from the coupling of the orbital (quadrupolar) moment of the e_g orbital of Mn³⁺ ion to the elastic strain has been observed. The anomalous Young's modulus properties imply the electron-phonon coupling due to the JT effect may play an important role in the system.     

Metastability in the resistance of polycrystalline La0.8Li0.2MnO3

Bulk polycrystalline La_0.8Li_0.2MnO₃ is found to switch between a low‐resistance state and a high‐resistance state on thermal cycling. The low‐temperature, high‐resistance state exhibits strong electroresistance whereas the high‐temperature, low‐resistance state does not. The change in resistance between the two distinct states is of two orders of magnitude. It is proposed that the observed metastability may serve as the basis for resistive thermal‐switching devices.

     

The correlation between structure and magnetic properties in the manganites La0.7Ca0.3−xTexMnO3 (0≤x≤0.15)

The effect of Te-doping at La-site on structural, magnetic and transport properties in the manganites La_0.7Ca_0.3−xTe_xMnO₃ (0≤x≤0.15) has been investigated. All samples show an orthorhombic structure (O′-Pbnm) at room temperature. It shows that the Mn-O-Mn bond angle decreases and the Mn-O bond length increases with the increase in the Te content. All samples exhibit an insulator-metal (I-M) transition and the resistivity increases with the increase in the Te-doping level. Additionally, the Curie temperature T_c decreases and the transition becomes broader with increasing Te-doping level, in contrast, the magnetization of Te-doping samples at low temperatures decrease with increasing x as x≤0.10 and then increase with further increasing x to 0.15. The results are discussed in terms of Jahn-Teller (JT) vibrational anisotropy Q₃/Q₂ and the opening of the new DE channel between Mn²⁺-O-Mn³⁺ due to the introduction of Mn²⁺ ions because of the substitution of Te⁴⁺ ions for Ca²⁺ ions.     

Effect of Al doping on the magnetic and electrical properties of layered perovskite La1.3Sr1.7Mn2−xAlxO7

The effect of Al substitution for Mn site in layered manganese oxides La_1.3Sr_1.7Mn_2−xAl_xO₇ on the magnetic and electrical properties has been investigated. It is interesting that all the samples undergo a similar and complex transition with lowing temperature; they transform from the two-dimensional short-range ferromagnetic order at T*, then enter the three-dimensional long-range ferromagnetic state at T_C, at last they display the canted antiferromagnetic state below T_N. T*, T_C and T_N are all reduced with Al content. Resistivity increases sharply with increasing Al concentration, and the metal-insulator transition disappears when x reaches 10%. Additionally, magnetoresistance (MR) effect is weakened. Al substitution dilutes the magnetic active Mn-O-Mn network and weakens the double exchange interaction, and further suppresses FM ordering and metallic conduction. Owing to the anisotropic interaction in the layered perovskite, the magnetic and electrical properties are more sensitive to Al doping level than those in ABO₃-type perovskite.     

Enhanced magneto-electrical properties and room temperature magnetoresistance in lightly doped manganite thin films

We report the effect of compressive strain on magnetic and magneto-electrical properties of lightly doped manganite La_0.88Sr_0.12MnO₃ thin films. Films, having 5-60 nm thickness, were grown on (001) LaAlO₃ and (001) SrTiO₃ substrate by DC-magnetron sputtering. These films show a magnetoresistance as high as ∼65% at room temperature and insulator-metal transition temperature . Further, we demonstrate that a small variation in strain causes significant changes in their properties. We have discussed the possible origin of these features and compared with the reported literature.     

Elastic properties of lanthanum manganites

We give a brief overview of the data on elastic properties of the CMR manganites published during last decade. The main emphasis is put on the results obtained for single crystals.     

Local structure study of T′ cuprate superconductors

Undoped T′-type cuprates are believed to be charge transfer (Mott-Hubbard) insulators. Recent observation of undoped superconducting T′-type cuprates suggests half-filling weakly correlated metallic electron states (Tsukada et al.), which is against the fundamental picture. Here we report the local structures of undoped T′-(La³⁺,Y³⁺)₂CuO₄ and doped T′-(La³⁺,Ce⁴⁺)₂CuO₄ thin-film single crystals by polarized extended X-ray absorption fine structure (EXAS). The in-plane (E//ab) polarized Cu K-EXAFS data shows a remarkable Cu-O bond distance displacement. The broadened Cu-O distribution suggests a large local lattice distortion in T′-type structures, which might influence the electronic structure.     

Double M–I transitions and low-temperature resistivity minimum of La2/3Ca1/3Mn1−xCoxO3(0⩽x⩽0.15) manganite

A series of Mn-site Co-doping samples La_2/3Ca_1/3Mn_1−xCo_xO₃ (0?x?0.15) have been prepared. The structure, magnetic and transport properties of this system have been systematically investigated. All the samples showed good single phase, and the lattice parameters decreased with the increase of doping concentration x. Only one paramagnetic–ferromagnetic transition was observed. The Curie temperature T_C decreases gradually and the transition width becomes wider with the increase of x. The abnormal transport properties were induced by Co doping, characterized by the double metal–insulator transitions and low-temperature minimum behavior. The present results are discussed and possible explanations were given based on the related theory and previous reported results.     

Structural, magnetic and transport properties in the manganites La0.7Sr0.3−xTexMnO3 (0≤x≤0.15)

The effect of Te-doping at La-site on structural, magnetic and transport properties in the manganites La_0.7Sr_0.3−xTe_xMnO₃ (0≤x≤0.15) has been investigated. All samples show a rhombohedral structure with the space group . It shows that the Mn-O-Mn bond angle decreases and the Mn-O bond length increases with the increase of Te content. The Curie temperature T_C decreases with increasing Te-doping level, in contrast, the magnetization magnitude of Te-doping samples at low temperatures increase with increasing x as x≤0.05 and then decrease with further increasing x to 0.15. The results are discussed in terms of the combined effects of the opening of the new double exchange (DE) channel between Mn²⁺-O-Mn³⁺ due to the introduction of Mn²⁺ ions because of the substitution of Te⁴⁺ for Sr²⁺ and the reduction of the transfer integral b due to the decrease of the Mn-O-Mn bond angle.     

The effect of Gd-doping on the charge ordering state of Bi0.3−xGdxCa0.7MnO3 (0≤x≤0.30)

The effect of Gd-doping on the charge ordering (CO) state in perovskite-type manganates Bi_0.3−xGd_xCa_0.7MnO₃ with x=0, 0.02, 0.05, 0.1, 0.3 has been investigated by transport and magnetic property measurements. It is found that CO temperature (T_CO) and antiferromagnetic (AFM) ordering temperature T_N occurring below T_CO decrease obviously with increasing Gd-doping level. Accompanying the variation of T_CO, the increased magnetization and the decreased resistivity are observed. In addition, the increased magnetic inhomogeneity has been also observed in the samples based on the difference between the zero-field-cooling (ZFC) magnetization M_ZFC and field-cooling (FC) magnetization M_FC, which is ascribed to the competition between ferromagnetic (FM) phase induced by Gd-doping and CO AFM phase. The experimental results indicate that the Bi³⁺ lone pair electron with 6s² character plays a dominating role on the CO state of Bi_0.3Ca_0.7MnO₃.     

Local structures of Mn in dilute magnetic semiconductor ZnMnO

The local structures of Mn atoms in (Zn, Mn)O have been investigated by extended x-ray absorption fine structure (EXAFS). For samples with lower Mn concentrations (<11%), EXAFS results show that the majority of Mn atoms are incorporated at Zn substitutional sites. Ferromagnetic behavior has been observed in those ZnMnO samples. It is the direct evidence to suppose that the ferromagnetism in (Zn, Mn)O is not due to Mn-related secondary phases but is intrinsic. When the Mn concentration is increased to 20% or higher, the EXAFS simulation shows the formation of MnO secondary phases and the films show a change from ferromagnetic (FM) to antiferromagnetic (AFM) state. We suggest that Mn-related secondary phases might be responsible for the transformation from FM to AFM state.     

Investigation of FIB irradiation damage in La0.7Sr0.3MnO3 thin films

In this work we analyse systematically how morphological and magnetotransport properties of manganite thin films are affected by the damage induced by focused ion beam (FIB) irradiation. We irradiate different areas of the same sample with doses ranging from 5×10¹² to 3×10¹⁷ ions/cm² and we find that the film becomes swollen for doses up to 10¹⁶ ions/cm² and is eventually eroded by ion milling for further irradiation. On the other hand, transport properties are much more sensitive to FIB irradiation: the metal–insulator transition temperature is found to decrease monotonically with increasing doses up to 1.8×10¹³ ions/cm². At doses higher than 5.6×10¹³ ions/cm² the metallic state is completely suppressed and likely, also ferromagnetism.     

Electrical behavior of some silver-doped lanthanum-based CMR materials

With a view to understand the structural, magnetic and electrical properties of La_1−xAg_xMnO₃ (x=0.05-0.3), a series of samples were prepared by polyvinyl alcohol (PVA) gel route. It has been found that both the metal-insulator and ferro- to paramagnetic transition temperatures after increasing up to the composition x=0.20, are found to remain constant thereafter. The electrical resistivity vs. temperature plot of the sample x=0.10 is found to exhibit an insulating behavior below 36 K, while the sample, x=0.20 exhibits two peaks, and the observed behavior is explained on the basis of the phase separation model. The low-temperature (T<T_P), electrical resistivity data were analyzed by a theoretical model, ρ=ρ₀+ρ₂T²+ρ_4.5T^4.5, indicating the importance of grain/domain boundary effects, electron-electron and two-magnon scattering processes. The low-temperature resistivity data (T<50 K) were fitted to an equation, which is based on the combined effect of weak localization, electron-electron and electron-phonon scattering.     

FMR fine structure – A tool to investigate the spatial magnetic phase separation phenomena in manganites

An original approach is proposed to study the magnetic phase separation phenomenon. It is based on the registration of the noise‐like FMR Fine Structure (FMR FS) caused by the magnetic interparticle dipole–dipole interaction between spatially separated ferromagnetic regions. Data obtained for a La_0.7Pb_0.3MnO₃ single crystal point to the existence of spatially separated ferromagnetic regions. It is shown that FMR FS of the La_0.7Pb_0.3MnO₃ single crystal is temperature reversible and disappears at the maximum of magnetoresistance.

     

Thermoelectric power of potassium doped lanthanum manganites at low temperatures

The temperature-dependent resistivity and thermoelectric power of monovalent (K) doped La_1−xK_xMnO₃ polycrystalline pellets (x=0.05, 0.10 and 0.15) between 50 and 300 K are reported. K substitution enhances the conductivity of this system. Curie temperature (T_C) also increases from 260 to 309 K with increasing K content. In the paramagnetic region (T>T_C), the electrical resistivity is well represented by adiabatic polaron hopping, while in the ferromagnetic region (T<T_C), the resistivity data show a nearly perfect fit for all the samples to an expression containing, the residual resistivity, spin-wave and two-magnon scattering and the term associated with small-polaron metallic conduction, which involves a relaxation time due to a soft optical phonon mode. Small polaron hopping mechanism is found to fit well to the thermoelectric power (S) data for T>T_C whereas at low temperatures (T<T_C) in ferromagnetic region (S_FM), S_FM is well explained with the spin-wave fluctuation and electron–magnon scattering. Both, resistivity and thermopower data over the entire temperature range (50–300 K) are also examined in light of a two-phase model based on an effective medium approximation.