Shear strain in Nd0.5Ca0.5MnO3 at high pressures

High-pressure x-ray powder diffraction has been measured on the half doped rare earth manganite Nd0.5Ca0.5MnO3 up to a pressure of 15 GPa. We report the presence of a quantifiable amount of shear distortion of the MnO6 octahedra in Nd0.5Ca0.5MnO3 at high pressures. The lattice strain of Nd0.5Ca0.5MnO3 is minimal at a crossover pressure of p* approximately 7 GPa, with the same lattice strain above and below this pressure achieved by shear and Jahn-Teller-type distortions, respectively. The increase in shear strain with increasing pressure provides a mechanism for the insulating behavior of manganites at high pressures that has not been considered before.     

XAFS investigation of the role of orientational disorder in the stabilization of the ferromagnetic metallic phase in nanoparticles of La(0.5)Ca(0.5)MnO3

The inclusion of the contribution of Jahn-Teller distortion of MnO(6) units, in addition to double-exchange, has been largely successful in explaining the magneto-transport behavior of manganites. However, our recent experiments on La(0.5)Ca(0.5)MnO(3) demonstrated the limitation of these factors in explaining the radical difference between the magneto-transport properties of bulk and nanocrystalline forms. While bulk La(0.5)Ca(0.5)MnO(3) exhibits insulator character (4-300 K) and an anti-ferromagnetic-ferromagnetic transition at 200 K, the nanocrystalline form stabilizes in a metallic ferromagnetic phase (4-300 K). This is counter-intuitive since large Jahn-Teller distortion, which promotes anti-ferromagnetism or insulator character, exists in the nanocrystals too (as indicated by x-ray diffraction results). In this work, we resolve this paradox by considering the role of structural disorder. Employing x-ray absorption spectroscopy, we establish that the disorder in inter-octahedral coupling is enhanced by 57% in the nanocrystals, as the octahedral units are randomly oriented with respect to each other. This orientational disorder promotes metallic ferromagnetism by destroying the stringent orbital ordering that is needed for anti-ferromagnetism and the co-operative nature of the orbital order.     

Size control of charge-orbital order in half-doped manganite La0.5Ca0.5MnO3

Motivated by recent experimental results, we study the effect of size reduction on half-doped manganite, La(0.5)Ca(0.5)MnO(3), using the combination of density-functional theory (DFT) and dynamical mean-field theory (DMFT). We find that upon size reduction the charge-ordered antiferromagnetic phase, observed in bulk, is destabilized, giving rise to the stability of a ferromagnetic metallic state. Our theoretical results, carried out on a defect-free nanocluster in isolation, establish the structural changes that follow upon size reduction to be responsible for this. Our study further points out the effect of size reduction to be distinctively different from application of hydrostatic pressure. Interestingly, our DFT+DMFT study additionally reports the correlation-driven stability of the charge-orbitally ordered state in bulk La(0.5) Ca(0.5) MnO(3), even in the absence of long-range magnetic order.     

Nd0.5Ca0.5Mn1-xAlxO3 (x=0,0.03)体系中电荷有序态的超声研究

系统研究了Nd0．5Ca0．5Mn1-xAlxO3(x=0,0．03)单相多晶样品在低温下的电磁性质和超声特性．电阻和磁化率测量表明,Nd0．5Ca0.5O3体系在TCO-257 K处发生了电荷有序相变．超声声速从室温开始随着温度的降低逐渐减小,并在TCO附近达到最小,之后,随着温度的进一步降低,声速急刷增加,同时伴随着一个尖锐的超声衰减峰出现.TCO附近的超声异常表明体系中存在着强烈的电-声子相互作用,该电-声子耦合来源于Mn3 的Jahn-Teller效应．在低温下,出现了另一个超声衰减峰,它的出现归结为反铁磁相与顺磁相之间的相分离现象.随着Al在Mn位的掺入,超声声速的最低点和衰减峰向低温移动,表明体系中的电荷有序态和反铁磁相均被部分抑制,     

The ac conductivity and complex impedance of CuO-doped (Ba0.5Sr0.5)TiO3 ceramic

The AC conductivity and complex impedance spectroscopy of CuO-doped (Ba_0.5Sr_0.5)TiO₃ ceramic were investigated. X-ray diffraction analysis showed that CuO-doped (Ba_0.5Sr_0.5)TiO₃ has a perovskite structure without any pyrochlore phase. Frequency dependent dielectric permittivity was discussed at a different temperature range. The activation energy was calculated and discussed through the Arrehnius equation from the ac conductivity with different frequency plots. CuO-doped (Ba_0.5Sr_0.5)TiO₃ ceramics have a negative temperature coefficient of resistivity. Dependence of impedance spectroscopy on frequency and temperature showed that the conduction process in the CuO-doped (Ba_0.5Sr_0.5)TiO₃ ceramic follows the thermally activated conduction mechanism.     

Irreversible metamagnetic transition and magnetic memory in small-bandwidth manganite Pr(1-x)Ca(x)MnO3 (x = 0.0-0.5)

The present paper reports detailed structural and magnetic characterization of the low-bandwidth manganite Pr(1-x)Ca(x)MnO(3) (with x = 0.0-0.5) (PCMO) polycrystalline samples. With increasing Ca content, reduction of the unit cell volume and improvement in perovskite structure symmetry was observed at room temperature. Magnetic characterization shows the signature of coexisting AFM-FM ordering and spin-glass phase at the low doping range (x = 0.0-0.2) while increased hole doping (x = 0.3-0.5) leads to charge ordering, training effect and an irreversible metamagnetic phenomenon. The large irreversible metamagnetism in the CO phase of PCMO and the corresponding spin memory effect is a direct consequence of hysteretic first-order phase transition arising from the weakening of the CO state under the external magnetic field and trapping of the spins due to a strong pinning potential in the material.     

Noise probe of the dynamic phase separation in La(2/3)Ca(1/3)MnO3

Giant random telegraph noise (RTN) in the resistance fluctuation of a macroscopic film of perovskite-type manganese oxide La(2/3)Ca(1/3)MnO3 has been observed at various temperatures ranging from 4 to 170 K, well below the Curie temperature ( T(C) approximately 210 K). The amplitudes of the two-level fluctuations vary from 0.01% to 0.2%. We discuss the origin of the RTN to be a dynamic mixed-phase percolative conduction process, where manganese clusters switch back and forth between two phases that differ in their conductivity and magnetization.     

Charge ordering fluctuation and optical pseudogap in La(1-x)Ca(x)MnO(3)

Optical spectroscopy was used to investigate the optical gap ( 2 Delta) due to charge ordering (CO) and related pseudogap developments with x and temperature ( T) in La(1-x)Ca(x)MnO(3) ( 0.48< or =x< or =0.67). Surprisingly, we found 2 Delta/k(B)T(CO) is as large as 30 for x approximately 0.5, and decreases rapidly with increasing x. Simultaneously, the optical pseudogap, possibly starting from T* far above T(CO) becomes drastically enhanced near x = 0.5, producing non-BCS T-dependence of 2 Delta with the large magnitude far above T(CO), and systematic increase of T* for x approximately 0.5. These results unequivocally indicate systematically enhanced CO correlation when x approaches 0.5 even though T(CO) decreases.     

La_(0.5)M_(0.5)MnO_3(M=Sr,Ba)的水热合成与表征

我们用水热法合成了钙钛矿锰氧化物La0.5M0.5MnO3(M=Sr,Ba)系列样品,研究了制备条件对结构、形貌和磁性的影响.     

Optimization of Pr(0.9)Ca(0.1)MnO(3) thin films and observation of coexisting spin-glass and ferromagnetic phases at low temperature

Optimization of thin films of small bandwidth manganite, Pr(1-x)Ca(x)MnO3 (for x = 0.1), and their magnetic properties are investigated. Using different pulsed laser deposition (PLD) conditions, several films were deposited from the stoichiometric target material on SrTiO3 (001) substrate and their thorough structural and magnetic characterizations were carried out using x-ray diffraction, atomic force microscopy, x-ray photoelectron spectroscopy (XPS), SQUID magnetometry and ac susceptibility measurements. A systematic investigation shows that irrespective of the growth temperature (between 550 and 750?°C), all the as-deposited films have twin boundaries and magnetic double phases. Post-annealing in partial or full oxygen pressure removes the extra phase and the twin boundaries. Zero-field-cooled magnetization data show an antiferromagnetic to paramagnetic transition at around 100 K whereas the field-cooled magnetization data exhibit a paramagnetic to ferromagnetic transition close to 120 K. However, depending on the oxygen treatments, the saturation magnetization and Curie temperature of the films change significantly. Redistribution of oxygen vacancies due to annealing treatments leading to a change in ratio of Mn3+ and Mn4+ in the films is observed from XPS measurements. Low temperature (below 100 K) dc magnetization of these films shows metamagnetic transition, high coercivity and irreversibility magnetizations, indicating the presence of a spin-glass phase at low temperature. The frequency dependent shift in spin-glass freezing temperature from ac susceptibility measurement confirms the coexistence of spin-glass and ferromagnetic phases in these samples at low temperature.     

Epitaxial growth of MnO/Ag(0 0 1) films

Among the 3d transition metal monoxides, MnO plays a particular role due to the high spin ground state of the 3d⁵ configuration of Mn²⁺. Therefore, MnO could be an excellent model system to investigate many kinds of spin dependent or magnetic interactions between electrons in the surface of an antiferromagnetic solid. But in practice, most experimental investigations on crystalline MnO(0 0 1) surfaces suffer from the bad structural quality of the surface of cleaved bulk crystals. In this paper, we report on the epitaxial growth of MnO/Ag(0 0 1) surfaces, which is proved to be a method to produce crystalline MnO(0 0 1) surfaces of high structural order.     

Tuning the phase transition dynamics by variation of cooling field and metastable phase fraction in Al doped Pr(0.5)Ca(0.5)MnO(3)

We report the effect of field, temperature and thermal history on the time dependence in resistivity and magnetization in the phase separated state of Al doped Pr(0.5)Ca(0.5)MnO(3). The rate of time dependence in resistivity is much higher than that of magnetization and it exhibits a different cooling field dependence due to percolation effects. Our analysis shows that the time dependence in physical properties depends on the phase transition dynamics, which can be effectively tuned by variation of temperature, cooling field and metastable phase fraction. The phase transition dynamics can be broadly divided into the arrested and unarrested regimes, and in the arrested regime this dynamics is mainly determined by time taken in the growth of critical nuclei. An increase in cooling field and/or temperature shifts this dynamics from the arrested to unarrested regime, and in this regime, this dynamics is determined by the thermodynamically allowed rate of formation of critical nuclei, which in turn depends on the cooling field and available metastable phase fraction. At a given temperature, a decrease in metastable phase fraction shifts the crossover from arrested to unarrested regimes towards lower cooling field. It is rather significant that in spite of the metastable phase fraction calculated from resistivity being somewhat off that of magnetization, their cooling field dependence exhibits a striking similarity, which indicates that the dynamics in arrested and unarrested regimes are so different that it comes out vividly provided that the measurements are performed around the percolation threshold.     

Electrical and structural properties of (Pb0.5Fe0.5)Sr2(RE0.5Ca0.5)Cu2Oz (RE: rare-earth element)

(Pb_0.5Fe_0.5)Sr₂(Y_0.5Ca_0.5)Cu₂O_z is known to have so-called “1-2-1-2” crystallographic structure which is one of typical structures of high-temperature superconducting cuprates (HTSC’s). While it possesses structural characteristics of HTSC’s, superconductivity has not been reported so far. In this study, phase formation, electrical properties and structural characteristics of this compound are investigated. It is shown that this phase can be synthesized using several kinds of rare-earth element and that post-annealing in O₂ is effective for decreasing electrical resistivity. The latter suggests that this “1-2-1-2” can be made superconducting by appropriate carrier-doping into the [Cu–O₂] plane. Preliminary Rietveld structural refinement using powder X-ray diffraction data show that the initial structural model based on the nominal (Pb,Fe)Sr₂(Y,Ca)Cu₂O_z is basically correct except for true site-distribution of Fe and Cu.     

Thermal and electronic transport properties and two-phase mixtures in La(5/8-x)Pr(x)Ca(3/8)MnO3

We measured thermal conductivity kappa, thermoelectric power S, and electric conductivity sigma of La(5/8-x)Pr(x)Ca(3/8)MnO3, showing an intricate interplay between metallic ferromagnetism (FM) and charge ordering (CO) instability. The change of kappa, S, and sigma with temperature (T) and x agrees well with the effective medium theories for binary metal-insulator mixtures. This agreement clearly demonstrates that with the variation of T as well as x, the relative volumes of FM and CO phases drastically change and percolative metal-insulator transition occurs in the mixture of FM and CO domains.     

Dielectric properties and electrical conduction of La2O3-doped (Bi0.5Na0.5)0.94Ba0.06TiO3 ceramics

La₂O₃ (2 wt%)-doped (Bi_0.5Na_0.5)_0.94Ba_0.06TiO₃ (abbreviated as BNBT6) lead-free piezoelectric ceramics were synthesized by conventional solid-state reaction. X-ray diffraction (XRD) patterns indicated that La₂O₃ has diffused into the lattice of BNBT6 ceramics and formed a solid solution with a pure perovskite structure. Addition of La₂O₃ decreased the piezoelectric properties and electrical conductivity. It was used to study the electrical conductivity of the La₂O₃-doped BNBT6 lead-free piezoelectric ceramics combined with electrical modulus and impedance plots at the temperature range over 788–873 K. The values of activation energy derived from the electrical impedance and modulus were found to be 0.51 and 0.50 eV, respectively. The discrepancy between activation energy of relaxation frequency and the activation energy (0.25 eV) of dc electrical conductivity might have been due to a short-range migration or hopping of single ionized oxygen vacancy and a long-range migration or hopping of charge carriers over the whole disordered system, respectively.     

Observation of Griffiths phase in antiferromagnetic La(0.32)Eu(0.68)MnO3

We report the Griffiths phase (GP) scenario for isovalent doping in antiferromagnetic La(0.32)Eu(0.68)MnO(3). Rietveld refinement of structural data displays strong crystal structural distortion. The dc and ac magnetic studies nicely demonstrate unambiguous aspects of robust GP. The presence of ferromagnetically correlated spin clusters is found above the transition temperature, T(N). The disorder-driven phase inhomogeneity is correlated to strong structural distortion, giving rise to the GP. This is an unique example in manganite where double exchange interaction does not play any role for the observed phenomenon.     

Oxygen stripes in La0.5Ca0.5MnO3 from ab initio calculations

We investigate the electronic, magnetic, and orbital properties of La0.5Ca0.5MnO3 perovskite by means of an ab initio electronic structure calculation within the Hartree-Fock approximation. Using the experimental crystal structure reported by Radaelli et al. [Phys. Rev. B 55, 3015 (1997)]], we find a charge-ordering stripelike ground state. The periodicity of the stripes, and the insulating magnetic structure, consisting of antiferromagnetically coupled zigzag chains, are in agreement with neutron x-ray and electron diffraction experiments. However, the detailed structure is more complex than that envisaged by simple models of charge and orbital order on Mn d levels alone, and is better described as a charge-density wave of oxygen holes, coupled to the Mn spin/orbital order.     

Current oscillation and low-field colossal magnetoresistance effect in phase-separated manganites

Current-induced switching from a metallic to an insulating state is observed in phase-separated states of (La(1-y)Pr(y))0.7Ca0.3MnO3 (y=0.7) and Nd(0.5)Ca(0.5)Mn(1-z)Cr(z)O3 (z=0.03) crystals. The application of magnetic fields to this current-induced insulating state causes a pronounced low-field negative magnetoresistance effect [rho(H)/rho(0)=10(-3) at H=1 kOe]. The application of a constant voltage also causes the breakdown of the Ohmic relation above a threshold voltage. At voltages higher than this threshold value, oscillations in currents are observed. This oscillation is well reproduced by a simple model of local switching of a percolative conduction path.     

The microscopic features of (Li0.5La0.5)TiO3

In ionic conducting materials, the crystal structure is closely related to the ionic conductivity. In this research we studied the microscopic features of Li_0.5La_0.5TiO₃ which exhibited a lithium ionic conductivity as high as 1×10⁻³ Scm⁻¹ at room temperature by XRD, TEM and SIMS. It was found that the superstructure was caused by the ordering of La⁺³ and vacancy, producing the 2a_p×2a_p×2a_p unit cell. This ordering was found to be regular in microscopic region, but became irregular in macroscopic region. Li⁺ showed a random distribution which meet the needs for the fast ionic conduction. The second phase was found to be Li₂TiO₃ which existed in the grain boundary junctions.     

High resolution study of permanent photoinduced reflectivity changes and charge-order domain switching in Bi(0.3)Ca(0.7)MnO(3)

We report near-field and far-field optical microscopic studies of photoinduced effects in charge-ordered Bi(0.3)Ca(0.7)MnO(3). Unlike previously reported transient photoinduced effects in manganites, we have observed permanent reflectivity changes following local sample illumination with 488 nm light. High-resolution images of exposed regions reveal optical contrast on a submicrometer scale. This observation indicates that photonic band-gap structures may be created using holographic recording in manganites. We also present images of photoinduced charge-order domain switching in Bi(0.3)Ca(0.7)MnO(3).