L. D. Yao

Enhanced magnetoresistance of La2/3Ca1/3MnO3/CeO2 nanocystalline composites synthesized by polymer-network gel method

Using a dispersing particle polymer-network gel method, we prepared xLa2∕3Ca1∕3MnO3∕(1−x)CeO2 nanocystalline composites, where the average grain sizes of La2∕3Ca1∕3MnO3 and CeO2 are about 30nm and less than 10nm, respectively. The magnetoresistance of the sintered composites is explored as a function of the ratio of metal/insulator, temperature, and magnetic field. The system exhibits metallic percolation threshold at x=xp∼35%, around which the high field magnetoresistance has a maximum (∼70%) at 50K and 5T, and the low field magnetoresistance value is close to 16% at 5K and 0.05T.

Conjectured orbital ordering behavior of Nd0.5Sr0.5MnO3 under high pressures

We have synthesized Nd0.5Sr0.5MnO3 polycrystalline bulk samples by solid state reaction, which show ferromagnetic (FM), A-type antiferromagnetic (AF) and charge ordering charge exchange (CE)-type AF transitions on cooling from room temperature to 78K. We consider that the abundant magnetic structures are caused by phase segregation. We have investigated and conjectured the orbital ordering behavior under hydrostatic pressures up to 7.5GPa and found that low pressure favors the A-type AF phase with d(x2−y2) orbital ordering and the CE-type AF phase with d(3x2−r2)∕d(3y2−r2) orbital ordering, while suppressing the FM phase with disordered orbital ordering; however, high pressure favors the A-type phase and suppresses the CE-type AF phase. Transport property enhancement and large resistance changes under high pressures are observed.

Structural defects in multiferroic BiMnO3 studied by transmission electron microscopy and electron energy-loss spectroscopy

The multiferroic material BiMnO3 synthesized under high pressure has been systematically studied by transmission electron microscopy and electron energy-loss spectroscopy, and some important structural defects are revealed in this multiferroic material. The frequently observed defects are characterized to be Σ3(11¯1) twin boundaries, Ruddlesden-Popper [Acta Crystallogr. 11, 54 (1958)] antiphase boundaries, and ap⟨001⟩p superdislocations connected with a small segment of Ruddlesden-Popper defect. These defects are present initially in the as-synthesized sample. In addition, we find that ordered voids (oxygen vacancies) are easily introduced into the multiferroic BiMnO3 by electron-beam irradiation.

Phase separation in Eu0.58Sr0.42MnO3 under pressure

Resistivity measurements of Eu0.58Sr0.42MnO3 were carried out at different magnetic fields under ambient pressure and 1 GPa, respectively. The critical temperature TC of the ferromagnetic metallic state is 128.5 K under 1 GPa and 7 T. The antiferromagnetic insulating phase can be transformed into the ferromagnetic metallic phase by either magnetic field or pressure. Different from the effect of the magnetic field, a plateau is observed in the temperature dependence of the resistivity curve in the lower temperature under pressure, which indicates a phase separation. Magnetic phase diagrams at ambient pressure and 1 GPa are established.

Structures and physical properties of LaxCa4−xMn3O10 (x=0,0.1) synthesized under high pressure

The nominal composition compounds LaxCa4−xMn3O10, (x=0,0.1) were synthesized using solid-state reaction under high pressure. The structures of the two samples have been studied by x-ray powder diffraction, transmission electron microscopy, and high-resolution electron microscopy. In the case of the nominal composition La0.1Ca3.9Mn3O10, besides the phase with n=2 member of Ruddlesden-Popper (RP) series, the phase with the ideal n=3 member RP structure (space group I4∕mmm) was formed. Moreover, it is interesting that some intergrowth or modulation structures were observed in some areas of n=3 phase. Based on the analysis of magnetic and electrical properties, LaxCa4−xMn3O10 undergoes from a paramagnetic-antiferromagnetic transition (x=0) at 104K to paramagnetic-weak ferromagnetic transition (x=0.1) at 113K ascribed to existence of La doped n=3 phase LaxCa4−xMn3O10 and intergrowth or modulation structures. Similar conductive mechanism, two-dimensional variable range hopping, is shown above magnetic transitio...

Enhanced magnetoresistance of La2/3Ca1/3MnO3/Sr2FeMoO6 core/cell nanocrystalline composites synthesized by polymer-network sol-gel method

This paper reports that the homogeneous nanocrystalline LCMO(core)/SFMO(shell) and SFMO(core)/LCMO(shell) series composites are successively synthesized using polymer-network sol-gel method. With the increase of SFMO content in the composites, the remanence magnetization M-r increases while the coercivity H-c decreases. This fact indicates that the ferromagnetic phase boosts up. Moreover, the LFMR (1 T) of the composites succeeds the preponderances of both SFMO and LCMO; i.e., the magnetoresistance (MR) value increases from 300 to 5 K and keeps a high level. In particular, the MR value of the LS-8 composite reaches 55% at 5 K and 7 T.

Charge ordering in PrCuMn6O12 and its behavior under pressure

Polycrystalline PrCuMn6O12 sample has been successfully synthesized. It has been characterized by x-ray diffraction (XRD), magnetic and magnetotransport measurements at ambient pressure, and transport property measurements under pressures. From the refinements of XRD data, a cubic phase and a trigonal phase with charge ordering state were identified at higher and lower temperatures, respectively. The temperature dependence of the resistance indicates that the phase transition is a first-order transition. Both effective paramagnetic moments of the trigonal and cubic phases can be explained by orbital moment freezing theory. The transport property of the trigonal phase has been fitted with Mott’s law for variable range hopping. Though the magnetoresistance value is not large, the pressure-induced resistance value change of PrCuMn6O12 is positive and achieves 264.8%. The cubic phase is suppressed under pressure and transformed to the trigonal phase at 15 kbar, indicating favor of the charge ordering state un...

Microstructure and ferromagnetic clusters in slightly doped LaxBa4−xMn3O10

The nominal composition LaxBa4−xMn3O10 (x=0–0.2) compounds were synthesized using solid-state reaction under high pressure. The structures of the two single-phase samples LaxBa4−xMn3O10 (x=0,0.05) have been studied by powder x-ray diffraction, electron diffraction, and high-resolution transmission electron microscopy (HRTEM). For the x=0.05 sample, some intergrowth structures and stacking faults were revealed by HRTEM observations. According to the results of electron energy-loss spectroscopy (EELS) analysis, the slight doping of La into the parent phase compound Ba4Mn3O10 was confirmed. Moreover, the L3∕L2 intensity ratio of Mn-L2,3 edges in the EELS spectra indicates the variation of Mn ionic valence state induced by the doping of La. Based on analysis of magnetic properties, the magnetization for the x=0 sample shows a broad peak centered around 170K and a sharp upturn at low temperature of 15K, which is due to a short-range low-dimensional antiferromagnetic ordering. For the x=0.05 sample, the slight ...