The structural phase transition of Pr(1-x)Ca(x)MnO(3)(x = 3/8) was investigated by means of low-temperature transmission electron microscopy. Superlattice reflection spots with a modulation wave vector q(1) = (0,1/2,0) appeared below 230 K, indicating formation of the d(3x(2-r(2))/d(3y(2)-r(2)) type of charge/orbital ordering. Below 150 K, a new series of superlattice reflection spots with a modulation wave vector q(2) = (1/4,1/4,1/2) appeared, suggesting an additional ordering of excess 1/8 Mn(3+), necessary due to the deviation of x from 1/2, with the occupation of the d(3z(2-r(2)) type of e(g) orbital.

Transmission-electron-microscopy measurements characterizing the structural features of chargeordering (CO) phases in La͑Ca͒MnO 3 and Pr͑Ca͒MnO 3 are presented. A rich variety of structural phenomena resulting from CO transition have been observed at low temperatures. The most interesting of all is the presence of a new structural modulation on the b ‫ء‬ -c ‫ء‬ plane. This modulation shows some structural properties in sharp contrast with those of the known d z 2 ͑Mn 31 ͒-orbital-ordering state. We herein interpret this new structural modulation in terms of the small-polaron ordering.[S0031-9007(99)08685-8] PACS numbers: 75.30.Kz, 64.70.Rh, 71.38. + i, 75.50.Cc The nature of the quasiparticles that originate from the doped holes in the high-temperature superconductors and colossal magnetoresistance manganites is believed to be of central importance for the understanding of the unusual physical properties unveiled in these systems [1][2][3]. Charge localization and the stripe correlations of concentrated holes and spins have attracted much attention in the recent studies, particularly because of their possible role on high-T c superconductivity [4][5][6]. In manganese systems, charge ordering (CO), as well as its magnetic-field-melting mechanism, has also been extensively investigated in connection with the colossal magnetoresistance [7-10]. An ordered structure observed in the CO states of manganites has been successfully explained in terms of the ionic ordering of Mn 31 and Mn 41 associated with the d z 2 ͑Mn 31 ͒ orbital ordering of e g electrons [10-12]. Furthermore, a number of physical evidences have strongly suggested the presence of small polarons in the hole-doped manganites [13,14]. However, direct observations of this kind of small polarons and their structural effects have not yet been achieved. In this Letter, we report a new structural modulation observed in the ͑La, Ca͒MnO 3 and ͑Pr, Ca͒MnO 3 materials and explain it in terms of the small-polaron ordering.Both ceramic and single-crystalline samples of ͑La, Ca͒MnO 3 and ͑Pr, Ca͒MnO 3 were used for the present study. The ceramics samples were prepared by a conventional solid-state reaction method and the singlecrystalline samples were melt grown by the floatingzone method. At low temperatures, the samples of La 12x Ca x MnO 3 with 0.5 # x # 1.0 and Pr 12x Ca x MnO 3 with 0.3 # x # 1.0 are known as CO insulators [15,16]. Experimental details of transmission-electron-microscopy (TEM) observations will be described elsewhere [17].In order to reveal the structural distortion in correlation with the CO transition in manganites, we have performed extensive low-temperature TEM studies on a series of ͑La, Ca͒MnO 3 and ͑Pr, Ca͒MnO 3 samples. A rich variety of microstructural phenomena have been observed. The most important of all is the presence of incommensurate structural modulations in the low-temperature CO phases. Based on the systematic analysis of the experimental data, two distinctive structural modulations were determined. The first is the well-kno...

Ferromagnetic resonance ͑FMR͒ in diluted Fe nanogranular films with low volume fractions of Fe (v) has been studied. Films prepared by a co-sputtering technique are composed of amorphous Fe nanoparticles embedded in SiO 2 glass matrices. The resonance of the FMR uniform mode is found to be strongly affected by the v, the temperature, and the angle between the film plane and the applied magnetic field. The present study suggests that the values of these parameters should be carefully selected in order to realize left-handed materials using diluted ferromagnetic-metal nanogranular films.

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Using synchrotron radiation x-ray photoemission spectroscopy and high-resolution transmission electron microscopy, we have studied Ca-rich Bi-based superconducting whiskers grown by an Al2O3-seeded glassy quenched platelet method. The Ca-rich Bi-based superconducting whiskers show a high critical current density of 2×105 A/cm2 at 40 K in self-field. We found that excess Ca2+ ions substitute for the Sr2+ sites and cause pillar-shaped nanocrystalline domains with shorter period modulation embedded in the base crystalline. The embedded nanocrystalline domains can result in structural distorted defects which work as strong pinning center.