The time decay of the thermoremanent magnetization(TRM) of the Fe 80−x Ni x Cr 20 (14 ≤ x ≤ 30) alloys has been measured for four different magnetic phases within the fcc γ-phase using a SQUID magnetometer. In the spin-glass phase(SG)(X=19) very distinct ageing effects are observed where M(t) can be described as M (t) = M 0 (t/t w ) −γ exp[−(t/τ ) 1−n ] for the entire time domain. In the reentrant spin-glass(RSG)(X= 23 and 26), M(t) can be better represented by the stretched exponential with an addition of a constant term which can be well explained by the Gabay-Toulouse(GT) model.We have also measured the linear and non-linear ac susceptibilities for the sample X=23 and confirmed the presence of the ferromagnetic(FM) ordering down to the lowest temperature. In the RSG(X=23), the TRM shows a minimum near T c and a local maximum just above T c . In the FM phase (X=30) the popular prediction of the power law decay of the TRM is observed. The latter is indistinguishable from the stretched exponential in the antiferromagnetic(AF) phase (X=14).

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...

International audiencehfagnetic relaxation experiments constit.ute a unique method of determining the nature of fluctuations in dissipative magnetic systems. At high temperatures these fluctuations are thermal and strongly temperature dependent. At low temperatures, where quantum fluctuations dominate, magnetic relaxation becomes independent of temperature. Such behavior has been observed in many systems. In this review we emphasize the study of low temperature relaxation in ferromagnetic nanoparticles, layers, and multilayers (including " domain wall junctions "), and large single crystals. The results of magnetic relaxation experiments are shown to agree with theoretical predictions of quantum tunneling of the magnetization. When dissipation becomes important, in large and complex systems, a time dependent WKB exponent needs to be introduced

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