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We have studied the ferromagnetic and antiferromagnetic fluctuations in the charge-ordered Pr0.65Ca0.35MnO3 antiferromagnet by triple-axis neutron spectrometry. Whereas ferromagnetic fluctuations are observed above and below the charge-ordering transition (TCO), the antiferromagnetic fluctuations develop only below TCO .T he dynamical exponent z of both ferromagnetic and antiferromagnetic fluctuations are determined. The ferromagnetic fluctuations are not completely suppressed below TCO and their correlation lengths are short-ranged at all temperatures. The results are discussed with respect to the Zener polaron model recently introduced to describe the charge-ordered state of Pr0.6Ca0.40MnO3.

We have studied the magnetism of the half-doped charge ordered manganite YBaMn2O6. A formation of ferromagnetic plaquettes of four Mn atoms in the charge ordered phase below T_{CO} approximately 480 K is inferred from high temperature magnetic susceptibility data and the magnetic structure, as determined by neutron powder diffraction at T=1.5 K. The results indicate that new fourfold Mn paramagnetic units form between T_{N}

We report the detailed study of dielectric response of ${\mathrm{Pr}}_{0.6}{\mathrm{Ca}}_{0.4}\mathrm{Mn}{\mathrm{O}}_{3}$ (PCMO), a member of the manganite family showing colossal magnetoresistance. Measurements have been performed on four polycrystalline samples and four single crystals, allowing us to compare and extract the essence of dielectric response in the material. High-frequency dielectric function is found to be ${\ensuremath{\epsilon}}_{\mathrm{HF}}=30$, as expected for the perovskite material. Dielectric relaxation is found in the frequency window of $20\phantom{\rule{0.3em}{0ex}}\mathrm{Hz}\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}1\phantom{\rule{0.3em}{0ex}}\mathrm{MHz}$ at temperatures of $50--200\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ that yields to colossal low-frequency dielectric function, i.e., the static dielectric constant. The static dielectric constant is always colossal, but varies considerably in different samples from $\ensuremath{\epsilon}(0)={10}^{3}\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}{10}^{5}$. The measured data can be simulated very well by blocking (surface barrier) capacitance in series with sample resistance. This indicates that the large dielectric constant in PCMO arises from the Schottky barriers at electrical contacts. Measurements in magnetic field and with dc bias support this interpretation. Colossal magnetocapacitance observed in the title compound is thus attributed to extrinsic effects. Weak anomaly at the charge ordering temperature can also be attributed to interplay of sample and contact resistance. We comment on our results in the framework of related studies by other groups.