Navegando por Autor "Ratkovski, Danilo Roberto"

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    Magnetic irreversibility and spin dynamics in nanoparticles of iron-doped europium chromite
    (2017) Ratkovski, Danilo Roberto; Marín Ramirez, Juan Marcos; Ribeiro, Pablo Rafael Trajano; Pessoni, Hermínia Veridiana dos Santos; Franco Júnior, Adolfo; Machado, Fernando Luis de Araújo
    The magnetic properties of nanoparticles of Eu1 xFexCrO3 (0 x 1) prepared by a combustion reaction technique were investigated. An irreversible behavior in the magnetization was observed for a sample with x ¼ 0:10 below the Neel temperature ( TN) yielding a complex phase diagram. The irreversibility data was fitted to a de Almeida-Thouless line by using f ¼ 3:0 for the critical exponent and a glassy temperature TG of 174.4 K for H 40 kOe. The dynamics of the spins was investigated by measuring the ac magnetic susceptibility (cac) near TN for frequencies (f) in the range 10 104 Hz. The maximum in cac was found to shift to higher values of T for increasing values of f, yielding x0:003 for the shift per decade of f parameter. The Voguel-Fulcher law and a power-law were used for analyzing cac yielding t0 ¼ 1:8 10 9 s (¼ 1:6 10 15 s) for the characteristic relaxation time, Ea=kB ¼ 39:23 K for the activation energy and TG ¼ 165:9K(¼ 167:5 K) for the Voguel-Fulcher (power-law) model. Moreover, the power-law yielded zn ¼ 5:62 for the product of the dynamical critical exponent (n) with the one associated to the correlation length (z). The dependence of TN with the bonding angles Cr3þ O2 Cr3þ was investigated for as-prepared samples yielding a trend contrary to the expected. The overall results were accounted for by taking into consideration the microstrain introduced by the sample preparation technique and by the ionic dopping.
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    On the correlation of the magnetic properties with the structural parameters in Fe-doped europium chromite
    (2021) Ratkovski, Danilo Roberto; Marin Ramirez, Juan Marcos; França, Emanoel Laurertan Tavares; Franco Júnior, Adolfo; Machado, Fernando L. A.
    In the present work, the dynamics of the spins and the structural parameters of thermal treated samples of Eu1−xFexCrO3 (x = 0, 0.1 and 0.2) were investigated. The ac-magnetic susceptibility (χac) was measured near TN for frequencies (f ) in the range 10 - 104 Hz, magnitude of the ac magnetic field of 10 Oe and for 5 ≤ T ≤ 300 K. X-ray diffraction data were used for determining the lattice parameters and the bonding angle θB (Cr– O(2) – Cr) for 100 ≤ T ≤ 300 K. The maximum in χac was found to shift to higher values of T for increasing values of f . The Vogel-Fulcher law was used for analyzing χac yielding values for the characteristic relaxation time τ0, activation energy Ea/kB and glassy temperature TG, respectively, in the ranges 2.10 - 3.96 ps, 46.5 - 47.2K and 169.9 -176.1 K. The super-exchange parameter J ∼ cos4[(180 − θB)/2]/d7, where d is the length of the bound Cr-O(2), was also obtained yielding a good correlation with the corresponding values of TN.
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    On the magnetic properties of the multiferroic ceramics Bi0:99Y0:01Fe1-xNixO3 Bi0:99Y0:01Fe1 xNixO3
    (2018) Ratkovski, Danilo Roberto; Ribeiro, P. R. T.; Machado, Fernando Luis de Araujo; Banerjee, Prasun; Franco Júnior, Adolfo
    Multiferroic ceramics of Bi0:99Y0:01Fe1 xNixO3 with 0:01 6 x 6 0:05 were synthesized by using a modified solid state reaction method. The crystalline structure and the morphology of the samples were investi gated by X-ray diffraction (XRD) and by scanning electron microscopy (SEM). The addition of Y and Ni to the bismuth ferrite (BiFeO3) was found to decrease the average grain size. Ac magnetic susceptibility and the zero-field-cooled (ZFC) and field-cooled (FC) magnetizations were measured for temperatures in the range 5 6 T 6 300 K. Hysteresis loops and an irreversible behavior in the temperature dependence of the magnetization not present in pure BiFeO3 were observed in the doped samples. However, the ferro magnetism was found more likely to be due to the presence of small amounts of magnetite. Nevertheless, the determination of the amount of Fe3O4 in these composite materials is important because it influences the magnetoelectric coupling which is important for some technological applications