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The response of magnetic system of lanthanum manganites doped with Ca, Ba, and Pb to static and dynamic magnetic fields was studied. The methods of magnetic resonance and antiresonance, penetration of radio-frequency electromagnetic waves as well as the standard methods of dc and ac magnetic susceptibility measurements were applied in order to investigate both linear and nonlinear dynamical magnetization processes. The high-frequency response of manganites doped by different metals is compared to peculiarities of their electronic structure and to dc magnetotransport properties. A complicated nonlinear dynamical magnetic behavior of Pb-doped manganites, which includes variation of magnetization which is incommensurate to the applied external ac field, efficient generation of harmonics, and magnetic aftereffect, were investigated. Special attention was paid to the temperature range of magnetic phase transition in which the influence of temporal and spatial inhomogeneities on dynamical magnetic properties were investigated experimentally and analyzed. Nonlinear electromagnetic phenomena in manganites are of considerable interest. Penetration of electromagnetic field through a lanthanum manganite plate was studied in [1]. In this work we report about several types of instabilities and the extremely strong generation of harmonics which are observed upon excitation of a transformer-type sample by ac current of varying frequency. The experimental investigations were carried out in the frequency range of 800 Hz to 180 kHz using the toroidal manganite core on which several coils were wound (the transformer-type sample). The primary coil was excited by variable frequency generator. The external permanent magnetic field H was applied perpendicular to the plane of the core, therefore, perpendicular to the ac magnetic field. The bulk sintered polycrystalline samples of nominal composition La0.60Pb0.40MnO3 were prepared by wet chemical route. The final sintering was carried out at 9500C in the oxygen flow during 12 hours. The module and phase of the transmission coefficient was measured as a function of the strength of the dc magnetic field and the frequency of the excitation voltage. For the frequencies above few kilohertz several types of instabilities were clearly seen besides the ordinary monotonic decrease of the output signal with the dc magnetic field increasing. The start of appearance of instabilities is accompanied by the extremely strong effect of generation of harmonics. Both even and odd harmonics were observed with the amplitudes up to 30% from the signal of the fundamental frequency. When the supplying voltage and the external field H exceeded some threshold values, the bifurcation of the saddle-focus type appeared and the phase portrait of oscillations became very complicated. The self-detection effect was observed in the exciting ac current. A kink point appears in the ac current oscillogram, which reflects the impact excitation of the secondary coil. The estimates of the dynamic magnetic permeability were carried out. The imaginary part of the magnetic permeability is comparable to real part at the frequencies above several kilohertz. The work is partially supported by RFBR and "Quantum Macrophysics" Program of the Russian Academy of Sciences. [1] A.Rinkevich, A.Nossov, V.Ustinov, V.Vassiliev, S.Petukhov. Penetration of the electromagnetic waves through doped lanthanum manganites, J. Appl. Phys., Vol.91, No.6, pp.3693-3697, 2002 |