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Nanoparticles of Co1 –xMgxFe2O4 with x equal to 0, 0.2, 0.4, 0.6, 0.8, and 1.0 have been synthesized. For all values of x, they are nanocrystals with a cobalt ferrite structure and an average linear size ( nm. Based on the analysis of the Mössbauer effect spectra, the Co2+ ions were shown to occupy only octahedral positions at all values of x. The experimentally obtained dependence of the nanoparticles magnetization on x corresponds to the dependence calculated using the Mössbauer effect data, except for the sample with . The effective magnetic anisotropy constant estimated for 0 K from the analysis of the coercive force temperature dependences decreases from at to erg/cm3 at and drops sharply to erg/cm3 at . [ABSTRACT FROM AUTHOR]

The effect of the size of Bi2Fe4O9 and BiFeO3 nanoparticles on the magnetoelectric interaction in the Bi2Fe4O9/BiFeO3 composite with a percentage ratio of 67/33 has been studied. The electrostriction and electric polarization on electric and magnetic field in wide temperature range has been measured. The hysteresis of the polarization and I‒V characteristics has been found. Temperature ranges with activation and hopping types of conductivity have been found. The mechanism of electric polarization and the crossover temperature from dipole polarization to migration polarization at 260 K have been established. Linear and quadratic contributions to the magnetoelectric effect have been found. Below 120 K the linear contribution is an order of magnitude greater than the quadratic contribution and above 240 K the quadratic contribution to the ME effect prevails. Models have been proposed to explain the enhancement of the magnetoelectric effect as a result of the migration polarization in mullite and linear magnetoelectric effect in bismuth ferrite. The correlation of temperatures of the extremum of the temperature coefficient of the electrical resistance and the magnetic phase transition in mullite at 260 K indicates a polaron-type conductivity and a strong electron‒phonon interaction. A change in the sign of the electrostriction coefficient upon heating and the compression temperature of the composite in an electric field was found.Graphical Abstract: The effect of the size of Bi2Fe4O9 and BiFeO3 nanoparticles on the magnetoelectric interaction in the Bi2Fe4O9/BiFeO3 composite with a percentage ratio of 67/33 has been studied. The electrostriction and electric polarization on electric and magnetic field in wide temperature range has been measured. The hysteresis of the polarization and I‒V characteristics has been found. Temperature ranges with activation and hopping types of conductivity have been found. The mechanism of electric polarization and the crossover temperature from dipole polarization to migration polarization at 260 K have been established. Linear and quadratic contributions to the magnetoelectric effect have been found. Below 120 K the linear contribution is an order of magnitude greater than the quadratic contribution and above 240 K the quadratic contribution to the ME effect prevails. Models have been proposed to explain the enhancement of the magnetoelectric effect as a result of the migration polarization in mullite and linear magnetoelectric effect in bismuth ferrite. The correlation of temperatures of the extremum of the temperature coefficient of the electrical resistance and the magnetic phase transition in mullite at 260 K indicates a polaron-type conductivity and a strong electron‒phonon interaction. A change in the sign of the electrostriction coefficient upon heating and the compression temperature of the composite in an electric field was found. [ABSTRACT FROM AUTHOR]