The Investigations On Magnetoelectric Coupling Effect Of CaBaCo₄O₇ Single Crystals

CaBaCo₄O₇ oxide has rich physical properties,especially its high polarization along the c-axis,and it has received extensive attention and research.Although the single crystal sample shows obvious anisotropy in the magnetic and pyroelectric current measurements along different directions,is there any polarization on the a and b axes,the cause of the polarization,and the nature of magnetoelectric coupling hasn not been identified.Based on this,the CaBaCo₄O₇ single crystal was grown using the optical floating zone method,and the magnetic,dielectric,pyroelectric,thermal expansion and magnetostrictive properties of the single crystal were analyzed in detail after orientated and investigated the source of the polarization systematically.By investigating the impact of temperature and magnetic field on the magnetism,dielectric,the pyroelectric（magnetic）current and Lattice distortion of the single crystals,the anisotropy properties of single crystal was measured.The investigations that the c-axis of CaBaCo₄O₇ single crystal is the oritention of the easy polarization and along the b-axis single crystal can be easily magnetized.At the temperature of 64 K,the transition from the ferrimagnetic state to the paramagnetic state is a first-order phase transition,where there are anomalous change of dielectric constant,lattice distortion,and spontaneous polarization.The pyroelectric current signal cannot be reversed by the reversal of the polarization electric field,indicating that CaBaCo₄O₇ single crystal is a pyroelectric material.The single crystal has magnetodielectric effects in each directions.Under the conditions that the electric field is parallel to the magnetic field,the magneto-dielectric effects along the a and b axes are less than about 10%,Along the c axis,the magnetodielectric effect is more obvious: at 64 K with the magnitude of the magnetic field of 0.68 T The dielectric constant can be changed by 128.3%.The axis of easy magnetization is perpendicular to the axis of easy polarization and the lattice distortion is near the magnetic order temperature.Based on these results and the analysis of the existing source mechanism of the polarization,We believe that the polarization of the CaBaCo₄O₇ single crystal is derived from the exchange stretching mechanism,and the exchange interaction produces elongation or contraction on the specific axis of the single crystal sample.By comparing and analyzing the changes of the magnetic,dielectric and expansion characteristics of the single crystal with the temperature and magnetic field in the a-axis direction,it was found that the single crystal has undergone magnetomagnetic transformation along this direction.Based on this,We proposes a spin-ordered model with Co1 and Co4 positions in this direction first,and a spin-ordered after Co2 and Co3 positions.