Anomalous Magnetism And Magnetoelectric Effect In The Perovskite Oxides R(Cr,Mn)O₃ System

Multiferroics are the compounds in which the ferroelectric (or antiferroelectric) and ferromagnetic (or antiferromagnetic) order coexist simultaneously in a certain temperature range. The coexistence of the two order parameters may result in the coupling interaction between them.In detail, the ferroelectric polarization may change the magnetic property by redistributing the spin order, correspondingly,the fluctuation of the spin order may induce the dielectric anomaly or the ferroelectric relaxation through the magnetostrictive effect or electron-phonon interaction.The dielectric anomaly at the magnetic transition temperature observed in experiment is indicative of the inherent magnetoelectric(ME) coupling in multiferroics. Furthermore, the application of an external magnetic field will induce the dielectric change, named as the magnetocapaciance (MC) effect. And the magnetocapaciance effect is another research focus for perovskite manganites except for the giant magnetoresistance effect. In technologic area, the ability to couple with either the magnetic or the elelctric polarization offers an extra freedom in the design of convention actuators, transducers, and storage devices. A number of device applications have been suggested for ferroelectromagnets, including multiple state memory elements, ferromagnetic resonance devices controlled by electric field, and variable transducers with either magnetically modulated piezoelectricity or electrically modulated pizeomagnetism.The investigation of the magnetoelectric effect has important fundamental values and extensive technological applications. In this dissertation, based on the preparation of high quality polycrystalline samples, the novel anomalous magnetic and magnetoelectric effect for perovskite oxides R(Cr,Mn)O3 system were studied. The main results are summarized as follows:1. The dc/ac magnetization and specific heat measurement indicate that Cr3+ orders canted antiferromagnetic with a weak ferromagnetism at TN =133 K, Around TSR≈22K, ErCrO3 undergoes a spin reorientation(SR) fromГ4 toГ1. And the stability of the ferromagneticГ4 phase increases with increasing applied field. Furthermore, the electric polarization Ps increases rapidly with decreasing temperature around TN =133 K. This anomalie is clear indication of the ME coupling. And the result that the applied magnetic field can suppresses the polarization is additional evidence with the ME effect in ErCrO3.2. In low temperature range, the magnetization of the perovskite YbCrO3 chromites strongly show that there exist two complex sequences of magnetic transitions with the characteristics of magnetization reversal and Yb3+/Cr3+ spin ordering at different temperature respectively. The antiferromagnetic-like transition around TN=118K is attributed to the antiferromagnetic ordering of the Cr3+ spins and a negative magnetization, accompanied with a tendency to the plateau below 10K caused by the Yb3+ ordering. This can be interpreted by accounting for the molecular field approximation by assuming that the induced polarization of the paramagnetic ytterbium ions is opposite to the ferromagnetic moment of the chromium ions. When the Cr and Yb substructures of ordering and polarization are equivalent, the Yb substructure moments totally overcome the Cr moments at the compensation temperature (Tcomp=18.5K). As soon as the polarization of the Yb3+ moments exceed the ferromagnetic component of the Cr3+ sublattice (T