The Modulation Of The High Magnetic Field And Doping On The D-f Interaction And Multiferroicity In RFeO₃

The rare-earth orthoferrites RFeO₃ with perovskite structure possess rich physical properties and potential applications,which have been concerned widely by researchers in the material science and condensed matter physics due to the existence of the complex magnetic interactions（i.e.,Fe-Fe,Fe-R and R-R）in this system.However,the earlier studies on RFeO₃ system were mainly focused in the lower magnetic field range.The relevant research reports on the system in the high field（up to about 60.0 T）are few.Hence,the investigations of the magnetic phase transitions induced by pulsed high magnetic fields in the RFeO₃ system will be beneficial to understand the complex interactions of the 3d-4f,and provide experimental basis datum for further investigation of the magneto-electric coupling effect in this system.Besides,there are few reports about the magnetodielectric effect,which is one of the strongest evidence for the existence of a magneto-electric coupling effect,of this system at low temperatures.Therefore,studying the magnetic dielectric behavior of RFeO₃ materials will promote the exploration of the magneto-electric coupling effect in RFeO₃ system and its physical mechanism.In this dissertation,the magnetic and magnetodielectric behaviors of Nd Fe O₃,ErFe O₃single crystals and their doped samples were investigated based on the pulsed high magnetic field and dielectric measurement system.The purposes of this dissertation are to deepen the understanding of the complex interactions of the 3d-4f in RFeO₃,and to explore the possible magnetoelectric coupling effect.The main research contents of this dissertation are as following:First of all,the magnetic properties of ErFe O₃ single crystal samples under static magnetic field and pulsed high magnetic field are investigated in detail,and the complete magnetic phase diagram is constructed.The results confirm that the spin reorientation and spin switch transitions occur almost exclusively within the ac plane.In addition to the temperature induced spin reorientation transition ofΓ₂（?）Γ₄ type,the spin reorientation transition also can be driven by the external magnetic field.When the magnetic field is applied along the b axis,the spins of Er³⁺and Fe³⁺ions successively deviate from the ac plane with the increase of magnetic field and finally form a helical structure.The measurement results of the magnetostriction confirms that there exists a strong coupling between the spin and lattice in the ErFe O₃ single crystal.Secondly,the effect of ion doping in different sites on the magnetization behavior of ErFe O₃ single crystal is investigated.Mn³⁺-dopant reduces the net magnetic moment of Fe³⁺sublattice and the Jahn-Teller effect of Mn³⁺ions induces local distortion,which has great impact on the superexchange interaction and the magnetic anisotropy,enhances the strength of the antiferromagnetic coupling between rare earth ions and weakens the strength of the antiferromagnetic coupling between Er³⁺and Fe³⁺sublattice.As a result,the critical temperature of the spin reorientation and spin switch transition undergo changes.The Dy³⁺-dopant has weak influence on the interaction between rare earth ions,but weakens the interaction energy between 3d-4f,resulting in that the spin orientation and spin switch transition shifted to a lower temperature region.In addition,there exists a new Dy³⁺-Dy³⁺interaction and an additional magnetic phase transition is observed when the magnetic field is parallel to the b axis.Thirdly,the magnetic phase transition behavior of Nd Fe O₃ single crystal samples under static magnetic field and pulsed high magnetic field were explored.The results show that Nd Fe O₃ single crystal exhibits spin reorientation,negative magnetization and spin switch transitions under the modulation of temperature and magnetic field.In addition to the temperature induced spin reorientation transition ofΓ₂（?）Γ₄ type,the spin reorientation transition also can be driven by the external magnetic field along a axis or c axis.When the magnetic field is parallel to a axis,the magnetization behavior of Nd Fe O₃ is closely related to the sweep rate of the magnetic field.When the temperature is lower than the compensation temperature T_comp,the multistep phase transition behavior induced by magnetic field emerges.As the temperature rises above the T_comp,the multistep phase transition disappears and becomes irreversible.This peculiar magnetization phenomenon results from the competition between antiferromagnetism and weak ferromagnetism in the Nd Fe O₃.Finally,the dielectric properties of ErFe O₃ and its doped samples and Nd Fe O₃ single crystal samples are studied in detail.The experimental results show that the ErFe O₃ single crystal and its doped samples exhibit colossal dielectric constant and dielectric relaxation behavior at room temperature,and nonlinear magnetodielectric effect at low temperature.When the electric field is parallel to the c axis,the dielectric behavior of the sample is closely related to magnetization behavior.The magnetodielectric effect induced by a magnetic phase transition can be observed at the critical magnetic fields B_a2 and B_c1,indicating the existence of magnetoelectric coupling effect.While the colossal dielectric constant and dielectric relaxation behavior,as well as the abnormal dielectric behavior induced by a magnetic phase transition are not found in the Nd Fe O₃ single crystal.The anisotropic and nonlinear magnetodielectric effects observed at low temperatures may be ascribed to the spin-phonon couplings.