The Investigation Of Multiferroicity In The Spin Frustrated Materials

Spin frustration, or competing interaction, is common in systems with multifold degrees of freedom. Spin frustration is a long lasting hot topic in condensed matter physics, and it arises from the fact that each of the interactions in the competition tends to favor its own characteristic spatial correlations. Typically, two scenarios contribute to the frustration:one is the incompatibility between lattice structure and antiferromagnetism, and the other is from the mutual competitions of the nearest neighbor spins. Perhaps the most exciting event in spin frustrated systems in the recent ten years is the discovery of multiferroics with strong magnetoelectric coupling, indicating the intrinsic correlation between magnetism and ferroelectricity. The various order parameters behind the fantastic phenomenon, especially the mutual competition and modulation between the spin, lattice, charge, and orbit, arouse great interests.Multiferroics have been formally defined as materials that exhibit more than two primary ferroic order parameters simultaneously in a single phase. The ferroelectric polarization arises from spiral spin order or some specific spin order plus lattice distortion, where the space and time inversion symmetry are simultaneously broken. In this thesis, we focus on multiferroicity in several spin frustrated transition metal magnetic compounds. On the one hand, we try to explore the multiferroicity and hidden mystery in spin frustrated Dy2T2O7and FeCr2S4. On the other hand, we pay our attention to the typical multiferroics Ca3CoMnO6and GdMnO3in order to investigate the ferroelectricity improvement and correlated competition among the mutual magnetic orders. The whole thesis schedule is as follows:Chapter one starts from several basic concepts on magnetism and magnetic interactions, followed closely by a brief highlight on the research track of spin frustration. A detailed review on the physics of spin frustration and associated magnetic phase transitions in several typical spin frustrated systems, such as triangular lattice, Kagome lattice, pyrochlore structure, and spinel structure, is presented. Subsequently, a short introduction to the basic concepts and research history of multiferroics and their association with spin frustration is made. In the last section of this chapter, we give a review on several representative multiferroics and clarify the coupling between spin frustration, lattice, charge, orbit, and spin freedoms in these materials.In chapter two, attention is paid to polycrystalline spin ice Dy2Ti2O7. The magnetism, specific heat, dielectricity, ferroelectricity, and magnetoelectric coupling are systematically studied. The ferroelectricity in Dy2Ti2O7is demonstrated by a series of characterizations, revealing that the spontaneous electric polarization emerges subsequently at temperature T=Tc1～25K and T=Tc2～13K. It is argued that the high temperature ferroelectric phase may originate from the Dy-O tetrahedral distortion, while the phase below Tc2～13K comes from the excitation of magnetic monopoles. The macroscopic polarization and multiferroicity can be modulated by external electric field and magnetic field, due to the modulated direction and density of the monopoles. Besides, a series of experiments on the role of magnetic ions Dy3+in determining the multiferroicity of Dy2Ti2O7are performed. The related ferroelectricity of pure Y2Ti2O7, Gd3+and Tb3+doped Dy2Ti2O7are explored. At last, Monte Carlo simulation is applied to simulate the density of the magnetic monopoles in response to temperature, and the temperature dependent polarization is evaluated, roughly consistent with experiment results.Possible modulation of the spin frustration and ferroelectricity by ionic disorder in multiferroic Ca3CoMnO6is addressed in chapter three. Ca3CoMnO6is a well known multiferroics, in which the collinear spin order induces the ferroelectric polarization. The symmetrical exchange striction mechanism contributes to the polarization, which is strongly dependent on the Ising↑↑↓↓long range spin order and Co/Mn ionic order. It was reported that the long range order is abruptly lost in the narrow vicinity of Co/Mn=1and the magnetic state becomes incommensurate. The long range order can be recovered once Co/Mn deviates from one, thus leading to the macroscopic polarization. Our data show that Mn deficiency in Ca3CoMnO6can change the interchain superexchange interactions to release the spin frustration, and give an enhanced ferroelectric polarization. Meanwhile, the ferroelectric transition can be enhanced from16K to32K. Subsequently, the ferroelectricity in Mn-deficient Ca3CoMnO6upon the Fe substitution of Mn is investigated, and the enhanced ferroelectricity is obtained. Therefore, it is a powerful approach to enhance the ferroelectricity by introducing Co/Mn ions deficiency in Ca3CoMnO6.Chapter four deals with the magnetism, specific heat, dielectricity, ferroelectricity, and magnetoelectric coupling in spinel chalcogenide FeCr2S4. Extraordinary magnetic phases with mutual coupling between spin, orbit, and lattice are available, allowing FeCr2S4to be a candidate for exploring the mutual competition and modulation among the various order parameters. Our results show that FeCr2S4is also a multiferroic with both ferroelectricity and ferrimagnetism firmly coupled. The Fe2+and Cr3+spins transfer from the collinear ferrimagnetism to the noncollinear conical spin order below50K. Accompanied by the enhancement of spin-orbital coupling, FeCr2S4evolves into ferroelectric phase at Fe2+orbital ordering termperature T=8.5K. Such a ferroelectric phase on one hand originates from the two opposite conical spin structures of Fe2+and Cr3+sublattices, whose propagation vectors remain stable below6K, thus contributing to the nonzero polarization. On the other hand, the Jahn-Teller effect of Fe2+sublattice also gives rise to the polarization along with the orbital ordering of Fe2+. Therefore, our data on FeCr2S4evidence the strong coupling between ferroelectricity, orbit, lattice, and magnetism.In chapter five, we pay close attention to the ferroelctricity of polycrystalline orthogonal perovskite GdMnO3. Due to the quenched disorder and defects, GdMnO3may accommodate the cycloidal spin order in addition to the A-type AFM order at Mn sites. Our results show complicated responses of the pyroelectric current to magnetic field, temperature, and pyroelectric mode, indicating multi-fold magnetoelectric responses of GdMnO3. The two-orbit double exchange model and measured hysteresis loops of polarization against magnetic field indicate the coexistence of A-type AFM and cycloidal spin order in the Mn site. The cycloidal spin order is the main source for ferroelectricity. It should be noted that the Mn cycloidal spin order can be modulated by the Gd3+AFM order at low temperature via the Gd3+-Mn3+spin interaction.The sixth chapter is devoted to the conclusion and perspectives to the future work.