| The so-called multiferroics refer to those materials in which two or more primary ferroic properties such as ferroelectric or anti-ferroelectric, ferromagnetic or anti-ferromagnetic, ferroelastic, and ferrotoroidic properties, are united in the same phase. At present, the most interested materials are those with ferroelectric polarization well turned by external magnetic field and with magnetization well controlled by external electric field. We mainly deal with the so-called type-Ⅱ multiferroics in which the ferroelectric polarization is generated by some specific spin orders. On the on hand, such kind of materials may offer the advantages of both magnetic random access memory (MRAM) and ferroelectric random access memory (FeRAM). For example, it is claimed that in these materials the spin states can be reversed by writing the polarization states via electric field so that the high energy and complicated writing procedure in MRAMs can be avoided, benefiting to opportunities for further high performance multi-functional devices. On the other hand, multiferroics itself also provide attractive platforms for updating our understanding of ferroelectricity, magnetism, and spintronics etc.In spite of well promised future of type-Ⅱ multiferroics, quite a lot of chanllenges for practicl applications remain to be overcome. One key issue is the low Curie temperature Tc for ferroelectricity together with small polarization and weak ferromagnetism. The electric control of the spin states is an unsolved issue either. In this work, one representative system of type-II multiferroics, manganese tungstate MnW04 (MWO), will be chosen as an objective platform and a series of experimental works on the ferroelectricity generation and its magnetic origin as well as possible approaches to manipulate the multiferroicity will be performed. First, we investigate carefully the spin state dependence of the ferroelectricity including the path-dependence of the spin ordering sequence and polarization responses. Second, we deal with the minor Mn/W nonstoichiometry and its impact on the multiferroic behaviors. Third, we explore the Ru4+ substitution of Mn2+ in two different approaches. To the end, we reach a comprehensive understanding of the microscopic mechanisms of multiferroicity in these schemes. The whole thesis is organized as follows:Chapter One begins with a brief introduction to recent experimental and theoretical background on multiferroics. As the objective platform, we focus on MW and special attention is paid to the physical properties and multiferroic characters of MWO. It is understood from literature that upon cooling, MWO first enters a very narrow collinear but incommensurate (ICM) antiferromagnetic (AFM) structure (AF3 phase) with the wave vector q3=(0.214,0.5,-0.457) and sinusoidally modulated spin moment at T=TAF3-13.5K from the high-T paramagnetic phase, and then an ICM noncollinear spin structure (AF2 phase) with the wave vector q2=q3 is favored at T<TAF2-12.6K, leaving only a ～1.0K gap between the two consecutive magnetic transitions. At T<TAF1～7.8K, this ferroelectric (FE) AF2 phase is again replaced by a commensurate (CM) AFM phase (AF1) which is non-ferroelectric. From the viewpoint of multiferroic, the first issue we need to deal with is the stability of the AF2 phase and possible routes to enhance the FE polarization.In Chapter Two, we investigate the electric field control of the spin states in MWO. The fact that all the three antiferromagnetic phases (AF1, AF2 and AF3) are highly frustrated and close in energy make it possible to tune the magnetic structure by means of electric field via the magnetoelectric coupling. We address the magnetic phase transitions in MWO in response to electric field which is applied via various roadmaps, and it is demonstrated that an electric field as low as 10kV/cm is sufficient to enhance the stability of the ferroelectric AF2 phase in compensation of the non-ferroelectric AF1 ground state phase. This work suggests that electric field induced electrostatic energy, which is yet small due to the weak electric polarization induced magnetically, may effectively tune the magnetism of highly frustrated multiferroic materials.The multiferroic phase stability of MWO in response to the non-stoichiometry of Mn and W is addressed in Chapter Three. It is observed that the non-stoichiometry does not affect the ferroelectric transition point (the AF3-AF2 transition point) and the AF2-AF1 transition point, but the non-ferroelectric AF1 phase is partially replaced by the ferroelectric AF2 phase. The measured electric polarization is slightly enhanced with increasing stoichiometric deviation. While the lattice contraction is believed to be responsible for the destabilization of the non-ferroelectric AF1 phase, the appearance of Mn/W vacancies releases the spin frustration and stabilizes the AF2 configuration and thus the enhanced polarization.In Chapter Four, we focus on Mn1-xRuxWO4+δ where the one-to-one replacement of Mn by Ru is realized without inducing Mn vacancy. It is revealed that the Ru substitution in Mn1-xRuxWO4+δ does contribute to the electric polarization and partially suppress the non-ferroelectric AF1 phase, resulting in the multiferroic phase coexistence at low temperature. While the proposed structural model excludes the contribution of the spin-orbit coupling to the modulated electric polarization, the lattice distortion and strengthened robustness of the AF2 spin structure is supposed to be responsible for the enhanced polarization and multiferroic phase coexistence.Based on the above two chapters, in Chapter Five, we investigate the 1:2 Ru substitution of Mn (for charge neutrality) in polycrystalline MWO (Mn1-xRux/2WO4) in order to unveil the consequence of the co-existence of Ru ions and Mn vacancies to the magnetic transitions and ferroelectricity. It is found that is gradually suppressed and disappeared at x=0.20 while the high-T AF3 phase remains roughly unaffected. More importantly, we observe remarkable enhancement of the FE polarization. The value of polarization at T=2K at x=0.20 reaches up to 60μC/m2, almost one order of magnitude higher than pure MWO in polycrystalline form. According to the discussion in Chapters Three and Four, here we argue that the coexistence of Ru ions and Mn vacancies result in the measured phenomenons. First, both the two facts results the lattice constraction and second, they weakens the interactions between the spins and stablizes the AF2 phase, and all of these are beneficial to the enlarged polarization.The Sixth Chapter is devoted to the conclusion and perspectives to the future work. |
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