The Investigation Of Multiferroicity In The Spin Frustrated Manganite DyMn₂O₅

The investigation on multiferroic materials has been always a hot topic in condensed matter physics for recent ten years. Multiferroics have been formally defined as materials that simultaneously exhibit more than one primary ferroic order parameter in a single phase. The ferroelectric polarization arises from specific spin order, or collinear spin order plus lattice distortion, where the spatial and time inversion symmetries are simultaneously broken. In these materials, electric polarization P is believed to be intrinsically correlated with particular spin order, and thus the cross-coupling between ferroelectricity and magnetism is significant, allowing possible magnetic control of electric polarization or/and electric control of magnetism. In this thesis, we focus on multiferroicity of DyMn2O5 (DMO), a representative member of the spin frustrated transition metal manganite compounds RMn2O5 where R is the rare-earth ion. First, the ferrielectric model is explicitly proposed to explain the ferrielectricity of DMO. Then, typical strategies of chemical substitution were used to modulate the reversal of electric polarization. The motivations and main results of this thesis are highlighted as the following:In Chapter One, the concept and progress of multiferroic researches are summarized first. Then the mechanism of the spin frustrated multiferroics is described, followed by highlighting some typical multiferroic materials with strong coupling between the spin and charge order parameters, such as the lattice, charge, orbit and spin. The background for RMm2O5 is introduced in the subsequent part. At last, the application of multiferroics is briefly described.In Chapter Two, the temperature-dependent electric polarization of DMO is investigated using the pyroelectric current method and the positive-up-negative-down (PUND) method. It is revealed that DyMn2O5 does exhibit the unusual ferrielectricity rather than ferroelectricity. The two ferroelectric sublattices are believed to be generated from the symmetric exchange-striction mechanisms associated with the Mn-Mn spin interactions and Dy-Mn spin interactions, respectively. The path-dependent electric polarization features the first-order magnetic transitions in the low temperature regime. The magnetoelectric effect is mainly attributed to the Dy spin order which is sensitive to magnetic field. The present experiments may be helpful for clarifying the puzzling issues on the multiferroicity in DyMn2O5 and other RMn2O5 multiferroics.In Chapter Three, attention is paid to reversing ferroelectric polarization in multiferroic DMO by nonmagnetic Al substitution of Mn. We have investigated in details the effects of Al3+ substitution of Mn ions on the magnetic and ferroelectric behaviors in multiferroic DMO, based on the proposed model on ferrielectricity generation. It is revealed that structurally the Al3+ substitution favors the replacement of Mn3+sites rather than Mn4+ions, and makes the lattice contracting slightly. This tiny structural distortion disables the independent Dy spin ordering which enters below TDy～8K. In consequence, the ferrielectric lattice decomposes gradually into a normal ferroelectric lattice by disappearance of the electric polarization component PDM due to the gradual disordering of the ↓↓↑ or ↑↑↓ collinear Mn3+(Al3+)-Mn4+-Mn3+(Al3+) spin blocks, while the ↓↓↑ or ↑↑↓ collinear Dy3+-Mn4+-Dy3+spin blocks are maintained. It is demonstrated that the simple strategy of Al-substitution of Mn can be an effective approach to tune the electric polarization and reverse it from negative value to positive one. The present work provides a comprehensive understanding of the multiferroicity in DMO and may shed light on efficient approaches to improve the multiferroic performances of the whole RMn2O5 systems.In Chapter Four, based on the proposed model for ferrielectricity generation, another alternative to tune the multiferroics of DMO is substituting Dy3+by nonmagnetic Y3+ion. It is revealed that structurally the Y3+substitution enables a slight contraction of the lattice, and the temperature of the independent Dy spin ordering is shifted to the lower temperature side upon the increasing Y substitution level. In consequence, the electric polarization component PDM is decreased gradually due to the disordering of the ↓↓↑ or ↑↑↓ collinear Dy3+(Y3+)-Mn4+-Dy 3+(Y3+) spin blocks, while the ↓↓↑ or ↑↑↓ collinear Mn3+-Mn4+-Mn3+spin blocks are maintained. Due to the inhomogeneity of Y3+substitution, the ferrieletricity model is applied to explain the ferroelectricity of the present Y-doped DMO. It is demonstrated that the Y-substitution of Dy can be an effective approach to tune the electric polarization.In Chapter Five, the PUND method is comprehensively used to explore the ferrielectricity in DMO. It is demonstrated that the ferroelectric transition points are consistent with earlier reports, while the phase below the Dy3+independent spin ordering may be nonferroeletric, consistent with our pyroelectric current results. The P-E loop is characterized by a slightly deformed hysteresis loops, which may evidence the ferrieletricity of DMO.The Chapter Six is devoted to the conclusion and perspectives to the future work.