| Multiferroicity refers to coexistence and coupling among various ferroic orders.The cross-control of the ferroic orders in multiferroics is significant,which can be utilized to design memory devices with high density,high speed,and low power.For instance,in multiferroic materials containing ferroelectricity and magnetism,the magnetization M(polarization P)can be manipulated by electric field E(magnetic field H).Among,the multiferroics with multiple spin lattices showing abundant physics have attracted great attention in the past few years.The primarily physical properties of these materials are embodied in several aspects.First,the interactions between multiple spin lattices are strong,usually yielding higher Curie temperature TC.Second,the diverse combinations between multiple spin lattices will lead to complex multiferroic phenomena and physics.Third,the different moments in various spin lattices may generate a net M,which is in favor of enhancing the ferromagnetism.With these motivations,multiferroicity and its modulation of Fe2Mo3O8 and Ni3TeO6 have been studied extensively.The main results and innovations of the thesis can be summarized as following:(1)Magnetoelectric(ME)properties of Fe2Mo3O8 single crystal were explored systematically under steady-field and pulsed-field.It is revealed that an antiferromagnetic(AFM)order is formed below TN=60 K,which can be excited to a ferrimagnetic(FIM)state by an out-of-plane field H.Concurrently,the AFM-FIM transition produces sudden change in permittivity εr and polarization P,unveiling the strong ME coupling in this system.In addition,the pulsed-field experiments demonstrate that a novel inverted behavior of the linear ME coefficient α3 presents in Fe2Mo3O8,i.e.,the sign of α3 changes from negative to positive when H increases to-40 T.Imporatntly,a giant linear ME effect with α3～-430 ps/m at 4.2 K is identified,which is about 1-2 orders of magnitude larger than the values in literature.A detailed phase diagram of Fe2Mo3O8 is established.These findings indicate that exchange striction mechanism is an efficient route to obtain remarkable linear ME effect.(2)(Fe1-xZnx)2Mo3O8(0.05≤x≤0.5)single crystals were fabricated by the chemical vapor transport technique and their ME properties were investigated in detail.The AFM interaction in Fe2Mo3O8 is highly damaged by doping nonmagnetic Zn2+ion.However,the change of M modulated by E becomes stronger when introducing a small amount of Zn2+ dopants.Meanwhile,the excited-field He for E-control of M reduces greatly.It offers a feasible method to achieve remarkable electro-control of M with low H.In addition,it can be confirmed that magnetodielectric(MD)effect and E-control of M are closely associated,i.e.,the stronger the MD coupling is,the more significant the E-control of M is.We also find the microscopic ME origin in Fe2Mo3O8 changes as increasing T,which originates from the enhanced spin fluctuation and short-range magnetic interaction.Thus,(Fe1-xZnx)2Mo3O8 could be good candidates of realizing electric-control of magnetization due to the spin-lattice coupling,which pave the way to explore low-field electric control of magnetization in multiferroics.(3)The ME properties of Ni2MnTeO6 single crystal were measured systematically.In Ni2MnTeO6,the anisotropic magnetic exchange is highly influenced by thermal fluctuation.Therefore,the magnetocrystalline anisotropy of Ni2MnTeO6 is changed upon cooling.It forms a canted ↑↑↓↓↓↑ magnetic ground state,in which the DM interaction appears.The DM vector is crucial to the successive two-step spin-flop transition observed below～20 K.The ME phase diagram of Ni2MnTeO6 is well established according to the magnetism,di electricity and ferroelectricity measurements.Besides,it also manifests that substituting Ni2+ by some Mn2+ ions enriches the multiferroic phase of Ni3TeO6,but weakens the ME coupling slightly.These results indicate that by magnetic ion doping,the magnetic exchange interactions can be enriched,and thus new multiferroic phase may be realized,which is significant for exploring new multiferroics and ME coupling.(4)The ME characters of NiCo2TeO6 single crystal along different orientations were explored systematically.It can be concluded that NiCo2TeO6 possesses easy-plane magnetic anisotropy.The in-plane field H induces clear ME coupling effect along the c-axis and ab-plane.It means ferro-toroidal moment T=P × M,one origin of the non-reciprocity,appears in NiCo2TeO6.Additionally,E-control of Mis greatly improved with increasing T,while the necessary excited-field He is small,which makes it possible to achieve significant ME cross-control at high T and low H.Our results imply that the coexistence of time and space symmetry broken is not only significant for the ME effect,but also crucial for realizing novel physical phenomena such as non-reciprocal transport,which is important for exploiting multiferroic physics. |
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