The Preliminary Study On Two New Multiferroic Materials

The correlation/coupling between order parameters in solid state electronic system,which offers exotic quantum and magnetic transitions,is one of the core issues in condensed matter physics.The high concentration on ferroic order coupling,especially the exploration on intrinsic coupling between spin order and ferroelectric order(called multiferroicity)started in 2003.In the past 15 years,the emerging new physical phenomena and mechanisms have been deserved for deep exploration and understanding.In type Ⅱ multiferroics,ferroelectricity originates from specific spin order,in which the space and time inversion symmetries are simultaneously broken.Therefore,type Ⅱ multiferroics are expected to be good candidates for next generation of practical applications.Nevertheless,it is still tough for these applications due to the low ferroelectric transition temperature,small ferroelectric polarization,and intrinsic antiferromagnetic ordering,which highly limits the potential application devices.Motivated by progress on multiferroic materials in the last five years,we focus on two new multiferroic systems in this thesis.On the one hand,we try to synthesize high temperature multiferroics with high magnetic order and coexistence of ferroelectricity and ferromagnetism in hexagonal rare earth oxide h-RFeO3.On the other hand,we pay our attention to double perovskite Y2CoMnO6,in which the multiferroic property is still under debate.We try to shade light on the possible magnetoelectric coupling by investigating high-quality single crystals.The thesis is organized as the following:Chapter 1 starts from the basic concepts on multiferroic,followed closely by a detailed review on the physics and progress.We will give a short review on several representative multiferroics,including type Ⅰ multiferroics BiFeO3 and h-YMnO3,and two type Ⅱ materials TbMnO3 and Ca3CoMnO6.The inter-coupling between spin,lattice,charge,and orbital orders of freedom in these materials will be clarified.In Chapter2,attention is paid to the relevant experimental methods and devices used in the research,such as sample preparation,microstructure characterization,magnetic and electrical properties measurement.In Chapter3,we will systematically study the multiferroic properties of polycrystalline(Yb,Sc)FeO3.It is well known that h-RMnO3 usually has very high ferroelectric Curie temperature(TC～1000 K),but their antiferromagnetic Neel temperature are always below 100K.h-RFeO3 also offers the same geometric ferroelectricity mechanism as h-RMnO3,and the Fe3+-Fe3+ exchange interaction is stronger than Mn3+-Mn3+,hence higher Neel temperature could be desired.Although the ground state structure of RFeO3 family is orthorhombic,a metastable hexagonal structure can be still obtained in strained films.The latest studies also confirmed that hexagonal phase can be stabilized by a proper chemical doping or by sol-gel technique,while the Neel temperature seems difficult to be improved any more(highest TN～172K).In addition,it is noted that since most of the works are focused on LuFeO3,but Lu3+ is nonmagnetic,the 4f-3d coupling effect has not drawn enough attentions.Here we focus on h-YbFeO3,in which the magnetic structure and evidence of two steps of ferroelectric polarization are still inconclusive.In this thesis,we try to stabilize hexagonal phase through doping Sc3+,and explore systemically the multiferroic property.The Chapter 4 focuses on the possible multiferroic properties in Y2CoMnO6 single crystals.Y2CoMnO6 also exhibits ↑↑↓↓ magnetic structure and Co2+/Mn4+ charge ordering along c direction as that in Ca3CoMnO6.Recently,the double-perovskite compounds Lu2CoMnO6 and Yb2CoMnO6 offering the ↑↑↓↓ spin order are reported as new member of type Ⅱ multiferroics.However,the existing contradictory results on ferroelectricity,ferroelectric direction,and magnetic ground state becomes more and more questioned.By growing Y2CoMnO6 single crystals,we try to figure out whether the ferroelectricity is intrinsic or not.The effect of antisite disorder on magnetic and electric properties will also be discussed.The Chapter 5 is devoted to the conclusion and perspectives to the future work.