First-Principles Calculations On Structural, Magnetic And Multiferroic Properties Of Rare-earth Based Perovskites

Multiferroic materials, which exhibit both magnetic and ferroelectric order, have recently attracted great attention for their potential applications in spintronics. Designing and exploring novel multiferroic materials has thus become a hot topic. The functional perovskite oxides as potential candidates for multiferroics is at the heart of such research field. In addition, the coupling among lattice, charge, spin and orbital degrees of freedom in perovskite oxides has also attracted scientific attention. This work focuses on rare-earth ferrites RFeO3, as well as the double perovskites R2NiMnO6 and R2CoMnO6 (R= rare-earth ion). By first-principles calculation, crystal structure and physical properties are investigated. Three kinds of "pressure" are considered in the present work:chemical pressure, hydrostatic pressure and misfit strain. We call them "modification pressures", since all of them can affect cell volume and physical properties of perovskites. We try to explore the possible magnetic, ferroelectric, multiferroic and other electric properties of perovskites, by applying these three kinds of "pressure". Using first-principles simulation, we present the following conclusions.The effects of chemical and hydrostatic pressures are not equivalent on the crystal structure and physical properties of double perovskites. On the one hand, compared to the hydrostatic pressure, the effect of chemical pressure is more obvious on octahedral tilting and less obvious on bond length (as in Fe-O, Ni-0 and Mn-O bonds). On the other hand, there is always a linear effect on the structural parameters of perovskites for hydrostatic pressure but not for chemical pressure:for example, neither the lattice parameter b nor the bond length varies monotonically with chemical pressure. As for magnetic properties of perovskites, the effects of chemical and hydrostatic pressures can be both similar (i.e., weak magnetization of RFeO3) and different (i.e., magnetic Curie temperature of R2NiMnO6).The atomistic theory of hybrid improper ferroelectricity in (A’BO3)1/(A"BO3)1 perovskite superlattices is given. Two energy terms combining the tilting of BO6 octahedral (the anti-phase tilting ωR and in-phase tilting ωM) and antiferroelectric motions of A-site ions is demonstrated. Here, the trilinear coupling ωRωMP is responsible for ferroelectricity in (001)-oriented (A’BO3)1/(A"BO3)1 superlattices. Meanwhile, (001)-oriented (A’BO3)m/(A"BO3)n superlattices (both A’BO3 and A"BO3 have Pbnm symmetry and a-a-c+ tilting) show ferroelectric order with Pmc21 symmetry if and only if both m and n are odd number.A completely new idea has been raised to build the near room-temperature multiferroic materials, namely, combining two paraelectric ferromagnetic rare-earth based double perovskites to form the ferroelectric ferromagnetic superlattice. The improper ferroelectriticy is induced by the unequal antiferroelectric displacements of two different rare-earth ions. By first-principles and Monte Carlo simulations, we have predicted novel near room-temperature multiferroic R2NiMnO6/La2NiMnO6 superlattices. Such superlattices, in which ferromagnetic and ferroelectric orders coexist and are tunable, is very promising for room-temperature multiferroics research.The epitaxial misfit strain can induce ferroelectricity in paraelectric rare-earth ferrites, making RFeO3 multiferroics. The polarization and Neel temperature are tunable by chemical and epitaxial misfit strain. First-principles calculations indicate (1) at large enough compressive strain, the resulting ferroelectric phase of CeFeO3, PrFeO3, NdFeO3 and SmFeO3 is tetragonal P4mm; and (2) at large enough tensile strain, the resulting ferroelectric phase of RFeO3 (R= Ce～Er) is P21am(Ⅰ). The multiphase boundary will also occur in the vicinity of critical tensile strain.A high-spin to low-spin transition induced by hydrostatic pressure is predicted in double perovskites R2CoMnO6 (R= Nd, Sm, Gd, Tb, Dy, Ho and Er). This is a first-order phase transition, with a discontinuous change of crystal structure, magnetic and electronic properties in R2CoMnO6. For R= Nd, Sm, Gd and Tb, the high-spin to low-spin transition is also an insulator to half-metal transition, while for R= Dy, Ho and Er, this transition is also an insulator-semiconductor transition.