One of the major breakthroughs associated with multiferroicity in recent years is the discovery of ferroelectricity generated by specific magnetic structures in some magnetic insulating oxides such as rare-earth manganites RMnO3 and RMn2O5. An unresolved issue is the small electric polarization. Relatively large electric polarization and strong magnetoelectric coupling have been found in those manganites of double magnetic ions: magnetic rare-earth R ion and Mn ion, due to the strong R-Mn (4f-3d) interactions. DyMn2O5 is a representative example. We unveil in this work the ferrielectric nature of DyMn2O5, in which the two ferroelectric sublattices with opposite electric polarizations constitute the ferrielectric state. One sublattice has its polarization generated by the symmetric exchange striction from the Mn-Mn interactions, while the polarization of the other sublattice is attributed to the symmetric exchange striction from the Dy-Mn interactions. We present detailed measurements on the electric polarization as a function of temperature, magnetic field, and measuring paths. The present experiments may be helpful for clarifying the puzzling issues on the multiferroicity in DyMn2O5 and other RMn2O5 multiferroics.

Hexagonal LuFeO3 has drawn a lot of research attention due to its contentious room-temperature multiferroicity. Due to the unstability of hexagonal phase in the bulk form, most experimental studies focused on LuFeO3 thin films which can be stabilized by strain using proper substrates. Here we report on the hexagonal phase stabilization, magnetism, and magnetoelectric coupling of bulk LuFeO3 by partial Sc-substitution of Lu. First, our first-principles calculations show that the hexagonal structure can be stabilized by partial Sc substitution, while the multiferroic properties including the noncollinear magnetic order and geometric ferroelectricity remain robustly unaffected. Therefore, Lu1-xScxFeO3 can act as a platform to check the multiferroicity of LuFeO3 and related materials in the bulk form.Second, the magnetic characterizations on bulk Lu1-xScxFeO3 demonstrate a magnetic anomaly (probable antiferromagnetic ordering) above room temperature,~425-445 K, followed by magnetic transitions in low temperatures (~167-172 K). In addition, a magnetoelectric response is observed in the low temperature region. Our study provides useful information on the multiferroic physics of hexagonal RFeO3 and related systems.PACS numbers: 75.85.+t, 71.15.Mb,

The eg-orbital double-exchange mechanism as the core of physics of colossal magnetoresistance (CMR) manganites is well known, which usually covers up the role of super-exchange at the t2g-orbitals. The role of the double-exchange mechanism is maximized in La0.7Ca0.3MnO3, leading to the concurrent metal-insulator transition and ferromagnetic transition as well as CMR effect. In this work, by a set of synchronous Ru-substitution and Ca-substitution experiments on La0.7–yCa0.3+yMn1–yRuyO3, we demonstrate that the optimal ferromagnetism in La0.7Ca0.3MnO3 can be further enhanced. It is also found that the metal-insulator transition and magnetic transition can be separately modulated. By well-designed experimental schemes with which the Mn3+-Mn4+ double-exchange is damaged as weakly as possible, it is revealed that this ferromagnetism enhancement is attributed to the Mn-Ru t2g ferromagnetic super-exchange. The present work allows a platform on which the electro-transport and magnetism of rare-earth manganites can be controlled by means of the t2g-orbital physics of strongly correlated transition metal oxides.

[Abstract] So far most of earlier works on the effect of chemical substitution in multiferroic MnWO 4 have focused on the 3d transition metal substitution of Mn. In this work we investigate the Ru substitution of Mn in polycrystalline Mn 1-x Ru x/2 WO 4 in order to unveil the consequence of 4d transition metal substitution in terms of magnetic transitions and ferroelectricity. It is found that the Ru substitution substantially reshuffles the magnetic frustration and stabilizes the incommensurate helical spin order phase (AF2) by partially suppressing the collinear spin order phase (AF1 phase). The coexistence of the AF2 phase and AF1 phase at low temperature is suggested. Consequently, the ferroelectric polarization is remarkably enhanced, in accompanying with significant response of the polarization to magnetic field. It is argued that the structural distortion and enhanced spin-orbital coupling associated with the Ru substitution may be responsible for this ferroelectricity enhancement.