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Searching for low-power-consuming and high-efficient methods for well controllable driving of skyrmion motion is one of the most concerned issues for future spintronic applications, raising high concern with an appreciated choice of magnetic media and driving scenario. In this work, we propose a novel scenario of spin wave driven skyrmion motion in a ferrimagnetic (FiM) lattice with the net angular momentum {\delta}s. We investigate theoretically the effect of both {\delta}s and the circular polarization of spin wave on the skyrmion dynamics. It is revealed that the momentum onto the skyrmion imposed by the excited spin wave can be partitioned into a ferromagnetic term plus an antiferromagnetic term. The ratio of these two terms and consequently the Hall angle of skyrmion motion can be formulated as the functions of {\delta}s, demonstrating the key role of {\delta}s as an effective control-parameter for the skyrmion motion. Moreover, the spin wave frequency dependent skyrmion motion is discussed, predicting the frequency enhanced skyrmion Hall motion. This work thus represents an essential contribution to understand the skyrmion dynamics in a FiM lattice., Comment: 25 pages, 4 figures

The hexagonal rare-earth ferrite $R\mathrm{Fe}{\mathrm{O}}_{3}$ family represents a unique class of multiferroics exhibiting weak ferromagnetism, and a strong coupling between magnetism and structural trimerization is predicted. However, the hexagonal structure for $R\mathrm{Fe}{\mathrm{O}}_{3}$ remains metastable in conventional conditions. We have succeeded in stabilizing the hexagonal structure of polycrystalline $\mathrm{Yb}\mathrm{Fe}{\mathrm{O}}_{3}$ by partial Sc substitution of Yb. Using bulk magnetometry and neutron diffraction, we find that ${\mathrm{Yb}}_{0.42}{\mathrm{Sc}}_{0.58}\mathrm{Fe}{\mathrm{O}}_{3}$ orders into a canted antiferromagnetic state with the N\'eel temperature ${T}_{N}\ensuremath{\sim}165\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, below which the ${\mathrm{Fe}}^{3+}$ moments form the triangular configuration in the $ab$ plane and their in-plane projections are parallel to the [100] axis, consistent with magnetic space group $P{6}_{3}{c}^{\ensuremath{'}}{m}^{\ensuremath{'}}$. It is determined that the spin canting is aligned along the $c$ axis, giving rise to the weak ferromagnetism. Furthermore, the ${\mathrm{Fe}}^{3+}$ moments reorient toward a new direction below reorientation temperature ${T}_{R}\ensuremath{\sim}40\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, satisfying magnetic subgroup $P{6}_{3}$, while the ${\mathrm{Yb}}^{3+}$ moments order independently and ferrimagnetically along the $c$ axis at the characteristic temperature ${T}_{\mathrm{Yb}}\ensuremath{\sim}15\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. Interestingly, reproducible modulation of electric polarization induced by magnetic field at low temperature is achieved, suggesting that the delicate structural distortion associated with two-up/one-down buckling of the Yb/Sc planes and tilting of the $\mathrm{Fe}{\mathrm{O}}_{5}$ bipyramids may mediate the coupling between ferroelectric and magnetic orders under magnetic field. The present work represents substantial progress to search for high-temperature multiferroics in hexagonal ferrites and related materials.

Earlier neutron scattering studies suggested the coexistence of complex stripelike and zigzaglike antiferromagnetic orders in polar honeycomb lattice ${\mathrm{Ni}}_{2}{\mathrm{Mo}}_{3}{\mathrm{O}}_{8}$, while its magnetoelectric (ME) behavior as an emergent effect is thus of high interest. Here we report our observations of two metamagnetic transitions and novel ME responses of ${\mathrm{Ni}}_{2}{\mathrm{Mo}}_{3}{\mathrm{O}}_{8}$ single crystals against high magnetic field $H$ up to \ensuremath{\sim}60 T. The $c$-axis (polar axis) spontaneous electric polarization ${P}_{\mathrm{spin}}$, emerging at the magnetic N\'eel temperature ${T}_{N}\ensuremath{\sim}5.5\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, and its remarkable response to $H$ applied along the $c$ axis ($H//c$) and $a$ axis ($H//a$), respectively, provide the clear evidence for the magnetism-driven ferroelectricity. While the magnetism exhibits the in-plane anisotropy to some extent, the magnetic field dependencies of magnetization and electric polarization in the low-field region and high-field region are distinctly different. In the low-field region where a weak spin-flop type metamagnetic transition occurs, the electric polarization response shows the parabolic dependence of magnetic field applied along both the $c$ axis and in-plane $a$ axis. The second metamagnetic transition happens when the magnetic field extends up to the high-field region where the magnetization and electric polarization response at low temperature are characterized by an extraordinarily broad plateau for the magnetic field along the $c$ axis but roughly linear dependence for field along the $a$ axis. These unusual phenomena are discussed, based on the symmetry-related local ME tensor analysis, and it is suggested that both the spin current and p-d hybridization mechanisms may contribute to the spontaneous electric polarization and ME responses. The present work demonstrates ${\mathrm{Ni}}_{2}{\mathrm{Mo}}_{3}{\mathrm{O}}_{8}$ as a unique multiferroic and promising platform for exploring the rich spin-1 physics and ME phenomena in honeycomb lattice.

Among numerous multiferroic phenomena observed in spin frustrated lattice, giant magnetoelectricity in honeycomb lattice ${(\mathrm{Fe},\phantom{\rule{0.28em}{0ex}}\mathrm{Mn})}_{2}{\mathrm{Mo}}_{3}{\mathrm{O}}_{8}$ has stimulated great interest and substantial effort in searching for novel members in this 238 family. In this work, we synthesize successfully compound ${\mathrm{Co}}_{2}{\mathrm{Mo}}_{3}{\mathrm{O}}_{8}$, a structural analogue of ${\mathrm{Fe}}_{2}{\mathrm{Mo}}_{3}{\mathrm{O}}_{8}$, and present a series of characterizations on its structural, magnetic, and electric properties. An antiferromagnetic transition takes place at the Neel temperature ${T}_{N}=39\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ with appearance of electric polarization and dielectric anomaly, which provides clear evidence of simultaneous magnetic and ferroelectric transitions. The neutron powder diffraction (NPD) and magnetic susceptibility data confirm the $c$-axis collinear antiferromagnetic orders and emergent ferroelectric polarization. In particular, such antiferromagnetic order is relatively robust against magnetic field up to 9 T, different from ${\mathrm{Fe}}_{2}{\mathrm{Mo}}_{3}{\mathrm{O}}_{8}$ with ferrimagnetic transition or ${\mathrm{Mn}}_{2}{\mathrm{Mo}}_{3}{\mathrm{O}}_{8}$ with spin flop in the low-field region. Our data on single crystals demonstrate the second-order magnetoelectric effect in terms of magnetic field dependence of ferroelectric polarization response, while no linear magnetoelectric response is allowed. It is suggested that ${\mathrm{Co}}_{2}{\mathrm{Mo}}_{3}{\mathrm{O}}_{8}$ provides a unique platform on which rich multiferroic physics in the presence of collinear magnetic order can be explored.

We report successive electric-polarization $(P)$ switches in the $S=1/2$ quantum magnet ${\mathrm{Co}}_{2}{\mathrm{V}}_{2}{\mathrm{O}}_{7}$ by application of a magnetic field along the $b$ axis, where a 1/2-magnetization plateau is seen at 5.4--11.6 T. Polarization reversal appears at $\ensuremath{\sim}5$ T from $\ensuremath{-}P\ensuremath{\parallel}b$ to $+P\ensuremath{\parallel}b$, leading to an irreversible magnetoelectric history involved with a memory effect, while the polarization flop in fields of 12--17 T is identified by a transition from the $P\ensuremath{\parallel}b$ to $P\ensuremath{\parallel}ac$ plane, different from those reported previously. These intriguing magnetoelectric phenomena are owing to the unique nature of the skew-chain-like magnetic structure of ${\mathrm{Co}}_{2}{\mathrm{V}}_{2}{\mathrm{O}}_{7}$ and can be understood by a change in symmetry of the magnetic order in applied fields. The emergent ferroelectricities deviating from the half-plateau state may arise from magnon Bose-Einstein condensation in this quantum magnet.

Low-dimensional frustrated antiferromagnet is a good model system to study exotic quantum physics. Here we report the observation of half magnetization plateau and ferroelectricity which emerge simultaneously in the bond-alternating skew-chain compounds ${R}_{2}{\mathrm{V}}_{2}{\mathrm{O}}_{7}\phantom{\rule{4pt}{0ex}}(R=\mathrm{Ni},\text{Co})$ induced by high magnetic fields. The half plateau is stabilized in fields of 8--30 T (7--12 T) for $\mathrm{N}{\mathrm{i}}_{2}{\mathrm{V}}_{2}{\mathrm{O}}_{7}\phantom{\rule{4pt}{0ex}}(\mathrm{C}{\mathrm{o}}_{2}{\mathrm{V}}_{2}{\mathrm{O}}_{7})$, whereas two magnetic field induced ferroelectricities are located below and above this plateau. The remarkable high-field ferroelectricity for Ni or Co compound is about $50\text{\ensuremath{-}}60\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{C}/{\mathrm{m}}^{2}$ for the polycrystalline sample. The resulting magnetic field-temperature $(H\text{\ensuremath{-}}T)$ phase diagrams are very complex and several distinct phase transitions are observed. The forced electric polarization by sweeping magnetic fields evidences a clear magnetoelectric coupling in $\mathrm{C}{\mathrm{o}}_{2}{\mathrm{V}}_{2}{\mathrm{O}}_{7}$ associated with the low-field ferroelectricity. Our magnetization data also reveal that $\mathrm{C}{\mathrm{o}}_{2}{\mathrm{V}}_{2}{\mathrm{O}}_{7}$ produces an effective spin-1/2 behavior at the magnetic ground state. These experimental findings improve the knowledge to multiferroics and pave the way for exploring the quantum state of frustrated antiferromagnets.

$\mathrm{C}{\mathrm{a}}_{3}\mathrm{T}{\mathrm{i}}_{2}{\mathrm{O}}_{7}$ is an experimentally confirmed hybrid improper ferroelectric material, in which the electric polarization is induced by a combination of the coherent $\mathrm{Ti}{\mathrm{O}}_{6}$ octahedral rotation and tilting. In this work, we investigate the tuning of ferroelectricity of $\mathrm{C}{\mathrm{a}}_{3}\mathrm{T}{\mathrm{i}}_{2}{\mathrm{O}}_{7}$ using isovalent substitutions on Ca sites. Due to the size mismatch, larger/smaller alkaline earths prefer ${A}^{\ensuremath{'}}/A$ sites, respectively, allowing the possibility for site-selective substitutions. Without extra carriers, such site-selected isovalent substitutions can significantly tune the $\mathrm{Ti}{\mathrm{O}}_{6}$ octahedral rotation and tilting, and thus change the structure and polarization. Using the first-principles calculations, our study reveals that three substituted cases (Sr, Mg, and Sr+Mg) show divergent physical behaviors. In particular, ${(\mathrm{CaTi}{\mathrm{O}}_{3})}_{2}\mathrm{SrO}$ becomes nonpolar, which can reasonably explain the suppression of polarization upon Sr substitution observed in experiment. In contrast, the polarization in ${(\mathrm{MgTi}{\mathrm{O}}_{3})}_{2}\mathrm{CaO}$ is almost doubled upon substitutions, while the estimated coercivity for ferroelectric switching does not change. The ${(\mathrm{MgTi}{\mathrm{O}}_{3})}_{2}\mathrm{SrO}$ remains polar but its structural space group changes, with moderate increased polarization and possible different ferroelectric switching paths. Our study reveals the subtle ferroelectricity in the ${A}_{3}\mathrm{T}{\mathrm{i}}_{2}{\mathrm{O}}_{7}$ family and suggests one more practical route to tune hybrid improper ferroelectricity, in addition to the strain effect.

We present a comprehensive study of the magnetic properties of the long-range-ordered quasi-one-dimensional ${J}_{1}\ensuremath{-}{J}_{2}$ systems with a newly developed torque equilibrium spin-wave expansion approach, which can describe the spin Casimir and magnon decay effects in a unified framework. While the framework does not lose generality, our discussion will be restricted to two representative systems, each of which has only one type of interchain coupling (${J}_{3}$ or ${J}_{4}$) and is referred to as the ${J}_{3}$ or ${J}_{4}$ system, respectively. In spite of the long-range spiral order, the dynamical properties of these systems turn out to be highly nontrivial due to the incommensurate noncollinear spin configuration and the strong quantum fluctuation effects enhanced by the frustration and low dimensionality. Both the systems show prominent spin Casimir effects induced by the vacuum fluctuation of the spin waves and related modification of the ordering vector, Lifshitz point position, and sublattice magnetization. In addition to these static properties, the dynamical behaviors of these systems are also remarkable. Significant and spontaneous magnon decay effects are manifested in the quantum corrections to the excitation spectrum, including the broadening of the spectrum linewidth and downward renormalization of the excitation energy. Furthermore, the excitation spectrum appears to be very sensitive to the types of the interchain coupling and manifests three distinct features: (i) the magnon decay patterns between the ${J}_{3}$ and ${J}_{4}$ systems are very different, (ii) the renormalized spectrum and the overall decay rate of the ${J}_{3}$ and ${J}_{4}$ systems show very different sensitivity to the magnetic anisotropy, and (iii) there is a nearly flat mode in the renormalized magnon spectrum of the ${J}_{4}$ system along the X-M direction. By adjusting the strength of magnetic anisotropy and varying the approximation scheme, it is revealed that these striking distinct features are quite robust and have deep connection with both the spin Casimir and the magnon decay effects. Thus these special consequences of the interchain coupling on the spin-wave dynamics may serve as a set of probes for different types of interchain couplings in experiments. Finally, to guide experimental measurements such as inelastic neutron scattering in realistic materials and complement our theoretical framework, we develop an analytical theory of the dynamical structure factor within the torque equilibrium formulism and provide explicit results of the quasi-one-dimensional ${J}_{1}\ensuremath{-}{J}_{2}$ systems.

In this work, we study a biquadratic Heisenberg model with coupled orbital degree of freedom using Monte Carlo simulation in order to investigate the phase transitions in iron-based superconductors. The antiferro-quadrupolar state, which may be related to the magnetism of FeSe [Phys. Rev. Lett. 115, 116401 (2015)], is stabilized by the anisotropic biquadratic interaction induced by a ferro-orbital-ordered state. It is revealed that the orbital and nematic transitions occur at the same temperature for all the cases, supporting the mechanism of the orbital-driven nematicity as revealed in most recent experiments [Nat. Mater. 14, 210 (2015)]. In addition, it is suggested that the orbital interaction may lead to the separation of the structural and magnetic phase transitions as observed in many families of iron pnictides., Comment: 20 pages, 7 figures, to be published in Physical Review B

As the first experimentally confirmed ferroelectric metal, ${\mathrm{LiOsO}}_{3}$ has received extensive research attention recently. Using density-functional calculations, we perform a systematic study on the origin of the metallic ferroelectricity in ${\mathrm{LiOsO}}_{3}$. We confirm that the ferroelectric transition in this compound is order-disorder-like. By doing electron screening analysis, we unambiguously demonstrate that the long-range ferroelectric order in ${\mathrm{LiOsO}}_{3}$ results from the incomplete screening of the dipole-dipole interaction along the nearest-neighboring Li-Li chain direction. We conclude that highly anisotropic screening and local dipole-dipole interactions are the two most important keys to form ${\mathrm{LiOsO}}_{3}$-type metallic ferroelectricity.

Polycrystalline ${\mathrm{La}}_{0.5}{\mathrm{Sr}}_{0.5}{\mathrm{MnO}}_{3\ensuremath{-}\ensuremath{\delta}}$ thin films deposited on quartz wafers at 680 \ifmmode^\circ\else\textdegree\fi{}C and various oxygen pressures P by pulsed laser deposition are prepared. The effects of oxygen nonstoichiometry on the microstructural, electrotransport and low-field magnetotransport property of the thin films are investigated in details. A structural distortion from the stoichiometric lattice is identified for the samples deposited at $Pl0.1\mathrm{mbar}.$ It is verified that the thin-film conductivity over the Curie point follows variable-range hopping. The carrier density at the Fermi surface falls and the metal-insulating transition shifts toward low temperature with decreasing P, with a jump at $P=0.1\mathrm{mbar}.$ Enhanced low-field magnetoresistance at low temperature is achieved for $Pg0.1\mathrm{mbar}.$ Oxygen overdeficiency at $Pl~0.1\mathrm{mbar}$ essentially prohibits the spin reordering. The temperature dependence of the electro- and magnetotransport properties is explained by the two-channel model where the insulating channels and metallic ones coexist in parallel.

An elastic Ising model for CoMnO(6) chain is proposed to explain the ferroelectricity induced by collinear magnetic order in Ca(3)CoMnO(6), and then a mean-field theory with interchain spin interactions based on this model is developed. With inclusion of the dynamics of polarization domains at finite temperature, we address the rationality of our theory by the good agreement of the calculated electric polarization and dielectric susceptibility with the reported data on Ca(3)Co(2-x)Mn(x)O(6) (x approximate to 0.96) [Y. J. Choi, H. T. Yi, S. Lee, Q. Huang, V. Kiryukhin, and S.-W. Cheong, Phys. Rev. Lett. 100, 047601 (2008)], a typical diatomic Ising spin chain system, while the predicted magnetic susceptibility shows some difference from experiment, the reason of which is discussed.

A two-dimensional Ising-like model for a triangular spin-chain lattice, where each spin-chain is treated as a rigid giant spin, is proposed to investigate the magnetization of a triangular spin-chain lattice by Monte Carlo simulation. The simulations show the steplike evolution of the magnetization at low temperature against an external magnetic field, namely two steps above 10 K and four steps below 10 K, in quantitative agreement with experiments on a Ca3Co2O6 compound. It is argued that the interchain interaction and magnetic inhomogeneity of the lattice are two important ingredients to induce the intriguing steplike feature of the magnetization below 10 K.

The electric dipole configuration and dielectric susceptibility of a Ginzburg-Landau model ferroelectric lattice with randomly distributed defects are simulated using the Monte Carlo method. The simulated characteristics of the lattice configuration and dielectric susceptibility indicate that the model lattice evolves from a normal ferroelectric state to a typical relaxor state with increasing defect concentration. Consequently, the energy and dielectric susceptibility characteristics associated with the ferroelectric phase transitions become smeared. The simulated results approve the applicability of the Ginzburg-Landau model in approaching relaxor ferroelectrics.