Nonreciprocal directional dichroism in multiferroics

The pulse profiles of the Crab pulsar(as well as some other pulsars) vary with time. They can lead to a major source of intrinsic timing noise, which lacks a detailed physical model. The phase separation ? between the first left peak(P1) and the second right peak(P2) is a key parameter that shows the variations of pulse profiles for the Crab pulsar. It was found that the evolution of ? has a tendency with increasing rates of 0.82?± 0.25?, 0.80?± 0.54?, and 0.77?± 0.28?per century for the 2-6, 6-15, and15-60 ke V bands, respectively. Furthermore, the flux ratios(P2/P1) of X-ray pulse profiles in the three bands were calculated, and the derived flux ratios were consistent with the radio and X-ray measurements of the Insight-HXMT. In addition to discovering the physical origin of the pulse changes, the high-SNR X-ray pulse profiles were simulated in the annular gap model, and two model parameters(e.g., the maximum emission heights of the two peaks) were observed to slightly affect the variations of peak separation. We fitted the long-term variations of emission heights of the two peaks and discovered that the emission heights showed increasing tendencies with time. Variations of these emission heights induced a characteristic period derivative, and a complete formula for both the magnetic dipole radiation and wind-particle-induced variations of the moment of inertia was used for the pulsar's spin-down to obtain the variation rate ˙α of the magnetic inclination angle, which was-1.60?per century. Intrinsic timing noise is observed to be mainly induced by the variations of pulse profiles, which might correlate with a characteristic spin period derivative arising from the fluctuations of the emission regions. This work will lay a foundation for understanding the origin of intrinsic timing noise and making high-precision timing models in the future.     

X-ray directional dichroism of a polar ferrimagnet

In a polar ferrimagnet GaFeO3, we have found a novel magneto-optical effect, termed x-ray nonreciprocal directional dichroism (XNDD), that the x-ray absorption at around the K edge of an Fe ion depends on whether the x-ray propagation vector is parallel or antiparallel to the outer product of the magnetization and electric-polarization vectors. The XNDD spectroscopy as demonstrated here can be a useful tool to probe the local magnetism in noncentrosymmetric systems such as magnetic interfaces and nanostructures.     

Flexomagnetoelectric interaction in multiferroics

We present a theoretical analysis of phase separation in the presence of a spatially periodic forcing of wavenumber q traveling with a velocity v. By an analytical and numerical study of a suitably generalized 2d-Cahn-Hilliard model we find as a function of the forcing amplitude and the velocity three different regimes of phase separation. For a sufficiently large forcing amplitude a spatially periodic phase separation of the forcing wavenumber takes place, which is dragged by the forcing with some phase delay. These locked solutions are only stable in a subrange of their existence and beyond their existence range the solutions are dragged irregularly during the initial transient period and otherwise rather regular. In the range of unstable locked solutions a coarsening dynamics similar to the unforced case takes place. For small and large values of the forcing wavenumber analytical approximations of the nonlinear solutions as well as for the range of existence and stability have been derived.     

Coupled caloric effects in multiferroics

We propose a conception – coupled caloric effect   where the enhanced caloric effects originate from the coupling among magnetic, ferroelectric, and structural degrees of freedom. Specifically, as the magneto-electric case, the magnitude of the coupled caloric effect was evaluated for a ferromagnetic–ferroelectric system using a phenomenological calculation based on Landau phase transition theory. The isothermal entropy change is greatly enhanced by increasing the magneto-electric coupling strength. This work indicates that the caloric effect in a ferromagnetic–ferroelectric coupled system consists of pure magnetic entropy change (Δ_SM$\mathrm{\Delta }{S}_M$), pure ferroelectric one (Δ_SE$\mathrm{\Delta }{S}_E$), and coupled one (Δ_SME$\mathrm{\Delta }{S}_{\mathit{ME}}$) that plays a significant part. The counterpart of the last one in magneto-structural coupled system was usually neglected. Our study provides a route to energy-efficient refrigeration via realization of coupling among various ferroic orders.     

Nonreciprocal Transmission and Nonreciprocal Entanglement in a Spinning Microwave Magnomechanical System

This study presents nonreciprocal transmission and nonreciprocal magnon–phonon entanglement in a spinning microwave magnomechanical system. This system consists of microwave photons, magnon modes, and phonons. These are created by the vibrational mode of a yttrium iron garnet sphere. This investigation reveals that nonreciprocity is caused by the light that is circulating in a resonator that is experiencing a Fizeau shift. This leads to a difference in the effective detuning frequency of the photon for forwarding and backward drives. A super-strong transmission isolation rate (>100 dB) and a strong entanglement isolation rate (≈50 dB) are obtained by applying the experimental parameters. This scheme opens a new route for exploiting a variety of nonreciprocal effects, and it provides the theoretical basis for the design and realization of magnetically controllable isolators and diodes.     

Nonreciprocal devices in integrated optics

This article reviews the solutions that have been studied for the implementation of nonreciprocal devices in integrated optics. These components, either isolators or circulators, use the nonreciprocal interaction between light and a magnetic medium. The only two isolators that have been experimented on to date are described in detail with their advantages and drawbacks, and some solutions are proposed to overcome the difficulties encountered.     

Solitons of electric polarization in multiferroics

Complete analysis of the properties of breathers in spiral multiferroic structures in the sine-Gordon model has been presented. The methods of the excitation and detection of the breathers in the external electric and magnetic fields have been discussed.     

Advances in ab-initio theory of multiferroics

Within the broad class of multiferroics (compounds showing a coexistence of magnetism and ferroelectricity), we focus on the subclass of ??improper electronic ferroelectrics??, i.e. correlated materials where electronic degrees of freedom (such as spin, charge or orbital) drive ferroelectricity. In particular, in spin-induced ferroelectrics, there is not only a coexistence of the two intriguing magnetic and dipolar orders; rather, there is such an intimate link that one drives the other, suggesting a giant magnetoelectric coupling. Via first-principles approaches based on density functional theory, we review the microscopic mechanisms at the basis of multiferroicity in several compounds, ranging from transition metal oxides to organic multiferroics to organic-inorganic hybrids.     

Nonreciprocal devices in integrated optics

This article reviews the solutions that have been studied for the implementation of nonreciprocal devices in integrated optics. These components, either isolators or circulators, use the nonreciprocal interaction between light and a magnetic medium. The only two isolators that have been experimented on to date are described in detail with their advantages and drawbacks, and some solutions are proposed to overcome the difficulties encountered.     

Domains and domain walls in multiferroics

Multiferroics are gathering solid-state matter in which several types of orders are simultaneously allowed, as ferroelectricity, ferromagnetism (or antiferromagnetism), ferroelasticity, or ferrotoroidicity. Among all, the ferroelectric/ferromagnetic couple is the most intensively studied because of potential applications in novel low-power magnetoelectric devices. Switching of one order thanks to the other necessarily proceeds via the nucleation and growth of coupled domains. This review is an introduction to the basics of ferroelectric/ferromagnetic domain formation and to the recent microscopy techniques devoted to domains imaging, providing new insights into the archetypal multiferroic domain morphologies. Some relevant examples are also given to illustrate some of the unexpected properties of domain walls, as well as the way these domain walls can be manipulated altogether thanks to various types of magnetoelectric coupling.     

Room-temperature giant magnetotranstance effect in single-phase multiferroics

Single-phase multiferroic materials are usually considered useless because of the weak magnetoelectric effects,low operating temperature,and small electric polarization induced by magnetic orders.As a result,current studies on applications of the magnetoelectric effects are mainly focusing on multiferroic heterostructures and composites.Here we report a room-temperature giant effect in response to external magnetic fields in single-phase multiferroics.A low magnetic field of 1000 Oe applied on the spin-driven multiferroic hexaferrites BaSrCo_2 Fe_(11)AlO_(22) and Ba_(0.9)Sr_(1.1)Co_2 Fe_(11)AlO_(22) is able to cause a huge change in the linear magnetoelectric coefficient(α_E=dE/dH) by several orders,leading to a giant magnetotranstance(GMT) effect at room temperature.The GMT effect is comparable to the well-known giant magnetoresistance(GMR) effect in magnetic multilayers,and thus opens up a door toward practical applications for single-phase multiferroics.     

Nonlinear magnetoelectric effect in composite multiferroics

The theoretical and experimental studies of the nonlinear magnetoelectric effect in composite multiferroics in the low-frequency spectral region and in the electromechanical resonance region have been performed. It has been shown that such structures demonstrate a nonlinear magnetoelectric effect, which is quadratic in ac magnetic field strength at weak magnetic fields. In the region of the electromechanical resonance, the resonance excitation of an electric field occurs by means of ac magnetic field at a frequency lower than the resonance frequency by a factor of two. In the low-frequency spectral region, there is a difference of amplitude values of two neighboring voltage maxima due to the superposition of signals from the linear and nonlinear effects, and the difference is proportional to the dc magnetic field strength in weak fields. The results of the experimental study of the two-layer permendur-lead zirconate titanate structure are presented.     

Elastic anomalies at phase transitions in multiferroics

The temperature dependences of the elastic modulus in multiferroics-magnetoelectrics are analyzed, in which magnetic and ferroelectric orderings appear as the result of two successive phase transitions. The analytical relationships for the elastic modulus near the phase transitions to ordered states are obtained for the cases of either linear-quadratic or biquadratic contributions to magneto- and electroelastic coupling. The explicit dependence of the elastic modulus in the multiferroic phase on the magnetoelectric coupling constant was found. It is shown that the characteristic elastic properties in multiferroics can be treated using the Landau theory without taking into account fluctuations. The analysis includes changes in the phase diagrams due to the magneto- and electroelastic coupling.     

Nonreciprocal cell for neutrons

The effect of nonreciprocal transmission of thermal neutrons (λ = 3–6 Å) through a system of magnetic mirrors with a noncoplanar distribution of the magnetic induction is predicted and observed experimentally. The relative difference between the transmittances for the direct and inverse processes reaches 75%. Thereby, the feasibility of a nonreciprocal cell for spin-1/2 particles is demonstrated.     

Nonreciprocal effects in a plasma waveguide

The nonreciprocal effects in a waveguide completely filled with plasma are investigated. The basic assumptions and equations concerning the reason for the occurrence of such effects are presented. The results of the numerical calculations for different values of parameters are given. The main features of the nonreciprocal effects involving the electromagnetic waves in a plasma waveguide are summarized. It is shown that such effects exist only for nonsymmetrical waves of the HE type. Two frequency regions are found in which only one nonsymmetrical HE-mode can propagate along the field, while against the field no one mode of any type can     

Coupling of replicate order-parameters in incommensurate multiferroics

The specific properties of incommensurate multiferroic phases resulting from the coupling of order-parameter replicates are worked out using the illustrative example of iron vanadate. The phase difference between the order-parameter copies induces an additional broken symmetry phase corresponding to the lowest symmetry of the system and varies critically at the transition to the multiferroic phase. It reflects the temperature dependence of the angle between paired spins in the antiferromagnetic spiral structure. Expressing the transition order-parameters in terms of spin-density waves allows us to show that isotropic exchange interactions contribute to the stabilization of the ferroelectric phase.     

Microscopic origin of magnetoelectric coupling in noncollinear multiferroics

An universal microscopic mechanism to understand the interplay between the electric and magnetic degrees of freedom in noncollinear multiferroics is developed. In a system with a strong spin-orbit coupling, we show that there is a pure electric mechanism that the ferroelectricity is generated by noncollinear magnetism through an electric current cancellation process, which saves the pure electric energy. This mechanism provides a simple estimation and sets a physical limitation of the value of ferroelectricity in noncollinear multiferroic materials.