Optical non-reciprocity in magnetic structures related to high-Tc superconductors

Recent neutron scattering [1,2], and optical measurements [3,4] have detected evidence in underdoped cuprate superconductors for a phase transition near the pseudogap onset temperature T* to a time reversal-breaking state. The neutron scattering indicates antiferromagnetic ordering, while it is often assumed that optical polarization rotation requires at least a weak ferromagnetic component. In this note we identify several antiferromagnetic structures, compatible with neutron scattering data, that allow intrinsic polarization rotation through the magnetoelectic effect

Interfacial magnetization in exchange-coupled Fe/Cr/Fe structures investigated by second harmonic generation

We have studied the magnetic field dependences of magnetic optical second harmonic generation (SHG) in MBE-grown Fe/Cr/Fe/Ag/GaAs(100) heterostructures displaying both bilinear and biquadratic interlayer exchange coupling. The magnetic field H was applied in the (100) surface plane along both easy ([001]) and hard ([110]) axes of the in-plane fourfold magnetic anisotropy. The SHG has been measured in reflection at near normal incidence for different polarization combinations (pp,ps,ss,sp) of the fundamental and second harmonic light in longitudinal and transversal geometries. The magnetic field variation of the SHG signal clearly reflects the field-induced transformations of the magnetic state at the interfaces in the trilayer. It strongly depends on the configuration of light polarization, experimental geometry (longitudinal or transversal), and orientation of the magnetic field H relative to the crystal axes. In contrast to linear magneto-optical Kerr effect, which is odd in magnetic field, magnetic SHG is either even in H or does not display a definite parity at all, depending on the polarization configuration. We interpret the data based on a model accounting for nonmagnetic and magnetic contributions to SHG from the surface and interfaces described by C-4v point symmetry. Taking into account the changes of the mutual orientation of interfacial magnetizations allows us to describe the general features of the measured field dependences of SHG

Study of charge state of polarization domain walls in organic ferroelectric 2-methylbenzimidazole crystals

The work is supported by the RFBR grant № 16-02-00399

THIN FILMS OF A NEW ORGANIC SINGLE-COMPONENT FERROELECTRIC 2-METHYLBENZIMIDAZOLE

Subject of Research.We present results of structural and dielectric study of organic ferroelectric 2-methylbenzimidazole (MBI) thin films. Method. The films have been grown on substrates of leuco-sapphire, fused and crystalline silica, neodymium gallate, bismuth germanate, gold, aluminium, platinum. The films have been grown by two different methods: substrate covering by ethanol solution of MBI and subsequent ethanol evaporation; sublimation at the temperature near 375 K under atmospheric pressure. Crystallographic orientation studies have been performed by means of «DRON-3» X-ray diffractometer, block structure of the films has been determined by «LaboPol-3» polarizing microscope. Small-signal dielectric response has been received with the use of «MIT 9216A» digital LCR-meter, while strong-signal dielectric response has been studied by Sawyer-Tower circuit. Main Resuts. We have shown that the films obtained by evaporation are continuous and textured. Obtained film structure depends on the concentration of the solution. Films may consist of blocks that are splitted crystals like spherulite. Spontaneous polarization components in such films may be directed both perpendicularly and in the film plane. We have also obtained structures consisting of single-crystal blocks with spontaneous polarization components being allocated in the film plane. Block sizes vary from a few to hundreds of microns. Films obtained by sublimation are amorphous or dendritic. The dielectric properties of the films obtained by evaporation have been studied. We have shown that the dielectric constant and dielectric loss tangent increase under heating. The dielectric hysteresis loops are observed at the temperature equal to 291-379 K. The remnant polarization increases with temperature for constant amplitude of the external electric field, and achieves 4.5mC/cm2, while the coercive field remains constant. We propose that such behavior is explained by increase of the number of crystallites with switchable polarization due to decrease in the coercive field under heating. The remnant polarization decreases with frequency increase. Practical Relevance. Proposed method of ferroelectric films manufacture is characterized by low cost and convenience. Unlike many other ferroelectrics, the films contain no lead and rare metals. MBI films demonstrate the low value of the coercive fields. This paper may be useful for electronic components developers

Magnetoelectric Control of Domain Walls in a Ferrite Garnet Film

The effect of magnetic domain boundaries displacement induced by electric field is observed in epitaxial ferrite garnet films (on substrates with the (210) crystallographic orientation). The effect is odd with respect to the electric field (the direction of wall displacement changes with the polarity of the voltage) and even with respect to the magnetization in domains. The inhomogeneous magnetoelectric interaction as a possible mechanism of the effect is proposed.Comment: 5 pages, 2 figure

Theory of Ferromagnetism in Ca1-xLaxB6

Novel ferromagnetism in Ca$_{1-x}$La$_{x}$B$_6$ is studied in terms of the Ginzburg-Landau theory for excitonic order parameters, taking into account symmetry of the wavefunctions. We found that the minima of the free energy break both inversion and time-reversal symmetries, while the product of these two remains preserved. This explains various novelties of the ferromagnetism and predicts a number of magnetic properties, including the magnetoelectric effect, which can be tested experimentally.Comment: 5 pages, accepted for publication in Phys.Rev.Let

Theory of Non-Reciprocal Optical Effects in Antiferromagnets: The Case Cr_2O_3

A microscopic model of non-reciprocal optical effects in antiferromagnets is developed by considering the case of Cr_2O_3 where such effects have been observed. These effects are due to a direct coupling between light and the antiferromagnetic order parameter. This coupling is mediated by the spin-orbit interaction and involves an interplay between the breaking of inversion symmetry due to the antiferromagnetic order parameter and the trigonal field contribution to the ligand field at the magnetic ion. We evaluate the matrix elements relevant for the non-reciprocal second harmonic generation and gyrotropic birefringence.Comment: accepted for publication in Phys. Rev.

Magnetization dynamics induced by ultrashort optical pulses in Fe/Cr thin films

The magnetization dynamics of single-crystalline Fe(001) thin films with Cr cap layers has been studied by an all-optical time-resolved pump-probe technique. The system is characterized by a fourfold in-plane magnetic anisotropy. We observed long-lived (similar to 1 ns) magnetization oscillations caused by the ultrafast (similar to 0.15 ps) optical pulse excitation. The oscillations are associated with the temporal variation of the magnetization component M-z normal to the film surface. The phase of the oscillations is independent of the polarization state of the pump beam giving evidence for a predominantly thermal origin of the excitation. The amplitude of the oscillations considerably depends on the in-plane orientation and magnitude of the magnetic field. The azimuthal variation of the oscillation frequency at constant magnetic field follows the fourfold in-plane magnetic anisotropy. Angle and field variations of the frequency are well described by a uniform precession mode known from the theory of ferromagnetic resonance. Our analysis indicates that the precession amplitude is determined by the frequency of the uniform mode and an in-plane tilting of the effective magnetic field directly caused by the pumping light beam

Domain structure in biaxial Fe/Cr films induced by lateral fluctuations of the magnetic anisotropy

The magnetic microstructure of single-crystalline Fe(001) thin films has been studied by magnetic second harmonic generation and conventional magneto-optical Kerr effect methods. The layers were grown on GaAs/Fe/Ag(100) substrates, capped by a Cr overlayer, and displayed a fourfold in-plane magnetic anisotropy. We observe the formation of a multidomain structure at magnetic fields vertical bar H vertical bar <= 0.5 kOe, when the field is applied in a narrow range of azimuthal angles close to the hard axis direction (vertical bar Delta xi vertical bar < 1 degrees). The domains are characterized by the same longitudinal magnetization component M-y and transverse components M-x of opposite signs. We developed an approach to extract the change of the relative contributions of domains with different magnetization directions during magnetization reversal from the experimental data. The formation of a domain structure can be explained by taking into account the lateral fluctuations of the magnetic anisotropy energy caused by defects and stress at the interfaces and the role of the biquadratic exchange coupling between interfacial Fe and Cr magnetic moments