Nonlinear optical properties of a regular array of triangular-shaped vortex magnetic nanoparticles is studied using the optical second harmonic generation (SHG) technique. We demonstrate that the SHG azimuthal anisotropy is consistent with the 3$m$ symmetry of individual Co nanodots placed in a square surface lattice. Qualitatively different SHG magnetic hysteresis loops are obtained for circular and linear polarizations of the fundamental radiation. In the first case, a wide SHG hysteresis at zero DC magnetic field $H$ is observed, which is attributed to a macroscopic magnetic toroid moment in Co nanodots induced by a noncentrosymmetric distribution of the magnetization. On the contrary, for the linear pump polarization the SHG loop is similar to observed commonly in linear magnetooptics for vortex magnetic structures and reveals a rather narrow width at $H=0$. A phenomenological SHG description based on the introduction of the SHG polarization induced by a magnetic toroid moment in vortex magnetic nanostructures is presented.

We report an experimental study of the optical properties of a two-dimensional square lattice of triangle Co and CoFe nanoparticles with a vortex magnetization distribution. We demonstrate that the intensity of light scattered in the diffraction maxima depends on the vorticity of the particles’ magnetization and can be manipulated by applying an external magnetic field. The experimental results can be understood in terms of simple phenomenological consideration.

Linear and nonlinear optical properties of a thin planar $\mathrm{CoFe}/{\mathrm{Al}}_2{\mathrm{O}}_3/\mathrm{CoFe}$ structure for different angles of incidence of the fundamental radiation are studied by means of the magneto-optical Kerr effect and magnetization-induced second-harmonic generation (SHG). The composition of this multilayered structure supports the appearance of the antiferromagnetic state, which leads to nonreciprocal optical effects and is discussed in terms of the magnetic toroid moment. We show that in accordance with the phenomenological description, SHG magnetic hysteresis loops for a linearly polarized pump beam are determined by linear in magnetization $M$ components of the SHG polarization, while for a circularly polarized pump such a description necessarily involves both linear and quadratic in $M$ components of the nonlinear polarization. Symmetry analysis of the magnetization-induced SHG supports the differences between the cases of linear and circular polarization of the incident radiation observed in the experiment, as well as a partial suppression of first in magnetization SHG contributions. Relative values of the linear and quadratic in magnetization components of the SHG polarization are estimated for the trilayer structure and for a reference homogeneous ferromagnetic film.

We report on the design of a spintronic emitter based on the Pt(3 nm)/Co(3 nm) structure, which enables the control over terahertz radiation polarization. Utilizing the field-induced magnetization rotation that takes place at low magnetic fields of up to 250 Oe at room temperature, we have achieved the full range of terahertz polarization rotation from 0° to 360°. This rotation became possible due to the uniaxial magnetic anisotropy induced in the plane of the cobalt film during its fabrication. We evaluated the efficiency of the Co/Pt structure in generating terahertz radiation and found that the terahertz pulse energy flux reaches ∼160 nJ/cm2 at an excitation flux of 4 mJ/cm2.

We report a study of interlayer exchange interaction in multilayer [Co/Pt]_n/Pt/Co structures. The structures consist of a periodic [Co/Pt]_n multilayer film with a perpendicular anisotropy and a thick Co layer with an in-plane anisotropy. The subsystems are separated by a Pt spacer with variable thickness. The magnetooptical Kerr effect and the ferromagnetic resonance measurements show the essentially non-collinear state of magnetic moments of the layers and strong exchange coupling between the [Co/Pt]_n and the Co subsystems. A simple model based on the Landau-Lifshitz-Gilbert equation shows that the exchange coupling is ferromagnetic. The exchange constant is estimated in this simple model. The estimated value is $J=2\hspace{0.17em}{\text{erg}/\text{cm}}^2$ .

We show that terahertz (THz) emission spectroscopy is a method to probe the in-plane magnetic state in spintronic heterostructures with exchange bias between the constituent antiferromagnetic and ferromagnetic layers. We demonstrate that the intense laser excitation can switch and reversibly reset the direction of the pinning axis arising from the exchange bias in technologically relevant tunnel magnetoresistance elements. By applying the double-pump THz emission spectroscopy, we reveal that this switching exhibits a threshold character in the pump intensity.