Scientific results:
- We have built a theory of the functioning of spin-transfer oscillators, detectors, emitters, resonators and gates based on antiferromagnetic dielectrics operating in the terahertz range. More specifically:
- Our researchers have conducted a study of the transfer, conversion and spin current and its control in antiferromagnetics (AFMs). We conducted a study of of spin moment transfer in dielectric AFMs. We produced a theoretical model that describes spin current transformation in a thin layer of an AFM dielectric with two-axis anisotropy that accounts for various factors: intra-sublattice exchange, magnetic anisotropy, elastic interaction, spin current, Zeeman field, the electromagnetic field of the excitation pulse. By successive consideration of the piezoelectric effect in the piezoelectric-antiferromagnetic heterostructure when elastic deformations are induced in a layer of a ferromagnetic through deformations in a layer of a piezoelectric and the magnetoelastic effect it has been determined that the electric field in the piezoelectric layer can be used to induce magnetic anisotropy fields and to change the magnetic resonance frequency and to change the critical current required to excite auto-oscillations. This result was demonstrated on the example of the Pt/NiO/PZT-5H heterostructure.
- To implement a detector of THz oscillations that can be adjusted for various frequencies based on one-axis crystals on antiferromagnetics, the polarization of incident radiation should be circular or elliptical. Linearly polarized radiation for single-layer AFM can be rectified when an external magnetic field is applied. If the frequency of a detector is adjusted using an external constant field, the degeneracy of normal oscillation modes of AFM is removed and two frequencies with left and right circular polarization are formed. Circular polarization of an external variable electromagnetic field ensures the maximum value of rectified current compared with elliptical and linear polarizations.
- The Laboratory has developed models of spin current converters, emitters and spin current gates based antiferromagnetic dielectrics. We produced equations describing the dynamics of magnetization of antiferromagnetic sublattices accounting for the contribution of electric current in a platinum layer, static mechanical strains in the antiferromagnetic layer and deformations caused by the electrical field in the piezoelectric layer. On the basis of the produced equations we found two solutions describing dying oscillations and auto-oscillations of magnetization. We found conditions of the realization of these solutions as well as the dependencies of the frequency and amplitude of oscillations on the value of electric current in platinum and the electric field on the piezoelectric. We have obtained graphical dependencies of the frequency of magnetic oscillations in an antiferromagnetic for the material parameters of antiferromagnetic NiO and PZT-5H piezoceramics at various values of external electric currents supplied to the piezoelectric. It has been shown that there are regions of subcritical and supercritical currents: at subcritical currents the system is in the state of dying oscillations, at supercritical currents auto-oscillations are implemented.
- We have researchers a model of the excitation of terahertz nonlinear spin excitations in a resonator relying on «canted antiferromagnetic–heavy metal» heterostructures. It has been demonstrated that when the pumping pulse amplitude grows, the value of the response of the spin system increases linearly on the main (resonance) antiferromagnetic mode and quadratically on the second harmonic. We have demonstrated the possibility of controlling magnetization switching in sublattices in a antiferromagnetic with weak ferromagnetism (Dzyaloshinskii–Moriya interaction, for instance, hematite) by terahertz pulses of the magnetic field.
- During experimental research we have obtained the following results:
- As a result of conducted experimental research, we have experimentally studied the quasistatic processes of remagnetization of parabola-shaped magnetoelastic nanostrips. It has been shown that disruption stable magnetization states in a nanostrip can be achieved using a static magnetic field applied perpendicularly to the light axis of the ferromagnetic. In this case it is possible to split parabola-shaped nanostrips into two domains; the domain boundary in the equilibrium state is located in the center of the nanostrip, in its narrow part. Further changing the magnetization states allows to manipulate them (for instance, moving the domain boundary) by homogeneous mechanical stress induced by applying an electrical field to the dielectric substrate. Experiments in the micro- and nanostructuring of samples for their use as the basis of elements of micro- and nanomagnonics were conducted using CoPt films with a thickness of 8 nm. A feature of the films is the presence of perpendicular magnetic anisotropy and a coercivity field of several hundred Oersted, which allows to create reconfigurable domain structures in films that are stable in the absence of an external magnetic field. We have studied the possibility of using pulse force nanolithography by the probe of an atomic force microscope (AFM) to create nanodimensional regions with an increased density of flowing current. To conduct the experiments, we had preliminarily prepared (using photolithography and subsequent ion etching) samples in the form of Hall bridges with a channel width of 5 μm. Then, using an AFM probe, we performed necessary sections.
- For the first time we have observed the acoustic generation of parametric spin waves – ASWs in volume acoustic spin wave resonators containing YIG in contact with a Pt film. The detection of parametric YIG was performed by using the spin pumping and ISHE created by them. Electric detection using this method with parametric ASWs, as far as is known, have only been theoretically discussed earlier. The efficiency of ASW generation under conditions of piezoelectric excitation of a high-overtone bulk acoustic resonator–HBAR – that has been demonstrated in the linear mode as well, in this case also manifests itself in low values of threshold power, ~ 0.1 mW. We found a number of features in the behavior of the voltage of spin pumping in a magnetic field that are in good agreement with various schemes of parametric excitation depending on the position of dispersion diagrams with respect to the resonance frequency of pumping.
- We have developed an experimental methodology for researching nanosecond and subnanosecond processes in magnetic micro- and nanostructures using polarization microscopy and the Faraday effect. The methodology is applicable to researching processes in magnetic dielectrics in pulse magnetic fields with pulse durations ranging from 2 ns to 2 μs. As the master oscillator we used a generator of delays and pulses with four independent channels of pulses at the output of the generator and 8 delay channels. This allows to implement various experimental conditions both in terms of impact on the researched sample and in terms of the synchronization of the operation of the elements of the device. It is possible to research the behavior of the magnetic system of the sample under the influence of (a) one pulse of a magnetic field, (b) a sequence of short pulses (from 2 ns) following at short tine intervals (from 1 ns), and (c) a combination of a long pulse of the field with simultaneous short pulses of the field. The (а) option is related to the generally accepted method of researching magnetic materials, the (b) and (c) options are planned to be used to seek conditions of magnetization state switching with minimum energy expenditure. To ensure the conditions for the study of subnanosecond processes, we used pulse sharpening modules with a rise time of pulses not exceeding 100 ps. At such rise/duration increase times for the creation of magnetic fields we used low-inductance devices for forming magnetic fields – single-turn flat coils, we modeled magnetic fields created by them. The inhomogeneity of created magnetic fields during research of microstructures amounted to about 5 per cent. To produce pulses with durations of over 50 ns and amplitudes of up to 200 Oe, we used an amplifier with a rise time of 5 ns. To demagnetize researched microstructures in intervals between the pulses of the field, we envisioned a block for forming radio-pulses with declining amplitudes. We have developed and created a system for registering processes of changes in the magnetization state in micro- and nanostructured using the magneto-optical method. The system for registering remagnetizing signals includes a photoelectronic multiplier with a rise time of 570 ps, a stroboscopic oscilloscope with a bandwidth of 15 GHz, which ensures a temporal resolution not worse than 1 ns when registering remagnetization signals. Our researchers have implemented the possibility of directly registering dynamic domains using a polarization microscope with a spatial resolution of 30 nm and a temporal resolution of 5 ns. as a light source we used a pulse laser with a wavelength of 527 nm and a pulse duration at half-height amounting to 5 ns. We have ensured synchronicity of the operation of the blocks that form remagnetizing pulses and demagnetizing radio-pulses, the modulator of the PEM, a stroboscopic oscilloscope, pulse laser. This allowed to refine a methodology for the research of dynamic processes in magnetic dielectrics – ferrite films with a garnet structure with a thickness of several micrometers. We have studied the remagnetization processes in microstructures depending on the amplitude of the pulses of the field. Our researchers have studied the dynamic properties of domain boundaries (velocity, mobility), which is necessary to justify the possibilities of applying domain boundaries in magnonic devices as the basis for future reconfigurable spin nanowaveguides.
- The Laboratory has conducted a series of experiments in ultra-fast motion of the domain walls in ferromagnetics near the point of compensation of the angular and spin moments. It was of specific interest to study the range of temperatures around the points of magnetic and spin compensation that precisely balances both the magnetic moment and the angular moment of the system and leading to a special nature of magnetic switching when the domain wall is moved. We observed a temperature-dependent dynamic of the domain wall in a temperature range that encompasses the points of compensation of both the angular moment and magnetization in a garnet film and reaching the Curie temperature. We have demonstrated a sharp difference between the mobility of the domain wall and the maximum attainable velocity in the vicinity of these two compensation points. A high mobility of the domain wall in weak applied magnetic fields has been discovered.
- Our researchers have developed a neuromorphic-interference paradigm of computation and signal processing that features the simultaneous use of terahertz-range oscillators excited by AFM current and AFM buses (a system of AFM waveguides). We proposed a concept of an ultra-fast neuromorphic computational processor with optical and electrical control based on an antiferromagnetic/heavy metal heterostructure (AFM/TM). Artificial neurons based on AFM/TM are excited by short pulses in the terahertz range, launching precession in the AFM. Displacement current int he TM layer can be used to change the resonance precession frequency. The transformation of magnetization precession in electric current in the TM layer occurs due to the inverse spin Hall effect. Therefore we proposed a model of a neuromorphic processor that consists of exciting artificial neurons based on AFM – oscillators and processing neurons – detectors. It has been demonstrated that by using optical excitation can significantly increase the data processing rate in neuromorphic computations with low power consumption. We researched examples of implementations of simple logical operations (OR, AND).
- We have developed prototypes of magnetic waveguide and logical devices for neuromorphic magnonic signal processes. We proposed prototypes of magnonic spin waveguides and spin wave logical devices based on domain walls as well as methods of changing the position of domain walls. In particular, since the dynamic interaction of acoustic and magnetic systems is of high interest now, by choosing the angle of incidence, amplitude and frequency of an transverse acoustic wave it is possible to control the motion of the domain wall. It has been established that at sufficiently high amplitudes of shift offset the speed of forced movement of the domain boundary can reach significant fractions of the speed of sound. It has been demonstrated that this occurs due to the specific conditions of resonance that depend on the frequency of the wave, its incidence angle and the amplitude of shift offset, which leads to full reflection of the wave and the maximum effect. Most interestingly, in the interaction of elastic and magnetic subsystems a strong nonlinearity manifests itself, which is expressed in the negative inclination of the resonance peak of reflection and the s-shaped dependence of the speed of the domain wall on the amplitude of shear offset that is characteristic of nonlinear system.
Education and retraining of personnel:
- Every year we conduct a scientific seminar and workshop «Spin photonics» on the grounds of the Institute of the Institute of Radio-engineering and Electronics of the Russian Academy of Sciences.
- Employees of the Laboratory have developed and launched the following disciplines at HSE University: «Basics of magnetism» and «Nanomagnetism and spintronics» for third-year bachelor’s degree students, as well as a specialized case study in spin photonics course for bachelor’s and master’s degree students.
Cooperation:
Moscow Institute of Physics and Technology, N. G. Chernyshevsky Saratov State University, Institute for Physics of Microstructures of the Russian Academy of Sciences, Far Eastern Federal University (Russia): joint research.
We are also actively collaborating with the Research and Practice Center of the National Academy of Sciences of Belarus.
