Magnetic Multilayered Structures
Electrodynamic Properties of Magnetic Multilayered Structures
Tech Area / Field
- PHY-SSP/Solid State Physics/Physics
8 Project completed
Senior Project Manager
Lapidus O V
Russian Academy of Sciences / Institute of Radioengineering and Electronics / Fryazino Branch, Russia, Moscow reg., Fryazino
- Russian Academy of Sciences / Institute of Radioengineering and Electronics, Russia, Moscow\nRussian Academy of Sciences / Institute of Radioengineering and Electronics / Saratov Branch (SB IRE RAS), Russia, Saratov reg., Saratov
- University of Salford, UK, Salford\nUniversity of Oxford / Department of Physics, UK, Oxford\nOakland University/Departament of Physics, USA, NY, Rochester\nSt Andrews University / School of Physics and Astronomy, UK, St Andrews\nColorado State University, USA, CO, Fort Collins
Project summaryAims of the Project
The aim of the project is to investigate the electrodynamic properties of multilayered magnetic structures (MMS). Such structures can be of different composition and geometry and present new classes of materials not found in nature. Their striking electrodynamic properties are unique and important from the fundamental point of view. In addition, exploiting these properties, we try to develop new electronic components for state-of-the-art commercial communication systems.
In particular, two types of MMS will be considered: 1) those made of metal, dielectric and semiconductor films, which are grown by molecular beam epitaxy (MBE) and magnetron sputtering, and 2) those made of doped yttrium-iron-garnet (YIG) films, which are grown by liquid phase epitaxy (LPE). Depending on the particular MMS, we will pursue our investigations within three areas.
First, we shall investigate and explain the effects related to the spin-polarization of charge carriers in the MMS. It is planned to prepare structures, where the ferromagnetic conducting layers are separated by non-magnetic spacer layers (in particular dielectric or semiconductor layers) and to study the ac and dc currents through the interfaces. Two interesting phenomena can be observed in such structures: the effect of giant magnetoresistance and that of spin-injection. We believe that these phenomena are interdependent. Therefore, we would like to: 1) clarify the basic coupling mechanisms between the ferromagnetic layers for different compositions, temperatures, and electrical and magnetic bias fields, thus determining the relation between the injection level and the magnetic state of the structure; 2) investigate the energy spectrum and kinetics of the injected electrons; and 3) determine the optimal conditions for a high spin-injection level and/or strong giant magnetoresistance effect.
Second, it is planned to obtain information on the high-frequency and microwave properties of the MMS. Therefore, the main types of magnetic and magnetoelastic resonances need to be determined, by analyzing the frequency spectra and temperature dependence of such resonances. Further, we are interested in the dispersion and temperature characteristics of propagating spin-waves. This allows to develop effective methods to excite the different types of resonances and short exchange-dominated spin-waves. Further, we will study the influence of boundary inhomogeneities (including artificially created ones) on the wave and oscillation properties of the MMS as well as non-linear and parametric effects, appearing at high amplitudes of the spin-wave oscillations.
Third, we shall research in the optical properties of the magnetic thin-film structures, namely, 1) their waveguide properties, 2) the efficiency of light scattering off magnetic and magnetoelastic oscillations, and 3) magneto-optical Faraday and Kerr effects (in particular, the influence of the interfaces). This will provide important information on how to process signals with the MMS, e.g. signal modulation and its filtering in space.
The gained insight in these three areas of research will be further used to develop devices with new principles of operation. Samples of these devices shall demonstrate their functioning and technical characteristics. The aim is to focus on inexpensive and widely used materials, whose properties allow the devices to operate at room temperatures and frequencies that are common to commercial communication systems. In this way, the devices will be of general commercial interest and able to compete on the world market for electronic components.
State of the Research and the Problems
MMS have attracted much attention because of their unique electrodynamic properties. During the past ten years, most of the researches on metallic multilayers and magnetic tunnel structures, has been devoted to the giant magnetoresistance effect and its implementation in recording heads, magnetic sensors, etc. Other fundamental properties of the MMS have been studied to a much lesser extent. So far, there is no clear understanding as concerns: 1) the relationship between giant magnetoresistance and spin-injection, 2) the mechanisms and the kinetics of spin-injection, 3) the influence of spin-injection on the energy spectrum and its spatial non-uniformity, and 4) the relationship between the spin-injection and the magnetic state of the structure. Only very recently, an interesting prediction has been made. It describes the possibility to switch between the ferromagnetic and antiferromagnetic states in a magnetic tunnel junction, by driving a tunnel current across the interfaces . The validity of this prediction has not been tested experimentally yet. To answer this and the listed problems in detail, one has to investigate into the microwave and optical properties of such MMS.
On the other hand, yttrium iron garnet (YIG) is a well researched material with unique microwave properties due to the low loss of magnetic excitations therein. Single crystal YIG spheres have therefore found use in military applications (such as tunable oscillators and filters). Similarly, one could use single crystal epitaxial YIG films , as shown in Ref. . As the scientists from the IRE RAS have a long standing tradition of researching in this field, there is much evidence that a number of devices for civil (non-military) application can be developed on the basis of MMS which are made from doped YIG. For example, doping YIG with Ga and Sc allows to reduce the operating frequency  and make devices for the high-frequency band common to commercial communication systems. Doping YIG films with Lu and Bi, makes it possible to prepare high quality magnetooptic materials with small loss for spin-waves . Thus, it is possible to obtain materials for a high-frequency processing of optical signals. Moreover, the growing conditions of the MMS can be chosen to create spatially inhomogeneous structures with a varying anisotropy across the film thickness. This allows to excite spin-waves of short wavelengths  and magnetoelastic waves  in a very effective manner. We expect to develop a new class of devices based on such MMS. On the other hand, much experience in studying non-linear phenomena , opens different possibilities to construct a non-linear signal-to-noise enhancer 9] used in satellite TV systems .
However, besides all the potential applications of MMS based on dopped YIG films, our experience in studying non-uniform structures and two-layer epitaxial structures with interlayer exchange coupling , equips us with the necessary methods to carry out microwave diagnostics of a much broader range of MMS, including metalic multilayers and magnetic tunnel junctions.
Brief Description of the Research
The research will be carried out in four mutually connected directions.
The first task of our research is concerned with the investigation of MMS made of YIG and grown by LPE. There are structures of four types containing: 1) YIG films doped with Ga and Sc ions, in which the magnetic resonances and waves will be studied at the relatively low frequencies (HF and UHF range); 2) YIG films, whose magnetic properties are non-homogeneous over the film thickness, and in which short spin-waves and intense resonances will be excited and investigated; 3) several contacting YIG films with different magnetic parameters, in which the interlayer interaction will be studied; and 4) non-uniform YIG films doped with Lu and Bi for magneto-optical investigations. Involved in this work will be the groups at the FIRE, SIRE and MIRE.
The second task of research is concerned MMS made of metal, dielectric and semiconductor films, which are grown by MBE and magnetron sputtering. The materials used should be inexpensive and widely used. We plan to investigate in the following directions: 1) development of a technology for ultra-thin film growth on oriented and specially prepared substrates (Chapter 3); 2) development of novel methods to test the MMS by probing them with spin-waves and magnetoelastic waves; 3) experimental and theoretical investigation of magnetic resonance spectra, dispersion characteristics of traveling waves, non-linear resonances and waves, and magneto-optic effects; 4) temperature dependence of these properties at different geometry and composition; and 5) experiments on the magneto-resistance and development of magneto-resistance sensors. Involved in this work will be the groups at the SIRE and MIRE.
The third direction of research is concerned the effect of spin-injection in MMS. We plan to: 1) prepare the MMS, test them, and find the optimal choice of their composition and geometry; 2) investigate theoretically the spin-injection mechanisms at different temperatures and bias fields, the electron kinetics and their energy spectra close to the interfaces, estimate the injection level depending on the current, and clarify the relation between the injection level and the structure of the magnetic state; 3) investigate experimentally the mechanisms and parameters of spin-injection by FMR and optical methods, and measure the current-voltage and capacitance-voltage characteristics; 4) compare the experimental and theoretical results in order to clarify features such as the non-uniformity of the magnetic state at strong currents and the possibility to amplify and generate electromagnetic waves at these conditions. Involved in this work will be the group at the FIRE.
The fourth task of research is concerned with optical properties of MMS. We plan to conduct: 1) measurements of the optical parameters of the structures at different optical wavelengths and theoretical investigations describing light propagation in the structures; 2) experimental and theoretical investigations of light interacting with magnetic oscillations and waves in the structures, especially non-linear waves and oscillations; 3) the preparation of structures used in magneto-optical Bragg cells; 4) study of the produced Bragg cells, optimization of their characteristics and development of schemes for their use in optical communication channels; 5) development of mathematical models on signal processing in optical communication channels consisting of magneto-optical devices; 6) development of magneto-optical visualization methods suitable for investigating magnetic thin films used in magnetic storage devices and signal processing devices. Involved in this work will be the groups at the MIRE and SIRE.
Results of investigations will be presented as reports, publications and devices.
Potential applications: 1. Specially prepared YIG films – commercial communication systems; 2. Spin-transistors and other spin-injection devices – signal processing, sensors; 3. Thin-film multi-layered structures – magneto-resistance sensors in memory devices; 4. Magneto-optical Bragg cells – components for optical communication systems.
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