Persons

The base element of micromagnetic devices are the layered spin-valve structures. Small sizes, compatibility with the CMOS technology, scaling ability and various work conditions make the spin-valve structures a universal component of modern microelectronics. The purpose of present work is the analysis, systematization and generalization of the data of the work theoretical bases, experimental data and the application of spin valves. In the review, the hard disc drives, random-access magnetoresistive memory, the spin-transfer nano-oscillators, the magnetic biosensors, as well as various computing systems, operating on the principles of stochastic and deterministic logic, have been considered. The key theoretical works devoted to giant magnetoresistance and spin transfer have been used. The data on various types of the hard-disc readheads have been systematized, their architecture and parameters have been compared, and it has been shown how modern scientific research of nanomagnetic phenomena accelerates the growth rate of the recording density. The analysis of modern research devoted to magnetoresistive random access memory has been carried out. The problems of energy efficiency and increasing the degree of the integration for these devices have been discussed. The latest achievements in the field of materials, geometry and the properties of the spin-transfer nano-oscillators, as well as the problems and prospects for the development of this technology have been considered. The analysis of theoretical and experimental works, in which the spin-gate structures have been used to perform the logical operations of Boolean and non-Boolean logic, has been carried out. It has been shown how the probabilistic nature of the unstable switching of spin gates is used in the operation of the unconventional computing systems, namely, neuromorphic or Bayesian networks. The principles of operation of the spin valves as magnetic biosensors have been considered and the advantages of their application have been discussed.

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The magnetoresistive random access memory (MRAM) has some advantages over other types of memory. However, MRAM has a significant drawback: the values of current density and the magnetic field, that must be applied to switch the free layer of the spin valve, which is included into the MRAM cell, are too high. The dependence of the current density and of the switching magnetic field on the magnetic parameters, from which the ferromagnetic layers of the spin valve have been fabricated, has been analyzed. The comparison of the critical characteristics of the spin valve with the longitudinal anisotropy of the ferromagnetic layers, made on the basis of different materials has shown that the perspective materials for the spin valve are cobalt, iron, cobalt ferrobates and cobalt alloys with gadolinium. For these materials the bifurcation diagrams of the equations, describing the valve switching process, have been created and analyzed. On the basis of the study four optimal switching modes have been selected. The comparison of the values of the external magnetic field and the injection current corresponding to the stable switching of the magnetosensitive memory cell has been performed for various materials. The numerical estimations of the MRAM switching time have been executed and the conditions for the optimal cell operation have been found. The studies performed have shown that the materials for the spin valve fabrication are most suitable - FeCoB and CoGd annealed at the temperatures of 300 °C and 200 °C, respectively.

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In the years since the discovery of giant magnetoresistance, a layered spin-valve structure has found application in a large number of microelectronic devices such as magnetoresistive random access memory, a variety of spin-transfer nanooscillators and stochastic binary neurons used to implement the probabilistic spin logic. Therefore, the identification of the basic modes of operation of the spin valve for these microelectronic devices is an important task. To solve this problem, a mathematical model of a spin valve, of square cross-section, with longitudinal anisotropy, placed in a magnetic field perpendicular to the anisotropy axis and the plane of the layers has been studied. A qualitative analysis of the obtained system of equations describing the dynamics of the magnetization vector of the free layer of the spin valve has allowed determination of the equilibrium states of the magnetization in the free layer of the spin-valve structure. Bifurcation analysis of the dynamic system of equations revealed conditions for changing the type of singular points of the system. Based on the study of the dynamics of the magnetization vector in the free layer, the most suitable modes for the operation of the magnetoresistive memory, spin-transfer nanooscillator and binary stochastic neuron have been chosen. The ranges of the magnetic field and current corresponding to the optimal performance parameters of these devices including the frequency and amplitude characteristics of spin-transfer nano-oscillators have been determined.

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The magnetic random-access memory has significant advantages over already existing memory types. The study of the magnetization-vector dynamics in the magnetic random-access memory (MRAM) using the Stoner - Wohlfahrth approximation is reduced to the analysis of a nonlinear dynamic system of three equations. For simplification, the influence of the functional form of the Slon-czewski - Berger current term on the dynamics of the magnetization vector when switching cell of the magnetic random-access memory has been studied. For this purpose the expansion of the current term into the Taylor series in the vicinity of singular points of the dynamical system has been carried out. Thus, depending on the number of recorded terms of the Taylor expansion, a set of dynamical systems has been obtained, which has been studied with use of the qualitative analysis methods. The analysis of relative errors of calculations has been performed. The current term has been expanded into a Taylor series from zero up to the fourth degree. The singular points of the set of equations describing the magnetization dynamics in a three-layer structure Co/Cu/Co for different approximations of the current term, have been found. The bifurcation diagrams of the dynamic system have been built and analyzed. It is shown that the calculations of the dynamics of the magnetization vector of the valve structure, as well as critical currents and field of the MRAM cells, permit to use the decomposition into the Taylor series of the Slonczewski-Berger current term from the fourth power and higher.

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Modern microelectronic devices based on the layered spin-valve structures are characterized by low power consumption, high reliability and wide temperature range. The study of the spin valve modes and the investigation of the possibilities of controlling these modes are of practical interest. In the work the spin valve modes operation, which are the base for magnetoresistive random-access memory MRAM, a binary stochastic neuron p-bit and a variety of the spin-transfer nano-oscillators STNOs, have been considered. For this purpose, a mathematical model of a spin valve with longitudinal anisotropy, placed in a magnetic field perpendicular to the axis of anisotropy and parallel to the plane of the layers has been constructed. A system of equations, describing the dynamics of the magnetization vector of the free layer of the spin valve has been obtained. The qualitative analysis of this system has enabled us to determine the equilibrium state of the free layer magnetization for the spin-valve structure. Based on the bifurcation analysis of the dynamic system of equations the conditions for changing the type of singular points of the system have been determined. The study on the dynamics of the magnetization vector of the free layer of the spin valve has helped to identify the main modes of its work as a part of the magnetoresistive random access memory, a binary stochastic neuron, spin-transfer nano-oscillators and to determine the ranges of the current and the magnetic field corresponding to the modes. For the spin-transfer nano-oscillators, also, the frequency and amplitude characteristics have been calculated. The proposed structure with planar anisotropy, placed in the field, which is perpendicular to the anisotropy axis, from the point of view of its application as a spin-transfer nano-oscillator will be more preferable than the structure with the field applied in parallel to anisotropy axis.

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