### Browsing by Author "Uzdin, V. M."

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Item Crossover temperature for quantum tunnelling in spin systems(2016-09-15) Vlasov, S. M.; Bessarab, P. F.; Uzdin, V. M.; Jónsson, H.; St. Petersburg National Research University of Information Technologies, Mechanics and Optics (ITMO); St. Petersburg State University; Department of Applied PhysicsWe derive an analytical expression for the crossover temperature corresponding to the transition from classical activation mechanism to temperature assisted quantum tunnelling in spin systems. The crossover temperature depends on the magnetic configuration and Hessian at the first order saddle point on the energy surface of the system. The theory is applied to several single spin models, including a system with four-fold anisotropy. Good agreement is obtained with experimental results for a molecular magnet containing Mn4.Item Energy surface and transition rates in a hexagonal element of spin ice(2017-10-28) Liashko, S. Y.; Uzdin, V. M.; Jónsson, H.; University of Iceland; St. Petersburg National Research University of Information Technologies, Mechanics and Optics (ITMO); Department of Applied PhysicsThe energy surface of a hexagonal element of Kagome spin ice consisting of six prolate magnetic islands is investigated as a function of applied magnetic field. Minimum energy paths for magnetic reversals are determined to estimate energy barriers and the transition rates estimated using harmonic transition state theory for magnetic systems. The overall transition rate between equivalent ground states is calculated using the stationary state approximation including all possible transition paths. The calculated transition rates are in close agreement with reported experimental measurements taken in the absence of an applied field. Predictions are made for the change in the energy landscape and transition rates as a magnetic field is applied.Item Fast and robust algorithm for energy minimization of spin systems applied in an analysis of high temperature spin configurations in terms of skyrmion density(Elsevier Science B.V., 2021-03) Ivanov, A. V.; Uzdin, V. M.; Jónsson, H.; Department of Applied Physics; Multiscale Statistical and Quantum Physics; University of Iceland; St. Petersburg State UniversityAn algorithm for the minimization of the energy of magnetic systems is presented and applied to the analysis of thermal configurations of a ferromagnet to identify inherent structures, i.e. the nearest local energy minima, as a function of temperature. Over a rather narrow temperature interval, skyrmions appear and reach a high temperature limit for the skyrmion density. In addition, the performance of the algorithm is further demonstrated in a self-consistent field calculation of a skyrmion in an itinerant magnet. The algorithm is based on a geometric approach in which the curvature of the spherical domain is taken into account and as a result the length of the magnetic moments is preserved in every iteration. In the limit of infinitesimal rotations, the minimization path coincides with that obtained using damped spin dynamics while the use of limited-memory quasi-newton minimization algorithms, such as the limited-memory Broyden–Fletcher–Goldfarb–Shanno (LBFGS) algorithm, significantly accelerates the convergence.Item Fully self-consistent calculations of magnetic structure within non-collinear Alexander-Anderson model(ST PETERSBURG NATL RESEARCH UNIV INFORMATION TECHNOLOGIES, MECH & OPTICS, 2020-02) Ivanov, A.; Bessarab, P. F.; Jonsson, Hannes; Uzdin, V. M.; University of Iceland; St. Petersburg National Research University of Information Technologies, Mechanics and Optics (ITMO); Multiscale Statistical and Quantum Physics; Department of Applied PhysicsAn implementation of the non-collinear Alexander-Anderson model for itinerant electrons in magnetic systems is presented where self-consistency is reached for specified directions of the magnetic moments. This is achieved by means of Lagrange multipliers and a variational principle for determining the transverse and longitudinal components of the magnetic moments as well as the average number of d-electrons using direct optimisation. Various optimisation algorithms are compared and the limited memory Broyden-Fletcher-Goldfarb-Shanno algorithm is found to give the best performance. An application to antiferromagnetic Cr crystal is presented where spin-dynamics and curvature of the energy surface are calculated to compare results obtained with and without the constraints on the orientation of the magnetic moments.Item Models of the energy landscape for an element of shakti spin ice(ST PETERSBURG NATL RESEARCH UNIV INFORMATION TECHNOLOGIES, MECH & OPTICS, 2018-12) Arnalds, U. B.; Liashko, S. Y.; Bessarab, P. F.; Uzdin, V. M.; Jonsson, H.; University of Iceland; St. Petersburg National Research University of Information Technologies, Mechanics and Optics (ITMO); St. Petersburg State University; Multiscale Statistical and Quantum Physics; Department of Applied PhysicsMicromagnetic calculations are compared with faster model calculations of interacting nanoscopic magnetic islands representing an element of a shakti spin ice lattice. Several pathways for transitions between equivalent ground states are studied. The model calculations describe the interaction between the islands either with the point dipole approximation, or with a dumbbell approximation where the distance between the two poles is optimized to match the micromagnetic results. The closest agreement in the energy of both local minima as well as transition state configurations where one macrospin has rotated by 90 degrees is obtained with a dumbbell model where the distance between the poles is ca. 20 % smaller than the island length.Item Rate of thermal transitions in kagome spin ice(2016-09-15) Liashko, S. Y.; Uzdin, V. M.; Jónsson, H.; St. Petersburg National Research University of Information Technologies, Mechanics and Optics (ITMO); St. Petersburg State University; Department of Applied PhysicsThe rate of thermal transitions in a kagome spin ice element is calculated using harmonic transition state theory for magnetic systems. Each element consists of six prolate magnetic islands. Minimum energy paths on the multidimensional energy surface are found to estimate activation energy. Vibrational frequencies are also calculated to estimate the rate of the various transitions. An overall transition rate between equivalent ground states is calculated by using the stationary state approximation including all possible transition paths. The resulting transition rate is in a good agreement with experimentally measured lifetime.Item Stability of long-lived antiskyrmions in the Mn-Pt-Sn tetragonal Heusler material(American Physical Society, 2020-10-26) Potkina, M. N.; Lobanov, I. S.; Tretiakov, O. A.; Jónsson, H.; Uzdin, V. M.; Department of Applied Physics; Multiscale Statistical and Quantum Physics; University of Iceland; St. Petersburg State University; University of New South WalesThe lifetime of antiskyrmions at room temperature in an Mn-Pt-Sn tetragonal Heusler material has been calculated using an atomic scale representation including nearly a million spins. The evaluation of the pre-exponential factor in the Arrhenius rate expression for this large system is made possible by implementation of harmonic transition state theory that avoids evaluation of the eigenvalues of the Hessian matrix. The parameter values in the extended Heisenberg Hamiltonian, including anisotropic Dzyaloshinskii-Moriya interaction, are chosen to reproduce experimental observations [Nayak et al., Nature (London) 548, 561 (2017)10.1038/nature23466], in particular, the 150-nm diameter. The calculated results are consistent with the long lifetime observed in the laboratory and this exceptional stability of the antiskyrmions is found to result from large activation energy for collapse due to strong exchange coupling while the pre-exponential factor in the Arrhenius expression for the lifetime is found to have a typical magnitude of 10-12 s, despite the large number of spins. The long lifetime is, therefore, found to result from energetic effects rather than entropic effects in this system.Item Tip-surface interaction and rate of magnetic transitions(2016-09-15) Ivanov, A. V.; Bessarab, P. F.; Uzdin, V. M.; Jónsson, H.; University of Iceland; KTH Royal Institute of Technology; St. Petersburg National Research University of Information Technologies, Mechanics and Optics (ITMO); Department of Applied PhysicsCalculations of the interaction between a magnetic tip of an atomic force microscope with a magnetic surface using the non-collinear extension of the Alexander-Anderson model are described. The mechanism and rate of thermally activated magnetic transitions in a cluster of atoms at the tip is investigated. The results are compared with experimental data and found to be in good agreement with measured lifetimes [R. Schmidt et. al., Phys. Rev. B 86, 174402 (2012).]. The results are also compared with previously reported density functional theory calculations.