On the nonlinear NMR and magnon BEC in antiferromagnetic materials with coupled electron-nuclear spin precession

We present a new study of nonlinear NMR and Bose-Einstein Condensation (BEC) of nuclear spin waves in antiferromagnetic MnCO3 with coupled electron and nuclear spins. In particular, we show that the observed behaviour of NMR signals strongly contradicts the conventional description of paramagnetic ensembles of noninteracting spins based on the phenomenological Bloch equations. We present a new theoretical description of the coupled electron-nuclear spin precession, which takes into account an indirect relaxation of nuclear spins via the electron subsystem. We show that the magnitude of the nuclear magnetization is conserved for arbitrary large excitation powers, which is drastically different from the conventional heating scenario derived from the Bloch equations. This provides strong evidence that the coherent precession of macroscopic nuclear magnetization observed experimentally can be identified with BEC of nuclear spin waves with k=0.Comment: 12 pages, 8 figure

Magnon caustics in face centered cubic ferromagnets

Caustic phenomenon in magnon propagation picture in face centered cubic lattice is investigated. The employed model takes into account the Heisenberg exchange interaction between an atom's spin and it's nearest and next nearest neighbors. Caustic directions are defined by exploring the corresponding dispersion relations. On the basis of face centered cubic lattice of EuS ferromagnet, such singular peculiarities are investigated for different Heisenberg exchange parameters. Magnon energy regions where the caustics can be observed are defined together with the caustic directions in characteristical crystallographic planes {100} and {110}. It was obtained that caustics can be observed if the magnon frequency is about hundreds of gigahertz, and the widths of the caustic direction regions, that are determined by the magnon frequencies, are of the order of dozens of degrees. © Published under licence by IOP Publishing Ltd.Russian Foundation for Basic Research, RFBR: 18-32-00139Ural Branch, Russian Academy of Sciences, UB RAS: No.18-10-2-37The research was carried out within the state assignment of FASO of Russia (theme “Function” AAAA-A19-119012990095-0), supported by RFBR (project No. 18-32-00139), and by UB RAS Project No.18-10-2-37

Nuclear magnetic relaxation induced by the relaxation of electron spins

© 2017, Pleiades Publishing, Inc.A physical mechanism responsible for the relaxation of nuclear spins coupled by the hyperfine interaction to relaxed electron spins in materials with spin ordering is proposed. The rate of such induced nuclear spin relaxation is proportional to the dynamic shift of the nuclear magnetic resonance (NMR) frequency. Therefore, its maximum effect on the NMR signal should be expected in the case of nuclear spin waves existing in the system. Our estimates demonstrate that the induced relaxation can be much more efficient than that occurring due to the Bloch mechanism. Moreover, there is a qualitative difference between the induced and Bloch relaxations. The dynamics of nuclear spin sublattices under conditions of the induced relaxation is reduced to the rotation of m1 and m2 vectors without any changes in their lengths (m12(t) = m22(t) = m02(t)= const). This means that the excitation of NMR signals by the resonant magnetic field does not change the temperature Tn of the nuclear spin system. This is a manifestation of the qualitative difference between the induced and Bloch relaxations. Indeed, for the latter, the increase in Tn accompanying the saturation of NMR signals is the dominant effect

Features of focusing magnetoelastic waves in YIG crystals

The focusing features and caustic of magnetoelastic waves in Y3Fe5O12 (YIG) crystals in the long wavelength approximation are investigated. It is shown that the interaction of phonon and magnetic subsystems leads to pronounced anisotropic properties of magnetoelastic waves. Four magnetoelastic eigenmodes are realized in the crystal, and two of them possess a focusing and caustic in the vicinity of magnetoelastic resonance point. The region of frequencies and wavenumbers of magnetoelastic waves is obtained, where the caustic can be observed. The directions of the focusing and caustic are defined. © Published under licence by IOP Publishing Ltd.Russian Academy of Medical Sciences, RAMSRussian Foundation for Basic Research, RFBR: 18-32-00139The research was carried out within the state assignment of Minobrnauki of Russia (theme “Function” AAAA-A19-119012990095-0) and project No. 32-1.1.3.5 of the Program of Basic Research of the Presidium of the Russian Academy of Science, supported in part by RFBR (project No. 18-32-00139)

Autoresonant excitation of dark solitons

Continuouslyphase-locked (autoresonant) dark solitons of the defocusing nonlinear Schrodinger equation are excited and controlled by driving the system by a slowly chirped wavelike perturbation. The theory of these excitations is developed using Whitham's averaged variational principle and compared with numerical simulations. The problem of the threshold for transition to autoresonance in the driven system is studied in detail, focusing on the regime when the weakly nonlinear frequency shift in the problem differs from the typical quadratic dependence on the wave amplitude. The numerical simulations in this regime show a deviation of the autoresonance threshold on the driving amplitude from the usual 3/4 power dependence on the driving frequency chirp rate. The theory of this effect is suggested. © 2015 American Physical Society

CAUSTIC OF MAGNETOELASTIC WAVES IN A THICK YIG FILM

The caustic of magnetoelastic waves (MEW) propagating in a thick Y3Fe5O12 film is in-vestigated. The frequency range is obtained where the MEW caustic occurs. The caustics directions as a function of the wave frequency are calculated.The research was carried out within the state assignment of FASO of Russia (theme ``Function'' AAAA-A19-119012990095-0), supported by Basic Research Program of the Presidium of the Russian Academy of Sciences (project No. 13-1.1.3.5)

Putting the “Sensory” Into Sensorimotor Control: The Role of Sensorimotor Integration in Goal-Directed Hand Movements After Stroke

Integration of sensory and motor information is one-step, among others, that underlies the successful production of goal-directed hand movements necessary for interacting with our environment. Disruption of sensorimotor integration is prevalent in many neurologic disorders, including stroke. In most stroke survivors, persistent paresis of the hand reduces function and overall quality of life. Current rehabilitative methods are based on neuroplastic principles to promote motor learning that focuses on regaining motor function lost due to paresis, but the sensory contributions to motor control and learning are often overlooked and currently understudied. There is a need to evaluate and understand the contribution of both sensory and motor function in the rehabilitation of skilled hand movements after stroke. Here, we will highlight the importance of integration of sensory and motor information to produce skilled hand movements in healthy individuals and individuals after stroke. We will then discuss how compromised sensorimotor integration influences relearning of skilled hand movements after stroke. Finally, we will propose an approach to target sensorimotor integration through manipulation of sensory input and motor output that may have therapeutic implications

PECULIARITIES OF THE ANISOTROPY OF MAGNETOELASTIC WAVES SPECTRUM IN GALFENOL

The magnetoelastic waves in a bulk galfenol sample in the vicinity of magnetoelastic resonance are investigated. It is shown, that magnetoelastic waves with frequencies close to magnetoelastic resonance possess focusing effect: they do not propagate uniformly, but mainly along specific directions.The research was carried out within the state assignment of FASO of Russia (theme "Function'' AAAA-A19-119012990095-0), supported by Basic Research Program of the Presidium of the Russian Academy of Sciences (project No. 13-1.1.3.5)