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

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

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)

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)

Motor Skill Acquisition Promotes Human Brain Myelin Plasticity

Experience-dependent structural changes are widely evident in gray matter. Using diffusion weighted imaging (DWI), the neuroplastic effect of motor training on white matter in the brain has been demonstrated. However, in humans it is not known whether specific features of white matter relate to motor skill acquisition or if these structural changes are associated to functional network connectivity. Myelin can be objectively quantified in vivo and used to index specific experience-dependent change. In the current study, seventeen healthy young adults completed ten sessions of visuomotor skill training (10,000 total movements) using the right arm. Multicomponent relaxation imaging was performed before and after training. Significant increases in myelin water fraction, a quantitative measure of myelin, were observed in task dependent brain regions (left intraparietal sulcus [IPS] and left parieto-occipital sulcus). In addition, the rate of motor skill acquisition and overall change in myelin water fraction in the left IPS were negatively related, suggesting that a slower rate of learning resulted in greater neuroplastic change. This study provides the first evidence for experience-dependent changes in myelin that are associated with changes in skilled movements in healthy young adults

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)

THE EFFECT OF DISSIPATION ON THE CAUSTICS OF MAGNETOELASTIC WAVES IN FERROMAGNETS

The report is devoted to the invetigation of damping influence to wave's caustics in elastically anisotropic magnetic materials. Based on phenomenological model, we defined the dependence of caustic patterns on damping parameter and elasic anisotropy.Результаты исследования получены в рамках государственного задания Министерства науки и высшего образования Российской Федерации (тема «Функция» № 122021000035–6) и при поддержке гранта РНФ (проект № 22-19-00355)