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

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

Spin Susceptibility of Ga-Stabilized delta-Pu Probed by {69}^Ga NMR

Spin susceptibility of stabilized \delta phase in the Pu-Ga alloy is studied by measuring {69,71}^Ga NMR spectra and nuclear spin-lattice relaxation rate {69}T_{1}^{-1} in the temperature range 5 - 350 K. The shift ({69}^K) of the {69,71}^Ga NMR line and {69}^T_{1}^{-1} are controlled correspondingly by the static and the fluctuating in time parts of local magnetic field arisen at nonmagnetic gallium due to transferred hyperfine coupling with the nearest f electron environment of the more magnetic Pu. The nonmonotonic with a maximum around 150 K behavior of {69}^K(T) \chi_{s,5f}(T) is attributed to the peculiarities in temperature dependence of the f electron spin susceptibility \chi_{s,5f}(T) in \delta phase of plutonium. The temperature reversibility being observed in {69}^K(T) data provides strong evidence for an electronic instability developed with T in f electron bands near the Fermi energy and accompanied with a pseudogap-like decrease of \chi_{s,5f}(T) at T<150 K. The NMR data at high temperature are in favor of the mainly localized character of 5f electrons in \delta phase of the alloy with characteristic spin-fluctuation energy \Gamma(T) T^{0.35(5)}, which is close to $\Gamma(T) T^{0.5} predicted by Cox et al. [J. Appl. Phys. 57, 3166 (1985)] for 3D Kondo-system above T_Kondo}. The dynamic spin correlations of 5f electrons become essential to consider for {69}^T_{1}^{-1}(T) only at T<100 K. However, no NMR evidences favoring formation of the static magnetic order in \delta-Pu were revealed down to 5K .Comment: 6 pages, 4 figure

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

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

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