Multiquantum triple-pulse spin-echo signals from quadrupole nuclei in magnetically ordered substances

We analyze theoretically the formation of NMR pulse responses from a quadrupole spin system in which the inhomogeneous broadening of a spectral line is due to both magnetic and electric quadrupole interactions. We derive formulas for the moments of formation of multiquantum echo signals in the case of three exciting pulses. For the first time we detected in experiments multiquantum spin-echo signals from copper nuclei in ferromagnetic copper sulfochromite in the cases of double-pulse and triple-pulse excitations. We find that there is good agreement between the calculated and experimentally observed moments of echo signal formation. Zh. éksp. Teor. Fiz. 115, 2106–2112 (June 1999)     

Spectra of multiquantum echo signals from quadrupole nuclei with half-integral spin in magnetically ordered materials

The numerical simulation of two-pulse echo signals at times 2τ, 4τ, and 6τ for the I=5/2 spin and at time 2τ, 4τ, and 8τ for the I=7/2 spin (τ is the time interval between exciting pulses) is carried out. It is shown that a delay by 2τ in the moment of formation of the echo results in the disappearance of extreme quadrupole satellites in the NMR spectrum obtained by recording the frequency dependence of the echo amplitude. The echoes at the maximum possible time of formation (2I+1)τ are only observed at the frequency of the purely magnetic spectroscopic transition $ \pm \frac{1}{2} \rightleftarrows \mp \frac{1}{2}$ ; no such echoes are observed at the quadrupole satellite frequencies. The computations are compared with the experimental results obtained for the ⁵⁵Mn nuclei (spin I=5/2) in the perovskite GdCu₃Mn₄O₁₂ and the spinel Li_0.5Fe_2.5O₄: Mn.     

Nuclear orientation and nuclear spin-lattice relaxation in insulating magnetically ordered crystals

The mechanism of nuclear spin-lattice relaxation in insulating magnetically ordered crystals at low temperatures is discussed. In 3-dimensional systems the relaxation time T₁ is long, but in the lower dimensional systems it can be quite short making them possible candidates for hosts in on-line experiments. An experiment in 2-dimensional⁵⁴Mn−Mn (COOCH₃)₂·4H₂O is discussed with particular reference to the relatively short value of T₁ in low applied fields. A preliminary experiment in the 1-dimensional system⁵⁴Mn−(CH₃)₄NMnCl₃ (TMMC) is also described. Pulsed NMRON measurements in⁵⁴Mn−MnCl₂·4H₂O are outlined and the advant-systems with fast relaxation emphasized.     

Neutral fermions in paramagnetic insulators

We discuss sources of neutral fermions in paramagnetic insulators which apparently play a major role in high-T_c superconductivity     

Spin transport in antiferromagnetic insulators

Electrical spin, which is the key element of spintronics, has been regarded as a powerful substitute for the electrical charge in the next generation of information technology, in which spin plays the role of the carrier of information and/or energy in a similar way to the electrical charge in electronics. Spin-transport phenomena in different materials are central topics of spintronics. Unlike electrical charge, spin transport does not depend on electron motion, particularly spin can be transported in insulators without accompanying Joule heating. Therefore, insulators are considered to be ideal materials for spin conductors, in which magnetic insulators are the most compelling systems. Recently, we experimentally studied and theoretically discussed spin transport in various antiferromagnetic systems and identified spin susceptibility and the Néel vector as the most important factors for spin transport in antiferromagnetic systems. Herein, we summarize our experimental results, physical nature, and puzzles unknown. Further challenges and potential applications are also discussed.     

Electron paramagnetic resonance in Kondo insulators

This review is devoted to the application of electron paramagnetic resonance (EPR) in the study of fluctuating-valence materials, which are characterized by a narrow gap in the electron energy spectrum (Kondo insulators or Kondo semiconductors). The authors’ papers on studying classical objects of this field of solid-state physics, SmB₆ and YbB₁₂, are considered as an illustration of the potentiality of the EPR method. Temperature dependences of the gap width in these materials were obtained, the static and dynamic Jahn-Teller effects on Sm³⁺ ions in SmB₆ were detected, and the formation of Yb³⁺ ion pairs and the spontaneous breaking of cubic symmetry in YbB₁₂ were observed. The results obtained indicate that preference should be given to the exciton-polaron model developed by Kikoin et al. for the ground state of Kondo insulators.     

Nonsecular contribution to the decay of spin-echo signals of quadrupolar nuclei in magnetically ordered materials

We analyze the influence of fluctuations of the nonsecular part of the spin Hamiltonian on the decay of ordinary and multiquantum signals of the two-pulse spin echo in a quadrupole spin system with an inhomogeneously broadened spectral line. Expressions are obtained for the rate of decay of an echo in the case of selective excitation of a signal from quadrupole nuclei with arbitrary spin. These expressions are then used to analyze the experimentally observed ordinary and multiquantum echo signals from quadrupole nuclei with spin I=3/2 (⁵³Cr, ⁶³Cu, and ⁶⁵Cu) in ferromagnetic chromium chalcogenide spinels. Zh. éksp. Teor. Fiz. 116, 204–216 (July 1999)     

Anomalously strong relaxation of the polarization of muons in the magnetically ordered and paramagnetic states of the TbMnO<Subscript>3</Subscript> multiferroic

An anomalously strong relaxation of the muon polarization in a magnetically ordered state in the TbMnO₃ multiferroic has been revealed by the method below the μSR Néel temperature (42 K). Such a relaxation is due to the muon channel of relaxation of the polarization and the interaction of the magnetic moment of the muon with inhomogeneities of the internal magnetic field of an ordered state in the form of a cycloid. Above the Néel temperature, beginning with temperatures depending on the applied magnetic field, a two-phase state has been revealed where one phase has an anomalously strong relaxation of the muon polarization for a paramagnetic state. These features of the paramagnetic state are due to short-range magnetic order domains that appear in strongly frustrated TbMnO₃. A true paramagnetic state has been observed only at T ≥ 150 K.     

Spin waves in paramagnetic metals

The excitation of spin waves in paramagnetic metals by circularly-polarized electromagnetic waves is considered. An investigation is made of the effect of boundary conditions for the magnetic moment on the amplitude of the electromagnetic-spin waves in a metal. The same investigation is carried out for the surface impedance.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 7–10, January, 1974.The author acknowledges A. Ya. Blank for proposing the subject and for numerous discussions.     

Effects of paramagnetic cations on the nonexponential spin-lattice relaxation of rare spin nuclei in solids

Nonexponential spin-lattice relaxation is often observed for rare spin nuclei in the solid state. Deviation from single-component decay may be amplified by the coupling of rare spin nuclei to paramagnetic centers. Nonexponential spin-lattice relaxation was observed in derivatized silica gels resins. This phenomenon was localized and enhanced when paramagnetic transition metal cations were bound to surface functional groups. A stretched exponential analysis method was determined to be robust in fitting nonexponential relaxation curves for silica gels both with and without bound paramagnetic ions. Spin-lattice relaxation rates (T₁⁻¹) for functional group nuclei increased as a function of percent surface coverage with metal ion. The magnitude of the relaxation rate increase was dependent upon internuclear distances from the paramagnetic center. At low surface coverages, a semi-random distribution of paramagnetic centers increased the degree of stretching of spin-lattice relaxation decays, as measured by decreases in the calculated stretching parameter β. At higher surface coverages, calculated β values reached a limiting value, indicating that while the spin-diffusion mechanism in metal-ex-changed silica gels is restricted, it is not completely diminished.     

Muon level-crossing resonance in magnetically ordered materials

The positive muon is widely used as a microscopic probe of internal fields at interstitial sites in magnetically ordered materials. Recently, we have demonstrated that the hyperfine fields on the neighboring host nuclear spins can be measured using a novel muon level-crossing resonance technique, thus providing a more detailed picture of the electronic and magnetic environment around the muon. In this paper I will describe the fundamentals of muon level-crossing resonance as applied to magnetically ordered materials, and report an example in MnF₂.     

Quantum magnetic impurities in magnetically ordered systems

We discuss the problem of a spin 1/2 impurity immersed in a spin S magnetically ordered background. We show that the problem maps onto a generalization of the dissipative two level system with two independent heat baths, associated with the Goldstone modes of the magnet, that couple to different components of the impurity spin operator. Using analytical perturbative renormalization group methods and accurate numerical renormalization group we show that contrary to other dissipative models there is quantum frustration of decoherence and quasiscaling even in the strong coupling regime. We make predictions for the behavior of the impurity magnetic susceptibility. Our results may also have relevance to quantum computation.     

Spin waves in paramagnetic Boltzmann gases

As the temperature is lowered we get an interesting temperature region? _d?T?? ²/mr ₀ ² (where? _d is the quantum degeneracy temperature,m the mass of a gas molecule,r ₀ the radius of interparticle interaction) in which the thermal de Broglie wavelength Λ of a particle is considerably greater than its sizer ₀ though Λ turns out to be less than the mean interparticle distanceN ^?1/3?Λ?r ₀. Although the gas molecules obey the classical Boltzmann-Maxwell statistics the system as a whole begins to exhibit a larger number of essentially quantum macroscopic collective features. One of the most interesting and dramatic features is the possibility of propagation of weakly damped spin oscillations in spin-polarized gases (external magnetic field, optical pumping). Such oscillations can propagate both in the low-frequencyθτ?1 regime and the high frequencyθτ?1. The last case is highly non-trivial for a Boltzmann gas with a short range interaction between particles. The weakness of relaxation damping of spin modes implies that the degree of polarization is high enough 1>/|α|?|a|/Λ, whereα=(N ₊?N _?)N,a is the two-particles-wave scattering length. Under these conditions the spectrum of magnons has the form (Bashkin 1981, 1984; Lhuillier and Laloe 1982) 1 $$\omega = \Omega _H + \left( {{{K^2 \nu _{\rm T}^2 } \mathord{\left/ {\vphantom {{K^2 \nu _{\rm T}^2 } {\Omega _{int} }}} \right. \kern-\nulldelimiterspace} {\Omega _{int} }}} \right)\left( {{{1 - i} \mathord{\left/ {\vphantom {{1 - i} {\Omega _{int} }}} \right. \kern-\nulldelimiterspace} {\Omega _{int} }}\tau } \right), \Omega _{int} = {{ - 4\pi ahN\alpha } \mathord{\left/ {\vphantom {{ - 4\pi ahN\alpha } m}} \right. \kern-\nulldelimiterspace} m}, \nu _{\rm T}^2 = {T \mathord{\left/ {\vphantom {T m}} \right. \kern-\nulldelimiterspace} m}$$ where Ω _H is the Larmor precession frequency for spins in the magnetic fieldH. Collisionless Landau damping restricts the region of applicability of (1) with not too large wave vectorsKv _T?|Ω_int|. The existence of collective spin waves has been experimentally confirmed in NMR-experiments with gaseous atomic hydrogen H↑ (Johnsonet al 1984). The presence of undamped spin oscillations means automatically the existence of long range correlations for transverse magnetization. Such correlations decrease with the distance according to the power law 2 $$\delta _{ik} \left( r \right) = 2\left| a \right|\frac{{\left( {\beta N\alpha } \right)^2 }}{\gamma }\delta _{ik} $$ . Hereβ is the molecule magnetic moment. Spin waves being even damped can nevertheless reveal themselves atT?? ²/mr ₀ ² or when |α|?r ₀/Λ. The first experimental discovery or damped spin waves in gaseous³He↑ has been done in Nacheret al 1984. Oscillations of magnetization can also propagate in some condensed media such as liquid³He-⁴He solutions, semimagnetic semiconductors etc.     

Spin fluctuations and the superconducting state in doped insulators

We propose a model of electron pairing via spin fluctuations in doped insulators. The bare states for the superconducting condensate correspond to impurity bands in the original band gap of the undoped material. We obtain a complete set of equations for the superconducting state. We show that fermion pairing in impurity bands of extended states is possible, and thus so is superconductivity, if localized spin-0 bosons are produced. The latter are necessarily accompanied by localized spin-1 bosons, which are responsible for the relationship between singlet and triplet pairing channels of quasiparticles. Zh. éksp. Teor. Fiz. 114, 1765–1784 (November 1998)     

Spin relaxation in fluids

The effect of spin relaxation in fluids on perturbed angular distributions is discussed. The first example, deals with dynamical perturbations resulting from collision-induced charge-transfer process in gases. A brief discussion of relaxation effects caused by molecular motion in non-metallic liquids is followed by a presentation of a new approach to the theory of quadrupole relaxation in liquid metals.