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.     

Pulsed nuclear magnetic resonance signals in magnetically ordered materials

We have obtained an analytical expression for nuclear precession and nuclear echo signals generated in magnetically ordered materials upon resonant excitation of the nuclear subsystem by two pulses of identical amplitude but different durations. We show that in a nuclear subsystem with inhomogeneous broadening of the spectroscopic transition and an inhomogeneous gain distribution, the two-pulse precession and echo signals are split into four and nine components respectively. We have analytically established a correlation between the macroscopic parameters of the components of the two-pulse signals (relative amplitudes, signal formation times) and the microscopic parameters of the magnetically ordered media (inhomogeneous half-width of the spectral line, half-width of the gain distribution function, average gain). The theoretically calculated formation times for the components of the nuclear precession and nuclear echo signals agree with the experimental data obtained for the alloy FeNiCo (70% Co). __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 4, pp. 529–538, July–August, 2008.     

Three-pulse spin echo signals from quadrupolar nuclei in magnetic materials

The time evolutions of the three-pulse spin echo signals from quadrupolar nuclei ⁶³Cu and ⁵³Cr in ferromagnetic CuCr₂S₄:Sb have been investigated at the temperature T=77 K. The experimental results were well explained by the developed theory of the time evolutions of the three-pulse echoes. The main assumption of this theory is the assumption that the temporal fluctuations in the electron magnetization lead to the fluctuations in the hyperfine and quadrupole interaction Hamiltonians.     

Single-pulse nuclear echo signals in magnetically ordered media

An analytical expression for the amplitude of a single-pulse nuclear echo signal generated in magnetically ordered materials is obtained taking into account the inhomogeneous broadening of the spectroscopic transition and the inhomogeneous distribution of the gain with an average value of greater than unity. It is shown that, in this signal, summation of the oscillations of nuclear magnetic moments with equal amplitudes and phases occurs at each instant of time. The cause of the effective suppression of the nuclear magnetic moment oscillations in the initial portion of the free precession signal is revealed analytically. The dependence of the amplitude of the one-pulse echo signal on the strength of an external alternating magnetic field, the pulse duration, and the width of the gain distribution is determined. The results obtained are compared with the experimental data for a Co₂MnSi ferromagnetic polycrystalline sample.     

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.     

Evidences of defect contribution in magnetically ordered Sm-implanted GaN

Samarium (Sm) ions of 200 keV in energy were implanted into highly-resistive molecular-beam-epitaxy grown GaN thin films with a focused-ion-beam implanter at room temperature. The implantation doses range between 10¹⁴ and 10¹⁶ cm⁻². X-ray diffraction revealed Sm incorporation into GaN matrix without secondary phase. Raman-scattering spectroscopy identified impurity-independent defect-related oscillation modes. Slight decrease in band gap and significant reduction in transmittance were observed by optical transmission spectroscopy. Photoluminescence spectra showed emission peaks related to background p-type impurity. Ferromagnetic hysteresis loops were recorded from GaN implanted with highest Sm dose, and magnetic ordering was observed from Sm-implanted GaN with dose of and above 10¹⁵ cm⁻². The long-range magnetic ordering can be attributed to interaction of Sm ions through the implantation-induced Ga vacancy.     

On the theory of multipulsed excitation of spin-echo and free-induction decay signals in angular distribution of oriented nuclei radiation

A theoretical study of the effects of multipulsed NMR on the angular distribution of radiations from oriented nuclei is reported. The perturbation factors of the angular correlation function may be written in general form for any scheme of multipulsed excitation of a nuclear spin system. The irreversible relaxation is taken into account. The experimentally realized situations of spin-echo and free induction decay in angular distribution of nuclear radiation are considered.     

Spin relaxation of quadrupole nuclei in paramagnetic and magnetically ordered insulators

The longitudinal and transverse spin relaxation through a (generally anisotropic) electron-nucleus interaction in paramagnetic and magnetically ordered insulators is theoretically studied for nuclei with a resolved quadrupole structure. Expressions are derived for the relaxation rates of both the transverse nuclear magnetization components when individual transitions are excited in the quadrupole structure and the total longitudinal nuclear magnetization component. These expressions are reduced to a form that contains the Fourier transforms of the time correlation functions only for the electron spins. Given the specific form of these correlation functions corresponding to different phase states of the electron spins and different origins of their fluctuations, the temperature dependences of the nuclear relaxation rates are ascertained in various cases, including those for dipole and isotropic hyperfine interactions. Calculations are performed for arbitrary electron and half-integer nuclear spins by taking into account the possible quadrupole splitting of the NMR spectrum without any restriction on the smallness of the ratio ?ω s/k_BT (ω_s is the resonance frequency of the electron spins). The derived expressions are compared with available experimental data on the longitudinal and transverse nuclear relaxation in colossal-magnetoresistance lanthanum manganites in the part of their phase diagram where the corresponding samples are either paramagnetic or magnetically ordered insulators and near the points of transition to an ordered state. Interpretations alternative to the existing ones are offered.     

Mössbauer spectroscopic studies of magnetically ordered biological materials

This paper discusses recent work showing the application of Mössbauer spectroscopy to the study of the properties of the magnetically ordered materials which occur in a variety of biological systems. These materials display a diversity of behaviour which provides good examples of the various possibilities which can arise with iron-containing particles of different compositions and sizes.     

Circular dichroism in 4f photoemission from magnetically ordered rare-earth materials

4f Photoemission (PE) spectra from magnetically ordered rare-earth materials using circularly polarized X-rays exhibit strong Magnetic Circular Dichroism (MCD), i.e., the intensities of the individual multiplet lines depend on the relative orientation of sample magnetization and photon spin. On the example of the wellresolved Tb 4f PE multiplet, it is shown that in relevant cases 4f PE lines are essentially only observed for one magnetization direction, either parallel or antiparallel to the photon spin. These large MCD effects in 4f PE open new perspectives in the analysis of surface and thin-film magnetism and provide a sensor for the degree of circular polarization of soft X-rays over a wide photon-energy range. To demonstrate the potential of MCD in 4f PE as a magnetometer, we studied Gd(0001) and Tb(0001), where the magnetization of the topmost atomic (0001) layer can be easily separated from the bulk magnetization via the surface core-level shift. In multicomponent magnetic thin films containing different rare-earth elements 4f PE allows to monitor magnetization in an element-specilic way, e.g., in case of the hetero-magnetic interface 1 ML En/Gd(0001).     

Triple-quantum MAS-NMR of quadrupolar nuclei

From two-dimensional multiquantum NMR spectra of quadrupolar nuclei, it is now possible to obtain much greater resolution than in a classical single-quantum magic-angle spinning or variable-angle spinning spectrum. We describe here a very simple pulse scheme which efficiently excites the desired multiquantum NMR coherence and a new acquisition procedure which yields to pure-absorption mode 2D spectra. Experimental spectra for ⁸⁷Rb in polycrystalline rubidium nitrate illustrate the method.     

From crystalline to glassy gallium fluoride materials: an NMR study of Ga and Ga quadrupolar nuclei

Owing to the implementation of acquisition techniques specific for nuclei with very large quadrupolar interaction (full shifted echo and variable offset cumulative spectra (VOCS)), NMR spectra of ⁶⁹Ga and ⁷¹Ga are obtained in crystallised (PbGaF₅, Pb₃Ga₂F₁₂, Pb₉Ga₂F₂₄ and CsZnGaF₆) and glassy (PbF₂–ZnF₂–GaF₃) gallium fluorides. Simulations of both static (full echo or VOCS) and 15 kHz MAS spectra allow to obtain consistent determinations of isotropic chemical shifts and very large quadrupolar parameters (ν_Q up to 14 MHz). In the crystalline compounds whose structures are unknown, the number and the local symmetry of the different gallium sites are tentatively worked out. For the glassy systems, a continuous Czjzek's distribution of the NMR quadrupolar parameters accounts for the particular shape of the NMR spectrum.     

Electric quadrupolar contribution to the nuclear spin-lattice relaxation of Ir in Fe

We report on the first quantitative determination of the electric quadrupolar contribution to the nuclear spin-lattice relaxation in a transition metal. For 186Ir and 189Ir in Fe we have determined the magnetic and the electric quadrupolar part of the relaxation for magnetic fields between 0.01 and 2 T. The quadrupolar part gives information on the role of the orbital motion of the electrons for the relaxation process. Our results prove that the unexpected high relaxation rates in Fe and their magnetic field dependence are due to a nonorbital relaxation mechanism.     

Signal enhancement in solid-state NMR of quadrupolar nuclei

Recent progress in the development and application of signal enhancement methods for NMR of quadrupolar nuclei in solids is presented. First, various pulse schemes for manipulating the populations of the satellite transitions in order to increase the signal of the central transition (CT) in stationary and rotating solids are evaluated (e.g., double-frequency sweeps, hyperbolic secant pulses). Second, the utility of the quadrupolar Carr–Purcell–Meiboom–Gill (QCPMG) and WURST-QCPMG pulse sequences for the rapid and efficient acquisition of particularly broad CT powder patterns is discussed. Third, less frequently used experiments involving polarization transfer from abundant nuclear spins (cross-polarization) or from unpaired electrons (dynamic nuclear polarization) are assessed in the context of recent examples. Advantages and disadvantages of particular enhancement schemes are highlighted and an outlook on possible future directions for the signal enhancement of quadrupolar nuclei in solids is offered.     

Electromagnetic excitation of ultrasound in magnetically ordered dielectrics

The electromagnetic excitation of sound in magnetically ordered dielectrics—ferro-and antiferromagnets—is investigated theoretically. It is shown that sound generation in dielectrics by the Lorentz mechanism (displacement current) is much less efficient than in metals. The magnetoelastic mechanism of sound excitation in dielectrics is just as efficient as in metals. In antiferromagnets the amplitude of the excited sound depends on the relaxation parameter in the magnetic subsystem. The sound excitation efficiency increases as the orientational phase transition point or the ferromagnetic resonance frequency is approached. Zh. éksp. Teor. Fiz. 111, 1810–1816 (May 1997)     

From crystalline to glassy gallium fluoride materials: an NMR study of 69Ga and 71Ga quadrupolar nuclei

Owing to the implementation of acquisition techniques specific for nuclei with very large quadrupolar interaction (full shifted echo and variable offset cumulative spectra (VOCS)), NMR spectra of 69Ga and 71Ga are obtained in crystallised (PbGaF5, Pb3Ga2F12, Pb9Ga2F24 and CsZnGaF6) and glassy (PbF2-ZnF2-GaF3) gallium fluorides. Simulations of both static (full echo or VOCS) and 15 kHz MAS spectra allow to obtain consistent determinations of isotropic chemical shifts and very large quadrupolar parameters (nuQ up to 14 MHz). In the crystalline compounds whose structures are unknown, the number and the local symmetry of the different gallium sites are tentatively worked out. For the glassy systems, a continuous Czjzek's distribution of the NMR quadrupolar parameters accounts for the particular shape of the NMR spectrum.