Pulsed NMRON measurements on insulators

Selective single and double quantum excitation pulsed NMRON has been utilized to obtain rotation patterns, free induction decays, and spin echoes in antiferromagnetic⁵⁴Mn−MnCl₂·4H₂O and the quasi-2 dimensional ferromagnet⁵⁴Mn−Mn(COOCH₃)₂·4H₂O. The pulsed technique is well suited to observing fast spin-lattice relaxation and T₁ values down to 100 ms have been measured in these systems. These short times may make magnetic insulators viable hosts for on-line experiments. The dependence on field and temperature of the⁵⁴Mn spin-lattice relaxation time T₁ has been investigated and a T₁ minimum at high fields observed in both systems. A spin-spin relaxation time T₂≈50 μs is measured in⁵⁴Mn−MnCl₂·4H₂O. The observation of NMRON in the paramagnetic phase of MnCl₂·4H₂O allows the resonant frequencies in the antiferromagnetic and paramagnetic phases to be compared, yielding a value for the zero point spin deviation in the former phase.     

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.     

In situ 54Mn NMRON Studies of the Mixed Halide Mn(Br X Cl1−X )2 · 4H2O in Applied Magnetic Fields

⁵⁴Mn NMRON in the mixed halide antiferromagnet, Mn(Br _X Cl_1−X )₂ · 4H₂O, for varying external field, is reported. Significant qualitative differences are found between these NMRON transitions and those of the two terminal compounds, especially in respect to line widths. A tentative assignment is made to most of the observed NMRON transitions out to 0.8 T. An unidentified lower frequency inhomogeneous signal, present only at the lowest temperatures, with no counterpart in the terminal compounds, is also reported.     

NMRON studies of the antiferromagnetic and paramagnetic phases of54Mn-MnCl2 · 4H2O

Pulsed NMRON, CW NMRON and thermal NMR-NO methods have been utilized to study⁵⁴Mn-MnCl₂ · 4H₂O. The⁵⁴Mn spin-lattice relaxation timeT ₁ in zero applied field has been measured between 35 and 90 mK in the antiferromagnetic phase. Above 65 mK the dominant relaxation mechanism is a Raman process with the electronic magnons, but at lower temperatures a direct process takes over. NMRON has been observed for the first time in the paramagnetic phase, and a line width of 300 kHz, with both homogeneous and inhomogeneous contributions, is observed. In the antiferromagnetic phase the line width is 35 kHz, and there are also homogeneous and inhomogeneous contributions. The dependence ofT ₁ for the⁵⁴Mn spins on field and temperature was studied in the paramagnetic phase. AT ₁ minimum centred atB ₀=2.64 T was observed. The hyperfine parameter <⁵⁴ AS>/h=−513.6(3) MHz in the paramagnetic phase, and comparison with the value in the antiferromagnetic phase gives 0.013(1) for the zero point spin deviation.     

Impurities in Magnetic Insulators Studied by Low-Temperature Nuclear Orientation

Although the effects of impurities in metallic magnets have been extensively studied there have been far fewer experiments on impurities in insulating magnets, and, in an effort to understand this problem, we have investigated the effect of impurities in various systems. In MnCl₂⋅4H₂O samples doped with 1.6, 4.0 and 6.2% Co, the ⁵⁴Mn NMRON line is significantly broadened compared to the line in the pure crystal and shows structure on the low frequency side. The nuclear spin-lattice relaxation time is also lower by an order of magnitude. ⁵⁴Mn has also been doped into FeCl₂⋅4H₂O for which a 4-sublattice structure has been proposed and some details of this structure have been obtained. This revised version was published online in September 2006 with corrections to the Cover Date.     

NMRON studies of the quasi-2-dimensional ferromagnet 54Mn-Mn(COOCH3)2·4H2O

Pulsed and continuous wave NMRON, and NMR thermally detected by Nuclear Orientation have been used to investigate the magnetic properties of the quasi-2-dimensional ferromagnet ⁵⁴Mn-Mn(COOCH₃)·4H₂O. The NMR frequencies of both the ⁵⁴Mn spins and the abundant ⁵⁵Mn spins have been determined for the two crystalline lattice sites Mn1 and Mn2. The line widths of the ⁵⁴Mn resonances are homogeneously broadened to about 300 kHz, and the ⁵⁵Mn resonances are significantly pulled down in frequency. The spin-lattice relaxation times in low applied magnetic fields are short with T₁ for the Mn2 site being much less than the value for the Mn1 site. Level crossing is observed between the Mn1 and Mn2 resonances in an applied field of 2.6 T. This revised version was published online in August 2006 with corrections to the Cover Date.     

Irreducible tensor operators and selective/hard single and double resonance experiments in insulators: Applications to NMRON and MQ-NMR

Recently, pulsed NMRON experiments have been carried out on trace amounts of radioactive⁵⁴Mn in the antiferromagnet MnCl₂4H₂O at 500 MHz (Le Gros et al. [1]). In this compound, the quadrupole splitting between the two lowest NMR transitions is ≈3 MHz, which precludes the use of non-selective (hard) rf pulses. Yet within the restricted 2*2 manifold, associated with a given transition, the nuclear rotation is “hard”. In this paper, the theory of “selective-hard” NMRON and MQ-NMR experiments is developed within the framework of irreducible tensor operators. In essence, the theory extends the early work of Jaynes [4] to deal with the higher-order multipolar states created during the course of a given NMR experiment. Several new pulsed NMRON and MQ-NMR experiments are proposed. For example, it is demonstrated how “ouble resonance”, “selective-hard” experiments on the pseudo spin-1 manifold spanned by |±1> and |0> Zeeman states of any integer spinI could be used to extract small chemical shifts in the face of very large quadrupole splittings.     

High precision54Mn NMRON in antiferromagnetic MnCl2.4H2O,using a new method of nuclear spin cooling in the mK region

A practical method of nuclear spin cooling in the mK region, based on dramatic reduction in the spin-lattice relaxation time T₁ at the antiferromagnetic-spin flop and spin flopparamagnetic phase transitions, is described. Far lower temperatures (down to 13 mK) than have been obtained to date (～ 90 mK) are achieved in reasonable times, greatly increasing the sensitivity of NMRON. Following resonance, active isotope spins can be rapidly reset (in times ～ 1 second) to thermal equilibrium. For⁵⁴Mn in MnCl₂.4H₂O, hyperfine parameters A=?203.75 MHz, P=0.155 MHz and the exchange/ crystal field parameter combination B_ex + 4¦D¦/gμ_B=1.355 T have been determined from good agreement between perturbation theory to second order and detailed NMRON results in applied fields parallel to the easy axis in the range 0-0.7 T. Splitting of observed lines in zero applied field (? 3.05 MHz) is mainly due to the pseudo-quadrupole (second order magnetic) interaction. Small third order effects (～0.05 MHz) have been resolved.     

Effective orientation of water in 1,4-dioxane···water: the rotational spectrum of the H2 17O isotopologue

The measurements of the rotational spectrum of 1,4-dioxane-water complex, performed by pulsed jet Fourier transform microwave spectroscopy, have been extended to the 7–18.5 GHz frequency region and to two additional isotopologues, 1,4-dioxane···H₂ ¹⁷O and 1,4-dioxane···HOD. The effective orientation of water in the complex has been obtained from the ¹⁷O quadrupole coupling constants.     

The effect of applied fields on the NMRON of54Mn-MnCl2·4H2O

The effect on the most intense M=3→2 NMRON line of⁵⁴MnMnCl₂. 4H₂O resulting from a magnetic field B₀ applied along the crystalline c- and b-axes is observed. Possible mechanisms for these results are discussed.     

Nuclear orientation and NMRON studies of tetrahydrate manganese salts

Two tetrahydrate manganese salts, MnBr₂·4H₂O and Mn(COOCH₃)₂·4H₂O have been studied by nuclear orientation. Two⁵⁴Mn NMRON lines were observed in the bromide at 499.75(4) MHz and 501.6 (1) MHz allowing hyperfine parameters to be estimated. The spin-flop transition was also observed in this salt. Although considerable -ray anisotropy was observed in the acetate no resonances have yet been observed.     

Die Eigenschwingungen des Metall-Hydratkomplexes in BeSO4·4H2O und BeSO4·4D2O

Infrared reflection spectra of single crystals of BeSO₄·4H₂O and BeSO₄·4D₂O have been obtained in polarized light at 300°K and at 14°K in the region between 4000 cm^?1 and 300 cm^?1. By a Kronig-Kramers analysis, the frequencies of the infrared active transitions have been calculated. These transitions are attributed to internal vibrations of the water molecules and sulfate ions and, in the region between 1000 cm^?1 and 300 cm^?1, especially to internal and external vibrations of the tetrahedral Be⁺⁺·4aqu-complexes. The vibrational modes of these complexes consist of a superposition of translational and librational modes of the water molecules and translational modes of the central Be⁺⁺-ion. The vibrational frequencies and normal modes of this complex have been calculated in a central-force model, and force-constants have been determined by fitting the calculated frequencies to the observed spectra. The calculations have shown that the modes, which comprise mainly translational motions of the water molecules, are strongly coupled with librational motions of the water molecules. On the other hand, there exist pure librational modes with practically no admixture of translational motions. The optimum sets of force constants for the BeSO₄·4H₂O crystal and the BeSO₄·4D₂O crystal differ in a manner which can be understood under the assumption that the dimensions of the Be(D₂O)₄ complex are about 0.1 Å larger than those of the Be(H₂O)₄ complex. Some arguments supporting this conclusion will be discussed.     

Absorptionsspektrum des Ce3+-Ions und Schwingungsspektrum im CeCl3·7H2O und CeCl3·7D2O

The absorption spectra of CeCl₃·7 H₂O, LaCl₃·7 H₂O, CeCl₃·7 D₂O and LaCl₃·7 D₂O have been measured at low temperatures in the region of the 4 f→4 f infrared transitions. The energy levels of the Ce³⁺-ion in these compounds have been derived. The infrared phonon transitions have also been determined from the reflection spectra of these crystals measured in the region between 4000 cm^?1 and 50 cm^?1. Some of the energy levels of the Ce³⁺-ions in the deuterated crystals are shifted for about 10 wavenumbers against those in CeCl₃·7 H₂O. These shifts are probably due to an interaction of the electronic levels with resonant phonon-states in one or the other of the two compounds.     

The hydrates and deuterates of ferrous sulfate (FeSO4): a Raman spectroscopic study

A comparative analysis has been carried out on the Raman spectra of FeSO₄·nH₂O (n = 1, 4, 7) including the ²D‐analogs. The effects of changing the degrees of hydration have been found from the lattice, SO₄²⁻ internal, and H₂O internal modes. Increasing degrees of hydration shift the intense ν₁(SO₄) peak to lower wavenumbers and reduce the amount of splitting on the ν₃(SO₄) peaks. Some of the water librational bands cause the broadening of the ν₄(SO₄) peaks in FeSO₄·7H₂O and the ν₂(SO₄) peaks in FeSO₄·7D₂O. The ν₂(H₂O) band in FeSO₄·H₂O is red‐shifted in excess of 100 cm⁻¹ relative to the unperturbed H₂O band. Between 240 and 190 K and between 140 and 90 K in the spectra of FeSO₄.4H₂O, two potential phase transitions have been identified from the changes in the lattice and water‐stretching regions. The resolution of the ν₁(H₂O) and ν₃(H₂O) bands in FeSO₄·4H₂O and FeSO₄·H₂O also improved sharply at low temperatures. The capability of distinguishing various forms of FeSO₄ hydrates unambiguously makes the Raman technique a potential analytical tool for the identification of sulfate minerals on planetary surfaces. Copyright © 2006 John Wiley & Sons, Ltd.     

Pulsed nuclear magnetic resonance of oriented nuclei in an insulating magnetic material:54Mn−MnCl2.4H2O

The first observation of pulsed nuclear magnetic resonance of oriented nuclei in an insulating magnetic material is reported. The system studied was the ordered antiferromagnet⁵⁴Mn−MnCl₂.4H₂O. The response of the⁵⁴Mn spins to a single pulse of variable length, the free induction decay, and Hahn spin-echo have all been observed. A discussion of the transverse decoherent relaxation timeT ₂ in MnCl₂.4H₂O is given.     

Raman spectroscopic study of the hydrogen and arsenate bonding environment in isostructural synthetic arsenates of the variscite group—M3+ AsO4·2H2O (M3+ = Fe,Al, In and Ga): implications for arsenic release in water

The Raman spectra of synthetic compounds equivalent to the variscite group: FeAsO₄·2H₂O AlAsO₄·2H₂O, GaAsO₄·2H₂O, and InAsO₄·2H₂O are reported. In particular, upon comparison of FeAsO₄·2H₂O to AlAsO₄·2H₂O, it is observed that the Type II (weak) H‐bond lengths in the latter are slightly longer, which is postulated to affect the stability (As release) in water at pH 5 and 7. Arsenate stretching and bending vibrations were found to be distinct in terms of spectral structure and therefore well suited for fingerprinting. The calculated As O bond strengths from existing crystallographic data showed no significant variations. The strongest ν₁ (AsO₄³⁻) stretch was used to monitor the As O bonding interactions in the four As O M units, where a shift of 114 cm⁻¹ was observed in the order FeAsO₄·2H₂O (lowest) < InAsO₄·2H₂O < GaAsO₄·2H₂O < AlAsO₄·2H₂O (highest); this order also followed exactly the measured arsenic release of these phases. This shift in ν₁ (AsO₄³⁻) position was rationalized to stem from the differences in the electronegativities of the M³⁺ cations. The trends mentioned above were verified and found to also hold for the isostructural phosphate analogues strengite (FePO₄·2H₂O) and variscite (AlPO₄·2H₂O) using published data. Therefore, it is postulated that, as observed with the stability of solution complexes, there may be a correlation between the electronegativity of the M³⁺ cation in these isostructural phases and their stability (As or P release) in water. Copyright © 2010 John Wiley & Sons, Ltd.     

Vibrational spectra of three Zirconium phosphates

The analysis of the IR and Raman spectra of the ion exchanger α-Zr(HPO₄)₂ · H₂O and its half and fully exchanged phases, ZrKH(PO₄)₂ and Zr(NH₄PO₄)₂ · H₂O, have been reported. The splitting of the degenerate and nondegenerate modes indicates the presence of two crystallographically distinct anions in these compounds. The HPO^2-₄ ion has stronger P-O bond in ZrKH(PO₄)₂. One of the three crystallographically distinct water molecules in α-Zr(HPO₄)₂ · H₂O can be considered as a free water molecule. The water molecules have significantly different hydrogen bond strengths. In Zr(NH₄PO₄)₂ · H₂O bands indicate the presence of two distinct NH⁺₄ ions and water molecules. The PO^3-₄ ion in ZrKH(PO₄)₂ is linearly distorted.     

Electron paramagnetic resonance of vanadyl ion impurities in crystalline solids

The electron paramagnetic resonance of VO²⁺ ion impurities has been studied in certain crystalline solids at ~ 9.45 GHz. VO²⁺ shows an isotropic spectra in Mg(ClO₄)₂·6H₂O, RbBr, RbI, CsCl, thiourea, NH₄HC₂O₄·$\text{1}\text{2}$H₂O and urea oxalate at room temperature, and has preferential orientation in MgSeO₄·6H₂O, KHC₂O₄, Rb₂SO₄ and (NH₄)₂M″(SeO₄)₂·6H₂O (M″ = Zn, Co) single crystals. The line broadening of the EPR spectra of VO²⁺ in (NH₄)₂Co(SeO₄)₂·6H₂O observed on cooling the crystal is explained on the basis of host spin lattice relaxation narrowing. The EPR spectra have been analysed and the spin-Hamiltonian parameters evaluated.     

The Studies of Enhanced Fluorescence in the Two Novel Ternary Rare-Earth Complex Systems

Two novel ternary rare-earth complexes SmL₅·L^’·(ClO₄)₂·7H₂O and EuL₅·L^’·(ClO₄)₂·6H₂O (the first ligand L = C₆H₅COCH₂SOCH₂COC₆H₅, the second ligand L^’ = C₆H₄OHCOO⁻) were synthesized and characterized by element analysis, molar conductivity, coordination titration analysis, IR, TG-DSC, ¹HNMR and UV spectra. The detailed luminescence studies on the rare-earth complexes showed that the ternary rare-earth complexes presented stronger fluorescence intensities, longer lifetimes, and higher fluorescence quantum efficiencies than the binary rare-earth materials. After the introduction of the second ligand salicylic acid group, the relative emission intensities and fluorescence lifetimes of the ternary complexes LnL₅·L^’·(ClO₄)₂·nH₂O (Ln = Sm, Eu; n = 7, 6) enhanced more obviously than the binary complexes LnL₅·(ClO₄)₃·2H₂O. This indicated that the presence of both organic ligand bis(benzoylmethyl) sulfoxide and the second ligand salicylic acid could sensitize fluorescence intensities of rare-earth ions, and the introduction of salicylic acid group was a benefit for the fluorescence properties of the ternary rare-earth complexes. The fluorescence spectra, fluorescence lifetime and phosphorescence spectra were also discussed.