Second-order quadrupole splittings of the95Mo NMR signal in Mo(CO)6 and their reduction by sample spinning

The lifetimes of higher lying¹S_0?,¹D_2?, and¹F ₃ ⁰ -levels and of the 5s 7s³S₁- and 5p^{2 3}P₁-state in the Sr I-spectrum have been measured by a time resolved registration of the fluorescent intensity. The¹S_0? and¹D₂-levels were populated selectively by a two step optical excitation using two pulsed dye lasers. In the case of the¹F ₃ ⁰ -levels and of the³S_1?,³P₁-states the first excitation step was performed by a discharge, for the second, selective step a pulsed dye laser was used. The agreement of the experimental results with theoretically obtained lifetimes is satisfactory only in some cases.     

NMR in 55Mn2+ nuclei in the quasi-one-dimensional antiferromagnetic CsMnBr3

The NMR spectrum of the quasi-one-dimensional easy-plane antiferromagnetic CsMnBr₃, which has trigonal spin lattice, is investigated in detail. The measurements were performed on a wide-band NMR decimeter microwave-band spectrometer over a wide range of magnetic fields at temperatures 1.3–4.2 K. All three branches of the NMR spectrum previously found by us [JETP Lett. 64, 225 (1996)] are severely distorted because of the dynamic interaction with the Goldstone mode in the antiferromagnetic resonance spectrum. The experimental results in fields up to 40 kOe are described satisfactorily by an equation obtained by Zaliznyak et al. [JETP Lett. 64, 473 (1996)]. Formulas are obtained in our work that agree very well with experiment at all fields up to the “collapse” field H _c of all sublattices. The unbiased NMR frequency in CsMnBr₃ is determined to be v _n0=416 MHz (T=1.3 K) in zero external magnetic field, and in this way the reduction in the spontaneous moment due to the quasi-one-dimensional nature of the system of Mn²⁺ spins, which according to our data amounts to 28%, is determined more accurately. The field dependences of the directions of the magnetic sublattices with respect to the magnetic field are obtained from the NMR spectra, confirming the equations of Chubukov [J. Phys. Condens. Matter 21, 441 (1988)]. The results on the field dependence of the width and intensities of the NMR lines are discussed, along with three observed anomalies: 1) a strong increase in the NMR frequency for nuclei in sublattices that are perpendicular to the magnetic field; 2) the nonmonotonic temperature dependence of the resonance field for the lower branch of the spectrum; 3) the presence of two branches of the NMR spectrum in large H _c fields, in which the CsMnBr₃ must be a quasi-one-dimensional antiferromagnetic. Zh. éksp. Teor. Fiz. 113, 352–368 (January 1998) Deceased.     

Observation of magnetic Mn sites in cubic and hexagonal HoMn2: 55Mn NMR study

⁵⁵Mn nuclear magnetic resonance has been measured for both cubic C15 and hexagonal C14 HoMn₂. In the ordered state, we found a high-frequency signal, which can be assigned to magnetic Mn atoms, for both C15 and C14 phases together with a low-frequency signal from non-magnetic Mn atoms. The results of the spin-spin relaxation time T₂ in the ordered state and the NMR spectra in the paramagnetic state are also given to discuss the magnetic instability and the magnetic structure.     

The influence of substitution of Mn ions on quadrupole splitting of EuFeO3 perovskite oxide

Samples of EuFe_1-xMn_xO₃( =0.0–1.0) were prepared. XRD measurements show that a Jahn–Teller distortion occurs at =0.6. As increases, the Mössbauer spectra of the samples change from a simple six line spectrum for =0.0 to a doublet for 0.5 and QS increase from -0.02 mm/s to 1.05 mm/s for =0.8, which is caused by the decrease of the Curie temperature and by the Jahn–Teller distortion, respectively.     

NMR on 55Mn in the ferromagnetic Ni-substituted manganite La0.6Pb0.4Mn0.86Ni0.14O3

NMR on ⁵⁵Mn in the single-crystal manganite La_0.6Pb_0.4Mn_0.86Ni_0.14O₃ (T _C =242 K), which exhibits metallic conductivity below T _C , is investigated in the temperature range 61–215 K. At low temperatures, together with a line corresponding to the averaged hyperfine field at the ⁵⁵Mn nuclei (the averaging is due to the motion of electronic holes along Mn sites), the NMR spectrum also contains two lines corresponding to localized states Mn⁴⁺ and Mn³⁺. In the temperature range 100–200 K it is found that the complicated NMR spectrum is transformed into a single line on account of a delocalization of the holes in the e _g orbitals of manganese. A comparison of the NMR data with the temperature dependence of the resistivity suggsets that a wide distribution of charge-carrier mobilities exists in the crystal. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 8, 522–527 (25 October 1999)     

Investigation of the dynamic susceptibility of antiferromagnetic MnCO3 near the NMR of Mn55

The real part χ″ of the dynamic magnetic susceptibility of antiferromagnetic MnCO₃ is measured near the NMR of Mn⁵⁵. Two new phenomena are discovered: χ″ vanishes at the unshifted NMR frequency in the region of large dynamic frequency shift (DFS) of NMR, and the NMR line is broadened as the shift varies from large to small values. The results obtained are described satisfactorily by the concept of the DFS of NMR developed in the works of E. A. Turov, M. I. Kurkin, and V. I. Tsifrinovich. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 9, 636–640 (10 November 1996)     

Magnetic-field-induced quadrupole splitting in gaseous and liquid 131Xe NMR: quadratic and quartic field dependence

A theory for the magnetic-field-dependent quadrupole splitting in the 131Xe NMR spectra in isotropic media is presented and tested by ab initio electronic structure calculations. Evidence exists only for even-power magnetic field dependence. The dominant mechanism is verified to be the electric field gradient caused by the diamagnetic distortion of the atomic electron cloud, quadratic in the magnetic field. The computed results are in excellent agreement with the recent experiment by Meersmann and Haake.     

Mn55 NMR investigation of the suppression of quantum fluctuations in quasi-one-dimensional antiferromagnets by a magnetic field

The NMR of Mn⁵⁵ nuclei is measured in the quasi-one-dimensional antiferromagnets CsMnBr₃, RbMnBr₃, and CsMnI₃ in magnetic fields upto 8 T attemperatures in the range 1.3–4.2 K. The average moments of the magnetic sublattices and their field dependences, which turned out to be comparatively strong and different for magnetically non-equivalent Mn²⁺ ions, are determined from the hyperfine-field data obtained. As a result, the magnetizations of separate sublattices in an external magnetic field ∼8 T differ by more than 5%. The results obtained agree qualitatively with the theory of the suppression of quantum fluctuations by a magnetic field. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 11, 724–729 (10 December 1997) Deceased.     

Nuclear quadrupole effects in orientation-modulated NMR

A quantum-statistical method is used to study the structure of the NMR spectrum under adiabatic orientation-modulated (OM) magnetically anisotropic Hamiltonian of a nuclear system by rotational oscillation of a crystal about a fixed axis. A general expression is obtained for an NMR absorption signal, in phase with the OM, which reflects the main anomalies of such a spectrum. The results are used to interpret the OM NMR spectrum in spin systems with weak axisymmetric nuclear quadrupole interactions.Translated from Izvestiya Vysshykh Uchebnykh Zavedenii, Fizika, No. 4, pp. 48–52, April, 1990.     

Determination of the quadrupole splitting distributions of the A- and B-site ferrous ions in Fe2TiO4

Fe₂TiO₄ has been investigated using⁵⁷Fe Mössbauer spectroscopy with the aim of determining the shape and the temperature dependence of the separate A- and B-site quadrupole splitting distributions. This has been impossible hereto because both contributions are strongly overlapping and the normal doublet spectra do not contain enough information to differentiate between both. It is found that, through th use of external magnetic fields, the necessary information to perform the separation is unveiled. Furthermore, the change in shape as a function of temperature of the distributions thus obtained is well understood from the known behaviour of Fe²⁺.     

Point charge model analysis of the quadrupole splitting in normal spinels

The point charge model analysis was used to gain a clear understanding of the primary contributions to the quadrupole splitting (Δ) in the normal spinels ZnFe₂O₄ and CdFe₂O₄. The large difference in Δ between zinc and cadmium ferrite is not able to be explained using a monopole/dipole model based solely on their varying lattice constants and oxygen parameters. It is suggested that a modification of the effective charges within each crystal and/or a subtle twisting of the oxygen sub-lattice is required to accurately predict Δ in these spinels.     

A direct determination of the quadrupole splitting in ferromagnetic amorphous metals

We introduce a new method for direct determination of the quadrupole splitting of amorphous alloys in the ferromagnetic state. The radio frequency induced collapse of the magnetic hyperfine structure leaves pure quadrapole split Mössbauer spectra. This allows the direct evaluation of the distribution of quadrupole splittings and isomer shift.     

A Mössbauer investigation of the quadrupole splitting in β-FeOOH

High resolution ⁵⁷Fe Mössbauer effect measurements at room temperature show that the spectra of pure, synthetic β-FeOOH in its paramagnetic state consist of a superposition of two discrete doublets, due to the existence of two different electric field gradients (EFG) which are proven to be related to the presence of halogen ions in this compound.     

NMR in the quadrupole regime: Measurement of quadrupole and chemical shift parameters in single-crystal paradibromobenzene

The exact theory for the frequency of transition between the two lowest levels of a spin $\text{I =}\text{3}\text{2}$ nucleus experiencing a large asymmetric electric field gradient, an applied magnetic field, and an anistropic chemical interaction was presented in an earlier paper. Using the assumption that the quadrupolar and chemical shift tensors have the same principal axis system, the Hamiltonian was solved exactly — analytically for the applied field aligned along each of the three axes of the quadrupolar principal axis system, and numerically for arbitrary orientations.This theory is reviewed here and applied to our room-temperature experiments in single-crystal paradibromobenzene. The self-consistent least-squares fit to the field-dependencies and simultaneously the angular dependence (rotational pattern) of the resonance frequency was performed using the literature value for the pure quadrupole frequency $\text{ν}{}_{\mathrm{Q}}\text{(1 +}\text{η}{}^2\text{3}\text{)}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{= 223·8}\text{MHz}$. The fit yielded values for the quadrupolar asymmetry η = 0·0461 ± 0·0004 and the chemical shift components σ_x = ?0·001 ± 0·001, σ_v = σ_z = 0·000 ± 0·001. Our value for η is in good agreement with values determined by other methods; it and our shift values are consistent with the information obtained by this method using a powdered specimen.The process of using the NMR signal itself to align the specimen yielded sufficient information for an unambiguous determination of the Euler angles of orientation of the crystal in its mounting within ± 0.6°.     

Short range order in metallic glasses from quadrupole splitting distributions

Changes are observed in the short range order of several Fe based amorphous alloy systems through the quadrupole splitting distributions. The quadrupole splittings are determined directly in the ferromagnetic state by using the radio frequency collapse of the magnetic splitting.     

55Mn NMR study of the effect of a constant magnetic field on the phase separation of the ferromagnetic phase in manganites

The ⁵⁵Mn NMR studies of nano-sized and polycrystalline La_{1 ? x} Ca _x MnO₃ samples have been performed. It has been shown that a decrease in the average size of the nanoparticles leads to the disappearance of the phase separation of the ferromagnetic phase, which is likely a general property of manganites. The studies in constant magnetic fields have demonstrated that the phase separation under consideration occurs within domains rather than as a result of the separation of the samples into domains and domains walls.     

Hyperfine splitting of muonium in SiO2 powder

The hyperfine splitting of Mu in evacuated SiO₂ powder has been measured over the temperature range 17–300 K using the two frequency method. Above 100 K it is consistent with the vacuum value (4463.3 MHz) indicating the Mu atoms are moving freely between the particle grains. At lower temperatures it drops rapidly to 4437±4 MHz at 17 K. The effect is attributed to thermal adsorption on the SiO₂ surface. Measurements were also made on an Ar coated powder between 7–62 K. The hyperfine splitting is consistent with the vacuum value over this range.