NQR indications of unconventional magnetism in some bismuth-based diamagnets

An evidence that local ordered magnetic fields from 30 to 200 G exist in diamagnets α-Bi₂O₃, Bi₃O₄Br, Bi₄Ge₃O₁₂, Bi₂Al₄O₉ which comprise neither d- nor f-elements was earlier given by the zerofield and Zeeman-perturbed²⁰⁹Bi nuclear quadrupole resonance (NQR) spectra. With a view to find similar spectroscopic effects in other compounds, we examined the²⁰⁹Bi NQR Zeeman-perturbed spectra of the Bi₃B₅O₁₂ oriented single crystal as well as the zero-field spin echo envelopes in Bi₃B₅O₁₂ and Bi₂Ge₃O₉. Distinctive modulations were displayed by the zero-field²⁰⁹Bi spin echo envelopes in Bi₂Ge₃O₉ powder. The modeling of the spin echo envelopes within the density matrix formalism explained the observed effect by the presence of local ordered magnetic field of the order of 65 G at the Bi atoms. The zero-field modulations of the²⁰⁹Bi spin echo envelopes were also observed in Bi₃B₅O₁₂ indicating the presence of the internal source of line splitting. This finding and considerable deviation of the resonance intensity ratios from that in a pure NQR, found in the zero-field²⁰⁹Bi spectrum of the single crystal, were understood as indirect evidences that a local ordered magnetic field exists also in Bi₃B₅O₁₂. The zero-field²⁰⁹Bi spin echo envelopes assigned to the lines split by the local magnetic fields in α-Bi₂O₃ and Bi₄Ge₃O₁₂ were observed to display modulations on the appropriate curves.     

Anomalous magnetism and 209Bi nuclear spin relaxation in Bi4Ge3O12 crystals

The quadrupole ²⁰⁹Bi spin–spin and spin–lattice relaxation were studied within 4.2–300 K for pure and doped Bi₄Ge₃O₁₂ single crystals which exhibit, as was previously found, anomalous magnetic properties. The results revealed an unexpectedly strong influence of minor amounts of paramagnetic dopants (0.015–0.5 mol.%) on the relaxation processes. Various mechanisms (quadrupole, crystal electric field, electron spin fluctuations) govern the spin–lattice relaxation time T ₁ in pure and doped samples. Unlike T ₁, the spin–spin relaxation time T ₂ for pure and Nd-doped samples was weakly dependent on temperature within 4.2–300 K. Doping Bi₄Ge₃O₁₂ with paramagnetic atoms strongly elongated T ₂. The elongation, although not so strong, was also observed for pure and doped crystals under the influence of weak (～30 Oe) external magnetic fields. To confirm the conclusion about strong influence of crystal field effects on the temperature dependence of T ₁ in the temperature range 4.2–77 K, the magnetization vs. temperature and magnetic field was measured for Nd- and Gd-doped Bi₄Ge₃O₁₂ crystals using a SQUID magnetometer. The temperature behavior of magnetic susceptibility for the Nd-doped crystal was consistent with the presence of the crystal electric field effects. For the Gd-doped crystal, the Brillouin formula perfectly fitted the curve of magnetization vs. magnetic field, which pointed to the absence of the crystal electric field contribution into the spin–lattice relaxation process in this sample.     

Local ordered magnetic fields in bismuth dielectrics. 209Bi NQR in Bi4Ge3O12 single crystal

The results of the ²⁰⁹Bi NQR experiments carried out on α-Bi₂O₃, Bi₃O₄Br, Bi₂M₄O₉ (M=Al, Ga), Bi₂Ge₃O₉, and Bi₄Ge₃O₁₂ showed that these compounds are not diamagnets in the conventional sense. The Zeeman perturbed zero-field ²⁰⁹Bi NQR spectra gave an indication that local ordered magnetic fields of the order of 30–200 G exist in these substances comprising neither transition nor rare earth elements. Further aspects of this principally new phenomenon of yet unknown nature were studied by the ²⁰⁹Bi NQR experiments on the Bi₄Ge₃O₁₂ single crystal in external magnetic fields below 500 Oe, applied at various orientations with respect to the crystal axes. The spectral patterns of dramatically increased intensity and multiplicity, caused by the unknown magnetism of the compound, were observed in applied fields and modeled. Based on the results of the modeling, the conclusions were made that at least four magnetically non-equivalent Bi sites characterized by antiferromagnetically ordered local fields of the order of 30 G are present in the Bi₄Ge₃O₁₂ crystal; the intensity increase was interpreted to arise from the change in orientation of the electric field gradient axes at the Bi site upon applying an external magnetic field.     

<Superscript>209</Superscript>Bi NMR spectrum of BiFeO<Subscript>3</Subscript> in the presence of spatial modulation of hyperfine fields

A complicated spin-echo spectrum was observed for ²⁰⁹Bi nuclei in a ferroelectric antiferromagnet BiFeO₃ in zero external field. This spectrum is the first example of nuclear quadrupole resonance of a system of diamagnetic atoms in a hyperfine magnetic field produced by the spatially modulated Fe³⁺ spin system and varying in orientation and magnitude. An attempt is undertaken to theoretically simulate the spectrum.     

Local Magnetic Fields in Some Bismuth-Based Diamagnets

Evidence that local ordered magnetic fields from 30 to 250 G exist in bismuth-based diamagnetic compounds comprising neither d- nor f-elements was given by ²⁰⁹Bi NQR spectroscopy and supported by SQUID measurements of α-Bi₂O₃. The NQR experiments involved a study of the zero-field line shapes, analysis of the Zeeman-perturbed patterns, and examination of the zero-field spin-echo envelopes in single crystals and powders. The results of the experiments followed by computer modeling of the observed spectra were interpreted assuming that ordered magnetic fields are located at the bismuth sites in α-Bi₂O₃, Bi₃O₄Br, Bi₂Al₄O₉, Bi₄Ge₃O₁₂, Bi₂Ge₃O₉ and perhaps in Bi₃B₅O₁₂. A survey of the related ²⁰⁹Bi NQR data is here presented.     

Mössbauer study of cubic pyrochlores,Bi2−xYxFeSbO7

The solid solutions Bi_2?xY_xFeSbO₇ (x = 0,1,2) which are cubic pyrochlores of the type A₂B₂O₇ have been prepared for the first time and the lattice parameters determined. The ⁵⁷Fe Mössbauer spectra have been recorded both in the paramagnetic and magnetically ordered states. From the values of the Mössbauer parameters it is found that Fe ions are in the high spin trivalent state. The large value for the quadrupole splitting of these compounds is attributed to the highly distorted BO₆ octahedron in these compounds. The quadrupole splitting decreases with increase of Bi concentration and therefore, the BO₆ octahedron is found to have the lowest distortion for the composition Bi₂FeSbO₇. The magnetic transition temperature and the value of the magnetic hyperfine fields have been determined. The distribution of Fe³⁺ and Sb⁵⁺ ions in B sites gives rise to the distribution in magnetic hyperfine fields.     

Hyperfine field and electric quadrupole interaction of51V in YFe2 and HoFe2

Spin echo measurements were made on Y(Fe_0.99V_0.01)₂, (Y_0.97Ho_0.03) (Fe_0.99V_0.01)₂ and Ho(Fe_0.96 V_0.04)₂ compounds. The addition of Ho to YFe₂ causes a spin reorientation from <111> to <100> but this is not reflected in the⁸⁹Y or⁵¹V hyperfine fields. A well resolved⁵¹V NMR line was observed at 51.5 MHz and 60.0 MHz respectively for Y and Ho Laves compounds but with no evidence of quadrupole satellites. Small quadrupole splittings ¦2P_t¦ = 171 ±5 kHz and 178 ±5 kHz respectively for Y and Ho compounds were deduced from the oscillatory decay of the⁵¹V spin echo after a two pulse sequence.On leave from Department of Physics, University College, The University of New South Wales, Australian Defence Force Academy, Campbell, ACT, Australia.     

NMR spin echoes and quadrupole effects in nickel single crystals

NMR spin echo measurements of naturally abundant⁶¹Ni have been performed with Néel-type nickel single crystals at 4.2 K. The NMR excitation condition could properly be chosen in order to get signals either from nuclei situated within magnetic domains or within domain walls. In both cases a quadrupole splitting of the NMR line could be observed. By applying an external magnetic field, the direction of the domain magnetization could be varied with respect to the fcc crystal lattice. From the variation of the quadrupole splitting with the crystallographic direction, the tensor of the electric field gradient (EFG) has been derived. The quadrupole splittings and the corresponding field gradients in the principal axis system of the EFG are: Δv_Q kHz, Δv_Q kHz, Δv_Q kHz, V_<111>=(6.6±0.5)·10¹⁸ V/m², V_<110>=4.6±0.5)·10¹⁸ V/m², V_<112>=(2.0±0.5)·10¹⁸ V/m².     

Rescaling of 2D ESEEM Data as a Tool for Inverse Problem Solving

A rescaling procedure is proposed for electron spin echo envelope modulation spectra observed at several electron paramagnetic resonance transitions. Analytical expressions describing the relations between the rescaled frequencies and hyperfine and quadrupole parameters of the remote nucleus are obtained. The dependences of the rescaled data on the external magnetic field and spin projections of the ion nucleus and the remote nuclei are used to derive the parameters of the nuclear state in the crystals Y₂SiO₅ and YVO₄ doped by ion Nd³⁺.     

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)     

High spin states and neutron multiplicities after pion capture in181Ta and209Bi

The absorption of stoppedπ ^? in¹⁸¹Ta and²⁰⁹Bi has been investigated by studying prompt and delayedγ-ray spectra. Absolute cross-sections for the yield of isotopes per capturedπ ^? in (π ^?, xn) reactions, as well as the relative probability of populating nuclear states of different spins have been measured for the hafnium and lead isotopes, respectively. A spin as high as 20 has been observed in the production of²⁰⁴Pb. The ground-state rotational bands of the hafnium isotopes are excited to spin values up to 16. Neutron multiplicities as large as 15 have been observed for both targets. A neutron multiplicity of ?8 is most probable for both tantalum and bismuth targets. The strong interaction monopole energy shift? ₀ and widthΓ ⁰ for the 4f level are found to beε ₀(¹⁸¹Ta)=540±100eV; ?₀(²⁰⁹Bi)=1790±150 eV;Γ ₀(¹⁸¹Ta)=225±57 eV;Γ ₀(²⁰⁹Bi) =1166±70 eV. The quadrupole moments, determined from the hyperfine splitting of the 4f pionic atom level, areQ=3.30±0.06b andQ=}-0.50±0.08b for¹⁸¹Ta and²⁰⁹Bi, respectively.     

Search for magnetic interaction in In doped AlN using perturbed angular correlation spectroscopy

The possible presence of a large magnetic field due to spin polarization of a Cd nucleus (decay product of ¹¹¹In) at an Al substitutional site in AlN is investigated with perturbed angular correlation (PAC) spectroscopy. The PAC spectra of ¹¹¹In/¹¹¹Cd in AlN show two probe environments: a weak quadrupole interaction (quadrupole interaction constant, $\nu _{\rm Q}^{\,\,\,\rm lattice} = 30$  MHz) due to ¹¹¹In probes at a defect free Al substitutional site and an unknown large interaction ( $\nu _{\rm Q}^{\,\,\,\rm complex} = 300$  MHz) tentatively attributed to a nearest neighbour pair between ¹¹¹In and a nitrogen vacancy (V_N) aligned along the c-axis. Surprisingly, in density functional theory (DFT) calculations, such a large electric field gradient (EFG) could not be reproduced. However, an inclusion of spin polarization in the calculations indicates a strong magnetic field at ~50 % of the ¹¹¹In/¹¹¹Cd site. An attempt to verify the presence of the strong magnetic field and to explain the origin of the strong interaction is made. Orientation measurements show, the large interaction is not characterised by a magnetic interaction and is predominantly due to the EFG. However, in the presence of an external magnetic field, the strong interaction probe environment becomes more uniform and the EFG increases by ~10 %. This definitely hints towards some sort of magnetic interaction at the strong interaction probe site.     

Zum selektiven NMR-Spinecho-Verhalten bei Systemen mit I = 1 im Festkörper am Beispiel des Kaliumoxalatmonohydrats

Investigation of the Selective N.M.R. Spin Echo Response of I = 1 Systems in Solids Demonstrated on Potassium Oxalate Monohydrate The theory of the nonselective spin echo response in solids for dipolar-coupled spins I = 1 with quadrupole interaction by BODEN et al. [5] is modified for the case of selective irradiation to only one line of the n.m.r. spectrum. The theoretical results are compared with ¹H-n.m.r. experiments on dipolar-coupled protonpairs of a (KCOO)₂ · H₂O single crystal. This is possible because in this material the interpair dipolar interaction is substantially smaller than the intrapair dipolar interaction. An echo response results which has some similarities to the behaviour of spin systems with heteronuclear coupling.     

Mössbauer Effect Study of Eu0.88Fe4Sb12 Skutterudite

The partly filled skutterudites Eu_0.88Fe₄Sb₁₂ were investigated by ⁵⁷Fe Mössbauer spectroscopy in the temperature range 4.2 K ≤ T ≤ 295 K and in external fields up to 13.5 T. The results favour a statistical distribution of Eu and voids in the Fe near neighbour shell. Below 82 ± 1 K magnetic order is present. Debye temperatures Θ _D = 460 ± 20 K and 165 ± 30 K were obtained for Fe with completely occupied Eu sites and for Fe with vacancies in the R-neighbour shell, respectively. The temperature dependence of the quadrupole splitting reflects the thermal expansion of the lattice. The induced hyperfine fields at 4.2 K are negative and differ by roughly a factor of two for the two Fe surroundings.     

Motions of the flux lines in high-T c superconductors from NMR linewidth,relaxation and spin echo decay

The motion of the flux lines (FL) in high temperature superconductors and their relationship with the NMR quantities are reviewed and discussed in the light of recent⁸⁹Y NMR experiments in YBCO-type compounds. In particular measurements involving the⁸⁹Y spin echo attenuation induced both by the thermal excitation of the FL’s and by motions driven by DC current and pulsed magnetic fields are presented, with preliminary results and lines of interpretation. Flux line motion as observed with¹⁹⁹Hg NMR in HgBa₂CuO_4+δ high temperature superconductor is discussed.     

<Superscript>53</Superscript>Cr NMR study of CuCrO<Subscript>2</Subscript> multiferroic

The magnetically ordered phase of the CuCrO₂ single crystal has been studied by the nuclear magnetic resonance (NMR) method on ⁵³Cr nuclei in the absence of an external magnetic field. The ⁵³Cr NMR spectrum is observed in the frequency range ν_res = 61–66 MHz. The shape of the spectrum depends on the delay tdel between pulses in the pulse sequence τ_π/2–t _del–τ_π–t _del–echo. The spin–spin and spin–lattice relaxation times have been measured. Components of the electric field gradient, hyperfine fields, and the magnetic moment on chromium atoms have been estimated.     

The 59Co nuclear magnetic resonances in the intermetallic compounds

The anisotropic hyperfine fields of the intermetallic compounds have been studied by the spin echo method and were obtained from the NMR spectra of ⁵⁹Co nuclei in ThCo₅ and YCo₅ which have the high anisotropy fields resulting from the orbital moments. The intermetallic compounds, ThCo_5.3 and YCo_5.3, have been also investigated.     

Spin re-orientation in FeCr2S4

The spinel FeCr₂S₄has been studied intensely for its peculiar magnetic and local structural changes which are sensitively influenced by the Jahn?CTeller properties of Fe^2?+?in tetrahedral sulfur coordination. Recent muon spin rotation data give strong evidence that the commonly assumed collinear magnetic structure of this compound is only found between the Curie temperature T_C = 165 K and 50 K. For lower temperatures a helical structure has been proposed. We present new Mössbauer spectroscopic data taken on the same sample as used for muon spin rotation. Also the hyperfine spectra revealing non-equivalent iron sites support the appearance of a spin re-orientation around 50 K which may be related to the onset of short-range orbital order. Below 20 K severe dynamic broadenings are found which may indicate orbital fluctuations. Orbital order occurs around 11 K accompanied by severe changes in the crystalline electric field ground state as traced from quadrupole interaction.     

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.     

Directional behavior of nuclear quadrupole resonance in YBa2Cu3O6

Powder samples of YBa₂Cu₃O₆ were magnetically aligned and the anisotropies in the systems were studied by means of Cu(1) nuclear quadrupole resonance (NQR) in the absence of external magnetic fields. Our room temperature measurements of the NQR lineshapes and the spin–lattice and spin–spin relaxation times as a function of the aligning magnetic field indicate that full microscopic alignment can be achieved by using a magnetic field of about 4.7 T, for which doublet line patterns arising from a hyperfine splitting were observed.