V. G. Orlov

Magnetic properties of amorphous PrNi5

Abstract The drastic changes in magnetic properties of amorphous PrNi5 as compared with the paramagnetic crystalline analogue have been observed. Nonlinear magnetization curves and change of the sign of curvature in the Belov-Arrott plots give evidence of a ferromagnetic transition in the amorphous PrNi5. The experimental results are discussed in the framework of the phenomenological model of the crystalline field effects in amorphous PrNi5.

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.

Study of local magnetic fields in the oxide α-Bi2O3 byNQR andμSR techniques

NQR andμSR investigations of the local magnetic field inα-Bi2O3 were performed. In theNQR experiments onα-Bi2O3 which is usually considered as diamagnetic, the splitting of the spectral lines revealed a local field on the bismuth nuclei. The internal magnetic field obtained byμSR significantly exceeds the dipole field from Bi nuclear magnetic moments. A possible source of the local magnetic fields is partial covalent bonds inα-Bi2O3.

Inelastic Neutron Magnetic Scattering from Amorphous and Polycrystalline PrNi5

The temperature dependence of inelastic neutron scattering spectra from amorphous Pr16.7Nia.3 and polycrystalline PrNi5 was studied using the thermal neutron time- of-flight spectrometer IN4 at the HFR of the Institut Laue-Langevin in Grenoble. For polycrystalline PrNis there are 3 well-resolved crystal field transitions in the energy range investigated (1.7 to 15 meV) and two further transitions unresolved in the wings of the elastic peak. In contrast to these very pronounced peaks in the dynamic structure factor S(8, w) of the sample after crystallization, S(8, w) of as-sputtered amorphous Pr16.7Nia.3 shows no structure in the energy region investigated, but some additional intensity of magnetic origin near the elastic region appears. We analyse this intensity of the experimental spectra of the amorphous sample by an averaging procedure of the phenomenological crystalline-electric-field (CEF) Hamiltonian. The coefficients of this Hamiltonian take into account the possibility of deformations of the nearest surrounding of the Pr3+ ion in the amorphous sample. The averaging of deformations of two types was carried out: without and with distortions of the local crystalline symmetry, the latter being responsible for the absence of any CEF peaks between 3 and 15meV and the intensity increase in the vicinity of the elastic peak in the spectra of the amorphous sample.

The crystalline electric field and Nd antiferromagnetism in Nd2CuO4

Abstract The scale of the crystalline electric field effects in the Nd2CuO4 compound is discussed. The RE contribution to the magnetic susceptibility of Nd2CuO4 and spontaneous magnetic moment of the Nd subsystem have been calculated. At temperatures higher than TN for the Nd ions induced by the dipole magnetic field of Cu spins turned out to be much less as compared with the experimental one.

Quadrupole spin-echo envelopes for Bi4Ge3O12 single crystals doped with the atoms of transition and rare-earth elements

The influence of a weak (below 50 Oe) constant magnetic field on a quadrupole spin-echo envelope was studied for an undoped single crystal Bi4Ge3O12, in which local magnetic fields on the order of 20–30 G were previously found, as well as for single Bi4Ge3O12 crystals doped with the atoms of transition and rare-earth elements. In all of these cases, the spin-echo envelopes were strongly influenced. A considerable increase in the nuclear spin-spin relaxation time T2 was observed for the undoped sample upon the switching of weak external magnetic fields. For the doped samples, the spin-echo envelope decay became much slower already in the zero field. The external magnetic fields exhibited a markedly weaker influence on the spin-echo envelope for the doped samples.

209Bi NQR and Magnetic Properties of Bismuth Oxide-Based Compounds

Abstract The NQR line shapes of 209Bi in the α-Bi2O3-based mixed oxides Bi2O3 • 2M2O3 (M = Al, Ga). Bi2O3 • 3GeO2, and 2Bi2O3 • 3GeO2 are recorded in zero and weak magnetic fields (Hext < 500 Oe) and compared with the results of computer simulation. Splittings and line shape asymmetry, exhibited by the resonances, suggest that internal magnetic fields, similar to those earlier reported for α-Bi2O3 and Bi3O4Br, may exist in these compounds. In the spectra of single crystal Bi4Ge3O12, the line multiplicity in external magnetic fields is higher than simulated, which might results from domains in the crystal. In external magnetic fields a notable increase in the line intensity was observed, the effect depending on the mutual orientation of the EFG axes and the fields perturbing the nuclear spin system.

Structural Relaxation and Amorphous-To-Crystal Transition: Change of Local Atomic Topology Studied by Neutron Inelastic Magnetic Scattering

The possibility of a rearrangement of bond angles in amorphous Pr16.7Ni83.3 towards a local bond angle distribution similar to that in crystalline PrNi5 during a structural relaxation of the amorphous sample induced by different heat treatment below the crystallization temperature Tx was studied by neutron inelastic magnetic scattering using the high sensitivity of crystal field splittings to local disorder. In spite of a large range of annealing temperatures Tann up to 0.97?Tx no change of the neutron magnetic inelastic spectra could be observed at Tann?<?Tx, indicating that a substantial bond orientational order towards that of the crystallized sample is not induced during structural relaxation, but only takes place during the crystallization process itself. These results can reasonably be described by a model calculation, in which randomization of the crystalline lattice with respect to bond length and bond angles (breaking of the hexagonal crystalline symmetry) it taken into account.

Nuclear quadrupole spin-lattice relaxation in Bi4Ge3O12 single crystals doped with atoms of d or f elements. Crystal field effects in compounds exhibiting anomalous magnetic properties

The nuclear quadrupole spin-lattice relaxation was studied in the range 4.2–300 K for single crystals of Bi4Ge3O12 doped with minor amounts (the tenth fractions of mol%) of paramagnetic atoms of Cr, Nd, and Gd. Unusual spin dynamic features were recently found for these crystals at room temperature: a dramatic (up to 8-fold) increase in the effective nuclear quadrupole spin-spin relaxation time T2* occurred upon doping the pure Bi4Ge3O12 sample. Unlike T2*, the effective spin-lattice relaxation time T1* at room temperature differs insignificantly for both doped and pure samples. But at lower temperatures, the samples exhibit considerably different behavior of the spin-lattice relaxation with temperature, which is caused by different contributions to the relaxation process of the dopant paramagnetic atoms. The distinctive maximum in the temperature dependence of the spin-lattice relaxation time for the Nd-doped crystal is shown to result from the crystal electric field effects.

NEW MAGNETIC PHENOMENA IN BI-BASED DIELECTRICS

Abstract The 209 Bi resonance line shape evolution in the single crystal Bi4Ge3O12 under the action of radio-frequency, external, and local magnetic fields at their various mutual orientations was observed experimentally and modelled to determine the strength and orientation of the local field with respect to the principal components of the EFG at the Bi sites (θ i , φ i ) . The new phenomena were observed in Bi4Ge3O12 using 209 Bi NQR in weak static magnetic fields: an increase in spectroscopic multiplicity as compared to the expected Zeeman pattern in external magnetic field and significant growth in the integral resonance intensity compared to that in zero field. The former phenomenon is evidently caused by the presence of the local magnetic field at the Bi sites. The intensity growth of the resonances in external field is due to high sensitivity of the transition probabilities to the values of angular variables, which can be responsible for eight–tenfold increase in the integral resonance intensity upon switching external magnetic field.