Untersuchungen an magnetischen Phasen intermetallischer Erbiumverbindungen

The Mössbauer effect of the 80.6 keVγ-transition of Er¹⁶⁶ has been employed to study the temperature dependence of the nuclear hyperfine splitting of Er¹⁶⁶. The Er isotope was introduced into the intermetallic compounds ErCo₂, ErCo_0.5Ni_0.5, ErCoNi, ErNi₂ and ErAg. Except for ErCo₂, all the compounds exhibit a behaviour typical for electronic relaxation phenomena. The system ErCo₂ shows at 35 °K a magnetic phase transition of the first order which is discussed in the framework of the theory ofBean andRodbell. Calorimetric data, low-temperature X-ray measurements and magnetization data are in agreement with the Mössbauer data. In ErAg and in ErNi₂ the Er³⁺-moment is partially quenched by the interaction with the crystalline electric field. The ErCo₂ data may be interpreted in terms of an undisturbed \({}^4I_{15/2^ - } \) multiplet.     

Optical detected ESR in the4 S 3/2 excited state of Er:YCl3

The electron spin resonance in the⁴ S _3/2 excited state of Er³⁺ in yttrium trichloride was studied by optical detection techniques. From angular dependence of the resonance field the principle value of theg-tensor in direction of the twofold crystal axis was deduced to beg∥=3.350±0.004 and the perpendicular valueg⊥ in the crystallographica-b-plane was extrapolated to beg⊥=2.857±0.004. The lifetime of the excited state is found to be temperature independent with τ _r =(1.62±0.02)·10^?3 sec and the spin lattice relaxation timeT ₁ was determined in the temperature region 1.5 to 2.1 °K by observing the recovery of the fluorescent light signal after a microwave saturation pulse was switched off.T ₁ is found to follow a direct process with \(T_1^{ - 1} = k \cdot cth\left( {\frac{{\rlap{--} h\omega }}{{2kT}}} \right)\) .     

Pion–Photon Transition Form Factor

An analysis of all available data (CELLO, CLEO, B A B AR) in the range [1÷ 40] GeV² for the pion–photon transition form factor in terms of light-cone sum rules with next-to-leading-order accuracy is discussed, including twist-four contributions and next-to-next-to-leading order and twist-six corrections—the latter two via uncertainties. The antithetic trend between the B A B AR data for the γ*γπ ⁰ and those for the γ*γ η(η′) transition is pointed out, emphasizing the underlying antagonistic mechanisms: endpoint enhancement for the first and endpoint-suppression for the second—each associated with pseudoscalar meson distribution amplitudes with distinct endpoint characteristics.     

An analysis of the binary and ternary fission of uranium induced by 12.2 GeV protons

In this work the binary and ternary fission of uranium induced by 12.2 GeV protons has been investigated with a polycarbonate detector registering the tracks of particles with mass numbersA>16. The basic characteristics of binary and ternary fission are discussed and the corresponding cross-sections are given. The values of the cross-sections for binary and ternary fission are calculated to be σ _b =(915 ± 120) mb and σ _t -(12.8 ± 2.5) mb, respectively.     

Magnetic hyperfine field at Cr site in AgCrO2 given by Perturbed angular correlations

This work presents an electric field gradient and magnetic hyperfine field study, in the AgCrO₂ multiferroic with triangular spin lattice. The temperature dependence of the electric field gradient (efg) and magnetic hyperfine field (mhf) at Cr site was studied at isolde via perturbed angular correlation measurements with the ¹¹¹In probe, at room temperature and below the Néel temperature (T?≤?21 K) down to 12 K. The results show the presence of two distinct local environments. One axial symmetric efg with a very low mhf, and a non axially symmetric efg with a much higher one. The temperature dependences of mhf magnitude and of the angle between the mhf and the principle component of the efg are investigated.     

Spin-reorientation transition at isoelectronic substitution in double-layer manganites (La1−z Prz)1.2Sr1.8Mn2O7

The transport, magnetic, and thermal properties of single crystal double-layer manganites of the Ruddlesden-Popper series (La_1?z Pr_z)_1.2Sr_1.8Mn₂O₇ (z=0.1 or 0.4) were studied. The compounds exhibit the colossal negative magnetoresistance effect in the region of a transition into a ferromagnetic state. Upon the isoelectronic substitution of Pr³⁺ for La³⁺, the Curie temperature decreases, while the easy magnetization axis rotates from the ab plane to the c axis. The observed effect is related to a change in the occupancy of $d{x^2-y^2} $ and $d{3z^2-r^2} $ orbitals as a result of stretching of the MnO₆ octahedra.     

Precision determination of high-Z K-shell fluorescence yields from195Au,207Bi,and235Np decays

A new method utilizing a cooled, high resolution, windowless Si(Li) x-ray and Auger electron detector has been developed for the accurate measurement of high-Z K-shell fluorescence yields. With this method, values of theK-fluorescence yield ω _k have been determined with high accuracy atZ=78, 82, and 92 from the radioactive decay of carrier-free¹⁹⁵Au,²⁰⁷Bi, and²³⁵Np, respectively. The values of ω _k are 0.968±0.008, 0.972±0.008, and 0.970±0.005, respectively where the error limits represent 95% confidence. In addition, relativeK-Auger electron group intensities atZ=92 were measured and found to be(K-LL)∶(K-LX)∶(K-XY)=100∶(65.2±6.0)∶ (8.1±0.8). The experimental results for ω _k are compared with the relativistic calculations of Bhalla, Ramsdale and, Rosner, and satisfactory agreement is found.     

Determination of theg j -factor in the72 P 3/2-state of133Cs by optical double resonance in a strong magnetic field

The hfs in the 7² P _3/2-state of¹³³Cs has been investigated by optical double resonance in a strong magnetic field. From the positions of the magnetic dipole transitions Δm _j =± 1, Δm _i =0 the magnetic hfs coupling con slanta (7² P _3/2)=16.591(25) MHz and theg _j -factorg _j (7² P _3/2)=1.33410(15) could be derived. Contrarily to recent measurements,g _j agrees well with the value calculated from the Lande formula.     

Investigation of the two quasiparticle rotational band at 1172 keV in172Yb and determination of theg R -factor of the 3+ 3 band head

Investigations of the rotational band based on the 3⁺ 3 two quasiparticle state in¹⁷²Yb have shown that some of its properties can be well described by the collective model of Bohr and Mottelson. As a result of these investigations we have derived the ratio (g _k – g _R )/Q _o =? (0.0136 ± 0.0007). The magneticg-factor of the band head has also been determined by an integral angular correlation measurement perturbed by an external magnetic field. As a result we foundg=0.201 ± 0.030. Using this value and our result of (g _k – g _R )/Q _o we have calculated theg _R -factor of the two quasiparticle state asg _R =0.283 ± 0.018 which is out of the errors smaller than theg _R -factor of the ground state rotational band. An analysis of our angular correlation experiments gave for the mixing parameter δ of theK-forbidden 1094 keV intraband transition: δ=? (3.63 _?0.06 ^+0.14 ) in agreement with the results of other authors.     

The Kepler-Coulomb problem on SO(2, 2) hyperboloid

In this note the Kepler-Coulomb problem in hyperbolic space H ₂ ² : z ₀ ² + z ₁ ² ? z ₂ ² ? z ₃ ² = R ² is discussed.     

The determination by an independent method ofP k electron capture probabilities in133Ba and139Ce decays gamma decay of133Ba

The electron capture decays of¹³³Ba (10.4 y) and¹³⁹Ce (140 d) were investigated with high resolution Ge(Li) detectors. By x-ray — γ-ray coincidences values of the quantityP _k ω _k ? _k for various transitions in the decay of¹³³Ba and¹³⁹Ce are obtained. The factor ω _k ? _k was measured independently by means ofK-conversion electron-K x-ray coincidences and was then used to determineP _k , theK-capture probability. The independent measurement of the product ω _k ? _k , together with recent accurate values of ω _k , provides a new method for the accurate calibration of semiconductor x-ray detectors. In the decay of¹³⁹Ce, a value ofP _k =0.78±0.03 is found (where the error represents twice the standard deviation) from whichQ _ec =326 _?30 ⁺⁷⁰ keV to the¹³⁹La ground-state is found by use of theory. In¹³³Ba decay, values are found ofP _k 1=0.72±0.04 for theEC transition to the 437 keV level in¹³³Cs,P _k 2=0.80±0.07 to the 384 keV level, and from an independent measurement, the ratioP _k 1/P _k 2=0.87±0.07. From these results the ground-state value ofQ _ec =522 _?10 ⁺²⁰ keV is derived from theory for¹³³Ba decay. The gamma spectrum of¹³³Ba also was remeasured. From the present gamma intensities and previous conversion electron intensities, new values forK-shell conversion coefficients are obtained. Previously reported γ-rays at 35 and 391 keV are not confirmed.     

Gadolinium as a ferromagnetic host for IMPAC experiments

The properties of ferromagnetic Gd as a host for IMPAC measurements have been investigated. The transient and internal magnetic fields at Cd, Nd, Sm, Dy, Er, Yb and Hf nuclei recoil implanted into polarized Gd at 80 K have been studied by the IMPAC technique. All available experimental transient field data for Gd have been analysed in the framework of the Lindhard-Winther theory. Empirical values of the parametersv _p andC _ion C _atom have been deduced which give good agreement between experiments and theory. Internal magnetic fields at rare-earth nuclei in magnetized Gd at 80 K have been deduced. The results areH _h.f. (NdGd)=?1370±440 kG,H _h.f.(SmGd)=?1440±120 kG,H _h.f.(DyGd)=1410±400 kG,H _h.f.(ErGd)=2310±420 kG andH _h.f.(YbGd)=?216±32 kG. The signs of these fields are, except for Yb which is in a 2⁺ ionic state, consistent with a ferromagnetic coupling between the 4f spins of the implanted ion and the Gd host. The deduced internal field at Hf in Gd is ?440±90 kG. The observed time-dependent interactions for rare-earth nuclei in ferromagnetic Gd are consistent with the Abragam-Pound theory. For the Cd isotopes,g-factors of the first 2⁺ states were deduced from the experiments. The results areg(¹¹⁰Cd)=0.49±0.11,g(¹¹⁴Cd)=0.34±0.09 andg(¹¹⁶Cd)=0.41±0.11. The use of transient magnetic fields forg-factor measurements on high-spin rotational states is discussed.     

90° pulsed magnetization of ferrite-garnet films with easy-plane anisotropy

The process of pulsed 90° magnetization of ferrite-garnet films was studied. These films, in addition to easy-plane anisotropy, have biaxial anisotropy in the film plane with an effective field H _K2 ? 40–55 Oe. the pulsed magnetization curve contains two portions separated by a kink observed at a field pulse amplitude H _p=H _p ^* ? 16–18 Oe. An analysis of the magnetization signals showed that the restoring torque, which is mainly caused by biaxial-anisotropy forces, is overcome in fields H _p ≥ H _p ^* and that magnetization rotation occurs. In fields H _p < H _p ^* , the magnetization vector rotates at the initial stage only and the angle of rotation ?_in is less than 25°–26°. The field H _p ^* and angle ?_in are calculated. The results of the calculations are confirmed by experimental data. In fields H _p > H _p ^* , the process of magnetization is accompanied by oscillations of the magnetization vector. In contrast to free magnetization oscillations, these oscillations are nonlinear and the frequency of the first harmonic (≈5 × 10⁸ Hz) is much lower than that for free oscillations, (7–12) × 10⁸ Hz. Oscillations are excited at a pulse rise time of ≈6 ns.     

Influence of nonmagnetic impurities on the kondo effect

The influence of scattering by nonmagnetic impurities is studied in perturbation theory. While the finite lifetime of the electrons in intermediate states due to scattering by nonmagnetic impurities does not lead to a change in the logT-behaviour of the third-order self-energy, certain vertex-corrections give rise to an additional term which varies like 1/√T at low temperatures. Similar correction terms are found to occur in the higher order self-energy contributions. Although these terms diverge more strongly atT=0 than the logarithmic contributions they are quite small at finite temperatures since they depend on the lifetime τ of the electrons through a factor of (? _F τ)^?5/2 (? _F Fermi energy). The possibility of observing these interference effects experimentally is discussed.     

Magnetic properties of nanoparticles of Prussian blue-based molecular magnets M 3[Cr(CN)6]2⋅zH2O (M=Fe, Co, and Ni)

The nanoparticles of Prussian blue-based molecular magnets, M ₃[Cr(CN)₆]₂?zH₂O (where M=Fe, Co, and Ni), prepared by a slow addition (drop by drop) of chemicals using the co-precipitation method, are investigated by means of X-ray diffraction, infra red spectroscopy and dc magnetization measurement techniques. The formation of nanoparticles has been confirmed by scanning electron microscopy, whereas the characteristic peak, observed in the range of 1900–2300 cm^?1 in the infrared spectra, corresponds to the CN stretching frequency of $\mbox{Cr}^{\mathrm{+III}}$ –CN– $M^{\mathrm{+II}}$ , and confirms the formation of Prussian blue compounds. The results, derived from the Rietveld refinement of X-ray diffraction patterns, reveal that all samples are nanocrystalline in nature with a face-centered cubic crystal structure of space group Fm3m. The particle size and the lattice constants decrease with an increasing atomic number of the transition metals (M=Fe, Co and Ni). The magnetization data show a magnetically ordered state of all nanoparticle samples with a low coercivity (except for the Fe₃[Cr(CN)₆]₂?zH₂O) as well as the remanent magnetization. In addition, by varying M with Fe, Co and Ni, the magnetic ordering temperature increases from ～12 to ～28 K, whereas the maximum magnetization and the coercive field decrease from ～14 to ～4.5 μ_B/f.u. and ～554 to ～22 Oe, respectively. The observed magnetization behavior has been discussed in terms of the structural changes due to the decreasing particle size as well as the varying nature of the metal ions.     

Mössbauer study on Fe0.068Ni0.932Cl2 with competing spin anisotropies

Mössbauer measurements on Fe_0.068Ni_0.932Cl₂ were done at temperatures between 55K and 4.2K. We have found that below T_N-50K an observed spectrum is composed of two kinds of spectra and that the composition ratio changes gradually with temperature. The average angle of Fe²⁺ spins, \(\overline {\theta _{Fe} } \) , with the c-axis is smaller than \(\overline {\theta _c } \) by ?15° at each temperature below T_N, where \(\overline {\theta _c } \) is the average angle of total spins with the c-axis in this system obtained from the neutron scattering measurements. This is reasonably understood if we take into account that Fe²⁺ spins have the strong uniaxial anisotropy along the c-axis. We discuss the coexistence of the two kinds of spectra by considering the exchange energy of Fe²⁺ spins and the local magnetic anisotropy.     

Hyperfine interaction of Ce and La in 3-d ferromagnets

NMR/ON measurements on¹⁴¹CeFe show the sign of the hyperfine field of CeFe to be negative. For the¹⁴¹Ce nucleus a g-factor of ¦g_N¦=0.311±0.011 is found. With this g-value a hyperfine field of H_hf=?41±2 T for CeFe is derived. Low temperature nuclear orientation experiments on¹⁴¹CeCo and¹⁴⁰LaFe yield ¦H_hf¦=30±3 T and ¦H_hf¦=46±5 T respectively. The valence of cerium impurities in Fe, Co and Ni is discussed.     

g-factors of rotational states in 176Hf, 177Hf,and 180Hf

g-factors of rotational states in ¹⁷⁶Hf and ¹⁸⁰Hf were measured with the twelve detector IPAC-apparatus of our laboratory [1]. The natural radioactivity 3.78 · 10¹⁰y ¹⁷⁶Lu and the 5.5 h isomer ^180mHf were used which populate the ground-state rotational bands of ¹⁷⁶Hf and ¹⁸⁰Hf. The integral rotations of γ-γ directional correlations in strong external magnetic fields and in static hyperfine fields of (Lu → Hf)Fe₂ and HfFe₂ were observed. The following results were obtained: The hyperfine field in (Lu → Hf)Fe₂ was calibrated by observing the integral rotation of the 9/2^? first excited state of ¹⁷⁷Hf populated in the decay of 6.7d ¹⁷⁷Lu. The g-factor of this state was redetermined in an external magnetic field as Finally the g-factor of the 2 ₁ ⁺ state of ¹⁷⁶Hf was derived from the measured g(2 ₁ ⁺ ) of ¹⁸⁰Hf by use of the precisely known ratio g(2 ₁ ⁺ , ¹⁷⁶Hf)/g(2₁ ⁺, ¹⁸⁰Hf) [2] as      

EPR investigations of SiC and SiOC nanometric powders

EPR investigations are performed in SiC and SiOC nanometric powders annealed between 1200 and 1800°C. By using different EPR frequency bands and a suitable spectra analysis, three quite different paramagnetic defects with well defined $\tilde g_i $ (i=1, 2, 3) and hyperfine $\tilde A_i $ (i=1, 2) tensors account for the EPR signal in these materials. The defects are characterized by $\tilde g_1 $ (g ₁ ^‖ =2.0046(3), g ₁ ^? =2.0023(3)), $\tilde g_2 $ (g ₂ ^‖ =2.0037(3), g ₂ ^? =2.0028(3)) and isotropic $\tilde g_3 $ (2.0030(3)) tensors. In SiC powders, the defects assignment is discussed with respect to the different SiC forms, namely α-SiC and β-SiC polytypes as well as amorphous SiC and carbon present in minor concentration in the network. In SiOC powders, the above defects are evidenced only at high annealing temperature (T _a≥1200°C) when the oxygen contained is highly reduced.