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In the garnet structure compound Yb3Ga5O12, the Yb3+ ions (ground state effective spin S' = 1/2) are situated on two interpenetrating corner sharing triangular sublattices such that frustrated magnetic interactions are possible. Previous specific heat measurements evidenced the development of short range magnetic correlations below 0.5K and a lambda-transition at 54mK (Filippi et al. J. Phys. C: Solid State Physics 13 (1980) 1277). From 170-Yb M"ossbauer spectroscopy measurements down to 36mK, we find there is no static magnetic order at temperatures below that of the lambda-transition. Below 0.3K, the fluctuation frequency of the short range correlated Yb3+ moments progressively slows down and as the temperature tends to 0, the frequency tends to a quasi-saturated value of 3 x 10^9 s^-1. We also examined the Yb3+ paramagnetic relaxation rates up to 300K using 172-Yb perturbed angular correlation measurements: they evidence phonon driven processes., 6 pages, 5 figures

Using neutron diffraction, ${}^{170}\mathrm{Yb}$ M\"ossbauer and muon spin relaxation spectroscopies, we have examined the pyrochlore ${\mathrm{Yb}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$, where the ${\mathrm{Yb}}^{3+}{S}^{\ensuremath{'}}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1/2$ ground state has planar anisotropy. Below $\ensuremath{\sim}0.24\mathrm{K}$, the temperature of the known specific-heat $\ensuremath{\lambda}$ transition, there is no long range magnetic order. We show that the transition corresponds to a first-order change in the fluctuation rate of the ${\mathrm{Yb}}^{3+}$ spins. Above the transition temperature, the rate, in the GHz range, follows a thermal excitation law, whereas below, the rate, in the MHz range, is temperature independent, indicative of a quantum fluctuation regime.

Orthorhombic EuPdSb is known to undergo two magnetic transitions, at 12 K and at T N≃ 18 K, and in phase III (T < 12 K), single crystal magnetisation data have shown that the spin structure is collinear antiferromagnetic, with magnetic moments along the crystal a axis. From a 151Eu Mossbauer absorption study, we show that, at any temperature within phase III, all the moments have equal sizes, and that in phase II (12 K< T

URL: http://www-spht.cea.fr/articles/s01/071; International audience; In the pyrochlore-structure compounds R$_{2}$Ti$_{2}$O$_{7}$, the rare-earth (R) sublattice forms a network of corner-sharing tetrahedra such that the magnetic interactions may be geometrically frustrated. The low-temperature magnetic properties of these compounds are fashioned both by the frustration and by the intrinsic properties of the rare earth, that is, by the degeneracy and anisotropy of the rare-earth crystal-field ground state and by the nature, size and strength of the inter-ionic magnetic coupling. For Yb$_{2}$Ti$_{2}$O$_{7}$, we combine $^{170}$Yb Mössbauer spectroscopy, $^{172}$Yb perturbed angular correlation, magnetization and susceptibility measurements to establish the Yb $^{3+}$ crystal-field level scheme and to show that the crystal-field ground state is a well isolated Kramers doublet having a planar anisotropy. The main contribution to the Yb $^{3+}$ -Yb $^{3+}$ coupling is the exchange interaction which is ferromagnetic. We describe the frustration-related low temperature ( $

Using Mossbauer spectroscopy, we follow the thermal dependence of the fluctuation rate of the Cu(2) produced internal (molecular) field acting on a 170 Yb 3+ probe dilutely substituted at the Y 3+ sites. We use the fluctuations of this field to track the collective fluctuations of the nearest neighbour correlated Cu(2) moments. Results are presented for oxygen levels x ⋍ 6.0 and 6.35 and for T ⩽ K.

The authors report on {sup 170}Yb Moessbauer spectroscopy data in the cubic fcc alloy YbPtBi, in the temperature range 0.06--30 K. The zero field spectra show a two component structure above 1.5 K, which can be analyzed as due to 85% of Yb{sup 3+} ions at a site with cubic symmetry and 15% at a site with non-cubic symmetry. Below 0.3 K, the spectra are resolved and can also be interpreted in terms of two sites, but with a dominant non-cubic site ({approximately} 60%) with Yb ions bearing small magnetic moments. The spectra with a large applied magnetic field give information on the Yb{sup 3+} C.E.F. level scheme and on the exchange interaction. The relation of the Moessbauer data with the results of {micro}SR measurements is discussed.

We present some results concerning the local magnetic properties of doped superconducting cuprates obtained using Mossbauer spectroscopy. These results which follow on from those presented at the Erice Workshop [1] concern the La2−x Sr x CuO4 system studied using the 57Fe probe and the YBa2Cu3O x system studied using the 170Yb probe.

In the pyrochlore Yb(2)GaSbO(7), the Yb(3+) sublattice forms a network of corner sharing tetrahedra and the second sublattice is made up of disordered, non-magnetic Ga(3+) and Sb(5+) ions. We have examined this compound using magnetic susceptibility, (170)Yb Mössbauer spectroscopy (down to 0.03 K) and muon spin relaxation (μSR) (down to 0.02 K) measurements. We establish the size of the Yb(3+) magnetic moments and that of the Yb(3+)-Yb(3+) coupling. At low temperatures, the correlated moments fluctuate between directions that are well tilted relative to the local [111] axis. The lattice disorder does not quench the frustration induced low temperature spin fluctuations but it does remove the first order dynamic transition that is present in the crystallographically ordered counterpart Yb(2)Ti(2)O(7). Below 1.0 K, the fluctuation rate of the correlated moments decreases progressively as the temperature is reduced and the moments remain dynamic down to 0.02 K where the rate is 7 × 10(7) s(-1). Magnetic frustration is operative in Yb(2)GaSbO(7) where the Yb(3+)-Yb(3+) interaction is antiferromagnetic as it is in Yb(2)Ti(2)O(7) where the interaction is ferromagnetic.

170 Yb Mossbauer measurements in YbBa 2 Cu 3 O 6 and on Yb 3+ ions substituted into TmBa 2 Cu 3 O 6 show that the ordered Cu(2) sublattice produces a molecular field at the rare earth site. The Yb 3+ ion is not at a centre of magnetic inversion symmetry relative to the Cu(2) moments.

Mossbauer emission measurements show that the isolated 57 Fe 3+ probe pins a Cu spin-glass cluster in superconducting La 2− x Sr x CuO 4 for x x t ( x t is roughly at the centre of the superconducting x range in the phase diagram), whereas for x > x t the probe is essentially coupled with an induced localized Cu magnetic moment.

When the Zn 2+ level is increased in YBa 2 (Cu 1− x Zn x ) 3 O 7 , we observe that the thermal variation of 1/T 1 for the S ′= 1 2 ground state of Yb 3+ probes progressively crosses over from a dependence showing gap-like features ( x = 0) to one showing gapless features ( x = 0.06 and above).

Using Mossbauer spectroscopy, we have followed the internal (molecular) field produced by the short-range correlated Cu(2) moments on 170 Yb 3+ probes diluted at the Y 3+ sites in YBa 2 Cu 3 O x as a function of oxygen level and temperature. At intermediate oxygen levels two local situations coexist, attributed respectively to local magnetic (spin-glass like) and local superconducting behaviour. We monitor the oxygen dependence of the relative amounts of the two fractions.

The compound YbNiSn is studied by means of 170 Yb Mossbauer spectroscopy and single crystal magnetisation measurements. The ferromagnetic Yb 3+ magnetic moment is found to lie along the c -axis and its saturated value is 0.85 μ B . The Curie temperature is T c = 5.65 K. The results show that the exchange coupling is anisotropic and that the ferromagnetic moment is reduced by Kondo fluctuations with a Kondo temperature T 0 = 2 K.

We have measured the low-field (3–30 Oe) magnetisation of GdBa 2 Cu 3 O 7- δ single crystals from 1 K to above T c . T he Gd 3+ contribution cannot be detected when the single crystals are superconducting, while it is easily observed in a superconducting powder. In a single crystal made non-superconducting by deoxygenation, the susceptibility variation is characteristic of Gd 3+ antiferromagnetic ordering along the c -axis. The reduction of the Gd 3+ magnetic response in the superconducting state is found to be related to the amplitude of the shielding effect, even in field-cooled measurements where large flux trapping occurs.

169Tm Mossbauer absorption measurements have been made on the system TmxSe in order to investigate the variation of the local properties with stoichiometry. Five samples spanning the whole range of composition where the overall symmetry remains cubic were examined from 1.4 to 295 K. For the samples with the highest lattice constants, a = 5.712 and 5.711 A, there is no evidence of any significant departure from local cubic symmetry in the paramagnetic region, and in the magnetically saturated region we observe a distribution of hyperfine fields. For each of the samples with intermediate lattice constants (a = 5.685 and 5.672 A) a distribution of quadrupole splitting is observed in the paramagnetic region probably related to a distribution of noncubic environments. Surprisingly, for the most nonstoichiometric sample (a = 5.626 A) this distribution vanishes and there is only one dominant noncubic site present. A room temperature X-ray examination of this sample shows the presence of sharp superstructure reflections indicating a cationic vacancy segregation in every second {111} plane within a multi-domain structure. For the three smallest lattice parameters samples, in the paramagnetic region at low temperatures, the Mossbauer line shapes are influenced by magnetic relaxation. Adequate fits are obtained using a fluctuating hyperfine field model.

We report and compare Mossbauer effect measurements on 169 Tm in TmPd 2 Si 2 and on 170 Yb both diluted into the TmPd 2 Si 2 and in YbPd 2 Si 2 . For Tm the data is interpreted in terms of a Tm 3+ charge state and the crystal field parameters, the magnetic ordering temperature and the paramagnetic relaxation rates are obtained. The results on Yb in both the dilute and concentrated cases show no evidence of any magnetic character. The thermal variation of the quadrupole hyperfine interaction is treated in terms of intermediate valence Yb within the interconfiguration fluctuation model and yields a hybridization width of approximately the same size as the expected total energy splitting of the Yb 3+ 1 2 F 7/2 configuration (∼100 K). The intermediate valence behaviour is observed on Yb in both the dilute and concentrated samples. In an appendix we give the theoretical line shapes, calculated in the slow electronic relaxation limit, for a Tm 3+ ground state non-Kramers doublet whose degeneracy is lifted by the crystal field. This analysis predicts absorption lines at Doppler velocities in excess of ±100 cm/s Comparaison des mesures d'effet Mossbauer sur 169 Tm dans TmPd 2 Si 2 et son 170 Yb dilue dans TmPd 2 Si 2 et dans YbPd 2 Si 2 . Interpretation par un etat de charge Tm 3+ , dans le cas de Tm, et determination des parametres de champ cristallin, de la temperature d'ordre magnetique et des taux de relaxation para magnetique. Aucune manifestation de caractere magnetique dans le cas de Yb; analyse de la variation thermique de l'interaction hyperfine quadripolaire en termes de valence intermediaire de Yb, dans le cadre du modele de fluctuations interconfigurations; determination d'une largeur d'hybridation comparable a la separation en energie totale pour la configuration 2 F 712 de Yb 3+ (∼100 K)

170Yb Mossbauer measurements show that the heavy electron compound YbAs undergoes a first order paramagnetic-antiferromagnetic transition centered at TN=0.58K. The saturated 4f-shell magnetic moments are reduced by 47% relative to the value predicted by a crystal field model. A study of the line shapes below TN in the presence of an external field shows that the RKKY exchange energy is about 20 times bigger than kB TN. These results show that YbAs presents a magnetic order frustrated by the Kondo effect.

In YbPd, $^{170}\mathrm{Yb}$ M\"ossbauer measurements show that two different Yb charge states, one magnetic and one nonmagnetic, are present in essentially equal proportions. For the magnetic fraction the saturated spontaneous magnetic moment is $1.15{\ensuremath{\mu}}_{\mathrm{B}}$, which is lower than that in any of the ${\mathrm{Yb}}^{3+}$ cubic crystal-field levels, and there is no evidence of any anisotropy in the $4f$-shell charge distribution. These results suggest the presence of $4f$-conduction-band hybridization. Both above and below the ordering temperature of the magnetic fraction, the remaining Yb possess a negligibly small magnetic susceptibility and an isomer-shift value characteristic of a charge state close to ${\mathrm{Yb}}^{2+}$.

Mossbauer measurements are reported on 169Tm3+ in TmPO4 and on 170Yb3+ in TmXO4 and in YbXO4 (X = P, V). The measured hyperfine parameters are used to provide information concerning the electronic properties of the rare earth ions. A set of crystalline electric field (CEF) parameters is obtained for Tm3+ in TmPO4 and previously proposed CEF parameters for Tm3+ in TmVO4 are discussed. The measured ground state properties of Yb 3+ diluted into the two TmXO4 can be generated by sets of CEF parameters close to those obtained for Tm3+ in the corresponding matrix. Spin-spin relaxation rates were measured in the paramagnetic YbXO 4 and the origin of the rate in YbVO4 is discussed. Magnetic ordering is observed in YbVO4 below about 0.15 K and also in YbPO 4 in a comparable temperature zone.

169 Tm Mossbauer measurements are reported for the distorted perovskite compounds TmAlO 3 , TmFeO 3 , TmCrO 3 and TmVO 3 and have been analyzed to provide information concerning the crystal field and magnetic properties of the Tm 3+ ion. Tm 3+ has no magnetic moment in TmAlO 3 and low temperature saturated moments of 0.09, 0.8 and 2.1μ B in TmFeO 3 , TmCrO 3 and TmVO 3 , respectively. In TmFeO 3 and TmCrO 3 the magnitudes and directions of the Tm 3+ moments are examined in terms of previously proposed magnetic structural models. In TmVO 3 , the crystal field properties are very different to those in the ferrite and chromite and a reorientation of the Tm 3+ magnetic moments, accompanied by a sharp increase in their size, is observed near 10 to 15 K.

M\"ossbauer-effect measurements have been carried out on $^{170}\mathrm{Yb}$ in some rare-earth orthoaluminates ($R\mathrm{Al}{\mathrm{O}}_{3}$). We examine three different cases where the ${\mathrm{Yb}}^{3+}$ ion possesses high uniaxial anisotropy. For the first case, where the probe ion is diluted into a nonmagnetic matrix (YAl${\mathrm{O}}_{3}$: ${\mathrm{Yb}}^{3+}$, TmAl${\mathrm{O}}_{3}$: ${\mathrm{Yb}}^{3+}$) we examine phonon-driven temperature-dependent relaxation. For the second case, YbAl${\mathrm{O}}_{3}$, we observe magnetic ordering below 0.8 K and in the spin-spin driven relaxation region we observe two temperature zones where the relaxation rate has, respectively, a conventional temperature-independent value and anomalous temperature-dependent values. For the third case, that of ${\mathrm{Yb}}_{x}{\mathrm{Y}}_{1\ensuremath{-}x}\mathrm{Al}{\mathrm{O}}_{3}$, we study the dependence of the relaxation rate on the ${\mathrm{Yb}}^{3+}$ concentration. For YbAl${\mathrm{O}}_{3}$, using a formalism based on the Fermi golden rule, we have calculated the relaxation rates in the paramagnetic region arising from different types of interactions between the spins. We find that exchange with nearest neighbors in the $a\ensuremath{-}b$ plane is the dominant relaxation mechanism. The origins of the moment lowering and short-range ordering observed in this system are also discussed.

151 Eu Mossbauer spectroscopy measurements on two amorphous samples of EuPd x ( x = 2.4 and 3.0) prepared by sputtering, show the presence of inhomogeneous mixed valent (Eu 2+ and Eu 3+ ) states with sample averaged valencies 2.25 and 2.85 respectively. Magnetic ordering and hyperfine fields are visible on the Eu 2+ components. The two charge states have the same effective Debye temperature. The amorphous and recrystallised samples are compared.

EPR measurements have been made of the spin Hamiltonian parameters of Mn2+ at the three different sites in each of the four diamagnetic garnets LuAlG, YAlG, LuGaG and YGaG. The temperature variation of all the second order crystal field parameters together with those of the fourth order crystal field parameter and the hyperfine splitting parameter for octahedral sites in YGaG have been followed over a range 4·2-550°K. The magnitude of the various parameters and their temperature variations are discussed.

The dependence of the EPR resonance field on sample shape has been measured for Fe 3+ and Yb 3+ ions substituted into thullium gallium garnet. The results enable the second order paramagnetism of the lowest singlet Tm 3+ energy level to be found.

Measurements of the electric field gradient on 170Yb in YbPd2Si2, YbCu2Si2 and YbCuAl are interpreted within the interconfiguration fluctuation model and provide clear evidence of the persistence of crystal field effects in the Intermediate Valence(IV) state. IV is observed on Yb diluted into TmPd2Si2. The average magnetic hyperfine constant for Yb in YbCuAl has been measured.

170 Yb Mossbauer experiments confirm that the heavy-electron type compound YbP is magnetically ordered below 0.41 K. The saturated spontaneous moment of Yb is much smaller than that of the Yb 3+ Γ 6 doublet, which is the high-temperature ground-state level.

The high critical temperature superconductor YbBa 2 Cu 3 O 7−x has been studied over the range 0.05 to 95K using 170 Yb Mossbauer absorption spectroscopy. Magnetic ordering occurs within the Yb 3+ sublattice at 0.35K. The saturated magnetic moments are 1.7μ B . At all temperatures the hyperfine parameters show the presence of distributions attributed to some disordering of the oxygen vacancies.

Mössbauer study on Yb170 and magnetic susceptibility measurements show that YbMo6S8 contains both Yb2+ and Yb3+ ions with nearly equal proportions. The Yb2+ ions are submitted to a very strong electric field gradient with axial symmetry and move preferentially in the plane perpendicular to the gradient axis (which is probably the hexagonal axis c). The Yb3+ magnetic moments order perpendicularly to the c axis below TN=2.75 K and their magnitude is 1.7 μB near zero Kelvin. Ytterbium impurities also appear as Yb2+ and Yb3+ ions in TmMo6S 8, where the Yb3+ g-tensor is slightly anisotropic (g ∥ = 2.32, g〉 = 3.64). Tm 169 Mössbauer measurements on TmMo6S8 show that only one Tm site is present.

The high- T c superconductor DyBa 2 Cu 3 O 7− x (SC) and its oxygen deficient tetragonal non-superconducting counterpart (NSC) have been studied by DC magnetisation measurements down to 0.3 K and by 161 Dy Mossbauer spectroscopy down to 0.05 K. The low-temperature Dy 3+ magnetic properties, which are similar in the two compounds, show the influence of crystal fields and Dy-Dy magnetic correlations. The Dy 3+ paramagnetic fluctuation rates also show the influence of these correlations. Magnetic ordering occurs within the Dy sublattices near 1 K. The Dy 3+ saturated magnetic moments show fairly large distributions around mean values of 7.0 (SC) and 7.3 μ B (NSC). These distributions are attributed to crystal field inhomogeneities linked to the sample microstructure. In the SC we compare the average field penetration at the Dy sites with the percentage of the bulk Meissner effect. This suggests that the supercurrents are such that they preferentially shield the rare earth sites.

Although, according to macroscopic measurements, the intermetallics CeRu2Si2, UGe2 and YbNiSn have very different electronic ground states, muon spin relaxation measurements show that these three compounds have an anomalous quasi-elastic spectral weight of magnetic excitations in the transfer energy range of 10 neV.

170Yb Mossbauer measurements on the heavy-electron compounds YbP, YbAs, and YbSb have probed the competition between RKKY exchange interactions and Kondo hybridisation. Magnetic ordering occurs below TN ∼ 0.5 K with reduced magnetic moments; the RKKY exchange interaction was measured and found to be much greater than kBTN.

For the high Tc compound YbBa2Cu3O7−x, Mossbauer absorption measurements show that magnetic ordering occurs within the Yb3+ sublattice at 0.35K (μsat=1.7μB). For ErBa2Cu3O7−x the Er3+ sublattice orders at 0.7K (μsat=4μB). Measurements on57Fe diluted into YBa2Cu3O7−x shows that no ordered magnetic moments exist within the Cu sublattices down to at least 4.2K.

169 Tm 3+ Mossbauer quadrupole splitting data for Tm 3 Al 5 O 12 and Tm 3 Ga 5 O 12 measured from 4.2 to 950 K have been analysed to provide crystal field constants for the two lattices which are also compatible with susceptibility data.

In YbPd, 170 Yb Mossbauer measurements show that two different Yb charge states, one magnetic and one non-magnetic, are present in essentially equal proportions. The magnetic fraction, which orders at 1.9 K, shows evidence of 4f-conduction band hybridisation. Both above and below 1.9 K the remaining Yb possess a negligibly small magnetic susceptibility.

Exchange shifts in Tm 3 Ga 5 O 12 have been examined for Cr 3+ and reexamined for Yb 3+ . Line broadening arising from cross relaxation to thermally populated Tm 3+ levels is present for Cr 3+ but absent for Yb 3+ and also for Gd 3+ . These differences cannot be related uniquely to the form of the spectral functions associated with the excited Tm 3+ energy levels.