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Neutron powder diffraction measurements (NPD) and Mossbauer spectroscopy (MS) have been performed to determine the magnetic phase diagram of the YBa2(Cu1−xFex)3Oy system with 0 ⩽ x ⩽ 0.16 and an oxygen content y close to 6.3. These phases were deoxygenated from the initial oxygen-rich state (y ≈ 7), by a low temperature process. This thermal treatment leaves the iron distribution identical to the one of the original oxygenated compound, whereas the usual high temperature method leads to a modification of the iron distribution. The distribution of iron on the two copper sites, Cu(1) and Cu(2), was carefully checked by both methods. The thermal dependence of the mean magnetic moments on the Cu(1) and Cu(2) sites was determined from Rietveld refinements. For x > 0.04, a collinear phase (AF′2) is observed with a doubling of the cell along the c axis and a ferromagnetic coupling between Cu(1) and Cu(2) sites. At lower iron content, several transitions are observed as the temperature decreases, from a paramagnetic to a collinear structure without any moment on the Cu(1) site, then to a canted structure and at low temperature the AF′2 structure is stabilized.

Mossbauer relaxation measurements have been made on 170 Yb3+ in DyAlO3 , TbAlO3 and HoAlO3 . In DyAlO3 and TbAlO3 between 30 K and TN and in HoAlO3 between 30 and 10K conventional temperature inde pendent spin-spin driven relaxation is observed. In HoAlO3 below 10K, the relaxation rate is tempe rature dependent. This behaviour is interpreted in terms of coupling between Yb3+ and the collective excitations of the two singlet level ground states of the Ho3+ ions.

From 170Yb3+ Mossbauer measurements on Yb2Ni17 down to 0.05 K we have obtained the saturated 4f shell magnetic and quadrupole moments at the two Yb3+ sites. The combined influence of the Yb3+ crystal field and the Ni derived exchange interactions leaves an essentially pure fully stretched | J z = 7 2 〉 state at one site and a mixed state with a 40% reduction of the magnetic moment at the other site. The fully stretched state experiences a saturated molecular field of near 0.5 T. From the magnetisation measurements we obtain the magnetic ordering temperature of the Ni sublattice (145 K) and find that, in contrast to the other R2Ni17, the Yb3+ and Ni sublattice moments do not show a compensation temperature.

From measurements of the thermal dependence of 1/ T 1 the electronic fluctuation rate for the S′ = 1 2 ground state of 170 Yb 3+ Miossbauer probes in YBa 2 (Cu 1− x Zn x ) 3 O 7 , we find that the addition of Zn 2+ introduces a density of states at the Fermi level in the superconducting state. Gapless behaviour is reached when x reaches 0.06. We compare the information obtained from the 170 Yb 3+ probe when T c is reduced by adding Zn 2+ and/or by reducing the oxygen level.

We report a new Mossbauer “relaxation window” which allows the fluctuation rate of a molecular field acting on an impurity in a strongly correlated spin system to be obtained. For a molecular field of 1 T, the fluctuation rate can be measured up to 10 THz, which is three orders of magnitude higher than for the classical relaxation window. The calculated line shapes show that Ising-like fluctuations of the molecular field can be distinguished from other types of fluctuations (in-plane or in space). An application and some other possible uses of this new “relaxation window” are discussed.

We have recorded at ISIS zero field and longitudinalμSR relaxation spectra on a single crystal of the Van- Vleck paramagnetPrNi 5 from room temperature down to 0.09K. Our data clearly indicate that at low temperature the rare earth spin dynamics is quasi-static, i.e. its correlation time is larger than 10−6 s. From theμSR methodological point of view, this work provides the first example of aμSR study of the dynamics of an hyperfine-enhanced nuclear dipole system.

The magnetic properties of the rhombohedral intermetallic compound Yb 3 Pd 4 have been investigated using 170 Yb Mossbauer spectroscopy and neutron diffraction. Magnetic ordering of the Yb 3+ moments sets in below T N =3.2 K, the saturated moment per Yb 3+ being μ s ⋍ 0.6μ B . The magnetic structure is antiferromagnetic and we discuss the possible moment orientations. In the paramagnetic phase, the thermal variation of the Yb 3+ 4f shell quadrupolar moment has been measured. cea.fr.

A sample of isocubanite (CuFe2S3) collected in a suboceanic hydrothermal deposit was found to have transformed formed after a few months into a very intimate mixture of similar amounts of chalcopyrite (CuFeS2) and a new phase of composition close to CuFe3S4. This new phase is not a disordered solid solution like isocubanite, but has a tetragonal superstructure. Its magnetization, electric resistivity, 57Fe Mossbauer spectra, X-ray and neutron diffraction spectra, have been studied at various temperatures. The Fe occupies two different crystallographic sites with, at room temperature, a common mixed valency (nominally 2.33+). Three transitions are observed on cooling: at TN ≅ 285 K where one of the Fe sites becomes antiferromagnetic; then at Tt ≅ 190 K, where the second Fe site becomes ferromagnetic with medium-range ordering; finally at Tv ≅ 105 K, where a first-order Verwey transition results in the appearance of integral valencies Fe2+ on one site and Fe3+ on the other, and in a rearrangement of the magnetic ordering to a ferrimagnetic structure.

We report a zero fieldμSR study of single crystals of theRENi5 ferromagnets where RE=Gd, Er, Dy or Tm. Our work points out the strong influence of the crystal electric field on the rare earth total momentum dynamics: whereas the spin-lattice relaxation process belowTc is controlled by a two magnon mechanism for a weakly anisotropic magnet (GdNi5), the phonons drive the relaxation for a strongly anisotropic magnet such asErNi5. ForTmNi5 a comparison is made between the fluctuation time measured byμSR and169Tm Mossbauer spectroscopy.

We have used 170Yb3+ Mossbauer spectroscopy on Yb3+ diluted at the rare-earth sites in Pr-xYxBa2Cu3O7 and in PrBa2Cu3O6 to examine the magnetic properties associated with the (hybridized) Pr3+ ions. Magnetic ordering in the non-superconducting compounds PrBa2Cu3O7 (T> = 16 K) and PrBa2Cu3O7 (T = 10 K) is identified by the appearance of a Pr3+-derive d molecular field and by a change in the Yb3+ paramagnetic relaxation rates. Evidence is presented suggesting the ordered Pr3+ magentic moments are aligned well away from the c-axis. The splitting of the three lowest crystal-field levels in PrBa2Cu3O6 is estimated to be about half that (∼ 50 K) in PrBa2Cu3O7. In the superconducting samples of Pr1-xYxBa2Cu3O7, the Yb3+ l ocal probes give evidence for an inhomogeneous behavior linked to statistical variations in the Pr3+/Y3+ occupancies. With increasing x, both the volume of the sample showing local Pr3+ magnetic order and the average ordering temperature decreases. There appears to be an anti-correlation between the superconducting transition temperature and the volume of the sample showing Pr3+ magnetic order, suggesting the local Pr3+ concentration needed to remove superconductivity (in favor of Cu (2)-based magnetic order) is near that needed to support local Pr3+ magnetic order.

We have used 170Yb Mossbauer spectroscopy on Yb3+ diluted at the Y3+ sites to study YBa2(Cu0.94Zn0.06)3Ox. In the fully oxygenated sample, the presence of Zn2+ introduces a Korringa dependence of the Yb3+ paramagnetic relaxation rate, indicating an increase in the density of states at the Fermi level, which is visible both above and below Tc (32 K). A molecular field coming from the Cu (2) is observed on all the Yb3+ probes in non-superconducting samples with x = 6.0 and 6.5 showing that at these oxygen levels, all the Cu (2)'s are locally magnetically correlated. For x = 6.88 molecular fields suggesting there is phase separation between local magnetic and non-magnetic superconducting regions. For x = 7.0, none of the Cu(2)'s are magnetically correlated. The general features of the oxygen-level dependence of the local magnetic properties are similar in the zinc-substituted and in the zinc-free samples, but magnetic ordering persists to higher oxygen levels when Zn2+ is present.

The Kondo lattices YbP and YbN (nonstoichiometric samples) have been investigated by 170Yb Mossbauer spectroscopy, in the temperature range 0.085 K to 80 K in zero magnetic field in YbN and with an applied magnetic field at T=0.045 K in YbP and YbN, and by neutron diffraction in YbN. In both compounds, the antiferromagnetic transition at TN≌0.7 K is first order and occurs through a progressive growth of paramagnetic domains as the temperature increases. The measured exchange interaction is much larger than kBTN in both compounds and it is found to be isotropic in YbP and anisotropic in YbN. The saturated magnetic moment in both compounds is reduced by 50% with respect to the value calculated from the crystal field and exchange interactions. These properties are interpreted in terms of the competition between the Kondo effect and the RKKY exchange interaction; using a model mean field NCA calculation to describe the magnetic phase, the Kondo temperatures TK are found to be near 5–10 K.

We report 170 Yb Mossbauer spectroscopy and magnetisation measurements in the magnetically ordered Kondo lattices YbP, YbN, YbAuCu 4 and YbNiSn, which reveal hybridisation-induced reductions of the Yb spontaneous moment and of the magnetic ordering temperature. Using the self-consistent NCA method with a RKKY derived molecular field, we calculate these reductions and compare them with experiment.

We present an investigation of the low temperature properties of the ternary intermetallic alloy YbNiSn by X-ray and neutron diffraction, resistivity, magnetisation and 170Yb Mossbauer spectroscopy measurements. It is shown that YbNiSn is a ferromagnet (Tc = 5.65 K) with the Yb3+ magnetic moments lying along the orthorhombic c axis. At low temperatures a strong anisotropy of the interionic exchange coupling is evidenced (λc ≃ 100 kOe/μB, λa ≃ 15 kOe/μB) which competes with the Crystal Electric Field anisotropy (gc ≃ 1.67, ga ≃ 3.72), leading to the unusual situation where the ferromagnetic moments do not lie along the easy magnetisation axis a. Hybridisation of the 4f electrons with band electrons leads to a reduction of the spontaneous moment (saturated value 0.85μB) and is likely to be the origin of the large value of the molecular field constant λc, as well as of the anisotropy of the exchange interaction and of the resistivity.

J. Phys. I France APRIL PAGE Classification Physics Abstracts 75.30M Magnetic ordering Bonville(~), B. Canaud(~), J. HamnJann(~), Jdhanno(~), A. Severing(~) and Z. Fisk(~) P. G. Service les Los Laue-Langevin, RAsumA temp6rature basse gn6tique (TN Yb~+ K), nous rapport par 4vidence en France au notable M6ssbauer l~°Yb sur propr16t6s des h antiferroma- YbAuCu4. Dans la phase de 20 $l du spontan6 h saturation moment lourds r4duction T7 de champ cristallin, et magn4tique entre les ions. Nous fondamental l'interaction de U-S-A- December1991) 61ectrons l'4tat de form Mexico New spectroscopie par une mesurons moment anisotropie une Grenoble, pr6sentons une 4tude du compos6 cubique k Nous D6partement de Recherche sur l'Etat Condens6, de Saclay, 91191 Gif-sur-Yvette, France accepted in final October Imbert(~), P. d'Etudes Centre Hodges(~), J.A. Laboratory, Los Alamos, National Alamos (RecHved Condens6, Physique de l'Etat et les Moldcules, de Atomes Institut de hybridisation in YbAuCu4 and mettons nous interpr4tons conduction, d'hybridation entre les 41ectrons 4f et les 41ectrons de termes propri4t4s de type 41ectron lourd. L'dchelle d'4ner#e kBTo de l'hybridation estim4e de l'ordre de 0,3 K. Les spectres M6ssbauer dans les phases ordonn4e est et parama- gn4tique pr4sentent des 41argissements de rare inhomog+nes qui sont interpr6t4s en termes de distribution de locales contraintes suivant les axes du cristal, d'4nergie magn4to-41astique comportement compte ce en qui rend des moyenne o,4 K. Abstract. We report on spontaneous sizeable 4f anisotropy of electron The energy ordered arising energy JOURNAL moment DE with and from kBTo scale electron of distribution a PHYSIQUE I of local K. oA T 2, N' 4, APR~ lW2 heavy K), we to that of the is spectroscopy material electron measure magnetic hybridisation, hybridisation paramagnetic phases o.2 respect M6ssbauer cubic the interionic the conduction °Yb a magnetic properties of magnetically ordered phase (TN perature a crystal which the In the antiferro- Yb~+ saturated reduction be o.3 for the K. heavy electron The inhomogeneous line broadenings strains along the ill ii crystal axes, show of the low tem- T7 ground state and we evidence a We interpret these features in terms of accounts to of YbAuCu4. field interaction. estimated $l investigation M6ssbauer which of mean properties. spectra are in the interpreted as magneto-elastic

We have performed Mossbauer measurements on oriented powders of 57Fe-doped La2−xSrxCuO4 (x=0 and 0.2 ) and 57Co-doped La2CuO4. We find that the main axis OZ of the 57Fe electric field gradient in the series lies at an angle β=26°±5° with respect to the crystal c axis, which is much less than recently published values. Our data were corrected for a fraction of unoriented crystallites, which we find to be systematically present, but which was unsuspected in previous experiments. We describe a convenient X-ray examination procedure for detecting and evaluating the unoriented fraction. We discuss possible origins of the angle β and we examine the relative disposition of the direction of c, OZ and of the hyperfine field on 57Fe in La2CuO 4 below TN. We also consider the influence of the oxygen stoichiometry on the symmetry at the Fe site.

From Mossbauer measurements on 166 Er 3+ diluted into YBa 2 Cu 3 O x with x ≅7.0 (superconducting) we obtain the hyperfine tensor and the g tensor of the Er 3+ crystal field ground doublet: A ab = 1610, A c = 935 MHz , g ab = 8.0, g c = 4.5. When the Er 3+ is diluted into a sample with x ≅6.0 (non-superconducting) we find that it now experiences a molecular field of approximately 200 G attributed to a net exchange coupling with its magnetically ordered Cu(2) neighbours. The nonzero value for this field shows that the arrangement of the Cu(2) moments does not have inversion symmetry about the Er 3+ ion. From 166 Er 3+ measurements at 0.05 K on grain oriented ErBa 2 Cu 3 O x ( x ≅7.0 and 6.0) we confirm that for x ≅7.0 the ordered Er 3+ magnetic moments are aligned close to directions perpendicular to the c -axis and we show that for x ≅6.0 the additional Cu(2)-derived molecular field does not modify this alignment. We suggest however, that the appearance of this molecular field as the oxygen level is lowered below x ≅6.4 could be a factor contributing to the known x dependence of the breakdown of the 3-dimensional correlations within the Er 3+ sublattice. For the grain oriented samples we describe an X-ray diffraction analysis procedure which gives a comprehensive assessment of the quality of the grain orientation.

We extend the method used to measure the internal (molecular) field on 170 Yb 3+ diluted for Y 3+ in YBa 2 Cu 3 O x described in the preceding paper (Part 1) to the study of dynamical effects. (The method exploits the field dependence of the 170 Yb 3+ electronuclear levels and the measurements are made by Mossbauer spectroscopy. By studying the fluctuation rates of the Cu(2) derived molecular field acting on the 170 Yb 3+ probes, we follow the fluctuation rates of the short range correlated Cu(2) moments as a function of oxygen content (6.0⩽ x ⩽7.0) and temperature (⪅70 K). When x ∼6.0 the correlated Cu(2) moments are static or fluctuate at frequencies below the lower limit of the accessible window. For x ≅6.35, the average fluctuation rate shows a strong thermal dependence decreasing through the accessible range of 10 12 to 5×10 8 s -1 as the temperature is lowered to 1.4 K and the system shows spin glass-like behaviour. At 1.4 K as x is increased above about 6.5, we observe that a fraction of the probes no long show spin glass-like behaviour and show no evidence of any fields fluctuating in the accessible frequency range. We identify these probes as being situated in the superconducting regions of the sample where at temperatures much lower than the gap there are not expected to be any Cu(2)-based magnetic excitations. From these local probe measurements we find that the amount of the sample that is superconducting depends strongly on x increasing from essentially zero at x ≅6.50 to about 50% at x ≅6.56, to about 75% at x ≅6.66, and reaches 100% as x approaches 7.0. The remaining fraction of the probes which shows a concomitant decrease in importance, continues to show spin glass-like behaviour with a strong thermal dependence of the average local fluctuation rates.

We describe a method, based on the field dependence of the electro-nuclear levels of 170Yb3+ diluted for yttrium in YBa2Cu3Ox, which probes magnetic fields at the rate earth site. Measurements were made using 170Yb Mossbauer spectroscopy. When x ≅ 6.0, we have evidenced an internal (molecular) field on the Yb3+ attributed to the net exchange interaction with the ordered Cu(2) sublattice. The field has an average size of 1600 G and lies at an average angle of near 45° to the c-axis. Up to a least 60 K it remains static on the frequency scale of the experiment (5×108s−1). Some possible origins of this previously unsuspected field are discussed.

We present some selected results obtained using rare earth Mossbauer spectroscopy, on YBa2Cu3Ox partially and/or totally substituted by Yb3+, Er3+ or Dy3+. We report measurements of the single ion crystal field anisotropy and of the size, direction and ordering temperature of the rare earth sublattice magnetic moments. We use dilute170Yb3+ ions to probe both the Cu(2) derived internal (molecular) fields and the fluctuation frequencies of the short range correlated Cu(2) moments. We monitor these frequencies as the carrier level is changed up towards the threshold needed to stabilise superconductivity.

The magnetically ordered phase and the Yb 3+ paramagnetic relaxation in the compound YbNi 2 Si 2 have been investigated by 170 Yb Mossbauer spectroscopy between 0.05 and 80 K. The magnetic ordering transition is found to occur at T N = 2.1 K, the Yb 3+ magnetic moments being perpendicular to the fourfold symmetry axis ( c -axis). Above T N and upto 30 K, the thermal variation of the Yb 3+ paramagnetic relaxation rate is a superposition of an interionic exchange term (spin-spin relaxation) and of a T -linear conduction electron-4f spin exchange term (Korringa relaxation).

Experimental data in the hybridized compound YbPd 2 Si 2 is compared with the results obtained with a recently proposed hybridisation model, based on the large-degeneracy expansion approximation, which takes into account the crystal field splittings of the rare earth ion energy levels. With a unique set of parameters, satisfactory agreement is simultaneously obtained for the thermal variation of the electronic specific heat, of the magnetic susceptibility and of the electric field gradient at the Yb site, as well as for the field variation of the low temperature magnetisation

Experimental data in the hybridized compound YbPd2Si2 is compared with the results obtained with a recently proposed hybridisation model, based on the “large-degeneracy expansion” approximation, which takes into account the crystal field splittings of the rare earth ion energy levels. With a unique set of parameters, satisfactory agreement is simultaneously obtained for the thermal variation of the electronic specific heat, of the magnetic susceptibility and of the electric field gradient at the Yb site, as well as for the field variation of the low temperature magnetisation.

We have performed Mossbauer absorption and emission experiments on57Fe impurities in ZnTe. A transient Fe1+ charge state has been observed below 130 K in the emission spectra. The dynamics of the Fe1+−Fe2+ charge transfer was shown to obey an activation process with an activation energy of 0.09 eV. Low temperature Raman relaxation rates within the Fe2+ spin-orbit levels are found to be at least 100 times faster in ZnTe than in ZnS.

From Mossbauer and resistivity measurements on57Fe-doped La2−xSrxCuO4−σ samples, we have determined the phase diagram of the system for 0 0.15, a Fe-induced spin-glass state is observed. Recent data on oriented powder samples are discussed.

The magnetic properties and magnetic structure of sternbergite (crystallographic space groupe Ccmb) have been studied by single crystal neutron diffraction and Mossbauer spectroscopy. The magnetic space group is Cc′m′b′ and the moments are, at least in first approximation, along the b axis. All Fe atoms are equivalent, with intermediate nominal valency (2.5)+. They are paired by sharing one d electron, the magnetic coupling within a pair being ferromagnetic. This behaviour is similar to that of the analogous — but not isomorphous — compound cubanite CuFe2S3.

The intermetallic compounds YbPt3 and Yb3Pt5, which present magnetic ordering below 0.25 K and 0.95 K respectively, have been investigated by 170Yb Mossbauer spectroscopy at very low temperatures. In YbPt3, the electronic ground Kramers doublet of Yb3+ has been determined, as well as its exchange splitting and spin-spin relaxation rate; the lowest Yb3+ crystal field excitation has been estimated. The surprising finding that the crystal electric field has strong axial character in YbPt3, instead of the expected cubic symmetry, is discussed.

Mossbauer absorption and emission experiments have been performed on 57 Fe impurities in ZnTe. Raman relaxation rates in Fe 3d 6 (Fe 2+ ) ions are found at least 100 times larger in ZnTe than those previously measured in ZnS. A transient Fe 3d 7 (Fe 1+ ) charge state has been observed in emission spectra below 130 K. The Fe 1+ -Fe 2+ transition process involves an activation energy of 0.09±0.02 eV. Below 50 K the evolution of the spectra is compatible with the appearance of a new charge state Fe 3d 8 (Fe 0 ). An increased “p” character source shows lines associated with Fe 3d 5 (Fe 3+ ) and Fe 3d 6 (Fe 2+ ).

Fe 3+ , Fe 2+ , Fe 1+ , and very probably also Fe 0 centers have been observed by Mossbauer absorption or emission spectroscopy in 57 Fe- (or 57 Co) doped ZnTe, depending on the experimental conditions. Isomer shift measurements show that the iron bondings are more covalent than in ZnS. Low-temperature Raman relaxation rates within the Fe 2+ spin-orbit levels are found to be at least 100 times larger in ZnTe than in ZnS. A transient one-electron trap Fe 1+ state was observed after electron capture decay of 57 Co below 130 K in stoichiometric ZnTe. The Fe 1+ −Fe 2+ transition was shown to obey an activation process with an energy of 0.09 ± 0.02 eV. A transient two-electron trap Fe 0 state seems to have also been detected as a percursor of the Fe 1+ state in low-temperature emission spectra. In a ZnTe semiconductor with enhanced p character, the acceptor states Fe 1+ and Fe 0 are no longer observed after the 57 Co decay, whereas a donor state Fe 3+ appears.

Specific heat and 170Yb Mossbauer spectroscopy measurements performed in cubic YbPdSb show that the Yb3+ CEF ground state is the Γ8 quartet with a Kondo temperature T K ⋍ 8 K . A first-order magnetic transition is observed at T = 1 K; at T = 0.05 K, the Yb3+ spontaneous magnetic moment is 1.3 μB, markedly smaller than the saturated Γ8 magnetic moment (∼ 2 μB). The 170Yb Mossbauer spectra with applied magnetic field in the magnetically ordered phase show that the RKKY exchange is anisotropic and provide an estimation for TRKKY, which is found to be of the same order of magnitude as TK.

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.

We present muon spin relaxation measurements performed on crystals of the heavy fermion superconductor UPt$_3$. In zero applied field, contrary to a previous report, we do not observe an increase of the internal magnetic field in the lower superconducting phase (the B phase). Our result gives an experimental upper bound of the magnetic field that could be associated with the superconducting state., Comment: 4 pages, REVTeX 3.0, 2 PostScript figures

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.

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.

A detailed Mossbauer study of 170Yb diluted in CaF2 has been performed on CaF 2 (170Yb) absorbers as well as on CaF 2 (170Tm) sources. Various informations have been obtained about valency states of Ytterbium, relative proportions of the different sites, hyperfine parameters, relaxation of Yb3+, concentration and after decay effects etc— The isomer shift of Yb3+ and Yb2+ in cubic sites in CaF2 differ only by 0.29±0.10 mm s ; an accurate determination of the nuclear factor, g ( 170 Yb , 84 keV ) = 0.334±0.002 , has been derived from the slow relaxation Hf splitting of cubic Yb3+.

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.

Fe2+ impurities in ZnS blende exhibit low temperature Mossbauer spectra which are remarkably different in source and in absorber experiments. In source experiments below 5K we observed a complex quadrupole structure, which is partially due to the slow relaxation contribution of the first excited triplet Λ4 of Fe2+, whose population appears to be out of thermal equilibrium. The corresponding reduction factor q due to the dynamic Jahn—Teller coupling has been estimated to be about 0.91.

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 ${\mathrm{Yb}}^{2+}$.