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From Yb170 Mossbauer measurements on YbNi5, we establish: the Yb3+ ground state Kramers doublet is well isolated and has uniaxial anisotropy, the compound magnetically orders at 0.55 K, the Yb3+ moments are aligned along the hexagonal axis, they have a saturated value of 3.9μB and a thermal variation close to a mean field S = 1/2 law. We obtain the strength of the Yb3+–Yb3+ coupling which we find is considerably smaller than that between the rare earths (R3+) in the other RNi5. From a study of the spin dynamics, we find that dynamic short range order is present just above the ordering temperature and that above 2 K, the relaxation rate of the paramagnetic Yb3+ spins follows a T-linear Korringa law with a relatively pronounced slope which we link to a high density of states at the Fermi level. Magnetic susceptibility and magnetisation measurements establish the ferromagnetic nature of the Yb3+–Yb3+ coupling. We comment on the Yb3+ crystal field properties.

Ruthenium-doped yttrium gallium garnet single crystals were grown from a PbO/PbF2/B2O3 flux. Most samples are blue, occasionally green, orange or lemon yellow, depending upon the growth temperature and the amount of a divalent or tetravalent counterion. A spectroscopic study was carried out using several complementary techniques (optical absorption, MCD, and EPR). Low temperature MCD enabled discrimination among those absorption bands correlated to paramagnetic ions (Ru3+ or Ru5+) or diamagnetic Ru4+ at the octahedral site. Ru3+ dominates in the lemon yellow sample with a nominal Ge/Ru ratio of ≈7, whereas Ru5+ is responsible for the orange colour in a crystal with Ca/Ru ≈7. The strongest MCD features above 2.2 eV could be rationalised in terms of oxygen-to-Run+ (n = 3–5) charge transfer transitions in octahedral complexes, whereas the 2 eV band of blue crystals is tentatively assigned to an intervalence transition implying Ru4+. It is suggested that YIG films co-doped with ruthenium and a large amount of calcium or germanium might present interesting Faraday effect properties.

We show that, in rare earth based insulators, measurement of the thermal dependence of the muon spin-lattice relaxation rate at high temperature provides information on the nature of the magnetic correlations and on the crystal-field energy splitting, if any.

We have examined the low temperature magnetic properties of the geometrically frustrated antiferromagnetic pyrochlores Gd2Sn2O7 and Gd2Ti2O7 using specific heat, 155Gd M?ssbauer, magnetic susceptibility and magnetization measurements. For Gd2Sn2O7, the specific heat evidences a single, strongly first order magnetic transition near 1.0?K; in Gd2Ti2O7, we confirm the presence of both the transition near 1.0?K and the second transition near 0.75?K. Below 1?K, magnetic irreversibilities are present in both compounds, but their signature (the difference between the FC and ZFC branches) is more marked in Gd2Sn2O7. At 0.03?K in each compound, the M?ssbauer data show that the four Gd3+ of a tetrahedron carry moments of equal sizes and on a frequency scale of 120 ? 106?s?1 each is oriented perpendicular to the local direction. In Gd2Ti2O7, the M?ssbauer data also indicates that the transition at 0.75?K involves a small change in the magnetic structure.

We have probed the low temperature spin dynamics in two pyrochlore lattice compounds R2Ti2O7, where the rare earth (R3+) form a sublattice of tetrahedra linked by their corners such that geometrically derived magnetic frustration is possible. In Yb2Ti2O7 (Yb3+: S′=1/2, XY anisotropy), we show that below 0.24 K , the temperature of the known specific heat λ transition, there is no long range order. The transition, in fact, corresponds to a first-order change in the fluctuation rate of the Yb3+ spins. Above the transition temperature, the rate is in the GHz range and it follows a thermal excitation law, whereas below the transition, the rate is in the MHz range and it is temperature independent. In Gd2Ti2O7, where the S-state Gd3+ ions are antiferromagnetically coupled, there is a magnetic phase transition near 1 K . As the temperature is lowered through the critical region, λZ, the spin-lattice relaxation increases. On lowering the temperature to 21 mK , λZ tends to a temperature independent value. Concomitant with the persisting electronic spin fluctuations, we observe oscillating components evidencing long-range spin correlations. Therefore, temperature independent spin dynamics is observed even after passing through a magnetic phase transition and even when the magnetic correlations are long range.

We report on very low temperature (27 mK) Mossbauer absorption spectra on Gd 155 in the Heisenberg frustrated antiferromagnets Gd2 Sn2 O7 (GdSn) and Gd2 Ti2 O7. We show that electronic fluctuations of correlated Gd3+ moments are present near T=0 in GdSn by observing that the hyperfine levels are out of equilibrium. Using a model spin 1/2 system, submitted to a magnetic (hyperfine) field reversing randomly in time, we find an analytical expression for the stationary probability distribution of the level populations (or magnetization). This quantity is a simple function of the ratio of the nuclear relaxation time to the average electronic flip time. These two time scales are found to be of the same magnitude in GdSn, and we discuss a relevant mechanism for nuclear relaxation and the influence of electronic fluctuations on the nuclear specific heat.

The Ni ({S} = 1) chain compound Yb2BaNiO5 (space group: Immm) is the end magnetic member of the isomorphous rare earth series. We have studied its magnetic properties using 170Yb Mossbauer spectroscopy and neutron diffraction. Magnetic order is induced into the Ni2+ chains through the polarization of the Yb3+ and leads to an ordering temperature of 8.8 K. The Ni2+ moments, saturated value 1.1 µB, are antiferromagnetically aligned close to the c-axis and the Yb3+ moments, saturated value 0.6 µB, are antiferromagnetically aligned at an angle of 55° to the c-axis and close to the (ac) plane. The thermal evolution of the Yb3+ moment is well described with a phenomenological molecular-field model. The value derived for the effective polarizing field acting on the Ni2+ when expressed per Yb3+ moment is much higher than the equivalent value for any of the other rare earths.

In a previous study of the Er3+ spin relaxation rate in ErNi2B2C (Bonville et al., Z. Phys. B 101, 511 (1996)), we found that its thermal dependence showed an anomaly at the superconducting transition temperature (\(\) = 10.5 K). This behaviour could be related either to a superconductivity-related change in the density of conduction band states to which the Er3+ spin is coupled or alternatively to the slowing down of the Er3+ spin fluctuations due to the development of short range magnetic order above the long range ordering temperature (\(\)\(\) 6 K). To identify the origin of the anomaly, we compared the results of a 166Er Mossbauer study on Er(Ni0.975Co0.025)2B2C where \(\) is depressed to 4 K and \(\) remains near 6 K, and on Er0.2Y0.8Ni2B2C where \(\) = 14.2 K and there is no long range order. We conclude the relaxation anomaly is due to the onset of short range magnetic order. The normal state coupling of the 4f-conduction electron exchange in the partially diluted sample is \(\) = 0.015. We also present 166Er Mossbauer measurements of the hyperfine field in single crystal ErNi2B2C which provide information concerning the direction of the magnetically ordered Er3+ moments in the superconducting phase.

We report low temperature 170Yb Mossbauer spectroscopy and magnetization measurements on two tetragonal non-superconducting quaternary borocarbides, YbNi2B2C and YbNiBC, and present LIII edge X-ray absorption data in YbNiBC. Both compounds show evidence of 4f-conduction band hybridisation. In the heavy electron YbNi2B2C, measurements down to 0.023 K show that only a small hyperfine interaction is present and we estimate an upper limit of 0.05–0.1 μ B for any Yb moment. In YbNiBC, magnetic ordering is observed below 4.2 K, in agreement with previous specific heat measurements. The single ion easy magnetization axis lies along the c axis but the ordered Yb moments lie in the basal (a, b) plane. Their (extrapolated) saturated value is unusually low (0.35 μ B). The Yb-Yb exchange interaction is very large and highly anisotropic with the dominant component lying in the basal plane. We also report 166Er Mossbauer and specific heat data in the ferromagnet ErNiBC.

We describe a method for measuring the local fields arising from the vortex lattice in YBa 2 Cu 3 O 7− δ which is based on the field dependence of the electro-nuclear energy levels of non-perturbing 170 Yb 3+ Mossbauer probes. From 4.2 K measurements of the second moment of the field distribution after a field-cooled (FC) treatment, we obtain the bulk penetration lengths ( λ ab ) for under-, optimum- and overdoped sintered samples and we estimate the evolution of the condensate density with doping. The overdoped samples were obtained by increasing the oxygen content or by adding 20% Ca. We find the presence of Ca increases the condensate density above the level obtained by adding oxygen to fill the chains and the condensate density of the planes increases at the same time that T c decreases. For an optimally doped grain-oriented sample of YBa 2 Cu 3 O 7− δ , we obtain the 4.2 K bulk penetration lengths ( λ ab and λ c ) and we also obtain estimates for the surface penetration lengths by measuring the induction appearing in the surface layers of the grains when fields below H c1 are applied after a zero-field cooling treatment. The surface penetration lengths are found to be not representative of the lengths obtained from within the bulk.

We report a 57 Fe Mossbauer study of superconducting samples of YBa 2 Cu 3 O 7 substituted with 1% Fe and also, in addition, up to 4% Co. Increasing the Co level has no influence on the 57 Fe quadrupole interactions but strongly increases the saturated 57 Fe hyperfine fields. For the sample with 4% Co, where the increased hyperfine field leads to a reduced interference between the magnetic subspectra, we carry out a quantitative analysis of the thermal dependence of the fluctuation rate of the 57 Fe hyperfine field. We compare the properties of the short-range correlated magnetic order introduced by the Co/Fe as sensed by the perturbing 57 Fe probe with those reported previously for the non-perturbing Mossbauer probe 170 Yb in the same system.

We have used paramagnetic170Yb3+Mossbauer probes to examine two related properties of Y0.8Ca0.2Ba2Cu3Oxas a function of the doping level. First, by measuring the low temperature thermal dependence of\(1/T_1^{el} \)of the probes in overdoped samples, we obtain information concerning the density of states below Tc. Second, by measuring the internal (molecular) field appearing on the probes in underdoped samples, we examine the fluctuating short range correlated Cu(2) magnetic order which is progressively introduced as the doping level is reduced. This magnetic order microscopically cohabits with superconductivity.

We investigated the annealing temperature, the carbon non-stoichiometry and the Fe doping effects on the physical properties of the superconductor HoNi 2 B 2 C 3 (1 ≤ × ≤ 1.35), using X-ray diffraction, magnetisation or electrical resistance measurements and 57 Fe Mossbauer spectroscopy. We show that the spontaneous re-entrant behaviour of HoNi 2 B 2 C which has been attributed to the occurrence of incommensurate magnetic states destructive to superconductivity, is strongly enhanced by quenching or by decreasing the annealing temperature. The re-entrant behaviour also increases with the carbon content and the 57 Fe (0.5 at.%) doping. We conclude that the chemical defects introduced by quenching, deviation from stoichiometry or Fe doping, depress superconductivity and favour the appearance of the re-entrant behaviour. Our 57 Fe Mossbauer investigation shows that the presence of the incommensurate magnetic orders in the re-entrant region does not lead to the introduction of a hyperfine field at the Fe probe.

We have examined the local magnetic order which is introduced into YBa{sub 2}Cu{sub 3}O{sub 7} by substituting low levels (1{endash}4thinsp{percent}) of Co or Fe. The order is evidenced through the appearance of an internal field on the spin of {sup 170}Yb{sup 3+} M{umlt o}ssbauer probes diluted at the Y{sup 3+} sites. Low substitution levels only marginally influence the superconducting properties, yet introduce local magnetic order over the whole sample volume. The local order concerns the Cu sublattices and fluctuates with rates that depend on the substituted ion, its concentration, and the temperature. {copyright} {ital 1997} {ital The American Physical Society}

The recent discovery of superconductivity (SC) in the quaternary borocarbide system Y–Ni–B–C has opened up a new area of research in SC. SC mainly occurs only for the end members of rare earths (R) and magnetism and SC coexist whenRis a magnetic ion (Ho, Er, Tm, and later in Dy). We will discuss and give some new results on the effects of annealing on the impurity phases and on low Fe doping rates on the coexistence between SC and magnetism in materials withR=Ho. Superconductivity has been reported in Y–Pd–B–C system with aTc(22 to 23 K) higher than the record for bulk intermetallics. The composition and structure of the SC phase(s) is still a topic of controversy that we will briefly discuss, giving new results.

From magnetic, specific heat, 170Yb Mossbauer effect, neutron diffraction, and muon spin relaxation measurements on polycrystalline Yb2Sn2O7, we show that below the first order transition at 0.15 K all of the Yb3+ ions are long-range magnetically ordered and each has a moment of 1.1?B which lies at ?10° to a common fourfold cubic axis. The four sublattice moments have four different directions away from this axis and are therefore noncoplanar. We term this arrangement splayed ferromagnetism. This ground state has a dynamical component with a fluctuation rate in the megahertz range. The net ferromagnetic exchange interaction has an anisotropy that favors the local threefold axis. We discuss our results in terms of the phase diagram proposed by Savary and Balents [ Phys. Rev. Lett. 108 037202 (2012)] for a pyrochlore lattice of Kramers 1/2 effective spins.

166Er Mossbauer absorption spectra have been recorded in the superconductor ErNi2B2C (Tc = 10 K), in zero field and in the temperature range 1.4 K–40 K. The spectra in the magnetically ordered phase (TN ≃ 5.5 K) are characteristic of an incommensurate magnetic structure with a maximum Er3+ moment of 8.2 μB. We show that the magnetic transition is first order, with a small temperature range where magnetically ordered and paramagnetic domains coexist. The analysis of the magnetic and quadrupolar hyperfine interactions below and above TN suggests that the Er3+ ground doublet is close to |J = 15/2; Jz = ± 1/2 > and shows that there is a low lying (≃ 10 K) excited doublet. The measured Er3+ electronic relaxation rate 1/T1 shows an anomaly at 10 K (Tc suggesting that the conduction electrons that are exchange coupled to the 4f spin take part in the formation of the superconducting state. We also present specific heat data for ErxY1 − xNi2B2C and Er0.2Lu0.8Ni2B2C which also evidence a low lying doublet and from which we propose a modified (improved) Er3+ crystal field level scheme (0, 9, 62, 71, 73, 181, 212 and 216 K) where the energies of the higher levels are consistent with published neutron scattering data.

We report lattice parameter and superconducting temperature transition ( T c ) measurements on ErNi 2 − x M 2− x B 2 C (M  Co, Pd, Pt), x = 0.20; M  Co, x = 0.05). We discuss the origin of T c lowering which is relatively modest for th substitutions Pd and Pt (d T c /d x ∼ − 12 K) and more pronounced for the non-isoelectronic substitution of Co (d T c /d x ∼ − 120 K). We also report 166 Er Mossbauer measurements for M = Co, x = 0.20 which evidence a main Er 3+ ordering temperature near 5 K and a saturated magnetic moment of 8μ B .

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.

The magnetic interactions in superconducting YbBa 2 Cu 3 O 7 and non-superconducting YbBa 2 Cu 3 O 6 have been examined using 170 Yb 3+ Mossbauer spectroscopy on grain-oriented samples. In YbBa 2 Cu 3 O 7 where the Yb 3+ sublattice orders at 0.35 K and the saturated magnetic moment is 1.75 μ B , the saturated molecular field is 0.4 T and it is directed perpendicular to the c-axis. In YbBa 2 Cu 3 O 6 , where the Cu(2) sublattice also orders, the average saturated molecular field on Yb 3+ is 0.2 T. It is disordered in direction and contains contributions from both the Yb 3+ and the Cu(2) 2+ sublattices. The presence of a Cu(2) 2+ derived molecular field shows the Yb 3+ ions are not at a centre of magnetic inversion symmetry relative to the ordered Cu(2) 2+ moments

Des experiences d'emission Mossbauer effectuees sur des impuretes quasi-isolees de 57 Fe substituees a Cu dans La 2-x Sr x CuO 4 montrent un changement rapide du comportement magnetique local quand la teneur x en Sr excede une valeur effective x t , situee approximativement au centre du domaine en x associe a la supraconductivite dans le diagramme de phase. Pour x

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.

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.

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 R2In compounds (R = rare earth) form an isostructural series which presents a great variety of magnetic behaviors depending on the nature of the rare earth. In this paper we present a study of Er2In by means of magnetic and transport measurements, Mossbauer spectroscopy and neutron diffraction. Our results reveal that this compound orders ferromagnetically below Tc = 40 K: the saturated moments at both the Er3+ sites are 8.0μB and they lie close to the basal plane.

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.

For all the Rare-Earth (R) the R2In form in the same crystal structure (P63/mmc). These compounds show a great variety of magnetic behaviors. When the temperature decreases, the magnetic susceptibility of Er2InTb2In and Gd2In increases, passes through a maximum then decreases. For Gd2In this behavior was associated with change from a paramagnetic to a ferromagnetic then to an antiferromagnetic state. We have performed magnetic, transport (Tb, Er), Mössbauer spectroscopy (Er) and powder neutron diffraction experiments (Gd, Tb, Er) on these compounds. Unlike Gd2In the resistivity of Tb2In and Er2In does not reveal any anomaly at the temperature where the susceptibility begins to decrease and the Tb2In and Er2In magnetizations show the same behavior at all temperatures in the ordered region. Neutron diffraction experiments reveal ferromagnetic and antiferromagnetic structures at low temperature.

The magnetic properties of the Ni spin chain compounds R 2 BaNiO 5 (R=Gd, Dy, Yb) were investigated by means of Mossbauer spectroscopy and neutron diffraction . The data provide the size and direction of the R magnetic moments and an estimate of the second order CEF parameters. The T -dependence of sublattice magnetizations are discussed in the context of the “Haldane chain in a staggered field” model (Yokoo et al., Phys. Rev. B 58 (1998) 14424).

It has been reported that transport properties of polycrystalline EuPdSb shows anomalies at around 12 and 18 K. Magnetic and transport properties on single crystal of EuPdSb are reported for the first time in this paper. It is revealed that magnetic susceptibility along the a -axis shows strong anisotropy below 12 K. Such features are also observed in high-field magnetization using pulsed field. Especially, three jump steps of magnetization are observed for applying field along the a -axis. 151 Eu Mossbauer study implies that Neel temperature of EuPdSb is 18 K and magnetic phase transition occurs at 12 K, which is consistent with the specific heat measurement.

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.

We have examined the magnetic properties of polycrystalline, superconducting ${\text{YBa}}_{2}{({\text{Cu}}_{0.96}{\text{Ni}}_{0.04})}_{3}{\text{O}}_{y}$ $(y\ensuremath{\sim}7,\text{ }{T}_{\text{sc}}\ensuremath{\sim}75\text{ }\text{K})$ using two local probe techniques: $^{170}\text{Y}\text{b}$ M\"ossbauer down to 0.1 K and muon-spin relaxation $(\ensuremath{\mu}\text{SR})$ down to 1.5 K. At 0.1 K, the $^{170}\text{Y}\text{b}$ measurements show the Cu(2) over essentially all the sample volume carry magnetically correlated moments which are static on the time scale ${10}^{\ensuremath{-}9}\text{ }\text{s}$. The moments show a distribution in size. The correlations are probably short range. As the temperature increases, the correlated moments are observed to fluctuate with measurable rates (in the GHz range) which increase as the temperature increases and which show a wide distribution. The $\ensuremath{\mu}\text{SR}$ measurements also evidence that the fluctuation rates increase with increasing temperature and there is a distribution. The evidenced fluctuating, correlated Cu(2) moments coexist at an atomic level with superconductivity.

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 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.

The magnetic properties of Yb 2 BaCuO 5 have been examined using 170 Yb Mossbauer spectroscopy down to 0.05 K and magnetic susceptibility measurements down to 1.5 K. The magnetic polarisation of the Yb 3+ at each of the two sites is due uniquely to couplings with the Cu 2+ moments which are shown to magnetically order at 15 K. The saturated molecular fields at the two Yb 3+ sites are 12 and 22 kG and produce saturated Yb 3+ magnetic moments of 2.2 and 1.1 micro B , respectively.

We introduce the technique of studying the field dependence of the electro-nuclear energy levels of a rare earth to measure the magnetic field present at the rare earth/yttrium site in YBa2Cu3Ox. Measurements were made by170Yb Mossbauer spectroscopy. The hyperfine spectrum of the ground state Kramers doublet for Yb3+ ions diluted into this matrix is sensitive to fields in the range 100 to 2000G. Flux penetration and trapping at the local site level have been measured in superconducting samples. A molecular field exists on the rare earth site in non superconducting samples suggesting that the ordered Cu2 magnetic moments are intrinsically non colinear.

The spin liquid properties of Gd3Ga5O12 have been examined using 155Gd Mossbauer spectroscopy down to 0.027 K. Information has been obtained concerning both the directional properties of the short range correlated moments and the thermal dependence of their spin fluctuation rates. Each Gd3+ spin (S=7/2) is found to be confined to a plane and its fluctuation rate decreases from approximately 2.8 x 10(9) s(-1) at 0.4 K to approximately 0.03 x 10(9) s(-1) at 0.09 K following a close-to-quadratic thermal dependence.

The very low temperature properties of two pyrochlore compounds, Yb2Ti2O7 and Gd2Sn2O7, were investigated using an ensemble of microscopic and bulk techniques. In both compounds, a first order transition is evidenced, as well as spin dynamics persisting down to the 20mK range. The transition however has a quite different character in the two materials: whereas that in Gd2Sn2O7$ (at 1K) is a magnetic transition towards long range order, that in Yb2Ti2O7 (at 0.24K) is reminiscent of the liquid-gas transition, in the sense that it involves a 4 orders of magnitude drop of the spin fluctuation frequency; furthermore, no long range order is observed. These unusual features we attribute to frustration of the antiferromagnetic exchange interaction in the pyrochlore lattice., 20 pages, 18 figures, proceedings of the Zakopane School of Physics (2003)

Using 170-Yb and 155-Gd Mossbauer measurements down to 0.03K, we have examined the semiconducting pyrochlore Yb2Mo2O7 where the Mo intra-sublattice interaction is anti-ferromagnetic and the metallic pyrochlore Gd2Mo2O7 where this interaction is ferromagnetic. Additional information was obtained from susceptibility, magnetisation and 172-Yb perturbed angular correlation measurements. The microscopic measurements evidence lattice disorder which is important in Yb2Mo2O7 and modest in Gd2Mo2O7. Magnetic irreversibilities occur at 17K in Yb2Mo2O7 and at 75K in Gd2Mo2O7 and below these temperatures the rare earths carry magnetic moments which are induced through couplings with the Mo sublattice. In Gd2Mo2O7, we observe the steady state Gd hyperfine populations at 0.027K are out of thermal equilibrium, indicating that Gd and Mo spin fluctuations persist at very low temperatures. Frustration is thus operative in this essentially isotropic pyrochlore where the dominant Mo intra-sublattice interaction is ferromagnetic.