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[1] Ferrihydrite is an important iron oxyhydroxide for earth and environmental sciences, biology, and technology. Nevertheless, its mineral structure remains a matter of debate. The stumbling block is whether a significant amount of tetrahedrally coordinated iron is present. Here we present the first X-ray magnetic circular dichroism (XMCD) measurements performed on a well characterized synthetic sample of 6-line ferrihydrite, at both K and L2,3energy edges of iron. XMCD results demonstrate unambiguously the presence of tetrahedrally coordinated Fe(III) in the mineral structure, in quantities compatible with the latest extended X-ray absorption fine structure (EXAFS) analyses suggesting a concentration of 20–30%. Moreover, we find an antiferromagnetic coupling between tetrahedral and octahedral sublattices, with the octahedral sublattice parallel to the external magnetic field.

Uniform magnetic nanoneedles have been prepared by hydrogen reduction of elongated nanoarchitectures. These precursors are as-prepared or cobalt-coated aggregates of highly oriented haematite nanocrystals (∼5 nm). The final materials are flattened nanoneedles formed by chains of assembled Fe or FeCo single-domain nanocrystals. The microstructural properties of such nanoneedles were tailored using renewed and improved synthetic strategies. In this fashion, the needle elongation and composition, the crystallite size (from 15 up to 30 nm), the nanocrystal orientation (with the 〈110〉 or 〈001〉 directions roughly along the long axis of the nanoneedle) and their type of arrangement (single chains, frustrated double chains and double chains) were controlled by modifying the reduction time, the axial ratio of the precursor haematite and the presence of additional coatings of aluminum or yttrium compounds. The values of the coercivity H(C) found for these nanoneedles are compared with the values predicted by the chain of spheres model assuming a symmetric fanning mechanism for magnetization reversal.

Inelastic neutron scattering experiments have been carried out in the ``ordered spin ice'' Tb${}_{2}$Sn${}_{2}$O${}_{7}$, aiming at a detailed understanding of the low-energy magnetic excitations in this highly frustrated magnet. Besides the crystal-field transition lines, we report on a well-defined low-energy line that develops in the ordered phase. We show that the intensity of this transition cannot be understood within the trigonal crystal-field scheme and argue that an interaction, which involves the spin lattice coupling through a low-temperature distortion of the trigonal crystal field, is necessary to explain the data. Based on random-phase approximation spin-dynamics calculations, taking into account the actual crystal-field scheme as well as this distortion, we successfully describe the neutron data, quantitatively consistent with the experimental observations.

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.

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.

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 report muon spin resonance experiments on MgxCu4-x(OH)(6)Cl-2 with x similar to 1, a material isostructural to Herbertsmithite exhibiting regular kagome planes of S = 1/2 ions (Cu2+), and therefore a candidate for a spin-liquid ground state. We evidence the absence of any magnetic ordering down to 20 mK (similar to J/10(4)). We present a detailed investigation of the spin dynamics on well-characterized samples in zero and applied longitudinal fields, and a defect-based interpretation is proposed to explain the unconventional dynamics observed in the quantum spin-liquid phase.

The structural and magnetic properties of Yb(2)Al(1-x)Mg(x)Si(2) (x = 0, 0.5 and 1), crystallizing in the tetragonal Mo(2)FeB(2)-type structure, are reported in this work. Yb(2)AlSi(2) exhibits a Pauli paramagnetic ground state arising due to spin/valence fluctuations induced by a significant Yb 4f conduction band hybridization. High-field magnetization (up to 120 kOe) indicates a nearly temperature-independent susceptibility of 8.6 × 10(-3) emu/Yb mol below 10 K. On the other hand, Yb ions in Yb(2)MgSi(2) order antiferromagnetically at a relatively high temperature T(N) of 9.5 K. The intermediate composition alloy Yb(2)Al(0.5)Mg(0.5)Si(2) is a Kondo lattice, antiferromagnet with T(N) = 5.5 K. The coefficient of the linear term of the electronic heat capacity, γ, of Yb(2)AlSi(2) is found to be 305 mJ mol(-1) K(-2), indicating a significant electronic mass enhancement due to strong electronic correlations. Below 12 K, an additional contribution to the heat capacity of the form T(3)lnT is observed. The (170)Yb Mossbauer spectra in the ordered state of Yb(2)MgSi(2) indicate a strong coupling of the 4f quadrupolar moment with the magnetic moment through a magneto-elastic coupling.

We report the discovery of two consecutive, pressure driven magnetic instablities in Yb2Pd2Sn. They emerge in a non-Fermi liquid environment at the initial and the final point of a dome-like, single magnetic phase at pressures pc1 = 1 GPa and pc2 = 4 GPa. This singular behavior of Yb compounds is supposed to result from mutually competing, pressure modi ed energy scales, which in case of Yb2Pd2Sn cause a sign change of the pressure dependence of the Kondo temperature TK and magnetic ordering temperature TN.

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.

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 studied the field-induced magnetic structures of the ${\text{Tb}}_{2}{\text{Ti}}_{2}{\text{O}}_{7}$ pyrochlore by single-crystal neutron diffraction with a magnetic field applied along a [110] axis, focusing on the influence of a small misalignment. Both induced magnetic structures with $\mathbit{k}=\mathbf{0}$ and $\mathbit{k}=(0,0,1)$ propagation vectors are found to be sensitive to the misalignment, which controls the magnitude and orientation of the Tb moments involved in the $\ensuremath{\beta}$ chains, with local [111] anisotropy axis perpendicular to the field. For $\mathbit{k}=\mathbf{0}$, spin-icelike structures are observed for a misalignment of a few degrees whereas other structures, where the $\text{Tb-}\ensuremath{\beta}$ moments flip by ``melting'' on the field axis, occur when the field is perfectly aligned. The field evolution of the $\mathbit{k}=\mathbf{0}$ structure is well reproduced by a molecular field model with anisotropic exchange. We give a complete symmetry analysis of the $\mathbit{k}=\mathbf{0}$ and $\mathbit{k}=(0,0,1)$ magnetic structures, both being described by the basis functions of single irreducible representations.

We have measured neutron-diffraction patterns in a single-crystal sample of the pyrochlore compound ${\text{Er}}_{2}{\text{Ti}}_{2}{\text{O}}_{7}$ in the antiferromagnetic phase $(T=0.3\text{ }\text{K})$, as a function of the magnetic field, up to 6 T, applied along the [110] direction. We determine all the characteristics of the magnetic structure throughout the critical point at ${H}_{c}=2\text{ }\text{T}$. As a main result, all Er moments align along the field at ${H}_{c}$ and their values reach a minimum. Using a four-sublattice self-consistent calculation, we show that the evolution of the magnetic structure and the value of the critical field are rather well reproduced using the same anisotropic exchange tensor as that accounting for the local paramagnetic susceptibility. In contrast, an isotropic exchange tensor does not yield the correct moment variations through the critical point. The model also accounts semiquantitatively for other experimental data obtained in previous works, such as the field dependence of the heat capacity, energy of the dispersionless inelastic modes and transition temperature.

The Fe-based langasites are the only reported compounds presenting a magnetic ordering in this rich family, besides being well known for piezoelectric properties and optical activity. The structural, magnetic, and dielectric properties of the Fe-langasite compounds, with various substitution of nonmagnetic cations, have been studied with x-ray and neutron-diffraction, magnetostatic measurements, M\"ossbauer spectroscopy, and dielectric measurements. The title compounds (trigonal space group $P321$) display a helical magnetic order with signatures of frustration below ${T}_{N}\ensuremath{\approx}24--35\text{ }\text{K}$, where an anomaly of the dielectric permittivity is observed. The influence of the cationic substitutions and the nature of the magnetoelectric coupling are hereafter addressed.

57Fe Mossbauer spectroscopy measurements are presented in the underdoped Ba(Fe{1-x}Cox)2As2 series for x=0.014 (T_c < 1.4K) and x=0.03 and 0.045 (T_c ~ 2 and 12K respectively). The spectral shapes in the so-called spin-density wave (SDW) phase are interpreted in terms of incommensurate modulation of the magnetic structure, and allow the shape of the modulation to be determined. In undoped BaFe2As2, the magnetic structure is commensurate, and we find that incommensurability is present at the lowest doping level (x=0.014). As Co doping increases, the low temperature modulation progressively loses its "squaredness" and tends to a sine-wave. The same trend occurs for a given doping level, as temperature increases. We find that a magnetic hyperfine component persists far above the SDW transition, its intensity being progressively tranferred to a paramagnetic component on heating., 7 pages, 8 figures, published in EPL

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.

We present an interpretation of published neutron inelastic scattering spectra in the Kondo lattices YbPd2Si2 and YbAgCu4 obtained in terms of the Anderson impurity model, describing the hybridisation of the 4f Yb electrons with the band electrons, and also including the crystal electric field interaction. In YbPd2Si2, the tetragonal crystal field parameters were determined. In YbAgCu4 the crystal field interaction was taken to exist by analogy with the isoelectronic compound YbAuCu4 where it has been identified. Both compounds can be described by a Kondo temperature,T0=60 K and a Yb valency very close to 3.

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

170Yb Mossbauer spectroscopy measurements down to 0.025 K and neutron diffraction measurements down to 1.7 K have been performed in the heavy electron compound YbPtAl (TN≃ 5.8 K). Below TN the neutron data show that the magnetic structure is incommensurate, with a wave-vector k =(0.30, 0, 0) . At 0.025 K, the Mossbauer data reveal that the magnetic structure is modulated and that it is not an antiphase structure. YbPtAl is a clear example of a Kramers local moment system where the Kondo singlet ground state allows a modulated magnetic structure to persist down to 0 K.

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.

Single crystals of EuPtSi$_3$, which crystallize in the BaNiSn$_3$-type crystal structure, have been grown by high temperature solution growth method using molten Sn as the solvent. EuPtSi$_3$ which lacks the inversion symmetry and has only one Eu site in the unit cell is found to be an antiferromagnet with two successive magnetic transitions at $T_{\rm N1}$ = 17 K and $T_{\rm N2}$ = 16 K, as inferred from magnetic susceptibility, heat capacity and $^{151}$Eu M\"ossbauer measurements. The isothermal magnetization data for $H \parallel$ [001] reveal a metamagnetic transition at a critical field $H_{\rm c}$ = 1 T. The magnetization saturates to a moment value of 6.43 $\mu_{\rm B}$/Eu above 5.9 T (9.2 T) for $H \parallel $ [001] ([100]), indicating that these fields are spin-flip fields for the divalent Eu moments along the two axes. The origin of this anisotropic behaviour is discussed. A magnetic (H, T) phase diagram has been constructed from the temperature dependence of isothermal magnetization data. The reduced jump in the heat capacity at $T_{\rm N1}$ indicates a transition to an incommensurate, amplitude modulated antiferromagnetic structure. The shape of the hyperfine field split M\"ossbauer spectrum at $T_{\rm N1}$ provides additional support for the proposed nature of this magnetic transition., Comment: 6 pages, 6 figures. Submitted to Phys. Rev. B

We studied the field induced magnetic order in ${R}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$ pyrochlore compounds with either uniaxial ($R=\mathrm{Ho}$, Tb) or planar ($R=\mathrm{Er}$, Yb) anisotropy, by polarized neutron diffraction. The determination of the local susceptibility tensor ${{\ensuremath{\chi}}_{\ensuremath{\Vert}},{\ensuremath{\chi}}_{\ensuremath{\perp}}}$ provides a universal description of the field induced structures in the paramagnetic phase (2--270 K), whatever the field value (1--7 T) and direction. Comparison of the thermal variations of ${\ensuremath{\chi}}_{\ensuremath{\parallel}}$ and ${\ensuremath{\chi}}_{\ensuremath{\perp}}$ with calculations using the rare earth crystal field shows that exchange and dipolar interactions must be taken into account. We determine the molecular field tensor in each case and show that it can be strongly anisotropic.

International audience; The potential relevance of the Ba 3NbFe 3Si 2O 14 langasite in the field of multiferroism was investigated. Interesting properties were expected, in particular in view of the frustrating stacking of its magnetic Fe 3+ cations into triangular planes of triangle units. We hereafter report results of specific heat and magnetic measurements, Mössbauer spectroscopy and neutron diffraction. A peculiar magnetic structure is found out at low temperature. The Fe 3+ magnetic moments adopt a triangular 120° configuration within each triangle, which is in-phase propagated in each triangular plane and is helically modulated from plane to plane.

We used a local susceptibility approach in extensive polarized neutron diffraction studies of the spin liquid \tbti. For a magnetic field applied along the [110] and [111] directions, we found that, at high temperature, all Tb moments are collinear and parallel to the field. With decreasing temperature, the Tb moments reorient from the field direction to their local anisotropy axes. For the [110] field direction, the field induced magnetic structure at 10 K is spin ice-like, but with two types of Tb moments of very different magnitudes. For a field along [111], the magnetic structure resembles the so-called "one in-three out" found in spin ices, with the difference that all Tb moments have an additional component along the [111] direction due to the magnetic field. The temperature evolution of the local susceptibilities clearly demonstrates a progressive change from Heisenberg to Ising behavior of the Tb moments when lowering the temperature, which appears to be a crystal field effect., Conference Proceeding of Polarized Neutrons in Condensed Matter Investigations (Tokai, Japan, 2008)

The spin dynamics in the geometrically frustrated pyrochlore antiferromagnet ${\text{Gd}}_{2}{\text{Sn}}_{2}{\text{O}}_{7}$ is studied by means of the electron spin resonance. In the ordered phase $({T}_{N}=1\text{ }\text{K})$, we have detected three gapped resonance modes. Their values agree well with the developed spin-wave theory which takes into account the Heisenberg nearest-neighbor exchange, the single-ion anisotropy, and the long-range dipolar interactions. The theory also predicts a fourth lowest-frequency gap, which lies beyond the experimental range of frequencies, but determines the exponential decrease in the specific heat at low temperature.

We report the effect of replacing Cu by Pd in RCu5 (R ¼ Pr, Nd, Sm and Eu). The parent RCu5 compounds crystallize in the hexagonal CaCu5-type structure. The hexagonal symmetry is retained in PrCu4Pd and EuCu5� xPdx (x ¼ 1 and 2) but the crystal structure changes to cubic AuBe5-type in PrCu3Pd2, NdCu5� xPdx (x ¼ 1 and 2) and SmCu4Pd. Substitution with Pd leads to lattice expansion and modifies the magnetic behavior. While PrCu5 is known to be a van-Vleck paramagnet with a singlet ground state, PrCu4Pd and PrCu3Pd2 show ferromagnetic-like behavior at low temperatures. SmCu4Pd orders ferromagnetically near 28 K in contrast to the antiferromagnetic nature of the parent SmCu5. The divalent nature of the Eu ions in EuCu5 is retained in the ternary alloys, but the Curie temperature is reduced from 57 to 24.5 and 14.5 K in EuCu4Pd and EuCu3Pd2, respectively, inferred from the location of peak in the heat capacity of

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.

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

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 ordering of the hexagonal multiferroic compound ${\text{YbMnO}}_{3}$ has been studied between 100 and 1.5 K by combining neutron powder diffraction, $^{170}\text{Y}\text{b}$ M\"ossbauer spectroscopy, and magnetization measurements. The Yb moments of the two crystallographic sites order at two different temperatures: the $4b$ site together with the Mn moments (at ${T}_{N}\ensuremath{\simeq}85\text{ }\text{K}$) and the $2a$ site well below (at 3.5 K). The temperature dependences of the Mn and Yb moments are explained within a molecular-field model, showing that the $4b$ and $2a$ sites order via Yb-Mn and Yb-Yb interactions, respectively. A simple picture taking into account the local Mn environment of the rare-earth $R$ $(4b)$ ion is proposed to couple $R$ and Mn orders in hexagonal $R{\text{MnO}}_{3}$ manganites. The nature and symmetry of the $R\text{-Mn}$ interactions yielding the $R$ order are discussed.

We have studied magnetically frustrated ${\text{Tb}}_{2}{\text{Sn}}_{2}{\text{O}}_{7}$ by neutron diffraction and high-resolution energy-resolved neutron scattering. At 0.1 K, we observed short-range magnetic correlations with a typical scale of $4\text{ }\text{\AA{}}$, close to the near-neighbor distance between ${\text{Tb}}^{3+}$ ions. This short-range order coexists with ferromagnetic correlations and long-range spin ice order at the scales of 18 and $190\text{ }\text{\AA{}}$, respectively. Spin dynamics was investigated at a time scale down to ${10}^{\ensuremath{-}9}\text{ }\text{s}$ by energy-resolved experiments on a backscattering spectrometer. We observed a freezing of the spin dynamics for all length scales with a strong slowing down of the spin fluctuations when long-range order settles in. We discuss the spin fluctuations remaining in the ground state, in comparison with previous data obtained by muon spectroscopy.

We have studied the field-induced magnetic structures in ${\mathrm{Tb}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$, in a wide temperature ($0.3lTl270\text{ }\text{ }\mathrm{K}$) and field ($0lHl7\text{ }\text{ }\mathrm{T}$) range, by single crystal polarized and unpolarized neutron diffraction, with $\mathbf{H}\ensuremath{\parallel}[110]$ axis. A ferromagneticlike structure with $\mathbf{k}=\mathbf{0}$ propagation vector is induced, whose local order at low field and low temperature is akin to spin ice. The four Tb ions separate in $\ensuremath{\alpha}$ and $\ensuremath{\beta}$ chains having different values of the magnetic moments, which is quantitatively explained by taking the crystal field anisotropy into account. Above 2 T and below 2 K, an antiferromagneticlike structure with $\mathbf{k}=(0,0,1)$ is induced besides the $\mathbf{k}=\mathbf{0}$ structure. It shows a reentrant behavior and extends over a finite length scale. It occurs together with a broadening of the nuclear peaks, which suggests a field-induced distortion and magnetostriction effect.

FeII − III hydroxycarbonate Fe\(^{\rm II}_{4}\)Fe\(^{\rm III}_{2}\)(OH)12CO3, green rust GR(CO\(_{3}^{2-})\), reveals a ferrimagnetic behaviour. Moments that lie within two-dimensional cation layers are parallel for same species and antiparallel between FeII and FeIII. Respective ordering temperatures are 5.2 and 7 K. A sextet with distribution from 350 to 580 kOe for FeIII and an octet reflecting a mixture of states with field of 130 kOe and quadrupole splitting of −3.0 mm s − 1 for FeII are observed at 1.4 K. Ferric oxyhydroxycarbonate Fe\(^{\rm III}_{6}\)O12H8CO3 is ferromagnetic and displays at 4 K a sextet with field between 400 and 500 kOe (maximum at 480 kOe) and transition at 80 K. GR(CO\(_{3}^{2-})\) deprotonation gives magnetic domains with compositions at x = 1/3, 2/3 and 1 due to long range order.

The strongly correlated disordered phase of two highly frustrated pyrochlore magnets Gd2Ti2O7 and Gd2Sn2O7 is probed using electron-spin resonance in the temperature range 1.3-30 K. The deviation of the absorption line from the paramagnetic position nu=gamma H observed in both compounds below the Curie-Weiss temperature Theta(CW)similar or equal to 10 K suggests an opening up of a gap in the excitation spectra. On cooling to 1.3 K (which is above the ordering transition T-N similar or equal to 1.0 K) the resonance spectrum is transformed into a wideband of excitations with the gap amounting to Delta similar or equal to 26 GHz (1.2 K) in Gd2Ti2O7 and 18 GHz (0.8 K) in Gd2Sn2O7. The gaps increase linearly with the external magnetic field. For Gd2Ti2O7 this branch coexists with an additional nearly paramagnetic line absent in Gd2Sn2O7. These low-lying excitations with gaps, which are preformed above the ordering transition, may be interpreted as collective spin modes split by the single-ion anisotropy.

170Yb Mossbauer spectroscopy, temperature dependent X-ray, magnetisation and specific heat data are presented in the hexagonal intermetallic YbAl3C3, in order to shed light on the isostructural transition occurring near 80 K and to investigate the electronic state of the Yb ion above and below the transition. In the low temperature phase, we find that there occurs an atomic rearrangement in the hexagonal unit cell, leading to a strong symmetry lowering at the Yb site. We show that no magnetic ordering of the Yb3+ moments occurs down to 0.04 K, and we discuss this finding in terms of 4f-conduction electron hybridisation and geometric frustration.

We have identified a new compound of composition U 2 Fe 3 Ge, which crystallises in the hexagonal, C14 Laves phase MgZn 2 type structure. Fe and Ge occupy randomly the sites of the Zn sublattice. The lattice volume increases by about 4.3% compared to the parent cubic Laves phase compound UFe 2 . The magnetic properties of U 2 Fe 3 Ge have been probed by using magnetization, resistivity, heat capacity and 57 Fe Mossbauer spectroscopy. The compound orders ferromagnetically near 55 K in a state which is magnetically weaker than in UFe 2 . The latter is inferred from a lower saturated magnetic moment and hyperfine field at the Fe site. Resistivity shows a T 2 behaviour at low temperatures. Both the coefficient of the T 2 term in the resistivity and the Sommerfeld coefficient, γ , in U 2 Fe 3 Ge are slightly higher compared to that in UFe 2 .

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 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.