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International audience; Cobalt ferrite nanoparticles (d ∼ 11 nm, σd = 0.5) produced by coprecipitation at room temperature are heat treated up to 1000 °C after a preliminary dispersion in a sol-gel silica matrix to avoid aggregation and coarsening. This protected annealing allows for a significant increase of the crystallinity of the particles, as demonstrated by combined x-ray diffraction and transmission electron microscopy experiments. A large increase in the coercive field value is reported, from 1.0 to 1.6 Tesla after annealing at 600 °C. This enhanced coercivity can be explained by the cumulative effect of an increased magnetic anisotropy and of a decreased saturation magnetization (Ms). Mössbauer spectroscopy experiments show that these evolutions of the Ms and anisotropy constant (K) values originate from a small increase in canting angles and in inversion degree, i.e. a displacement of Co2+ ions from tetrahedral to octahedral sites of the spinel lattice. This study emphasizes the little impact of an improved crystallinity on saturation magnetization and canting angle values in coprecipitated CoFe2O4 nanoparticles and highlights that the main source of magnetic disorder is associated with the distribution of Co and Fe within the cationic sites.

International audience; Control of interactions between nanomaterials and cells remains a biomedical challenge. A strategy is proposed to modulate the intralysosomal distribution of nanoparticles through the design of 3D suprastructures built by hydrophilic nanocrystals (NCs) coated with alkyl chains. The intracellular fate of two water‐dispersible architectures of self‐assembled hydrophobic magnetic NCs: hollow deformable shells (colloidosomes) or solid fcc particles (supraballs) is compared. These two self‐assemblies display increased cellular uptake by tumor cells compared to dispersions of the water‐soluble NC building blocks. Moreover, the self‐assembly structures increase the NCs density in lysosomes and close to the lysosome membrane. Importantly, the structural organization of NCs in colloidosomes and supraballs are maintained in lysosomes up to 8 days after internalization, whereas initially dispersed hydrophilic NCs are randomly aggregated. Supraballs and colloidosomes are differently sensed by cells due to their different architectures and mechanical properties. Flexible and soft colloidosomes deform and spread along the biological membranes. In contrast, the more rigid supraballs remain spherical. By subjecting the internalized suprastructures to a magnetic field, they both align and form long chains. Overall, it is highlighted that the mechanical and topological properties of the self‐assemblies direct their intracellular fate allowing the control intralysosomal density, ordering, and localization of NCs.

We compare here exchange bias (EB) properties of chemically synthesized core-shell nanoparticles (NPs), based either on a core of soft ferrite (MnFe2O4) or hard ferrite (CoFe2O4) protected by a maghemite shell (γ-Fe2O3). These NPs dispersed in acidic solutions are electrostatically stabilized, yielding to stable colloidal dispersions with a strong interparticle repulsion and negligible dipolar interactions in the probed range of temperatures. Field cooled (FC) magnetic hysteresis loops of non-textured frozen dispersions (with magnetic anisotropy axis of NPs distributed at random) and those of a powder based on the same NPs present a shift along the H-axis, expressing the coupling between the spin-ordered cores and the disordered surface layer of the NPs. The bias field is found to present a maximum, larger for NPs based on harder ferrite core. It is obtained for a cooling field of the order of one half of the anisotropy field, which is much larger for the CoFe2O4 cores than for MnFe2O4 ones. In powders, particles are in contact leading to an interparticle exchange which is not present in the dilute solutions where exchange bias properties are only due to an intraparticle exchange between core and surface. The thermal dependence of the bias field is well described by a reduced exponential behavior with a characteristic freezing temperature of about 8 K.

International audience; We report on diffuse neutron scattering experiments providing evidence for the presence of random strains in the quantum spin-ice candidate Pr2Zr2O7. Since Pr3+ is a non-Kramers ion, the strain deeply modifies the picture of Ising magnetic moments governing the low-temperature properties of this material. It is shown that the derived strain distribution accounts for the temperature dependence of the specific heat and of the spin-excitation spectra. Taking advantage of mean-field and spin-dynamics simulations, we argue that the randomness in Pr2Zr2O7 promotes a new state of matter, which is disordered yet characterized by short-range antiferroquadrupolar correlations, and from which emerge spin-ice-like excitations. Thus, this study gives an original research route in the field of quantum spin ice.

We study the evolution of helical magnetism in MnGe chiral magnet upon partial substitution of Mn for $3d$-Co and $4d$-Rh ions. At high doping levels, we observe spin helices with very long periods---more than ten times larger than in the pure compound---and sizable ordered moments. This behavior calls for a change in the energy balance of interactions leading to the stabilization of the observed magnetic structures. Strikingly, neutron scattering unambiguously shows a double periodicity in the observed spectra at $x=0.5$ and g0.2 for Co- and Rh-doping, respectively. In analogy with observations made in smectic liquid crystals, we suggest that it may reveal the presence of magnetic ``twist grain boundary'' phases, involving a dense short-range correlated network of magnetic screw dislocations. The dislocation cores are here tentatively described as smooth textures, made of nonradial double-core skyrmions.

International audience; We investigate the local structure of nanoparticles based on a manganese ferrite core surrounded or not by a maghemite layer obtained after hydrothermal surface treatment. Results of X-ray powder diffraction (XRD) and neutron powder diffraction (NPD) measurements are crossed with those of in-field Mö ssbauer spectroscopy and X-ray absorption spectroscopy (XANES/EXAFS) to study the valence state of Mn ions and the cation distribution at interstitial sites of the core−shell nano-particle structure. Linear combination fitting of XANES data clearly indicates the existence of mixed valence states of Mn cations in the Mn ferrite phase. As a direct consequence, it induces nonequilibrium cation distributions in the nanoparticle core with the presence of a large amount of Mn cations at octahedral sites. The quantitative results of the inversion degree given by NPD, Mö ssbauer spectroscopy measurements, and EXAFS are in good accordance. It is also shown that both the proportions of each oxidation degree of Mn ions and their location at tetrahedral or octahedral sites of the spinel nanocrystal core can be modified by increasing the duration of the surface treatment.

International audience; Blue-emitting BaMgAl 10 O 17 :Eu 2þ (BAM:Eu), suitable for applications in a next generation of Hg-free lamps based on UV LEDs, was prepared by a microwave induced solution combustion synthesis, using urea as combustion fuel and nitrates as oxidizers. Purity control of the as-synthesized blue phosphor was undertaken by a washing step followed by a reduction one. Structural and morphological properties of the outcoming phosphors have been considered. Synthesis process allows producing a well-crystallized and nanostructured BAM phase within only few minutes. The influence of reduction treatment on the relative amounts of Eu 2þ /Eu 3þ in our samples has been investigated through an original study by magnetization and M€ ossbauer spectroscopy. Furthermore, a complete optical study has been carried out and allowed us to determine the europium localization in the three possible sites in BAM matrix. The percentage of Eu 2þ increased twofold after the reduction treatment, entailing an increase in the lumi-nescence efficiency upon UV excitation. Finally, temperature-dependent luminescence of combustion-derived powders has been studied till 170 C and compared to that of commercial BAM:Eu. MISCS-derived phosphors present a higher thermal stability than commercial one: whereas the emission efficiency of this last was reduced by 64%, the one of combustion-derived BAM:Eu experienced an only 12% decline. Furthermore, while commercial BAM suffered from a severe blue-shift with increasing temperature , our phosphors keep its color quality with a good stability of the photometric parameters.

International audience; In the orthorhombic manganites o-RMnO3, where R is a heavy rare earth (R = Gd-Yb), the Mn3+ sublattice is known to undergo two magnetic transitions. The low temperature phase has an antiferromagnetic structure (collinear or elliptical), which has been well characterized by neutron diffraction in most of these compounds. The intermediate phase, occurring in a narrow temperature range (a few K), is documented for R = Gd-Ho as a collinear modulated structure, incommensurate with the lattice spacings. We report here on a 57Fe M\"ossbauer study of 2% 57Fe doped o-YbMnO3, where the spin only Fe3+ ion plays the role of a magnetic probe. From the analysis of the shape of the magnetic hyperfine M\"ossbauer spectra, we show that the magnetic structure of the intermediate phase in o-YbMnO3 (38.0 K < T < 41.5 K) is also modulated and incommensurate.

In this work, we revisit the crystal field acting on the non-Kramers Pr3+ ion (4f2) in the quantum spin ice candidate Pr2Zr2O7 using both a standard calculation restricted to the ground spin-orbit multiplet and intermediate coupling states in the full basis of the f 2 configuration. Analysis of the thermal variation of the polycrystal magnetic susceptibility and of the local susceptibilities χ⊥ and χ// determined by means of polarized neutron diffraction experiments reveals that the effective antiferromagnetic exchange is strongly depleted at low temperature with respect to its high-temperature value.We then discuss the influence of crystal field imperfections arising from residual strains, which are especially important for a non-Kramers ion. We find that they are an essential ingredient to account for the very low temperature M(H) magnetization curves, showing that the saturation is not achieved even at 8 T. Furthermore, as possible candidates to qualitatively understand the Curie-like behavior observed below 0.5 K, we discuss the influence of the magnetic hyperfine interaction.

We present an experimental study of the quantum spin ice candidate pyrochlore compound ${\mathrm{Pr}}_{2}{\mathrm{Zr}}_{2}{\mathrm{O}}_{7}$ by means of magnetization measurements, specific heat, and neutron scattering up to 12 T and down to 60 mK. When the field is applied along the $[111]$ and $[1\overline{1}0]$ directions, $\mathbf{k}=0$ field-induced structures settle in. We find that the ordered moment rises slowly, even at very low temperature, in agreement with macroscopic magnetization. Interestingly, for $H\ensuremath{\parallel}[1\overline{1}0]$, the ordered moment appears on the so-called $\ensuremath{\alpha}$ chains only. The spin excitation spectrum is essentially inelastic and consists in a broad flat mode centered at about 0.4 meV with a magnetic structure factor which resembles the spin ice pattern. For $H\ensuremath{\parallel}[1\overline{1}0]$ (at least up to 2.5 T), we find that a well-defined mode forms from this broad response, whose energy increases with $H$, in the same way as the temperature of the specific-heat anomaly. We finally discuss these results in the light of mean field calculations and propose an interpretation where quadrupolar interactions play a major role, overcoming the magnetic exchange. In this picture, the spin ice pattern appears shifted up to finite energy because of those interactions. We then propose a range of acceptable parameters for ${\mathrm{Pr}}_{2}{\mathrm{Zr}}_{2}{\mathrm{O}}_{7}$ that allow to reproduce several experimental features observed under field. With these parameters, the actual ground state of this material would be an antiferroquadrupolar liquid with spin-ice-like excitations.

We present here a neutron diffraction study, both in zero field and as a function of magnetic field, of the magnetic structure of the tetragonal intermetallic ${\mathrm{EuNiGe}}_{3}$ on a single crystalline sample. This material is known to undergo a cascade of transitions, first at 13.2 K towards an incommensurate modulated magnetic structure, then at 10.5 K to an antiferromagnetic structure. We show here that the low-temperature phase presents a spiral moment arrangement with wave vector $\mathbit{k}=(\frac{1}{4},\ensuremath{\delta},0)$. For a magnetic field applied along the tetragonal $c$ axis, the square root of the scattering intensity of the (1 0 1) reflection matches very well the complex metamagnetic behavior of the magnetization along $c$ measured previously. For the magnetic field applied along the $b$ axis, two magnetic transitions are observed below the transition to a fully polarized state.

We have studied by muon spin resonance ({\mu}SR) the helical ground state and fluctuating chiral phase recently observed in the MnGe chiral magnet. At low temperature, the muon polarization shows double period oscillations at short time scales. Their analysis, akin to that recently developed for MnSi [A. Amato et al., Phys. Rev. B 89, 184425 (2014)], provides an estimation of the field distribution induced by the Mn helical order at the muon site. The refined muon position agrees nicely with ab initio calculations. With increasing temperature, an inhomogeneous fluctuating chiral phase sets in, characterized by two well separated frequency ranges which coexist in the sample. Rapid and slow fluctuations, respectively associated with short range and long range ordered helices, coexist in a large temperature range below T$_{N}$ = 170 K. We discuss the results with respect to MnSi, taking the short helical period, metastable quenched state and peculiar band structure of MnGe into account., Comment: 13 pages, 11 figures

International audience; Lepidocrocite, a widespread environmentally relevant iron oxyhydroxide, has been investigated for decades using 57Fe Mössbauer spectroscopy and magnetic measurements. However, a coherent and comprehensive interpretation of all the data is still lacking due to seemingly contradictory interpretations. On one hand, temperature dependence of magnetic susceptibility and Mössbauer spectra resemble those of superparamagnetic nanoparticles with diameters less than 10 nm even though physically particles are lath-shaped with lengths on the order of 100-300 nm. On the other hand, in-field Mössbauer spectra show that lepidocrocite is an antiferromagnet and becomes paramagnetic above 50-70 K, a temperature close to the blocking temperature deduced from susceptibility data. The present study investigates a well-characterized synthetic sample of lepidocrocite, includes modelling of Mössbauer spectra and dc and ac magnetization data, and proposes a solution to this paradox. The new data are coherent with the presence of two entities in lepidocrocite: a bulk antiferromagnetic matrix and sparse ferrimagnetic nanosized inclusions (d = 3.4 nm), akin to maghemite, embedded within. The presence of nanosized ferrimagnetic inclusions is confirmed for the first time by Mössbauer spectroscopy.

This paper reports original results on the synthesis of Carbon/Nitrogen/Iron-based Oxygen Reduction Reaction (ORR) electrocatalysts by CO2 laser pyrolysis. Precursors consisted of two different liquid mixtures containing FeOOH nanoparticles or iron III acetylacetonate as iron precursors, being fed to the reactor as an aerosol of liquid droplets. Carbon and nitrogen were brought by pyridine or a mixture of pyridine and ethanol depending on the iron precursor involved. The use of ammonia as laser energy transfer agent also provided a potential nitrogen source. For each liquid precursor mixture, several syntheses were conducted through the step-by-step modification of NH3 flow volume fraction, so-called R parameter. We found that various feature such as the synthesis production yield or the nanomaterial iron and carbon content, showed identical trends as a function of R for each liquid precursor mixture. The obtained nanomaterials consisted in composite nanostructures in which iron based nanoparticles are, to varying degrees, encapsulated by a presumably nitrogen doped carbon shell. Combining X-ray diffraction and Mossbauer spectroscopy with acid leaching treatment and extensive XPS surface analysis allowed the difficult question of the nature of the formed iron phases to be addressed. Besides metal and carbide iron phases, data suggest the formation of iron nitride phase at high R values. Interestingly, electrochemical measurements reveal that the higher R the higher the onset potential for the ORR, what suggests the need of iron-nitride phase existence for the formation of active sites towards the ORR.

The fluorescence and phosphorescence properties of Europium-doped M Al 2 Si 2 O 8 ( M =Ca, Sr, Ba) are reinvestigated and discussed on the basis of the propensity of an activator to agglomerate with an oxygen vacancy. Due to a stronger attraction of the anion vacancy towards Eu 2+ cations going from BaAl 2 Si 2 O 8 to SrAl 2 Si 2 O 8 and CaAl 2 Si 2 O 8 host lattices, the interpretation of the fluorescence spectra turns out to be less trivial in the Ca and Sr host lattices than in the Ba one and requests the account for Eu 2+ cations lying at alkaline-earth sites with or without vacancy in their neighborhood. Phosphorescence in these compounds is highlighted.

We have examined the magnetic properties of the heavy electron compounds YbAgGe and YbPtIn by 170Yb Mossbauer spectroscopy down to 0.1 K, and the crystal field properties of YbAgGe by Perturbed Angular Correlations (PAC) measurements up to 900 K. In YbAgGe, we show that each of the two magnetically ordered phases below 0.8 K involves a specific incommensurate modulation of the Yb moment. An analysis of existing low temperature specific heat data suggests the persistence of fluctuations of the correlated Yb spins down to 0.1 K. The PAC data allow to discriminate among proposed Yb3+ crystal field level schemes. In YbPtIn, we show that the low temperature magnetic order phase has an antiferro-para structure, where zero moment Yb ions coexist with large moment ones, and that a 90° moment reorientation occurs at 1.4 K.

We have studied the spin correlations with $\bf{k}$= ($\frac12$, $\frac12$, $\frac12$) propagation vector which appear below 0.4\, K in \tbti\ spin liquid by combining powder neutron diffraction and specific heat on Tb$_{2+x}$Ti$_{2-x}$O$_{7+y}$ samples with $x$=0, 0.01, -0.01. The $\bf{k}$= ($\frac12$, $\frac12$, $\frac12$) order clearly appears on all neutron patterns by subtracting a pattern at 1.2(1)\,K. Refining the subtracted patterns at 0.07\,K yields two possible spin structures, with spin-ice-like and monopole-like correlations respectively. Mesoscopic correlations involve Tb moments of 1 to 2 \mub\ ordered on a length scale of about 20 \AA. In addition, long range order involving a small spin component of 0.1 to 0.2 \mub\ is detected for the $x$= 0 and 0.01 samples showing a peak in the specific heat. Comparison with previous single crystals data suggests that the ($\frac12$, $\frac12$, $\frac12$) order settles in through nanometric spin textures with dominant spin ice character and correlated orientations, analogous to nanomagnetic twins.

We present detailed investigations on single crystals of quaternary EuRhAl4Si2 and EuIrAl4Si2. The two compounds order antiferromagnetically at TN1 = 11.7 and 14.7 K, respectively, each undergoing two magnetic transitions. The magnetic properties in the ordered state present a large anisotropy despite Eu2+being an S-state ion for which the single-ion anisotropy is expected to be weak. Two features in the magnetization measured along the c-axis are prominent. At 1.8 K, a ferromagnetic-like jump occurs at very low field to a value one third of the saturation magnetization (1/3 M0) followed by a wide plateau up to 2 T for Rh and 4 T for Ir-compound. At this field value, a sharp hysteretic spin-flop transition occurs to a fully saturated state (M0). Surprisingly, the magnetization does not return to origin when the field is reduced to zero in the return cycle, as expected in an antiferromagnet. Instead, a remnant magnetization 1/3 M0 is observed and the magnetic loop around the origin shows hysteresis. This suggests that the zero field magnetic structure has a ferromagnetic component and we present a model with up to third neighbor exchange and dipolar interaction which reproduces the magnetization curves and hints to an “up-up-down” magnetic structure in zero field.

International audience; EuAl2Si2 is known to crystallize in the CaAl2Si2-type trigonal structure. We have grown single crystals of EuAl2Si2 by flux method, using Al-Si eutectic (87.8% Al) as self-flux, and investigated their anisotropic magnetic properties by means of magnetization, electrical resistivity and heat capacity in zero and applied magnetic fields, and 151Eu Mössbauer spectroscopy. Magnetic susceptibility data show an antiferromagnetic transition at TN = 33.3 K in agreement with the previously reported value on polycrystalline sample. The isothermal magnetization at 2 K measured along and perpendicular to the c-axis shows anisotropic behaviour, which is rather unexpected as Eu2+ is an S-state ion. The spin flip fields along the two directions are 2.8 and 4.8 T, respectively, while two closely spaced spin-flop transitions in the ab-plane are observed near 1.4 and 1.6 T. The electrical resistivity shows an upturn between TN and 60 K as the temperature is lowered below ~ 60 K, suggesting the presence of antiferromagnetic correlations in the paramagnetic state. Magnetoresistivity at 2 K in 14 T is nearly 1070 % for H // [0001]. The results of heat capacity and 151Eu Mössbauer spectroscopy are in conformity with a bulk transition at 33.3 K.

We report the magnetic properties of two Eu based compounds, single crystalline EuIrGe$_3$ and EuRhGe$_3$, inferred from magnetisation, electrical transport, heat capacity and $^{151}$Eu M\"{o}ssbauer spectroscopy. These previously known compounds crystallise in the non-centrosymmetric, tetragonal, $I4mm$, BaNiSn$_3$-type structure. Single crystals of EuIrGe$_3$ and EuRhGe$_3$ were grown using high temperature solution growth method using In as flux. EuIrGe$_3$ exhibits two magnetic orderings at $T_{\rm N1}$ = 12.4 K, and $T_{\rm N2}$ = 7.3 K. On the other hand EuRhGe$_3$ presents a single magnetic transition with a $T_{\rm N}$ = 12 K. $^{151}$Eu M\"{o}ssbauer spectra present evidence for a cascade of transitions from paramagnetic to incommensurate amplitude modulated followed by an equal moment antiferromagnetic phase at lower temperatures in EuIrGe$_3$, the transitions having a substantial first order character. On the other hand the $^{151}$Eu M\"{o}ssbauer spectra at 4.2 and 9 K in EuRhGe$_3$ present evidence of a single magnetic transition. In both compounds a superzone gap is observed for the current density $J\parallel$ [001], which enhances with transverse magnetic field. The magnetisation measured up to 14 T shows the occurrence of field induced transitions, which are well documented in the magnetotransport data as well. The magnetic phase diagram constructed from these data is complex, revealing the presence of many phases in the $H-T$ phase space.

A tungstated zirconia (WZ) catalyst with iron promoter used for the conversion of n-pentane into isopentane has been characterized by Mössbauer spectroscopy. The Mössbauer spectra have been recorded in zero magnetic field in the temperature range 0.05-295 K and with a magnetic field up to 7 T between 4.2 and 50 K. Both the recording of Mössbauer spectra with an applied magnetic field and at extremely low temperature allowed for the demonstration that iron is present in the catalysts as (i) hematite (alpha-Fe2O3) particles a few 10 nm in size, (ii) very small oligomeric Fe(III) species, probably in solid solution in zirconia, and (iii) Fe(III) oxide clusters showing magnetic ordering, probably embedded in the first surface layer and thus forming "rafts". These latter clusters form two ensembles with quite different sizes: one with diameters of about 3 nm, the other with diameters larger than 30 nm. These results are in agreement with those recently obtained by X-ray absorption spectroscopy and electron paramagnetic resonance.

Single-ion anisotropy is of importance for the magnetic ordering of the frustrated pyrochlore antiferromagnets Gd2Ti2O7 and Gd2Sn2O7. The anisotropy parameters for the Gd2Sn2O7 were measured using the electron spin resonance (ESR) technique. The anisotropy was found to be of the easy plane type, with the main constant D=140mK. This value is 35% smaller than the value of the corresponding anisotropy constant in the related compound Gd2Ti2O7.

We report $^{35}\mathrm{Cl}$ NMR, ESR, $\ensuremath{\mu}\mathrm{SR}$, and specific-heat measurements on the $S=\frac{1}{2}$ frustrated kagome magnet kapellasite $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Cu}}_{3}{\text{Zn(OH)}}_{6}{\mathrm{Cl}}_{2}$, where a gapless spin-liquid phase is stabilized by a set of competing exchange interactions. Our measurements confirm the ferromagnetic character of the nearest-neighbor exchange interaction ${J}_{1}$ and give an energy scale for the competing interactions $|J|\ensuremath{\sim}10$ K. The study of the temperature-dependent ESR line shift reveals a moderate symmetric exchange anisotropy term $D$, with $|D/J|\ensuremath{\sim}3$%. These findings validate a posteriori the use of the ${J}_{1}\text{\ensuremath{-}}{J}_{2}\text{\ensuremath{-}}{J}_{d}$ Heisenberg model to describe the magnetic properties of kapellasite [Bernu et al., Phys. Rev. B 87, 155107 (2013)]. We further confirm that the main deviation from this model is the severe random depletion of the magnetic kagome lattice by 27%, due to Cu/Zn site mixing, and specifically address the effect of this disorder by $^{35}\mathrm{Cl}$ NMR, performed on an oriented polycrystalline sample. Surprisingly, while being very sensitive to local structural deformations, our NMR measurements demonstrate that the system remains homogeneous with a unique spin susceptibility at high temperature, despite a variety of magnetic environments. Unconventional spin dynamics is further revealed by NMR and $\ensuremath{\mu}\mathrm{SR}$ in the low-$T$, correlated, spin-liquid regime, where a broad distribution of spin-lattice relaxation times is observed. We ascribe this to the presence of local low-energy modes.

Pharmaceutical grade magnetic colloidal dispersions have been prepared from iron alloys synthesized by laser pyrolysis. The colloids were obtained by simultaneous dispersion and coating of the particles with dextran in a strong alkaline solution. Both powders and dispersions have been analyzed in terms of microstructural characteristics, chemical composition and magnetic properties. The powders consist of uniform spherical nanoparticles (12 nm of diameter) showing a metallic core encapsulated into an iron-oxide shell. On the other hand, the colloidal dispersions consist of magnetic particles-aggregates with hydrodynamic sizes of � 75 nm. Magnetic resonance images of rats were taken after the intravenously administration of the Fe colloidal dispersions, and compared with those obtained using a commercial iron oxide magnetic resonance imaging contrast agent. The results showed a contrast improvement of 60% in the liver with respect to the commercial sample, which suggests that this product could be a suitable contrast agent for NMR imaging of liver and spleen. r 2005 Elsevier Ltd. All rights reserved.

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 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.

Examples of materials where an ``order by disorder'' mechanism is at play to select a particular ground state are scarce. It has recently been proposed, however, that the antiferromagnetic XY pyrochlore ${\mathrm{Er}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$ reveals a most convincing case of this mechanism, with the observation of a spin gap at zone centers having recently been interpreted as a corroboration of this physics. Here we argue, however, that the anisotropy generated by the interaction-induced admixing between the crystal-field ground and excited levels provides for an alternative mechanism. It especially predicts the opening of a spin gap of about 15 $\ensuremath{\mu}\mathrm{eV}$, which is of the same order of magnitude as the one observed experimentally. We report high-resolution inelastic neutron scattering data which can be well understood within this scenario.

International audience; Single crystals of EuNiGe 3 crystallizing in the non-centrosymmetric BaNiSn 3-type structure have been grown using In flux, enabling us to explore the anisotropic magnetic properties which was not possible with previously reported polycrystalline samples. The EuNiGe 3 single crystalline sample is found to order antiferromagnetically at 13.2 K as revealed from the magnetic susceptibility, heat capacity and electrical resistivity data. The low temperature magnetization M(H) is distinctly different for field parallel to ab-plane and c-axis; the ab-plane magnetization varies nearly linearly with field before the occurrence of an induced ferromagnetic phase (spin-flip) at 6.2 Tesla; on the other hand M(H) along the c-axis is accompanied by two metamagnetic transitions followed by a spin-flip at 4.1 T. A model including anisotropic exchange and dipole-dipole interactions reproduces the main features of magnetization plots but falls short of full representation. (H,T) phase diagrams have been constructed for the field applied along the principal directions. From the 151 Eu Mössbauer spectra, we determine that the 13.2 K transition leads to an incommensu-rate antiferromagnetic intermediate phase followed by a transition near 10.5 K to a commensurate antiferromagnetic configuration.

We present a 57Fe M\"ossbauer spectroscopy study of the two incommensurate magnetic phases in the multiferroic material FeVO4. We devise lineshapes appropriate for planar elliptical and collinear modulated magnetic structures and show that they reproduce very well the M\"ossbauer spectra in FeVO4, in full qualitative agreement with a previous neutron diffraction study. Quantitatively, our spectra provide precise determinations of the characteristics of the elliptical and modulated structures which are in good agreement with the neutron diffraction results. We find that the hyperfine field elliptical modulation persists as T goes to 0, which we attribute to an anisotropy of the hyperfine interaction since a moment modulation is forbidden at T=0 for a spin only ion like Fe3+., Comment: 14 pages, 5 figures

We have studied the MnGe chiral magnet below ${T}_{\text{N}}=170$ K, by magnetic measurements, M\"ossbauer spectroscopy, and by neutron diffraction at ambient and under nonhydrostatic pressure. At ambient pressure, we observe the coexistence of two magnetic phases belonging to the same crystal phase in a large temperature range (down to 100 K) below ${T}_{\mathrm{N}}$: ferromagnetically correlated rapidly fluctuating spins coexist with frozen spins involved in the helical order. Applying a uniaxial pressure component induces a strong magnetic texture, where most of the helical axes reorient along the stress axis. The magnetic texture persists in the fluctuating chiral state up to ${T}_{\mathrm{N}}$. Our results suggest that the zero field ground state at ambient pressure is a multidomain state consisting of helical domains with random orientations rather than a three-dimensional skyrmion lattice. They show the presence of an unusually broad transition to paramagnetism with a dynamical phase separation triggered by temperature.

The magnetic behaviour of nanoparticles of antiferromagnetic artificial ferritin, with a mean Fe loading of 410 atoms per core, has been investigated by 57Fe Mossbauer absorption spectroscopy down to very low temperature (34 mK ) and by magnetometry down to 2.5 K . In previous experiments of frequency-dependent magnetic susceptibility χ(ω) and magnetic noise S(ω) performed at 25 mK in similar artificial ferritin particles, a resonance observed at a frequency of about 10 8 Hz was claimed to be due to a macroscopic quantum coherent state. In this work, we measured on the one hand the parameters which enter the theoretical expression for the tunnel splitting and for the incoherent tunnel fluctuation rate, namely the anisotropy energy density in ferritin, the transverse antiferromagnetic susceptibility and the excess magnetic moment due to uncompensated spins. We show that, using these measured values, the tunnel splitting is expected to be orders of magnitude lower than claimed in the above mentioned χ(ω) experiment. On the other hand, assuming that the tunnel splitting has the value claimed in the above mentioned χ(ω) experiments, we performed a search for incoherent tunnel fluctuations in these artificial ferritin particles by performing a 57Fe Mossbauer spectrum at 34 mK . Mossbauer spectroscopy on 57Fe has a “window” of measurement of fluctuation frequencies centered around 10 8 Hz , and we show that the expected signature of incoherent tunnel fluctuations is absent from the 34 mK Mossbauer spectrum. These experiments cast a doubt about the previous observation of a macroscopic quantum coherent state in ferritin.

We present measurements in the YbCu5-xAlx series, down to the 50 mK range, using 170Yb Mossbauer absorption spectroscopy and magnetisation measurements. In this series, the hybridisation between the Yb 4f electrons and the conduction electrons is known to decrease as the Al content x increases. We apply the variational solution of the impurity Kondo problem to the interpretation of our data. We show that the Kondo temperature can be derived from the measured 4f quadrupole moment and, for the magnetically ordered compounds (\(\)), we obtain the exchange energy as a function of the Al content. Our findings are in general agreement with Doniach's model describing the onset of magnetic ordering according to the relative values of the Kondo and exchange energy scales.

Specific heat and muon spin rotation and relaxation data are presented for two geometrically frustrated systems: Gd2Ti2O7 which antiferromagnetically orders and Yb2Ti2O7 which presents dynamical short range correlations at low temperature. The muon data help to characterize the spin dynamics of these two compounds.

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.

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.

Er${}_{2}$Sn${}_{2}$O${}_{7}$ is considered, together with Er${}_{2}$Ti${}_{2}$O${}_{7}$, as a realization of the $XY$ antiferromagnet on the pyrochlore lattice. We present magnetization measurements confirming that Er${}_{2}$Sn${}_{2}$O${}_{7}$ does not order down to 100 mK but exhibits a freezing below 200 mK. Our neutron scattering experiments evidence the strong $XY$ character of the Er${}^{3+}$moment and point out the existence of short-range correlations in which the magnetic moments are in peculiar configurations, the Palmer-Chalker states, predicted theoretically for an $XY$ pyrochlore antiferromagnet with dipolar interactions. Our estimation of the Er${}_{2}$Sn${}_{2}$O${}_{7}$ parameters confirm the role of the latter interactions on top of relatively weak and isotropic exchange couplings.

The dynamical magnetic correlations in ${\mathrm{Tb}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$ have been investigated using polarized inelastic neutron scattering. Dispersive excitations are observed, emerging from pinch points in reciprocal space and characterized by an anisotropic spectral weight. Anomalies in the crystal field and phonon excitation spectrum at Brillouin zone centers are also reported. These findings suggest that Coulomb phases, although they present a disordered ground state with dipolar correlations, allow the propagation of collective excitations. They also point out a strong spin-lattice coupling, which likely drives effective interactions between the $4f$ quadrupolar moments.

The vanadium oxyfluoride [NH(4)](2)[C(7)H(14)N][V(7)O(6)F(18)] (DQVOF) is a geometrically frustrated magnetic bilayer material. The structure consists of S = 1/2 kagome planes of V(4+) d(1) ions with S = 1 V(3+) d(2) ions located between the kagome layers. Muon spin relaxation measurements demonstrate the absence of spin freezing down to 40 mK despite an energy scale of 60 K for antiferromagnetic exchange interactions. From magnetization and heat capacity measurements we conclude that the S = 1 spins of the interplane V(3+) ions are weakly coupled to the kagome layers, such that DQVOF can be viewed as an experimental model for S = 1/2 kagome physics, and that it displays a gapless spin liquid ground state.

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.

We have studied the low-energy spin dynamics between 4.6 and 0.07 K in a Tb${}_{2}$Ti${}_{2}$O${}_{7}$ single-crystal sample by means of inelastic neutron scattering experiments. The spectra consist in a dual response, with a static and an inelastic contribution, showing striking $Q$ dependencies. We propose an interpretation involving an anisotropic exchange interaction in combination with a breaking of the threefold symmetry at the rare earth site. Simulations of the $Q$-dependent scattering in the random phase approximation account well for the inelastic response.

We present local probe results on the honeycomb lattice antiferromagnet Ba(3)CuSb(2)O(9). Muon spin relaxation measurements in a zero field down to 20 mK show unequivocally that there is a total absence of spin freezing in the ground state. Sb NMR measurements allow us to track the intrinsic susceptibility of the lattice, which shows a maximum at around 55 K and drops to zero in the low-temperature limit. The spin-lattice relaxation rate shows two characteristic energy scales, including a field-dependent crossover to exponential low-temperature behavior, implying gapped magnetic excitations.

Ho${}_{2}$Ti${}_{2}$O${}_{7}$ and Dy${}_{2}$Ti${}_{2}$O${}_{7}$ spin ices exhibit elementary excitations akin to magnetic monopoles. Here, we focus on Tb${}_{2}$Ti${}_{2}$O${}_{7}$ spin liquid, where correlated magnetic moments keep fluctuating down to very low temperatures. Using a monopole picture, we have reanalyzed the field-induced magnetic structure previously determined by neutron diffraction in Tb${}_{2}$Ti${}_{2}$O${}_{7}$. We show that under a high field applied along a [110] direction, Tb${}_{2}$Ti${}_{2}$O${}_{7}$ orders as a three-dimensional arrangement of monopole and antimonopole double layers. In contrast, Ho${}_{2}$Ti${}_{2}$O${}_{7}$ spin ice in the same conditions behaves as a monopole-free state. By symmetry analysis, we derived the distortions compatible with the observed magnetic structure of Tb${}_{2}$Ti${}_{2}$O${}_{7}$, which can be related to the appearance of the double-layered monopolar order.

[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.

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.

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.