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International audience; The development of rapid, sensitive, portable and inexpensive early diagnostic techniques is a real challenge in the fields of health, defense and in the environment. The current global pandemic has also shown the need for such tests. The World Health Organization has defined ASSURED criteria (affordable, sensitive, specific, user-friendly, rapid and robust, equipment-free and deliverable to end-users) that field diagnostic tests must fulfill, which proves the real need in terms of public health. Giant magnetoresistance (GMR) sensors, which have flourished in a wide variety of spintronic applications (automobile industry, Information Technology, etc.), also have real potential in the field of health, particularly for the development of early diagnostic point-of-care devices. This work presents a new type of innovative biochip, consisting of GMR sensors arranged on both sides of a microfluidic channel which allow on the one hand to count magnetic objects one by one but also to better distinguish false positives (aggregates of beads, etc.) from labelled biological targets of interest by determining their magnetic moment. We present the operating principle of this new tool and its great potential as a versatile diagnostic test.

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.

Mastering and understanding the magnetic couplings between magnetic electrodes separated by organic layers are crucial for developing new hybrid spintronic devices. We study the magnetic exchange interactions in organic-inorganic heterojunctions and unveil the possibility of controlling the strength of the magnetic exchange coupling between two ferromagnetic electrodes across $\ensuremath{\pi}$-conjugated molecules' ($\ensuremath{\alpha}$-sexithiophene or para-sexiphenyl) ultrathin film. In $\mathrm{F}{\mathrm{e}}_{3}{\mathrm{O}}_{4}/\ensuremath{\pi}$-conjugated molecules/Co magnetic tunnel junctions, an antiferromagnetic interlayer exchange coupling with variable strength is observed according to the nature of the aromatic rings (thiophene or phenyl groups). The underlying physical mechanism is revealed by ab initio calculations relating the strength of magnetic coupling to the spin moment penetration into a molecular layer at the molecule/Co interface. The prospect that magnetic coupling between two ferromagnetic electrodes can be mediated and tuned by organic molecules opens different perspectives in the way magnetization of organic tunnel junctions or spin valves can be driven.

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.

MFe2O4 (M: Co2+, Ni2+) nanoparticles with different sizes and crystal-chemistry have been synthesized under solvothermal conditions. The inversion degree (x) of the mixed spinel structure (Fe1–xMx)[Fe1+xM1–x]O4 has been investigated by XAS and Mossbauer spectroscopies obtaining values between 0.20 and 0.30 in the case of Co-ferrite samples and between 0.00 and 0.16 for the Ni-ferrite ones. In order to have a better understanding of the superparamagnetic behavior of the nanosized samples, it was necessary to do high-temperature magnetic measurements. Although Co-ferrite samples present higher magnetization values, their effective superparamagnetic moment is similar to those found for the Ni-ferrite ones. This suggests that the dipolar interactions may be stronger in the Ni-ferrite system, probably due to a less-effective anisotropy that can be considered an impediment to the particle interactions. This fact is supported by the effective volume/single-particle volume ratio that presents similar values for b...

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.

‘Multiferroic’ materials possessing simultaneously magnetic and ferroelectric orders are scarce and most of them order at low temperatures. So far, bismuth ferrite, BiFeO 3 , is the only reliable room-temperature multiferroic: it is ferroelectric and antiferromagnetic. The absence of a net magnetisation in this compound is a problem when one wants to use the magneto-electric effect to address magnetic information with an electric field for potential applications in spintronic devices. We show here that β-NaFeO 2 is also a multiferroic material at room-temperature but with the most interesting extra property of showing weak ferromagnetism. This makes it a potentially very promising material for applications and a model compound for fundamental studies of the interaction between ferroelectricity and magnetism.

It is usually considered that nanoparticles synthesized by low-temperature routes present structural disorder, from extended defects to local rearrangements (e.g., vacancy ordering or inversion in spinel ferrites), that may severely impact their magnetic properties. In the present work, we have investigated the influence of postsynthesis thermal treatments on 7-nm-sized γ-Fe2O3 nanoparticles prepared by room temperature coprecipitation of ferric and ferrous salts in alkaline medium, followed by the dispersion of the preformed particles in a sol–gel silica binder. Such protected annealing in a refractory matrix prevents coalescence and growth, thus preserving the mean size and size distribution of the pristine particles. Structural characterizations show that heat treatments up to 1000 °C turned the raw grains into well-crystallized particles without transformation into hematite. This strategy thus allows accounting for the influence of structural rearrangements on magnetic properties at fixed particle siz...

Here, we report a phase-solution annealing-induced structural transition of 7 nm-Co nanocrystals from the fcc polycrystalline phase to the hcp single-crystalline phase. For any annealing temperature, contrary to what was down in our previous paper (Langmuir 2011, 27, 5014), the same solvent (octyl ether) is used preventing any change in adsorbates related to various solvents on the nanocrystal surface. A careful transmission electron microscopy study, combined with the electron diffraction, confirms the nanocrystal recrystallization mechanism. The annealing process results in neither coalescence nor oxidation. The converted nanocrystals can be easily manipulated and due to their low size dispersion self-organize on an amorphous-carbon-coated grid. Magnetic property investigations, keeping the same nanocrystal environment, show that the structural transition is accompanied by a significant increase in both the blocking temperature (to a near room-temperature value) and the coercivity.

Here we report on the preparation and structural, magnetic and electrical characterization of BiFeO3 and Bi0.9Ca0.1FeO3 ceramic multiferroic samples. We suggest that Ca-doping creates oxygen vacancies and destabilizes the BiFeO3 spiral magnetic structure. We also study resistive switching effects in Bi0.9Ca0.1FeO3 with metallic electrodes, finding that the appearance of the effect is dependant on the fabrication procedure of the metallic electrode. On the basis of these observations, we critically revise some assumptions in currently available models of resistive switching of complex oxides.

Zn ferrite nanoparticles ranging from 3 to 19 nm have been obtained under solvothermal conditions. In addition, two samples of ZnFe2O4/SiO2 nanocomposite of 11 and 20 nm ferrite particle size have ...

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.

In this work, the nature of the deposit found inside an industrial cooling circuit (which consists of a mixture of different iron containing phases) has been characterized in detail by 57Fe Mossbauer spectroscopy. Electron Paramagnetic Resonance spectroscopy was also used to check for the presence of other metals, mainly manganese and copper, detected by the Inductive Coupled Plasma method. We conclude that the deposit contains a large amount of Fe(III), probably consisiting of ferrihydrite nanoparticles and of goethite, either bulk or as large particles. It also contains traces of an Fe(II) species (about 3%), probably adsorbed on the iron oxides. Mn(II) and traces of Cu(II) are also present.

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.

International audience; A ferrimagnetic behaviour is observed in green rust GR(CO32−), the layered double hydroxide FeII–III hydroxycarbonate FeII4FeIII2(OH)12CO3, by using Mössbauer spectroscopy and susceptibility measurements. Moments lying within two-dimensional cation layers are parallel for same species and antiparallel between FeII and FeIII that lead to two magnetic ordering temperatures, 5.2 and 7 K, respectively. Mössbauer spectrum shows at 1.4 K a sextet with field distribution from 350 to 580 kOe for ferric ions, and an octet reflecting a mixture of states with field of 130 kOe and quadrupole splitting of −3.0 mm s−1 for ferrous ions. Ferric oxyhydroxycarbonate GR(CO32−)∗, FeIII6O12H8CO3, displays at 4 K a sextet with field distribution between 400 and 500 kOe. Transition is at around 80 K. Spectra of deprotonated GR(CO32−)∗ at a ferric molar fraction x = 0.88 display a ferric sextet behaving similarly with transition at around 70 K and two paramagnetic doublets, one ferrous and one ferric that remain at 4.2 K. This suggests that the deprotonation of GR(CO32−) gives magnetic domains of definite compositions at x = 1/3, 2/3 and 1 and long range order of FeIII ions along a translation a × √3 if a is the parameter of the cation hexagonal pavement is confirmed.

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.

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.

International audience; Inspired by metallic alloys in atomic solids, two distinct metallic nanoparticles are used, considered as “artificial metal atoms,” to engineer ordered binary nanoparticle alloys at the mesoscale, called binary supracrystals. Here, ferromagnetic 7.2 nm Co nanoparticles are used as large “A” site particles, while either ferromagnetic 4.6 nm Co or nonmagnetic 4.0 nm Ag nanoparticles are used as small “B” site particles to fabricate long-range ordered binary supracrystals with a stoichiometry of AB2 and AB13. The interparticle distances between 7.2 nm Co nanoparticles within the Co/Ag binary supracrystals can be tuned by a control of crystal structure from AB2 (CoAg2) to AB13 (CoAg13). A decrease of magnetic coupling between Co nanoparticles is observed as the Co–Co interparticle distance increases. Furthermore, by alloying 7.2 and 4.6 nm Co nanoparticles to form AB2 (CoCo2) binary supracrystals, a collective magnetic behavior of these two particle types, due to the dipolar interaction, is evidenced by observing a single peak in the zero-field-cooled magnetization curve. Compared with the CoAg2 binary supracrystals, a spin orientation effect in sublattice that reduces the dipolar interactions in the supracrystals is uncovered in CoCo2 binary supracrystals.

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.

57Fe Mossbauer spectra are presented from (H3O)Fe3(SO4)2(OH)6 (or H3O-jarosite), which is a model kagome antiferromagnet which features geometrical frustration and spin-glass-like behaviour. Dynamic scaling of the freezing temperature as a function of frequency is observed over a large frequency range, which indicates the presence of a spin-glass transition. A fast relaxation model between “up” and “down” states, separated by an energy gap, is presented to account for the shape of the Mossbauer spectra below the freezing temperature. From a calculation of the Electric Field Gradient tensor, it is suggested that H3O-jarosite is an XY-Heisenberg antiferromagnet, where the Fe3+ moments lie in the kagome planes.

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.

A novel nonaqueous route has been applied for the preparation of nanocrystalline magnetite. In a simple one-pot reaction process, iron(III) acetylacetonate was dissolved in benzyl alcohol and treated in an autoclave between 175 and 200 °C. This approach leads to monocrystalline magnetite particles with sizes ranging from 12 to 25 nm, as evidenced by X-ray analysis, HRTEM, and Raman and Mossbauer spectroscopy. The isolated particles can be redispersed either in polar or nonpolar solvents by coating them just after synthesis with undecanoic acid or dopamine. Simple sedimentation after redispersion in hexane can be used to lower the polydispersity of the sample.

Uniform FeCo magnetic nanoneedles of ?nm in width and axial ratio have been obtained by Co-coating haematite and subsequent hydrogen reduction in two steps. By this method, FeCo needle-like nanoparticles were obtained with Co contents up to 10% evenly distributed inside the particles. Higher Co contents up to 30% were incorporated by coating the Co-doped magnetite nanoneedles. No segregation of a secondary phase was observed in any case and the morphology of the particles was preserved without adding any extra element. Structural changes during the reduction process have been followed by several techniques. Although protected by an oxide layer, the final metal nanoparticles behave as single crystals, but they are composed of sub-crystals with the same crystallographic orientation and a mean diameter that decreases as the Co content increases. We found the highest reported saturation magnetization values for samples of similar size (180?emu?g?1). The evolution of the coercivity with the cobalt modification could arise from the changes of the microstructure and the contributions of shape and crystalline anisotropies. Time dependence magnetization measurements showed the thermal activation to be related to the presence of crystallographic sub-units within the nanoneedles.

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.

Yttria-coated FeCo nanoneedles with a Co content up to 30 mol % were obtained by thermal reduction of oxyhydroxide precursors. Incorporation of Co and Y elements, with the adequate sequential distribution, to the Fe-based oxyhydroxides was carried out by an oxidation−precipitation method followed by a template method. Special emphasis has been set to correlate the physicochemical characteristics with the magnetic properties of the obtained samples. In this way, Mossbauer and X-ray photoelectron spectroscopy have allowed us to determine that the FeCo metallic core of the particles consists of areas with Fe and Co atoms randomly distributed and also forming clusters. We also established that the improvement in coercivity in metallic particles coated with yttria instead alumina could be associated with the higher degree of thermal protection supplied by yttria. We found that the highest values of coercivity (1550 Oe) are obtained for the yttria-coated FeCo metallic samples with a Co content of 20 mol %. Fina...

Biocompatible magnetic dispersions have been prepared from γ-Fe2O3 nanoparticles (5 nm) synthesized by continuous laser pyrolysis of Fe(CO)5 vapours. The feasibility of using these dispersions as magnetic resonance imaging (MRI) contrast agents has been analysed in terms of chemical structure, magnetic properties, 1H NMR relaxation times and biokinetics. The magnetic nanoparticles were dispersed in a strong alkaline solution in the presence of dextran, yielding stable colloids in a single step. The dispersions consist of particle-aggregates 25 nm in diameter measured using transmission electron microscope and a hydrodynamic diameter of 42 nm measured using photon correlation spectroscopy. The magnetic and relaxometric properties of the dispersions were of the same order of magnitude as those of commercial contrast agents produced using coprecipitation. However, these dispersions, when injected intravenously in rats at standard doses showed a mono-exponential blood clearance instead of a biexponential one, with a blood half-life of 7 ± 1 min. Furthermore, an important enhancement of the image contrast was observed after the injection, mainly located at the liver and the spleen of the rat. In conclusion, the laser pyrolysis technique seems to be a good alternative to the coprecipitation method for producing MRI contrast agents, with the advantage of being a continuous synthesis method that leads to very uniform particles capable of being dispersed and therefore transformed in a biocompatible magnetic liquid.

Ellipsoidal metal nanoparticles about 200 nm in length and with different axial ratios were obtained by reduction with hydrogen of an iron oxide. These metal particles were stabilized without the presence of an antisintering and protecting layer of aluminium or yttrium oxide, giving rise to a significant improvement of the magnetic properties. The precursors were uniform ellipsoidal haematite particles synthesized by forced hydrolysis of iron perchlorate in the presence of urea and phosphate ions. A detailed characterization of the nanoparticles was carried out to correlate the microstructure of the haematite precursors with the structural and magnetic properties of the final metal particles. It was observed that the single-crystal character of the particles is preserved during the transformation of iron oxide to metal. The resulting metal particles consist of a metal core of α-Fe and an oxide layer about 5 nm thick, with a spinel structure. The magnetic properties of this material showed very high saturation magnetization () and coercivity values increasing from 1000 to 1200 Oe as the particle axial ratio increases. Measurements of the time dependence of the magnetization yielded activation volumes eight and five times smaller than the particle physical volumes, suggesting a mechanism of incoherent reversal of the magnetization.

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.

The intermetallic bronze YbPd3S4 is shown to be a heterogeneous mixed-valence system, by means of 170Yb Mossbauer spectroscopy and x-ray LIII-edge absorption and magnetic measurements. Two valence states coexist in this compound: Yb3+ and close-to-divalent Yb. The trivalent fraction (about 50%) undergoes a transition to magnetic ordering at about 2 K, with the Γ7 doublet as the ground crystal field state. The possibility of charge (or valence) ordering is discussed.

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.

Acicular metallic particles of different sizes were obtained by thermal treatment under a hydrogen flow of undoped and Al-doped goethite precursors. The acicular goethite particles were synthesized by the controlled oxidation−precipitation of iron (II) sulfate and aluminum nitrate solutions by addition of a base. Microstructural studies showed that the Al-doped goethite precursors consist of acicular particles with the aluminum mainly concentrated at the particle outer layers even though it was initially added during the synthesis. The mechanism for this rather atypical enrichment of aluminum can be explained in terms of the different stability of an Al-carbonate green rust phase formed during the goethite synthesis. Moreover, it was observed that the aluminum enrichment on the goethite particle surface is later enhanced during the reduction to iron. A detailed characterization of the final metal particles was carried out to correlate their chemical and magnetic properties with the structural properties o...

We report resistive switching effects in polycrystalline samples of the multiferroic Bi 0.9 Ca 0.1 FeO 3 with silver electrodes. Mossbauer spectroscopy shows that upon Ca-doping the Fe remains in a 3+ valence state, suggesting charge compensation through the creation of large amounts of oxygen vacancies. Electrical characterization shows that the oxide/metal resistance can be switched between high and low resistance states by applying voltage pulses. This process was shown to be forming free and a strong relaxation after switching was found. We rationalize our results by considering oxygen vacancies migration to and from the metal–oxide interface, resulting in variations of the Schottky potential barrier height that modulate the interface resistance.

The purple hydrogenperoxo species [Fe(bztpen)(OOH)]2+ (1) can be generated in methanol solution by treatment of [Fe(bztpen)Cl]3+ with a large excess of hydrogen peroxide [A. Hazell, C. J. McKenzie, L. P. Nielsen, S. Schindler, M. Weitzer, J. Chem. Soc., Dalton Trans.2002, 310−317]. Addition of 30 equivalents of triethylamine to a solution of [Fe(bztpen)(OOH)]2+ gives the [Fe(bztpen)(O2)]+ (2) species, in which it has been proposed that the peroxo ligand is coordinated in the η2 mode. We have synthesized both 100% 57Fe-enriched non-heme FeIII peroxo species and studied them by EPR and Mossbauer spectroscopy. Species 1 displays a typical S = 1/2 low-spin ferric EPR spectrum, the g values of which have been analysed in terms of the Griffith model [J. S. Griffith, Proc. R. Soc. London, A1956, 234, 23−36]. We have thus obtained insight into the electronic structure of 1. The EPR spectrum of 2 is characteristic of a high-spin FeIII species in a nearly axial ligand field. We have determined all Mossbauer parameters for both complexes by a numerical treatment of applied field Mossbauer data. Species 1 exhibits an isomer shift δ/Fe = 0.16 mm/s and a quadrupole splitting ΔEQ = −2.08 mm/s (T = 4.2 K). In the case of 2, we obtain an isomer shift δ/Fe = 0.63 mm/s and a quadrupole splitting ΔEQ = 1.12 mm/s (T = 4.2 K). These values are close to those published very recently by Bill et al. for [Fe(trispicMeen)(OOH)]2+ (3) and [Fe(trispicMeen)(η2−OO)]+ (4) species, where bztpen and trispicMeen ligands differ by a noncoordinating group [A. J. Simaan, F. Banse, J.-J. Girerd, K. Wieghardt, E. Bill, Inorg. Chem.2001, 40, 6538−6540]. We have also used the extension of the Griffith model developed by Lang et al. for Mossbauer spectroscopy [W. T. Oosterhuis, G. Lang, Phys. Rev. 1969, 178, 439−456] to evaluate the 57Fe hyperfine tensor of 1 and reproduce the experimental data. On comparison with the Mossbauer data available for η2-peroxo FeIII complexes, our Mossbauer study of 2 agrees well with a high-spin FeIII side-on peroxo complex. Very characteristic δ/Fe and ΔEQ ranges for hydrogenperoxo and side-on peroxo FeIII species are now available, which should aid in the detection of comparable species formed in biological systems. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

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.