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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.

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.

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.

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

Measurements by 170Yb Mossbauer spectroscopy down to 0.025 K and by neutron diffraction down to 1.7 K have been performed in the heavy electron compound YbPtAl (TN 5.8 K). At 0.025 K, the Mossbauer spectrum evidences a non-square modulation of the magnetic moment, incommensurate with the lattice spacings. This is the first clear example of a Kramers local moment Yb-based system where the Kondo singlet ground state allows a modulated magnetic structure to persist to T → 0 K. The 3rd- and 5th-order harmonics of the modulation could be measured, and their thermal variation was followed from 0.025 K to 5 K. An abrupt decrease of both harmonics at 3 K is concomitant with a peak in the specific heat, suggesting a first-order phase transition where the modulation transforms into a pure sine wave.

Positive muon spectroscopy (μSR) and 170Yb Mossbauer absorption measurements have been performed in the cubic Kondo insulator YbB12 down to the 50 mK temperature range. We observe a paramagnetic fluctuation mode at low temperature, with a weak moment amplitude ( ≈ 10−2 μB) and a slow fluctuation frequency ( ≈ 60 MHz) which remains constant between 0.04 K and 4.2 K. No indication of a magnetic phase transition is found down to 0.04 K.

We report muon spin rotation measurements of the temperature dependence and anisotropy of the magnetic field penetration lengths in the heavy fermion superconductor . We observe a strong axial anisotropy. At 0.05 K we obtain for the penetration length parallel and perpendicular to the c axis (150) A and (130) A, respectively. at low temperatures excludes a superconducting order parameter in the B phase with only a line of nodes in the equatorial plane of the Fermi surface. The combined analysis of and measured in the B phase favours an hybrid order parameter with point nodes at the poles and a line of nodes at the equatorial plane. The A phase is characterized by a larger density of nodes than the B phase.

We present a positive-muon spectroscopy study of the cubic Kondo lattice , where the crystal-field splittings of the ion amount to a few tens of K. The transition from the paramagnetic to the antiferromagnetic state is detected at . The zero-field data provide evidence of an unusual temperature dependence of the mean nuclear dipole field at the muon site. The longitudinal relaxation rate as well as the published quasi-elastic neutron linewidth, both recorded far into the paramagnetic region (, are analysed in detail. We show that the classical kf exchange interaction cannot explain the observed thermal variation of the dynamical and neutron widths. We perform a calculation of the 4f excitation spectra using the expansion, applied to the Anderson one-impurity Hamiltonian in the non-crossing approximation (NCA), in the presence of crystal-field splittings, and compare the results with the observed thermal variations of the dynamical widths. Whereas the NCA reasonably explains the neutron measurements, it strongly overestimates the low-temperature width. We discuss different possible reasons for this discrepancy.

We investigate the physical meaning of the longitudinal muon spin-relaxation rate measured for a paramagnet at temperatures high with respect to the magnetic phase transition temperature. The depolarization rate is shown to be a function of the relaxation rates of the quasi-elastic and inelastic magnetic excitations. Using this analysis, we consider recently published data recorded on the strongly correlated electron systems CeNiSn and .

A microscopic investigation of the cubic ternary alloys YbPdBi and YbNiSb, using 170Yb Mossbauer spectroscopy, reveals that the point symmetry at the site of the Yb3+ is far from cubic in both compounds. An analysis of the thermal variation of the quadrupolar 4f moment shows that the energy of the first crystal electric field transition is close to 20 K in YbPdBi and to 10 K in YbNiSb. Specific heat measurements are also presented in YbPdBi, and magnetic ordering is detected below 1 K in YbPdBi, with saturated Yb3+ moments of 1.25 mu B, and below 0.85 K in YbNiSb, with saturated moments of 1 mu B. A tentative explanation of the Yb site symmetry breaking is made in terms of a Jahn-Teller distortion within the Gamma 8 cubic quartet.

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

We report on the physical properties of single crystalline EuRhSi$_3$ and polycrystalline EuIrSi$_3$, inferred from magnetisation, electrical transport, heat capacity and $^{151}$Eu M\"ossbauer spectroscopy. These previously known compounds crystallise in the tetragonal BaNiSn$_3$-type structure. The single crystal magnetisation in EuRhSi$_3$ has a strongly anisotropic behaviour at 2 K with a spin-flop field of 13 T, and we present a model of these magnetic properties which allows the exchange constants to be determined. In both compounds, specific heat shows the presence of a cascade of two close transitions near 50 K, and the $^{151}$Eu M\"ossbauer spectra demonstrate that the intermediate phase has an incommensurate amplitude modulated structure. We find anomalously large values, with respect to other members of the series, for the RKKY N\'eel temperature, for the spin-flop field (13 T), for the spin-wave gap ($\simeq$ 20-25 K) inferred from both resistivity and specific heat data, for the spin-disorder resistivity in EuRhSi$_3$ ($\simeq 35$ $\mu$Ohm.cm) and for the saturated hyperfine field (52 T). We show that all these quantities depend on the electronic density of states at the Fermi level, implying that the latter must be strongly enhanced in these two materials. EuIrSi$_3$ exhibits a giant magnetoresistance ratio, with values exceeding 600 % at 2 K in a field of 14 T., Comment: 6 pages, 8 figures

We present a model which accounts for the high field magnetisation at very low temperature in two pyrochlore frustrated systems, Er2Ti2O7 and Tb2Ti2O7. The two compounds present very different ground states: Er2Ti2O7, which has a planar crystal field anisotropy, is an antiferromagnet with T_N=1.2K, whereas Tb2Ti2O7 is expected to have Ising character and shows no magnetic ordering down to 0.05K, being thus labelled a ``spin liquid''. Our model is a mean field self-consistent calculation involving the 4 rare earth sites of a tetrahedron, the building unit of the pyrochlore lattice. It includes the full crystal field hamiltonian, the infinite range dipolar interaction and anisotropic nearest neighbour exchange described by a 3-component tensor. For Er2Ti2O7, we discuss the equivalence of our treatment of the exchange tensor, taken to be diagonal in a frame linked to a rare earth - rare earth bond, with the pseudo-spin hamiltonian recently developped for Kramers doublets in a pyrochlore lattice. In Tb2Ti2O7, an essential ingredient of our model is a symmetry breaking developping at very low temperature. We compare its prediction for the isothermal magnetisation with that of ``the quantum spin ice'' model.

The magnetic properties of single crystalline EuPtGe$_3$, crystallizing in the non-centrosymmetric BaNiSn$_3$-type crystal structure, have been studied by means of magnetisation, electrical resistivity, heat capacity and $^{151}$Eu M\"ossbauer spectroscopy. The susceptibility and heat capacity data indicate a magnetic transition at $T_{\rm N}$ = 11\,K. The M\"ossbauer data confirm this conclusion, but evidence a slight first order character of the transition. Analysing the magnetisation data using a mean field model with two antiferromagnetically coupled sublattices allows to explain some aspects of the magnetic behaviour, and to derive the first and second neighbour exchange integrals in EuPtGe$_3$.

The pyrochlore material Tb2Ti2O7, although possessing a negative paramagnetic Curie temperature witnessing the presence of antiferromagnetic exchange interactions, shows no long range magnetic order of Tb3+ moments down to 0.05K. The reason for this behaviour is not thoroughly understood up to now. We present here a model based on the presence of a distortion from trigonal symmetry at low temperature, making this material a two-singlet system. Application of a 4-site mean field theory allows us to construct the phase diagram for this system and to understand how, in the presence of a distortion, Tb2Ti2O7 can remain in a paramagnetic spin liquid phase down to T = 0. Although the existence of a static distortion at low temperature in these materials is not yet convincingly assessed, we believe that our model can give useful hints towards an understanding of this interesting material.

ZnFe2O4 nanoparticles with a mean size ranging from 3 nm to 10 nm, have been prepared using the solvothermal method. The TEM images reveal a narrow particle size distribution for all samples. Bulk magnetic measurements indicate superparamagnetic behaviour with a blocking temperature (TB) that increases from 10 K to 22 K upon increasing the particle size. At temperatures lower than TB the nanoparticles are in the "frozen" regime and the coercive field changes from 145 Oe to 220 Oe for nanoparticles with sizes from 3 nm to 10 nm. On heating, the Mossbauer spectroscopy data show a progressive crossover from a magnetic to a quadrupolar hyperfine interaction, with a blocking temperature coherent with that obtained from magnetization measurements. The spinel inversion parameter, which appears to be a function of the particle size, has been estimated from neutron diffraction experiments.

The local Yb(3+) magnetic susceptibility tensor was recently measured in the frustrated pyrochlore compound Yb(2)Ti(2)O(7) by means of in-field polarized neutron scattering in a single crystal. A very anisotropic effective exchange tensor was derived for the Yb(3+) ion. Using this result, we reinterpret here the data for the powder susceptibility in Yb(2)Ti(2)O(7). We show that, in the case of a well-isolated Kramers doublet with anisotropic g and exchange tensors, the inverse susceptibility for a powder sample does not strictly obey a Curie-Weiss law at low temperature. We discuss the consequences regarding the paramagnetic Curie temperature, usually taken as a measure of the exchange/dipolar interaction, and the exotic 'slow fluctuation' ground state of Yb(2)Ti(2)O(7).

We have studied the field induced magnetic structures (H//[110]) in Tb2Ti2O7 in a wide temperature (0.2

We report on a procedure for increasing the Co content in acicular Fe–Co nanoparticles (70 nm length and ~5 axial ratio) obtained by thermal reduction of Co-doped goethite nanoparticles coated with Co(OH)2 layers. It has been found that the diffusion of Co cations located onto the particle surface of the precursor to the inner part can be promoted by separating reduction in two steps, first from the precursor to Co-doped magnetite and, finally, from this phase to metal. Furthermore, the uniformity in the Co distribution in the final metallic alloy could be improved by annealing the Co-doped magnetite obtained from the first reduction step, resulting in an important increase of coercivity. The enhancement of the magnetic properties is mainly discussed in terms of the reversal magnetization mechanism into the metallic alloy. The hard magnetic properties resulting from the higher Co content and its homogeneous distribution in the final Fe–Co alloy, along with the reduction in the particle size, make our acicular Fe–Co nanoparticles suitable for high-density magnetic recording applications.

We present a comparative study of (Tb 0.8 La 0.2 ) 2 Mo 2 O 7 and Gd 2 Mo 2 O 7 situated at the verge of a Mott transition from ferromagnetic metal to insulating spin glass, which can be tuned by the rare-earth ionic radius. We probe the spin correlations and fluctuations versus temperature and pressure thanks to neutron scattering and μSR. The ambient pressure state of ferromagnetic character shows striking differences between the two compounds, both in the static and dynamic properties, showing the influence of the Tb crystal field anisotropy. Under pressure, both compounds transform into a spin glass state. We also studied the chemically ordered spin glass Tb 2 Mo 2 O 7 with pressure and temperature. We observe mesoscopic ferromagnetic correlations between Tb moments, together with short-range correlations. All persist down to very low temperature (40 mK), showing that the spin glass order corresponds to the magnetic ground state. Under pressure, the lengthscale of the mesoscopic correlations is strongly reduced, whereas the short-range correlations are unchanged.

A single-ion anisotropy of the planar type is found in the pyrochlore antiferromagnet oxides Gd2M2O7 (M = Ti,Sn); its strength is comparable with the strength of the exchange coupling. Models considering the effects of the planar anisotropy predict the appearance of a transverse magnetization in an applied magnetic field. A detailed experimental study of Gd2Ti2O7 single crystals reveals that a transverse magnetization is really present at low temperatures. The magnetic phase diagram of Gd2Ti2O7 is refined.

170Yb Mossbauer experiments in the heavy-electron compound YbSi are presented in the temperature range 0.045-100 K. Magnetic ordering of the Yb moments below 1.6 K is confirmed and a strong moment reduction in the magnetic phase is demonstrated. The crystal-field excitation is estimated to be 2.5 K. YbSi is thus a heavy-electron material where Kondo-frustrated magnetic ordering occurs due to the competition between RKKY exchange and Kondo coupling.