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In the heavy-fermion system $Yb_2Pd_2In_{1-x}Sn_x$, the interplay of crystal-field splitting, Kondo effect, and Ruderman-Kittel-Kasuya-Yosida interactions leads to complex chemical-, pressure-, and magnetic-field phase diagrams, still to be explored in full detail. By using a series of techniques, we show that even modest changes of parameters other than temperature are sufficient to induce multiple quantum-critical transitions in this highly susceptible heavy-fermion family. In particular, we show that, above $\sim 10$ kbar, hydrostatic pressure not only induces an antiferromagnetic phase at low temperature, but it likely leads to a reorientation of the Yb magnetic moments and/or the competition among different antiferromagnetic configurations.

We report on a study on a representative set of FeP-based MnFePSi samples by means of Mn NMR in both zero and applied magnetic field. The first-order nature of the magnetic transition is demonstrated by truncated order parameter curves with a large value of the local ordered moment at the Curie point, even at compositions where the transition appears second order from magnetic measurements. No weak ferromagnetic order could be detected at Si-poor compositions showing the kinetic arrest phenomenon, but rather the phase separation of fully ferromagnetic domains from volume fractions where Mn spins are fluctuating. The more pronounced decrease of the ordered moment at the $3f$ sites than at the $3g$ sites on approaching ${T}_{C}$, characteristic of the mixed magnetism of these materials, is shown to be driven by the drop of the $3f$ spin density instead of enhanced spin fluctuations. The temperature-driven $3f$ spin extinction is demonstrated to evolve into a truly nonmagnetic state of the $3f$ Mn ions well above ${T}_{C}$, in agreement with theoretical models and in contrast with previous experiments who detected just a partial moment quenching. Besides ``normal'' $3f$ Mn ions undergoing a magnetic to spinless state transition, NMR in Mn-rich compositions detects the disproportionation at $3f$ sites of a significant minority Mn fraction with negligible hyperfine couplings, which retains its diamagnetic character independent of temperature. Such a diamagnetic fraction qualitatively accounts for the reduced average $3f$ moment previously reported at large Mn concentrations.

Muon spin rotation and relaxation studies have been performed on a "111" family of iron-based superconductors NaFe_1-xNi_xAs. Static magnetic order was characterized by obtaining the temperature and doping dependences of the local ordered magnetic moment size and the volume fraction of the magnetically ordered regions. For x = 0 and 0.4 %, a transition to a nearly-homogeneous long range magnetically ordered state is observed, while for higher x than 0.4 % magnetic order becomes more disordered and is completely suppressed for x = 1.5 %. The magnetic volume fraction continuously decreases with increasing x. The combination of magnetic and superconducting volumes implies that a spatially-overlapping coexistence of magnetism and superconductivity spans a large region of the T-x phase diagram for NaFe_1-xNi_xAs . A strong reduction of both the ordered moment size and the volume fraction is observed below the superconducting T_C for x = 0.6, 1.0, and 1.3 %, in contrast to other iron pnictides in which one of these two parameters exhibits a reduction below TC, but not both. The suppression of magnetic order is further enhanced with increased Ni doping, leading to a reentrant non-magnetic state below T_C for x = 1.3 %. The reentrant behavior indicates an interplay between antiferromagnetism and superconductivity involving competition for the same electrons. These observations are consistent with the sign-changing s-wave superconducting state, which is expected to appear on the verge of microscopic coexistence and phase separation with magnetism. We also present a universal linear relationship between the local ordered moment size and the antiferromagnetic ordering temperature TN across a variety of iron-based superconductors. We argue that this linear relationship is consistent with an itinerant-electron approach, in which Fermi surface nesting drives antiferromagnetic ordering.

55 Mn NMR was used to perform the atomic-scale study of the anisotropic properties of Mn 5 Ge 3 /Ge(111) epitaxial films with thicknesses between 9 and 300 nm. The NMR spectra have been recorded as a function of strong external magnetic field applied in the film plane and perpendicular to it. Two 55 Mn NMR resonances have been observed, corresponding to the two manganese sites 4d and 6g, in the hexagonal D8 8 structure; in zero field their frequency is centered around 207.5 and 428 MHz, respectively. The anisotropy of 55 Mn hyperfine fields between the hexagonal c direction and the c plane at both Mn sites was evidenced and attributed to the anisotropic term due to the unquenched Mn orbital momentum. The anisotropy of the orbital contribution to hyperfine fields was determined as 1.52 T in the 4d site and up to 2.77 T in the 6g site. The 4d site reveals a quadrupolar interaction due to the strong electric field gradient: V zz = 5.3 × 10 19 V/m 2 in this site, which is shown to be oriented along the hexagonal c axis.

Muon spin rotation ($\ensuremath{\mu}\mathrm{SR}$) and $^{19}\mathrm{F}$ nuclear magnetic resonance (NMR) measurements were performed to investigate the effect of Mn for Fe substitutions in ${\mathrm{La}}_{1\ensuremath{-}y}{\mathrm{Y}}_{y}{\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}{\mathrm{AsO}}_{0.89}{\mathrm{F}}_{0.11}$ superconductors. While for $y=0$ a very low critical concentration of Mn ($x=0.2%$) is needed to quench superconductivity, as $y$ increases the negative chemical pressure introduced by Y for La substitution stabilizes superconductivity and for $y=20%$ it is suppressed at Mn contents an order of magnitude larger. A magnetic phase arises once superconductivity is suppressed both for $y=0$ and for $y=20%$. Low-energy spin fluctuations give rise to a peak in $^{19}\mathrm{F}$ NMR $1/{T}_{1}$ with an onset well above the superconducting transition temperature and whose magnitude increases with $x$. Also the static magnetic correlations probed by $^{19}\mathrm{F}$ NMR linewidth measurements show a marked increase with Mn content. The disruption of superconductivity and the onset of the magnetic ground state are discussed in the light of the proximity of ${\mathrm{LaFeAsO}}_{0.89}{\mathrm{F}}_{0.11}$ to a quantum critical point.

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

We report the results of a muon-spin relaxation (μSR) investigation of La2−xSrxCoO4, an antiferromagnetic insulating series which has been shown to support charge ordered and magnetic stripe phases and an hourglass magnetic excitation spectrum. We present a revised magnetic phase diagram, which shows that the suppression of the magnetic ordering temperature is highly sensitive to small concentrations of holes. Distinct behavior within an intermediate x range (0.2≤x≲0.6) suggests that the putative stripe ordered phase extends to lower x than previously thought. Further charge doping (0.67≤x≤0.9) prevents magnetic ordering for T≳1.5K.

We investigate the magnetic properties and the phonon-induced relaxation dynamics of the first regular ${\mathrm{Cr}}_{9}$ antiferromagnetic (AF) ring, which represents a prototype frustrated AF ring. Geometrical frustration in ${\mathrm{Cr}}_{9}$ yields an energy spectrum with twofold degenerate low-lying levels and a low-spin ground state. The electronic relaxation dynamics is probed by ${}^{1}\mathrm{H}$-NMR through the temperature dependence of the spin-lattice relaxation rate $1/{T}_{1}$. We develop a microscopic model that reproduces $1/{T}_{1}(T)$ curves, taking also into account the wipeout effect. By interpreting these measurements we determine the spin-phonon coupling strength and we investigate the decay of the cluster magnetization due to the spin-phonon interaction. We find that at very low temperatures, the relaxation is characterized by a single dominating Arrhenius-type relaxation process, whereas several relevant processes emerge at higher temperatures. In addition, we calculate the temperature and magnetic field dependence of level lifetimes.

The temperature-dependent behavior of the Rh(111)-(2\ifmmode\times\else\texttimes\fi{}2)-1O phase was investigated by He-atom scattering (HAS) and low-energy electron diffraction. The adsorption system undergoes an order-disorder phase transition at ${\mathrm{T}}_{\mathrm{c}}$=280\ifmmode\pm\else\textpm\fi{}5 K, with critical exponents found to be consistent with the four-state Potts model. Beyond the phase transition the HAS specular peak intensity exhibits a strong and reversible increase. This finding points toward a reduction of the surface charge-density corrugation induced by the phase transition itself. Around 160 K, hydrogen adsorbed on the Rh(111)-(2x2)-1O surface reacts with oxygen to form water, and drives the overlayer in an out-of-equilibrium condition which is characterized by a dramatic domain-wall proliferation.

We report on a systematic study of the structural, magnetic, and transport properties of high-purity 1T-VS2 powder samples prepared under high pressure. The results differ notably from those previously obtained by deintercalating Li from LiVS2. First, no charge-density wave (CDW) is found by transmission electron microscopy down to 94 K, though ab initio phonon calculations unveil a latent CDW instability driven by an acoustic phonon softening at the wave vector qCDW≈(0.21,0.21,0) previously reported in deintercalated samples. A further indication of latent lattice instability is given by an anomalous expansion of the V-S bond distance at low temperature. Second, infrared optical absorption and electrical resistivity measurements give evidence of nonmetallic properties, consistent with the observation of no CDW phase. On the other hand, magnetic susceptibility and NMR data suggest the coexistence of localized moments with metallic carriers, in agreement with ab initio band structure calculations. This discrepancy is reconciled by a picture of electron localization induced by disorder or electronic correlations leading to a phase separation of metallic and nonmetallic domains in the nm scale. We conclude that 1T-VS2 is at the verge of a CDW transition and suggest that residual electronic doping in Li deintercalated samples stabilizes a uniform CDW phase with metallic properties.

We have measured the nuclear spin-lattice relaxation of 59Co in the geometrically frustrated Ising-type spin-chain system Ca3Co2O6 as a function of temperature and applied magnetic field. In the nearly-saturated ferrimagnetic and ferromagnetic regimes considered here, the field-dependent energy scales governing the spin-lattice relaxation rates reflect the energy cost of a spin flip in the Co ions adjacent to the probed nuclei. This results in a thermally activated form for the nuclear relaxation rates, which can be exploited as a local probe of the exchange interactions. In particular, the measurements enable the values of intra- and interchain exchange constants J1, J2+J3 to be directly extracted from the data. By using a quasi-one-dimensional model, we then determine the value of J2 consistent with the spin-density-wave order observed in zero field.

We present the results on heavily underdoped Y1−xCaxBa2Cu3O6+y, which provide the evidence that the doping mechanism (cation substitution or oxygen loading) directly determines whether the corresponding injected mobile holes contribute to superconductivity or only to high-temperature transport. We argue that this hole tagging is a signature of the complexities of single-hole doping in Mott insulators, and it calls for a subtler description of the correlated bands than the usual one. We also map in great detail the underdoped superconducting phase diagram Tc vs hole doping, which shows that the total number of mobile holes is not the driving parameter for superconductivity.

The oxygen-order dependent emergence of superconductivity in YBa2Cu3O6+x is studied, for the first time in a comparative way, on pair samples having the same oxygen content and thermal history, but different Cu(1)Ox chain arrangements deriving from their intercalated and deintercalated nature. Structural and electronic non-equivalence of pairs samples is detected in the critical region and found to be related, on microscopic scale, to a different average chain length, which, on being experimentally determined by nuclear quadrupole resonance (NQR), sheds new light on the concept of critical chain length for hole doping efficiency.

The antiferromagnetic manganite ${\mathrm{Pr}}_{0.5}{\mathrm{Sr}}_{0.5}\ensuremath{-}{\mathrm{MnO}}_{3}$ was investigated at low temperature by means of magnetometry and ${}^{55}\mathrm{Mn}$ NMR. A field-induced transition to a ferromagnetic state is detected by magnetization measurements at a threshold field of a few tesla. NMR shows that the ferromagnetic phase develops from zero field by the nucleation of microscopic ferromagnetic domains, consisting of an inhomogeneous mixture of tilted and fully aligned parts. At the threshold the NMR spectrum changes discontinuously into that of a homogeneous, fully aligned, ferromagnetic state, suggesting a percolative origin for the ferromagnetic transition.

NMR and high-resolution x-ray diffraction of several lithium-doped fullerides ${\mathrm{Li}}_{x}{\mathrm{C}}_{60}$ show that the stoichiometry $x=12$ represents a stable Li doping for a high symmetry ${\mathrm{C}}_{60}$-like phase, which might extend to much larger x. ${\mathrm{Li}}_{12}{\mathrm{C}}_{60}$ is fcc at high temperature, and it distorts to tetragonal upon cooling. We have evidence from ${}^{13}\mathrm{C}$ NMR that the ${\mathrm{C}}_{60}$ units are static in the tetragonal phase at least up to $T=373 \mathrm{K}.$ Rietvelt refinement of the fcc phase favors a hybrid cluster intercalated structure with shortening of selected Li-C distances.

We present ${}^{139}\mathrm{La}$ NMR experiments on five powder samples of lanthanum manganites, with a ${\mathrm{Mn}}^{4+}$ concentration ranging from the antiferromagnetic-insulator (AFM) to the ferromagnetic-conducting (FM) region of the phase diagram. We measure a positive hyperfine coupling $\mathcal{C}=0.13 \mathrm{T}/{\ensuremath{\mu}}_{B}.$ A signal from nuclei in a FM environment is present at all compositions, as evidenced by a hyperfine frequency in zero-field experiments, by a positive hyperfine shift in NMR experiments below ${T}_{c},$ and by a paramagnetic frequency shift following Curie-Weiss law. A signal from nuclei in an AFM environment is identified by a similar negative intercept Curie-Weiss law. The NMR spectra reveal a large temperature dependent fraction of static spin defects below ${T}_{c}$ in the FM domains. Nuclear relaxation indicates that the FM regions are influenced by diffusing, AFM-correlated excitations, while the AFM regions probe spin fluctuations from diffusing, FM correlated excitations. These results are interpreted in terms of electronic inhomogeneities due to the presence of a magnetic small polaron.

We investigate the magnetic ordering and dynamics of the stripe phase of La 5/3 Sr 1/3 CoO 4 , a material shown to have an hourglass magnetic excitation spectrum. A combination of muon-spin relaxation, nuclear magnetic resonance, and magnetic susceptibility measurements strongly suggest that the physics is determined by a partially disordered configuration of charge and spin stripes whose frustrated magnetic degrees of freedom are dynamic at high temperature and which undergo an ordering transition around 35 K with coexisting dynamics that freeze out in a glassy manner as the temperature is further reduced.

{sup 55}Mn NMR experiments were carried out at 1.3{endash}4.2 K on six samples of lanthanum manganites, with a hole doping concentration ranging from the antiferromagnetic-insulator to the ferromagnetic-conductor region of the phase diagram. The dependence of the resonance frequencies on external fields is either as expected for ferromagnets (FM) or for antiferromagnets (AF). No indication of a canted phase could be detected. FM and AF resonances were found to coexist in all samples and are grouped in three broad bands, whose frequencies do not depend in first order on hole concentration. Strong evidence of interaction between AF and FM regions was provided by longitudinal relaxation experiments in several samples. An explanation of this behavior is offered in terms of intrinsic phase separation of the holes into FM microdomains. We determine the {sup 55}Mn hyperfine coupling to be isotropic in all cases, with A=6.2T/{mu}{sub B} for the on-site term in the AF domains. {copyright} {ital 1997} {ital The American Physical Society}

We report on the magnetic and superconducting properties of LaO${}_{0.5}$F${}_{0.5}$BiS${}_{2}$ by means of zero- and transverse-field (ZF/TF) muon-spin spectroscopy measurements ($\ensuremath{\mu}$SR). Contrary to previous results on iron-based superconductors, measurements in zero field demonstrate the absence of magnetically ordered phases. TF-$\ensuremath{\mu}$SR data give access to the superfluid density, which shows a marked two-dimensional character with a dominant $s$-wave temperature behavior. The field dependence of the magnetic penetration depth confirms this finding and further suggests the presence of an anisotropic superconducting gap.

The appearance of static magnetism, nanoscopically coexisting with superconductivity, is shown to be a general feature of optimally electron-doped $L$Fe${}_{1\ensuremath{-}x}$Ru${}_{x}$AsO${}_{1\ensuremath{-}y}$F${}_{y}$ superconductors ($L$ represents a lanthanide ion) upon isovalent substitution of Fe by Ru. The magnetic ordering temperature ${T}_{N}$ and the magnitude of the internal field display a domelike dependence on $x$, peaked around $x=1/4$, with higher ${T}_{N}$ values for those materials characterized by a larger $z$ cell coordinate of As. Remarkably, the latter are also those with the highest superconducting transition temperatures (${T}_{c}$) for $x=0$. The reduction of ${T}_{c}(x)$ is found to be significant in the $x$ region of the phase diagram where the static magnetism develops. Upon increasing the Ru content superconductivity eventually disappears, but only at $x\ensuremath{\simeq}0.6$.

We investigate the direct correspondence between Co band ferromagnetism and structural parameters in the pnictide oxides $R\mathrm{CoPO}$ for different rare-earth ions ($R=\mathrm{La},\mathrm{Pr},\mathrm{Nd},\mathrm{Sm}$) by means of muon-spin spectroscopy and ab initio calculations, complementing our results published previously [G. Prando et al., Common effect of chemical and external pressures on the magnetic properties of $R\mathrm{CoPO}$ ($R=\mathrm{La},\mathrm{Pr}$), Phys. Rev. B 87, 064401 (2013)]. We find that both the transition temperature to the ferromagnetic phase ${T}_{\mathrm{C}}$ and the volume of the crystallographic unit cell $V$ are conveniently tuned by the $R$ ionic radius and/or external pressure. We report a linear correlation between ${T}_{\mathrm{C}}$ and $V$ and our ab initio calculations unambiguously demonstrate a full equivalence of chemical and external pressures. As such, we show that $R$ ions influence the ferromagnetic phase only via the induced structural shrinkage without involving any active role from the electronic $f$ degrees of freedom, which are only giving a sizable magnetic contribution at much lower temperatures.

Ac susceptibility and static magnetization measurements were performed in the nearly optimally doped LaFeAsO${}_{0.9}$F${}_{0.1}$ and CeFeAsO${}_{0.92}$F${}_{0.08}$ superconductors, complementing earlier results on SmFeAsO${}_{0.8}$F${}_{0.2}$ [Phys. Rev. B 83, 174514 (2011)]. The magnetic field-temperature phase diagram of the mixed superconducting state is drawn for the three materials, displaying a sizable reduction of the liquid phase upon increasing ${T}_{c}$ in the range of applied fields ($H\ensuremath{\leqslant}5$ T). This result indicates that SmFeAsO${}_{0.8}$F${}_{0.2}$ is the most interesting compound among the investigated ones in view of possible applications. The field-dependence of the intragrain depinning energy ${U}_{0}$ exhibits a common trend for all the samples with a typical crossover field value (2500 Oe $\ensuremath{\lesssim}{H}_{\mathrm{cr}}\ensuremath{\lesssim}5000$ Oe) separating regions where single and collective depinning processes are at work.

The effect of isoelectronic substitutions on the microscopic properties of LaFe${}_{1\ensuremath{-}x}$Ru${}_{x}$AsO, for $0\ensuremath{\le}x\ensuremath{\le}0.8$, has been investigated by means of $\ensuremath{\mu}$SR and ${}^{139}$La NMR. It was found that Ru substitution causes a progressive reduction of the N\'eel temperature (${T}_{N}$) and of the magnetic order parameter without leading to the onset of superconductivity. The temperature dependence of ${}^{139}$La nuclear spin-lattice relaxation rate $1/{T}_{1}$ can be suitably described within a two-band model. One band giving rise to the metallic antiferromagnetic ground state, while the other one is characterized by weakly correlated electrons. Ru for Fe substitution yields a progressive decrease of the density of states at the Fermi level close to the one derived from band structure calculations. The reduction of ${T}_{N}$ with increasing Ru content follows the predictions of the ${J}_{1}$-${J}_{2}$ model on a square lattice, which appears to be an effective framework to describe the magnetic properties of the magnetic ground state.

Ac susceptibility and static magnetization measurements were performed in the optimally doped SmFeAsO${}_{0.8}$F${}_{0.2}$ superconductor. The field-temperature phase diagram of the superconducting state was drawn, and, in particular, the features of the flux lines were derived. The dependence of the intragrain depinning energy on the magnetic field intensity was derived in the thermally activated flux-creep framework, enlightening a typical $1/H$ dependence in the high-field regime. The intragrain critical current density was extrapolated in the zero-temperature and zero-magnetic-field limit, showing a remarkably high value ${J}_{c0}(0)~2\ifmmode\times\else\texttimes\fi{}{10}^{7}$ A/cm${}^{2}$, which demonstrates that this material is rather interesting for potential future technological applications.

Zero-field muon spin rotation and magnetization measurements were performed in La${}_{2}$Cu${}_{1\ensuremath{-}x}{M}_{x}$O${}_{4}$, for $0\ensuremath{\leqslant}x\ensuremath{\leqslant}0.12$, where Cu${}^{2+}$ is replaced either by $M={\text{Zn}}^{2+}$ or by $M={\text{Mg}}^{2+}$ spinless impurity. It is shown that while the doping dependence of the sublattice magnetization [$M(x)$] is nearly the same for both compounds, the N\'eel temperature [${T}_{N}(x)$] decreases unambiguously more rapidly in the Zn-doped compound. This difference, not taken into account within a simple dilution model, is associated with the frustration induced by the Zn${}^{2+}$ impurity onto the Cu${}^{2+}$ antiferromagnetic lattice. In fact, from ${T}_{N}(x)$ and $M(x)$ the spin stiffness is derived and found to be reduced by Zn doping more significantly than expected within a dilution model. The effect of the structural modifications induced by doping on the exchange coupling is also discussed.

We report on a (59)Co NMR investigation of the trigonal cobaltate Ca(3)Co(2)O(6) carried out on a single crystal, providing precise determinations of the electric field gradient and chemical shift tensors, and of the internal magnetic fields at the nonmagnetic Co I sites, unavailable from former studies on powders. The magnetic-field-induced ferri-and ferromagnetic phases at intermediate temperature (e.g., 10 K) are identified by distinct internal fields, well accounted for by purely dipolar interactions. The vanishing transferred hyperfine field at the Co I site indicates that the Co(3+) (I) orbitals do not participate in the intrachain superexchange, in disagreement with a previous theoretical model. The strong Ising character of the system is confirmed experimentally by the field dependence of the resonance lines, indicating that local moments are saturated even at the phase boundaries. In the vicinity of the critical fields, nuclear spin-spin relaxations detect the spin-reversal dynamics of large magnetic assemblies, either Ising chain fragments or finite-size domains, which drive the metamagnetic transitions. Such collective excitations exhibit a glassy behavior, slowing down to subacoustic frequencies and freezing at low temperature. The relevance of such slow-fluctuation modes for the low-temperature multistep behavior reported in the magnetization is discussed.

Zero-field copper NMR measurements on ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6}$ up to 380 K are reported. The temperature dependence of the local magnetic moment at the Cu(2) plane site is compared with the predictions of spin-wave theory, based on the exchange coupling constants obtained from inelastic neutron scattering. The agreement is satisfactory, within the limits of the theoretical approximations, but discrepancies with elastic neutron-scattering results are pointed out and justified in terms of three-dimensional stacking defects.

We report extensive muon-spin-rotation measurements on the lightly doped Y1-xCaxBa2Cu3O6+y compound, which allows us to disentangle the effect of disorder, controlled by random Ca2+ substitution, from that of mere doping. A three-dimensional phase diagram of lightly doped cuprates is accurately drawn. It shows a quantum critical point around which a thermally activated antiferromagnetic phase competes with superconductivity. Disorder suppresses both the competing order parameters and the quantum critical point, unveiling an underlying frozen state.

We report on the coexistence of magnetic and superconducting states in ${\text{CeFeAsO}}_{1\ensuremath{-}x}{\text{F}}_{x}$ for $x=0.06(2)$, characterized by transition temperatures ${T}_{m}=30\text{ }\text{K}$ and ${T}_{c}=18\text{ }\text{K}$, respectively. Zero-field and transverse-field muon-spin-relaxation measurements show that below 10 K the two phases coexist within a nanoscopic scale over a large volume fraction. This result clarifies the nature of the magnetic-to-superconducting transition in the ${\text{CeFeAsO}}_{1\ensuremath{-}x}{\text{F}}_{x}$ phase diagram, by ruling out the presence of a quantum critical point which was suggested by earlier studies.

The effect of the proximity of Au nanoparticles on the transport and magnetic properties of ultrathin ${\text{La}}_{2/3}{\text{Sr}}_{1/3}{\text{MnO}}_{3}$ (LSMO) films has been investigated. We find a huge increase in the resistivity of the manganite (by four orders of magnitude for a Au nominal thickness of 2 nm), which is accompanied by a strong decrease in the Curie temperature. A combined scanning transmission electron microscopy and electron energy-loss spectroscopy analysis shows that interfaces are coherent and atomically sharp, and that the structural quality is very high. On the other hand, a strong reduction in the Mn oxidation state is seen upon Au capping. NMR data show a strong attenuation of the double exchange signal upon formation of Au nanoparticles. Ab initio calculations indicate a negligible influence of Au on LSMO at an ideal interface, with the LSMO surface magnetic and electronic properties essentially unchanged upon creation of the Au/LSMO interface. In view of these calculations, the experimental results cannot be explained in terms of purely electrostatic effects induced by the proximity of a noble metal. Here we propose that the main driving force underlying the observed change in physical properties is the high reactivity of Au nanoparticles, which can locally pump oxygen from the manganite, thus favoring a phase separation ensuing from O inhomogeneity which deteriorates the transport and electrical properties.

We present uSR investigations on SmFeAsO_1-xF_x showing coexistence of magnetic order and superconductivity only in a very narrow F-doping range. The sharp crossover between the two types of order is similar to that observed in LaFeAsO_1-xF_x, suggesting a common behavior for the 1111 pnictides. The analysis of the muon asymmetry demonstrates that the coexistence must be nanoscopic, i.e. the two phases must be finely interspersed over a typical length-scale of few nm. In this regime both the magnetic and the superconducting transition temperatures collapse to very low values. Our data suggest a competition between the two order parameters

We deeply investigated the properties of submonolayer distributions of isolated molecular Cr7Ni rings deposited on Au111 by liquid phase. X-ray absorption spectra measured at the Cr and Ni L2,3 edges show that the grafting of the Cr7Ni rings onto the gold surface does not affect the oxidation state and the local symmetry of the Cr and Ni sites. The circular dichroism shows a change in sign of the Ni magnetic moment. This is due to a reduction in the exchange coupling constants that, however, preserves the structure of the low-energy levels of the grafted rings, as corroborated by spin-Hamiltonian simulations and comparison with measurements on bulk sample. Density-functional theory calculations show that the Ni-Cr bond gets weaker with slight ring distortion suggesting possible explanation for the observed magnetic behavior. These results show that complex magnetic molecules can be grafted onto surfaces, and that changes in their magnetic behavior must be examined in individual cases.