Scientific Highlights 2008

The following selection of scientific highlights comprises either publications with NUM staff members as part of the list of authors or publications based on experiments, which were performed at one of the three user facilities operated by NUM: SINQ, SμS or Particle Physics.


Spin-State Polarons in Lightly-Hole-Doped LaCoO3

Inelastic neutron scattering (INS), electron spin resonance (ESR), and nuclear magnetic resonance (NMR) measurements were employed to establish the origin of the strong magnetic signal in lightly-hole-doped La1-xSrxCoO3, x~0.002. Both INS and ESR low temperature spectra show intense excitations with large effective g factors ~10–18. NMR data indicate the creation of extended magnetic clusters. From the Q dependence of the INS magnetic intensity, we conclude that the observed anomalies are caused by the formation of octahedrally shaped spin-state polarons comprising seven Co ions. The present INS, ESR, and NMR data give evidence for two regimes in the lightly-hole-doped samples: (i) T<35 K dominated by spin polarons; (ii) T>35 K dominated by thermally activated magnetic Co3+ ions.
Facility: SINQ

Reference: A. Podlesnyak et al, Physical Review Letters 101 , 247603 (2008)

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Thermodynamics of the Spin Luttinger Liquid in a Model Ladder Material

The phase diagram in temperature and magnetic field of the metal-organic, two-leg, spin-ladder compound (C5H12N)2CuBr4 is studied by measurements of the specific heat and the magnetocaloric effect. We demonstrate the presence of an extended spin Luttinger-liquid phase between two field-induced quantum critical points and over a broad range of temperature. Based on an ideal spin-ladder Hamiltonian, comprehensive numerical modeling of the ladder specific heat yields excellent quantitative agreement with the experimental data across the entire phase diagram.
Reference: C. Rüegg et al, Physical Review Letters 101 , 247202 (2008)

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Evidence for a Competition between the Superconducting State and the Pseudogap State of (BiPb)2(SrLa)2CuO(6+delta) from Muon Spin Rotation Experiments

The in-plane magnetic penetration depth lambda_(ab) in optimally doped (BiPb)2(SrLa)2CuO6+ (OP Bi2201) was studied by means of muon-spin rotation. The measurements of lambda_(ab)^(-2) (T) are inconsistent with a simple model of a d-wave order parameter and a uniform quasiparticle weight around the Fermi surface. The data are well described assuming the angular gap symmetry obtained in ARPES experiments [Phys. Rev. Lett. 98, 267004 (2007)], which suggest that the superconducting gap in OP Bi2201 exists only in segments of the Fermi surface near the nodes. The remaining parts of the Fermi surface, which are strongly affected by the pseudogap state, do not contribute significantly to the superconducting condensate.
Facility: SµS

Reference: R. Khasanov et al, Physical Review Letters 101 , 227002 (2008)

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Direct measurement of the electronic spin diffusion length in a fully functional organic spin valve by low-energy muon spin rotation

Electronic devices that use the spin degree of freedom hold unique prospects for future technology. The performance of these 'spintronic' devices relies heavily on the efficient transfer of spin polarization across different layers and interfaces. This complex transfer process depends on individual material properties and also, most importantly, on the structural and electronic properties of the interfaces between the different materials and defects that are common to real devices. Knowledge of these factors is especially important for the relatively new field of organic spintronics, where there is a severe lack of suitable experimental techniques that can yield depth-resolved information about the spin polarization of charge carriers within buried layers of real devices. Here, we present a new depth-resolved technique for measuring the spin polarization of current-injected electrons in an organic spin valve and find the temperature dependence of the measured spin diffusion length is correlated with the device magnetoresistance.
Facility: SµS

Reference: A.J. Drew et al, Nature Materials 8, 109 (2008)

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Microscopic Understanding of Negative Magnetization in Cu, Mn, and Fe Based Prussian Blue Analogues

A crossover of the field-cooled magnetization from positive to negative has been observed below the magnetic ordering temperature (17.9 K) in a multimetal Prussian Blue analogue (PBA), Cu0.73Mn0.77[Fe(CN)6]·zH2O. The reverse Monte Carlo (RMC) modeling (using the program RMCPOW) has been used to derive the various scattering contributions (e.g., nuclear diffuse, nuclear Bragg, magnetic diffuse, and magnetic Bragg) from the observed neutron diffraction patterns. The RMC analysis combined with the Rietveld refinement technique show an antiferromagnetic ordering of Mn moments with respect to the Cu as well as the Fe moments. Our study gives the first neutron magnetic structure evidence towards the microscopic understanding of the negative magnetization in the PBAs. This information can be effectively utilized to design suitable PBAs for making multifunctional devices.
Facility: SINQ

Reference: A. Kumar et al, Physical Review Letters 101 , 207206 (2008)

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Avoided crossing of rattler modes in thermoelectric materials

Engineering of materials with specific physical properties has recently focused on the effect of nano-sized 'guest domains' in a 'host matrix' that enable tuning of electrical, mechanical, photo-optical or thermal properties. A low thermal conductivity is a prerequisite for obtaining effective thermoelectric materials, and the challenge is to limit the conduction of heat by phonons, without simultaneously reducing the charge transport. This is named the 'phonon glass–electron crystal' concept and may be realized in host–guest systems. The guest entities are believed to have independent oscillations, so-called rattler modes, which scatter the acoustic phonons and reduce the thermal conductivity. We have investigated the phonon dispersion relation in the phonon glass–electron crystal material Ba8Ga16Ge30 using neutron triple-axis spectroscopy. The results disclose unambiguously the theoretically predicted avoided crossing of the rattler modes and the acoustic-phonon branches. The observed phonon lifetimes are longer than expected, and a new explanation for the low kappa_L is provided.
Facility: SINQ

Reference: M. Christensen et al, Nature Materials 7 , 811 (2008)

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Coupled Superconducting and Magnetic Order in CeCoIn5

Strong magnetic fluctuations can provide a coupling mechanism for electrons that leads to unconventional superconductivity. Magnetic order and superconductivity have been found to coexist in a number of magnetically mediated superconductors, but these order parameters generally compete. We report that close to the upper critical field, CeCoIn5 adopts a multicomponent ground state that simultaneously carries cooperating magnetic and superconducting orders. Suppressing superconductivity in a first-order transition at the upper critical field leads to the simultaneous collapse of the magnetic order, showing that superconductivity is necessary for the magnetic order. A symmetry analysis of the coupling between the magnetic order and the superconducting gap function suggests a form of superconductivity that is associated with a nonvanishing momentum.
Facility: SINQ

Reference: M. Kenzelmann et al, Science 321 , 1652 (2008)

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Nature of the Magnetic Order in Ca3Co2O6

We present a detailed powder and single-crystal neutron diffraction study of the spin chain compound Ca3Co2O6. Below 25 K, the system orders magnetically with a modulated partially disordered antiferromagnetic structure. We give a description of the magnetic interactions in the system which is consistent with this magnetic structure. Our study also reveals that the long-range magnetic order coexists with a shorter-range order with a correlation length scale of ~180 Å in the ab plane. Remarkably, on cooling, the volume of material exhibiting short-range order increases at the expense of the long-range order.
Facility: SINQ

Reference: S. Agrestini et al, Physical Review Letters 101 , 097207 (2008)

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Field and Temperature Dependence of the Superfluid Density in LaFeAsO1-xFx Superconductors: A Muon Spin Relaxation Study

We present zero field and transverse field muon spin relaxation experiments on the recently discovered Fe-based superconductor LaFeAsO1-xFx (x=0.075 and x=0.1). The temperature dependence of the deduced superfluid density is consistent with a BCS s-wave or a dirty d-wave gap function, while the field dependence strongly evidences unconventional superconductivity. We obtain the in-plane penetration depth of lambda_ab(0)=254(2) nm for x=0.1 and lambda_ab(0)=364(8) nm for x=0.075. Further evidence for unconventional superconductivity is provided by the ratio of Tc versus the superfluid density, which is close to the Uemura line of high-Tc cuprates.
Facility: SµS

Reference: H. Luetkens et al, Physical Review Letters 101 , 097009 (2008)

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Coherent d-Wave Superconducting Gap in Underdoped La2-xSrxCuO4 by Angle-Resolved Photoemission Spectroscopy

We present angle-resolved photoemission spectroscopy data on moderately underdoped La1.855Sr0.145CuO4 at temperatures below and above the superconducting transition temperature. Unlike previous studies of this material, we observe sharp spectral peaks along the entire underlying Fermi surface in the superconducting state. These peaks trace out an energy gap that follows a simple d-wave form, with a maximum superconducting gap of 14 meV. Our results are consistent with a single gap picture for the cuprates. Furthermore our data on the even more underdoped sample La1.895Sr0.105CuO4 also show sharp spectral peaks, even at the antinode, with a maximum superconducting gap of 26 meV.
Facility: SLS

Reference: M. Shi et al, Physical Review Letters 101 , 047002 (2008)

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Quantum Hall Effect of Massless Dirac Fermions in a Vanishing Magnetic Field

The effect of strong long-range disorder on the quantization of the Hall conductivity sigma_(xy) in graphene is studied numerically. It is shown that increasing Landau-level mixing progressively destroys all plateaus in sigma_(xy) except the plateaus at sigma_(xy)= F e^2/2h (per valley and per spin). The critical state at the Dirac point is robust to strong disorder and belongs to the universality class of the conventional plateau transitions in the integer quantum Hall effect. We propose that the breaking of time-reversal symmetry by ripples in graphene can realize this quantum critical point in a vanishing magnetic field.
Reference: K. Nomura et al, Physical Review Letters 100 , 246806 (2008)

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Dislocation Cross-Slip in Nanocrystalline fcc Metals

Constant strain rate molecular dynamics simulations of nanocrystalline Al demonstrate that a significant amount of dislocations that have nucleated at the grain boundaries, exhibit cross-slip via the Fleischer mechanism as they propagate through the grain. The grain boundary structure is found to strongly influence when and where cross-slip occurs, allowing the dislocation to avoid local stress concentrations that otherwise can act as strong pinning sites for dislocation propagation.
Reference: E. Bitzek et al, Physical Review Letters 100 , 235501 (2008)

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Effect of the alpha-gamma Phase Transition on the Stability of Dislocation Loops in bcc Iron

Body-centered-cubic iron develops an elastic instability, driven by spin fluctuations, near the alpha-gamma phase transition temperature Tc=912 °C that is associated with the dramatic reduction of the shear stiffness constant c'=(c11-c12)/2 near Tc. This reduction of c' has a profound effect on the temperature dependence of the anisotropic elastic self-energies of dislocations in iron. It also affects the relative stability of the a<100> and a/2<111> prismatic edge dislocation loops formed during irradiation. The difference between the anisotropic elastic free energies provides the fundamental explanation for the observed dominant occurrence of the a<100>, as opposed to the a/2<111>, Burgers vector configurations of prismatic dislocation loops in iron and iron-based alloys at high temperatures.
Reference: S.L. Dudarev et al, Physical Review Letters 100 , 135503 (2008)

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Irrational Versus Rational Charge and Statistics in Two-Dimensional Quantum Systems

We show that quasiparticle excitations with irrational charge and irrational exchange statistics exist in tight-binding systems described, in the continuum approximation, by the Dirac equation in (2+1)-dimensional space and time. These excitations can be deconfined at zero temperature, but when they are, the charge rerationalizes to the value 1/2 and the exchange statistics to that of “quartons” (half-semions).
Reference: C. Chamon et al, Physical Review Letters 100 , 110405 (2008)

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Magnetic and Orbital Ordering in the Spinel MnV2O4

Neutron inelastic scattering and diffraction techniques have been used to study the MnV2 O4 spinel system. Our measurements show the existence of two transitions to long-range ordered ferrimagnetic states, the first collinear and the second noncollinear. The lower temperature transition, characterized by development of antiferromagnetic components in the basal plane, is accompanied by a tetragonal distortion and the appearance of a gap in the magnetic excitation spectrum. The low-temperature noncollinear magnetic structure has been definitively resolved. Taken together, the crystal and magnetic structures indicate a staggered ordering of the V d orbitals. The anisotropy gap is a consequence of unquenched V orbital angular momentum.
Reference: V.O. Garlea et al, Physical Review Letters 100 , 066404 (2008)

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Manipulating the Magnetic Structure with Electric Fields in Multiferroic ErMn2O5

Based on measurements of soft x-ray magnetic diffraction under in situ applied electric field, we report on significant manipulation and exciting of commensurate magnetic order in multiferroic ErMn2O5. The induced magnetic scattering intensity arises at the commensurate magnetic Bragg position whereas the initial magnetic signal almost persists. We demonstrate the possibility to imprint a magnetic response function in ErMn2O5 by applying an electric field.
Facility: SLS

Reference: Y. Bodenthin et al, Physical Review Letters 100 , 027201 (2008)

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Unexpected High Stiffness of Ag and Au Nanoparticles

We studied the compressibility of silver (10 nm) and gold (30 nm) nanoparticles, n-Ag and n-Au, suspended in a methanol-ethanol mixture by x-ray diffraction (XRD) with synchrotron radiation at pressures up to 30 GPa. Unexpectedly for that size, the nanoparticles show a significantly higher stiffness than the corresponding bulk materials. The bulk modulus of n-Au, K0=290(8) GPa, shows an increase of ca. 60% and is in the order of W or Ir. The structural characterization of both kinds of nanoparticles by XRD and high-resolution electron microscopy identified polysynthetic domain twinning and lamellar defects as the main origin for the strong decrease in compressibility.
Facility: SLS

Reference: Q.F. Gu et al, Physical Review Letters 100 , 045502 (2008)

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Commensurate Spin Density Wave in LaFeAsO: A Local Probe Study

We present a detailed study on the magnetic order in the undoped mother compound LaFeAsO of the recently discovered Fe-based superconductor LaFeAsO1-xFx. In particular, we present local probe measurements of the magnetic properties of LaFeAsO by means of 57Fe Mössbauer spectroscopy and muon-spin relaxation in zero external field along with magnetization and resistivity studies. These experiments prove a commensurate static magnetic order with a strongly reduced ordered moment of 0.25(5)µB at the iron site below TN=138 K, well separated from a structural phase transition at TS=156 K. The temperature dependence of the sublattice magnetization is determined and compared to theory. Using a four-band spin density wave model both, the size of the order parameter and the quick saturation below TN are reproduced.
Facility: SµS

Reference: H. Klauss et al, Physical Review Letters 101 , 077005 (2008)

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Oxygen Isotope Effects on the Superconducting Transition and Magnetic States Within the Phase Diagram of Y1-xPrxBa2Cu3O7-delta

The various phases observed in all cuprate superconductors [superconducting (SC), spin-glass (SG), and antiferromagnetic (AFM)] were investigated with respect to oxygen-isotope (16O/18O) effects, using here as a prototype system of cuprates Y1-xPrxBa2Cu3O7-delta. All phases exhibit an isotope effect which is strongest where the respective phase terminates. In addition, the isotope effects on the magnetic phases (SG and AFM) are sign reversed as compared to the one on the superconducting phase. In the coexistence regime of the SG and SC phase a two-component behavior is observed where the isotope induced decrease of the superfluid density leads to a corresponding enhancement in the SG related density.
Facility: SµS

Reference: R. Khasanov et al, Physical Review Letters 101 , 077001 (2008)

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Spatially Resolved Inhomogeneous Ferromagnetism in (Ga,Mn)As Diluted Magnetic Semiconductors: A Microscopic Study by Muon Spin Relaxation

Thin epitaxial films of the diluted magnetic semiconductor (DMS) GaMnAs have been studied by low energy muon spin rotation and relaxation (LE-µSR) as well as by transport and magnetization measurement techniques. LE-µSR allows measurements of the distribution of magnetic field on the nanometer scale inaccessible to traditional macroscopic techniques. The spatial inhomogeneity of the magnetic field is resolved: although homogeneous above Tc, below Tc the DMS consists of ferromagnetic and paramagnetic regions of comparable volumes. In the ferromagnetic regions the local field inhomogeneity amounts to 0.03 T.
Facility: SµS

Reference: V.G. Storchak et al, Physical Review Letters 101 , 027202 (2008)

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Microscopic Evidence of Spin State Order and Spin State Phase Separation in Layered Cobaltites RBaCo2O5.5 with R=Y, Tb, Dy, and Ho

We report muon-spin relaxation measurements on the magnetic structures of RBaCo2O5.5 with R=Y, Tb, Dy, and Ho. Three different phases, one ferrimagnetic and two antiferromagnetic, are identified below 300 K. They consist of different ordered spin state arrangements of high-, intermediate-, and low-spin Co3+ of CoO6 octahedra. Phase separation into well separated regions with different spin state order is observed in the antiferromagnetic phases. The unusual strongly anisotropic magnetoresistance and its onset at the FM-AFM phase boundary is explained.
Facility: SµS

Reference: H. Luetkens et al, Physical Review Letters 101 , 017601 (2008)

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Depth-Dependent Spin Dynamics of Canonical Spin-Glass Films: A Low-Energy Muon-Spin-Rotation Study

We have performed depth dependent muon-spin-rotation and -relaxation studies of the dynamics of single layer films of AuFe and CuMn spin glasses as a function of thickness and of its behavior as a function of distance from the vacuum interface (5–70 nm). A significant reduction in the muon-spin relaxation rate as a function of temperature with respect to the bulk material is observed when the muons are stopped near (5–10 nm) the surface of the sample. A similar reduction is observed for the whole sample if the thickness is reduced to, e.g., 20 nm and less. This reflects an increased impurity spin dynamics (incomplete freezing) close to the surface although the freezing temperature is only modestly affected by the dimensional reduction.
Facility: SµS

Reference: E. Morenzoni et al, Physical Review Letters 100 , 147205 (2008)

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Easy-Axis Kagome Antiferromagnet: Local-Probe Study of Nd3Ga5SiO14

We report a local-probe investigation of the magnetically anisotropic kagome compound Nd3Ga5SiO14. Our zero-field muon spin relaxation (µSR) results provide direct evidence of a fluctuating collective paramagnetic state down to 60 mK, supported by a wipeout of the Ga nuclear magnetic resonance (NMR) signal below 25 K. At 60 mK a dynamics crossover to a much more static state is observed by µSR in magnetic fields above 0.5 T. Accordingly, the NMR signal is recovered at low T above a threshold field, revealing a rapid temperature and field variation of the magnetic fluctuations.
Facility: SµS

Reference: A. Zorko et al, Physical Review Letters 100 , 116601 (2008)

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Intrinsic Mobility Limit for Anisotropic Electron Transport in Alq3

Muon spin relaxation has been used to probe the charge carrier motion in the molecular conductor Alq3 (tris[8-hydroxy-quinoline] aluminum). At 290 K, the magnetic field dependence of the muon spin relaxation corresponds to that expected for highly anisotropic intermolecular electron hopping. Intermolecular mobility in the fast hopping direction has been found to be 0.23±0.03 cm2 V-1 s-1 in the absence of an electric- field gradient, increasing to 0.32±0.06 cm2 V-1 s-1 in an electric field gradient of 1 MV m-1. These intrinsic mobility values provide an estimate of the upper limit for mobility achievable in bulk material.
Facility: SµS

Reference: A.J. Drew et al, Physical Review Letters 100 , 116601 (2008)

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Two-Step Magnetic Ordering in Quasi-One-Dimensional Helimagnets: Possible Experimental Validation of Villain's Conjecture about a Chiral Spin Liquid Phase

Low-temperature specific heat, magnetic susceptibility, and zero-field muon spin resonance (µSR) measurements have been performed in the quasi-one-dimensional molecular helimagnetic compound Gd(hfac)3NITEt. The specific heat presents two anomalies at T0=2.19±0.02 K and TN=1.88±0.02 K, which both disappear upon the application of a weak magnetic field. Conversely, magnetic susceptibility and µSR data show the divergence of two-spin correlation functions only at TN=1.88±0.02 K. These results suggest an experimental validation of Villain's conjecture of a two-step magnetic ordering in quasi-one-dimensional XY helimagnets; i.e., the paramagnetic phase and the helical spin solid phase are separated by a chiral spin liquid phase, where translational invariance is broken without violation of rotational invariance.
Facility: SµS

Reference: F. Cinti et al, Physical Review Letters 100 , 057203 (2008)

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We prove the direct link between low-temperature (T) magnetism and high-T Na+ ordering in NaxCoO2 using the example of a so far unreported magnetic transition at 8 K which involves a weak ferromagnetic moment. The 8 K feature is characterized in detail and its dependence on a diffusive Na+ rearrangement around 200 K is demonstrated. Applying muons as local probes this process is shown to result in a reversible phase separation into distinct magnetic phases that can be controlled by specific cooling protocols. Thus the impact of ordered Na+ Coulomb potential on the CoO2 physics is evidenced opening new ways to experimentally revisit the NaxCoO2 phase diagram.
Facility: SµS

Reference: T.F. Schulze et al, Physical Review Letters 100 , 026407 (2008)

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Anomalous Temperature Evolution of the Internal Magnetic Field Distribution in the Charge-Ordered Triangular Antiferromagnet AgNiO2

Zero-field muon-spin relaxation measurements of the frustrated triangular quantum magnet AgNiO2 are consistent with a model of charge disproportionation that has been advanced to explain the structural and magnetic properties of this compound. Below an ordering temperature of TN=19.9(2) K we observe six distinct muon precession frequencies, due to the magnetic order, which can be accounted for with a model describing the probable muon sites. The precession frequencies show an unusual temperature evolution which is suggestive of the separate evolution of two opposing magnetic sublattices.
Facility: SµS

Reference: T. Lancaster et al, Physical Review Letters 100 , 017206 (2008)

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Electronic Liquid Crystal State in the High-Temperature Superconductor YBa2Cu3O6.45

Electronic phases with symmetry properties matching those of conventional liquid crystals have recently been discovered in transport experiments on semiconductor heterostructures and metal oxides at millikelvin temperatures. We report the spontaneous onset of a one- dimensional, incommensurate modulation of the spin system in the high- transition- temperature superconductor YBa2Cu3O6.45 upon cooling below similar to 150 kelvin, whereas static magnetic order is absent above 2 kelvin. The evolution of this modulation with temperature and doping parallels that of the in- plane anisotropy of the resistivity, indicating an electronic nematic phase that is stable over a wide temperature range. The results suggest that soft spin fluctuations are a microscopic route toward electronic liquid crystals and that nematic order can coexist with high- temperature superconductivity in underdoped cuprates.
Facility: SµS

Reference: V. Hinkov et al, Science 319 , 597 (2008)

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Neutron Decoherence Imaging for Visualizing Bulk Magnetic Domain Structures

Here we introduce a novel neutron imaging method, which is based on the effect that the spatial coherence of the neutron wave front can be changed through small-angle scattering of neutrons at magnetic domain walls in the specimen. We show that the technique can be used to visualize internal bulk magnetic domain structures that are difficult to access by other techniques. The method is transferable to a wide variety of specimens, extendable to three dimensions, and well suited for investigating materials under the influence of external parameters, as, e.g., external magnetic field, temperature, or pressure.
Facility: SINQ

Reference: C. Grünzweig et al, Physical Review Letters 101 , 025504 (2008)

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Quantum Magnets under Pressure: Controlling Elementary Excitations in TlCuCl3

We follow the evolution of the elementary excitations of the quantum antiferromagnet TlCuCl3 through the pressure-induced quantum critical point, which separates a dimer-based quantum disordered phase from a phase of long-ranged magnetic order. We demonstrate by neutron spectroscopy the continuous emergence in the weakly ordered state of a low-lying but massive excitation corresponding to longitudinal fluctuations of the magnetic moment. This mode is not present in a classical description of ordered magnets, but is a direct consequence of the quantum critical point.
Facility: SINQ

Reference: C. Rüegg et al, Physical Review Letters 100 , 205701 (2008)

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Uncovering Flux Line Correlations in Superconductors by Reverse Monte Carlo Refinement of Neutron Scattering Data

We describe the use of reverse Monte Carlo refinement to extract structural information from angle-resolved data of a Bragg peak. Starting with small-angle neutron scattering data, the positional order of an ensemble of flux lines in superconducting Nb is revealed. We discuss the uncovered correlation functions in the light of topical theories, in particular, the “Bragg glass” paradigm.
Facility: SINQ

Reference: M. Laver et al, Physical Review Letters 100 , 107001 (2008)

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Size Effect in the Spin Glass Magnetization of Thin AuFe Films as Studied by Polarized Neutron Reflectometry

We used polarized neutron reflectometry to determine the temperature dependence of the magnetization of thin AuFe films with 3% Fe concentration. We performed the measurements in a large magnetic field of 6 T in a temperature range from 295 to 2 K. For the films in the thickness range from 500 to 20 nm we observed a Brillouin-type behavior from 295 K down to 50 K and a constant magnetization of about 0.9µB per Fe atom below 30 K. However, for the 10 nm thick film we observed a Brillouin-type behavior down to 20 K and a constant magnetization of about 1.3µB per Fe atom below 20 K. These experiments are the first to show a finite-size effect in the magnetization of single spin-glass films in large magnetic fields. Furthermore, the ability to measure the deviation from the paramagnetic behavior enables us to prove the existence of the spin-glass state where other methods relying on a cusp-type behavior fail.
Reference: M. Saoudi et al, Physical Review Letters 100 , 057204 (2008)

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Superconducting Vortices in CeCoIn5 - Towards the Pauli-Limiting Field

Many superconducting materials allow the penetration of magnetic fields in a mixed state where the superfluid is threaded by a regular lattice of Abrikosov vortices each carrying one quantum of magnetic flux. While the phenomenological Ginzburg-Landau theory based on the concept of characteristic length scales has generally provided a good description of the Abrikosov vortex lat- tice state, the present neutron scattering measurements of the vortex lattice form factor in the heavy-fermion superconductor CeCoIn 5 , increases with increasing field - opposite to the expectations from the Abrikosov-Ginzburg- Landau paradigm. It is proposed that the anomalous field-dependence of the form factor arises from Pauli paramagnetic effects around the vortex cores and the proximity of the superconducting state to a quantum critical point.
Facility: SINQ

Reference: A. Bianchi et al, Science 319 , 177 (2008)

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