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Superconductors usually display either type-I or type-II superconductivity and the coexistence of these two types in the same material, for example, at different temperatures, is rare in nature. We employed the muon spin rotation ($\ensuremath{\mu}\mathrm{SR}$) technique to unveil the superconducting phase diagram of the dodecaboride ${\mathrm{ZrB}}_{12}$ and obtained clear evidence of both type-I and type-II characteristics. Most important, we found a region showing unusual behavior where the usually mutually exclusive $\ensuremath{\mu}\mathrm{SR}$ signatures of type-I and type-II superconductivity coexist. We reproduced that behavior in theoretical modeling that required taking into account multiple bands and multiple coherence lengths, which suggests that material has one coherence length larger and another smaller than the magnetic field penetration length (the type-1.5 regime). At stronger fields, a footprint of the type-II mixed state showing square flux-line lattice was also obtained using neutron diffraction.

In this paper, we present the observation of the electric field control on the charge-ordering and metamagnetic transitions during the magnetization measurements in a single crystal of Pr0.6Ca0.4MnO3 (PCMO). We have demonstrated that the complete melting of charge ordering can be realized in a single crystal of PCMO by applying a voltage as small as 2.5 V, which otherwise needs magnetic fields in excess of 11 T. The maximum change in magnetization with applied voltage occurs across the charge-ordering transition temperature (T-co = similar to 235 K). Even though the electric field does not seem to affect the magnetic ordering, we see a clear evidence at low temperatures for the occurrence of the metamagnetic transitions at higher fields with the application of electric field. (C) 2017 Elsevier B.V. All rights reserved.

We observe a hysteretic reorientation of the magnetic vortex lattice in the noncentrosymmetric superconductor ${\mathrm{Ru}}_{7}{\mathrm{B}}_{3}$, with the change in orientation driven by altering the magnetic field below ${T}_{\mathrm{c}}$. Normally a vortex lattice chooses either a single or degenerate set of orientations with respect to a crystal lattice at any given field or temperature, a behavior well described by prevailing phenomenological and microscopic theories. Here, in the absence of any typical VL structural transition, we observe a continuous rotation of the vortex lattice which exhibits a pronounced hysteresis and is driven by a change in magnetic field. We propose that this rotation is related to the spontaneous magnetic fields present in the superconducting phase, which are evidenced by the observation of time-reversal symmetry breaking, and the physics of broken inversion symmetry. Finally, we develop a model from the Ginzburg-Landau approach which shows that the coupling of these to the vortex lattice orientation can result in the rotation we observe.

We have studied the magnetic properties of a LuVO 3 single crystal. The compound shows an orbital ordering at T OO = 179 K followed by the antiferromagnetic spin ordering at T SO = 109 K. In the magnetically ordered regime, there appears an abrupt change at T o = 82.5 K in the magnetisation, indicating a first-order transition. The compound has very large negative Weiss temperature observed along all the main crystallographic axes, suggesting a strong antiferromagnetic correlations in the paramagnetic state. The observation of hysteresis curves in the collinear antiferromagnetic regime is discussed in terms of an inhomogeneity generating some spins with weak local fields in a strongly antiferromagnetic matrix.

The development of elemental analysis at the ISIS pulsed neutron and muon facility, STFC Rutherford Appleton Laboratory, UK, is described. Presented are recent results from the calibration of the new setup and this shows the technique can be a powerful tool in determining the composition of materials in bulk, not just at the surface. Moreover, this technique is non-destructive and should be sensitive to all elements (perhaps with the exception of H and He).

The noncentrosymmetric superconductor, Re3Ta, has been characterized in detail with a combination of magnetization, heat capacity, and electrical resistivity measurements, as well as a microscopic investigation of the internal magnetic fields using muon spin spectroscopy (μSR). In low applied fields, we observe 100% flux expulsion at a temperature of Tc = 4.68 K, which is concomitant with a sudden decrease of the electrical resistivity to zero and a sharp discontinuity in the heat capacity, confirming bulk superconductivity in this material. We find that Re3Ta is a poor metal, with superconductivity occurring in the dirty limit, and in which the disorder in the structure dominates the physical properties. Zero-field μSR shows that the superconducting state preserves time-reversal symmetry, and transverse-field measurements of the superfluid density are well described by an isotropic s-wave model. A careful analysis of the internal field distribution reveals a high level of disorder in the vortex lattice. Furthermore, we have combined the experimental data and calculated the effective mass, carrier density, and electronic mean-free path in this material, and ultimately show that Re3Ta lies close to the unconventional region of the Uemura plot.

The superconductivity in the rare-earth transition-metal ternary borides $R{\mathrm{RuB}}_{2}$ (where $R=\text{Lu}$ and Y) has been investigated using muon-spin rotation and relaxation. Measurements made in zero field suggest that time-reversal symmetry is preserved upon entering the superconducting state in both materials; a small difference in depolarization is observed above and below the superconducting transition in both compounds, however, this has been attributed to quasistatic magnetic fluctuations. Transverse-field measurements of the flux-line lattice indicate that the superconductivity in both materials is fully gapped, with a conventional $s$-wave pairing symmetry and BCS-like magnitudes for the zero-temperature gap energies. The electronic properties of the charge carriers in the superconducting state have been calculated, with effective masses ${m}^{*}/{m}_{\mathrm{e}}=9.8\ifmmode\pm\else\textpm\fi{}0.1$ and $15.0\ifmmode\pm\else\textpm\fi{}0.1$ in the Lu and Y compounds, respectively, with superconducting carrier densities ${n}_{\mathrm{s}}=(2.73\ifmmode\pm\else\textpm\fi{}0.04)\ifmmode\times\else\texttimes\fi{}{10}^{28}\phantom{\rule{0.28em}{0ex}}{\mathrm{m}}^{\ensuremath{-}3}$ and $(2.17\ifmmode\pm\else\textpm\fi{}0.02)\ifmmode\times\else\texttimes\fi{}{10}^{28}\phantom{\rule{0.28em}{0ex}}{\mathrm{m}}^{\ensuremath{-}3}$. The materials have been classified according to the Uemura scheme for superconductivity, with values for ${T}_{\mathrm{c}}/{T}_{\mathrm{F}}$ of $1/(414\ifmmode\pm\else\textpm\fi{}6)$ and $1/(304\ifmmode\pm\else\textpm\fi{}3)$, implying that the superconductivity may not be entirely conventional in nature.

The properties of the noncentrosymmetric superconductor ($\alpha$-$\textit{Mn}$ structure) Nb$_{0.5}$Os$_{0.5}$ is investigated using resistivity, magnetization, specific heat, and muon spin relaxation and rotation ($\mu$SR) measurements. These measurements suggest that Nb$_{0.5}$Os$_{0.5}$ is a weakly coupled ($\lambda_{e-ph}$ $\sim$ 0.53) type-II superconductor ($\kappa_{GL}$ $\approx$ 61) having a bulk superconducting transition temperature $T_c$ = 3.07 K. The specific heat data in the superconductive regime fits well with the single-gap BCS model indicating nodeless s-wave superconductivity in Nb$_{0.5}$Os$_{0.5}$. The $\mu$SR measurements also confirm $\textit{s}$-wave superconductivity with the preserved time-reversal symmetry., Comment: 7 Page, 8 Figures

The properties of the noncentrosymmetric superconductor (α-[Formula: see text] structure) Nb

We have investigated the superconducting state of the noncentrosymmetric superconductor Re6Ti (Tc = 6.0 K) using muon-spin rotation/relaxation (muSR) technique. The zero-field muon experiment shows the presence of spontaneous magnetic fields in the superconducting state, indicating time-reversal symmetry breaking (TRSB). However, the low-temperature transverse field muon measurements suggest nodeless s-wave superconductivity. The time reversal symmetry breaking further confirmed in the stoichiometric composition Re24Ti5. These results indicate that the pairing symmetry is not affected by spin-orbital coupling Re6X family of compounds. Altogether these studies suggest unconventional nature (TRSB) of superconductivity is intrinsic to Re6X family of compounds and paves the way for further studies of this family of materials., 6 Page, 4 Figure

The discovery of new families of unconventional superconductors is important both experimentally and theoretically, especially if it challenges current models and thinking. By using muon spin relaxation in zero field, time-reversal symmetry breaking has been observed in ${\mathrm{Re}}_{6}\mathrm{Hf}$. Moreover, the temperature dependence of the superfluid density exhibits $s$-wave superconductivity with an enhanced electron-phonon coupling. This, coupled with the results from isostructural ${\mathrm{Re}}_{6}\mathrm{Zr}$, shows that the ${\mathrm{Re}}_{6}X$ family is indeed a new and important group of unconventional superconductors.

We describe the crystal growth of high quality single crystals of the non-centrosymmetric superconductor, Ru7B3 by the floating zone technique, using an optical furnace equipped with xenon arc lamps. The crystals obtained are large and suitable for detailed measurements, and have been examined using x-ray Laue patterns. The superconducting properties of the crystals obtained have been investigated by magnetisation and resistivity measurements. Crystals have also been grown starting with enriched 11B isotope, making them suitable for neutron scattering experiments., Comment: 4 pages, 5 figures. Accepted for publication in Journal of Crystal Growth

The electron-quasiparticle interaction (EQI) spectral function has been measured for $\rm DyNi_2B_2C$ in the normal state at low temperatures by means of point-contact spectroscopy (PCS). A low-frequency peak is found around $eV\sim 5\ meV$. It becomes measurable at $T_m^*\simeq 15\ K$ and grows in intensity with constant width as the temperature is lowered. We argue that this peak arises from the strong interaction of conduction electron with coupled crystal-electric-field-phonon excitations whose branches cross at low energy. The comparison with PC spectra for $\rm HoNi_2B_2C$ suggests that a similar peak also exists for this compound. The magnitude of the point-contact EQI parameter $\lambda_{PC}$ for $\rm DyNi_2B_2C$ is estimated. $\copyright$2000 Published by Elsevier Science B.V. All rights reserved., Comment: 10 pages, 8 figures

We present a detailed investigation of the temperature and depth dependence of the magnetic properties of 3D topological Kondo insulator SmB6 , in particular near its surface. We find that local magnetic field fluctuations detected in the bulk are suppressed rapidly with decreasing depths, disappearing almost completely at the surface. We attribute the magnetic excitations to spin excitons in bulk SmB6 , which produce local magnetic fields of about ~1.8 mT fluctuating on a time scale of ~60 ns. We find that the excitonic fluctuations are suppressed when approaching the surface on a length scale of 40-90 nm, accompanied by a small enhancement in static magnetic fields. We associate this length scale to the size of the excitonic state., Comment: 5 pages, 5 figures, accepted for publication as a Rapid Communication in Phys. Rev. B

We have measured the lifetime and frequency of acoustic phonons in niobium by triple-axis neutron spectroscopy over a range of temperatures spanning the superconducting transition temperature, and have extracted quantitative information on the q and polarisation dependence of the electron-phonon interaction. The behaviour of the transverse modes supports a simple calculation based on the BCS theory of superconductivity, but the damping of a longitudinal mode appears to be unusually severe.

We have measured the complete crystalline electric field (CEF) spectrum of Nd 3+ in superconducting Nd 1.85 Ce 0.15 CuO 4 and in the non-superconducting parent compound, Nd 2 CuO 4 , by neutron inelastic scattering. The best description of the ground state excitations for both compounds is achieved by the addition of a molecular field parameter to the CEF Hamiltonian that takes account of exchange interactions from Nd or Cu spin ordering. This model predicts a small splitting of each of the five crystal field doublets, and the Schottky anomaly associated with the splitting of the ground state is consistent with earlier observations of peaks in the specific heat at low temperatures. The agreement between existing specific heat data and the Schottky anomaly calculated with the assumption of a temperature independent molecular field splitting is very good for Nd 1.85 Ce 0.15 CuO 4 , but for Nd 2 CuO 4 there are differences which may be due to magnetic ordering of the Nd ions and to the change in the molecular field splitting with temperature.

The low-energy magnetic excitations (ħω ≤ 2 meV) of polycrystalline samples of Nd2-xCexCuO4 with x = 0, 0.1 and 0.15 have been measured with high resolution inelastic neutron scattering using time-of-flight technique at temperatures below 0.3 K. All observed scattering originates from transitions within the ground state doublet of Nd3+. For Nd2CuO4 the response is inelastic, and shows at least three different modes, of which one is almost dispersionless. For x = 0.1 the excitation spectrum shifts towards lower energies, but the main features of the undoped compound are still visible, whereas for x = 0.15 the response is mainly quasi-elastic but remains localised in Q-space around the first magnetic Bragg point (0.5 0.5 1). A comparison with data obtained on a triple-axis-spectrometer on single crystals with x = 0.0 and x = 0.15 reveals that the response of the doped sample in the [0 0 1] direction is still inelastic.

The superconductivity of the noncentrosymmetric compound La(7)Ir(3) is investigated using muon spin rotation and relaxation. Zero-field measurements reveal the presence of spontaneous static or quasistatic magnetic fields below the superconducting transition temperature T(c)=2.25 K-a clear indication that the superconducting state breaks time-reversal symmetry. Furthermore, transverse-field rotation measurements suggest that the superconducting gap is isotropic and that the pairing symmetry of the superconducting electrons is predominantly s wave with an enhanced binding strength. The results indicate that the superconductivity in La(7)Ir(3) may be unconventional and paves the way for further studies of this family of materials.

The superconductivity of the noncentrosymmetric compound La(7)Ir(3) is investigated using muon spin rotation and relaxation. Zero-field measurements reveal the presence of spontaneous static or quasistatic magnetic fields below the superconducting transition temperature T(c)=2.25 K-a clear indication that the superconducting state breaks time-reversal symmetry. Furthermore, transverse-field rotation measurements suggest that the superconducting gap is isotropic and that the pairing symmetry of the superconducting electrons is predominantly s wave with an enhanced binding strength. The results indicate that the superconductivity in La(7)Ir(3) may be unconventional and paves the way for further studies of this family of materials.

We have measured the magnetic field (H, Comment: 11 pages, 6 figures. Changes made after referee comments

The first microscopic investigation of superconductivity in LaNiSn is reported using muon spin rotation. LaNiSn is found to be mainly a type I superconductor in an intermediate state with some evidence for type II behaviour at low temperatures, possibly due to a temperature dependent Ginzburg Landau parameter κ .

We describe neutron inelastic scattering measurements of spin waves in the quasi-two-dimensional antiferromagnet FeTa2O6. The intrinsic spin wave dispersion for a single domain is deduced from linear spin wave theory and used to determine the intra-plane exchange coefficients. Almost dispersion-free excitations are observed along the c* direction perpendicular to the magnetic planes. This justifies the neglect of inter-plane coupling, which merely stabilizes the ordered configuration below TN. Spin wave theory yields values for the nearest neighbour and next nearest neighbour exchange constants on perpendicular diagonals of J = −0.040(9) meV, J'/J = 0.20(0), J''/J = 0.23(9), and the anisotropy parameter D = 0.31(8) meV. The large value of D shows an Ising-like spin configuration to be consistent with the experimental data. Measurements of the temperature dependence of the scattering show that magnetic excitations persist to at least 20 K (over two times TN) due to the extensive 2D short range order. This is a highly unusual result, and has consequences for the understanding of two-dimensional spin systems.

Magnetic susceptibility, transport and heat capacity measurements of single crystal NaxCoO2 (x=0.71) are reported. A transition to a spin density wave (SDW) state at Tmag = 22 K is observable in all measurements, except chi(ac) data in which a cusp is observed at 4 K and attributed to a low temperature glassy phase. M(H) loops are hysteretic below 15 K. Both the SDW transition and low temperature hysteresis are only visible along the c-axis. The system also exhibits a substantial (~40%) positive magnetoresistance below this temperature. Calculations of the electronic heat capacity gamma above and below Tmag and the size of the jump in C indicate that the onset of the SDW brings about the opening of gap and the removal of part of the Fermi surface. Reduced in-plane electron-electron scattering counteracts the loss of carriers below the transition and as a result we see a net reduction in resistivity below Tmag. Sodium ordering transitions at higher temperatures are observable as peaks in the heat capacity with a corresponding increase in resistivity., Comment: 14 pages, 6 figures

The possibilities of two-dimensional (2D) short-range magnetic correlations and frustration effects in the mineral tapiolite are investigated using bulk-property measurements and neutron Laue diffraction. In this study of the magnetic properties of synthetic single-crystals of tapiolite, we find that single crystals of FeTa2O6 order antiferromagnetically at TN = 7.95 ± 0.05 K, with extensive two-dimensional correlations existing up to at least 40 K. Although we find no evidence that FeTa2O6 is magnetically frustrated, hallmarks of two-dimensional magnetism observed in our single-crystal data include: (i) broadening of the susceptibility maximum due to short-range correlations, (ii) a spin-flop transition and (iii) lambda anomalies in the heat capacity and d(χT)/dT. Complementary neutron Laue diffraction measurements reveal 1D magnetic diffuse scattering extending along the c* direction perpendicular to the magnetic planes. This magnetic diffuse scattering, observed for the first time using the neutron Laue technique by VIVALDI, arises directly as a result of 2D short-range spin correlations.

Compounds with a Kagome type lattice are known to exhibit magnetic frustration. Large single crystals of two compounds Ni3V2O8 and Co3V2O8, which are variants of a Kagome net lattice, have been grown successfully by the floating zone technique using an optical image furnace. The single crystals are of high quality and exhibit intriguing magnetic properties.

The magnetic behavior of the Ca3Co2O6 spin chain compound is characterized by a large Ising-like character of its ferromagnetic chains, set on a triangular lattice, that are antiferromagnetically coupled. At low temperature, T < 7 K, the 3D antiferromagnetic state evolves towards a spin frozen state. In this temperature range, magnetic field driven magnetization of single crystals (H // chains) exhibits stepped variations. The occurrence of these steps at regular intervals of the applied magnetic field, Hstep = 1.2 T, is reminiscent of the quantum tunneling of the magnetization (QTM) of molecular based magnets. Magnetization relaxation experiments also strongly support the occurrence of this quantum phenomenon. This first observation of QTM in a magnetic oxide belonging to the large family of A3BB′O6 compounds opens new opportunities to study a quantum effect in a very different class of materials from molecular magnets.

We report on investigations of the magnetic properties of the tetravalent praseodymium (Pr4+) compound Sr2PrO4 by dc and ac magnetic susceptibility and heat capacity measurements. The compound shows signatures of long range magnetic order at TN = 3 K in the heat capacity and magnetization data. In addition, the temperature dependence of the field cooled and the zero-field cooled susceptibility is hysteretic below 10 K. Heat capacity versus temperature data show that a significant part of the magnetic entropy is released above TN. This reveals that short range magnetic correlations among the Pr moments exist well above TN. The magnetic entropy obtained from the heat capacity measurements indicates that only the crystal field ground state of Pr with effective spin- is significantly populated at low temperatures.

Specific heat measurements were carried out on the closely related spin-chain oxides Ca3Co2O6 and Ca3CoRhO6. Both compounds consist of Ising magnetic chains that are arranged on a triangular lattice. The spin coupling along and between the chains are ferromagnetic and antiferromagnetic, respectively. Geometrical frustration is expected from the combination of these magnetic features. The present study reports that the specific heat data of these compounds exhibit similarities in the spin-freezing process at low T, whereas striking differences exist in the antiferromagnetic interchain ordering at higher T. These results are discussed in connection with previous magnetization and neutron diffraction data., 5 pages, 5 figures, accepted for publication in J. Phys.: Cond. Matt

Single crystal growth of several rare earth hexaborides has been carried out by the floating zone technique. A high power Xenon arc lamp image furnace was used for the crystal growth. Large high quality crystals of LaB$_{6}$, CeB$_{6}$, PrB$_{6}$ and NdB$_{6}$, about 1 cc in volume have been obtained. Crystals of all these compounds have also been grown using enriched $^{11}$B isotope for use in neutron scattering experiments., Comment: 3 pages, 3 figures, in press (J. Crystal Growth)

We report on polarized Raman scattering of single crystals of Nd1-xSrxMnO3 (x = 0.3, 0.5). Raman spectra of Nd0.7Sr0.3MnO3 show a significant change through the metal-insulator transition. In the ferromagnetic metallic phase phonon modes grow in intensity and number while the electronic continuum becomes more pronounced. We suggest that these effects are due to the strong competition between the localization and the delocalization of carriers which is the origin of the largest colossal magnetoresistance effect ever reported for the manganites. Raman spectra of Nd0.5Sr0.5MnO3, upon cooling through the charge-ordering temperature TCO = 148 K, exhibit several new lines which undergo a substantial hardening. This hardening is interpreted as a freezing of the Jahn-Teller distortions with a gradual decrease of a fraction of the ferromagnetic phase in the CE-type charge/orbital ordered state.

Single crystals of the non-centrosymmetric superconductor Nb 0.18 Re 0.82 have been synthesized by the floating zone technique using a four mirror optical furnace equipped with Xe arc lamps. The crystal shows a superconducting transition at ∼8.8 K. The quality and phase purity of the crystals were confirmed by x-ray Laue and Energy dispersive x-ray spectroscopy (EDAX) measurements. Large volumes of crystal obtained are suitable for detailed magnetic, transport and neutron scattering experiments.

We have performed extensive small-angle neutron scattering (SANS) diffraction studies of the vortex lattice in single crystal YNi2B2C for B‖c. High-resolution SANS, combined with a field-oscillation vortex lattice preparation technique, allows us to separate Bragg scattered intensities from two orthogonal domains and accurately determine the unit cell angle, β. The data suggest that upon increasing field there is a finite transition width where both low- and high-field distorted hexagonal vortex lattice phases, mutually rotated by 45°, coexist. The smooth variation of diffracted intensity from each phase through the transition corresponds to a redistribution of populations between the two types of domains.

We report polarized Raman scattering in single crystals of Nd0.7Sr0.3MnO3. The temperature dependence of the MnO6 octahedral bending and stretching modes observed in the XX spectra points to the existence of local lattice distortions, possibly polarons. The XY spectra have been analyzed using a collision-dominated model, which allows the extraction of the carrier scattering rate.

Transmittance of the colossal magnetoresistive compound Nd_0.5Sr_0.5MnO_3 showing metal-insulator phase transition has been studied by means of the submm- and mm-wavelength band spectroscopy. An unusually high transparency of the material provided direct evidence for the significant suppression of the coherent Drude weight in the ferromagnetic metallic state. Melting of the A-type antiferromagnetic states has been found to be responsible for a considerable increase in the microwave transmission, which was observed at the transition from the insulating to the metallic phase induced by magnetic field or temperature. This investigation confirmed a dominant role of the (x^2-y^2)-orbital degree of freedom in the low-energy optical properties of Nd_0.5Sr_0.5MnO_3 and other doped manganites with planar (x^2-y^2)-orbital order, as predicted theoretically. The results are discussed in terms of the orbital-liquid concept., Comment: 8 pages, 3 figures

The superconducting properties of CaTSi3 (where T = Pt and Ir) have been investigated using muon spectroscopy. Our muon-spin-relaxation results suggest that in both these superconductors time-reversal symmetry is preserved, while muon-spin-rotation data show that the temperature dependence of the superfluid density is consistent with an isotropic s-wave gap. The magnetic penetration depths determined from our transverse-field muon-spin-rotation spectra are found to be 448(6) and 150(7) nm for CaPtSi3 and CaIrSi3, respectively.

We present the results of a systematic investigation of the magnetic properties of the three-dimensional Kondo topological insulator SmB6 using magnetization and muon-spin relaxation/rotation (muSR) measurements. The muSR measurements exhibit magnetic field fluctuations in SmB6 below 15 K due to electronic moments present in the system. However, no evidence for magnetic ordering is found down to 19 mK. The observed magnetism in SmB6 is homogeneous in nature throughout the full volume of the sample. Bulk magnetization measurements on the same sample show consistent behavior. The agreement between muSR, magnetization, and NMR results strongly indicate the appearance of intrinsic bulk magnetic in-gap states associated with fluctuating magnetic fields in SmB6 at low temperature., 5 pages, 5 figures

The crystal structure of the manganite Nd0.5Sr0.5MnO3 is studied at temperature T=300 and 77.3 K by means of an x-ray diffractometer. It is shown that the transition from the ferromagnetic metallic state to the antiferromagnetic insulating charge-ordered state is accompanied by a lowering of the symmetry of the structure from orthorhombic to monoclinic. The space-group symmetry of the orthorhombic and monoclinic phases is identified as Imma and P21/m, respectively. Twinning of the crystal and the formation of a twin domain structure with coherent boundaries in the (00l) crystallographic planes are found.

The magnetization of Nd 2−2 x Sr 1+2 x Mn 2 O 7 layered manganite single crystals (with x =0.5, 0.4, 0.25) has been studied using high magnetic fields up to 30 T at 4.2 K. In these compounds, which are antiferromagnetic at low temperatures, induced metamagnetic transitions have been observed. These transitions become very sharp for doping level values in the range x =0.25–0.4, reaching saturation-like behavior and displaying a large hysteresis. Maximum field magnetization is much lower than expected for the full ferromagnetic configuration of the Nd and Mn moments, and a large high field susceptibility is observed beyond the transition. The observed magnetization at maximum field within the ab -plane is lower (up to a 17%) than along the c direction. However, the value of the critical magnetic field of the transition is in all the cases larger for the magnetization within ab than along c . These transitions have been ascribed to spin-flipping within the Mn moment sublattice.

Magnetization measurements have been performed on both pristine, as-produced single crystals of ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ and on crystals irradiated with energetic heavy ions. A scaling analysis applied to the hysteresis curves measured for both crystals enables the enhanced contribution to the critical current, due to the columnar defects, to be separated from that due to the residual pinning. Magnetic hysteresis, due to pinning almost entirely by the columnar defects, can be scaled between 4 K and 40 K, suggesting that the irradiation damage results in a single pinning mechanism over this temperature range. A temperature-dependent crossover field ${B}_{\mathrm{cr}}$ is determined, above which pinning due to the columnar defects is dramatically reduced. ${B}_{\mathrm{cr}}$ rises to several times the matching field ${B}_{\ensuremath{\varphi}}$ at low temperatures. These results indicate that pinning due to both types of defects appears as a linear summation in the overall pinning force and emphasizes the importance of both residual pinning and vortex-vortex interactions for fields close to ${B}_{\ensuremath{\varphi}}$ where a crossover from correlated to point disorder determined behavior is usually assumed.

Recent experimental results indicate that superconductivity in Sr2RuO4 is described by the p-wave E_u representation of the D_{4h} point group. Results on the vortex lattice structures for this representation are presented. The theoretical results are compared with experiment., Comment: 4 pages, 3 figures, M2S-HTSC-VI proceedings

The magnetic field dependence of the critical current is studied in single-crystal samples of the weak pinning type-II superconductor $2H\ensuremath{-}{\mathrm{NbSe}}_{2}$ in the high-temperature and the low-field region of the $(H,T)$ space. The experimental results demonstrate various pinning regimes: a collective pinned quasiordered solid in the intermediate-field range that is destabilized in favor of disordered vortex phases in both high fields near ${H}_{c2}$ and at low fields near ${H}_{c1}.$ The temperature evolution of the pinning behavior demonstrates how the amorphous limit (where the correlation volume is nearly field independent) is approached around the so-called nose region of the reentrant peak-effect boundary. In the high-field regime the rapid approach to the amorphous limit naturally yields a peak effect, i.e., a peak in the critical current. In the low-field regime the crossover to the individual pinning regime gives rise to a ``plateau effect.'' We show that with increasing effective pinning the peak effect shifts away from ${H}_{c2}$ and resembles a ``fishtail'' anomaly.

The results of systematic ultrasonic and magnetic studies of the magnetoresistive compound ${\mathrm{Nd}}_{0.5}{\mathrm{Sr}}_{0.5}{\mathrm{MnO}}_{3}$ are reported. With decreasing temperature the pronounced acoustic-mode hardening was observed at about ${T}_{\mathrm{CO}}=145$ K. The significant softening of the acoustic mode was observed in the vicinity of the insulator-to-metal transition induced by magnetic field. The $T\ensuremath{-}B$ phase diagram was obtained from the ultrasonic measurements. The change of sound velocity around ${T}_{\mathrm{CO}}$ was explained by a strong coupling between the acoustic phonons and the CO states. The peculiarities of the sound propagation at the phase transition can be attributed to the phase segregation in the ground state in ${\mathrm{Nd}}_{0.5}{\mathrm{Sr}}_{0.5}{\mathrm{MnO}}_{3}.$ An enhancement in magnetic correlations at temperatures below 20 K, as seen by magnetization and ultrasound measurements, was observed in ${\mathrm{Nd}}_{0.5}{\mathrm{Sr}}_{0.5}{\mathrm{MnO}}_{3}.$ We discuss this effect in terms of possible short-range magnetic ordering on the rare-earth sublattice.

We have measured the diffracted neutron scattering intensities from the square magnetic flux lattice in the perovskite superconductor ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$, which is thought to exhibit $p$-wave pairing with a two-component order parameter. The relative intensities of different flux lattice Bragg reflections over a wide range of field and temperature have been shown to be inconsistent with a single component Ginzburg-Landau theory but qualitatively agree with a two-component $p$-wave Ginzburg-Landau theory.

Neutron diffraction has been used to determine the magnetic order in the metamagnetic phases of ErNi2B2C, with the magnetic field applied parallel to the [0 1 0] and [1 1 0] crystallographic axes. Below 6 K, ErNi2B2C has an incommensurate antiferromagnetic structure with a modulation along the a-axis of wavevector δ=(0.55,0,0). For both orientations three first-order metamagnetic transitions are observed below 6 K as the applied field is increased. The first two transitions are to incommensurate antiferromagnetic states with different values of the a-axis modulation, and the third transition is to a saturated paramagnetic state in which the spins are aligned ferromagnetically by the applied field. With the field applied parallel to the [0 1 0] direction, which is the magnetically easy axis, the upper incommensurate state has an additional ferromagnetic component which is not observed with the field applied parallel to the [1 1 0] direction.

The peak effect (PE) region in a single crystal of ${\mathrm{Ca}}_{3}{\mathrm{Rh}}_{4}{\mathrm{Sn}}_{13}$ is shown to comprise two discontinuous first-order-like transitions located near its onset and peak positions, in accordance with a stepwise fracturing of the flux-line lattice. Magnetization response to thermal cycling across the onset position produces an open hysteresis loop, consistent with the notion of the fracturing. A thermomagnetic history dependence study shows that the critical current density ${J}_{c}(H,T)$ is path dependent over a large part of the $(H,T)$ parameter space. This path dependence ceases above the peak position of the peak effect, suggesting a complete amorphization of the flux-line lattice at ${(T}_{p}{,H}_{p})$ line. A plausible vortex phase diagram has been constructed for ${\mathrm{Ca}}_{3}{\mathrm{Rh}}_{4}{\mathrm{Sn}}_{13}$ in which phases like an elastic solid, a plastic solid, and pinned and unpinned amorphous states have been identified.

Pronounced thermomagnetic history effects, implying metastability of vortex phases, are observed in the low-Tc system 2H-NbSe2. Some special experimental consequences of metastability are summarized., © Elsevier

In the majority of the (RE)Ni 2 B 2 C family of superconductors (RE=Dy, Ho, Er, Tm, Y, Lu), a magnetic ordering of the RE moments (RE=Dy, Ho, Er, Tm) appears to directly influence the formation and structure of the vortex lattice as well as the weak residual flux pinning properties. To study the residual pinning in these materials, it is, therefore, necessary to investigate the non-magnetic members such as YNi 2 B 2 C or LuNi 2 B 2 C, without the influence of magnetic order. Here, we present data from local Hall probe and global magnetization measurements used to examine flux pinning and superconducting hysteresis in YNi 2 B 2 C ( T c ≈15.8 K). At high fields, a pronounced peak effect in the magnetization indicates that bulk pinning becomes significant as the vortex lattice softens for fields approaching B c2 . On the other hand, for small applied fields close to H c1 , direct measurements of the local induction using linear micro-Hall probe arrays show dome-like field profiles, as expected when surface and geometrical barrier effects dominate the vortex behaviour over bulk pinning. We discuss the competing roles of weak residual bulk pinning and surface and geometrical barrier effects in YNi 2 B 2 C.