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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.

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.

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 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.

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 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

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.

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.

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.

Neutron diffraction has been used to determine the magnetic phase diagram of ${\mathrm{HoNi}}_{2}{\mathrm{B}}_{2}\mathrm{C},$ in the plane of temperature and magnetic field up to 4.6 T, for two different crystallographic orientations relative to the applied magnetic field. For both orientations two first-order metamagnetic transitions are observed below 5.5 K as the applied field is increased. In both cases the first transition is to a state in which the magnetic structure is modulated along the c axis with wave vector $\ensuremath{\delta}=[0,0,0.667].$ With the field applied parallel to the $[11\ifmmode\bar\else\textasciimacron\fi{}0]$ direction the second transition is to a saturated paramagnetic state in which the spins are aligned ferromagnetically by the applied magnetic field. With the field parallel to the [0 1 0] direction, the second transition is into a state in which the magnetic structure is modulated along the b axis with wave vector $\ensuremath{\delta}=[0,0.610,0],$ in disagreement with a simplified theoretical model.

Thermal expansion measurements have been performed on single crystals of ${R}_{2\ensuremath{-}2x}{\mathrm{Sr}}_{1+2x}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7}$ ( $R\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}\mathrm{Pr}$, $\mathrm{Nd}$, and $\mathrm{Dy}$) over a wide temperature range (4--900 K). Our analysis allows us to give a general explanation for the large lattice anomalies observed in the layered manganites. Localization of the carriers gives rise to volume and anisotropic anomalies in the paramagnetic regime. The anisotropic distortion is enhanced with the establishment of long-range antiferromagnetism, whereas it is quenched by long-range ferromagnetic order.

In the high-{kappa} cubic superconductor V{sub 3}Si , phase transitions of the flux lattice structure occur as a function of applied field and temperature. With the field parallel to the fourfold [001] axis, the flux lattice transforms from triangular to square symmetry at approximately 1thinspthinspT. With the field parallel to the twofold [110] axis, the lattice, which is a nearly perfect hexagonal array at the lowest fields, distorts as the field is increased; the rate of increase in this distortion changes abruptly at 1.3thinspthinspT. As T{sub c} is approached, the system tends towards a more isotropic hexagonal array of the flux lines. These transitions are largely but not completely in agreement with a recent theory of the effects on the flux line arrangements of nonlocal electrodynamics in the London limit. {copyright} {ital 1999} {ital The American Physical Society }

A small angle neutron scattering study of the flux-line lattice in a large untwinned single crystal of ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ is presented. In fields parallel to the $c$ axis, diffraction spots are observed corresponding to four orientations of a hexagonal lattice, distorted by the $a\ensuremath{-}b$ anisotropy. A value for the anisotropy, the penetration depth ratio, of ${\ensuremath{\lambda}}_{a}/{\ensuremath{\lambda}}_{b}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1.18\left(2\right)$ was obtained. The high quality of the data is such that second-order diffraction is observed, indicating a well ordered FLL. With the field at 33\ifmmode^\circ\else\textdegree\fi{} to $c$, a field dependent reorientation of the lattice is observed around 3 T.

Rare-earth nickel boro carbides [sREdNi2B2C] are an exciting class of materials offering the possibility of coexistence and interactions between superconducting and magnetically ordered states at low temperatures. The RE site may be occupied by both nonmagnetic Y and Lu ions, as well as magnetic Ho, Er, Tm, or Dy ions presenting systems with varying types of magnetic order coinciding with the superconducting state [1 ‐6]. The superconducting transition temperatures, Tc, vary from about 15 K for RE › Y and Lu to 6 K for Dy, scaling roughly with the magnetic pair-breaking de Gennes factor for the RE ions indicating strong interactions between the isolated moments and superconducting electrons [4]. For RE › Ho, Er, and Tm, magnetic ordering occurs below Tc while for RE › Dy order is established above Tc. The magnetic order in these compounds is incommensurate in most cases, becoming antiferromagnetic and commensurate only with the underlying crystal structure at the lowest temperatures [5 ‐ 7]. The occurrence of magnetic order within the superconducting state has provoked much interest in the detailed mechanisms of the coupling between magnetism

Magnetic fields penetrate a type-II superconductor via quantized flux lines. Why these flux lines sometimes form square arrays, as in YNi{sub 2}B{sub 2}C with the field parallel to the {ital c} axis, rather than the expected hexagonal ordering, has long fascinated physicists and has eluded a simple explanation. Our latest measurements on YNi{sub 2}B{sub 2}C prove conclusively that the London penetration depth within the {ital a-b} plane is not isotropic. This anisotropy of the London depths implies that the cross section of an individual flux line has square symmetry, which makes a square lattice energetically favored over the more prosaic hexagonal ordering. {copyright} {ital 1998} {ital The American Physical Society}

We present the results of a neutron-powder-diffraction study in which we have examined the magnetic and structural properties of ${\mathrm{Pr}}_{0.6}({\mathrm{Ca}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{)}_{0.4}{\mathrm{MnO}}_{3}$ $(0l~xl~1)$ as a function of doping concentration x and temperature. For $xl~0.15$ charge ordering and a Jahn-Teller distortion below 250 K are evident from rapid changes in the lattice parameters and bond lengths as a function of temperature and a transition from a high-temperature orthorhombic to a low-temperature monoclinic structure. For $x=0.0$ the system orders antiferromagnetically at ${T}_{N}=170\mathrm{K}.$ At low temperature the system has a canted CE structure. As the temperature approaches ${T}_{N}$ there is a transition to a collinear magnetic arrangement. For Sr doping of up to $x=0.15$ the magnetic transition temperature remains almost constant but the magnetic arrangement now has a ferromagnetic component which persists up to ${T}_{N}.$ For $xg0.15$ the charge ordering is removed and the lattice parameters show a more normal temperature dependence. The materials are now simple ferromagnets and the ferromagnetic transition temperature ${T}_{C}$ increases rapidly with x.

Neutron-diffraction experiments have been performed on both polycrystalline and single-crystal samples of CeCuGa3. The results of the powder diffraction experiments show that CeCuGa3 adopts an incommensurate magnetic structure at 1.25 K. The propagation wave vector associated with the magnetic ordering k(m) is (0.176, 0.176, 0), and the magnitude of the magnetic moment in the ordered state is 1.24 +/- 0.02 mu(B). Single-crystal neutron diffraction confirmed the existence of magnetic satellite peaks at Q values determined by the wave vector k(m). Additional satellites have been observed in the single-crystal diffraction experiments with an incommensurate propagation vector of k(s) = (0.137, 0.137, 0). These are probably the result of a short-ranged modulation of the crystal structure.

High resolution, neutron small angle scattering studies have been performed to investigate the low field $(Bl250\mathrm{mT})$ region of the phase diagram for the vortex lattice in the superconducting state of ${\mathrm{YNi}}_{2}{\mathrm{B}}_{2}\mathrm{C}$. The data present clear evidence for a vortex lattice reorientation transition from a state with the diagonal of the rhombic unit cell along a $[110]$ direction to a $[100]$ direction. Above this transition the lattice distorts under the influence of the applied field until the apex angle becomes constant at some higher field. For $\mathbf{B}\ensuremath{\parallel}\mathbf{c}$ a square lattice configuration is formed. These experiments confirm qualitatively many of the predictions of a general model based on nonlocal corrections to the London model as applied to this and similar materials.

The peak effect in single crystals of the superconductor Ca${}_{3}$Rh${}_{4}$Sn${}_{13}$ has been investigated by dc magnetization, ac susceptibility, electrical resistivity, and critical current measurements in applied magnetic fields. The transition from a weakly pinned region to a strongly pinned region is mapped out in the $H$-$T$ phase diagram. The results on the superconducting stannides, Ca${}_{3}$Rh${}_{4}$Sn${}_{13}$ and Yb${}_{3}$Rh${}_{4}$Sn${}_{13},$ are found to be similar to that obtained on some heavy fermion and intermediate valence superconductors.

Neutron-diffraction measurements have been carried out to determine the nature of the magnetic ordering in ${\mathrm{TmNi}}_{2}$${\mathrm{B}}_{2}$C. ${\mathrm{TmNi}}_{2}$${\mathrm{B}}_{2}$C exhibits superconductivity below 11 K and magnetic ordering below 1.5 K. The magnetic structure is incommensurate and consists of ferromagnetic (110) planes of Tm moments aligned along the c axis with the magnitude of the moments modulated sinusoidally along the (110) direction, the modulation wave vector having a magnitude of 0.241 A${\mathrm{\r{}}}^{\mathrm{\ensuremath{-}}1}$. The magnetic structure as well as the modulation direction exhibited by ${\mathrm{TmNi}}_{2}$${\mathrm{B}}_{2}$C is different from that of other R${\mathrm{Ni}}_{2}$${\mathrm{B}}_{2}$C (R=Gd--Er) compounds. The modulated state formed by the Tm moments allows the superconductivity to coexist with the magnetic ordering below ${\mathit{T}}_{\mathit{N}}$. \textcopyright{} 1996 The American Physical Society.

The $c$-axis resistivity, ${\ensuremath{\rho}}_{c}(B, T, \ensuremath{\theta})$, where $\ensuremath{\theta}$ is the angle between the $c$ axis and the dc field, has been measured for ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$Ca${\mathrm{Cu}}_{2}$${\mathrm{O}}_{8}$ single crystals before and after the introduction of columnar defects by heavy ion irradiation. The effects of different columnar track density and angle with respect to the basal plane are also investigated. Uniaxial enhancement of the irreversibility line for fields below the matching field and parallel to the columnar defects is observed in out-of-plane transport measurements. Measurements in the flux transformer geometry confirm that the vortices are connected lines in the irradiated crystal. We have also attempted to reconcile $c$-axis data with the predictions of the Bose-glass theory for correlated disorder.

Magnetic and transport properties of single crystals of ${\mathrm{TbNi}}_{2}$${\mathrm{B}}_{2}$C have been investigated by ac susceptibility, dc magnetization, specific heat, and resistance measurements. The compound shows highly anisotropic magnetic properties which come about as a result of the Tb magnetic moments lying predominantly in the ab plane. The ac susceptibility and low-field dc magnetization measurements indicate the presence of two magnetic transitions; one due to the antiferromagnetic (AFM) ordering of the Tb moments around 15 K, and another at 5 K, which we believe is due to a spin reorientation of the Tb moments. The resistivity (${\mathrm{\ensuremath{\rho}}}_{\mathit{ab}}$) measurements show a sharp decrease in the resistance at 15 K but contain no observable features corresponding to the lower-temperature transition. The compound does not show superconductivity above 300 mK. Specific heat at low temperatures shows a large \ensuremath{\lambda}-type anomaly at 13.8 K in addition to the anomalies associated with the AFM ordering of the Tb moments at 15 K (${\mathit{T}}_{\mathrm{AF}}$) and the spin reorientation at 5 K. Magnetization (M) and ${\mathrm{\ensuremath{\rho}}}_{\mathit{ab}}$(T) measurements indicate a decrease in ${\mathit{T}}_{\mathrm{AF}}$ as a function of applied field (H) for H\ensuremath{\parallel}ab, whereas for H\ensuremath{\parallel}c, ${\mathit{T}}_{\mathrm{AF}}$ is independent of field. The M-H isotherms at 2 K show that for H\ensuremath{\parallel}ab the compound goes through a series of temperature-dependent metamagnetic states before finally saturating into a ferromagnetic state for H\ensuremath{\gtrsim}21 kOe. For H\ensuremath{\parallel}c the compound shows a linear M-H behavior expected for a normal AFM compound. The ${\mathrm{\ensuremath{\rho}}}_{\mathit{ab}}$(H) measurements show anomalies due to the metamagnetic transitions and a large negative magnetoresistance above 21 kOe for H\ensuremath{\parallel}ab. \textcopyright{} 1996 The American Physical Society.

A detailed study of the properties of ${\mathrm{Pr}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ca}}_{\mathit{x}}$${\mathrm{MnO}}_{3}$ shows that for a range of compositions (0.3\ensuremath{\le}x0.5) there is a first-order magnetic-field-induced insulator-metal transition. For the x=0.4 composition the magnetic and charge ordering (CO) effects are decoupled with antiferromagnetic (AFM) ordering, ${\mathit{T}}_{\mathit{N}}$=170 K, developing at considerably lower temperatures than the CO state, ${\mathit{T}}_{\mathrm{CO}}$=250 K. Below the CO temperature, metamagnetic transitions exist that transform the magnetic correlations from either paramagnetic or AFM to ferromagnetic. Metastable conducting states are formed below 25 K.

We compare the low-temperature electronic properties of the centrosymmetric (CS) and noncentrosymmetric (NCS) phases of Re${}_{3}$W using muon-spin spectroscopy and heat capacity measurements. The zero-field $\ensuremath{\mu}$SR results indicate that time-reversal symmetry is preserved for both structures of Re${}_{3}$W. Transverse-field muon-spin rotation has been used to study the temperature dependence of the penetration depth $\ensuremath{\lambda}(T)$ in the mixed state. For both phases of Re${}_{3}$W, $\ensuremath{\lambda}(T)$ can be explained using a single-gap $s$-wave BCS model. The magnetic penetration depth at zero temperature $\ensuremath{\lambda}(0)$ is $164(7)$ and $418(6)$ nm for the centrosymmetric and noncentrosymmetric phases of Re${}_{3}$W, respectively. Low-temperature-specific heat data also provide evidence for an $s$-wave gap symmetry for the two phases of Re${}_{3}$W. Both the $\ensuremath{\mu}$SR and heat capacity data show that the CS material has a higher ${T}_{c}$ and a larger superconducting gap $\ensuremath{\Delta}(0)$ at 0 K than the NCS compound. The ratio $\ensuremath{\Delta}(0)/{k}_{B}{T}_{c}$ indicates that both phases of Re${}_{3}$W should be considered as strong-coupling superconductors.

We report the observation of positive magnetization on field cooling (PMFC) in low applied magnetic fields ($H\ensuremath{\leqslant}100$ Oe) in a single crystal of Ca${}_{3}$Rh${}_{4}$Sn${}_{13}$ near its superconducting transition temperature (${T}_{c}\ensuremath{\approx}8.35$ K). For 30 Oe $\ensuremath{\leqslant}H\ensuremath{\leqslant}100$ Oe, the PMFC response crosses over to a diamagnetic response as the temperature is lowered below 8 K. For 100 Oe $\ensuremath{\leqslant}H\ensuremath{\leqslant}300$ Oe, the diamagnetic response undergoes an unexpected reversal in its field dependence above a characteristic temperature (designated as ${T}_{\text{VL}}^{*}=7.9$ K), where the field-cooled cool-down magnetization curves intersect. The in-phase and out-of-phase ac susceptibility data confirm the change in the superconducting state across ${T}_{\text{VL}}^{*}$. We ascribe the PMFC response to a compression of magnetic flux caused by the nucleation of superconductivity at the surface of the sample. In very low fields ($H\ensuremath{\leqslant}20$ Oe), the PMFC response has an interesting oscillatory behavior, which persists up to about 7 K. The oscillatory nature underlines the interplay between competing responses contributing to the magnetization signal in the PMFC regime. We believe that the (i) counterintuitive field dependence of the diamagnetic response for $H\ensuremath{\geqslant}100$ Oe and above ${T}_{\text{VL}}^{*}$ (lasting up to ${T}_{c}$), (ii) the oscillatory character in the PMFC response at low fields, and (iii) the PMFC peaks near 8.2 K in 30 Oe $\ensuremath{\leqslant}H\ensuremath{\leqslant}100$ Oe provide support in favor of a theoretical scenario based on the Ginzburg-Landau equations. The scenario predicts the possibility of complex magnetic fluctuations associated with transformation between different metastable giant vortex states prior to transforming into the conventional vortex state as the sample is cooled below ${T}_{\text{VL}}^{*}$.

The superconducting properties of Lu${}_{2}$Fe${}_{3}$Si${}_{5}$ with ${T}_{c}=6.1$ K have been investigated using low-temperature transverse-field muon spin rotation ($\ensuremath{\mu}$SR) and specific-heat measurements. The magnetic penetration depth at zero temperature, $\ensuremath{\lambda}(0)$, is $353(1)$ nm. The temperature dependence of the magnetic penetration depth $\ensuremath{\lambda}(T)$ is consistent with a two-gap $s+s$-wave model. Low-temperature specific-heat measurements on the same sample also show evidence of two distinct superconducting gaps.

Single crystals of Gd(2)(Ti(1-x)Zr(x))(2)O(7) with x = 0.02 and 0.15 have been used to investigate the effects of Zr doping on the properties of the geometrically frustrated antiferromagnet Gd(2)Ti(2)O(7). Powder and single-crystal x-ray data, along with optical birefringence measurements, reveal that the x = 0.02 sample retains the cubic Fd (3) over barm structure of pure Gd(2)Ti(2)O(7), while the x = 0.15 composition adopts a tetragonal I4(1)/amd structure. Low-temperature magnetization and specific heat measurements show that for Gd(2)(Ti(0.98)Zr(0.02))(2)O(7) there are two magnetic transitions at T(N1) = 1.02 K and T(N2) = 0.70 K, but for Gd(2)(Ti(0.85)Zr(0.15))(2)O(7) a single transition is observed at T(N1) = 1.02 K. Changes in the specific heat with a magnetic field applied along the [110] and the [111] directions are used to construct the H-T phase diagrams for both samples.

The magnetic properties of ${\text{Co}}_{3}{\text{V}}_{2}{\text{O}}_{8}$ have been studied by single-crystal neutron diffraction. In zero magnetic field, the observed broadening of the magnetic Bragg peaks suggests the presence of disorder both in the low-temperature ferromagnetic and in the higher temperature antiferromagnetic state. The field dependence of the intensity and position of the magnetic reflections in ${\text{Co}}_{3}{\text{V}}_{2}{\text{O}}_{8}$ reveals a complex sequence of phase transitions in this Kagome staircase compound. For $H\ensuremath{\parallel}a$, a commensurate-incommensurate-commensurate transition is found in a field of 0.072 T in the antiferromagnetic phase at 7.5 K. For $H\ensuremath{\parallel}c$ at low temperature, an applied field induces an unusual transformation from a ferromagnetic to an antiferromagnetic state at about 1 T accompanied by a sharp increase in magnetization.

We report the results of magnetization, heat capacity, and neutron scattering studies of ${\text{LaVO}}_{3}$ single crystals. From the neutron-diffraction studies, it was found that the compound is magnetically ordered with a $C$-type antiferromagnetic spin structure at about 136 K. In the vicinity of the ordering temperature, we also observed hysteresis in the neutron-diffraction data measured on cooling and heating which indicates the first-order nature of the phase transition. In the antiferromagnetically ordered phase, the inelastic neutron scattering studies reveal the presence of a temperature independent $c$-axis spin-wave gap of about 6 meV which is similar to that previously reported for the sister compound ${\text{YVO}}_{3}$.

We have studied the heat capacity and magnetic properties of a EuVO3 single crystal. The compound undergoes an orbital ordering transition at T-OO=204 K, which is then followed by a spin ordering transition at T-SO=131 K. In the spin ordered region, we have found that the compound is very sensitive to small trapped fields present in the superconducting solenoid of a magnetometer during cooling. Large negative Weiss temperatures are seen along all principal crystallographic axes indicating that strong antiferromagnetic interactions are present in this compound. Nevertheless, at low temperatures, we observed open hysteresis loops with a relatively high remanent magnetization and coercivity. We discuss the results for EuVO3 in terms of an inhomogeneity with random fields.

Ultrafast midinfrared spectroscopy is used to probe dynamics in the intermediate bandwidth manganite ${\mathrm{Nd}}_{0.5}{\mathrm{Sr}}_{0.5}{\mathrm{MnO}}_{3}$. In the majority paramagnetic and ferromagnetic phases, the early time dynamics are consistent with the excitation and subsequent redressing of uncorrelated lattice polarons, with longer time dynamics related to spin-lattice thermalization. These polaron excitations reveal the intrinsically inhomogeneous nature of these phases. At lower temperatures we observe ultrafast melting of charge-orbital order, liberating quasiparticles that subsequently relax into bound polaronic states on a subpicosecond time scale. The temperature-dependent amplitude of the polaron excitations scales with the volume fraction of the CE phase. Thus, polaron dynamics, as measured using ultrafast spectroscopy, serve as a sensitive probe of phase inhomogeneity.

We report on a study of various $R\mathrm{V}{\mathrm{O}}_{3}$ single-crystal samples ($R=\mathrm{La}$, Nd, Sm, Gd, Er, and Y) which show temperature-induced magnetization reversal. For compounds with lighter rare earths ($R=\mathrm{La}$, Nd, and Sm), magnetization reversal can be observed for a magnetic field applied in the $ab$ plane and along the $c$ axis, whereas for the heavy rare earths ($R=\mathrm{Gd}$ and Er), magnetization reversal is only observed when the field is applied along the $a$ axis. $\mathrm{Y}\mathrm{V}{\mathrm{O}}_{3}$ has a magnetization reversal along all the main crystallographic axes in a modest applied field. We have found that some compounds are very sensitive to small trapped fields present in the superconducting solenoid of the magnetometer during the cooling. Based on the observed results, we argue that inhomogeniety caused by defects in the orbital sector in the quasi-one-dimensional orbital systems could account for the unusual magnetization reversal.

Polarized and unpolarized neutron-diffraction studies have been carried out on single crystals of the coupled spin tetrahedra systems Cu2Te2O5X2 (X=Cl, Br). A model of the magnetic structure associated with the propagation vectors k(') (Cl) approximate to (-0.150,0.422, (1)/(2)) and k'(Br) approximate to (-0.172,0.356, (1)/(2)) and stable below T-N = 18 K for X= Cl and T-N = 11 K for X = Br is proposed. A feature of the model, common to both the bromide and chloride, is a canted coplanar motif for the four Cu2+ spins on each tetrahedron which rotates on a helix from cell to cell following the propagation vector. The Cu2+ magnetic moment determined for X = Br, 0.395(5)mu(B), is significantly less than for X= Cl, 0.88 (1)mu(B) at 2 K. The magnetic structure of the chloride associated with the wave vector k' differs ;from that determined previously for the wave vector k approximate to (0.150,0.422, (1)/(2))

In ${\mathrm{V}}_{3}\mathrm{Si}$, a first-order structural phase transition from hexagonal to square flux-line lattice occurs at approximately 1 T with $H\ensuremath{\Vert}$ to the $a$ axis. In this paper, we demonstrate the reentrant structural transition in the flux-line lattice, which reverts to hexagonal symmetry as the magnetic field approached ${H}_{c2}(T)$. This behavior is described very well by a nonlocal London theory with thermal fluctuations. The phase diagram of the flux lattice topology is mapped out for this geometry.

Detailed high-resolution small-angle neutron diffraction measurements have been used to determine the structural phase diagram of the vortex lattice VL in YNi2B2C with H c. At low temperatures T Tc the first-order 45° reorientation transition at a field H1 T and second-order rhombic to square transition, H2 T have previously been described by the effects of Fermi surface FS anisotropy and nonlocality. H1 T decreases while H2 T increases with increasing temperature. Measurements of the VL structure as close to the upper critical field Tc2 H as is currently experimentally feasible show no evidence of significant upward curvature or reentrance of H2 that is expected when thermal fluctuations suppress nonlocality. For fields H1 T H H2 T the VL remains rhombic and shows no sign of becoming hexagonal close to Tc2 H . Our data suggest that an underlying electronic asymmetry, other than FS and nonlocal effects and likely due to an anisotropic superconducting gap, controls the VL structure close to Tc2 H .

Neutron inelastic scattering investigations have been performed on the spin tetrahedral system ${\mathrm{Cu}}_{2}{\mathrm{Te}}_{2}{\mathrm{O}}_{5}{X}_{2}$ $(X=\mathrm{Cl},\mathrm{Br})$. We report the observation of magnetic excitations with a dispersive component in both compounds, associated with the three-dimensional incommensurate magnetic order that develops below ${T}_{N}^{\mathit{Cl}}=18.2\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and ${T}_{N}^{\mathit{Br}}=11.4\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The excitation in ${\mathrm{Cu}}_{2}{\mathrm{Te}}_{2}{\mathrm{O}}_{5}{\mathrm{Cl}}_{2}$ softens as the temperature approaches ${T}_{N}^{\mathit{Cl}}$, leaving diffuse quasi-elastic scattering above the transition temperature. In the bromide, the excitations are present well above ${T}_{N}^{\mathit{Br}}$, which might be attributed to the presence of a degree of low dimensional correlations above ${T}_{N}^{\mathit{Br}}$ in this compound.

At low temperature $(T\ensuremath{\sim}1.5\phantom{\rule{0.3em}{0ex}}\mathrm{K})$, the isothermal magnetization curves of a phase-separated manganese oxide $({\mathrm{Pr}}_{0.6}{\mathrm{Ca}}_{0.4}{\mathrm{Mn}}_{0.96}{\mathrm{Ga}}_{0.04}{\mathrm{O}}_{3})$ are found to exhibit a large number of small steps. Analysis of this phenomenon shows several features which are typical of avalanchelike dynamics. These findings lend further support to the martensitic origin of the magnetization steps encountered in phase-separated manganites.

For the spin-chain compound ${\mathrm{Ca}}_{3}{\mathrm{Co}}_{2}{\mathrm{O}}_{6}$, the magnetization curves as a function of the magnetic field are strongly out-of-equilibrium at low temperature, and they exhibit several steps whose origins are still a matter for debate. In the present paper we report on a detailed investigation of the temperature and time dependence of these features. First, it is found that some of the magnetization steps can disappear as the characteristic time of the measurement is increased. A comparison of the influence of temperature and time points to the existence of a thermally activated process that plays an important role in determining the form of the magnetization curves. Second, direct investigations of the magnetic response as a function of time show that this thermally activated process competes with a second relaxation mechanism of a very different nature, which becomes dominant at the lowest temperatures. These results shed new light on the peculiar magnetization process of this geometrically frustrated, Ising-like spin-chain compound.

${\text{Gd}}_{2}{\text{Ti}}_{2}{\mathrm{O}}_{7}$ is a highly frustrated antiferromagnet on a pyrochlore lattice, where apart from the Heisenberg exchange the spins also interact via dipole-dipole forces. We report on low-temperature specific heat measurements performed on single crystals of ${\text{Gd}}_{2}{\text{Ti}}_{2}{\mathrm{O}}_{7}$ for three different directions of an applied magnetic field. The measurements reveal the strongly anisotropic behavior of ${\text{Gd}}_{2}{\text{Ti}}_{2}{\mathrm{O}}_{7}$ in a magnetic field despite the apparent absence of a significant single-ion anisotropy for ${\text{Gd}}^{3+}$. The $H\text{\ensuremath{-}}T$ phase diagrams are constructed for $H\ensuremath{\parallel}[111]$, $H\ensuremath{\parallel}[110]$, and $H\ensuremath{\parallel}[112]$. The results indicate that further theoretical work beyond a simple mean-field model is required.

The Heisenberg spin-chain compound ${\mathrm{Sr}}_{3}{\mathrm{CuPtO}}_{6}$ is investigated by magnetic susceptibility and heat capacity measurements. ${\mathrm{Sr}}_{3}{\mathrm{CuPtO}}_{6}$ has an unconventional chain structure in the sense that (i) the spin-half copper atoms are arranged in a zigzag chain structure and (ii) neighboring Cu atoms along the chains are separated by spin-zero platinum atoms. We report that this compound shows broad features in the temperature dependence of both the magnetic contribution to the heat capacity and the magnetic susceptibility. Despite the unconventional nature of the spin-chain structure, this set of data exhibits good agreement with theoretical models for a classical $S=\frac{1}{2}$ Heisenberg spin-chain compound. The values of the intrachain coupling constant, obtained by different techniques, are found to be very close to each other. The low-temperature heat capacity data (below $\ensuremath{\sim}6\mathrm{K})$ exhibit a deviation from the theoretically expected behavior, which could be related to a small energy gap in the spin excitation spectrum.

For certain combinations of temperature and magnetic field, the evolution with time of the magnetization of some phase-separated manganese oxides exhibits a unique steplike feature. This jump in the magnetization is proposed to correspond to a burstlike growth of the ferromagnetic fraction at the expense of the antiferromagnetic component, driven by the evolution of the strains at the interfaces between the two kinds of domains. These results bear a striking similarity with the phenomenon of an ``incubation time'' encountered in standard martensitic transformations.