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Local magnetic measurements vs temperature in an untwinned ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ crystal reveal an abrupt increase in the local magnetization at a field-dependent temperature, well below the melting line. At the same field and temperature a pronounced kink is observed in the local magnetization vs field curves. The line ${B}_{k}(T)$ describing the locations of these anomalies in the field-temperature phase diagram divides the vortex solid phase into two regions characterized by weak and strong pinning. A recently developed model describing the vortex solid-solid disorder-induced phase transition explains quantitatively the observed behavior of ${B}_{k}(T).$ From this behavior we infer that the microscopic origin of pinning in ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ is fluctuations in the charge-carrier mean free path.

We calculate the distribution of the current density $j$ in superconducting films along the direction of an external field applied perpendicular to the film plane. Our analysis reveals that in the presence of bulk pinning $j$ is inhomogeneous on a length scale of order the inter vortex distance. This inhomogeneity is significantly enhanced in the presence of surface pinning. We introduce new critical state model, which takes into account the current density variations throughout the film thickness, and show how these variations give rise to the experimentally observed thickness dependence of $% j$ and magnetic relaxation rate., RevTex, 9 PS figures. To appear in Phys. Rev. B

We report on measurements of the angular dependence of the irreversibility temperature {ital T}{sub irr}({theta}) in YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} thin films, defined by the onset of a third-harmonic signal and measured by a miniature Hall probe. From the functional form of {ital T}{sub irr}({theta}) we conclude that the origin of the irreversibility line in unirradiated films is a dynamic crossover from an unpinned to a pinned vortex liquid. In irradiated films the irreversibility temperature is determined by the trapping angle. {copyright} {ital 1996 The American Physical Society.}

Bi 2 Sr 2 CaCu 2 O 8 (BSCCO) crystals were irradiated with 5.8-GeV Pb ions to produce columnar defects either along the c direction or at 45 o with respect to it. Magnetization curves of the irradiated BSCCO crystals were measured with the applied field at different directions relative to the defects. The width of the magnetization curves, which reflects the pinning strength, is found to be the largest for fields along the defects. This unidirectional enhancement clearly indicates the line nature of the vortices in BSCCO

Defects in the form of cylindrical amorphous tracks were introduced into ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ crystals by irradiation with 5.3-GeV Pb ions. The columnar defects formed provide maximum possible pinning of flux lines parallel to the tracks and induce giant enhancement of magnetic irreversibility. However, even for this strong pinning, magnetic decay was measured at high temperatures. Relaxation is nonlogarithmic and exhibits an increase of the effective barrier for flux creep with decreasing persistent current. This behavior is interpreted in the framework of the nucleation creep model.

A superconducting ${\text{La}}_{1.84}{\text{Sr}}_{0.16}{\text{CuO}}_{4}$ film patterned into a network of $100\ifmmode\times\else\texttimes\fi{}100\text{ }{\text{nm}}^{2}$ noninteracting square loops exhibits large magnetoresistance oscillations superimposed on a background which increases monotonically with the applied magnetic field. Neither the oscillations amplitude nor its temperature dependence can be explained by the Little-Parks effect. Conversely, a good quantitative agreement is obtained with a recently proposed model ascribing the oscillations to the interaction between thermally excited moving vortices and the oscillating persistent currents induced in the loops. Extension of this model, allowing for direct interaction of the vortices and antivortices magnetic moment with the applied field, accounts quantitatively for the monotonic background as well. Analysis of the background indicates that in the patterned film both vortices and antivortices are present at comparable densities. This finding is consistent with the occurrence of Berezinskii-Kosterlitz-Thouless transition in ${\text{La}}_{1.84}{\text{Sr}}_{0.16}{\text{CuO}}_{4}$ films.

We present magnetic measurements in a single crystal of the newly discovered superconducting iron-pnictide $\text{Ba}{({\text{Fe}}_{0.925}{\text{Co}}_{0.075})}_{2}{\text{As}}_{2}$. The magnetization loops exhibit a second magnetization peak (SMP) similar to that observed in most high-temperature superconductors (HTSs). Magnetic relaxation measurements reveal a minimum in the normalized relaxation rate, $S=d\text{ }\text{ln}\text{ }M/d\text{ }\text{ln}\text{ }t$, located in between the SMP onset and the peak fields. The SMP in HTSs is commonly associated with the vortex order-disorder phase transition. However, in $\text{Ba}{({\text{Fe}}_{0.925}{\text{Co}}_{0.075})}_{2}{\text{As}}_{2}$ the onset and peak fields, as well as the minimum point in $S$, exhibit strong temperature dependence down to low temperatures, excluding the possibility for such a transition. We suggest that the SMP in $\text{Ba}{({\text{Fe}}_{0.925}{\text{Co}}_{0.075})}_{2}{\text{As}}_{2}$ is associated with a vortex structural phase transition from rhombic to square lattice taking place at field and temperatures corresponding to the minimum point of $S$. A theoretical fit to the transition line, based on a recent theoretical model for vortex structural phase transition, shows good agreement with the experimental results.

We investigated ${\mathrm{Pr}}_{2}$${\mathrm{BaO}}_{4}$ and ${\mathrm{PrBaO}}_{3}$ by several complementary experimental techniques. ${\mathrm{PrBaO}}_{3}$ crystallize in an orthorhombic structure. dc and ac susceptibility measurements exhibit irreversibility phenomena below ${\mathit{T}}_{\mathit{N}}$=11.7 K and reveal the magnetic structure of ${\mathrm{PrBaO}}_{3}$, which consists of an antiferromagnetic ordering together with a small canting of the spins which produces the ferromagnetic component. Both magnetic and specific-heat studies show that ${\mathit{T}}_{\mathit{N}}$ is not affected by the applied magnetic field up to 9 T. ${\mathrm{Pr}}_{2}$${\mathrm{BaO}}_{4}$ crystallizes in an orthorhombic structure where a=10.56 \AA{}, b=12.43 \AA{}, and c=3.617 \AA{} belongs to Pnam space group. dc and ac susceptibility and specific-heat measurements show that ${\mathrm{Pr}}_{2}$${\mathrm{BaO}}_{4}$ is paramagnetic down to 2 K, with a vanishing electronic specific-heat coefficient \ensuremath{\gamma}. X-ray photoemission spectroscopy shows that Pr is mainly trivalent in ${\mathrm{Pr}}_{2}$${\mathrm{BaO}}_{4}$ and close to being tetravalent in ${\mathrm{PrBaO}}_{3}$. The pronounced ferromagnetic-like behavior found below 11.7 K in the ${\mathrm{Pr}}_{\mathit{x}}$${\mathit{R}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{\mathit{z}}$ system is due to the impurity of ${\mathrm{PrBaO}}_{3}$.

We present a study of the field dependence of the remanent nonlinear magnetic response induced in sintered Y-Ba-Cu-O samples by momentary application of a dc magnetic field H. The results show that, contrary to theoretical predictions, the remanent nonlinear response depends strongly on H. We attribute this behavior to weak links subjected to internal fields induced by flux trapped within the grains. To support this view, we present an extensive comparison between ac and dc data in the same samples. This comparison clarifies the role of grains and of weak links in various field regimes, and demonstrates the relationship between the remanent nonlinear response and the trapped flux.

Fingerlike flux patterns, normally observed in superconducting films, are demonstrated in a bulk material. We observed such patterns in ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}\mathrm{Ca}{\mathrm{Cu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ crystals with an artificial barrier separating regions of different magnetic diffusivity, created by irradiating one-half of the crystal with heavy ions. Flux penetration from the nonirradiated part of the crystal, through the barrier, into the irradiated part forms fingerlike patterns in a wide range of temperatures and field ramping rates. The mechanism responsible for the observed patterns is discussed.

Vector magnetization measurements in ${\mathrm{La}}_{1.85}{\mathrm{Sr}}_{0.15}\mathrm{Cu}{\mathrm{O}}_{4}$ under fields slightly tilted from the crystallographic $ab$ plane reveal flux rotation with distinctive behavior around ${T}^{*}\ensuremath{\approx}15\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The temperature dependence of the onset-field ${H}^{*}$ for flux rotation and the dependence of ${H}^{*}$ upon the tilting angle are markedly different below and above ${T}^{*}$, exhibiting pronounced anomalies in the crossover regime. Possible mechanisms for these anomalies, such as dimensional crossover in superconductivity or the anomalous behavior of the in-plane penetration depth due to coexistence of two superconducting gaps, are discussed.

A theory of structural phase transition of the vortex lattice in tetragonal superconductors is constructed based on the self consistent harmonic approximation for lattice anharmonicities, within the fourfold-symmetric generalization of the London model. Thermal fluctuations on the mesoscopic scale are strong enough to affect the location of the square to rhomb transition line in the $T\text{\ensuremath{-}}H$ plane. We find that the slope of the transition line is generally negative: thermal fluctuations favor the more symmetric square lattice. The calculated transition line is concave, and fits the experimental line deduced in LaSCO crystals from the onset of the second magnetization peak. Near the transition line the ``squash'' modulus ${C}_{\mathit{sq}}=2({C}_{11}+{C}_{12})\ensuremath{-}{C}_{66}$ is softened leading to enhancement of the critical current, born out in the experiment as a second magnetization peak.

The dynamics of transient disordered vortex states in ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ was magneto-optically traced in three experiments: (i) during continuous injection of transient vortex states while ramping up the external magnetic field, (ii) during annealing of injected transient states while keeping the external field constant, and (iii) during annealing of transient ``supercooled'' disordered states while ramping down the external field. The results reveal frontlike propagation [experiment (i)] or retreat [experiments (ii) and (iii)] of the transient vortex states, at a rate governed by the rate of change of the external field, the annealing time $\ensuremath{\tau}$ of the transient states and the creep rate. The experimental results are theoretically analyzed in terms of competition between generation and annealing of transient disordered vortex states. Extraction of the annealing time $\ensuremath{\tau}$ from the above three experiments yields the same results for $\ensuremath{\tau}$ as a function of the induction B and temperature T. Knowledge of $\ensuremath{\tau}(B,T)$ allows for correct determination of the thermodynamic order-disorder vortex phase-transition line.

High-temporal resolution magneto-optical measurements in Bi 2 Sr 2 CaCu 2 O 8 + δ demonstrate that transient vortex states govern its magnetic behavior near the order-disorder vortex phase transition. These data enable direct measurement of the annealing time T of the transient states as a function of the induction, temperature, and electric field. The behavior of τ reveals fundamental limitations of common experimental procedures in characterizing the disorder-driven vortex phase transition.

Magneto-optical snapshots of the induction distribution across a Bi 2 Sr 2 CaCu 2 O 8 + δ crystal subjected to ramped magnetic fields reveal strong effects of transient disordered vortex states near the disorder-driven vortex phase transition. These transient states, predominantly generated by edge contamination during field ascending and by supercooling of the disordered vortex lattice during field descending, cause smearing of the transition and shift of its onset towards lower inductions. With decreasing temperature, these effects are more pronounced as the lifetime of the transient vortex states increases. Similar effects are observed with increasing field sweep rates, as transient states with shorter lifetime play a role. These observations clarify previously reported puzzles, e.g., the absence of the second magnetization peak at low temperatures, its appearance at longer times, and the shift of its onset to lower inductions for higher field sweeping rates.

We report on magnetization measurements in $({\mathrm{La}}_{0.937}{\mathrm{Sr}}_{0.063}{)}_{2}{\mathrm{CuO}}_{4}$ crystals with the field either parallel or perpendicular to the c axis. A second magnetization peak (``fishtail''), interpreted as indicating a vortex order-disorder transition, is observed in both directions. Differences in the details of the fishtail anomaly in the two directions (width, temperature and time dependence, and history effects) are attributed to the anisotropy and the twin boundaries in the ab plane. The transition fields in both directions, although different in magnitude, exhibit similar qualitative behavior, namely, a strong decrease with temperature in the entire measured range. This unique behavior is explained postulating that both thermally and disordered induced fluctuations contribute to the destruction of the vortex lattice. The resulting ``pinned liquid'' disordered state is characterized by large thermal fluctuations and irreversible magnetic behavior.

The diversity of vortex melting and solid-solid transition lines measured in different high-T c superconductors is explained, postulating a unified order-disorder phase transition driven by both thermally- and disorder-induced fluctuations. The temperature dependence of the transition line and the nature of the disordered phase (solid, liquid, or pinned liquid) are determined by the relative contributions of these fluctuations and by the pinning mechanism. By varying the pinning mechanism and the pinning strength one obtains a spectrum of monotonic and nonmonotonic transition lines similar to those measured in Bi 2 Sr 2 CaCu 2 O 8 , YBa 2 Cu 3 O 7 - Φ , Nd 1 . 8 5 Ce 0 . 1 5 CuO 4 - Φ , Bi 1 . 6 Pb 0 . 4 Sr 2 CaCu 2 O 8 + Φ , and (La 0 . 9 3 7 Sr 0 . 0 6 3 ) 2 CuO 4 .

Magnetization measurements in ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4}$ crystals indicate vortex order-disorder transition manifested by a sharp kink in the second magnetization peak. The transition field exhibits unique temperature dependence, namely a strong decrease with temperature in the entire measured range. This behavior rules out the conventional interpretation of a disorder-driven transition into an entangled vortex solid phase. It is shown that the transition in ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4}$ is driven by both thermally- and disorder-induced fluctuations, resulting in a pinned liquid state. We conclude that vortex solid-liquid, solid-solid and solid to pinned-liquid transitions are different manifestations of the same thermodynamic order-disorder transition, distinguished by the relative contributions of thermal and disorder-induced fluctuations.

Magnetization measurements as a function of temperature are reported for ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6.5}$ crystal ${(T}_{c}=45.2 \mathrm{K})$ for fields between 0.2 and 3.5 T. All isochamps for $Hg1$ T intersect at ${T}_{2D}^{\ensuremath{\star}}\ensuremath{\simeq}42.8$ K, implying a fluctuation contribution to the magnetization. These curves collapse into a single curve when magnetization and temperature are scaled according to the predicted ``two-dimensional (2D) scaling'' in the fluctuation regime. Surprisingly, the low-field curves also intersect, at ${T}_{3D}^{\ensuremath{\star}}\ensuremath{\simeq}43.4$ K, and they obey a 3D scaling. We provide a theoretical picture of the magnetization in the fluctuation regime based on the Lawrence-Doniach model. Within this model we calculate the field and temperature dependence of the magnetization. The two intersection points and the $2\stackrel{\ensuremath{\rightarrow}}{D}3\mathrm{D}$ crossover are consistent with the experimental observation.

We have studied the angular dependence of the magnetization of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ by rotating the sample relative to the applied magnetic field. At low temperatures and low fields the irreversible part of the field-cooled magnetization ${M}_{\mathrm{irr}}$ rotates with the sample as a rigid entity. The rigidity is broken and part of ${M}_{\mathrm{irr}}$ disappears above an angle ${\ensuremath{\varphi}}^{\mathrm{*}}$ which decreases with increasing either temperature or field. This behavior resembles that of spin glasses and differs qualitatively from the behavior found for Nb, a type-II superconductor.

Neutron scattering has been used to study the temperature dependence of the in-plane structure of the stage-1 graphite intercalation compound ${\mathrm{C}}_{9}$${\mathrm{OsF}}_{6}$. The low-temperature structure has a two-dimensional character and can be described as a nearly hexagonal domain-wall lattice in which sharp domain walls separate regions of (2\ifmmode\times\else\texttimes\fi{}2)R0\ifmmode^\circ\else\textdegree\fi{} structure on a 60-A\r{} scale. The domain-wall lattice is imperfect with a correlation length that only extends over a few such regions. The in-plane intercalant structure is disordered above 280 K and resistivity measurements show a clear hysteresis in the transition region above 250 K, indicating a first-order transition. The structural data are consistent with coexistence of a solidlike and fluidlike phase in the transition region, as expected for constant volume melting. The results suggest that this class of intercalation compounds is a good realization of a four-state Potts model with effective dimensionalities less than 3.

Measurements of the angular dependence of the remanent magnetization of Y-Ba-Cu-O and of Bi-Sr-Ca-Cu-O single crystals reveal uniaxial anisotropy for flux trapping; the anisotropy axis is the crystalline {bold c} axis. The amount of trapped flux is determined by the component of the field {bold H} along {bold c}. For flux trapping with {bold H} in the {ital a}-{ital b} plane, we observe unidirectional anisotropy; the anisotropy direction is defined by {bold H} during the cooling process.

The single-phase high-T/sub c/ superconductors YBa/sub 2/Cu/sub 3/O/sub 7-//sub x/S/sub x/ and YBa/sub 2/Cu/sub 3(1-//sub y//sub )/Fe/sub 3//sub y/O/sub 7/ have been prepared and studied by dc magnetometry. YBa/sub 2/Cu/sub 3/O/sub 6/S has a larger unit cell than YBa/sub 2/Cu/sub 3/O/sub 7/, and yet has the same T/sub c/; its phase transition is sharper and displays full Meissner effect. Substitution of Cu by Fe leads to a sharp decrease in T/sub c/.

ESR and susceptibility studies of highly oriented pyrolytic graphite--${\mathrm{OsF}}_{6}$(${\mathrm{C}}_{10}$${\mathrm{OsF}}_{6}$) stage-I graphite intercalated compounds are reported. The anisotropy in both ESR and magnetization are interpreted in terms of a quartet ground state of ${\mathrm{Os}}^{+5}$, 5${d}^{3}$ configuration, in axial local symmetry. The results yield information about charge transfer, anisotropic exchange, spin-orbit coupling, crystal-field splittings and molecular orientation.

Magnetic measurements on a high-T/sub c/ superconductor YBa/sub 2/Cu/sub 3/O/sub 7/ are reported. Diamagnetism sets in, at low fields, below a temperature Tapprox. =90 K. At low T the field-cooled magnetization becomes positive again. We show that the low-T phase is still superconducting and the apparent reentry behavior is due to strong paramagnetic contributions. Glassy features appear below a temperature whose field dependence differs substantially from that observed in spin glasses.