The transition temperature Tc~26 K of the recently discovered superconductor LaFeAs(O,F) has been demonstrated to be extremely sensitive to the lanthanide ion, reaching 55 K for the Sm containing oxypnictides. Therefore, it is important to determine how the moment on the lanthanide affects the overall magnetism in these systems. Here we report a neutron diffraction study of the Nd oxypnictides. Long ranged antiferromagnetic order is apparent in NdFeAsO below 1.96 K. Rietveld refinement shows that both Fe and Nd magnetic ordering are required to describe the observed data with the staggered moment 1.55(4) Bohr magneton per Nd and 0.9(1) Bohr magneton per Fe at 0.3 K. The other structural properties such as the tetragonal-orthorhombic distortion are found to be very similar to those in LaFeAsO. Neither the magnetic ordering nor the structural distortion occur in the superconducting sample NdFeAsO0.80F0.20 at any temperatures down to 1.5 K.
The vortex phase diagrams of NdFeAsO0.85F0.15 and NdFeAsO0.85 superconductors are determined from the analysis of resistivity and current-voltage (I-V) measurements in magnetic fields up to 9 T. A clear vortex glass to liquid transition is identified only in the oxygen deficient NdFeAsO0.85, in which I-V curves can be well scaled onto liquid and glass branches consistent with the vortex glass theory. With increasing magnetic field, the activation energy U0, deduced from the Arrhenius plots of resistivity based on the thermally activated flux-flow model (TAFF), decays more quickly for NdFeAsO0.…
We have systematically investigated the grain boundary (GB) angle dependence of transport properties for NdFeAs(O,F) fabricated on [001]-tilt symmetric MgO bicrystal substrates. In our previous study, NdFeAs(O,F) bicrystal films showed a weak-link behaviour even at a GB angle of 6°. However, this was caused by an extrinsic effect originating from the damage to both NdFeAs(O,F) and MgO substrate by excess F-diffusion along the grain boundary. To investigate the intrinsic nature of grain boundaries, we minimized the damage to NdFeAs(O,F) and MgO by reducing the deposition temperature of NdOF ove…
Measurements of the in-plane magnetic field penetration depth λ_{ab} in Fe-based superconductors with the nominal composition SmFeAsO_0.85 (T_c\simeq52K) and NdFeAsO_0.85 (T_c\simeq51K) were carried out by means of muon-spin-rotation. The absolute values of λ_{ab} at T=0 were found to be 189(5)nm and 195(5)nm for Sm and Nd substituted samples, respectively. The analysis of the magnetic penetration depth data within the Uemura classification scheme, which considers the correlation between the superconducting transition temperature T_c and the effective Fermi temperature T_F, reveal that both fa…
Photoemission spectroscopy with low-energy tunable photons on oxygen-deficient iron-based oxypnictide superconductors NdFeAsO0.85 (Tc=52K) reveals a distinct photon-energy dependence of the electronic structure near the Fermi level (EF). A clear shift of the leading-edge can be observed in the superconducting states with 9.5 eV photons, while a clear Fermi cutoff with little leading-edge shift can be observed with 6.0 eV photons. The results are indicative of the superconducting gap opening not on the hole-like ones around Gamma (0,0) point but on the electron-like sheets around M(pi,pi) point…
High resolution synchrotron X-ray powder diffraction (SXRPD) was used to study the temperature dependence of the oxygen deficient NdFeAsO$_{0.85}$ superconducting compound. By employing a dense temperature sampling we have managed to reveal unnoticed structural modifications that start around $\sim$180K, and disappear at the transition temperature. The data show minor changes of the structural characteristics in the Nd-O charge reservoir layer while in the superconducting Fe-As layer the FeAs$_{4}$ tetrahedron shows gradual modifications below $\sim$180K, which suddenly disappear at T$_{\rm c}…
The recently discovered high temperature superconductor F-doped LaFeAsO and related compounds represent a new class of superconductors with the highest transition temperature (Tc) apart from the cuprates. The studies ongoing worldwide are revealing that these Fe-based superconductors are forming a unique class of materials that are interesting from the viewpoint of applications. To exploit the high potential of the Fe-based superconductors for device applications, it is indispensable to establish a process that enables the growth of high quality thin films. Efforts of thin film preparation sta…
Using intrinsic multiple Andreev reflections effect (IMARE) spectroscopy, we studied ballistic superconductor - normal metal - superconductor (SnS) contacts in layered oxypnictide superconductors NdFeAsO$_{0.6}$H$_{0.36}$ with critical temperatures $T_c = 45-48$ K. We directly determined the magnitude of two bulk superconducting order parameters, the large gap $Δ_L \approx 10.4$ meV, and a possible small gap $Δ_S \approx 1.8$ meV, and their temperature dependence. Additionally, a resonant coupling with a characteristic bosonic mode was observed. The boson energy at 4.2 K, $\varepsilon_0 = 10.5…
Both DC and AC magnetization measurements were performed on the NdFeAsO0.88F0.12 superconductor to investigate the influence of magnetism on the superconducting properties of this system. The crossover of the ZFC and FC magnetic susceptibility curves under 7.5KOe was observed. The imaginary component of the first harmonics of the AC magnetic susceptibility, increases with the increasing DC field below 10K and shows frequency dependency under 7.5KOe at low temperature. The paramagnetism of Nd3+ ions tilts the magnetic hysteresis loops and broadens the hysteresis width. After correction for the …
Raman spectra of polycrystalline NdFeAsO1-xFx (x=0.0, 0.1, 0.2) compound have been systematically investigated as functions of temperature and fluorine concentration. Scanning electron microscopic and Raman microscopic characterization demonstrates that the polycrystalline samples mainly contain two phases, i.e. superconductor NdFeAsO1-xFx compound and a MnP-type FeAs phase, with dissimilar characteristic Raman bands. It was found that fluorine doping leads to structure disorder in the insulator Nd-O layers and high temperature coefficient of Fe-As vibrational mode.
Polycrystalline NdFeAsO0.88F0.12 superconductors prepared by high pressure (HP) and ambient pressure (AP) method were comparatively studied by magnetization and transport measurements. Upper critical field (Hc2), irreversibility field (Hirr) and the anisotropy parameter were estimated from resistance transition curves. The broadening of transition width was observed, and ascribed to both Hc2 anisotropy and superconductivity inhomogeneity of samples. Magnetic hysteresis loops (MHLs) in low fields were measured to detect the trace of weak-link behavior. The reclosed hysteresis loops in low field…
In this paper, nominal compositions of NdFeAsO0.8F0.2, NdFeAs0.95Sb0.05O0.8F0.2 and NdFeAs0.95P0.05O0.8F0.2 were prepared by one step solid state reaction method. The structural, electrical and morphological properties of samples were characterized through the XRD pattern, the 4 probe method and SEM, respectively. The crystal structure of our samples was tetragonal with P4/nmm:2 symmetry group. Also, the (x, y, z) and occupancy of ions and lattice parameters were changed by isovalent substitution of Phosphorus P and Antimony Sb in the NdFeAsO0.8F0.2 sample. The alfa and beta bond angles and Fe…
Single crystals of the oxypnictide superconductors SmFeAsO_(0.8)F_(0.2) and NdFeAsO_(0.8)F_(0.2) with T_c in the range of 44 K to 48 K were investigated by torque magnetometry. An analysis of the data in terms of a recently proposed model for the anisotropic magnetization in the superconducting state, treating the penetration depth anisotropy differently than the upper critical field anisotropy, provides evidence that in the oxypnictide superconductors two distinct anisotropies are present. As a result the penetration depth anisotropy differs significantly in magnitude and in temperature depen…
The flux pinning mechanism of NdO0.82F0.18FeAs superconductor made under high pressure, with a critical temperature, Tc, of 51 K, has been investigated in detail in this work. The field dependence of the magnetization and the temperature dependence of the magnetoresistivity were measured in fields up to 13 T. The field dependence of the critical current density, Jc(B), was analyzed within the collective pinning model. A crossover field, Bsb, from the single vortex to the small vortex bundle pinning regime was observed. The temperature dependence of Bsb(T) is in good agreement with the delta-l …
Magneto-optical imaging was used to study the local magnetization in polycrystalline NdFeAsO$_{0.9}$F$_{0.1}$ (NFAOF). Individual crystallites up to $\sim200\times100\times30$ $μm^{3}$ in size could be mapped at various temperatures. The in-grain, persistent current density is about $j\sim10^{5}$ A/cm$^{2}$ and the magnetic relaxation rate in a remanent state peaks at about $T_{m}\sim38$ K. By comparison with with the total magnetization measured in a bar-shaped, dense, polycrystalline sample, we suggest that NdFeAsO$_{0.9}$F$_{0.1}$ is similar to a layered high-$T_{c}$, compound such as Bi$_{…
Local structure of NdFeAsO$_{1-x}$F$_{x}$ ($x$=0.0, 0.05, 0.15 and 0.18) high temperature iron pnictide superconductor system is studied using arsenic $K$-edge extended x-ray absorption fine structure measurements as a function of temperature. Fe-As bondlength shows only a weak temperature and F-substitution dependence, consistent with the strong covalent nature of this bond. The temperature dependence of the mean-square relative-displacements of the Fe-As bondlength are well described by the correlated-Einstein model for all the samples, but with different Einstein-temperatures for the superc…
The structural, electronic, and magnetic properties of the superconducting NdFeAsO$_{1-x}$F$_{x}$ phases ($T_C=43 {\rm K}$ for $x=0.15$) have been investigated experimentally by high-resolution synchrotron x-ray powder diffraction, magnetization and $^{75}$As NMR measurements. Density-functional calculations were performed to calculate and analyze the electric field gradient and the density of states. Compared to LaFeAsO family, the NdFeAsO family shows a contraction of the lattice parameters with shorter rare-earth (RE)-As bond distances, an increased thickness of the As-Fe$_2$-As layer, and …
We report plane-polarized Raman scattering spectra of iron oxypnictide superconductor NdFeAsO$_{1-x}$F$_x$ single crystals with varying fluorine $x$ content. The spectra exhibit sharp and symmetrical phonon lines with a weak dependence on fluorine doping $x$. The temperature dependence does not show any phonon anomaly at the superconducting transition. The Fe related phonon intensity shows a strong resonant enhancement below 2 eV. We associate the resonant enhancement to the presence of an interband transition around 2 eV observed in optical conductivity. Our results point to a rather weak cou…
We report 75As-NQR/NMR studies on the oxygen-deficient iron(Fe)-based oxypnictide superconductors LaFeAsO_{0.6} (T_c=28 K) along with the results on LaFeAsO, LaFeAsO_{0.75}(T_c=20 K) and NdFeAsO_{0.6}(T_c=53 K). Nuclear spin-lattice relaxation rate 1/T_1 of 75As NQR at zero field on LaFeAsO_{0.6} has revealed a T^3 dependence below T_c upon cooling without the coherence peak just below T_c, evidencing the unconventional superconducting state with the line-node gap. We have found an intimate relationship between the nuclear quadrupole frequencyof 75As and T_c for four samples used in this study…
The newly discovered oxypnictide family of superconductors show very high critical temperatures of up to 55K. Whilst there is growing evidence that suggests a nodal order parameter, point contact Andreev reflection spectroscopy can provide crucial information such as the gap value and possibly the number of energy gaps involved. For the oxygen deficient NdFeAsO0.85 with a Tc of 45.5K, we show that there is clearly a gap value at 4.2K that is of the order of 7meV, consistent with previous studies on oxypnictides with lower Tc. Additionally, taking the spectra as a function of gold tip contact p…
The low-temperature magnetic structure of NdFeAsO has been revisited using neutron powder diffraction and symmetry analysis using the Sarah representational analysis program. Four magnetic models with one magnetic variable for each of the Nd and Fe sublattices were tested. The best fit was obtained using a model with Fe moments pointing along the c-direction, and Nd moments along the a-direction. This signals a significant interplay between rare-earth and transition metal magnetism, which results in a spin-reorientation of the Fe sublattice upon ordering of the Nd moments. All models that fit …
We measured the initial M-H curves for a sample of the newly discovered superconductor NdFeAsO0.82Fe0.18, which had a critical temperature, Tc, of 51 K, and was fabricated at the high pressure of 6 GPa. The lower critical field, Hc1, was extracted from the deviation point of the Meissner linearity in the M-H curves, which show linear temperature dependence in the low temperature region down to 5 K. The Hc1(T) indicates no s-wave superconductivity, but rather an unconventional superconductivity with a nodal gap structure. Furthermore, the linearity of Hc1 at low temperature does not hold at hig…
We report a comprehensive investigation of the suppression of the critical temperature Tc of NdFeAs(OF) single crystal by alpha-particle irradiation. Our data indicate that irradiation defects produce both nonmagnetic and magnetic scattering, resulting in the Kondo-like excess resistance $Δρ(T)\propto ln T$ over 2 decades in temperatures above $T_c$. Despite high densities of irradiation defects, the dose at which $T_c$ is suppressed to zero is comparable to that for MgB2 but is well above the corresponding values for cuprates.
We investigated transport and magnetic properties of NdFeAsO$_{0.9}$F$_{0.1}$ single crystal under hydrostatic pressures up to 50\,GPa. The ambient pressure superconductivity at $T_{c} \sim$ 45.4\,K is fully suppressed at $P_{c} \sim$ 21 GPa. Upon further increase of the pressure, the ferromagnetism associated with the order of rare-earth subsystem is induced at the border of superconductivity. Our finding is supported by the hysteresis in the magnetization $M$($H$) loops and the strong increase in the field cooled data, $M$($T$), toward low temperatures. We also show that the temperature evol…
The possibility of p-wave pairing in superconductors has been proposed more than five decades ago, but has not yet been convincingly demonstrated. One difficulty is that some p-wave states are thermodynamically indistinguishable from s-wave, while others are very similar to d-wave states. Here we studied the self-field critical current of NdFeAs(O,F) thin films in order to extract absolute values of the London penetration depth, the superconducting energy gap, and the relative jump in specific heat at the superconducting transition temperature, and find that all the deduced physical parameters…
We present results of transport and magnetic properties and heat capacity measurements on polycrystalline CeFeAsO, PrFeAsO, and NdFeAsO. These materials undergo structural phase transitions, spin density wave-like magnetic ordering of small moments on iron, and antiferromagnetic ordering of rare earth moments. The temperature dependence of the electrical resistivity, Seebeck coefficient, thermal conductivity, Hall coefficient, and magnetoresistance are reported. The magnetic behavior of the materials have been investigated using Mossbauer spectroscopy and magnetization measurements. Transport …
The structural and magnetic phase transitions have been studied on NdFeAsO single crystals by neutron and x-ray diffraction complemented by resistivity and specific heat measurements. Two low-temperature phase transitions have been observed in addition to the tetragonal-to-orthorhombic transition at T_S = 142 K and the onset of antiferromagnetic (AFM) Fe order below T_N = 137 K. The Fe moments order AFM in the well-known stripe-like structure in the (ab) plane, but change from AFM to ferromagnetic (FM) arrangement along the c direction below T* = 15 K accompanied by the onset of Nd AFM order b…
The Fe-based superconductors (FBS) present a large variety of compounds whose properties are affected to different extents by their crystal structures. Amongst them, the $\it{RE}$FeAs(O,F) ($\it{RE}$1111, where $\it{RE}$ is a rare earth element) is the family with the highest critical temperature $T_c$ but also with a large anisotropy and Josephson vortices as demonstrated in the flux-flow regime in Sm1111 ($T_c$ $\sim$ 55 K). Here we focus our attention on the pinning properties of the lower-$T_c$ Nd1111 in the flux-creep regime. We demonstrate that for H//c critical current density $J_c$ at …
We study the effect of the lattice structure on the spin-fluctuation mediated superconductivity in the iron pnictides adopting the five-band models of several virtual lattice structures of LaFeAsO as well as actual materials such as NdFeAsO and LaFePO obtained from the maximally-localized Wannier orbitals. Random phase approximation is applied to the models to solve the Eliashberg equation. This reveals that the gap function and the strength of the superconducting instability are determined by the cooperation or competition among multiple spin fluctuation modes arising from several nestings am…
The effect of $α$-particle irradiation on a NdFeAs(O,F) thin film has been investigated to determine how the introduction of defects affects basic superconducting properties, including the critical temperature $T_c$ and the upper critical field $H_{c2}$, and properties more of interest for applications, like the critical current density $J_c$ and the related pinning landscape. The irradiation-induced suppression of the film $T_c$ is significantly smaller than on a similarly damaged single crystal. Moreover $H_{c2}$ behaves differently, depending on the field orientation: for H//c the $H_{c2}$ …
We described the effect of nonmagnetic impurity on the superconductivity behavior of the NdFeAsO0.8F0.2 iron-based superconductor. The resistivity measurement showed that the superconductivity suppressed upon increasing the low amounts of calcium impurity (x<0.05). Also, the Tc decreased with the increase in the residual resistivity. Such behavior was qualitatively described by the Abrikosov-Gorkov theory and confirmed that these impurities act as scattering centers. For our samples, the exchange constant between the calcium and the conduction electron spins was estimated Jexc=|8| meV. Moreove…
Evolution of Fermi surface (FS) states of NdFeAs$_{1-x}$P$_x$O$_{0.9}$F$_{0.1}$ single crystals with As/P substitution has been investigated. The critical temperature $T_{\rm c}$ and the power law exponent ($n$) of temperature-dependent resistivity ($ρ(T) = ρ_0 + AT^n$) show a clear correlation above $x=$0.2, suggesting that $T_{\rm c}$ is enhanced with increasing bosonic fluctuation in the same type of FS state. Around $x=$0.2, all the transport properties show anomalies, indicating that $x$$\sim$0.2 is the critical composition of drastic FS change. The angle resolved photoemission spectrosco…
The Andreev reflection spectra were studied for the first time in the magnetic superconductor Dy0.6Y0.4Rh3.85Ru0.15B4. It is found that an external magnetic field (up to 0.7 Hc2) is a strong stimulator of superconductivity in contrast with traditional superconductors with a singlet pairing. The ratio 2Δ/kBTc \approx 4.0 obtained for some contacts is higher than the value 3.52 which is typical of conventional superconductors with a weak electron-phonon interaction. It has been proposed that in Dy0.6Y0.4Rh3.85Ru0.15B4 a triplet mechanism of superconducting pairing is realized.
The upper critical field, the lower critical field and the critical magnetic field ratio of anisotropic magnetic superconductors are calculated by Ginzburg-Landau theory analytically. The effect of the Ginzburg-Landau parameter(κ_0), magnetic susceptibility(χ) and magnetic-to- anisotropic parameter ratio(θ) on the critical field ratio are considered. We find that the value of critical magnetic field ratio with χ< 0, higher κ_0,and higher θcase are shown the higher value. And, the diamagnetic superconductors with highly anisotropic and the ferromagnetic superconductors are shown the highest and…
Magnetic response of a superconductor depends on the thermodynamic stability of vortex in the material. Here we show that the vortex stability has a close relation with the ratio of the magnetic field at the vortex core center to the thermodynamic critical field. This finding provides a new criterion for the classification of the superconductors according to their magnetic responses.
We consider ferromagnetic particles embedded into a superconductor and study the screening of their magnetic moments by the spins of the Cooper pairs in the superconductor. It is shown that a magnetic moment opposite to the one of the ferromagnetic particle is induced in the superconductor. In the case of a small itinerant ferromagnet grain and low temperatures the full screening of the magnetic moment takes place, \textit{% i.e} the absolute value of the total magnetic moment induced in the superconductor is equal to the one of the ferromagnetic particle. In type II superconductors the propos…
The upper and lower critical field, and the critical field ratio of an anisotropic two-band magnetic superconductors in Ginzburg-Landau(GL) scenery is derived analytically. The temperature-dependent on the upper critical field is investigated and applied to Fe-based superconductors. We find that the very high value of zero-temperature upper critical field in Fe-based superconductors can be found in the negative differential susceptibility region. The temperature-dependent upper critical field is presented in two formulas as in the empirical view and in the GL two-band view that agrees with the…
We report experimental evidence of the effect of an applied magnetic field on the non-thermal magnetic relaxation in a disk-shaped type-I lead superconductor. The time evolution of the irreversible magnetization proves to be logarithmic for a wide range of temperatures and magnetic field values along the descending branch of the hysteresis cycle. When the intensity of the magnetic field increases, the crossover temperature separating the thermal and non-thermal regimes of magnetic relaxation is found to decrease, whereas the rate at which such relaxation occurs is observed to increase. These r…
It is shown that the application of sufficiently strong magnetic field to the odd-frequency paired Pair Density Wave state described in Phys. Rev. B 94, 165114 (2016) leads to formation of a low temperature metallic state with zero Hall response. Applications of these ideas to the recent experiments on stripe-ordered La_{1.875}Ba_{0.125}CuO_4 are discussed.
It was observed that the trapped magnetic moment of HTS tablets or annuli increases in time (negative relaxation) if they are not completely magnetized by a pulsed magnetic field. It is shown, in the framework of the Bean critical-state model, that the radial temperature gradient appearing in tablets or annuli during a pulsed field magnetization can explain the negative magnetic relaxation in the superconductor.
The thermodynamical phenomenological theory of magnetically active superconducting materials and magnetic-superconducting heterostructures is presented. The materials may exhibit arbitrarily strong anisotropy, parametric or structural. Exact anisotropic similarity transformations are found for both the continuum and layered hybrid systems.
Using the $ϕ$-mapping method, we argue that ferromagnetic spin-triplet superconductors allow formation of unstable magnetic monopoles. In particular, we show that the limit points and the bifurcation points of the $ϕ$-mapping will serve as the interaction points of these magnetic monopoles.
Elastic neutron scattering simultaneously probes both the crystal structure and magnetic order in a material. Inelastic neutron scattering measures phonons and magnetic excitations. Here, we review the average composition, crystal structure and magnetic order in the 245 family of Fe-based superconductors and in related insulating compounds from neutron diffraction works. A three-dimensional phase-diagram summarizes various structural, magnetic and electronic properties as a function of the sample composition. A high pressure phase diagram for the superconductor is also provided. Magnetic excit…
The penetration of the magnetic field of an infinitesimal magnetic dipole into a bilayered type-II superconductor/soft-magnet heterostructure is studied on the basis of the classical London approach. The critical values of the dipole moment for the first appearance of a single magnetic vortex and, respectively, a magnetic vortex-antivortex pair in the superconductor constituent are obtained, when the magnetic dipole faces the superconductor or the soft-magnet constituent. This reveals that the soft-magnet constituent inhibits penetration of vortices into the superconductor constituent, when th…
Enhancement of flux pinning by magnetic nanoparticles embedded into the bulk of type-2 superconductor is studied both theoretically and experimentally. Magnetic part of the pinning force associated with the interaction between a spherical magnetic inclusion and an Abrikosov vortex was calculated in the London approximation. Calculations are supported by the experimental results obtained on sonochemically modified MgB2 superconductor with embedded magnetic Fe2O3 nanoparticles and compared to MgB2 with nonmagnetic Mo2O5 pinning centers of similar concentration and particle size distribution. It …
We present a comparative theoretical study of the effects of standard Anderson and magnetic disorders on the topological phases of two-dimensional Rashba spin-orbit coupled superconductors, with the initial state to be either topologically trivial or nontrivial. Using the self-consistent Born approximation approach, we show that the presence of Anderson disorders will drive a topological superconductor into a topologically trivial superconductor in the weak coupling limit. Even more strikingly, a topologically trivial superconductor can be driven into a topological superconductor upon diluted …
On the basis of exact solutions to the London equation the magnetic moment of a type II superconductor filament surrounded by a soft-magnet environment is calculated and the procedure of extracting the superconductor contribution from magnetic measurements is suggested. Comparison of theoretical results with experiments on MgB_2/Fe wires allows estimation of the value of critical current for the first magnetic flux penetration.
We investigate the magnetic field distribution near the surface of a magnetic superconductor when a magnetic source is placed close to the superconductor. The magnetic field distribution can be measured by magnetic force microscopy and scanning SQUID microscopy, from which one can extract both the penetration depth $λ_L$ and magnetic susceptibility $χ$. When the magnetic moments are parallel to the surface, one extracts $λ_L/\sqrt{1-4πχ}$. When the moments are perpendicular to the surface, one obtains $λ_L$. By changing the orientation of the crystal, one thus is able to extract both $χ$ and $…
We calculate the specific heat and susceptibility of rare-earth magnetic superconductors in the context of the Ginzburg-Landau theory. The specific heat and susceptibility are calculated both at the coexistence phase of antiferromagnetism and superconductivity and normal phase. Theoretical results are compared with experimental results, which agree excellently well.
In this report, the temperature distribution in the critical state of hard superconductors is investigated in the quasi-stationary approximation. It is shown that the temperature profile can be essentially inhomogeneous in the sample, which affects the conditions of initiation of a magnetic flux jumps.
The magnetic flux penetration into the half-space superconductor sample is studied in the flux flow regime in parallel geometry assuming that an external magnetic field increasing with time in accordance with the power law. Assuming that the flux flow resistivity as a power-law function of the magnetic field induction we found an exact analytical solution for the nonlinear local magnetic flux diffusion equation. The obtained solution describes space-time distribution of the magnetic induction in the sample.
Using a Ginsburg-Landau free energy functional, we study the $H-T$ phase diagram of a weak magnetic superconductor, where the magnetization from the magnetic component is marginal in supporting a spontaneous vortex phase in absence of external magnetic field. In particular, the competition between the spiral state and spontaneous vortex phase is analysed. Our theory is applied to understand the magnetic properties of $ErNi_2B_2C$.
The effect of suppression of static magnetization of an anisotropic hard superconductor by alternating magnetic field is analyzed theoretically. The magnetic moment suppression dynamics is described with respect to the magnetization loop of the superconductor. It is found that in some cases the magnetic moment varies nonmonotonically with the growth in amplitude $\mathrm{h}$ of the alternating field. Effect of transition, induced by $\mathrm{\mathbf{h}}(t)$, of superconductor form paramagnetic into the diamagnetic state is considered. The amplitude of alternating magnetic field $\mathrm{h}_{c}…
We report an unusual enhancement of the magnetic induction in single crystals of the magnetic superconductor RbEuFe$_4$As$_4$ , highlighting the interplay between superconducting and magnetic subsystems in this material. Contrary to the conventional Meissner expulsion of magnetic flux below the superconducting transition temperature, we observe a substantial boost of the magnetic flux density upon approaching the magnetic transition temperature, Tm. Direct imaging of the flux evolution with a magneto-optical technique, shows that the magnetic subsystem serves as an internal magnetic flux pump,…
In this review I discuss the application of scanning magnetic imaging to fundamental studies of superconductors, concentrating on three scanning magnetic microscopies - scanning SQUID microscopy (SSM), scanning Hall bar microscopy (SHM), and magnetic force microscopy (MFM). I briefly discuss the history, sensitivity, spatial resolution, invasiveness, and potential future developments of each technique. I then discuss a selection of applications of these microscopies.
In the framework of the London approximation the magnetic flux penetration into a type-II superconductor filament surrounded by a soft-magnet sheath and exposed to a transverse external magnetic field is studied. The lower transverse critical field as well as the critical field and the critical current of the first vortex nucleation at the superconductor/magnet interface are calculated on the basis of an exact solution for a vortex of arbitrary plane configuration. The Bean-Livingston barrier against the vortex nucleation is shown to strongly depend on the magnet sheath parameters.
We discuss the scenario of two-step magnetic ordering in layered high temperature superconductors after charge ordering. As the temperature decreases, the transition from 3D Heisenberg spin behavior to 2D XY coupling of the Cu spins occurs at Berezinskii-Kosterlitz-Thouless temperature in dielectric stripes. Further temperature decreasing leads to the 3D spin glass transition.
We study the proximity effect in a superconductor (S)-normal metal (N) bilayer under in-plane magnetic field. A compensation between the Zeeman effect and the energy splitting between bonding and anti-bonding levels leads to a magnetic field induced superconducting phase in the (H,T) phase diagram of the S-N bilayer well above the standard paramagnetic limit. We demonstrate that the presence of the non-magnetic impurities shrink the region of field induced superconductivity existence in S-N and S-S bilayers.
Superconductor-ferromagnet-superconductor Josephson junctions are known to exist in the $0$ and $π$ states with the transitions between them controlled by the temperature and ferromagnetic interlayer thickness. We demonstrate that these transitions can be controlled also by the external magnetic field directed perpendicular to the layers. By varying the ratio of diffusion coefficients in superconducting and ferromagnetic layers, these field-controlled transitions can be made detectable for arbitrary large value of the exchange energy in the ferromagnet. We also show that the $0$-$π$ transition…
This thesis describes experimental and theoretical study of static and dynamic aspects of the irreversible magnetic behavior of high-Tc superconductors. Experimentally, conventional magnetometry and novel Hall-probe array techniques are employed. Using both techniques extends significantly the experimental possibilities and yields a wealth of new experimental results.
