Isofield magnetization curves are obtained and analyzed for three single crystals of YBa2Cu3O{7-x}, ranging from optimally doped to very underdoped, as well as the BCS superconductor Nb, in the presence of magnetic fields applied both parallel and perpendicular to the $ab$ planes. Near Tc, the magnetization exhibits a temperature dependence \sqrt{M} [Ta(H)-T]^m. In accordance with recent theories, we associated Ta(H) with the onset of coherent phase fluctuations of the superconducting order parameter. For Nb and optimally doped YBaCuO, Ta(H) is essentially identical to the mean-field transition line Tc(H). The fitting exponent m=0.5 takes its mean-field value for Nb, and varies just slightly from 0.5 for optimally doped YBaCuO. However, underdoped YBCO samples exhibit anomalous behavior, with Ta(H)>Tc for H applied parallel to the c axis, suggesting that the magnetization is probing a region of temperatures above Tc where phase correlations persist. In this region, the fitting exponent falls in the range 0.5 < m < 0.8 for H\parallel c, compared with m~0. for $H\parallel ab planes. The results are interpreted in terms of an anisotropic pairing symmetry of the order parameter: d-wave along the ab planes and s-wave along the c axis.
We report on isofield magnetization curves obtained as a function of temperature in two single crystals of $Ba_{1-x}K_xFe_2As_2$ with superconducting transition temperature $T_c$=28K and 32.7 K. Results obtained for fields above 20 kOe show a well defined rounding effect on the reversible region extending 1-3 K above $T_c(H)$ masking the transition. This rounding appears to be due to three-dimensional critical fluctuations, as the higher field curves obey a well know scaling law for this type of critical fluctuations. We also analysed the asymptotic behavior of $\sqrt M$vs.T curves in the reve…
The field dependence of the vortex core size $ξ(B)$ is incorporated in the London model, in order to describe reversible magnetization $M(B,T)$ for a number of materials with large Ginzburg-Landau parameter $κ$. The dependence $ξ(B)$ is directly related to deviations in $M(\ln B)$ from linear behavior prescribed by the standard London model. A simple method to extract $ξ(B)$ from the magnetization data is proposed. For most materials examined, $ξ(B)$ so obtained decreases with increasing field and is in qualitative agreement both with behavior extracted from $μ$SR and small angle neutron scatt…
We present reversible magnetization measurements on MgB2 single crystals in magnetic fields up to 2.5 T applied parallel to the crystal's c-axis. This magnetization is analyzed in terms of the Hao-Clem model, and various superconducting parameters, such as the critical fields [Hc(0) and Hc2(0)], the characteristic lengths [xi(0) and lambda(0)], and the Ginzburg-Landau parameter, kappa, are derived. The temperature dependence of the magnetic penetration depth, lambda(T), obtained from the Hao-Clem analysis could not be explained by theories assuming a single gap. Our data are well described by …
By using a high quality Tl2Ba2Ca2Cu3O10 (Tl-2223) single crystal as an example, the magnetization vector was probed in the reversible region of highly anisotropic cuprate superconductors. For that, we have measured its components along and transverse to the applied magnetic field for different crystal orientations. The analysis shows that the angular dependence of the perpendicular component of the magnetization vector follows the one predicted by a London-like approach which includes a contribution associated with the thermal fluctuations of the 2D vortex positions. For the Tl-2223 crystal st…
We have studied the anisotropy of the London penetration depth, which was obtained from reversible magnetization measurements with the magnetic field both parallel and perpendicular to the c-axis. The anisotropy of the London penetration depth has a smaller magnitude than the anisotropy of the upper critical field and increases with temperature while that of the upper critical field decreases as reported earlier. This behavior is in sharp contrast with the behaviors of superconductors with one superconducting energy gap. The temperature dependence of the anisotropies of the London penetration …
Bulk polycrystalline HgBa(2)Ca(2)Cu(3)O(x) materials were irradiated with 0.8 GeV protons to form randomly oriented columnar defects, by induced fission of Hg-nuclei. Proton fluences from 0 to 35 times 10^{16} cm^{-2} were used to install defects with area densities up to a "matching field" of 3.4 Tesla. Studies were conducted on the dependence of the equilibrium magnetization and the intragrain persistent current density on temperature and applied magnetic field, at various defect densities. The magnetization was modeled using London theory with the addition of vortex-defect interactions.
We argue that the method, which was used for the analysis of equilibrium magnetization data in the original publication, is not adequate to the experimental situation. As a result, the temperature dependencies of the upper critical field Hc2(T) and the magnetic field penetration depth lambda(T), obtained in this work, are incorrect. Using a different approach, we reanalyze the presented experimental data and demonstrate that the normalized Hc2(T) curves are rather different from those presented in the original publication and do not follow predictions of the Werthamer-Helfand-Hohenberg theory.…
A recently developed technique for measuring the isothermal magnetocaloric coefficient ($M_T$) is applied to the study of a superconducting NdBa$_2$Cu$_3$O$_7$ single crystal. Results are compared with magnetization ($M$) and specific heat ($C$). In the reversible region both $C$ and $M_T$ follow the scaling law of the 3D-xy universality class. The anomalies connected with flux-line lattice melting are visible on $M_T(B)$ curves as peaks and steps, similar to $C(T)$ curves yet with much smaller background. At lower temperature, in the irreversible region the $M_T(B)$ behaviour resembles more t…
The temperature and field dependence of reversible magnetization have been measured on a YBa$_2$Cu$_3$O$_{7-δ}$ single crystal at six different doping concentrations. It is found that the data above 2 T can be described by the scaling law based on the GL-LLL (lowest Landau level approach based on Ginzburg-Landau theory) critical fluctuation theory yielding the values of the slope of upper critical field $-\mathrm{d}H_{\mathrm{c2}}(T)/\mathrm{d}T$ near $T_\mathrm{c}$. This set of values is self-consistent with that obtained in doing the universal scaling for the six samples. Based on a simple G…
We report the observation of positive magnetization signal at the onset of superconducting transition in isothemral and isomagnetic measurements in single crystal Niobium sphere. The PME displays two regimes, viz., low applied magnetic fields H < 4 Oe) and higher fields H > 120 Oe, and the PME signal is absent at the intermediate magnetic fields. Subsequently, the isothermal magnetization data shows the presence of positive signal at the superconductor to normal transition. Using this data we are able to plot the H(c3)-T(c3) line, at which the positive magnetization signal nucleates, in the ph…
We propose a simple scaling procedure for the normal-state magnetization Mn data collected as functions of temperature T in different magnetic fields H. As a result, the Mn(T) curves collected in different fields collapse on to a single Msc(T) line. In this representation, the onset of superconducting diamagnetism manifests itself by a sharp divergence of the Msc(T) curves for different H values. As will be demonstrated, this allows for a reliable determination of temperature Tonset, at which superconducting diamagnetism become observable.
The magnetization vector \vec M was measured in the reversible region of the mixed state of a high quality Tl2Ba2Ca2Cu3O10 single crystal as a function of temperature and for different magnetic field amplitudes and orientations. These measurements allowed to study the \vec M components perpendicular and parallel to the CuO2 layers (Mperp and Mpara, respectively) under arbitrary values for the corresponding components of the applied magnetic field, Hperp and Hpara. For temperatures close to Tc (in the critical fluctuation region) we observed Mperp(Hperp,Hpara) Mperp(Hperp,0) and Mpara(Hperp,Hpa…
In highly anisotropic cuprate superconductors it is generally accepted that the reversible magnetization vector, M, is essentially perpendicular to the superconducting CuO2 layers in a wide range of crystal orientations with respect to the applied magnetic field, H. In a recent work [J. Mosqueira et al., Phys. Rev. B 84, 134504 (2011)] it is shown, however, that the dependence of M on the H orientation in the reversible mixed state of a high-quality Tl-based cuprate presents a notable deviation from this behavior. Here we extend these measurements to the fluctuation region above Tc, in order t…
In the cuprate superconductors, Nernst and torque magnetization experiments have provided evidence that the disappearance of the Meissner effect at $T_c$ is caused by the loss of long-range phase coherence, rather than the vanishing of the pair condensate. Here we report a series of torque magnetization measurements on single crystals of $\mathrm{La_{2-x}Sr_xCuO_4}$ (LSCO), $\mathrm{Bi_2Sr_{2-y}La_yCuO_6}$ (Bi 2201), $\mathrm{Bi_2Sr_2CaCu_2O_{8+δ}}$ (Bi 2212) and optimal $\mathrm{YBa_2Cu_3O_7}$. Some of the measurements were taken to fields as high as 45 T. Focusing on the magnetization above …
The depairing (upper critical) field $H_{c2}$ in hole-doped cuprates has been inferred from magnetization curves $M$-$H$ measured by torque magnetometry in fields $H$ up to 45 T. We discuss the implications of the results for the pair binding energy, the Nernst onset temperature, fluctuations and the nature of the Meissner transition at $T_c$.
It is argued that the stress caused by vortex cores in the mixed state of superconductors may result in a field dependent contribution to the free energy and magnetization of measurable levels. For sufficiently strong stress dependence of the critical temperature, $\partial T_c/\partial p$, this contribution may result in the so-called "second peak" in the field dependence of the reversible magnetization, the effect often masked by vortex pinning and creep.
We present studies of the dc magnetization of thin platelike samples of the isotropic type II superconductor PbTl(10%), as a function of the angle between the normal to the sample and the applied magnetic field ${\bf H}$. We determine the magnetization vector ${\bf M}$ by measuring the components both parallel and normal to ${\bf H}$ in a SQUID magnetometer, and we further decompose it in its reversible and irreversible contributions. The behavior of the reversible magnetization is well understood in terms of minimization of the free energy taking into account geometrical effects. In the mixed…
We measured isofield magnetization curves as a function of temperature in two single crystal of deoxygenated YBaCuO with T_c = 52 and 41.5 K. Isofield MvsT were obtained for fields running from 0.05 to 4 kOe. The reversible region of the magnetization curves was analyzed in terms of a scaling proposed by Prange, but searching for the best exponent $\upsilon$. The scaling analysis carried out for each data sample set with $\upsilon$=0.669, which corresponds to the 3D-xy exponent, did not produced a collapsing of curves when applied to MvsT curves data obtained for the lowest fields. The resulti…
Using the Ginzburg-Landau theory in very general terms, we develop a simple scaling procedure which allows to establish the temperature dependence of the upper critical field and the value of the superconducting critical temperature Tc of type-II superconductors from measurements of the reversible isothermal magnetization. An analysis of existing experimental data shows that the normalized dependencies the upper critical field on T/Tc are practically identical for all families of high temperature superconductors at all temperatures for which the magnetization data are available.
We study the extent of the reversible region in the vortex phase diagram of recently available RbOs$_2$O$_6$ single crystals \cite{Rogacki08a} by means of bulk magnetization measurements. We found that the irreversible magnetic response sets in at a field $H_{\rm irr}(T) \sim 0.3 H_{\rm c2}(T)$ for $0.5 \lesssim T/T_{\rm c} \lesssim 0.8$, yielding a reversible vortex region that is uncommonly wide in comparison with typical low-$T_{\rm c}$ materials. The relevance of thermal fluctuations is limited since we estimate a Ginzburg number $G_{\rm i} = 5 \times 10^{-7}$. However, the relevance of qu…
Thermodynamic (specific heat, reversible magnetization, tunneling spectroscopy) and transport measurements have been performed on high quality (K,Ba)BiO$_3$ single crystals. The temperature dependence of the magnetic field $H_{Cp}$ corresponding to the onset of the specific heat anomaly presents a clear positive curvature. $H_{Cp}$ is significantly smaller than the field $H_Δ$ for which the superconducting gap vanishes but is closely related to the irreversibility line deduced from transport data. Moreover, the temperature dependence of the reversible magnetization present a strong deviation f…
An intriguingly complex phase diagram of Na-doped SrFe2As2 is uncovered using high-resolution thermal-expansion, magnetization and heat-capacity measurements. The detailed temperature dependence of the orthorhombic distortion and the anisotropy of the uniform magnetic susceptibility provide evidence for nine distinct electronic phases near the transition region between stripe antiferromagnetism and unconventional superconductivity. In particular, we report the finding of a new magnetic phase which competes surprisingly strongly with superconductivity. From theoretical studies we propose that t…
We present a quantitative comparison of the magnetization measured by spin-polarized neutron reflectivity (SPNR) and DC magnetometry on a 1370 Å -thick Nb superconducting film. As a function of magnetic field applied in the film plane, SPNR exhibits reversible behavior whereas the DC magnetization shows substantial hysteresis. The difference between these measurements is attributed to a rotation of vortex magnetic field out of the film plane as the applied field is reduced. Since SPNR measures only the magnetization parallel to the film plane whereas DC magnetization is strongly influenced by …
We study a high-TC superconducting (YBa2Cu3O7-d) / ferromagnetic (Co/Pt multilayer) hybrid which exhibits resistance switching driven by the magnetic history: depending on the direction of the external field, a pronounced decrease or increase of the mixed-state resistance is observed as magnetization reversal occurs within the Co/Pt multilayer. We demonstrate that stray magnetic fields cause these effects via i) creation of vortices/antivortices and ii) magnetostatic pinning of vortices that are induced by the external field.
We have observed the conduction electron spin resonance (CESR) in fine powders of MgB2 both in the superconducting and normal states. The Pauli susceptibility is chi_s=2.0*10^{-5} emu/mole in the temperature range of 450 to 600 K. The spin relaxation rate has an anomalous temperature dependence. The CESR measured below T_c at several frequencies suggests that MgB_2 is a strongly anisotropic superconductor with the upper critical field, H_c2, ranging between 2 and 16 T. The high-field reversible magnetization data of a randomly oriented powder sample are well described assuming that MgB_2 is an…
The filled skutterudite compound PrOs4Sb12, the first example of a Pr-based heavy fermion superconductor, displays superconductivity with $T_c\sim 1.85$ K and has an effective mass $m^* \sim$ {50 $m_e$}, where $m_e$ is the free electron mass. For magnetic fields above 4.5 T, sharp features in the normal state electrical resistivity, magnetization, specific heat, and thermal expansion data suggest the occurrence of a phase transition at high fields. This high field ordered phase in the normal state may originate from a combination of crystalline electric field enhanced Zeeman splitting and quad…
Paramagnetic Meissner effect was observed in single crystal Yb(3)Rh(4)Sn(13). While field cooling the sample, at the onset of the superconducting transition at 7.92 K, the DC magnetization first goes through a minimum at 7.85 K, followed by the peak of the PME signal at 7.6 K and then crosses over to the diamagnetic state. The magnetization vs field curves are reversible above the minimum and imbibe a pinning characteristic below 7.85 K. The minimum can be attributed to the opposing contribution to the dc magnetization signal from surface superconductivity. The subsequent flux compression whil…
The different pinning strengths of the flux line lattice in the peak effect (PE) region of a polycrystalline sample of CeRu$_2$ with a superconducting transition temperature {$T_c = 6.1$ K} have been probed by means of magnetization measurements with a SQUID magnetometer as the temperature $T$ and the magnetic field $H$ are varied. Magnetic relaxation measurements were used to monitor the flux line dynamics in the PE region. For {$T < 4.5$ K} and $H < H_P$, where $H_P$ is the field where the magnetization reaches a maximum in the PE region, the relaxation rate was found to be significantly lar…
Based on the self-consistent solution of a nonlinear system of one-dimensional GL-equations, the onset and destruction of superconductivity, the phase transitions and hysteresis phenomena are discussed for a cylinder (radius R) in an axial magnetic field (H) for arbitrary R,kappa,H,m (kappa$ is the GL-parameter, m is the total vorticity of the system). The edge-suppressed solutions (which are connected with the jumps of magnetization in the states with fixed vorticity m), the depressed solutions (responsible for the hysteresis in type-II superconductors), and the precursor solutions (which des…
We have measured the microwave dissipation at 10 GHz in a Bi2Sr2CaCu2O8 single crystal, between 75 and 50 K, as a function of the static magnetic field applied parallel to the c axis. We observe a clear-cut onset in the dissipation precisely at the melting field Hm(T), that we identify in the same sample by a step in the static magnetization. We show that this onset can be assigned to the c-axis resistivity becoming finite in our whole temperature range, whereas the in-plane resistivity displays a thermally activated behavior. This suggests that pinning is efficient even at 10 GHz. The normal …
High-density bulk materials of a newly discovered 40K intermetallic MgB2 superconductor were prepared by high pressure sintering. Superconducting transition with the onset temperature of 39K was confirmed by both magnetic and resistive measurements. Magnetization versus field (M-H) curve shows the behavior of a typical Type II superconductor and the lower critical field Hc1(0) estimated from M-H curve is 0.032T. The bulk sample shows good connection between grains and critical current density Jc estimated from the magnetization hysteresis using sample size was 2x104A/cm2 at 20K and 1T. Upper c…
A variational model is proposed to describe the magnetic properties of type-II superconductors in the entire field range between $H_{c1}$ and $H_{c2}$ for any values of the Ginzburg-Landau parameter $κ>1/\sqrt{2}$. The hexagonal unit cell of the triangular flux-line lattice is replaced by a circle of the same area, and the periodic solutions to the Ginzburg-Landau equations within this cell are approximated by rotationally symmetric solutions. The Ginzburg-Landau equations are solved by a trial function for the order parameter. The calculated spatial distributions of the order parameter and th…
We report on magnetic measurements as a function of field, temperature and time (relaxation) in superconducting Nb films of critical temperature Tc = 9.25 K. The magnetic measurements as a function of field exhibited a second magnetization peak (SMP) which in general is accompanied by thermomagnetic instabilities (TMIs). The lines where the SMP occurs and where the first flux jump in the virgin magnetization curves is observed, end at a characteristic point (To,Ho)=(7.2 K,80 Oe). Relaxation measurements showed that for T<To=7.2 K the activation energy Uo and the normalized relaxation rate S ex…
Dynamic and thermodynamic magnetization experiments on heavy-ion irradiated single crystalline Bi_{2}Sr_{2}CaCu_{2}O_{8+δ} are correlated in order to clarify the nature of the mixed state phase diagram. It is shown that whereas the entropy contribution to the free energy in the London regime plays a minor role in unirradiated crystals and irradiated crystals at fields close to or above the matching field $B_ϕ$, it becomes very important at low fields in irradiated crystals with high $B_ϕ$. The direct determination of the entropy contribution to the free energy from the reversible magnetization…
We report on magnetization data obtained as a function of temperature and magnetic field in Li2 (Pd0.8Pt0.2)3B and Li2Pd3B non-centro-symmetric superconductors. Reversible magnetization curves were plotted as M1/2 vs. T. This allows study of the asymptotic behavior of the averaged order parameter amplitude (gap) near the superconducting transition. Results of the analysis show, as expected, a mean field superconducting transition for Li2Pd3B. On contrary, a large deviation from the mean field behavior is revealed for Li2(Pd0.8Pt0.2)3B. This is interpreted as due to the strength of the non s-wa…
The recently discovered intermetallic superconductor MgB2 appears to have a highly anisotopic upper critical field with Hc2(max)/Hc2(min} = γ> 5. In order to determine the temperature dependence of both Hc2(max) and Hc2(min) we propose a method of extracting the superconducting anisotropy from the magnetization M(H,T) of randomly oriented powder samples. The method is based on two features in dM/dT the onset of diamagnetism at Tc(max), that is commonly associated with Hc2, and a kink in dM/dT at a lower temperature Tc(min). Results for LuNi2B2C and YNi2B2C powders are in agreement with anisotr…
We present magnetization measurements for the upper critical field $H_{c2}$ on aligned Na$_{x}$(H$_{3}$O)$_{z}$CoO$_{2}\cdot y$H$_{2}$O superconductors with magnetic field parallel to CoO$_2$ layers. The temperature-dependent part of the normal-state magnetization $M(H,T)$ can be well scaled with Brillouin function, suggesting the existence of impurity moments. By subtracting the contribution of the magnetic impurity moments, a well defined onset of superconducting diamagnetism allows the in-field superconducting transition temperature $T_{c}$($H$) accurately determined. No superconducting tra…
Cryogenic scanning tunneling microscopy and magnetization measurements were used to study the superconducting properties of MgB_2. The magnetization measurements show a sharp superconductor transition onset at T_c = 38.5 K, in agreement with previous works. The tunneling spectra exhibit BCS gap structures, with gap parameters in the range of 5 to 7 meV, yielding a ratio of 2delat/KT_c ~ 3-4. This suggests that MgB_2 is a conventional BCS (s-wave) superconductor, either in the weak-coupling or in the `intermediate-coupling` regime
We report on the systematic evolution of vortex pinning behavior in isovalent doped single crystals of BaFe2(As1-xPx)2. Proceeding from optimal doped to ovedoped samples, we find a clear transfor- mation of the magnetization hysteresis from a fishtail behavior to a distinct peak effect followed by a reversible magnetization and Bean Livingston surface barriers. Strong point pinning dominates the vortex behavior at low fields whereas weak collective pinning determines the behavior at higher fields. In addition to doping effects, we show that particle irradiation by energetic protons can tune vo…
Transport and thermodynamic properties of a sintered pellet of the newly discovered $MgB_2$ superconductor have been measured to determine the characteristic critical magnetic fields and critical current densities. Both resistive transition and magnetization data give similar values of the upper critical field, $H_{c2}$, with magnetization data giving $dH_{c2}/dT=0.44~T/K$ at the transition temperature of $T_c=40.2 K$. Close to the transition temperature, magnetization curves are thermodynamically reversible, but at low temperatures the trapped flux can be on the order of one Tesla. The value …
We studied three single crystals of YBa2Cu3O{7-x} with Tc= 62.5, 52, and 41 K, and a textured specimen of (Bi-Pb)2Sr2Ca2Cu3O10 with Tc=108 K, for H//c axis. The reversible data were interpreted in terms of 2D lowest-Landau-level fluctuation theory. The data were fit well by the 2D LLL expression for magnetization obtained by Tesanovic etal., producing reasonable values of kappa but larger values of dHc2/dT. Universality was studied by obtaining a simultaneous scaling of Y123 data and Bi2223. An expression for the 2D x-axis LLL scaling factor used to obtain the simultaneous scaling was extracte…
Reversible field-free superconducting diode effect controlled by an antiferromagnet
Institute of Physics, Czech Academy of Sciences（捷克科学院物理研究所）；Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University in Prague（布拉格捷克理工大学核科学与物理工程学院）；Niels Bohr Institute, University of Copenhagen（哥本哈根大学尼尔斯·玻尔研究所）；Faculty of Mathematics and Physics, Charles University（查理大学数学与物理学院）
