Magnetic rotation observation of the C2b3Σg− ← a3Πu transition using a color center laser

The magnetic rotation observation of the C₂b³Σ_g^? ← a³Π_u Ballik-Ramsay system using a color center laser is reported. This is the first detection of this system in absorption. Three bands, 0 ← 1, 1 ← 2, and 2 ← 3, were identified in the spectral range 3650–4030 cm^?1. The last two bands were observed for the first time. In magnetic rotation many satellite lines (ΔN ≠ ΔJ) which would be very weak in normal absorption have been observed with intensity comparable to the main branch lines. This permits a slight improvement in the accuracy of some of the fine structure constants. A variety of lineshapes are observed for the various branches by magnetic rotation. Because the b³Σ_g^? fine structure is small, giving a partial overlap, the peak frequency of a magnetic rotation signal usually does not correspond to the center frequency of the normal absorption signal of that transition. A computer program has been written to predict magnetic rotation lineshapes and obtain the peak frequency displacements. Various observed and calculated lineshapes are displayed and compared.     

Visualization of magnetic flux penetration and trapping in superconducting Bi2Sr2CaCu2Ox single crystals

Magnetic flux profiles in superconducting Bi₂Sr₂CaCu₂O_x single crystals have been observed using a magnetic garnet film as a magnetic field sensor. Measurements have been performed for the “zero-field-cooled” and “field-cooled” regimes as a function of temperature and magnetic field. An analysis of the observed patterns has revealed some peculiarities in the flux trapping effects. In particular, a nonuniform magnetic structure has been directly observed in the remanent state after “field-cooled” processes.     

Spatial variation of superconducting properties of Gd123 Bulk superconductors with magnetic particles addition

Magnetic flux trapping and the homogeneity of the flux pinning are essential problems in the practical application of high-temperature superconductors. We have conducted study on the role of addition of soft magnetic Fe-B alloy particles contribute to the enhancement of the critical current density (J_c) under wide-range of magnetic field. Magnetic flux trapping was enhanced in Gd123 bulk superconductor with suitable amount of magnetic particles addition. In addition, it can be effective as pinning center enhance the J_c of the bulk in both the a–b growth sector and the c-growth sector under magnetic field. However, the T_c of the Gd123 bulk was decreased obviously by addition of magnetic particles. The study on the spatial variation of superconducting properties indicates that the performance of the upper part of the bulk is better than the bottom. By comparing the superconducting properties of the Gd123 bulk with magnetic particles addition and without magnetic particles addition, we concluded that there is a trace of the formation of homogeneous pinning properties in the magnetic particles addition Gd123 bulk.     

Angular dependences of critical current density in YBCO thin films with crossed columnar defects

We investigated the dependences of the critical current density J_c on the magnetic field angle θ in YBa₂Cu₃O_7−δ thin films with the crossed configurations of the columnar defects (CDs). To install the crossed CDs, the films were irradiated using the high energetic Xe ions at two angles relative to the c-axis. The additional peak around the c-axis appears in the J_c(θ) for all irradiated films. In lower magnetic fields, the height of the J_c(θ) peak caused by the crossed CDs with the crossing angles θ_i = ±10° was higher than that for the parallel CDs. It is considered that the correlation of the flux pinning by the crossed CDs along the c-axis occurs even in the case of θ_i = ±25°, which was also suggested by the kink behaviors of the scaling parameters of the current–voltage characteristics near 1/3 of the matching field. In higher magnetic fields, on the other hand, the height and width of the J_c(θ) peak for the crossed CD configurations rapidly reduce with increasing the magnetic field compared to the parallel ones. In the crossed CD configurations, the dispersion in the direction of CDs would prevent the correlation of flux pinning along the c-axis in high magnetic fields, which occurs in the parallel CD configurations due to the collective pinning of flux lines including the interstitial flux lines between the directly pinned flux lines by CDs.     

Properties of superfine superconducting filaments embedded in normal matrix

The size-, surface- and proximity-effects are combined for calculating the critical temperature and some unusual electromagnetic properties of superfine superconducting NbTi filaments (i.e. with a diameterd _s < 0·1 μm) in the neighbourhood of normal metal (Cu). It is shown that the existence of the one-dimensional flux line lattice is responsible for the extremely large values of the critical current densities in low magnetic fields (B < 0·2B _c2,B _c2 — the upper critical magnetic field), for the maximum of the volume pinning force at low magnetic fields (B _m ≈ 0·1B _c2, comparing withB _m≈ 0·5B _c2 in usual macroscopic multifilamentary NbTi superconductors), as well as for the peculiar minimum of the magnetization after reversing the magnetic field direction. The possible role of the frozen-in flux lines in creating flux lines of opposite direction is sketched. It is proposed that for practical purposes the superconducting filaments should be coated with electrically insulating material (e.g. cupro-nickel or dielectric) to destroy the electro-magnetic coupling between the filaments and to ensure low ac losses in the composite.     

Metal magnetic memory effect caused by static tension load in a case-hardened steel

For investigating the magnetic abnormality influenced by stress in ferromagnetic materials, static tension tests on a case-hardened steel were carried out. Different loads, which covered tensile elastic loads up to plastic deformation and break, were applied. Meanwhile, the normal component of magnetic flux leakage, H_p(y), was measured by metal magnetic memory testing. The results indicate that H_p(y) values change with the tensile loads and positions. There exists a relationship between k, which is the inclination of the linear amplitude-locus magnetic flux leakage curve, and static tension load. A simple model is derived. Additionally, the mechanism of the magnetic memory effect can be explained by the theory of the interaction between dislocations and domains. The research provides the potential possibility of quantitative inspection for metal magnetic memory testing.     

Magnetic field buoyancy in accretion disks of young stars

Buoyancy of the fossil magnetic field in the accretion disks of young stars is investigated. It is assumed that the Parker instability leads to the formation of slender flux tubes of toroidal magnetic field in the regions of effective magnetic field generation. Stationary solution of the induction equation is written in the form in which the buoyancy is treated as the additional mechanism of the magnetic flux escape. We calculate the fossil magnetic field intensity in the accretion disks of young T Tauri stars for the cases when radius of the magnetic flux tubes a _mft = 0.1H,0.5H or 1H, where H is the accretion disk height scale. Calculations show that the buoyancy limits toroidal magnetic field growth, so that its strength is comparable with the vertical magnetic field strength for the case a _mft = 0.1H.     

Study of magnetic flux pinning in granular YBa2Cu3O7 − y /nanoZrO2 composites

In this work, the effect of ZrO₂ nanoparticles prepared in a low-pressure arc discharge plasma on magnetic flux pinning of granular YBa₂Cu₃O_{7 ? y} /nanoZrO₂ composites has been studied. It has been shown that the ZrO₂ nanoparticles do not change the superconducting transition and the microstructure of superconductors. At a temperature of 5 K, the addition of 0.5 and 1 wt % of ZrO₂ nanoparticles may lead to the additional effect of magnetic flux pinning and the increase in the critical current density J _c. The J _c value for composites with 1 wt % is two times larger than that for the reference sample. The fishtail effect is observed for YBa₂Cu₃O_{7 ? y} /nanoZrO₂ composites at the temperatures of 20 and 50 K. The problems associated with the additional effect of magnetic flux pinning of granular YBa₂Cu₃O_{7 ? y} /nanoZrO₂ composites and the appearance of the fishtail effect have been discussed.     

Study of the magnetic interaction in nanocrystalline Pr-Fe-Co-Nb-B permanent magnets

The magnetic properties of an isotropic, epoxy resin bonded magnets made from Pr-Fe-Co-Nb-B powder were investigated. The magnetization reversal process and magnetic parameters were examined by measurements of the initial magnetization curve, major and minor hysteresis loops and sets of recoil curves. From the initial magnetization curve and the field dependencies of the reversible and irreversible magnetization components derived from the recoil loops it was found that the magnetization reversal process is the combination of the nucleation of reversed domains and pinning of domain walls at the grain boundaries and the reversible rotation of magnetization vector in single domain grains. The interactions between grains were studied by means of δM plots. The nonlinear behavior of δM curve approve that the short range intergrain exchange coupling interactions are dominant in a field up to the sample coercivity.The interaction domains and fine magnetic structure were revealed as the evidence of exchange coupling between soft α-Fe and hard magnetic Nd₂Fe₁₄B grains.     

Spin-rotating magnet for the ΔσL(np) experiment at LHEP JINR

The spin rotating magnet (SRM) is purposed for the orientation rotation of the nucleon spins in the polarized beam from the transverse (T) direction with respect to the nucleon beam momentum to the longitudinal (L) one. The longitudinally polarized neutron beam was used in the experiment for measuring the total cross section difference Δσ_L(np) with parallel and antiparallel orientation of the participant L polarization. To perform the nucleon spin rotation in the polarized nucleon beam through the angle of 90° over the beam momentum region of ～1.8–5.5 GeV/c, a proper spin rotation device had to be prepared. For this purpose, the necessary calculations of corresponding values of the magnetic induction integral were carried out. Using the calculations the dipole magnet SP 57 type was chosen for the Δσ_L(np) experiment and the required reconstruction of its pole tips was also accomplished. After the SRM installation at the neutron beam line the appropriate apparatus set for the magnetic measurements was prepared and the precise measurements of the whole set of the SRM characteristics were performed. The obtained results for the SRM magnetic field parameters were successfully used during the Δσ_L(np) experimental runs to specify the current at this magnet coil corresponding to the calculated magnetic induction integral for the given neutron beam momentum.     

Magnetic anisotropy in multilayers

Magnetic anisotropy between in-plane and out of plane magnetic alignments is studied in a variety of multilayer systems using Mössbauer spectrosopy to observe the (Fe) magnetic orientation. The surface anisotropy in Fe/Au (1 1 1) multilayers is measured as K _s = 0.9 × 10^?3 Jm^?2. In Fe/Ni multilayers the dependence of magnetic orientation on external field applied normal to the layers enables volume and interface anisotropies K _v = (?5 ± 1) × 10⁴ Jm^?3 and K _s = (?0.6 ± 0.4)× 10^?3 Jm^?2 to be evaluated. In similar applied field experiments coherent rotation of the magnetic Fe and NiFe layers in Fe/Cu/NiFe/Cu multilayers was observed for intervening Cu layer thickness x = 5 Å but independent rotation for x = 50 Å. Out of plane magnetic components are observed for DyFe₂, YFe₂ thin films and DyFe₂/YFe₂ multilayers. In fields of up to 0.25 T applied inplane only the moments of the YFe₂ film showed significant rotation.     

Thermally induced magnetic flux in a superconducting ring

The thermally induced magnetic flux in a bi-metallic superconducting loop of Nb and Ta has been measured and compared with theory. The predicted logarithmic divergence of the flux near the T_c of Ta is found, however the magnitude of the flux is larger than predicted.     

Electromagnetic induction in conductors accelerated in magnetic fields amplified by flux compression

The initial boundary-value problem for the electromagnetic induction in a conducting slab ats(t)≦x≦s(t)+a resulting from its accelerated motionv={s(t), 0, 0} across a transverse magnetic fieldB={0,B(x,t), 0} is treated, when the latter is amplified by orders-of-magnitude with respect to its initial valueB(x,t=0)=B ₀(x) by flux compression in the gap between the moving conductor surfacex=s(t) and an ideal resting conductor atx=0. Two initial (t=0) configurations are considered, assuming that (I)B ₀ (step-shaped) has not yet and (II)B ₀ (uniform) has completely diffused into the conductor atx=s(t=0). By means of a time-dependent coordinate transformation ξ=[x ? s(t)]/a and Fourier series expansions, the electromagnetic fields in the moving conductor are represented as integralfunctionals of the magnetic fieldB ₁ (t) in the gap 0≦x≦s(t).B ₁ (t) is given analytically as solution of a singular Volterra integro-differential equation. The theory is valid for arbitrary (nonrelativistic) speeds.(t) and accelerationss(t)) of the moving conductor. Applications to explosion driven electric induction generators and magnetic flux experiments are discussed briefly.     

On the possibility of direct measurement of the magnetic field-induced phase transition in hematite by means of magnetooptical method

It is shown that a rotation ? and a deformation κ of the optical indicatrix appear during the transverse magnetic field-induced phase transition in hematite. Analytic expressions for ? and κ are deduced from the magnetization-dependent electromagnetic energy in the crystal. It is shown that during the phase transition, induced by increasing the temperature, the electromagnetic energy in the crystal. It is shown that during the phase transition, induced by increasing the temperature, the antiferromagnetic vector L = M₁ - M₂ is rotating from the three-fold C₃ axis toward the basal plane, which implies that the main axis of the optical indicatrix is not aligned in a general case with the magnetic field or the crystallographic axis although the magnetic moment (M₁ + M₂) is always parallel to the field. The linear magnetic birefringence is very sensitive to the magnetic phase in hematite, as described in previous experimental work, but the present analysis shows that a direct determination of the transverse field-induced phase transition can be obtained in hematite, by means of a magnetooptical method, only when large and non-uniform rotation (up to ninety degrees) and variation of the shape of the indicatrix are taken into account.     

Effect of helical force on the stationary convection in a rotating ferrofluid

In this paper we studied the onset of instability in a horizontal layer of a rotating ferrofluid in the presence of the helical force. The analytical expression of the Rayleigh number of the system is determined as a function of the dimensionless numbers obtained. Then, the effect of each dimensionless parameter is studied. The helical force, the binary parameter ψ then the magnetic parameters M₁, M₃ and ψ_m accelerate the onset of stationary convection whereas the rotation and the magnetic parameter M₂ delay it. Also all the magnetic parameters, the binary parameter and the rotation cause the convection rolls to shrink while only the helical force increases the size of these structures.     

Magnetoelectric effect and magnetization process in Fe5LiO8

After showing the existence of second order M.E. effect β, in Fe₅LiO₈ [1], we try to verify Lee and Rado's assumptions. These authors suppose that the existence of β is connected with the rotation of magnetic domains. We compare the magnetoelectric β and the magnetization σ curves vs the magnetic field H₀. The latter curve reveals the different magnetic modes, and therefore illustrates the behavior of the magnetic domains. This comparison has been made for several directions of the applied field in order to illustrate all possible cases of rotation of the domains and allows us to confirm the basic assumption.     

Maximal supercurrent of a Josephson junction in a field of magnetic particles

The maximal supercurrent I_m of a short Josephson junction formed by an edge contact of two superconducting films is calculated for the case where the junction is placed in a periodic field produced by a chain of magnetic nanoparticles. The commensurability effects occurring when the magnetic flux of a homogeneous external field H₀ through an elementary cell is equal to an integral number of magnetic flux quanta Φ₀ are considered. The effects give rise to additional maxima in the I_m(H₀) dependence.     

Physical processes in high-temperature superconductors at the interface between the vortex-filled and meissner regions

A technique has been proposed for investigating the magnetic microstate of high-temperature superconductors with a simultaneous analysis of the crystalline microstate of the sample with the aim of elucidating the specific features of the interaction between the crystalline and magnetic microstructures of polycrystalline high-temperature superconductors. Qualitatively new results have been obtained for samples with different microstructures. In particular, it has been found that the magnetic field dependences of the trapped magnetic flux density B _tr(H ₀) of polycrystalline and epitaxial films of high-temperature superconductors exhibit regular steps for both increasing and decreasing magnetic fields. The obtained results have demonstrated that, in strong magnetic fields, the studied epitaxial films, as well as bulk and thin-film polycrystalline high-temperature superconductors, “break down” into single domains, crystallites, and subcrystallites with different demagnetization factors. It has been revealed that the dependences B _tr(H ₀) also exhibit steps due to the simultaneous penetration of vortices into crystallites of approximately the same sizes and into more regularly arranged subcrystallites. As the quality of the samples increases, these steps become more pronounced because of the increase in the short-range order. The absence of steps in the dependence B _tr(H ₀) of the polycrystalline bulk samples clearly demonstrates the absence of long-range order in these samples. It is the vitreousness of the crystallographic microstructure of high-temperature superconductors which is responsible for the observed transformations in the vortex system. The similarity of the results obtained for samples with different microstructures indicates that the penetration (escape), distribution, and trapping of the magnetic flux in these samples occur through a universal mechanism. It has been found that the polycrystalline high-temperature superconductors are actually multi-step rather than two-step systems. It has been shown that the vitreousness of the microstructure of high-temperature superconductors and the presence of close-packed twin boundaries in samples lead to the penetration of a magnetic flux in the form of hypervortices into the sample and cause the formation of a superconducting glass state on a different physical basis as compared to the Ebner-Stroud model of a granulated glass.     

Self-similar distribution in “giant” magnetic flux creep

The problem of magnetic field penetration into the half-space is considered in parallel geometry in an external magnetic field increasing with time in accordance with the law B(0, t, τ₀ = B _{c 1} (1 + t/τ₀) ^m , m ≥ 0, t ≥ 0 (τ ₀ is the time of magnetic flux redistribution and B _{c 1} is the lower critical field). It is assumed that the flow of vortices is thermally activated in the “giant” creep mode (i.e., for weak pinning creep and high temperatures). A model equation is derived for describing the magnetic flux evolution. Analytic formulas are obtained for the depth and velocity of magnetic field penetration. It is shown that the giant creep regime is stable for 0 ≤ m ≤ 1/2.     

Phase transition in a vortice lattice in a Bi2212:Pb single crystal

The magnetooptical method was used to investigate the penetration of a magnetic flux into a single crystal of a high-temperature superconductor (Bi_0.84Pb_0.16)_2.2Sr₂CaCu₂O₈ in crossed magnetic fields. It is shown that at low temperatures the penetration of the magnetic flux is anisotropic: the flux moves preferentially along the magnetic field applied in the plane of the sample, and the anisotropy grows as the temperature increases. At a temperature T_m = 54±2 K, there occurs a sharp change in the character of penetration of the magnetic field into the superconductor; the direction of the flux ceases be dependent on the direction and magnitude of the magnetic field applied in the plane of the sample. In this case, the transition temperature T_m is independent of the applied magnetic field. The effect is interpreted in terms of the concepts of a phase transition in the system of vortices, which is related to a sharp decrease in the correlations in the position of vortices in various CuO planes, i.e., with the transition from three-dimensional to two-dimensional behavior of the vortex structure.