温度对垂直布洛赫线链解体临界面内磁场区间的影响

实验研究了温度对液相外延石榴石磁泡薄膜中条状普通硬磁畴壁内的垂直布洛赫线（ＶＢＬ）链解体临界面内磁场区间的影响。发现存在一对特征温度T⁽¹⁾₀ 和 T⁽²⁾₀（前者比后者略低但均高于室温），在从室温到T⁽²⁾₀的每个温度Ｔ下，使ＶＢＬ逐渐消失的面内磁场Ｈ_ip都分布于一个与Ｔ有关的区间[H_ip⁽¹⁾(T), H_ip⁽²⁾(T)]内，称为临界面内磁场区间：H_ip^{＜H_ip(1)(T)时，ＶＢＬ链保持不变;H_ip(1)(T)< H_ip < H_ip(2)(T)时，随着Ｈ_ip的增加，越来越多的ＶＢＬ消失;H_ip > H_ip(2)(T)时，所有ＶＢＬ都消失。H_ip(1)(T),H_ip(2)(T)及H_ip(2)(T)-H_ip(1)(T)均随Ｔ的升高而下降，前二者分别于T(1)₀ 和 T(2)₀降为零。比值H_ip(2)(T)/H_ip(1)(T)随Ｔ的升高而升高，在低温段（包括室温）升高缓慢且约为21/2,在T(1)₀附近急剧升高且至T(1)₀时趋于∞。对以上结果做了理论分析。 关键词：}     

Magnetic field and temperature-induced first-order transition in Gd5(Si1.5Ge2.5): a study of the electrical resistance behavior

Magnetic field (0–4 T) and temperature dependencies (4.2–320 K) of the electrical resistance of Gd₅(Si_1.5Ge_2.5), which undergoes a reversible first-order ferromagnetic↔paramagnetic phase transition, have been measured. The electrical resistance of Gd₅(Si_1.5Ge_2.5) indicates that the magnetic phase transition can be induced by both temperature and magnetic field. The temperature dependence of the electrical resistance, R(T), for heating at low temperatures in the zero magnetic field has the usual metallic character, but at a critical temperature of T_cr=216 K the resistance shows a 20% negative discontinuity due to the transition from the low-temperature high-resistance state to the high-temperature low-resistance state. The R(T) dependence for cooling shows a similar but positive 25% discontinuity at 198 K. The isothermal magnetic field dependence of the electrical resistance from 212T224 K indicates the presence of temperature-dependent critical magnetic fields which can reversibly transform the paramagnetic phase into the ferromagnetic phase and vice versa. The critical magnetic fields diagram determined from the isothermal magnetic field dependencies of the electrical resistance of Gd₅(Si_1.5Ge_2.5) shows that the FM↔PM transition in zero magnetic field on cooling and heating occurs at 206 and 213 K, respectively. The full isothermal magnetic filed hysteresis for the FM↔PM transition is 2 T, and the isofield temperature gap between critical magnetic fields is 7 K.     

Superconducting Properties of Indium Nanostructured in Pores of Thin Films of SiO<Subscript>2</Subscript> Microspheres

Samples of a superconducting indium nanocomposite based on a thin-film porous dielectric matrix prepared by the Langmuir–Blodgett method are obtained for the first time, and their low-temperature electrophysical and magnetic properties are studied. Films with thickness b ≤ 5 μm were made from silicon dioxide spheres with diameter D = 200 and 250 nm; indium was introduced into the pores of the films from the melt at a pressure of P ≤ 5 kbar. Thus, a three-dimensional weakly ordered structure of indium nanogranules was created in the pores, forming a continuous current-conducting grid. Measurements of the temperature and magnetic field dependences of the resistance and magnetic moment of the samples showed an increase in the critical parameters of the superconductivity state of nanostructured indium (critical temperature T_c ≤ 3.62 K and critical magnetic field H_c at T = 0 K H_c(0) ≤ 1700 Oe) with respect to the massive material (T_c = 3.41 K, H_c(0) = 280 Oe). In the dependence of the resistance on temperature and the magnetic field, a step transition to the superconductivity state associated with the nanocomposite structure was observed. A pronounced hysteresis M(H) is observed in the dependence of the magnetic moment M of the nanocomposite on the magnetic field at T < T_c, caused by the multiply connected structure of the current-conducting indium grid. The results obtained are interpreted taking into account the dimensional dependence of the superconducting characteristics of the nanocomposite.     

Non-planar spin texture driven dissipation in a ferromagnet-superconductor bilayer

We report the observation of a pronounced dip in the in-plane magnetic field (H_∥) dependence of the critical current density J_c(H) and a peak in resistance R(H) of a NbN-HoNi₅ bilayer at temperatures below the magnetic ordering temperature (T_Curie ≈ 3.5 K) of HoNi₅, which is lower than the onset temperature (≈9 K) of superconductivity in the NbN layer. The extrema in J_c(H) and R(H) appear at fields much below the upper critical field of NbN. We attribute these features to a coupling between localized out-of-plane moments present in the magnetic film and Pearl vortices of the superconducting layer. A spin re-orientation transition of the localized moments by H_∥ breaks this coupling, leading to the observed excess dissipation.     

Transport critical-current and voltage-current characteristics of Bi(2223) silver-clamped thick films

The transport critical current properties of Bi(2223) silver-clamped thick films are studied by the measurement of its dependence on magnetic field and temperature close to T _c. It is found that the transport critical current follows a power law J _c(1–T/T _c)^3/2 for the sample with J _c>2.0×10⁴ A/cm² (77K, zero field) and that J _c(H) is basically reversible for increasing and decreasing magnetic field. After the transport current exceeds the critical current, the voltage-current (V-I) characteristics show a flux-creep-like behaviour until they smoothly join the flux-flow state. From the measurement of V-I curves, the pinning property of the sample may be estimated using the flux-creep mode. The flux-flow resistance is found to have a nonlinear magnetic field dependence.     

The defects influence on domain wall propagation in bistable glass-coated microwires

We studied the domain wall (DW) dynamics of magnetically bistable amorphous glass-coated Fe₇₄B₁₃Si₁₁C₂ microwires. In according to our experimental results magnetic field dependences of DW velocity of studied microwires can be divided into two groups: with uniform or uniformly accelerated DW propagation along the microwire. Strong correlation between the type of the magnetic field dependence of domain wall velocity, v(H), and the distribution of the local nucleation fields has been observed.Moreover, we observed abrupt increasing of DW velocity (jump) on the magnetic field dependences of the domain wall velocity, v(H), for the both types of the v(H) dependences. At the same time usual linear increasing of the domain wall velocity with magnetic field persists below these jumps. It was found that the jump height correlates with the location of nucleation place of the new domain wall. We have measured local nucleation field distribution in all the microwires. From local nucleation field distribution we have obtained the DW nucleation locations and estimated the jump height     

Non-analytical angular dependence of the upper critical magnetic field in a quasi-one-dimensional superconductor

We have derived the so-called gap equation, which determines the upper critical magnetic field, perpendicular to conducting chains of a quasi-one-dimensional superconductor. By analyzing this equation at low temperatures, we have found that the calculated angular dependence of the upper critical magnetic field is qualitatively different than that in the so-called effective mass model. In particular, our theory predicts a non-analytical angular dependence of the upper critical magnetic field, H _c2(0) − H _c2(α) ∼ α^3/2, when magnetic field is close to some special crystallographic axis and makes an angle α with it. We discuss possible experiments on the superconductor (DMET)₂I₃ to discover this non-analytical dependence.     

Enhancement of the upper critical field of MgB2 by carbon-doping

We have measured the temperature dependence of the upper critical field, H_c2(T), of carbon-doped MgB₂. H_c2(T) does not follow the well-known Werthamer-Helfand-Hohenberg (WHH) result for a one-gap dirty superconductor but can be described well by the result of a recent theoretical calculation for a two-gap dirty superconductor. H_c2(0) of the carbon-doped material is determined to be between 29 and 38 T, substantially higher than that of pure MgB₂ (15-23 T).     

高Jc Tl系2223相银包套带材的磁通钉扎和耗散

测量了Ｔｌ系２２２３相银包套超导带（Ｊ_c＝１．５×１０^４Ａ·ｃｍ^-2，７７Ｋ，０Ｔ）在０－０．８Ｔ磁场下电阻转变展宽，实验结果引用热激活磁通蠕动模型加以解释。磁场平行于带面（Ｈ∥ａｂ面）和磁场垂直于带面（Ｈ⊥ａｂ面）两种情况下，激活能与磁场之间满足幂指数关系：Ｕ_0∥＝０．２１ H^-0.3(eV),Ｕ_0⊥＝０．１5 H^-0.4(eV)，其中Ｈ的单位为ｋＧ。 关键词：     

Doping dependence of the vortex glass and sublimation transitions in the high-T<Subscript>c</Subscript> superconductor La<Subscript>2-x</Subscript>Sr<Subscript>x</Subscript>CuO<Subscript>4</Subscript> as determined from macroscopic measurements

Magnetization and ac-susceptibility measurements are used to characterize the mixed phase of the high-temperature cuprate superconductor La_2-xSr_xCuO₄ over a large range of doping (0.075 0.20). The first order vortex lattice phase transition line H_FOT(T), the upper critical field H_c2(T) and the second peak H_sp(T) have been investigated up to high magnetic fields (8 Tesla applied perpendicular to the CuO₂ planes). Our results reveal a strong doping dependence of the magnetic phase diagram, which can mainly be explained by the increasing anisotropy with underdoping. Within our interpretation, the first order vortex lattice phase transition is due to the sublimation (rather than melting) of the vortex lattice into a gas of pancake vortices, whereas the second peak is related to the transition to a more disordered vortex glass state.     

Temperature dependence of coercivity and metamagnetism in Tb0.5Y0.5Ni

The temperature dependences of magnetic properties of the FeB structure compound Tb_0.5Y_0.5Ni are examined with the aim to elucidate the origin of intrinsic magnetic hardness in this compound. The easy a axis coercivity of an aligned powder sample is found to be strongly temperature dependent. A domain wall activation model is employed to obtain the parameters H₀(0)=30kOe, V_V(0)=0.24K^?1, and β=0.016K^?1, where H₀(0) is the coercive force at absolute zero, V_T(0) the domain wall activation parameter, and β the coefficient for the linear temperature dependence of the reduced anisotropy. From polycrystalline data, we find a strong temperature dependence of the c axis metamagnetic transition only for the ferromagnetic (F) to antiferromagnetic (AF) direction. From the relative constancy of the AF to F c axis metamagnetic transition compound to the a axis coercivity we conclude that the coercive force is not directly related to the local molecular fields as had been previously suggested.     

Study of the Nonlinear Response of Superconductors in the Microwave Band Using a Local Technique

Using a local technique, we study the microwave radiation power P₃(T,P,H_dc) at the triple frequency of the main signal as a function of the temperature T, the input power P, and the external magnetic field H_dc perpendicular to the superconductor surface for YBa₂Cu₃O₇ (YBCO) films, monocrystals, and polycrystals and for Nb films. The most distinctive feature of the temperature dependence P₃(T) of nonlinear response of superconductors is a maximum of the nonlinearity near the critical temperature T_c. Spatial distributions of the third-harmonic power are obtained for YBCO films at various temperatures. These distributions are indicative of a nonuniform distribution of T_c over the superconductor surface. Additional nonlinearity maxima are discovered in the dependence P₃(T) for YBCO films and monocrystals at temperatures about 2T_c/3. These maxima are probably related to the existence of several superconducting phases with different critical temperatures. For Nb films, the second nonlinearity maximum in the dependence P₃(T) appears only in the presence of an external magnetic field. The experimental data are interpreted within the framework of a two-fluid model of a superconductor, which takes into account the phenomenological nonlinear relationship between the vector potential A and the supercurrent j_s(A). The origin of nonlinearity in the studied superconductors is discussed.     

YBa2Cu3O7-δ外延薄膜的有效钉扎势

在0—7T磁场范围内，测量了不同测量电流密度下YBa₂Cu₃O_7-δ外延薄膜的电阻温度关系.实验结果表明，临界温度以下，混合态的耗散电阻率能很好地用热激活磁通蠕动描述.有效钉扎势的电流密度关系遵守Zeldov等人提出的对数关系，有效钉扎势的温度和磁场关系遵守U∝(1-T/T_c)H^-α关系，其中α=0.63，与热激活磁通点阵位错运动模型相一致，表明样品具有2D涡旋性质. 关键词：     

Mixed state and critical current in narrow semiconducting films

The mixed state of thin narrow superconducting films with an edge barrier placed in a transverse magnetic field is considered. The boundaries of the region for the existence of metastable mixed states with an assigned number of vortices N [H _min(N)⩽H⩽H _max(N)] are found. The magnetic-field dependence of the critical field is found for the films. The transition from the Meissner state to the static mixed state is discussed. Fiz. Tverd. Tela (St. Petersburg) 40, 1773–1777 (October 1998)     

Evidence for two superconducting gaps in the unconventional superconductor PrPt4Ge12

We report the results of magnetization measurements on the unconventional superconductor PrPt₄Ge₁₂ in its superconducting state. A distinct positive curvature is observed in the temperature dependence of the lower critical field H _C1(T) and the upper critical field H _C2(T) around 5.5?K. The zero field critical current density J _C(T, H?=?0) is found to vary nearly linearly with temperature. We argue that these observed features are due to the existence of two superconducting gaps in the material. The normalized superfluid density is estimated from the temperature dependence of H _C1. Our detailed analysis of the normalized superfluid density using a two-gap model indicates the presence of line nodes in the larger gap. The H _C2(T) line can be fitted with the Usadel equations, and the positive curvature in H _C2(T) is shown to be consistent with the two-gap picture of superconductivity.     

Ullah-Dorsey Scaling of Fluctuation Conductivity Near the Superconducting Transition in (LaSe)1.14(NbSe2)

Magnetotransport measurements with the quasi two-dimensional misfit-layer compound (LaSe)_1.14(NbSe₂) with critical temperature T _c = 1.23 K are presented. The temperature dependence of the upper critical magnetic field H _c2 has been determined from an analysis of magnetoresistance data through the scaling procedure obtained theroretically by Ullah and Dorsey for the fluctuation conductivity. We show that in contrast to a direct determination of H _c2(T) with a positive curvature the temperature dependence of H _c2 obtained via the scaling procedure reveals a negative curvature in agreement with the Werthammer-Helfand-Hohenberg theory for conventional type-II superconductors.     

High Field ESR System and Its Application to the Study of Quantum Spin Systems

Developments of the high field ESR system in Kobe University is presented. Using Gunn oscillators and backward traveling oscillators (BWO), we can cover the frequency region from 30 to 1183.6 GHz with the use of InSb detector. Pulsed magnetic field up to 30 T is available and we are now trying to extend the field up to 40 T. Temperature range is from 1.8 to 300 K. Using this system, we studied S=1/2 ladder like system Cu₂(C₅H₁₂N₂)₂Cl₄, and found a new magnetic transition at 10.1 T at 1.8 K. The temperature dependence of ESR in Cu₂(C₅H₁₂N₂)₂CI₄ shows g-shift below 8 K which corresponds to the maximum of the magnetic susceptibility. The g-shift below 8 K suggests the increase of the quantum fluctuation in the system, and the role of the quantum fluctuation in Cu₂(C₅H₁₂N₂)₂CI₄ is discussed.     

Unusual magnetization suppression induced by higher magnetic field in (La0.83Bi0.17)0.67Ca0.33MnO3

Magnetization behavior of (La_0.83Bi_0.17)_0.67Ca_0.33MnO₃ has been investigated in the temperature range from 100 to 180 K. A metamagnetic transition was observed in the temperature region, where the magnetization was measured after a zero-field-cooling from room temperature to a selected temperature. Experimental results show that, after a higher magnetization route, the field-increasing branches of the magnetization curves shows an unusual training effect: below a magnetic field H₀, the applied magnetic field enhances the value of magnetization; however, above H₀ the magnetic field suppresses the value, and the behavior cannot be totally attributed to the enhancement effect of the applied magnetic field on ferromagnetic phase fraction. It is proposed that, in the two-phase coexistence region, the higher magnetic field promotes the phase separation and leads to both the fraction of ferromagnetic domain and the stabilization of antiferromagnetic domain increase.     

在Tokamak装置中用中空超导环段冻结环向磁场的设想

本文提出一个设想:即将n个中空超导圆柱体放置于n饼纵场线圈之中,分段冻结环向磁场,则其所需要的电功率将比常规线圈降低到2/n倍。我们知道超导材料的冻结场可近似地写为。H_fj(r,T)=H_fj(r,0)[1-(T/(T_c))²。因此选定T=T₁,当轴向场H在大圆环的内壁达到H≥H_fj(r,T₁)时,中空体的内边缘达到临界场,则场进入中空区,再使H降到零,中     

Magnetic phase diagram of Ni(NO3)2 6NH3

The magnetic phase diagram of Ni(NO₃)₂6NH₃ was determined from the field and temperature dependence of the magnetic susceptibility. The zero temperature exchange and anisotropy fields were determined to be H_E(0) ≈ 26 kOe and H_A(0) ≈ 0.7 kOe respectively.