Tl2Ba2Ca2Cu3Oy超导体磁通钉扎能的分布

描述一种测量高温超导体磁通钉扎能U₀及其分布的新方法,此方法主要之点是测量磁化后样品的零场临界电流随时间的变化,利用这一方法测量Tl₂Ba₂Ca₂Cu₃O_y的U₀值分布(T=78K)。根据所得结果,对已报道的U₀值的分散性提出一种新解释。 关键词：     

Bi0.8Pb0.2SrCaCuOy超导体中磁通运动引起的内耗峰

在线性增加磁场的条件下,用变频倒扭摆测量了配比成份为(Bi_0.8Pb_0.2)SrCaCu₂O_y的试样在超导态时磁化过程中的低频内耗。测量温度为95K,试样处于零电阻状态,所用频率为0.5-5Hz.在内耗-场强H曲线上观测到内耗峰,它有如下特点:1)峰高随加场速率H的增大而增大,但随测量频率的增大而降低;2)峰值处的场强为4-6mT;3)当H由非零值突然改变至零时,磁化过程特征内耗消失;4)试样处于正常态时,上述特征内耗消失。认为超导体在磁化过程中的上述特征内耗是量子磁通的运动所引起来的。 关键词：     

弱场下熔融织构YBa2Cu3O7-δ样品局域磁通蠕动的实验研究

利用半导体霍尔探头测量法在液氮温度下研究了低场下圆柱状熔融织构YBa₂Cu₃O_7-δ样品的局域磁通蠕动行为.对瞬时加减场样品端面局域磁场的磁弛豫,磁滞回线以及不同扫场速率下局域磁场的变化进行了测量,观察到了对数时间磁弛豫的行为,发现由加场和去场数据所得的激活能U₀有明显的不同.磁滞回线测量中,局域点的磁场随外场几乎同步变化,不同扫场速率下局域磁场随外场变化的差别不大.实验结果表明,外场除对样品侧面外,对端面也有明显的穿 关键词： 局域磁通蠕动 激活能 磁弛豫     

钙钛矿(La1-yTby)0.67Sr0.33MnO3的超巨磁电阻效应

采用固相反应烧结法制成了钙钛矿(La_1-yTb_y)_0.67Sr_0.33MnO₃(y=0—1)多晶样品.研究了样品的微观结构,常温、低温下的磁性,样品的磁电阻随温度、成分的变化关系,电阻随温度变化特性等.并在y=0.40样品中,170K附近、最大磁化场为7T时,观察到了900%的巨磁电阻. 关键词：     

中子辐照熔融织构YBa2Cu3Oy的临界电流

测量了中子辐照熔融织构YBa₂Cu₃O_y样品从1.5K到85K之间一系列温度下的磁化曲线。磁场H平行于c轴时,在6K观察到磁通跳跃。中子辐照可使临界电流提高一个数量级。分析了临界电流的各向异性,辐照增强与磁场的关系和主要的钉扎中心。讨论了磁通跳跃的物理模型。 关键词：     

红色Bi4 Ge3 O12晶体在低温下的发光性能

用下降法制备了Bi₄Ge₃O₁₂晶体,发现生长出来的圆柱状晶体外侧呈现淡红色。对红色Bi₄Ge₃O₁₂晶体进行了低温(至8 K)下的近红外发射光谱及衰减寿命等测试分析。发现低温时(200 K以下)红色Bi₄Ge₃O₁₂在1 150 nm等波长处有较强的发射峰,强度随温度降低而增强,衰减时间为几百μs。     

2H-Nb0.9Ta0.1Se2中磁通涡旋输运的标度行为

测量了2H-Nb_0.9Ta_0.1Se₂单晶样品在不同电流下电压随磁场变化的曲线V(H),并从V(H)曲线得到V(I)数据.使用标度关系V=α(I-I_c)^β进行了拟合,得到了临界电流随磁场I_c(H)和微分电阻随磁场R_d的变化关系.在微分电阻随磁场变化的曲线中,电流较大时,靠近上临界磁场H_c2附近出现一个强峰,而在低电流下,该峰消失.同 关键词： 磁通涡旋 标度定律 临界电流峰效应     

金属玻璃(Fe1-xCox)78Si10B12的感生各向异性

本文研究了金属玻璃(Fe_1-xCO_x)₇₈Si₁₀B₁₂的磁化感生各向异性、应变感生各向异性随成分和温度的变化。磁化感生各向异性常数K_um为正值,x=0.7时为最大;不可逆的应变感生各向异性常数K_usi为正值,x=0.5时为最大;可逆的应变感生各向异性常数K_usr除了x>0.975区均为负值,在x=0.7时为最大;感生各向异性常数在温度变化时与M_s^α成正比,α在3.4和7.5之间随成分和退火工艺而变化。用短程有序模型解释了部分实验结果。 关键词：     

YBa2Cu3O7超导管内磁场与外磁场的关系

本文报道了当外磁场H_e较低时(H_e<4H_rm,其中H_rm是超导管的最大捕获磁场),超导管的内磁场H_i与时间及外磁场的关系可表示为H₀+B₀(lnt+1-exp(-t/τ)),式中B₀Int与磁通蠕动有关,B_c(1-exp(-t/τ))与外磁场引起的磁通扰动有关。扰动随时间很快衰减.H₀,B₀及1/τ随外磁场增加,在H_e>H_rm时,这几个量增加较大。当直流外磁场H_e<1/2H_rm时,超导管可将其完全屏蔽,对交流场不能完全屏蔽,但其屏蔽效果仍优于最好的金属屏蔽筒。 关键词：     

Nd2(Fe1-xMnx)14B的低温内禀矫顽力

当x<0.5时,Nd₂(Fe_1-xMn_x)₁₄B可形成四方晶体结构,空间群为P4₂/mnm。在低温下,该赝三元化合物的大块铸态样品具有高矫顽力。此矫顽力不依赖于热处理等工艺过程,因此具有内禀性质。内禀矫顽力 _iH_c与样品的成分有关。Nd₂(Fe_1-xMn_x)₁₄B的起始磁化曲线具有传播场H_p,并且H_p的数值与 _iH_c接近。这表明内禀矫顽力是由畴壁钉扎造成的。研究了 _iH_c与温度的变化关系,并估算了钉扎位垒的强度。测量了Nd₂(Fe_1-xMn_x)₁₄B的居里温度和饱和磁化强度。在此赝三元化合物中,交换作用随Mn对Fe的替换而急剧降低。这使得畴壁厚度变窄。Nd₂(Fe_1-xMn_x)₁₄B的磁化和反磁化行为可用窄畴壁的特征来解释。 关键词：     

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.     

Ultra-sharp metamagnetic transition in manganite Pr0.6Na0.4MnO3

We report the magnetic and electrical transport properties of manganite Pr_0.6Na_0.4MnO₃. At the temperature of 2 K, a field-induced steplike magnetization and resistivity transition are observed. The step transitions of magnetization and resistivity are shifted to higher fields as a result of field cooling, and transformed to a smooth broad one when the cooling field is higher than 20 kOe. Moreover, in a magnetic field slightly below the critical field, the magnetic and resistive relaxation exhibits a spontaneous step after a long incubation time when both the temperature and magnetic field are constant. Such steplike transitions are discussed in terms of a martensiticlike transformation associated with phase separation.     

Magnetic properties and magnetocaloric effects in Tb6Co1.67Si3 compound

Magnetic properties and magnetocaloric effects of Tb₆Co_1.67Si₃ have been investigated by magnetization measurement. This compound is of a hexagonal Ce$_{6}$Ni$_{2}$Si$_{3}$-type structure with a saturation magnetization of 187\,emu/g at 5\,K and a reversible second-order magnetic transition at Curie temperature $T_{\rm C} = 186$\,K. A magnetic entropy change $\Delta S = 7$\,J\,$\cdot$\,kg$^{-1}$\,$\cdot$\,K$^{-1}$ is observed for a magnetic field change from 0 to 5\,T. A large value of refrigerant capacity (RC) is found to be 330\,J/kg for fields ranging from 0 to 5\,T. The large RC, the reversible magnetization around $T_{\rm C}$ and the easy fabrication make the Tb₆Co_1.67Si₃ compound a suitable candidate for magnetic refrigerants in a corresponding temperature range.     

Magnetic dynamics of charge ordered Nd0.80Na0.20MnO3 compound

The magnetic dynamics of charge ordered Nd_0.8Na_0.2MnO₃ compound was studied by measuring the temperature variation of magnetization for different magnetic fields up to 7 T and, the field variation of magnetization at different temperatures down to 5 K. This sample exhibits a charge-ordering transition at 180 K, followed by a weak ferromagnetic (FM) transition at around 100 K and a spin glass like transition below 40 K. Suppression of charge-ordering and spin glass like transition and increase in FM T_C were observed with an increase in magnetic field. A reversible metamagnetic transition above a threshold field (H_f) of 4.5 T was observed at 130 K, followed by a saturation magnetization of 3.2 μ_B/f.u. However at 5 K, an irreversible field induced first order phase transition from charge ordered state to FM state was observed at H_f=5 T. For comparison, the temperature and field variations of magnetization were studied on a FM compound from the same series with the composition Nd_0.90Na_0.10MnO₃. A clear FM transition with a T_C of 113 K and a saturation magnetization of 4.3 μ_B/f.u was observed.     

Magnetic and magneto-transport investigations of Co37Ni34Al29 alloy

Ferromagnetic shape memory alloy with nominal composition Co₃₇Ni₃₄Al₂₉ is investigated by transport and magnetic measurements. The anomaly due to the martensitic transition is observed around 130-210 K. The thermal hysteresis, observed due to martensitic transition in the dc magnetization versus temperature data, gets suppressed at higher applied field. Below 50 K, magnetization varies with temperature perfectly as T^3/2, which signifies that spin wave excitations are largely responsible for thermal demagnetization. The sample shows small negative magneto-resistance, which varies non-monotonically with temperature showing largest value at around 200 K.     

High critical magnetic field superconductor La3S4

We report electrical conductivity, specific heat and magnetization measurements on La₃S₄ single crystals. The results show, that La₃S₄ is a strong coupling superconductor with a BCS coherence length of 132 Å. This extremely low value makes La₃S₄ an intrinsic high critical magnetic field superconductor with a Landau-Ginsburg parameter κ₀ of 20. For the temperature gradient of the upper critical magnetic field at the transition temperature we find values up to 35 kG/K.     

Phase coexistence in NaZn13-type LaFe11.4Al1.6C0.02 compound

In NaZn₁₃-type LaFe_11.4Al_1.6C_0.02 compound, a signature of weak ferromagnetism is observed at ∼100 K under a low field by ac magnetic-susceptibility and electrical-resistivity measurements, implying the coexistence of ferromagnetic (FM) and antiferromagnetic (AFM) phases. The hysteresis in isofield magnetization curves and large magnetic relaxation demonstrate the metastability of the magnetic state in the AFM-FM transition region. The variations of magnetization with temperature, time and field show distinct step-like behaviors, which is probably attributed to the discontinuous growth of ferromagnetic cluster in antiferromagnetic matrix.     

Ferromagnetic resonance of magnetic field oriented Fe3O4 nanoparticles in frozen ferrofluids

Measurements of the orientational dependence of the ferromagnetic resonance (FMR) spectra are made on Fe₃O₄ nanoparticles in ferrofluids solidified in dc magnetic fields. The in field solidification locks the direction of magnetization parallel to the direction of the cooling field enabling measurements as a function of orientation with respect to the direction of magnetization in the frozen state. The g value of the FMR spectra at 77 K is 2.16 and the anisotropy constant is −1.23 J/m³. A marked reduction of the difference between the field position in the parallel and perpendicular orientation onsets on warming to 140 K well below the melting temperature of the fluid carrier and is attributed to the onset of fluctuations in the direction of the magnetization in the solid phase. The phase transition of the magnetic symmetry observed in bulk Fe₃O₄ occurs at much lower temperature in the nanoparticles.