Magnetization Studies of Mercury-Cuprates and Its Precursors

The mercury-cuprates such as (Hg, Cr)/Sr-1201 and (Hg, Re)/Sr-1201 were synthesized byencapsulation-technique at 880–920°C for 12–16 h, and characterized by power XRD,M(T) andM(H). All the precursors (Sr₂CuO₃, Cr_0.30Sr₂CuO _y or Re_0.10Sr₂CuO_y) were found to be non-superconducting. The samples with optimal composition (Hg_0.70Cr_0.30)Sr₂CuO_4+δ [(Hg, Cr)/Sr-1201] and (Hg_0.90Re_0.10)Sr₂CuO_4+δ [(Hg, Re)/Sr-1201] are superconducting withT _c ^onset ∼ 58 and ∼ 54 K, respectively. At low temperatures (below 10 K), on cooling in D.C.-magnetic field of 4 kOe, the sample (Hg, Re)/Sr-1201 shows a dramatic decrease in diamagnetic signal. Further, this effect is also faintly reflected in theM(H) curve. Such a behaviour was not observed in the (Hg, Cr)/Sr-1201 cuprates. We ascribe this effect to a field enhanced paramagnetic contribution possibly arising from mixed valent (6+, 7+ dominant) Re ions in the (Hg, Re)/Sr-1201 cuprates.     

Magnetization Studies of Field-Induced Transitions by Using a Single-Turn Coil Technique

Some technical improvements for magnetization measurements using a vertical-type single-turn coil (V-STC) method have been attempted. We have chosen LaCoO₃, RbCoBr₃ and SrCu₂(BO₃)₂ that show interesting field induced magnetic transitions, as test materials. Intriguing features were clearly observed in magnetic fields of up to 100 T. The quality of the data is of comparable to those obtained by a conventional non-destructive pulse magnet.     

Exchange Bias and Its Phase Reversal in Zero Magnetization Admixed Rare-Earth Intermetallics

The exchange bias effect—an important attribute in many device applications— is traditionally discussed as a form of exchange anisotropy at the interface between the ferromagnetic/antiferromagnetic layers of two distinct systems. We report here on magnetically ordered alloys possessing the feature of unidirectional anisotropy that reverses its sign at a characteristic temperature. These alloys are admixed rare-earth (RE) intermetallics, comprising two dissimilar rare-earth ions, belonging to the two different halves of the $4f$ series. All $4f$ rare-earth ions exhibit ferromagnetic coupling between the spins of their ions and the conduction electrons. Magnetic moments of dissimilar RE ions in such admixed alloys, however, get antiferromagnetically coupled, and they display a magnetic compensation feature in which the field-induced reversal in the orientation of the magnetic moments of dissimilar rare-earth ions occurs at the compensation temperature $(T_{rm comp})$ . Above a threshold field, the conduction electron polarization also reverses its sign at $T_{rm comp}$. We suggest that this effect correlates with the observed phase reversal in the exchange bias field.      

电工钢片交流磁化曲线的计算机拟合及应用

本文介绍一种电工钢片交流磁化曲线的计算机拟合方法及其在磁测量计算过程中的运用     

MnFe2O4多晶微粉的共沉淀法制备及磁性研究

用化学共沉淀法制备了MnFe2O4铁酸盐多晶微粉,发现与NiFe2O4相比,MnFe2O4的制备有其独特之处:反应温度在80℃以上时,经沉淀洗涤后直接得到MnFe2O4纳米微粉,不需热处理过程;反应温度在80℃以下时,需要对沉淀洗涤后前驱体进行热处理才能得到MnFe2O4纳米微粉.利用振动样品磁强计测量了样品在室温下的磁性能;利用X射线衍射确定了粉体的组成、晶粒大小和晶格常数;利用扫描电子显微镜观察了颗粒的形貌.     

砝码磁化强度的测量结果不确定度分析

砝码是一种能复现给定质量值的实物量具,具有一定的物理特性和计量特性,本文详细介绍了砝码物理特性磁化强度的测量及测量结果不确定度分析。     

铁素体纤维的损伤与磁性

在铁素体纤维提取工艺理论分析的基础上,研究了提取过程中纤维的损伤机制,提出了提高纤维磁性的工艺措施。结果表明,纤维的损伤机制为氧化反应和置换反应,纤维中的氧、铜和吸附水导致其磁性下降。当mCr/m(Fe+Cr)在13%附近时,通过采取适当的工艺措施,纤维的比饱和磁化强度MS出现极大值(159 emu/g)。     

Stress Induced Enhancement of Magnetization Reversal Process of DyFeCo Films With Perpendicular Magnetization

A method to control magnetization reversal in magnetic films with perpendicular magnetic anisotropy was demonstrated in this study. Alloys of rare earth-transition metals (RE-TM) with their extremely large anomalous Hall coefficient and relatively large magnetostriction constants were suitable to observe the stress induced anisotropy using anomalous Hall effect. Stress applied to the RE-TM thin films significantly reduced the perpendicular coercivity and nucleation field of the film in comparison with that of the film in stress-free condition. Dy_x(Fe₉₀Co₁₀)_1-x thin films revealed large Hall voltage and remarkable change in the coercivity under the mechanical tensile stress     

Magnetization Time Lag Caused by Eddy Currents and Its Influence on High-Speed Magnetic Flux Leakage Testing

In high-speed magnetic flux leakage (MFL) testing, the tested workpieces pass rapidly through magnetizers. Thus, the magnetization time for workpieces is short. Because of the eddy current effect, the magnetic field inside the workpieces cannot instantly reach equilibrium, and if the magnetizing time is insufficient for the field to reach equilibrium, the MFL signals will be changed because of incomplete magnetization. In this article, the magnetization time lag caused by eddy currents and the influence of this lag on high-speed MFL testing is investigated. The time required for magnetic field to reach equilibrium in specimens, including steel bars and pipes, is obtained by theoretical calculations, finite element simulations, and experiments. The results indicate that the time required for a magnetic field inside a specimen to reach equilibrium is in the range of 50–100 ms. Using conventional magnetizers, the defect signals at testing speed of 10 m/s change because the workpiece reaches the detection zone before the magnetic field inside reaches the stable state. A simple solution is to increase the axial length of the magnetizing coil. After this procedure, signals obtained at 0.1 m/s and 10 m/s are almost identical.     

Photoinduced Magnetization Rotation and Precessional Motion of Magnetization in Ferromagnetic (Ga,Mn)As

Dynamics of photoinduced magnetization have been studied in ferromagnetic semiconductor (Ga,Mn)As thin films by time-resolved polar Kerr rotation. Ultrafast (150 fs) magnetization rotation has been realized using both chirality of excitation light and p–d exchange interaction. We also report on the occurrence of photoinduced magnetization precession in longer time domain. This is initiated by a change in hole concentration and resultant change in magnetic anisotropy. The overall behaviors of these photoinduced magnetization dynamics are discussed.     

Magnetization of dilute Al-Mn and Al-Fe alloys

The magnetization of very dilute Al-Mn and Al-Fe alloys was measured from 2 to 0.01 K in order to obtain a limit on how nonmagnetic such systems are. Results indicate that the impurity contribution to the magnetization is almost temperature independent and that the moment per impurity is less than 1.7 × 10^–2. The Al nuclear paramagnetism is mainly responsible for the uncertainty in this value.Supported by NSF grant DMR 76 21702.     

Magnetization reversal in ferromagnets

Quantum and classical reversal of magnetization in ferromagnets are considered. Expressions for the tunnelling and thermal activation rates of the magnetization reversal with the effect of magnetoelastic dissipation to the lowest order are presented with the prefactors included. The results are given for low temperatures, near the cross-over temperature and for high temperatures, so that an experimental check would be possible to perform. The results are within the transition state theory.     

Magnetization and Susceptibility Studies on BaZrO3-Doped YBa2Cu3O7 − x Bulk Superconductors

We studied the YBa₂Cu₃O_{7 ? x} bulk superconductor doped with BaZrO₃ up to 50 wt.%, obtained by solid-state reaction powder technology. From DC magnetization loops and low frequency AC susceptibility measurements we determined the influence of the BaZrO₃ doping level on the critical temperature, critical current density, field for full penetration, and intergrain lower critical field. The results show that even high content of BaZrO₃ does not lead to degradation of the superconducting properties of bulk YBa₂Cu₃O_{7 ? x} .     

Thermodynamic Magnetization of Normal and Superfluid 3He

We have measured the thermodynamic magnetization in the various phases of normal and superfluid ³ He. The A-B discontinuity differs from that inferred from nuclear-specific (NMR) experiments. We offer reasons why this apparent disagreement can be consistent with our current picture of super-fluid ³ He. One consequence of this measurement is an improved set of values for the phenomenological Landau-Ginzburg free energy -parameters.     

Relaxation and Magnetization Dynamics of Frustrated Spin-Chain Systems

A 3D model of the magnetization dynamics in frustrated triangular spin-chain systems Ca₃Co₂O₆ is proposed. This model is a generalization of the 2D model developed earlier. The spins are assumed to interact with the nearest neighbors and external agency (heat reservoir and external magnetic field) that causes them to change their states randomly with time. A probability of a single spin-flip process is assumed in a Glauber-like form. The 3D model allows describing the step-like behavior of a magnetization curves experimentally observed in a strong magnetic field.     

Magnetization in Ni and Ni-Fe thin films

Nickel films have been deposited in conventional high vacuum, ultra-high vacuum, and in controlled contaminant, ultra-high vacuum environments. High field saturation magnetization Msmeasurements on the films made in high vacuum and an oxygen-contaminated ultra-high vacuum system show that they posses low values of Msfor film thicknesses of 1000 Å. The normal moments observed for ultra-high vacuum deposited films lead to the conclusion that the low Msvalues are associated with the oxygen and are caused by a true loss of effective magnetization rather than coupled superparamagnetism or a ripple blocking field. Supplementary torque and ferromagnetic resonance studies on the Ni and two different Ni-Fe compositions also show that the grain boundary perpendicular anisotropy models are not particularly relevant for films made under such nominal technical vacuum conditions.     

微细条形NiFe薄膜元件在磁化和反磁化过程中磁畴结构的变迁过程

详细观察了长条形ＮｉＦｅ薄膜元件（宽１２０μｍ,厚４０ｎｍ）沿难轴方向（长方向）磁化和反磁化过程中磁畴结构变迁的全过程。观察表明,在磁化过程中,磁化转动和不可逆畴壁位移同时存在;在反磁化过程中,畴壁合并、封闭畴转变和畴壁极性转变是最主要的不可逆因素,也是造成元件输出讯号中Ｂａｒｋｈａｕｓｅｎ噪声的主要物理根源。     

Magnetization and pinning effect of a Josephson junction

The magnetization and the pinning effect of a Josephson junction have been analyzed by using a mechanical analog. The influences of an applied current on the magnetization and the critical current have been studied. The peak and the training effects have been observed when inhomogeneities are introduced into the junction. The behavior of the junction is qualitatively analogous to that of a type II superconductor. The mechanical model is a useful analog computer for studying precisely and visually the vortex behavior of the junction and is also helpful in understanding qualitatively that of a type II superconductor.     

Magnetization and remagnetization of anisotrophic magnetically hard ferrites

Magnetization curves and hysteresis loops of anisotropic hard ferrites were analyzed after the specimens had been demagnetized either thermally or by an ac field parallel or perpendicular to the preferred axis (PA) between -196°C and Curie temperature, or by a dc field. Each demagnetization yields a specific magnetization curve; below saturation fields, asymmetrically positioned and noncentrosymmetrical loops develop, except after ac demagnetization at 20°C (=measuring temperature) parallel to PA. This is due to domain configurations differently arranged and of different stability. The asymmetry is caused by "frozen" unstable configurations, which undergo an irreversible change due to the action of the field. Therefore, with reversed field not all of the previous possibilities for remagnetization are still available.