Magnetization processes of iron whiskers in small magnetic fields

The magnetization behaviour of iron whiskers in de magnetic fields has been investigated using the enhanced longitudinal magneto-optic Kerr effect [6]. The experimental results were compared with the phenomenological membrane model [1] and also with magneto static calculations.     

Equations of state along the road to Arrott's last plot

A modification of the Weiss theory of ferromagnetism is used to create an approximate equation of state that relates the magnetization, field and temperature. The high-temperature susceptibility, the critical region and the temperature dependence of the spontaneous magnetization to low temperature are found by making an approximation for the inverse Brillouin function and then adding critical exponents in the proper places. The coefficient of the Weiss effective field is treated as dependent on the magnetization, introducing an additional parameter. This equation of state is applied to results from studies of iron whiskers.     

Increase in the magnetization loop width in the Ba0.6K0.4BiO3 superconductor: Possible manifestation of phase separation

The magnetization of Ba_0.6K_0.4BiO₃ samples in fields up to 90 kOe in the temperature range from 2 to 30 K is investigated. It is shown that the observed increase in the width of the magnetization loop can be explained by a decrease in the phase nonuniformity upon an increase in the magnetic field. The asymmetric hysteretic dependence of magnetization with the secondary peak was successfully described by the extended critical state model taking into account the phase separation in the superconductor.     

Modeling magnetization transfer using a three-pool model and physically meaningful constraints on the fitting parameters.

A model for water-macromolecular magnetization transfer is presented which addresses the mechanism of coupling between the hydrogen populations and the extraction of physically meaningful parameters from experimental magnetization transfer data. Both physical exchange between bulk-solvent and site-specific hydration-layer hydrogens and intermolecular magnetic dipolar coupling between these specific hydration-layer-solvent and macromolecular hydrogens are explicitly included, leading to a three-pool model for magnetization transfer. It is shown that the three-pool model is well approximated by a two-pool model for coupling between the bulk-solvent and macromolecular hydrogens when the dipolar-coupled solvent hydrogens are a small fraction of the total solvent, and the solvent-macromolecular coupling constant includes both dipolar magnetic, kappa(dip), and physical exchange, kappa(ex), coupling rates. The model is also extended to multiple solvent systems. The model results in a set of coupled equations that predict magnetization transfer spectra as a function of temperature and composition. Physically meaningful constraints on the coupling and relaxation parameters are established for systems in which magnetization transfer has been observed including solvated cross-linked proteins and lipid bilayers. Using parameter estimates based on these constraints, empirical magnetization transfer spectra are well predicted by the model. It is found that the degree of magnetization transfer becomes independent of kappa(dip) and kappa(ex) when these parameters become greater than about 50 s(-1). In the semi-rigid cross-linked protein systems where the mobility of the macromolecular matrix is insensitive to temperature, the magnitude of the observed magnetization transfer is consistent with being limited by the intermolecular dipolar coupling and spin-lattice relaxation in the bulk-solvent phase.     

Negative magnetoresistance of iron single-crystal whiskers in the course of magnetization reversal

The change in the low-temperature resistance of iron single-crystal whiskers during magnetization reversal form a single-domain state to a state with a plane-parallel domain structure is studied theoretically. The negative magnetoresistance (～45%) is calculated from the Kubo formula with due regard for the change in the trajectories of conduction electrons in a magnetic induction field of domains. The magnetoresistance thus calculated is of the same order of magnitude as the magnetoresistance obtained in the experiment performed by Isin and Coleman.     

The magnetization behavior of nanocrystalline permanent magnets based on the Stoner–Wohlfarth Model

The magnetization behavior in nanocrystalline permanent magnets has been investigated using mean field Stoner–Wohlfarth model. The model is comparatively simple but allows the numerical treatment of extended nanostructures. The predominant intergrain exchange coupling is expressed by a mean field constant N_m in spite of no explicit relationship between N_m and the microstructure. By this model, the experimental phenomena, such as the remanence enhancement, the shape of demagnetization curve, the irreversible magnetization, the exchange bias field and their respective temperature dependence, can be well explained. The N_m dependence of magnetic properties has been extensively discussed in the present paper.     

Preparation and Dielectric Properties of Nanostructured ZnO Whiskers

By a novel controlled combustion synthesis method, a large number of nanostructured ZnO whiskers with different morphologies, such as tetra-needles, long-leg tetra-needles and multi-needles, are prepared without any additive in open air at high temperature. The morphologies and crystalline structures of the as-prepared ZnO nanostructured whiskers are investigated by SEM and XRD. The possible growth mechanism on the nanostructured ZnO whiskers is proposed. The experimental results indicate that the dielectric constants and losses of the nanostructured ZnO whiskers are very low, demonstrating that the nanostructured ZnO whiskers are low-loss materials for microwave absorption in X-band. However, obvious microwave absorption in nanostructured ZnO whiskers is observed. The quasi-microantenna model may be attributed to the microwave absorption of the ZnO whiskers.     

Magnetization plateaus and supersolid phases in an extended Shastry–Sutherland model

New magnetization plateaus and supersolid phases are identified in an extended Shastry–Sutherland model – with additional longer range interactions and exchange anisotropy – over a wide range of interaction parameters and an applied magnetic field. The model is appropriate for describing the low energy properties of some members of the rare earth tetraborides. Using a plaquette representation and exact mapping of ground state configurations in the Ising limit, supplemented by large scale quantum Monte Carlo simulations to include the effects of exchange interactions, we have identified several magnetization plateaus and associated spin supersolid phases. Our methods enable us to elucidate the underlying structure and mechanism of formation of the supersolid phases, thus gaining deeper understanding into their emergence from competing interactions. Additionally, in this regime we find evidence of a fractional plateau at 1∕9 saturation magnetization, which may hold the clue to understanding the fractional plateau in TmB₄ that exhibits magnetic hysteresis.     

Proton residual dipolar couplings by NMR magnetization exchange in cross-linked elastomers: determination and imaging.

Proton nuclear magnetic resonance (NMR) magnetization exchange is used to investigate residual dipolar couplings in a series of cross-linked poly(styrene-cobutadiene) elastomers. A new model for the dipolar unit is used for the evaluation of the signal decay in magnetization exchange experiments. It takes into account an extended residual dipolar coupling network along the polymer chain. It is shown that in the regime of short mixing times, information about the residual dipolar coupling between methine and methylene protons can be obtained which is not affected by other inter- and intramolecular dipolar couplings. The dynamic order parameter of methine-methylene protons is measured and correlated with cross-link density. This study certifies the quality of a filter for magnetization from residual dipolar couplings which exploit magnetization exchange. The filter can be used to generate contrast in NMR images of heterogeneous elastomers. The first proton NMR parameter image of a dynamic order parameter is presented for a phantom made from poly(styrene-cobutadiene) samples with different cross-link densities.     

原位生长高度定向ZnO晶须

采用大气压金属有机化合物化学气相沉积(AP-MOCVD)方法，以Zn(C₅H₇O₂)₂为原料，在玻璃基片上制备出高度定向的ZnO晶须.扫描电子显微镜观察发现晶须垂直基片取向生长，规则排列，长度、形状几乎一致.晶须直径为100nm—800nm,长径比为8—15，尖端曲率半径仅为50nm，甚至更小.x射线衍射(XRD)分析结果表明ZnO晶须为六方晶系纤锌矿结构，并沿c轴高度取向.采用热分析对反应前驱物进行了研究,同时也讨论 关键词： ZnO 取向生长 晶须 MOCVD     

The short-range van Hemmen model in a simple Kikuchi approximation: Comparison with the mean-field model and with spin-glasses

A simple first-order Kikuchi approximation is studied for the short-range (nearest-neighbour) version of the van Hemmen mean-field model originally proposed for spin-glasses. Although the approximation is very similar to the mean-field treatment, the phase diagrams from the two methods are drastically different. Previously reported results are reviewed and extended. The “reentrant” ferromagnetic to “spin-glass” transition found in zero magnetic field persists in small fields. The equilibrium magnetization displays a maximum as a function of temperature in the reentrance region. The characteristic S-shape of the magnetization versus field in the “spin-glass” region and magnetic hysteresis are observed. In addition, some exact results concerning the problem of the lower critical dimension of the short-range model are derived.     

Magnetization reversal processes in layered high-temperature superconductors with ferromagnetic impurities

The self-consistent interaction of a vortex system of a high-temperature superconductor and ferromagnetic impurities, including single impurities and their clusters, has been considered in the model of a layered high-temperature superconductor. For different temperatures and concentrations of ferromagnetic impurities, the magnetization reversal loops have been calculated by the Monte Carlo method taking into account an ensemble of ferromagnetic particles with different orientations of their easy magnetization axes with respect to the direction of an external magnetic field and for different magnetic anisotropy energies. It has been demonstrated that there is a nonlinear interaction of the high-temperature superconductor with ferromagnetic impurities, in which the initially thermodynamically reversible character of the magnetization reversal of the ferromagnetic ensemble can become irreversible. For a periodic lattice of clusters of ferromagnetic impurities, the magnetization curves of the high-temperature superconductor have been calculated for different sizes and configurations of the clusters. It has been revealed that, when extended defects are oriented parallel to the direction of the entrance of vortices in the sample, the length of the defects does not affect the remanent magnetization. It has been shown that the inclusion of the interaction between the magnetic moments inside the impurity cluster leads to a decrease in the magnetization reversal loop, the coercivity, and, accordingly, the energy loss due to magnetization reversal.     

Driving force and mechanism for spontaneous metal whisker formation

The room temperature spontaneous growth of low melting point metal whiskers, such as Sn, poses a serious reliability problem in the semiconducting industry; a problem that has become acute with the introduction of Pb-free technology. To date, this 50+ year old problem has resisted interpretation. Herein we show that the driving force is essentially a reaction between oxygen and the sprouting metal. The resulting volume expansion creates a compressive stress that pushes the whiskers up. The model proposed explains our observations on In and Sn whiskers and many past observations. The solution is in principle simple: diffusion of oxygen into the metal must be prevented or slowed down. This was demonstrated by coating the active surfaces with a polymer coating.     

交直流叠加磁化下恒导磁薄片的反常涡流损耗

本文利用一致畴转模型分析了在交直流叠加磁化下恒导磁薄片的低频涡流损耗。指出涡流损耗的反常不仅壁移,而且同样存在于畴转。其原因是畴转过程中畴内磁导率是张量,从而比经典模型增加了一项“侧向涡流”。导出了转动涡流损耗反常系数η_r的表达式;并将结果在某些应用领域中作了推广,得出了相应的结论。     

Subject index to volume 39

Measurements of the normal component of the stray-field at the surface of small permanent magnets were performed with a vibrating pick-up loop magnetometer with high geometrical resolution. They show a structure which reveals deviations from the desired magnetization configuration. A calculation method originally developed for soft magnetic materials was extended to calculations on permanent magnets. The real demagnetization characteristic of the material is introduced into the calculation using two independent susceptibility components. Thus the magnetic surface charges, the stray-field and the magnetization configuration can be calculated.     

Spin-seebeck effect: a phonon driven spin distribution

Here we report on measurements of the spin-Seebeck effect in GaMnAs over an extended temperature range alongside the thermal conductivity, specific heat, magnetization, and thermoelectric power. The amplitude of the spin-Seebeck effect in GaMnAs scales with the thermal conductivity of the GaAs substrate and the phonon-drag contribution to the thermoelectric power of the GaMnAs, demonstrating that phonons drive the spin redistribution. A phenomenological model involving phonon-magnon drag explains the spatial and temperature dependence of the measured spin distribution.     

二元金属须的强度和形变特点

测量了直径20—300微米的大量的Fe鬚、Fe-Cu鬚、Fe-Co鬚、Cu鬚和Cu-Co鬚的室温和高温拉断强度,结果指出,弥散型的二元金属鬚如Fe-Cu鬚和Cu-Co鬚的强度较高于单元金属鬚,而固溶型的二元金属鬚如Fe-Co鬚则否。根据应力应变曲线的测量和金相观测的结果,认为弥散型的Fe-Cu鬚的强度所以较高,可能是由于高强度的铁铜弥散混合层的出现,而它的范性所以较好,可能是由于这种鬚具有范性较好的铜心轴。讨论了弥散型和固溶型二元金属鬚在变粗当中所包含的基本过程,以及与它们所表现的强度和形变特点的联系。并且指出,生长弥散型混合鬚的原理和方法给制备强度较高和范性较好的复合材料提供了一个值得参考的途径。 关键词：     

Macroscopic and Microscopic Fields in High-Resolution Liquid NMR

The microscopic magnetic-induction field “seen by each nucleus” in a material medium and which is generated by a rapidly time-dependent spin magnetization gives rise to surprising new features in high-resolution nuclear magnetic resonance experiments. The purpose of the present paper is to show how the relations between the macroscopic average fields, the magnetization, and the microscopic fields (which were studied and clarified long ago at thermal equilibrium) can be extended to the present NMR context in which the magnetization can become rapidly time dependent and unrelated to thermal equilibrium properties.     

Modelling of self-diffusion and relaxation time NMR in multicompartment systems with cylindrical geometry

Multicompartment characteristics of relaxation and diffusion in a model for (plant) cells and tissues have been simulated as a means to test separating the signal into a set of these compartments. A numerical model of restricted diffusion and magnetization relaxation behavior in PFG-CPMG NMR experiments, based on Fick's second law of diffusion, has been extended for two-dimensional diffusion in systems with concentric cylindrical compartments separated by permeable walls. This model is applicable to a wide range of (cellular) systems and allows the exploration of temporal and spatial behavior of the magnetization with and without the influence of gradient pulses. Numerical simulations have been performed to show the correspondence between the obtained results and previously reported studies and to investigate the behavior of the apparent diffusion coefficients for the multicompartment systems with planar and cylindrical geometry. The results clearly demonstrate the importance of modelling two-dimensional diffusion in relation to the effect of restrictions, permeability of the membranes, and the bulk relaxation within the compartments. In addition, the consequences of analysis by multiexponential curve fitting are investigated.     

Magnetization and spin-spin energy diffusion in the XY model: a diagrammatic approach

It is shown that the diagrammatic cluster expansion technique for equilibrium averages of spin operators may be straightforwardly extended to the calculation of time-dependent correlation functions of spin operators. We use this technique to calculate exactly the first two non-vanishing moments of the spin-spin and energy-energy correlation functions of the XY model with arbitrary couplings, in the long-wavelength, infinite temperature limit appropriate for spin diffusion. These moments are then used to estimate the magnetization and spin-spin energy diffusion coefficients of the model using a phenomenological theory of Redfield. Qualitative agreement is obtained with recent experiments measuring diffusion of dipolar energy in calcium fluoride.