Mössbauer effect investigations of Co83Fe17/Au/Co/Au multilayers

[Co₈₃Fe₁₇/Au/Co/Au]_N sputter deposited multilayers displaying a giant magnetoresistance have been investigated. Complementary magnetic measurements were conducted in order to characterize a spin reorientation transition in Co₈₃Fe₁₇ layers sandwiched between Au spacers. The transition from a perpendicular magnetic anisotropy to easy-plane one takes place at the thickness of about 1 nm.     

Mechanochemical Synthesis and Nitrogenation of the Nd1.1Fe10CoTi Alloy for Permanent Magnet

In this work, the mechanochemical synthesis method was used for the first time to produce powders of the nanocrystalline Nd_1.1Fe₁₀CoTi compound from Nd₂O₃, Fe₂O₃, Co and TiO₂. High-energy-milled powders were heat treated at 1000 °C for 10 min to obtain the ThMn₁₂-type structure. Volume fraction of the 1:12 phase was found to be as high as 95.7% with 4.3% of a bcc phase also present. The nitrogenation process of the sample was carried out at 350 °C during 3, 6, 9 and 12 h using a static pressure of 80 kPa of N₂. The magnetic properties M_r, µ₀H_c, and (BH)_max were enhanced after nitrogenation, despite finding some residual nitrogen-free 1:12 phase. The magnetic values of a nitrogenated sample after 3 h were M_r = 75 Am² kg^–1, µ₀H_c = 0.500 T and (BH)_max = 58 kJ·m^–3. Samples were aligned under an applied field of 2 T after washing and were measured in a direction parallel to the applied field. The best value of (BH)_max ~ 114 kJ·m^–3 was obtained for 3 h and the highest µ₀H_c = 0.518 T for 6 h nitrogenation. SEM characterization revealed that the particles have a mean particle size around 360 nm and a rounded shape.     

Incorporation of Molecular Reorientation into Modeling Surface Pressure-Area Isotherms of Langmuir Monolayers

Langmuir monolayers can be assembled from molecules that change from a low-energy orientation occupying a large cross-sectional area to a high-energy orientation of small cross-sectional area as the lateral pressure grows. Examples include cyclosporin A, amphotericin B, nystatin, certain alpha-helical peptides, cholesterol oxydation products, dumbbell-shaped amphiphiles, organic–inorganic nanoparticles and hybrid molecular films. The transition between the two orientations leads to a shoulder in the surface pressure-area isotherm. We propose a theoretical model that describes the shoulder and can be used to extract the energy cost per molecule for the reorientation. Our two-state model is based on a lattice–sublattice approximation that hosts the two orientations and a corresponding free energy expression which we minimize with respect to the orientational distribution. Inter-molecular interactions other than steric repulsion are ignored. We provide an analysis of the model, including an analytic solution for one specific lateral pressure near a point of inflection in the surface pressure-area isotherm, and an approximate solution for the entire range of the lateral pressures. We also use our model to estimate energy costs associated with orientational transitions from previously reported experimental surface pressure-area isotherms.     

1H NMR study of molecular dynamics and phase transition in (NH4)2ZnBr4

The proton second moment (M ₂) and spin-lattice relaxation time (T ₁) have been measured in (NH₄)₂ZnBr₄ in the range 77–300 K. The room-temperature spectrum shows a structure which disappears around 243 K. The signal is strong and narrow even at 77 K. Proton T ₁ shows a maximum at 263 K, caused by spin rotation interaction and decreases with decreasing temperature till 235 K, where it shows a sudden increase. Below 235 K, again it decreases and shows a slope change around 216.5 K (reported T_c ). From 216.5 K, T ₁ decreases continuously without exhibiting any minimum down to 77 K. The narrow line at 77 K, and absence of a T ₁ minimum down to 77 K indicate the possibility of quantum mechanical tunnelling in this system. Motional parameters such as activation energy and pre-exponential factor have been evaluated for the reorientational motion of the NH⁺ ₄ ion.     

Structure and mobility in ferroelectric liquid crystalline elastomers

Time-resolved Fourier transform infrared (FTIR) spectroscopy with polarized light was employed to study the structure and mobility of a homologous series of ferroelectric liquid crystalline polymers (FLCPs) and ferroelectric liquid crystalline elastomers (FLCEs) in response to an external electric field. The chemical composition of the samples, besides the cross-linking units, is similar. For the elastomers, two different cross-linking architectures are realized: “intralayer” cross-linking leads to the formation of two-dimensional networks, whereas “interlayer” cross-linking forms three-dimensional networks. Due to its specificity, FTIR spectroscopy enables analysis of the reorientational dynamics for the different molecular moieties in detail, thus revealing information about reorientation times, angular excursion, and the phase relationship in the rearrangement of the various molecular groups. In comparison to the un-cross-linked FLCP, both elastomeric samples exhibited smaller reorientation angles and an increase of the reorientation times. In the case of the interlayer cross-linked FLCE, an elastic memory effect was observed: For the reversal from negative to positive field polarity, the reorientation times were longer than for those in the opposite direction. For the intralayer cross-linked sample, it was shown that the backbone molecules reorient slower than the other molecular units (“locomotive effect”). For the un-cross-linked FLCP and the two FLCE samples, different coupling mechanisms between the network and the mesogenic parts are derived from the measurements.     

Magnetic, magetostrictive properties, spin reorientation and M?ssbauer effect of Tb0.3Dy0.7?xPrx(Fe0.9Al0.1)1.95 alloys

The effect of Pr substitution for Dy on the magnetic and magnetostrictive properties, anisotropy, spin reorientation and M?ssbauer effect of a series of Tb_0.3Dy_0.7−x Pr _x (Fe_0.9Al_0.1)_1.95 (x=0, 0.1, 0.20, 0.25, 0.30, 0.35) alloys at room temperature have been investigated. It was found that a small amount of Pr substitution is beneficial to a decrease in the magnetocrystalline anisotropy for the Tb_0.3Dy_0.7−x Pr _x (Fe_0.9Al_0.1)_1.95 alloys. The magnetostriction decreases drastically with increasing x and the magnetostrictive effect disappears for x>0.2. However, the magnetostriction exhibits a slightly bigger value at x=0.1 than the free alloys and is saturated more easily with the magnetic field H. The saturation magnetization and Curie temperature decrease monotonously, but the spontaneous magnetostriction increases linearly with increasing x, whereas the spin reorientation temperature increases first, then decreases rapidly and reaches the maximum at x=0.1. The analysis of M?ssbauer spectra indicated that the easy magnetization direction in the {110} plane deviates slightly from the main axis of symmetry with the Pr concentration x, namely spin reorientation. Compared with Al substitution for Fe, the effect of Pr substitution for Dy on spin reorientation is relatively small. The hyperfine field increases with Pr concentration increasing, and the isomer shifts and the quadrupole splitting (QS) show weak concentration dependence. Supported by the National Natural Science Foundation of China (Grant No. 10574059), the Natural Science Foundation of Gansu Province (Grant No. 0710RJZA074), the Second Scientific Research Project of Bureau of Gansu Education and ‘Qing Lan’ Talent Engineering Funds of Lanzhou Jiaotong University     

Observation of spin reorientation in layered manganites La1.2Sr1.8(Mn1−yRuy)2O7 (0.0⩽y⩽0.2) by Lorentz transmission electron microscopy

The effect of Ru substitution for Mn in bilayered oxides La_1.2Sr_1.8(Mn_1−yRu_y)₂O₇ (0?y?0.2$0?y?0.2$) was investigated by magnetization measurements and low-temperature Lorentz transmission electron microscopy. It was found that the magnetic anisotropy is controlled by the Ru content y and temperature T. The easy axis changes from 〈1 1 0〉 for the y=0$y=0$ crystal to the c  -axis for y=0.2$y=0.2$, and it rotates away from the c-  axis for the y=0.05$y=0.05$ and y=0.07$y=0.07$ crystals with decreasing temperature. Furthermore, maze-shaped magnetic domain structures were observed in the (0 0 1) thin crystals with 0.05?y?0.2$0.05?y?0.2$. Changes in domain size and structure indicate that the uniaxial magnetic anisotropy becomes stronger as Ru content y increases.     

室温单分子偶极取向与量子化再取向动力学实验研究

对室温下染料单分子进行了偶极取向和偶极再取向动力学的实验研究.利用共焦扫描显微镜光学系统与荧光偏振探测分析相结合的方法分别测量了聚合体薄膜中的单分子和无聚合体薄膜的单分子偶极方向变化特性,经采样统计测量镶嵌于聚合体薄膜中的单分子发生偶极再取向的概率约为5%—9%,无聚合体薄膜的单分子发生偶极再取向的概率约为26%.通过测量单分子荧光的偏振度轨迹曲线发现,偶极再取向存在着在多个偏振态之间的量子化跳跃行为. 关键词： 单分子 偶极取向 再取向 量子化跳跃     

Temperature-driven spin reorientation transitionof magnetron sputtered nickel thin film

The temperature-driven spin reorientation transition of magnetron sputtered Ni/Si （111） systems has been studied. The relationship between ac initial susceptibility and temperature of nickel films with different thicknesses shows that the magnetization orientation changes from in-plane to out-of-plane with the increase of temperature. The temperature dependence of mugnetoelastic, magneto-crystalline, and magnetostatic anisotropies determines the direction of the reorientation transition. The temperature-driven spin reorientation transition is supported by Hall coefficient measurements which show that its temperature dependence is similar to that of susceptibility.     

A micromechanical continuum model for the tensile behavior of shape memory metal nanowires

We have previously discovered a novel shape memory effect and pseudoelastic behavior in single-crystalline face-centered-cubic metal (Cu, Ni, and Au) nanowires. Under tensile loading and unloading, these wires can undergo recoverable elongations of up to 50%, well beyond the recoverable strains of 5-8% typical for most bulk shape memory alloys. This phenomenon only exists at the nanoscale and is associated with a reversible lattice reorientation driven by the high surface-stress-induced internal stresses. We present here a micromechanical continuum model for the unique tensile behavior of these nanowires. Based on the first law of thermodynamics, this model decomposes the lattice reorientation process into two parts: a reversible, smooth transition between a series of phase-equilibrium states and a superimposed irreversible, dissipative twin boundary propagation process. The reversible part is modeled within the framework of strain energy functions with multiple local minima. The irreversible, dissipative nature of the twin boundary propagation is due to the ruggedness of strain energy curves associated with dislocation nucleation, glide, and annihilation. The model captures the major characteristics of the unique behavior due to lattice reorientation and accounts for the size and temperature effects, yielding results that are in excellent agreement with the results of molecular dynamics simulations.