Controlled growth and field emission of vertically aligned AlN nanostructures with different morphologies

The controllable growth of three different morphologies of AlN nanostructures (nanorod, nanotip and nanocrater) arrays are successfully realized by using chemical vapour deposition (CVD) technology. All three nanostructures are of single crystal h-AlN with a growth orientation of [001]. Their growth is attributed to the vapour-liquid-solid (VLS) mechanism. To investigate the factors affecting field emission (FE) properties of AlN nanostructures, we compare their FE behaviours in several aspects. Experimental results show that AlN nanocrater arrays possess the best FE properties, such as a threshold field of 7.2~V/μm and an emission current fluctuation lower than 4%. Moreover, the three AlN nanostructures all have good field emission properties compared with a number of other excellent cathode nanomaterials, which suggests that they are future promising FE nanomaterials.     

不同激光分布下Nd∶GdVO4晶体热焦距分析

为了研究方形激光晶体Nd∶GdVO4在超高斯泵浦光分布下的热效应,采用半解析的分析方法,进行了理论分析和实验验证.结果表明：当使用输出功率为15 W的半导体激光器端面中心入射时,在抽运端面中心获得1.38 μm最大热形变量（超高斯阶次为2时）和220 mm的热焦距（超高斯阶次为5时）.     

Ce3Rh3Si2和CeRh3Si2的磁性模拟和晶场效应研究

根据晶场理论,通过对D.Kaczorowski等人测量的磁化率倒数与温度关系曲线的模拟,得到了稀土化合物CeRh3Si2和Ce3Rh3Si2的晶场分裂能和相应波函数.对于CeRh3Si2和Ce3Rh3Si2,它们的第一激发能和总的激发能分别为176K,855K和77.5K,428K.计算表明,Kramers离子Ce3+在晶场效应的作用下,基态简并部分消除得到了三个双重态,模拟得到的化合物的磁化率倒数与温度关系曲线与实验曲线吻合较好.     

光磁共振实验中扫场和水平场关系的探究

光泵磁共振实验中,在磁共振信号观察时,常常会有光抽运信号同时出现的现象,这严重影响了实验的有效观察和准确测量。本文阐述了取合适的扫场和水平场的大小的关系,从而避免实验过程中磁共振信号和光抽运信号的同时出现,以确保实验的精准。     

High heat generation ability in AC magnetic field for nano-sized magnetic Y3Fe5O12 powder prepared by bead milling

Nano-sized magnetic Y₃Fe₅O₁₂ ferrite having a high heat generation ability in an AC magnetic field was prepared by bead milling. A commercial powder sample (non-milled sample) of ca. 2.9 μm in particle size did not show any temperature enhancement in the AC magnetic field. The heat generation ability in the AC magnetic field improved with a decrease in the average crystallite size for the bead-milled Y₃Fe₅O₁₂ ferrites. The highest heat ability in the AC magnetic field was for the fine Y₃Fe₅O₁₂ powder with a 15-nm crystallite size (the samples were milled for 4 h using 0.1 mm? beads). The heat generation ability of the excessively milled Y₃Fe₅O₁₂ samples decreased. The main reason for the high heat generation property of the milled samples was ascribed to an increase in the Néel relaxation of the superparamagnetic material. The heat generation ability was not influenced by the concentration of the ferrite powder. For the samples milled for 4 h using 0.1 mm? beads, the heat generation ability (W g⁻¹) was estimated using a 3.58×10⁻⁴ fH² frequency (f/kHz) and the magnetic field (H/kA m⁻¹), which is the highest reported value of superparamagnetic materials.     

Comparative study of heterogeneous magnetic state above TC in La0.82Sr0.18CoO3 cobaltite and La0.83Sr0.17MnO3 manganite

The magnetic, transport and structural properties are studied for La_0.83Sr_0.17MnO₃ and La_0.82Sr_0.18CoO₃ single crystals with nearly the same doping and the metallic ground state. Their comparisons have shown that ferromagnetic clusters originate in the paramagnetic matrix below Т^?>T_C in both samples and exhibit similar properties. This suggests the possible universality of such phenomena in doped mixed-valence oxides of transition metals with the perovskite-type structure. The cluster density increases on cooling and plays an important role on the physical properties of these systems. The differences in cluster evolutions and scenarios of their insulator–metal transitions are related to different magnetic behaviors of the matrixes in these crystals that is mainly due to distinct spin states of the Mn³⁺ and Co³⁺ ions.     

The frequency dependence of demagnetizing factor of a Fe-based amorphous ribbon

The frequency dependence of total and internal demagnetizing factors has been investigated. A minimum of the demagnetizing factor at optimal frequency of magnetization reversal is revealed. The influence of the demagnetizing factor on the frequency dependence of magnetic properties of amorphous ribbon is studied.     

Magnetic properties of the bond and crystal field dilution spin-3/2 Blume–Capel model in an external magnetic field

Magnetic properties of the bond and crystal field dilution spin-3/2 Blume–Capel model in an external magnetic field (h)$(h)$ on simple cubic lattice are studied by using the effective field theory. In the m−T$m-T$ plane, the degeneracy of the magnetization (m)$(m)$ is affected by the concentration of bond or crystal field dilution at low temperature (T)$(T)$. The magnetization curves can appear to fluctuate in certain regions of negative crystal field. In the m−h$m-h$ plane, the initial magnetization curve has an irregular behavior due to the introduction of bond dilution. The crystal field dilution has the influence on the process of magnetic domain displacement. In the χ−h$\chi -h$ plane, there exists one susceptibility (χ)$(\chi )$ shoulder and one step for different negative crystal field. The susceptibility curve takes on the feature of multi-peaks distribution under bond and crystal field dilution conditions.     

Magnetic field induced order–order phase transition in magnetocaloric systems Mn2−xFexAs0.5P0.5 and MnFeAsyP1−y

An analysis is presented of experimental and theoretical results of the MnFeAs_yP_1−y (0.15≤y≤0.66) and Mn_2−xFe_xAs_0.5P_0.5 (0.5≤x≤1.0) systems to identify main traits that underlie the mechanism of formation of different antiferromagnetic (AF) phases in the two systems. The discrepancy between the calculated from first principles and experimental values of the magnetic moment in the ferromagnetic phase with cation substitution in the system Mn_2−xFe_xAs_0.5P_0.5 is due to the appearance of a canted magnetic structure. In this case, the emergence of an AF phase with decreasing iron concentration precedes a significant change in the electronic d-band filling. In the model of the spiral structure in the system of itinerant electrons it is shown that the stabilization of the AF phase with decreasing arsenic concentration, while maintaining the number of d-electrons, is a consequence of changes in the shape of the density of electronic states that occur with a decrease in unit-cell volume.     

Dynamic compensation temperature in the mixed spin-1 and spin-2 Ising model in an oscillating field on alternate layers of a hexagonal lattice

The dynamic behavior of a mixed spin-1 and spin-2 Ising system with a crystal-field interaction in the presence of a time-dependent oscillating external magnetic field on a hexagonal lattice is studied by using the Glauber-type stochastic dynamics. The lattice is formed by alternate layers of spins σ=1$\sigma =1$ and S=2. The Hamiltonian model includes intersublattice, intrasublattice and crystal-field interactions. The set of mean-field dynamic equations is obtained by employing the Glauber transition rates. Firstly, we study time variations of the average sublattice magnetizations in order to find the phases in the system, and the thermal behavior of the average sublattice magnetizations in a period or the dynamic sublattice magnetizations to obtain the dynamic phase transition points as well as to characterize the nature (continuous and discontinuous) of transitions. Then, the behavior of the dynamic total magnetization as a function of the temperature is investigated to find the dynamic compensation points as well as determine the type of behavior. We also present the dynamic phase diagrams for both presence and absence of the dynamic compensation temperatures in the nine different planes. According to the values of Hamiltonian parameters, besides the paramagnetic (p), antiferromagnetic (af), ferrimagnetic (i) and non-magnetic (nm) fundamental phases, eight different mixed phases and the compensation temperature or L- and N-types behavior in the Néel classification nomenclature exist in the system.