Magnetic Excitations and the Curie Temperature of Decamethylchromocenium Tetracyanoethylenide

We describe the molecular ferromagnet decamethylchromocenium tetracyanoethanide [C_rC_p2^*] [TCNE] by a quasi-one-dimensional alternating-spin model with ferromagnetic exchange interaction. The spin-wave theory is used to discuss the magnetic excitation of the molecular ferromagnet. We obtain the magnetizations corresponding to different external magnetic fields without interchain exchange interaction which is consistent with the experimental results. The formula of the Curie temperature T_c is obtained by using the thermodynamic Green's function method. It is found to be related to the spin of donors, to the intrachain exchange interaction and to the spatial anisotropic parameter. The result obtained by this T_c formula is consistent with the experimental result T_c = 3.6 K for [C_rC_p2^*][TCNE].     

晶粒尺寸对BaTiO_3居里温度的影响

晶粒尺寸对ＢａＴｉＯ＿３居里温度的影响孟进芳，黄亚彬，张伟风（河南大学物理系开封４７５００１）ＥｆｆｅｃｔｓｏｆＧｒａｉｎＳｉｚｅｏｎＣｕｒｉｅＴｅｍｐｅｒａｔｕｒｅｏｆＢａＴｉＯ＿３￥ＭｅｎｇＪｉｎｆａｎｇ；ＨｕａｎｇＹａｂｉｎ；ａｎｄＺｈａｎｇＷ...     

稀磁半导体材料居里温度的极值点

以Zener模型为基础,考虑反铁磁性交换作用对DMS材料居里温度的影响,理论计算得到了居里温度关于掺杂浓度和反铁磁性交换作用的二元函数,对GaAs ∶ TM(Ga,TM)As (TM=Sc,Ti,V,Cr,Mn,Fe,Co,Ni)的居里温度做了详细分析得到:n型半导体居里温度有一个极大值,而p型掺杂是单调的递增。     

随机粗糙表面激光散射后向增强效应实验研究

随机粗糙表面激光散射后向增强效应实验研究吴振森（西安电子科技大学物理系西安７１００７１）ＥｘｐｅｒｉｍｅｎｔＲｅｓｅａｒｃｈｏｎＥｎｈａｎｃｅｄＢａｃｋｓｃａｔｅｒｉｎｇｏｆＲａｎｄｏｍＲｏｕｇｈＳｕｒｆａｃｅｓＷｕＺｈｅｎｓｅｎ（Ｄｅｐａｒｔｍｅｎ...     

Domain Structure of Hexaferrite BaFe12O19 Near the Curie Temperature

Physics of the Solid State - The features of the structure of domain walls in hexaferrite BaFe12O19 have been studied near the Curie temperature TC = 724 K. The studies have been performed on the...     

Magnetic susceptibility and electrical resistivity of Ni-Al alloys at high temperatures

Magnetic susceptibility and electrical resistivity of the Ni-Al alloy system are studied in wide temperature (700–800°C) and concentration (up to 25 at. %) intervals. It is shown that in the region of solid solutions, temperature dependencies are sufficiently well described by a modified Curie-Weiss law, taking into account the existence of a temperature-independent contribution. For liquid alloys, calculation of the dependence of the electrical resistivity on composition is performed according to the Faber-Ziman-Evans method. Magnetic and electrical characteristics obtained are explained on the basis of a modified Anderson model, with the interaction of virtual bound states of nickel with collective valent electrons of aluminum taken into account.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 23–28, March, 1982.     

Polyvinyl Alcohol Enhanced Fluorescent Sulfur Quantum Dots for Highly Sensitive Detection of Fe3+ and Temperature in Cells

As a new class of metal-free fluorescent dots, sulfur quantum dots (SQDs) have attracted more and more attention because of their unique properties and promising applications in many fields. However, synthesizing SQDs with high optical stability and stimuli-responsive fluorescence is still in its infancy. Herein, a simple one-pot strategy is reported for preparing polyvinyl alcohol (PVA)-capped SQDs from cheap elemental sulfur by employing PVA as ligand. The as-prepared SQDs with size in range of 1.5–5 nm show fine water dispersibility and stability, bright fluorescence, good optical stability, and low cytotoxicity, which make them potential for cell imaging. In addition, the SQDs show sensitive and selective fluorescence quenching behavior toward Fe³⁺ with a detection limit of 92 × 10⁻⁹ m . Meanwhile, the SQDs also present temperature-dependent fluorescence and hold promise for working as a nanothermometer. For the first time, the utilization of SQDs for monitoring of Fe³⁺ and temperature in the interior of cells is confirmed in this study. This work thus opens new opportunities for expanding the application fields of SQDs.     

Enhancement of Curie Temperature under Built-in Electric Field in Multi-Functional Janus Vanadium Dichalcogenides

Functionalized two-dimensional materials with multiferroicity are highly desired to be next-generation electronic devices. Here we theoretically predict a family of Janus vanadium dichalcogenides VXX’(X/X’ = S, Se, Te)monolayers with multiferroic properties, combing ferromagnetism, ferroelasticity and piezoelectricity. Due to the unpaired electrons on the V atom, the Janus VXX’ monolayers have intrinsic long-range ferromagnetic orders.Particularly, the Curie temperature of 1 T-VSeTe monolayer is...     

Observation of Highly Elongated Field-Reversing Electron Rings

Recent experiments on RECE-Christa showed trapping of field-reversing electron rings with large length-to-radius ratios L/R ? 6, compared with L/R ? 1-2 observed so far. These rings translated stably over axial distances of up to 20 ring radii. Their self-field 8Bz (initially up to 2.6 Bz0) decayed smoothly with L/R remaining roughly constant.     

In-Situ Alignment of MnBi Crystals Induced by High Magnetic Field above Curie Temperature

Above Curie temperature, MnBi crystals are aligned in situ along the c-axis in a Bi matrix by a high fabrication magnetic field H f of 10 T. Magnetic testing shows a pronounced anisotropy in magnetization in directions normal and parallel to the fabrication field, resulting from the alignment. The successful alignment m v result from the fact that the easy magnetization direction is along the c-axis of MnBi and the high fabrication field of 10 T is large enough to rotate the ）MnBi crystal to this direction even though the temperature is above the Curie temperature.     

高浓度Er/Yb共掺ZnO薄膜的结构及室温光致发光特性

采用射频磁控溅射方法制备了Er/Yb共掺ZnO薄膜,研究了退火处理对高浓度Er/Yb共掺ZnO薄膜的结构演化和光致发光(PL)特性的影响。X射线衍射分析结果表明:Er/Yb掺杂导致ZnO薄膜的晶粒细化及择优取向性消失,ZnO晶粒随退火温度的增加而逐渐长大。900℃退火时,出现Er3 、Yb3 偏析,退火温度高于1000℃时,薄膜与基体间发生了界面反应,1200℃时,ZnO完全转变为Zn2SiO4相。光致发光测量结果表明:高于900℃退火处理后,Er/Yb共掺ZnO薄膜在1540 nm附近具有明显的光致发光,发光强度在退火温度为1050℃时达到最大值;光致发光光谱呈现典型的晶体基质中Er3 离子发光光谱所具有的明锐多峰结构特征。此外,探讨了薄膜结构演化及其对光致发光光谱的影响。     

Oxidation of iron,cobalt and nickel at the Curie temperature

The large differences in the logarithmic oxidation kinetics reported as a change in activation energy with the oxidation rate decreasing from ferromagnetic to paramagnetic states are discussed for iron, cobalt and nickel. The observed anomalies at the Curie point are explained by a mechanism which takes into account both surface-state charge at the metal/oxide interface and space charge in the growing oxide.     

Carbon Encapsulation of High Entropy Alloy Nanoparticles with Extraordinary Coercivity and Saturation at Room Temperature

Owing to their unique properties and technological potential, high entropy alloys (HEAs) have become the subject of great interest in the materials science community. HEAs consist of more than four principle elements in equimolar ratio so their configurational entropy is intrinsically greater than one-principle element based. The increasing surface energy and chemical tendency toward clustering of like atoms at low dimension, however, make production of HEA-nanoparticles (HEA-NPs) extremely difficult. A facile production of HEA-NPs inside carbon nanotubes and nanoparticles is demonstrated in this work. Electron microscopic and elemental analyses confirm encapsulated to be solution phase; some embrace carbides and form multidomains with chemical composition ranging from quaternary to quinary phase. Multidomains and nonmagnetic centers create hardening thus promoting coercivity significantly at room temperature. Alloying induces electron redistribution into high spin states, accounting for observed high saturation. Configurational entropy of encapsulated HEA-NPs lies on a range comparable with bulk.     

Thermal Analysis of the Ferromagnetic Materials in the Region of Curie Temperature by Temperature-Modulated DTA

Thermal analysis has been employed to measure changes of the heat capacity of ferromagnetic materials such as alloys in the region of the Curie temperature. Thermal response for these materials is usually small in this region. The necessary increase in sensitivity was achieved by applying temperature modulation. Results obtained by this technique including temperature-modulated differential thermal analysis simultaneously with A.C. thermomagnetometry are presented.     

Magnetoresistance anomalies at the Curie point of some FeCr spinels

Magnetoresistance measurements on three sintered FeCr spinels up to H = 14.5 koe show clear peaks at the Curie points T_c. It is suggested that this effect is associated with Jahn-Teller lattice distortions and possibly with a non Néel spin configuration leading to large spin disorder at T_c.     

碲镉汞红外焦平面探测器封装用4J36合金深低温力学性能研究

为了提高碲镉汞红外焦平面探测器封装结构的可靠性,获得组件材料的深低温力学性能十分重要.4J36合金具有极低的膨胀系数且材料焊接性能较好,广泛应用于封装结构中的冷平台部分.本文在300 K～4.2 K温度区间内对4J36合金进行了热力学性能试验,获得了材料随温度变化的动态热力学参数;在300 K、77 K、4.2 K温度下对4J36试件进行了拉伸试验,获得了相应的工程应力应变曲线和力学性能参数;通过扫描电镜对断口形貌进行了微观结构分析,获得了4J36合金拉伸断口微观组织和性能的变化规律.试验结果表明4J36材料在300 K～4.2 K温度范围内平均线膨胀系数为2.08×10^-6/K,从300 K到77 K,4J36合金的比热容随温度降低而减小;在300K～4.2 K温度区间内,随温度降低,合金的抗拉强度和屈服强度显著提高,而弹性模量断后伸长率降低;由拉伸断口宏观和微观形貌看出在300 K、77 K和4.2 K下4J36合金拉伸试件的断裂模式均为韧性断裂.试验研究结果将对4J36合金在红外焦平面探测器组件的低温应用方面提供试验依据.     

Observation of magnetic cluster phase above Curie temperature in Fe2CrAl Heusler alloy

We present a detailed magnetic critical behavior study of Fe₂CrAl Heusler alloy, for the first time, with rigorous analysis of high precision magnetization data obtained over the critical temperature region. Our studies confirm that B2 type site-disordered in Fe₂CrAl alloy exhibits long-range ferromagnetic order below a well defined Curie temperature (T=208 K). Though the nature of this transition is found to be of second order, the estimated critical exponents β=0.42, γ=1.356 and δ=4.25, are in between the theoretically predicted values for three-dimensional Heisenberg and mean-field interaction models. However, it is noteworthy that the scaling relations are obeyed indicating renormalization of interactions around the Curie temperature (T_C), where magnetization data collapse into two separate branches, above and below T_C. This conclusively shows that calculated critical exponents as well as critical temperature are unambiguous and intrinsic to the system. However, magnetization vs temperature data shows another magnetic transition (at T=313 K) above the Curie temperature. It is shown that the short range magnetic correlation exist even beyond T_C with cluster moment ∼10² μ_B. This is attributed to a site disorder which results in formation of Cr clusters with short range ferromagnetic order.