外磁场下单离子各向异性海森堡铁磁体的低温磁性质

文章运用自旋波理论研究了在外磁场下有单离子各向异性的一维海森堡铁磁体的低温磁性质．通过霍斯坦因一普利马科夫变换得到近似的哈密顿量．详细讨论了磁化强度、磁化率、内能、比热等随温度、各向异性参数和外磁场的变化情况．推导出内能u（t）和比热cm（T）随温度的变化规律u（t）～ta与Cm（T）～tβ,并得出指数a,β与单离子各向异性参数以及外磁场的关系．文章的结果与采用其他理论以及蒙特卡罗模拟所得到的结果相符合．     

低温下的一维各向异性海森堡反铁磁体的磁性质

利用自旋波理论的方法研究了自旋S=1／2的一维各向异性海森堡反铁磁体的磁性质,推导得到了内能和比热与温度的关系：U（T）～Tα和Cm（T）～Tβ,同时研究还得出指数仅,届与各向异性参数、内磁场的关系．在零温和零磁场的条件下,计算得到基态能量和长程序,得出在各向异性参数值小于1时,长程序的值不会消失．在各向同性的情况下,长程序的结果与各向同性链模型的结果一致．     

自旋-3/2Ising模型的扩展Bethe-Peierls近似计算

采用扩展Bethe—Peierls近似方法,研究了二维正方晶格下自旋-3／2Ising模型的磁学性质,给出了不同单离子各向异性参数下的自由能、磁化强度、内能、比热等物理量随温度变化的规律．发现系统不存在一级相变,也不出现三相点．同时讨论了单离子各向异性对转变温度的影响．     

磁场对水内能作用的分子动力学模拟

将水系统视为正则系综 ,采用分子动力学模拟方法计算了磁场条件下水的内能、比热和径向分布函数。结果表明 ,磁场可以影响径向分布函数 g(r)的分布 ,使水的结构发生变化 ,从而导致水的内能、比热发生改变。水的内能、比热、径向分布函数伴峰的高度与磁场强度的关系均呈多极值特征 ,且在常温下磁感应强度为 0 .2 5T时 ,磁场作用最明显。     

磁场对水内能作用的分子动力学模拟

将水系统视为正则系统，采用分子动力学模型方法计算了磁场条件下水的内能，比热和径向分布函数，结果表明，磁场可以影响径向分布函数g(r)的分布，使水的结构发生变化，从而导致水的内能，比热发生改变，水的内能，比热，径向分布函数伴峰的高度与磁场强度的关系均呈多极值特征，且在常温下磁感应强度为0．25T时，磁场作用最明显。     

势阱中带电荷理想费米气体的热力学性质

从统计物理半经典近似方法出发,研究各向异性简谐势阱中理想带电荷费米气体系统的热力学性质,推导出了该体系的热力学势、比热、内能、磁场强度和磁化率随外加磁场的变化关系,进而分析了约束势阱对理想带电荷费米体系热力学性质的影响.     

在外磁场中两链自旋梯子模型的自旋波与比热

应用Jordan-Wigner变换和格林函数方法讨论了在外磁场中带有阻挫的两链自旋梯子模型的自旋波、子晶格磁矩、内能及比热,获得了比热与温度、外磁场的关系。结果显示：自旋波与波矢七成余弦关系,与外磁场成线性关系;当温度,趋近于0．5K和0．42K时,比热有明显的跃变。     

层状Heisenberg亚铁磁系统的温度补偿行为

采用双时温度格林函数方法讨论了具有层间耦合的蜂窝状晶格3/2和5/2混自旋亚铁磁Heisenberg系统全温区的磁性行为·讨论了晶体场单离子各向异性和层间耦合效应对系统磁矩、转变温度和补偿温度的影响·给出了子晶格磁矩和总磁矩在不同的晶体场单离子各向异性和层间耦合效应随温度变化曲线,系统的转变温度和补偿温度随晶体场单离子各向异性和层间耦合效应变化的相图·分析表明:系统存在多种磁矩曲线,当晶体场单离子各向异性和层间耦合超过最小值(D1min/J和J1min/J)后才会出现补偿现象,补偿温度随D1/J和J1/J的增大而减小,最后趋于不变·     

磁性化合物R2Fe17Nx（R=Er,Tm）自旋重取向的理论解释

用单离子模型计算了化合物R_2Fe_(17)N_x(R=Er,Tm)中稀土离子R~(3+)的磁晶各向异性稳定能随温度的变化关系。并考虑到铁次晶格的各向异性稳定能随温度的变化关系,成功地解释了化合物R_2Fe_(17)N_x(R=Er,Tm)中出现自旋重取向的现象。     

双钙钛矿Ba2CoWO6晶体制备及磁性研究

以BaCl₂为助熔剂，采用自助溶法制备了毫米尺寸的Ba₂CoWO₆单晶，并通过X射线衍射、比热和磁化率测量对晶体进行了结构表征和物性研究。利用脉冲强磁场技术对单晶进行了高达50 T的磁化测量，综合实验数据构建了Ba₂CoWO₆的磁场-温度(H-T)相图。结果表明，低温下Ba₂CoWO₆中Co²⁺离子处于自旋态S=3/2并伴随着有一定分量的轨道磁矩，且晶体不同轴向具有磁各向异性。在强磁场的驱动下，该材料在临界场H_c=30 T时发生铁磁相变，进入饱和态。     

横向Ising模型性质的理论计算

利用Bethe-Peierls近似,研究了配位数为6(简立方)和4(平面正方)晶格上的横向自旋1／2的Ising模型的磁学性质,在不同横向场参数下,计算了两个系统的自由能、横向和纵向磁化强度、内能和磁比热等物理量,发现横向磁场对系统的物理量有明显影响,并对系统自发磁化有抑制作用．     

基于有机分子亚铁磁体棱型自旋链外场下的热力学性质研究

基于近来有机高分子亚铁磁体在试验上成功的合成，本文采用量子转移矩阵方法，研究一维反铁磁-铁磁自旋1／2棱型自旋链在外场条件下低温下的热力学性质．磁化强度在低温下随外场的变化出现台阶式的平台结构，并表现出三个临界外场值，反映了系统磁相互耦合作用与热力学波动间的相互竞争；同时磁化率在外场下呈现双峰结构，其来源于系统在外场下的能隙和长程序参量的变化．本文的数值结果表明该棱型链与自旋1-1／2海森堡链性质一致．     

磁处理降低焊接残余应力中磁场方向的影响

为了解磁处理可降低铁磁性材料残余应力的内在机理和作用规律,通过实验研究了焊接试板在经不同磁场方向的磁场处理前后的残余应力分布,发现焊板平面内垂直于焊缝方向磁场处理后,焊缝区附近的残余应力有明显的降低;沿板厚方向磁场处理后,焊缝区附近残余应力也有所下降;而磁场方向沿焊缝方向进行处理,残余应力降低不明显。这说明磁场方向垂直于最大主应力方向时最有利于残余应力的降低。该文还初步分析了磁处理降低焊接残余应力时,磁场方向影响其效果的机理。     

Transition-metal-based magnetic refrigerants for room-temperature applications.

Magnetic refrigeration techniques based on the magnetocaloric effect (MCE) have recently been demonstrated as a promising alternative to conventional vapour-cycle refrigeration. In a material displaying the MCE, the alignment of randomly oriented magnetic moments by an external magnetic field results in heating. This heat can then be removed from the MCE material to the ambient atmosphere by heat transfer. If the magnetic field is subsequently turned off, the magnetic moments randomize again, which leads to cooling of the material below the ambient temperature. Here we report the discovery of a large magnetic entropy change in MnFeP0.45As0.55, a material that has a Curie temperature of about 300 K and which allows magnetic refrigeration at room temperature. The magnetic entropy changes reach values of 14.5 J K-1 kg-1 and 18 J K-1 kg-1 for field changes of 2 T and 5 T, respectively. The so-called giant-MCE material Gd5Ge2Si2 (ref. 2) displays similar entropy changes, but can only be used below room temperature. The refrigerant capacity of our material is also significantly greater than that of Gd (ref. 3). The large entropy change is attributed to a field-induced first-order phase transition enhancing the effect of the applied magnetic field.     

Electron dynamics in collisionless magnetic reconnection

Magnetic reconnection provides a physical mechanism for fast energy conversion from magnetic energy to plasma kinetic energy. It is closely associated with many explosive phenomena in space plasma, usually collisionless in character. For this reason, researchers have become more interested in collisionless magnetic reconnection. In this paper, the various roles of electron dynamics in collisionless magnetic reconnection are reviewed. First, at the ion inertial length scale, ions and electrons are decoupled. The resulting Hall effect determines the reconnection electric field. Moreover, electron motions determine the current system inside the reconnection plane and the electron density cavity along the separatrices. The current system in this plane produces an out-of-plane magnetic field. Second, at the electron inertial length scale, the anisotropy of electron pressure determines the magnitude of the reconnection electric field in this region. The production of energetic electrons, which is an important characteristic during magnetic reconnection, is accelerated by the reconnection electric field. In addition, the different topologies, temporal evolution and spatial distribution of the magnetic field affect the accelerating process of electrons and determine the final energy of the accelerated electrons. Third, we discuss results from simulations and spacecraft observations on the secondary magnetic islands produced due to secondary instabilities around the X point, and the associated energetic electrons. Furthermore, progress in laboratory plasma studies is also discussed in regard to electron dynamics during magnetic reconnection. Finally, some unresolved problems are presented.     

The variation in the magnetic fluctuation anisotropy across the front boundaries of magnetic clouds and its geomagnetic response

A statistical study of magnetic fluctuations near the front boundaries of magnetic clouds is approached with the method of Minimum Variance Analysis, based on the data of Imp8 and Wind spacecraft. New discoveries are that (1) fluctuation anisotropy tends to increase across the front boundaries of magnetic clouds; (2) there is a good correlation between the fluctuation anisotropy and the geomagnetic activity indices; and (3) in some cases, although there is southward field component immediately after the front boundary, K_p index descends (or D_st index ascends) with a corresponding decrease of the fluctuation anisotropy; in other cases with no distinct southward field component, K_p index ascends (or D_st index descends) with a corresponding increase of the fluctuation anisotropy. Thus we suggest that the fluctuation anisotropy might be a useful indicator in diagnosing the magnetic activities of magnetic clouds.