铁磁球/反铁磁纳米体系磁滞回线的Monte Carlo模拟

利用经典Heisenberg模型和Monte Carlo方法研究外磁场和反铁磁磁晶各向异性、交换相互作用对铁磁球均匀嵌入到反铁磁基体中的铁磁/反铁磁纳米体系磁滞回线的影响.模拟结果显示,外加反向最大磁场不同时,磁滞回线形状不同.当磁场正向增加时,体系的磁化强度会产生一个跃变,但跃变高度与反向场最大值无关.反铁磁磁晶各向异性越大,体系的交换偏置现象越明显,且磁化强度回到饱和值所需的外磁场越大.随着反铁磁基体交换相互作用的增大,在正向和负向磁场区域还可能出现新的磁滞现象.     

铁磁/反铁磁双层薄膜中自旋波禁闭现象研究

本文在一维海森堡模型的基础上,采用界面参数化方法,研究了非周期性边界条件下,铁磁/反铁磁双层薄膜中自旋波低温激发问题.发现体系中除了存在体自旋波外还存在禁闭自旋波.此类自旋波只能在一个子层中传播,在另一子层中没有激发.而且,禁闭在反铁磁层中自旋波的能量低于体自旋波的能量;禁闭在铁磁层中自旋波的能量高于体自旋波的能量.     

一种具有铁磁与反铁磁相互作用Ising模型的热力学性质

对最近邻自旋间具有反铁磁相互作用Ｊ_s，相隔偶数个自旋间又具有指数型长程铁磁相互作用Ｊ₁的一维Ｉｓｉｎｇ模型，利用积分方程的方法得出内能和比热的解析表达式。在Ｊ₁＝１，Ｊ_s分别为０．０５，０．１和０．３的条件下，作出零场时的内能和比热随温度参量变化的图形，并指出其相变临界点。在Ｊ₁＝１，Ｊ_s＝０．１的条件下，作出外磁场变化对内能和比热的影响图。鉴于系统在低温、零场或弱场以及较弱 关键词：     

相互作用突然开启后的反铁磁海森伯模型

本文以反铁磁海森伯模型为对象,设想了一个相互作用在t=0时刻开启的非平衡体系.在低温近似下,本文利用重整化群流方程方法讨论了该体系可观测量随时间的演化趋势.本文根据此类体系的特点,给出了获得算符时间演化形式的路径,并成功将其应用于本文所讨论的模型中.其中,本文着重介绍了如何在非平衡自旋体系中应用流方程方法,并利用该方法得到了随时间演化的基态磁化强度,同时论证了该方法处理此类系统的有效性.同时,在与平衡体系基态磁化强度的对比中,发现非平衡可观测量随时间震荡而无法收敛,这一点有别于相互作用突然开 关键词： 非平衡体系 反铁磁海森伯模型 流方程方法 磁化强度     

铁磁─反铁磁双层系统低温特性(英文)

采用自旋波的理论研究反铁磁层间耦合强度和不同自旋值对铁磁—反铁磁双层系统磁性质的影响，在层间反铁磁耦合情况下．得出了不同自旋值时每层子晶格交叉点的温度，在低温下表现出量子效应。     

近反铁磁费米液体的一个理论

我们为高温超导体的唯象模型，近反铁磁费米液体理论，建立了一个微观理论．我们用来描写高温超导体的微观模型由局域磁矩和掺杂到材料中的载流子两个子体系组成；我们用．ｓ－ｄ交换耦合来描写这两个子体系之间的相互作用．从这个微观模型出发，我们推导出了近反铁磁费米液体唯象模型中所假设的自旋磁化率．     

关于海森堡反铁磁链材料LiVGe2O6有限温度相变的理论研究

自旋s=1的海森堡反铁磁链材料LiVGe₂O₆的磁化率以及 核磁共振实验表明该材料在临界温度约为22 K时由顺磁相转变为反铁磁Néel相, 且低温磁激发谱存在能隙. 本文在已有模型哈密顿量的基础上提出了一个低能场论模型——Ginzburg-Landau理论来描述 这一反铁磁链材料, 并运用这一理论讨论了LiVGe₂O₆由于自发对称性破缺导致的有限温度相变及 相应的磁化率变化情况, 理论计算很好地解释了现有的实验结果. 关键词： 海森堡模型 σ模型')" href="#">O(3)非线性σ模型 有限温度相变     

锰酸钇薄膜中Mn3+离子d-d跃迁的超快光谱学研究

本文利用时间分辨光谱技术,系统研究了飞秒激光诱导YMnO₃薄膜中Mn³⁺离子3d轨道跃迁的 载流子动力学过程.当抽运光子能量为1.7 eV,对应于Mn³⁺离子的3d轨道跃迁, 抽运-探测零延迟时间处的透射率变化随着温度的降低逐渐减小. 这起源于低温下短程反铁磁有序诱导Mn³⁺离子d-d能级发生"蓝移". 载流子弛豫过程由快、慢两个过程组成,分别对应于电子-声子相互作用和自旋-声子相互作用. 实验发现,当温度低于80 K,电子-声子热化时间显著增加,表明低温下电子-声子的 耦合强度受长程反铁磁有序的影响.     

铁磁/反铁磁双层薄膜中自旋波特性研究

本文在一维海森堡模型的基础上,采用界面参数化方法,研究了非周期性边界条件下,铁磁/反铁磁双层薄膜中自旋波低温激发问题.重点讨论了表面各项异性对薄膜中自旋波谱、色散关系的影响.结果表明:表面各向异性对铁磁层中自旋波谱影响较大,对色散关系影响甚微。     

面心立方格子上Ising自旋Ⅰ(1)的反铁磁性——配分函数的级数展开法

本文采用与文献[1]完全相类似的方法作出一般化Ising模型s=1在面心立方格子(fcc)上最近邻相互作用的反铁磁系统的统计理论,即用Pad近似式处理配分函数的高、低温幂级数有限项展开式,得出其反铁磁-顺磁转变为一阶相变。T_c=1.33J/k,小于(1/2)相应的相变点。文中算出了相关的热力学量如内能、潜热、熵、比热、长程和短程序参量,以及磁化率等。值得指出的是:(1)fcc上反铁磁-顺磁相变属于一阶,这是由格子密堆积的拓扑特征所决定,与s的大小无关;(2)T_c(s)随s的增加而缓慢地下降。 关键词：     

具有四次幂交换作用的Heisenberg铁磁体的低温自旋波理论

本文讨论具有四次幂交换作用的Heisenberg铁磁体,用Малеев玻色变换,在低温区建立〈S_i^z〉的严格展式,正确地包括了动力学相互作用和运动学相互作用,得到自旋波谱和自发磁化的正确温度依赖关系。 关键词：     

Investigations of the sublattice magnetizations M(T) in antiferromagnets with fourth-order exchange interactions: EuSrTe

We present a neutron scattering study of the temperature and composition dependence of the MnO-type superstructure reflection intensities in the diamagnetically diluted antiferromagnetic compounds Eu_xSr_1-xTe. In these materials antiferromagnetic biquadratic and ferromagnetic three-spin interactions have been identified recently. These fourth-order non-Heisenberg interactions are able to create their own order parameter which is believed to govern the order of the transverse moment components and which, hence, is directed perpendicular to the common Heisenberg order parameter. The observed MnO-type diffraction intensities originate in the sublattice magnetizations, , of both order parameters. Due to the different composition dependencies for biquadratic interaction processes and three-spin interaction processes , the ferromagnetic three-spin interactions dominate for x > x c =0.85, while for x <0.85 the antiferromagnetic biquadratic interactions dominate. Associated with this sign change in the fourth-order interaction sum the transverse order parameter changes from the antiferromagnetic MnO type for x <0.85 to ferromagnetic for x >0.85. This is noticed as a sudden decrease of the low-temperature MnO scattering intensities at x c =0.85. Although susceptibility measurements reveal clearly a ferromagnetic component for x >0.85 no ferromagnetic Bragg intensities were observed in standard neutron scattering spectra using EuTe powder samples. We explain this by the competition of antiferromagnetic biquadratic and ferromagnetic three-spin interactions whereby a disturbed ferromagnetic superstructure may be generated which gives rise also to weak MnO-type diffraction lines. It is found that the resulting obeys a T² law until a temperature as large as 0.75T_N irrespective of the nature of the transverse order parameter. The T² law must, hence, be common to both types of order parameter showing that the fourth-order interactions re-define the spin dynamics of both completely. From the linear composition dependence of the normalized T² coefficient the existence of three-spin interactions is again confirmed. Received 23 July 1998 and Received in final form 12 October 1998     

Long range ferromagnetism in tunable cobalt(II) layered compounds up to 25 Å apart

We discuss the structure-magnetic property correlations for a series of layered cobalt(II) compounds, derived from the parent host Co(OH)₂. Intercalation takes place by exchange of OH⁻ by organic or inorganic anions resulting in interlayer spacing between 4.65 and 25.4 Å. For all the compounds, ferromagnetic in-plane interaction dominate the magnetic behavior at high temperature, but long range order occurs at low temperature; 3d antiferromagnetic order is observed for small basal spacing and ferromagnetic for large spacing. The nature of the magnetic interaction is discussed briefly.     

The thermodynamic one-particle Green function in the renormalized spin wave approximation for isotropic cubic ferromagnets

The thermodynamic one-particle Green function in the renormalized spin wave approximation for isotropic cubic ferromagnetic insulators with Dyson's spin wave theory as a base is derived. In quantitative respect, dynamic and kinematic effects of spin waves are approximated by the graphs deficient in the energy denominators, wherefore at low temperature kinematic interaction turns out to be too strong. As against the one-particle Green function for independent spin waves, dynamic interaction of ferromagnons is shown to effect the renormalization of the spin wave energy, whereas kinematic interaction directly modifies the average ferromagnon population numbers. In the matter of magnetization, its formula based on the Green function assumes a similar form as in the spin wave theory without interactions on the understanding that it remains valid within the entire range of temperatures from absolute zero up to the critical point.     

Effective self-consistent field and phase transitions in the <Emphasis Type="Italic">t-J</Emphasis> model

The influence of the kinematic interaction and the indirect exchange of electrons on the magnetic structure of a narrow-gap Hubbard magnet is investigated using the diagram technique of the perturbation theory. The summation of an infinite series of like diagrams makes it possible to separate the effective self-consistent filed. In contrast to the previous studies, the proposed theory contains only two self-consistent parameters, i.e., the chemical potential and the average spin. The obtained system of equations for these parameters admits the analytical solution at zero temperature. The competition of the simplest kinematic contribution and the indirect antiferromagnetic exchange is clearly demonstrated using the concentration dependences of the Néel temperature as an example. The boundaries of the existence of the homogeneous ferromagnetic, antiferromagnetic, and paramagnetic phases as a function of the electron concentration and the indirect exchange parameter are determined by numerically analyzing the internal energy.     

Froth-like Minimizers of a Non-Local Free Energy Functional with Competing Interactions

We investigate the ground and low energy states of a one dimensional non-local free energy functional describing at a mean field level a spin system with both ferromagnetic and antiferromagnetic interactions. In particular, the antiferromagnetic interaction is assumed to have a range much larger than the ferromagnetic one. The competition between these two effects is expected to lead to the spontaneous emergence of a regular alternation of long intervals on which the spin profile is magnetized either up or down, with an oscillation scale intermediate between the range of the ferromagnetic and that of the antiferromagnetic interaction. In this sense, the optimal or quasi-optimal profiles are “froth-like”: if seen on the scale of the antiferromagnetic potential they look neutral, but if seen at the microscope they actually consist of big bubbles of two different phases alternating among each other. In this paper we prove the validity of this picture, we compute the oscillation scale of the quasi-optimal profiles and we quantify their distance in norm from a reference periodic profile. The proof consists of two main steps: we first coarse grain the system on a scale intermediate between the range of the ferromagnetic potential and the expected optimal oscillation scale; in this way we reduce the original functional to an effective “sharp interface” one. Next, we study the latter by reflection positivity methods, which require as a key ingredient the exact locality of the short range term. Our proof has the conceptual interest of combining coarse graining with reflection positivity methods, an idea that is presumably useful in much more general contexts than the one studied here.     

Influences of Crystal-Field and Interlayer Coupling Interactions on Dynamic Phase Diagrams of a Mixed-Spin(3/2,2) Bilayer System

Influences of crystal-fields(D_A and D_B) and interlayer coupling interactions(J_3) on dynamic magnetic critical behaviors of a mixed-spin(3/2, 2) bilayer system under an oscillating magnetic field are investigated by the Glauber-type stochastic dynamics based on the mean-field theory. For this purpose, dynamic phase diagrams are constructed in the reduced temperature and magnetic field amplitude plane for the ferromagnetic/ferromagnetic(FM/FM),antiferromagnetic/ferromagnetic(AFM/FM) and AFM/AFM interactions in detail. We observe that the influences of D_A, D_B and J_3 interactions parameters on the behavior of the dynamic phase diagrams are very much.     

Quantum-mechanically induced asymmetry in the phase diagrams of spin-glass systems

The spin-1/2 quantum Heisenberg spin-glass system is studied in all spatial dimensions d by renormalization-group theory. Strongly asymmetric phase diagrams in temperature and antiferromagnetic bond probability p are obtained in dimensions d>or=3. The asymmetry at high temperatures approaching the pure ferromagnetic and antiferromagnetic systems disappears as d is increased. However, the asymmetry at low but finite temperatures remains in all dimensions, with the antiferromagnetic phase receding from the ferromagnetic phase. A finite-temperature second-order phase boundary directly between the ferromagnetic and antiferromagnetic phases occurs in d>or=6, resulting in a new multicritical point. In d=3, 4, 5, a paramagnetic phase reaching zero temperature intervenes asymmetrically between the ferromagnetic and reentrant antiferromagnetic phases. There is no spin-glass phase in any dimension.     

Biaxial magnetic structures in rare-earth compounds: GdMg and HoP

The flopside spin structure, where the magnetic moments form two sublattices which at low temperatures are mutually perpendicular was first found in HoP and then in other rare-earth pnictides. There are large orbital contributions to the magnetic moments of these compounds and it had been thought that they play an important role in stabilizing the flopside spin structure. However, recently this spin structure has been found in GdMg. As Gd³⁺ is an S-state ion, there are negligible orbital effects. We have developed a model Hamiltonian which is able to explain both the occurence of initially a ferromagnetic phase and then at low temperature the flopside spin structure in two very dissimilar compounds GdMg and HoP. For GdMg we find that the competition between the near neighbor ferromagnetic and antiferromagnetic bilinear exchange interactions is such that while they produce a transition to a ferromagnetic phase at 110 K, an unusually small amount of biquadratic (quadrupolar) coupling is able to stabilize a flopside phase at low temperature which is able to resist collapse in a field as large as 150 kOe. For HoP we find that although anisotropic bilinear pair interactions - as for example pseudo-dipole - exist, they cannot be the primary origin of the flopside phase; quadrupole pair interactions are essential to explain the appearance of first the ferromagnetic and then the flopside phases found in HoP. On the basis of our model calculations we are able to explain the data extant on these compounds and we make some predictions which are open to experimental verification.     

The phase diagram and susceptibility of LiNiO2

The phase diagram and the temperature dependence of the susceptibility of a four-site spin system with orbital degeneracy have been calculated by taking into account the triangular lattice symmetry. The results indicate that the ground state of the spin–orbital system can be spin disordered even in the case of the large Hund's interaction and the anisotropic orbital hopping. The susceptibility and the short correlation show antiferromagnetic in low temperature, but ferromagnetic in high temperature. These conclusions are in agreement with the recent experimental observations on the material LiNiO₂.