Magnetocaloric properties of frustrated tetrahedra-based spin nanoclusters

Magnetization, entropy and magnetocaloric properties of various geometrically frustrated tetrahedra-based Ising antiferromagnetic nanoclusters with corner-, edge-, and face-sharing topologies are studied by exact enumeration. It is found that the studied properties strongly depend on the nanocluster topology and can be very different from those of a single tetrahedron as well as the pyrochlore lattice formed by an infinite number of corner-sharing tetrahedra. From the magnetocaloric point of view an important difference from the latter two systems is the absence of the ground-state zero-magnetization plateau in most of the studied structures, which at low temperatures facilitates emergence of a giant magnetocaloric effect in a vanishing magnetic field in the adiabatic demagnetization process. Magnetic systems with such properties might be suitable candidates for technological application as efficient refrigerators to ultra-low temperatures.     

微波激发下非线性磁表面波传播特性

研究了微波激发下非线性反铁磁晶体与线性铁磁晶体交界面上transverseelectric(缩写为TE)表面波的传播特性.用反函数形式给出了反铁磁晶体中TE表面波磁场的精确解.分析了小功率和大功率传输时的场分布特征.求得了TE表面波的色散方程,进而讨论了系统的微波频率特性.分析表明,非线性TE表面波存在频率通带和禁带,其带宽是功率的函数,通过调节导波功率可以有效地实现通带和禁带之间的转换 关键词： 反铁磁晶体 铁磁晶体 非线性表面波     

Phase diagram of a diluted triangular lattice Ising antiferromagnet in a field

Magnetization processes and phase transitions in a geometrically frustrated triangular lattice Ising antiferromagnet in the presence of an external magnetic field and a random site dilution are studied by the use of an effective-field theory with correlations. We find that the interplay between the applied field and the frustration-relieving dilution results in peculiar phase diagrams in the temperature-field-dilution parameter space.     

双层反铁磁体K3Cu2F7 中轨道序驱动的自旋二聚化

基于自旋-轨道-晶格Hamilton量,应用团簇自洽场方法,研究了双层钙钛矿结构材料K₃Cu₂F₇基态的晶格、磁及轨道结构,发现近孤立的双层的对称破缺和Jahn-Teller晶格畸变使得Cu²⁺离子在每层内交替占据 z²-x²〉/ z²-y²〉轨道,进而导致双层的层间表现为强的反铁磁耦合,层内为弱的铁磁耦合.强反铁磁耦合导致层间     

Oxo-Hydroxoferrate K2−xFe4O7−x(OH)x: Hydroflux Synthesis,Chemical and Thermal Instability,Crystal and Magnetic Structures

The reaction of Fe(NO₃)₃⋅9 H₂O with KOH under hydroflux conditions at about 200 °C produces red crystals of K_2−xFe₄O_7−x(OH)_x in a quantitative yield. In the crystal structure, edge-sharing [FeO₆] octahedra form Fe₂O₆] honeycomb nets. Pillars consisting of pairs of vertex-sharing [FeO₄] tetrahedra link the honeycomb layers and form columnar halls in which the potassium ions are located. The trigonal (P 1m) and the hexagonal (P6₃/mcm) polytypes of K_2−xFe₄O_7−x(OH)_x show oriented intergrowth. The sub-stoichiometric potassium content (x≈0.3) is compensated by hydroxide ions. K_2−xFe₄O_7−x(OH)_x is an antiferromagnet above 2 K and its magnetic structure was determined by neutron powder diffraction. Under ambient conditions, K_2−xFe₄O_7−x(OH)_x hydrolyzes and K₂CO₃ ⋅ H₂O forms gradually on the surface of the K_2−xFe₄O_7−x(OH)_x crystals. Upon annealing at air at about 500 °C, the potassium atoms in the columnar halls start to order into a superstructure. The thermal decomposition of K_2−xFe₄O_7−x(OH)_x proceeds via a topotactic transformation into K_1+x′Fe₁₁O₁₇, adopting the rhombohedral β’’ or the hexagonal β-aluminate-type structure, before γ-Fe₂O₃ is formed above 950 °C, which then converts into thermodynamically stable α-Fe₂O₃.     

Long-range magnetic ordering of S=1/2 linear trimers in A3Cu3(PO4)4 (A=Ca, Sr, and Pb)

Magnetic properties of S=1/2 linear trimer cluster compounds A₃Cu₃(PO₄)₄ (A=Ca, Sr, and Pb) were investigated. Magnetic susceptibility data for the three compounds showed that paramagnetic copper spins form trimers with the total spin of 1/2 below about 45 K. Specific heat and magnetization measurements indicated that the trimer clusters undergo ferromagnetic long-range ordering at for A=Ca and antiferromagnetic long-range ordering at for A=Sr and for A=Pb. A₃Cu₃(PO₄)₄ exhibited 1/3-magnetization plateau at least up to magnetic field of 55 T at 1.3 and 4.2 K. A₃Cu₃(PO₄)₄ with A=Sr and Pb showed a spin-flop transition near 0.03 T in the antiferromagnetic state at 0.08 K. Specific heat data at magnetic fields clearly showed broad maxima at low temperatures due to the finite intra-chain interaction in one-dimensional arrays of the trimers.     

磁场诱导的单离子各向异性反铁磁链的比热

利用改进的Schwinger玻色子平均场理论,研究了横向外加磁场下自旋为1的单粒子各向异性海森堡反铁磁链的热力学性质.发现在外加磁场下预示这一体系中存在Luttinger液体相—完全极化相相变的比热奇性,并给出了相应的相图.研究还发现,单粒子各向异性对比热的影响较小,这和纵向场的结果完全不同.以上结果对整自旋的反铁磁链具有普适性,对实验研究具有指导性. 关键词： 反铁磁链 热力学性质 Schwinger 玻色子     

Spin-Wave Analysis of the Spin-1 Two-Dimensional Frustrated Heisenberg Antiferromagnet with the Single-Ion Anisotropy

In this paper, the low-temperature properties of the spin-1 two-dimensional frustrated Heisenberg antifer- romagnet with the single-ion anisotropy are investigated on a square lattice by using the spin-wave theory. The influence of the frustration and anisotropy is found in the thermodynamics of the model, such as the temperature dependence of the staggered magnetization and specific heat. For some selected values of the frustration and anisotropy parameters, the results for the specific heat are compared with those of existing theories and numerical estimates. Within a spin-wave analysis, we have found the evidence for an intermediate magnetically disorder phase to separate the Néel and collinear phases.     

Specific heat study of the complex phase transitions in magnetism and dielectricity on triangular lattice antiferromagnets

The complex phase transitions involving dielectricity and magnetism on distorted triangular lattice antiferromagnets, "KNiC_l3-family" crystals, which have the crystal structures derived from the prototype CsNiCl₃, were studied through the heat capacity measurements. These crystals are classified into characteristic three groups from the viewpoint of magnetic and structural properties clarified so far. The results of the dielectric constant measurement as a function of temperature are also presented in detail. The present calorimetric study reveaed that the structural successive phase transitions rather than the magnetic transitions in these crystals are recognized more distinctly as the presence of specific heat anomalies at the respective phase transition points. This revised version was published online in August 2006 with corrections to the Cover Date.