Neutron scattering investigations of multiferroic YMnO<Subscript>3</Subscript>

Multiferroic materials are the materials that show several cooperative ordering phenomena simultaneously. Here we consider only those materials that show both antiferromagnetism and ferroelectricity and in which the ordering parameters are coupled to each other. I shall review our recent neutron scattering investigations on multiferroic hexagonal manganites YMnO₃. We have determined the spin wave dispersions in YMnO₃ in symmetry directions by inelastic neutron scattering and have fitted the results with a simple nearest-neighbour anisotropic Heisenberg model. We have investigated the spin dynamics of YMnO₃ above T _N ∼ 70 K and have shown the existence of persistent albeit heavily damped spin waves.      

A synthetic iron phosphate mineral, spheniscidite, [NH4]+[Fe2(OH)(H2O)(PO4)2]?H2O, exhibiting reversible dehydration

Spheniscidite, a synthetic iron phosphate mineral has been synthesized by hydrothermal methods. The material is isotypic with another iron phosphate mineral, leucophosphite. Spheniscidite crystallizes in the monoclinic spacegroupP2₁/n. (a=9·845(1),b=9·771(3),c=9·897(1),β=102·9°,V=928·5(1),Z=4,M=372·2,d _calc=2·02 g cm⁻³ andR=0·02). The structure consists of a network of FeO₆ octahedra vertex-linked with PO₄ tetrahedra forming 8-membered one-dimensional channels in which the NH₄ ⁺ ions and H₂O molecules are located. The material exhibits reversible dehydration and good adsorption behaviour. Magnetic susceptibility measurements indicate that the solid orders antiferromagnetically.     

Synthesis, structures and magnetic properties of the new vanadates AgMnVO4 and RbMnVO4

The new compounds, AgMnVO₄ and RbMnVO₄ have been synthesized by solid state reaction route. Their crystal structures were determined from single-crystal X-ray diffraction data for RbMnVO₄ and powder X-ray diffraction data for AgMnVO₄. AgMnVO₄ crystallizes with the maricite-type structure in space group Pnma, a=9.5778(6) Å, b=6.8518(4) Å, c=5.3734(3) Å and Z=4. RbMnVO₄ crystallizes in space group P6₃ with a stuffed tridymite-type structure, a=11.2584(3), c=8.9893(13) Å and Z=8. A merohedral twinning was taken into account for its structural refinement. To our knowledge this is the first vanadate showing this structural type. AgMnVO₄ and RbMnVO₄ were characterized by magnetic susceptibility and specific heat measurements. AgMnVO₄ is antiferromagnetic with a Néel temperature of 12.3 K determined by specific heat measurements. RbMnVO₄ exhibits canted antiferromagnetism with a Néel temperature of 6.5 K.     

Block antiferromagnetism and possible ferroelectricity in KFe2Se2

Superconductors and multiferroics are two of the hottest branches in condensed matter physics. The connections between those two fields are fundamentally meaningful to unify the physical rules of correlated electrons. Recently, BaFe₂Se₃, was predicted to be multiferroic [Phys. Rev. Lett. 113 , 187204 (2014)] due to its unique one‐dimensional block‐type antiferromagnetism. Here, another iron‐selenide KFe₂Se₂, a parent state of iron‐based superconductor, is predicted to be multiferroic. Its two‐dimensional block‐type antiferromagnetism can generate a moderate electric dipole for each Fe–Se layer via the Fe–Se–Fe exchange striction. Different stacking configurations of these magnetic blocks give closely proximate energies and thus the ground state of KFe₂Se₂ may be switchable between antiferroelectric and ferroelectric phases.

Crystal structure of KFe₂Se₂. (a) Purple: K; green: Se; brown: Fe. Two Fe sheets in a minimum unit cell are indicated as A and B. (b) One Fe–Se layer with magnetism. Brown: spin up; blue: spin down. (c) A side view of Fe–Se bonds. The ionic displacements driven by exchange striction are indicated by arrows.     

Generalized Hartree-Fock theory and the Hubbard model

The familiar unrestricted Hartree-Fock variational principles is generalized to include quasi-free states. As we show, these are in one-to-one correspondence with the one-particle density matrices and these, in turn, provide a convenient formulation of a generalized Hartree-Fock variational principle, which includes the BCS theory as a special case. While this generalization is not new, it is not well known and we begin by elucidating it. The Hubbard model, with its particle-hole symmetry, is well suited to exploring this theory because BCS states for the attractive model turn into usual HF states for the repulsive model. We rigorously determine the true, unrestricted minimizers for zero and for nonzero temperature in several cases, notably the half-filled band. For the cases treated here, we can exactly determine all broken and unbroken spatial and gauge symmetries of the Hamiltonian.Dedicated to Philippe Choquard on his 65th birthday.     

Field-Induced Magnetic Properties in RCrO4 Oxides (R=Pr, Gd, Tb, Tm, and Yb)

RCrO₄ oxides (R=Pr, Gd, Tb, Tm, and Yb) have been synthesized at 773 K using the corresponding nitrates as precursors. X-ray diffraction data reveal that these samples are single phases and crystallize with the zircon-type structure, showing tetragonal symmetry, space group I4₁/amd. All the compounds are antiferromagnetic and the Néel temperature, which depends on the R³⁺ ion, takes values lower than 30 K. The presence of a canting appears to be responsible for the negative values of the magnetic susceptibility found below the compensation temperature. This uncommon phenomenon is named reversal of magnetization. It is field-dependent, being suppressed at 500 Oe for the TmCrO₄ compound. The highest value of the compensation temperature (24 K) corresponds to the YbCrO₄ oxide. A metamagnetic transition has been observed in all cases at critical fields ranging from 225 Oe (GdCrO₄) to 1600 Oe (YbCrO₄).     

Kristallstrukturverfeinerungen an Natriumtrifluorometallaten NaMF3 (M  Mg,Co, Ni,Zn): Oktaederkippung und Toleranzfaktor orthorhombischer Fluorperowskite

Crystal Structure Refinements of Sodium Trifluorometallates NaMF₃ (M ? Mg, Co, Ni, Zn): Tilting of Octahedra and Tolerance Factor of Orthorhombic Fluoroperovskites Based on newly measured X-ray single crystal data the crystal structures of the orthorhombic fluoroperovskites NaMF₃ (M ? Mg, Co, Ni, Zn) were refined in space group Pbnm (GdFeO₃ type, Z = 4). The octahedra are but slightly distorted; the average distances (and bridge angles M? F? M) are: Mg? F = 197.8 pm (150.9°), Co? F = 203.8 pm (146.1°), Ni? F = 200.3 pm (148.0°) and Zn? F = 202.3 pm (147.4°). With respect to the axes of the pseudocell (Z = 1) the octahedra are rotated by tilt angles which vary, including NaMnF₃ and NaFeF₃, between 14.6 and 18.8°. The tilting becomes the more pronounced, the smaller the tolerance factor of the compound, so as to yield uniformly average values of Na? F = 232 pm for the four shortest distances within the [NaF₈] coordination (distorted tetrahedron and bicapped trigonal prism, respectively). Measurements of magnetic powder susceptibilities show that tilting of octahedra is no sufficient cause to produce spin canting resulting in ferromagnetism: This property is confirmed only for NaNiF₃ (T_N = 150 K), whereas NaCoF₃ exhibits a sharp minimum in its χ^?l-T curve (T_N = 75 K) and remains uncanted antiferromagnetic then down to 4.2 K.     

正交多铁性材料DyMnO3的磁性质研究

基于密度泛函理论结合投影缀加平面波方法, 通过VASP软件包执行计算, 在分别考虑电子自旋阻挫共线与非共线的磁性结构基础上, 研究了正交结构下多铁性DyMnO₃材料在不同磁性构型下的晶格参数、总能、磁性、电子态密度和能带结构. 计算过程中选取广义梯度近似赝势, 同时使用局域自旋密度近似+U方法处理强关联作用下3d电子的计算结果. 计算结果表明: Mn离子为A型反铁磁态磁性构型的情况下能量最低结构最为稳定, Dy稀土离子磁性甚微, 可忽略不计; 当考虑电子自旋为非共线排列时, 正交DyMnO₃的总能提高、磁矩增大; 从电子结构图分析可知, 材料为间接能隙绝缘体, 能隙宽度约为0.38 eV, 加U后为1.36 eV, 导致晶格畸变的主要原因为Mn-3d与O-2p电子之间强烈的杂化作用. 关键词： 多铁性 反铁磁 密度泛函理论 非共线     

Fermion-Spin Approach to Study the Doping Dependence of Antiferromagnetism in the Copper Oxide Material

Within the fermion-spin theory, we study the ground-state properties of the copper oxide materials by considering quantum fluctuations of spinons in the random-phaseapproximation (RPA). The RPA ground-state at half-filling is the magnetized π-flux state with the energy E₀=-0.332J per bond and the staggered magnetization M = 0.327. Away from half-fiing this staggered magnetization vanishes around doping δ= 5% for the reasonable parameter t/J = 5, which is in very good agreement with the experiments on copper oxide materials. Our results indicate that both hole dopings and quantum fluctuations of spinons lead to a strong suppression of the antiferromagnetic long-range order.     

Edge and surface antiferromagnetism in ABO3 perovskite-type nanoparticle within the effective field theory

ABSTRACT

The edge and surface antiferromagnetic properties of the ABO₃ perovskite-type nanoparticle and their components (A, B and O) are investigated through the Effective Field Theory developed by Kaneyoshi. It is found that the Edge AntiFerroMagnetism (EAFM) causes a decrease in the Curie temperature and separation in the magnetizations. However, the Surface AntiFerroMagnetism (SAFM) causes a compensation behaviour (T_Comp?=?1.23) in the total magnetisation of the ABO₃ perovskite-type nanoparticle. The magnetisation of the central B component has been affected by the edge antiferromagnetism more strongly than by the surface antiferromagnetism. Moreover, coercivity (Hc) and remanence magnetizations (Mr) decrease for the EAFM, but they increase for the SAFM. Triple hysteresis behaviour is obtained for the SAFM. Therefore, it is suggested that there is a strong relationship between the compensation behaviour and the triple hysteresis behaviour.