Magnetic order and spin dynamics in ferroelectric HoMnO3

Hexagonal HoMnO3 is a frustrated antiferromagnet (T(N)=72 K) ferroelectric (T(C)=875 K) in which these two order parameters are coupled. Our neutron measurements of the spin-wave dispersion for the S=2 Mn3+ on the layered triangular lattice are well described by a two-dimensional nearest-neighbor Heisenberg exchange J=2.44 meV, and an anisotropy D that is 0.28 meV above the spin-reorientation transition at 40 K and 0.38 meV below. For H parallel c the magnetic structures and phase diagram have been determined, and reveal additional transitions below 8 K where the ferroelectrically displaced Ho3+ ions are ordered magnetically.     

Field-induced phase transition in Fe3BO6

The temperature dependence of the field-induced phase transition in Fe₃BO₆ has been measured in fields up to 14.7 T. The experimental value for the anisotropy is compared with the calculated result for the dipole-dipole energy.     

半掺杂Sm0.5Ca0.5MnO3体系的电荷有序和再入型自旋玻璃行为

研究了半掺杂锰氧化物Sm0.5Ca0.5MnO3体系的结构、输运和磁特性,结果表明,在半掺杂情况下,该体系呈现O′类正交结构,表明体系存在典型的Jahn-Teller效应畸变;输运结果在整个测量温区均呈现半导体导电行为,没有出现金属-绝缘体(M-I)转变和CMR效应;电荷有序转变发生在T=270K左右,反铁磁转变温度出现在200K附近,且表现出典型的再入型自旋玻璃(spin-glass)行为,自旋玻璃转变温度TSG在41K附近,同时,观察到了存在于该体系中负的磁化异常,表明半掺杂的Sm0.5Ca0.5MnO3基态存在有多种复杂而丰富的磁相互作用之间竞争机制,其研究将为强关联锰氧化物体系物理机理的理解提供丰富的实验资料.     

Field-induced magnetic order and simultaneous lattice deformation in TlCuCl(3)

We report the results of Cu and Cl nuclear magnetic resonance experiments and thermal expansion measurements in magnetic fields in the coupled dimer spin system TlCuCl3. We found that the field-induced antiferromagnetic transition as confirmed by the splitting of NMR lines is slightly discontinuous. The abrupt change of the electric field gradient at the Cl sites, as well as the sizable change of the lattice constants, across the phase boundary indicate that the magnetic order is accompanied by simultaneous lattice deformation.     

Field-induced phase separation in one dimension

Phase separation is induced in the one-dimensional Ising chain (or lattice-gas model of a fluid) by means of an external field that changes sign in the middle of the chain. The magnetization profile (or density profile of the analogous fluid) is obtained analytically. It is found to decay exponentially rapidly to the bulk-phase magnetizations (or densities), the exponential decay parameter being the correlation length in the bulk phases in the presence of the field. This is in accord with earlier theoretical ideas. The interfacial tension is also obtained analytically. In an appropriately defined limit of large neighboring-site spin-spin interactions and small external field the interface becomes infinitely broad while the amplitude of the profile and the interfacial tension both vanish, in close imitation of the approach to a critical point in a real fluid. In this asymptotic limit the interfacial tension is related to the amplitude of the profile in the way that is predicted by earlier theories of interfaces near critical points, with critical-point exponents now those appropriate to one dimension. The exact interfacial profile and tension are used to test several approximations, including a corrected form of the barometric law and local (square-gradient) and nonlocal forms of the van der Waals theory.     

Nature of Ho magnetism in multiferroic HoMnO3

Using x-ray resonant magnetic scattering and x-ray magnetic circular dichroism, techniques that are element specific, we have elucidated the role of Ho3+ in multiferroic HoMnO3. In zero field, Ho3+ orders antiferromagnetically with moments aligned along the hexagonal c direction below 40 K, and undergoes a transition to another magnetic structure below 4.5 K. In applied electric fields of up to 1 x 10(7) V/m, the magnetic structure of Ho3+ remains unchanged.     

Field-induced phase transitions in antiferroelectric liquid crystals

A theoretical study is made of the process by which an antiferroelectric smectic liquid crystal undergoes a field-induced transition to ferroelectric alignment. We find that for cells of moderate thickness the initial departure from antiferroelectric alignment occurs as a continuous Fréedericksz transition. The following transition from partial alignment to complete ferroelectric ordering may occur as either a first-order or continuous transition, depending on the relative strength of some of the model parameters. The case where the transition is continuous provides a possible mechanism for some recently observed thresholdless transitions in these systems.     

Field-induced supersolid phase in spin-one Heisenberg models

We use numerical methods to demonstrate that the phase diagram of S=1 Heisenberg models with uniaxial anisotropy contains an extended supersolid phase. We show that this Hamiltonian is a particular case of a more general and ubiquitous model that describes the low-energy spectrum of some isotropic and frustrated spin-dimer systems. This result is crucial for finding a spin supersolid state in real magnets.     

On a reentrant isotropic phase in disc-like liquid crystals

A binary mixture of two disc-like liquid crystals 2, 3, 7, 8, 12, 13-hexa-n-tetradecanoyloxy and 2, 3, 7, 8, 12, 13-hexa (4-n-dodecyloxybenzoyloxy) truxenes exhibits an isotropic phase above and below the temperature existence range of a columnar mesophase.     

Field-induced magnetization distribution and antiferroquadrupolar order in CeB(6)

The anisotropic Knight shift of implanted positive muons (micro(+)) in CeB(6) has been studied between 2.2 and 200 K in a field of 0.6 T. The results imply that the field-induced magnetization distribution is not only found at the Ce sites but also in other regions of the unit cell (e.g., near or inside the B6 molecule, as proposed by Saitoh et al. [J. Phys. Soc. Jpn. Suppl. 71, 106-108 (2002)]). While above the antiferroquadrupolar ordering temperature T(Q) approximately 3.55 K this additional magnetization appears to be antiparallel to the Ce moments, a drastic change is observed below T(Q), and the additional magnetization is eventually aligned parallel to the Ce moments.     

Field-induced magnetic phase transitions in the Yafet-Kittel model

The Yafet-Kittel model for a two-sublattice ferrimagnet with an antiferromagnetic exchange interaction in one of the sublattices was developed to describe magnetic-field-induced phase transitions in the isotropic and Ising cases. Depending on the relative values of the exchange parameters of the inter-sublattice interaction and the intra-sublattice interaction in the isotropic case, two types of magnetic phase diagrams with two types of second-order phase transitions are possible: to the noncollinear phase and to the spin-flip phase, and, in the Ising case, three types of magnetic phase diagrams with first-order phase transitions are possible. Fiz. Tverd. Tela (St. Petersburg) 41, 1797–1799 (October 1999)     

Muon investigation of HoMnO3 and YMnO3 hexagonal manganites

The magnetic properties of multiferroics HoMnO₃ and YMnO₃ have been investigated using the muon (μSR) method. Analysis of the dependence of the dynamical relaxation rate λ and characteristics of the distribution of local static fields makes it possible to more precisely determine the phase states of the samples under investigation.     

Observation of a reentrant twist grain boundary phase

We report the occurrence of a reentrant twist grain boundary phase, which we designate as Re- TGB(A). Microscopic observations on a nonsymmetric dimer showed the phase sequence Iso- N*- TGB(A)-Sm- A-Re- TGB(A)- TGB(C*). Here N* and Sm- A stand for the chiral nematic and smectic- A phases, TGB(A) is the twist grain boundary phase with smectic- A blocks, and TGB(C*) that with smectic- C* blocks and exhibiting features of both the smectic- C* and TGB phases. The reentrance of the TGB(A) phase is unambiguously demonstrated using x-ray diffraction, selective reflection, and optical rotation data.     

Field-induced phase transitions in magnetooptical films with a small positive anisotropy constant

We study phase transitions induced by a static magnetic field in magnetically uniaxial films with a small positive anisotropy constant. The phase diagram of these objects is determined in the H _∥-H _⊥ plane, where H _∥ and H _⊥ are, respectively, the components of the magnetizing field along and perpendicular to the surface normal. The stability boundary is located for all of the main types of domain configurations observed: a simple stripe domain structure, a stripe domain structure with periodic bending by surface distortions in the profile of the domain walls, and hexagonal lattices of cylindrical magnetic bubbles. Zh. éksp. Teor. Fiz. 111, 283–297 (January 1997)     

Field-induced phase transitions and giant magnetoresistance in Dy D{y_3}CoCo single crystals

Electrical resistivity and calorimetric measurements on Dy ₃ Coshow that below the Néel temperature (T N =44 K) the non-collinear antiferromagnetic structure exhibits field-induced magnetic phase transitions of a first-order type along all principal axes, accompanied by a strongly anisotropic giant magnetoresistance and by a change of the Sommerfeld coefficient of the specific heat. Quantum tunnelling of the magnetization appears to be possible for T < 0.6 K. Received 15 July 1999 and Received in final form 6 December 1999     

半掺杂Sm0.5Ca0.5MnO3体系的电荷有序和再入型自旋玻璃行为

研究了半掺杂锰氧化物Sm_0.5Ca_0.5MnO₃体系的结构、输运和磁特性，结果表明，在半掺杂情况下，该体系呈现O′类正交结构，表明体系存在典型的Jahn-Teller效应畸变；输运结果在整个测量温区均呈现半导体导电行为，没有出现金属－绝缘体(M-I)转变和CMR效应；电荷有序转变发生在T=270K左右，反铁磁转变温度出现在200K附近，且表现出典型的再入型自旋玻璃(spin-glass)行为，自旋玻璃转变温度T_SG在4 关键词： 自旋玻璃 电荷有序 负磁化现象 多相竞争