Inhomogeneous magnetostriction states in uniaxial ferromagnetic films

Surface magnetoelastic Love waves and nonuniform distributions of the magnetization and elastic strains are investigated in a uniaxial ferromagnetic film on a massive nonmagnetic substrate in a tangential external magnetic field. A new inhomogeneous phase is predicted having spatial modulation of the order parameter, arising from magnetostrictive coupling of the magnetization with lattice strains near the interface of the magnetoelastic and elastic media. It is shown that, at some critical magnetic field H _c, different from the orientational transition field in an isolated sample, a magnetoelastic Love wave propagating parallel to the magnetization vector in the film plane becomes unstable. The frequency and group velocity of the wave vanish at wave number k=k _c≠0 and the wave freezes, forming a domain structure localized in the film and adjoining substrate. Fiz. Tverd. Tela (St. Petersburg) 41, 665–671 (April 1999)     

Protected nodal electron pocket from multiple-Q ordering in underdoped high temperature superconductors

A multiple wave vector (Q) reconstruction of the Fermi surface is shown to yield a profoundly different electronic structure to that characteristic of single wave vector reconstruction, despite their proximity in energy. We consider the specific case in which ordering is generated by Q(x)=[2πa,0] and Q(y)=[0,2πb] (in which a=b=1/4)-similar to those identified in neutron diffraction and scanning tunneling microscopy experiments-and more generally show that an isolated pocket adjacent to the nodal point k(nodal)=[±π/2,±π/2] is a protected feature of such a multiple-Q model, potentially corresponding to the nodal "Fermi arcs" observed in photoemission and the small size of the electronic heat capacity found in high magnetic fields-importantly, containing electron carriers which can yield negative Hall and Seebeck coefficients observed in high magnetic fields.     

Origin of magnetic and magnetoelastic tweedlike precursor modulations in ferroic materials

Based on experimental observations of modulated magnetic patterns in a Co0.5Ni0.205Ga0.295 alloy, we propose a model to describe a (purely) magnetic tweed and a magnetoelastic tweed. The former arises above the Curie (or Néel) temperature due to magnetic disorder. The latter results from compositional fluctuations coupling to strain and then to magnetism through the magnetoelastic interaction above the structural transition temperature. We discuss the origin of purely magnetic and magnetoelastic precursor modulations and their experimental thermodynamic signatures.     

A Combined Study of the Pion’s Static Properties and form Factors

We study consistently the pion’s static observables and the elastic and γ* γ → π ⁰ transition form factors within a light-front model. Consistency requires that all calculations are performed within a given model with the same and single adjusted length or mass-scale parameter of the associated pion bound-state wave function. Our results agree well with all extent data including recent Belle data on the γ* γ → π ⁰ form factor at large q ², yet the BaBar data on this transition form factor resists a sensible comparison. We relax the initial constraint on the bound-state wave function and show the BaBar data can partially be accommodated. This, however, comes at the cost of a hard elastic form factor not in agreement with experiment. Moreover, the pion charge radius is about 40 % smaller than its experimentally determined value. It is argued that a decreasing charge radius produces an ever harder form factor with a bound-state amplitude difficultly reconcilable with soft QCD. We also discuss why vector dominance type models for the photon-quark vertex, based on analyticity and crossing symmetry, are unlikely to reproduce the litigious transition form factor data.     

Hybridization wave as the "hidden order" in URu2Si2

A phenomenological model for the "hidden order" transition in the heavy-Fermion material URu(2)Si(2) is introduced. The hidden order is identified as an incommensurate, momentum-carrying hybridization between the light hole band and the heavy electron band. This modulated hybridization appears after a Fano hybridization at higher temperatures takes place. We focus on the hybridization wave as the order parameter in URu(2)Si(2) and possibly other materials with similar band structures. The model is qualitatively consistent with numerous experimental results obtained from, e.g., neutron scattering and scanning tunneling microscopy. Specifically, we find a gaplike feature in the density of states and the appearance of features at an incommensurate vector Q(*)～0.6π/a(0). Finally, the model allows us to make various predictions which are amenable to current experiments.     

Effect of mechanical stresses on the magnetoelastic properties of TmVO4

In this paper the influence of mechanical stress on magnetoelastic properties, i.e., magnetostriction and thermal expansion in the neighborhood of a structural phase transition of the Jahn-Teller crystal TmVO₄ is investigated experimentally and theoretically. It is shown that the magnetoelastic properties of TmVO₄ for a magnetic field H∥[001] do not change the domain structure of the sample, which is rather well described when mechanical stresses in the crystal are taken into account using the parameter . Conversely, for magnetic fields along the direction of spontaneous strain [110] the magnetoelastic properties are primarily caused by reorientation of the Jahn-Teller domains and short-range order effects. It is shown that the “true” magnetostriction of a single-domain crystal for H∥[110] diverges at the phase transition point T _c=2.15 K in the absence of mechanical stresses and is strongly decreased by these stresses. Fiz. Tverd. Tela (St. Petersburg) 40, 701–705 (April 1998)     

Pulsed magnetization reversal of single-crystal iron borate in the presence of a transverse magnetic field

This paper describes the first investigations of the process of pulsed 180° magnetization reversal in iron borate in the presence of a transverse magnetic field. How the intensity of magnetoelastic oscillations depends on the amplitude of the magnetization reversal field and the duration of the primary period of the transient process is studied, and also the analysis of hodographs of the magnetization vector, which show that the primary reason why the pulsed magnetization reversal curve exhibits a kink is a decrease in the energy lost to excitation of magnetoelastic oscillations, caused by lagging of the elastic subsystem of the crystal behind the magnetic subsystem for magnetization reversal times less than 13–16 ns. Fiz. Tverd. Tela (St. Petersburg) 40, 305–309 (February 1998)     

Electronic structures of ternary iron arsenides A Fe 2 As 2 ( A = Ba,Ca, or Sr)

We have studied the electronic and magnetic structures of the ternary iron arsenides AFe₂As₂ (A = Ba, Ca, or Sr) using the first-principles density functional theory. The ground states of these compounds are in a collinear antiferromagnetic order, resulting from the interplay between the nearest and the next-nearest neighbor superexchange antiferromagnetic interactions bridged by As 4p orbitals. The correction from the spin–orbit interaction to the electronic band structure is given. The pressure can reduce dramatically the magnetic moment and diminish the collinear antiferromagnetic order. Based on the calculations, we propose that the low energy dynamics of these materials can be described effectively by a t−J_H−J₁−J₂-type model [arXiv: 0806.3526v2, 2008].     

Measurement of magnetic excitations in the two-dimensional antiferromagnetic Sr₂CuO₂Cl₂ insulator using resonant x-ray scattering: evidence for extended interactions

We measured the momentum dependence of magnetic excitations in the model spin-1/2 2D antiferromagnetic insulator Sr2CuO2Cl2 (SCOC). We identify a single-spin-wave feature and a multimagnon continuum, with different polarization dependences. The spin waves display a large (70 meV) dispersion between the zone-boundary points (π, 0) and (π/2, π/2). Employing an extended t-t'-t'-U one-band Hubbard model, we find significant electronic hopping beyond nearest-neighbor Cu ions, indicative of extended magnetic interactions. The spectral line shape at (π, 0) indicates sizable quantum effects in SCOC and probably more generally in the cuprates.     

High-energy non-resonant X-ray magnetic scattering from EuAs3

We have investigated non-resonant high energy X-ray magnetic scattering from EuAs₃ both in the antiferromagnetic and in the incommensurate phase by using an X-ray energy of 104 and 106 keV. In the antiferromagnetic phase, we obtained a signal to background ratio of about 10:1 for the magnetic Bragg peak at Q=(−1,0,1/2) and a maximum count rate of about 200 counts/s at T=3.1 K. To our knowledge this is the first reported observation of the non-resonant magnetic signal from a rare-earth ion at X-ray energy as high as 106 keV. The temperature dependence of the integrated intensity of the (−1,0,1/2); magnetic reflection has been measured and compared with that obtained previously by neutron diffraction. We measured the integrated intensities of several magnetic reflections from the antiferromagnetic phase and have compared them with those calculated from the magnetic structure model derived from neutron diffraction. The intensities of the magnetic satellite reflections from the incommensurate phase have been measured and have been found to be very weak. We also investigated the temperature variation of the lattice spacing close to the magnetic ordering transition and have found a large magnetoelastic anomaly at the lock-in phase transition.     

Angle-resolved photoemission spectroscopy of the iron-chalcogenide superconductor Fe1.03Te0.7Se0.3: strong coupling behavior and the universality of interband scattering

We have performed angle-resolved photoemission spectroscopy of the iron-chalcogenide superconductor Fe1.03Te0.7Se0.3 to investigate the electronic structure relevant to superconductivity. We observed a holelike Fermi surface (FS) and an electronlike FS at the Brillouin zone center and corner, respectively, which are nearly nested by the Q～(π,π) wave vector. We do not find evidence for the nesting instability with Q～(π+δ,0) reminiscent of the antiferromagnetic order in the parent compound Fe1+yTe. We have observed an isotropic superconducting (SC) gap along the holelike FS with the gap size Δ of ～4 meV (2Δ/kBTc ～ 7), demonstrating the strong-coupling superconductivity. The observed similarity of low-energy electronic excitations between iron-chalcogenides and iron-arsenides strongly suggests that common interactions which involve Q～(π,π) scattering are responsible for the SC pairing.     

Nature of unusual spontaneous and field-induced phase transitions in multiferroics RMn2O5

Complex magnetic, magnetoelectric and magnetoelastic studies of spontaneous and field-induced phase transitions in TmMn₂O₅ were carried out. In the vicinity of spontaneous phase transition temperatures (35 and 25 K) the magnetoelectric and magnetoelastic dependences demonstrated the jumps of polarization and magnetostriction induced by the field ∼150 kOe. These anomalies can be attributed to the influence of magnetic field on the conditions of incommensurate-commensurate phase transition at 35 K and the reverse one at 25 K. In b-axis dependences the magnetic field-induced spin-reorientation phase transition was also observed below 20 K. Finally the magnetoelectric anomaly associated with metamagnetic transition is observed below the temperature of rare-earth subsystem ordering at relatively small critical fields of 5 kOe. This variety of spontaneous and induced phase transitions in RMn₂O₅ stems from the interplay of three magnetic subsystems: Mn³⁺, Mn⁴⁺, R³⁺. The comparison with YMn₂O₅ highlights the role of rare earth in low-temperature region (metamagnetic and spin-reorientation phase transitions), while the phase transition at higher temperatures between incommensurate and commensurate phases should be ascribed to the different temperature dependences of Mn³⁺ and Mn⁴⁺ ions. The strong correlation of magnetoelastic and magnetoelectric properties observed in the whole class of RMn₂O₅ highlights their multiferroic nature.     

Thermal expansion and magnetostriction of GdAg2, and relations to the magnetoelastic paradox

The antiferromagnet GdAg₂ has been shown to be a good model system for the magnetoelastic paradox (MEP), because it exhibits large symmetry conserving magnetoelastic strains and the antiferromagnetic propagation vector breaks the tetragonal lattice symmetry (therefore a large symmetry breaking magnetoelastic strain can be expected in a single magnetic structure). As in many similar Gd based compounds no symmetry breaking strain has been found in the experiment. In order to investigate this MEP further, we have measured magnetostriction and magnetization on a textured polycrystal. The behaviour closely resembles that of GdNi₂B₂C, the prototype system for the magnetoelastic paradox (MEP). Our forced magnetostriction data indicate that the crystal distorts in applied magnetic field and gives further evidence that the MEP is a low field effect. The observed phase transitions are in agreement with available specific heat and neutron diffraction data. Moreover, the saturation magnetic field was measured in high pulsed magnetic fields and agrees well with the value calculated from the Standard Model of Rare Earth Magnetism (SMREM).     

Magnetostriction domain structure in a periodic system of magnetoelastic and elastic nonmagnetic layers

The spectrum of magnetoelastic waves in a periodic structure of alternating ferromagnetic and nonmagnetic layers was studied. In the case of ferromagnetic layers with easy magnetization axes parallel to the layer surfaces, an orientational phase transition induced by an external tangential magnetic field H_e was considered. The formation of an inhomogeneous phase with a spatially modulated order parameter, which is caused by the magnetization being coupled through magnetostriction to lattice strains near the interfaces separating the magnetoelastic from elastic media, is predicted. It is shown that at a certain critical field in excess of the orientational phase transition field in the system without magnetostriction, a magnetoelastic wave propagating in a direction parallel to the in-plane magnetization vector M becomes unstable at finite values of the wave vector and condenses into a magnetostriction domain structure. A phase diagram in the (L, T, H_e) coordinates is constructed, and the regions of existence of thermodynamically equilibrium collinear, canted, and domain phases are established (L and T are the thicknesses of the ferromagnetic and nonmagnetic layers, respectively).     

Incommensurate magnetic order in TbTe3

We report a neutron diffraction study of the magnetic phase transitions in the charge-density wave (CDW) TbTe(3) compound. We discover that in the paramagnetic phase there are strong 2D-like magnetic correlations, consistent with the pronounced anisotropy of the chemical structure. A long-range incommensurate magnetic order emerges in TbTe(3) at T(mag1) = 5.78 K as a result of continuous phase transitions. We observe that near the temperature T(mag1) the magnetic Bragg peaks appear around the position (0, 0, 0.24) (or its rational multiples), that is fairly close to the propagation vector (0,0,0.29) associated with the CDW phase transition in TbTe(3). This suggests that correlations leading to the long-range magnetic order in TbTe(3) are linked to the modulations that occur in the CDW state.     

磁性斯格明子晶格的磁弹现象与机理

近年来,人们在一些具有手性相互作用的磁性体材料及薄膜中成功观测到具有非平凡拓扑性质的二维自旋结构,称作磁性斯格明子.在大部分情况下,磁性斯格明子自发地聚集成一种晶格结构,称作斯格明子晶格.孤立的斯格明子由于其奇特的拓扑性质以及优异的电流驱动性质等"局域化特征"受到人们的广泛关注.与此相对,斯格明子晶格作为一种新颖的宏观磁性相,可能与材料固有的多场耦合性质发生相互作用进而引发许多奇特的宏观物理现象乃至新性质.在此范畴内,人们发现由于手征磁体内禀的磁弹耦合,斯格明子晶格不但对材料的力学性质产生影响,而且在外力作用下自身具备"层展的弹性性质".本文对相关现象进行梳理,并基于一种针对B20族手征磁体磁弹耦合效应普遍适用的热力学唯象模型,逐一简述对于不同类型的磁弹现象如何建模分析,进而给出其中一部分现象的实验与理论结果比对.最后,对这一领域的发展提出几个可供进一步探索的方向.     

Commensurate spin density wave in LaFeAsO: a local probe study

We present a detailed study on the magnetic order in the undoped mother compound LaFeAsO of the recently discovered Fe-based superconductor LaFeAsO1-xFx. In particular, we present local probe measurements of the magnetic properties of LaFeAsO by means of 57Fe M?ssbauer spectroscopy and muon-spin relaxation in zero external field along with magnetization and resistivity studies. These experiments prove a commensurate static magnetic order with a strongly reduced ordered moment of 0.25(5)muB at the iron site below T(N)=138 K, well separated from a structural phase transition at T(S)=156 K. The temperature dependence of the sublattice magnetization is determined and compared to theory. Using a four-band spin density wave model both, the size of the order parameter and the quick saturation below T(N) are reproduced.     

Determination of the magnetic structure of SmFe(3)(BO(3))(4) by neutron diffraction: comparison with other RFe(3)(BO(3))(4) iron borates

Temperature dependent neutron diffraction studies were performed on SmFe(3)(BO(3))(4). The crystallographic structure was determined to stay as R32 over the whole studied temperature range of 2?K?相似文献   

Reflection and transmission of SH-waves at a corrugated interface between two semi-infinite anisotropic magnetoelastic half-spaces

An analysis has been carried out for reflection and transmission of a plane SH-wave incident at a corrugated interface between two anisotropic magnetoelastic half-spaces. Rayleigh’s method of approximation is applied to derive the reflection and transmission coefficients for first order approximation of corrugation. The expressions for reflection and transmission coefficients for first order approximation of corrugation are obtained in closed form for a special type of interface. It is found that these coefficients are proportional to the amplitude of corrugation and are functions of magnetoelastic properties of materials of the half-spaces as well as the angle of incidence. Special cases are deduced for anisotropic and isotropic materials for particular case of corrugation. Numerical computations are performed for a specific model of two different anisotropic magnetoelastic media. The effects of anisotropic magnetoelastic coupling parameter, the angle at which wave crosses the magnetic field, frequency factor, wave length of corrugation, and the amplitude of corrugation are shown through figures.