Magnons versus free spinons in finite quantum frustrated antiferromagnets

We have investigated the validity of doping with a vacancy the J₁–J₃ frustrated Heisenberg model on a finite square lattice as a way to test the existence of fractional spin excitations. Using a generalized t–J₁–J₃ model we have computed the vacancy spectral functions in the self-consistent Born approximation. We have found that by including spiral fluctuations in the magnetic ground state, the spectral functions on finite systems agree very well with the unbiased exact ones. In contrast to the recent proposal that the quasiparticle weight reduction could be a signal of a spinon free excitation in finite systems, we have found strong evidence that such a reduction is due to the existence of spiral fluctuations.     

Tunneling conductance in topological insulator ferromagnet/p-wave superconductor junctions

The tunneling conductance in topological insulator (TI) ferromagnet/p-wave superconductor (FM/pS) junction is studied based on the Blonder–Tinkham–Klapwijk (BTK) theory. The Fermi energy mismatch between FM and pS as well as the finite quasiparticle lifetime are considered. Three kinds of pairings p_x, p_y, and p_x+ip_y-waves for pS are chosen. It is found that the spectrum strongly depend on the magnetic gap, the gate potential, the quasiparticle lifetime as well as the type of the pair potential symmetry. The pair potential symmetry drastically affects the formation of the zero-energy bound states dependent on the magneto effect or the Fermi energy mismatch effect. The finite quasiparticle lifetime effect can suppress the Andreev resonant scattering process at eV=Δ₀ and smear the dips in the conductance.     

Magnetic field and external pressure effects on the spiral order of polycrystalline MnCr2O4

In this paper, the magnetic and specific heat properties of polycrystalline MnCr₂O₄ were investigated, mainly by focusing on the spiral order transition around T_s. With increasing magnetic field, the magnetic anomaly around T_s in the M–T curve is gradually suppressed. However, an external magnetic field up to 5 T has no evident influence on the spiral magnetic transition as revealed by specific heat measurements. Upon cooling below T_s, AC susceptibility displays no frequency dependence, but the coercivity increases abruptly. Applying external pressure increases the coercivity at 5 K, implying the strengthening of the spiral order. It is suggested that with decreasing temperature across T_s, the spiral component develops in the direction perpendicular to the easy axis of the parent collinear ferri-magnetic phase and does not contribute to the saturation magnetization.     

Thermodynamic properties of LiCu2O2 multiferroic compound

A spin model of quasi-one-dimensional LiCu₂O₂ compound with ground state of ellipsoidal helical structure has been adopted. The helical axis is along the diagonal of CuO₄ squares. By taking into account the interchain coupling and exchange anisotropy, the exotic magnetic properties and ferroelectricity induced by spiral spin order have been studied by performing Monte Carlo simulation. The simulation results qualitatively reproduce the main characters of ferroelectric and magnetic behaviors of LiCu₂O₂ compound and confirm the low-temperature noncollinear spiral ordering. Furthermore, by performing the calculations of spin structure factor, we systematically investigate the effects of different exchange couplings on the lower-temperature magnetic transition, and find that the spiral spin order depends not only on the ratio of nearest and next-nearest neighbor inchain spin coupling but also strongly on the exchange anisotropy.     

Hartree-Fock theory of spiral magnetic order in the 2-d Hubbard model

The magnetic order in the 2-d Hubbard model is investigated within Hartree-Fock theory. For the class of states with uniform particle density and spiral arrangement of spins the phase diagram is obtained by minimizing the free energy. At zero temperature and large Hubbard interactionU there is a continuous transition from the antiferromagnetic solution at half filling over a spiral state of increasing wavelength along the diagonal of the lattice to the ferromagnetic state at doping _c 2t/U. At finite temperatureT, the antiferromagnetic state remains stable for doping smaller than _AF 2T/U. For intermediate values ofU and finite doping there exists also a phase with a spiral wave vector of the form Q=(Q, ).     

Peculiarities of the band magnetism in the higher manganese silicide

Magnetism of MnSi_x, x=1.746 (Mn₂₇Si₄₇) was investigated by SANS, neutron diffraction, magnetization and magnetic susceptibility measurements. MnSi_x single crystalline specimens were characterized by X-ray and neutron diffraction. A spiral spin structure with periodicity ?=(163±4) Å along the c axis and a spiral component of the magnetic moment per Mn of p_o=0.056 was determined. From the field dependence of ? it is indicated that the magnetic order below T_N=42 K is an incommensurate state. From the large difference of the magnetic moments in the paramagnetic state and ordered state MnSi_x is classified as an itinerant magnet. Below T_N the difference of the magnetic moments per Mn between p_o=0.056 at H = 0 from SANS and p_s = 0.014 at saturation field from magnetization measurements is explained by longitudinal spin fluctuations.     

Influence of the Jahn—Teller distortion on magnetic ordering in TbMn1-xFexO3

The Jahn-Teller distortion plays an important role in determining the exchange interaction in rare-earth manganites.In this work we study the influence of the Jahn-Teller distortion on the magnetic structures of TbMn1-xFexO3（x = 0,0.02,0.05,0.10,and 0.20） single crystals in the basal MnO2 plane.The decrease in the quadruple splitting with the increasing Fe doping indicates the reduction of the Jahn-Teller distortion,which makes the nearest neighboring（NN） FM interaction dominant over the next nearest neighbor（NNN） AFM interaction.This alteration is favorable for the development of A-type AFM ordering instead of the spiral magnetic ordering,which collapses when x ≥ 0.05.The analysis of dielectric data indicates that the ferroelectricity is arising from the peculiar spiral magnetic ordering.     

中频磁脉冲处理Fe52Co34Hf7B6Cu1非晶合金的正电子湮没研究

对熔体急冷法制备的非晶合金 Fe₅₂Co₃₄Hf₇B₆Cu₁ 进行了不同频率的中频磁脉冲处理, 用透射电子显微镜、穆斯堡尔谱、正电子湮没寿命谱等方法研究了处 理前后试样的微观结构及结构缺陷变化. 结果表明,经中频磁脉冲处理后,样品发生了部分纳米晶化, 晶化量随磁脉冲频率增加而增加, 当磁脉冲频率为2000 Hz时, 晶化量达33.1%; 在淬态非晶样品中, 正电子在类单空位中的湮没寿命τ₁为150.5 ps, 强度 I₁为77.7%, 在微孔洞中的湮没寿命τ₂为349.7 ps,强度I₂为22.3%; 随磁脉冲频率的增加, τ₁, τ₂值呈现减小的变化趋势, 与淬态非晶相比, I₁有所增加, I₂下降, τ₁, τ₂的平均值τ大幅降低.     

Magnetic Phase Transitions in Substituted Spinels of Zn1−x Cu χ Cr2 Se4; 0.0 ≤ x ≤ 0.3

Magnetic and crystallographic properties have been studied by neutron powder diffraction and measurements of magnetization and magnetization hysteresis-loops for substituted spinels of Zn_1?xCu_xCr₂Se₄ with 0.0≤x≤0.3. It is found that the Zn_0.85Cu_0.15Cr₂Se₄ spinel has two magnetic phase transitions at 23.0 K (Néel temperature; T _N) and 410 K (Curie temperature; T _C) and that the Zn_0.70Cu_0.30Cr₂Se₄ spinel has magnetic transitions at 24.5 K (T _N) and 415 K (T _C) on heating. The low-temperature magnetic phase transition is from a spiral antiferromagnet to a ferromagnet, and the high-temperature magnetic phase transition is from a ferromagnet to a paramagnet, while ZnCr₂Se₄ shows a magnetic phase transition only from a spiral antiferromagnet to a paramagnet at about 21.0 K. From neutron powder diffraction, it is also found that the spinels of Zn_1?x Cu _x Cr₂Se₄; 0.0 ≤ x ≤ 0.3. show satellite-like magnetic reflection having indexes (h ± Q, k, l) with Q = 0.470 below T _N and short-range order of spins (spin glass-like) above T _N. The incommensurate antiferromagnetic phase below T _N results from a spiral long-range order of the spins of Cr³⁺. The intermediate ferromagnetic phase between T _N and T _C is related not to the spiral spin order but to double-exchange magnetic interaction among Cr³⁺ and Cr⁴⁺ mediated by current carriers, positive holes, which is made by the substitution of Zn²⁺ ions with Cu¹⁺ ions in Zn_1?x Cu _x Cr₂Se₄.     

The influence of hexagonal anisotropy on the magnetic structure of antiferromagnetic rare earth metals and alloys

The effect of hexagonal base plane anisotropy on the antiferroniagnetic (AF) spiral spin structure of rare earth metals and alloys has been investigated. Minimization of the anisotropy energy is considered either by a reorientation of rigid coherent sections of the spiral or by a continuous modulation of the spiral periodicity. Both results indicate that when the spiral periodicity is commensurate with the hexagonal anisotropy the energy is greater than for spirals which are non-commensurate. The calculations predict that when the temperature is varied in the AF phase, the magnetic structure becomes unstable as a commensurate turn angle is approached, which may provide a qualitative explanation for the abrupt changes in the c-axis thermal expansion and temperature dependence of the elastic constant C₄₄ that have been observed experimentally in dysprosium and terbium-50% holmium. Further consequences of the spiral perturbation by hexagonal anisotropy are considered.     

Crystal and magnetic structure of the CoMn1−xTixSi system

The magnetic and crystallographic nature of the compounds CoMn_1?xTi_xSi (0?x?0.5) is studied by X-ray, neutron diffraction and magnetometric measurements. It is shown that the solid solutions arise for 0?x?0.5, with the crystal structure of the NiTiSi type. All compounds are antiferromagnets. The magnetic moment localized on Mn and Co atoms forms a double spiral in both sites. The dilution of the Mn sublattice by Ti atoms change only slightly the magnetic properties - Néel temperature and parameters of magnetic structure.     

Crystalline and magnetic structures in REAlxGa2 − x systems (RE = Nd,Er; 0 ⪕ x ⪕ 2): Part II: Magnetic structures

Part II describes results on bulk magnetic properties, transition temperatures, magnetic structures and magnetic moments found for the NdAl_xGa_{2 ? x} and ErAl_xGa_{2 ? x} pseudoternary solid solution systems (0 ? x ? 2). Characterization techniques include electron microprobe analysis, induction magnetometer, X-ray and neutron diffraction. We found ferromagnetic as well as antiferromagnetic order with parallel and incommensurate spiral structures. These spiral structures are of cycloidal or triangular screw axis type.     

Pinning of spiral fluxons by giant screw dislocations in YBa2Cu3O7 − δ single crystals: Josephson analog of the fishtail effect

By using a highly sensitive homemade AC magnetic susceptibility technique, the magnetic flux penetration has been measured in YBa₂Cu₃O_{7 ? δ} single crystals with giant screw dislocations (having the structure of the Archimedean spirals) exhibiting a = 3 spiral turnings, the pitch b = 18.7 μm and the step height c = 1.2 nm (the last parameter is responsible for creation of extended weak-link structure around the giant defects). The magnetic field applied parallel to the surface enters winding around the weak-link regions of the screw in the form of the so-called spiral Josephson fluxons characterized by the temperature dependent pitch b _f (T). For a given temperature, a stabilization of the fluxon structure occurs when b _f (T) matches b (meaning an optimal pinning by the screw dislocations) and manifests itself as a pronounced low-field peak in the dependence of the susceptibility on magnetic field (applied normally to the surface) in the form resembling the high-field (Abrikosov) fishtail effect.     

Penetration of electromagnetic field through the La-Er-Ba manganite far above the magnetic phase transition temperature

Using the technique of penetration of electromagnetic waves through the bulk samples of La_0.60Er_0.07Ba_0.33MnO₃ manganite, it is shown that in this class of strongly correlated materials the local dynamic magnetic ordering is preserved at the temperature by 66 K above the Curie temperature where the static long-range magnetic order is absent. In this temperature range the dynamic magnetic permeability exceeds unity and is frequency dependent. The average lifetime of local ordered states is estimated.     

Magnetic field effects on synthesis process of high-Tc superconductors

Materials synthesis processes in high magnetic fields are investigated for high-T_c superconductors at the High Field Laboratory for Superconducting Materials, Institute for Materials Research, Tohoku University. On a melt growth process of YBa₂Cu₃O₇ bulk in magnetic fields, it was found that the crystallinity is improved. These results are due to not only the magnetic field orientation but also the decrease of the crystal growth rate by the magnetic field, which is suggested from a differential temperature analysis measurement in magnetic fields. On a chemical vapor deposition process of YBa₂Cu₃O₇ films deposited on the polycrystalline Ag substrates in high magnetic fields, the microstructure changes drastically from the rectangular large grains with 10 μm in size to the small complex shaped grains. The spiral growth mode is observed for the samples prepared at 0 T but it changes into the island growth mode for that prepared in the presence of a magnetic filed. On a partial-melt process of Bi₂Sr₂CaCu₂O₈ tapes in magnetic fields, the homogeneous texture is obtained and it results in the increase of the critical current density.     

Magnetoelastic effects in terbium

The variation of the elastic modulus C₃₃ of terbium has been investigated as a function of temperature in the range 200–230 K, which includes the whole of the antiferromagnetic phase, and as a function of magnetic field applied along the easy magnetic direction, the b axis. Hysteresis in C₃₃ in the antiferromagnetic phase is interpreted in terms of spiral spin domains. The magnetic phase changes are reflected in anomalies in the elastic constant and these are used to produce a magnetic phase diagram of terbium. The final phase diagram has been compared with earlier measurements of magnetisation and magnetostriction.     

First-principles theory of magnetically induced ferroelectricity in TbMnO<Subscript>3</Subscript>

We study the polarization induced via spin-orbit interaction by a magnetic cycloidal order in orthorhombic TbMnO₃ using first-principle methods. The case of magnetic spiral lying in the b-c plane is analyzed, in which the pure electronic contribution to the polarization is shown to be small. We focus our attention on the lattice-mediated contribution, and study it’s dependence on the Coulomb interaction parameter U in the LDA+U method and on the wave-vector of the spin spiral. The role of the spin-orbit interaction on different sites is also analyzed.     

Effect of magnetic annealing on phonon behaviors of nanocrystalline BiFeO3

Influence of magnetic annealing at 823 K up to 10 T (T) on the phonon behaviors of nanocrystalline BiFeO₃ was investigated by Raman spectroscopy. The frequencies of fundamental Raman modes increase obviously with increasing annealing magnetic field, and the intensity of the 1260 cm⁻¹ two-phonon mode decreases. The pronounced anomalies of Raman phonon modes under magnetic annealing are attributed to the change of the spin-phonon coupling due to the modulation of spiral spin order. Furthermore, the temperature dependence of Raman peak positions, for the two prominent modes (147 and 176 cm⁻¹), show no notable anomaly around T_N except the sample annealed under 10 T magnetic field; meanwhile, in this sample, another obvious phonon anomaly occurs at ∼150 K (another magnetic phase transition point), which indicate that stronger magnetic annealing with 10 T intensely enhances the spin-phonon coupling, and possibly increases magnetoelectric coupling of nanocrystalline BiFeO₃ due to severely modulation of spiral spin order.     

A phenomenological theory for polarization flop in spiral multiferroic TbMnO<Subscript>3</Subscript>

A phenomenological Landau theory has been used to explain magnetic field-driven polarization flop in TbMnO ₃. The Néel wall-like magnetic structure in spiral multiferroics induces a space-dependent internal magnetic field which exerts a torque on spins to rotate bc-spiral to ab-spiral. The external magnetic field is argued to be competing with easy axis anisotropy and the system stabilizes when anisotropy is minimum. With the help of Landau free energy with DM magnetoelectric coupling and a general ansatz for magnetization, the phenomenon of polarization flop has been explained. Relation between T_flop and critical magnetic field has been established and found to be in good agreement with the experiment. This could be an indication that anisotropy of the system is temperature- and magnetic field-dependent.     

Spin-wave spectrum of a two-dimensional itinerant electron system: Analytic results for the incommensurate spiral phase in the strong-coupling limit

We study the zero-temperature spin fluctuations of a two-dimensional itinerant-electron system with an incommensurate magnetic ground state described by a single-band Hubbard Hamiltonian. We introduce the (broken-symmetry) magnetic phase at the mean-field (Hartree-Fock) level through a spiral spin configuration with characteristic wave vector Q different in general from the antiferromagnetic wave vector Q _AF, and consider spin fluctuations over and above it within the electronic random-phase (RPA) approximation. We obtain a closed system of equations for the generalized wave vector and frequency dependent susceptibilities, which are equivalent to the ones reported recently by Brenig. We obtain, in addition, analytic results for the spin-wave dispersion relation in the strong-coupling limit of the Hubbard Hamiltonian and find that at finite doping the spin-wave dispersion relation has a hybrid form between that associated with the (localized) Heisenberg model and that associated with the (long-range) RKKY exchange interaction. We also find an instability of the spin-wave spectrum in a finite region about the center of the Brillouin zone, which signals a physical instability toward a different spin- or, possibly, charge-ordered phase, as, for example, the stripe structures observed in the high-T _c materials. We expect, however, on physical grounds that for wave vectors external to this region the spin-wave spectrum that we have determined should survive consideration of more sophisticated mean-field solutions. Received 15 September 2000