KCoF_3的水热合成及发光性质

以水和乙醇混合液为反应溶剂,利用水热法合成了立方相钙钛矿结构的KCoF3氟配合物立方颗粒,发现晶粒的尺寸受反应时间的影响非常明显,并且晶体的发光性质与晶粒尺寸有密切的关系,尺寸为1.7μm的样品在429,590nm处有两处弱发光峰,纳米尺寸的颗粒则在423和614nm处有两处强发射,长波长处的发射与2A1g(2G)能级的跃迁有关;而在短波长处的弱发射则和2T1g(2G)能级相联系。     

Anisotropic magnetic exchange in orthorhombic RCu2 compounds (R = rare earth)

The magnetic excitations in the field induced ferromagnetic phase F3 of a NdCu₂ single crystal were investigated by means of inelastic neutron scattering experiments. A mean field model using the random phase approximation in connection with anisotropic magnetic bilinear R-R (R denotes a rare earth) exchange interactions is proposed to account for the observed dispersion. The relevance of this model to the analysis of the magnetic ordering process in other RCu₂ compounds is discussed. Received 21 April 1999     

Dual model for magnetic excitations and superconductivity in UPd 2 Al 3

The Heavy Fermion state in UPd₂Al₃ may be approximately described by a dual model where two of the three U-5 f electrons are in a localized state split by the crystalline electric field into two low lying singlets with a splitting energy Δ≃ 6 meV. The third 5 f electron has itinerant character and forms the Heavy Electron bands. Inelastic neutron scattering and tunneling experiments suggest that magnetic excitons, the collective propagating crystal field excitations of the localized 5 f electrons, mediate superconducting (sc) pairing in UPd₂Al₃. A theory for this novel mechanism is developed within a nonretarded approach. A model for the magnetic exciton bands is analyzed and compared with experiment. The sc pair potential which they mediate is derived and the gap equations are solved. It is shown that this mechanism favors an odd parity state which is nondegenerate due to the combined symmetry breaking by the crystalline electric field and the AF order parameter. A hybrid model including the spin fluctuation contribution to the pairing is also discussed. Received 22 October 2001 and Received in final form 28 February 2002     

Magnetization and magnetic excitations in HoAl2

The magnetization of HoAl₂ single crystals parallel and perpendicular to the applied field for three crystallographic directions has been measured between 4.2 and 300 K in fields up to 8 T. Further the spin wave excitations were investigated by inelastic neutron scattering at 5.2 K along <100>, <110> and <111> directions in reciprocal space. The results are interpreted consistently in terms of a cubic crystal field using a simple molecular field to calculate the magnetization and an isotropic Heisenberg exchange interaction to obtain the magnetic excitations.     

Magnetic excitations and phase transition of CsFeBr3 in an external magnetic field

In CSFeBr₃ the Fe²⁺ ion with effective spin one has locally a singlet ground state (m=0). The antiferromagnetic interactions between neighbouring Fe-ions are too weak as compared with the anisotropy constant to introduce long range order in the absence of an external field. By inelastic neutron scattering we studied the magnetic excitations in an external magnetic field up to 5 Tesla applied along thec-axis. A linear Zeeman splitting was observed with a Landé factorg=2.4. The field renormalizes the dispersion curves in such a way that the exchange interaction has decreasing influence with increasing field. Theoretical calculations according to the excitonic model of Lindgård describe the experimental results very well. At 4.1 Tesla a phase transition appears to a commensurate long range order with a 120° arrangement of the spins in the hexagonal plane. Within the limits of experimental observation this phase transition has no influence on the dynamical behaviour. No critical phenomena could be observed. The dynamical structure factor |G _j()|² of the lower Zeeman split modes decreases with increasing field.     

Anisotropic ferromagnet with biquadratic exchange

A formulation, using the double time Green’s function method, is presented for the anisotropic spin-one ferromagnetic system in the presence of Ising part of the biquadratic exchange and an expression is given for the critical temperatureT _c in terms of two parameters. One is associated with the uniaxial anisotropy (treated exactly) and the other one corresponds to the biquadratic interaction (treated in RPA). The bilinear Heisenberg exchange term is treated in Callen’s approximation emphasizing the role of intersite correlation for small parameters. The prediction thatT _c increases with the biquadratic exchange is found to be consistent with the recent calculation based on high temperature series expansion.     

Current-driven excitations in symmetric magnetic nanopillars

We study experimentally the current-driven magnetic excitations in symmetric Co/Cu/Co nanopillars. In contrast with all the previous observations where the current of only one polarity is capable of exciting a multilayer system saturated by an externally applied magnetic field, we observe that both polarities of the applied current trigger excitations in a symmetric multilayer. This may indicate that in symmetric structures the current propels high-frequency magnetic oscillations in all magnetic layers. We argue, however, that only one layer is excited in our multilayers but, interestingly, currents of opposite polarities excite different layers. This hypothesis is supported by modeling the spin accumulation in symmetric magnetic multilayers.     

Locally nonequilibrium magnetic excitations in insulators

The equation of motion for the magnetic moment vector M in a locally nonequilibrium medium is derived. The dispersion and attenuation of the coupled modes of the magnetic vector potential and magnetization are determined. It is shown that the continuous spectrum contains frequencies corresponding to undamped waves or constant-phase damped waves.     

On the magnetic excitations in nickel

By comparing measured and calculated versions of the generalized magnetic response function of nickel over a wide range of temperatures it is shown that the much-discussed simple model of an itinerant-electron ferromagnet based on a random-phase-approximation treatment of the Hubbard Hamiltonian accounts, in its essentials, for the magnetism of that substance. Thermal neutron inelastic scattering techniques were used to explore the whole Brillouin zone of wave vectors K in nickel up to energies ▄ω of some 2k _B T _C over the range of temperatures up to 2T _C; the computations were based on direct evaluation of the Lindhard expression for the generalized susceptibility, using a tight-binding band structure, followed by exchange enhancement of the spin part as per Izuyama, Kim and Kubo. The contribution to the neutron scattering from orbital motion of the electrons was estimated to be small, and in a special experimental investigation a satisfactorily low upper limit on this quantity for our present purposes was observed.

In the course of the article we attempt to illuminate with our computational results the theoretical content of the random-phase approximation, and to show how it accounts for spin waves, paramagnons and a variety of other phenomena. For instance at low temperatures, where the collective or spin-wave mode is the most prominent feature, the Stoner-mode states are so strongly mixed with the spin-wave states by the electron-electron interaction that little trace is left of the ‘band of Stoner modes’ often referred to in elementary accounts of this phenomenon. With increase of temperature, the severe broadening of the spin wave and the characteristic style of evolution of the magnetic response function through the critical temperature are correctly predicted, outside a narrow range of conditions about the critical point. The dramatic divergence of the susceptibility characteristic of the phase transition is confined much more closely towards the origin of K,ω space than could be accounted for by free-electron gas theory, and this is explained. On the other hand what appears to be a case of breakdown of the random-phase approximation is detected in the ferromagnetic state, where in a certain set of conditions at large K and small ω an excess of longitudinal spin correlation was measured over that expected theoretically.

Aside from the aforementioned aberrant region, however, absolute agreement between experiment and theory essentially within the statistical error is obtainable by assuming that the effective Coulomb interaction parameter I _eff(K) falls off gently between K=0 and K _max in a manner consistent with the predictions of Singwi et al. ; with increase of temperature, I _eff(0) declines gently and the amount of fall-off with K increases. It is shown that the theory of the Heisenberg ferromagnet fails entirely to account for the observations—indeed, fails to correlate any of the data on nickel—and that at least three-quarters of the known magnetic stiffness of nickel must be due to band-theory effects rather than to Heisenberg-type exchange integrals.     

Anisotropic exchange contributions to the magnetic susceptibility of transition metal ions in hexagonal close packed metals

When the effect of anisotropic exchange, together with crystal-field potentials is included in the Hamiltonian for magnetic impurity ions in a non-cubic metal, an additional contribution to the magnetic susceptibility is obtained. This produces a change in the temperature dependence of the susceptibility at low temperatures and gives an anisotropy varying as 1/T². The anisotropy in exchange, deduced from experiment, is 0.0086 per cent for Zn: Mn and between 2.4 and 7.2 per cent for Zn : Cr with J₆ > J_⊥ in both cases.     

Spectroscopy of magnetic excitations in magnetic superconductors using vortex motion

In magnetic superconductors a moving vortex lattice is accompanied by an ac magnetic field which leads to the generation of spin waves. At resonance conditions the dynamics of vortices in magnetic superconductors changes drastically, resulting in strong peaks in the dc I-V characteristics at voltages at which the washboard frequency of the vortex lattice matches the spin wave frequency omegaS(g), where g are the reciprocal vortex lattice vectors. We show that if the washboard frequency lies above the magnetic gap, measurement of the I-V characteristics provides a new method to obtain information on the spectrum of magnetic excitations in borocarbides and cuprate layered magnetic superconductors.     

Energy excitations in three-axial magnets with single-ion and exchange anisotropies

The energy spectra of three-axial ferromagnets like U₃P₄ and ferrimagnets like U₃Sb₄ are presented and their properties are discussed. In case of U₃Sb₄ we find an interesting example of a magnetic structure which, due to the single-ion anisotropy, is unstable in the spin wave approximation. The k² dependence of magnon energies requires re-examination of the temperature dependence of the magnetisation and of the specific heat.