Spin-wave Branches Research Articles

Strong change in spin dynamics close to percolation in La 1− xCa xMnO 3

In low doping regime, manganites present a very particular spin dynamics with two spin wave branches and peculiar static features associated to ferromagnetic clusters. We point out the existence of an average canted antiferromagnetic state in a large domain of concentration and an insulator ferromagnetic state preceeding the metal–insulator transition ( x≃0.2). The high-energy spin wave exhibits a huge evolution between x=0.1 and 0.15 announcing the disappearance of the antiferromagnetism in the system. The low-energy branch is still observed in these compounds and it can be related to static ferromagnetic clusters. The huge evolution of the high-energy spin wave can be interpreted by the fact that the percolation threshold of the ferromagnetic clusters has been reached, announcing the metal–insulator transition.

Magnetic coupling induced by hole doping in perovskitesLa1−xCaxMnO3:A neutron scattering study

Elastic and inelastic neutron scattering has been used to study static and dynamic magnetic properties in Ca-doped ${\mathrm{LaMnO}}_{3}$ perovskites in the region of low doping (5% and 8%) and to determine the phase diagram. In addition to the spin-wave branch related to the superexchange coupling between Mn ions (ferromagnetic in the basal planes and antiferromagnetic along the direction perpendicular to them), a new type of ferromagnetic spin excitation branch is found induced by hole doping. This is revealed by well-defined propagative spin wave modes with a gap and a nearly isotropic dispersion law at small q values: $\ensuremath{\omega}={\ensuremath{\omega}}_{0}{+Dq}^{2}.$ At ${T}_{N},$ these modes change from a collective to a diffusive character. The width in energy associated to the quasielastic scattering exhibits a similar dispersion law: $\ensuremath{\Gamma}={\ensuremath{\Gamma}}_{0}{+dq}^{2}.$ All these results are compared with other experimental results on similar compounds and discussed in the frame of existing theories.

Magnetic excitations and dynamical Jahn-Teller distortions inUO2

Inelastic neutron scattering with polarization analysis has been used to study the evolution of the magnetic response across the antiferromagnetic to paramagnetic transition in a single crystal of uranium dioxide ${(T}_{N}=30.8 \mathrm{K})$. The spin-wave dispersion curves have been determined at 16 K by measuring the spin-flip channel of the neutron cross section along the principal crystallographic directions. Evidence of magnon-phonon interactions along the $(0,0,\ensuremath{\xi})$ direction is given by the splitting of the lowest spin-wave branch, and by the Q dependence of the energy-integrated dynamic susceptibility. Above ${T}_{N},$ a magnetic inelastic response consisting of two dispersive peaks was observed between 3 and 10 meV. This signal was easily measurable even at 200 K, more than six times the N\'eel temperature, where spatial correlations between the uranium spins are essentially zero. We assume this result as evidence that in the time scale of our experiment the uranium triplet ground state is split into three singlets, due to a dynamical Jahn-Teller (JT) distortion of the oxygen cage which reduces the point symmetry at the uranium site. Since the position of the peaks and their dispersion are compatible to a $1\ensuremath{-}\mathbf{k}$ distortion along the $〈100〉$ direction, a picture emerges in which local, uncorrelated $1\ensuremath{-}\mathbf{k}$ dynamical JT distortions occur above ${T}_{N}$ along the three directions of the $〈100〉$ star; as ${T}_{N}$ is approached, a correlation builds up between the phases of the corresponding vibrations until, eventually, a static $3\ensuremath{-}\mathbf{k}$ structure is obtained below ${T}_{N}.$

Magnetoacoustic resonance on nuclear spin waves in the cubic antiferromagnet RbMnF3

Magnetoacoustic resonance on nuclear spin waves is measured in the cubic antiferromagnet RbMnF3. A resonance change with respect to a constant magnetic field H0 with maximum damping at H0≈4×103 Oe is observed in the amplitude of an acoustic pulse passing through a sample owing to excitation of nuclear spin waves under nuclear magnetoacoustic resonance conditions. A study of the angular dependence of the damping revealed a 90° periodicity consistent with the fact that the [001] direction, around which the rotation takes place, is a four-fold axis of the crystal. An analysis of the dispersion law for nuclear spin waves shows that longitudinal ultrasound propagating along the [001] axis perpendicular to H0 excites a branch of nuclear spin waves whose frequency depends on the magnitude of the constant magnetic field.

Anomalous spin dynamics in La 0.9Ca 0.1MnO 3

Elastic and inelastic neutron scattering has been measured in La 0.9Ca 0.1MnO 3 single crystal. The temperature dependence of the intensities of Bragg peaks (0 0 1) and (0 0 2) shows two magnetic transitions: a transition paramagnetic/ferromagnetic (F) at T C=138 K and a transition ferromagnetic/canted antiferromagnetic (AF) at T N=118 K. Moreover, spin waves connected to super exchange integrals, coupling localized spins of Mn 3+ ions, have been determined. In addition to these spin waves, as already known in low doping rate compounds, new magnetic excitations have been measured. They appear as eigenmodes of the whole system and have a F character. Their intensity reveals a finite correlation length and the dispersion law is nearly isotropic ( ω= ω 0+ Bq 2). This new spin wave branch could be a precursor effect of the transition insulator/metal expected around x∼18%.

Pseudodipolar interaction and antiferromagnetism inR2CuO4compounds(R=Pr,Nd,Sm,and Eu)

It is shown that pseudodipolar interaction allows one to explain static and dynamic properties of the noncollinear antiferromagnets, ${R}_{2}{\mathrm{CuO}}_{4},$ in the temperature range where the rare-earth (RE) angular momenta are slightly polarized by the antiferromagnetic (AF) ordered ${\mathrm{Cu}}^{2+}$ spins. The spin-wave spectrum is determined and the inelastic neutron scattering cross section is evaluated taking into account interference related to the noncollinear arrangement of the copper spins in the adjacent ${\mathrm{CuO}}_{2}$ planes. The detailed experimental data for low-energy inelastic neutron scattering in ${\mathrm{Pr}}_{2}{\mathrm{CuO}}_{4}$ are presented. Two in-plane spin-wave branches are observed in accordance with the theoretical predictions. Parameters of the intraplane and interplane pseudodipolar interactions are determined. The former appears by one order of magnitude larger than the theoretically one predicted for an isolated ${\mathrm{CuO}}_{2}$ plane. We confirm this result by the analysis of the elastic neutron scattering data in magnetic field. Comparison of the spin-wave neutron scattering in ${\mathrm{Pr}}_{2}{\mathrm{CuO}}_{4}$ and ${\mathrm{Nd}}_{2}{\mathrm{CuO}}_{4}$ published earlier reveals the strong dependence of the in-plane anisotropy on the type of the RE ion. General expression for the ground-state energy in magnetic field applied parallel to the ${\mathrm{CuO}}_{2}$ planes is derived and analyzed. It is shown that the spin-flop transition is of second order if the field is applied along the [1,1,0] direction.

Inelastic neutron scattering study of spin waves in the garnet with a triangular magnetic structure

The spin-wave spectrum of the antiferromagnet with the garnet structure has been studied by inelastic neutron scattering on the triple-axis spectrometers IN12 and IN14 at ILL. Magnon dispersion curves were measured along and directions up to energies of 0.3 THz. The spin-wave symmetry analysis, which reduced the number of independent exchange parameters to six, was performed to ensure correct description of the spin-wave branches. As determined by the fitting, five of the parameters are statistically significant and have been found to be , and . This quite unusual sequence of exchange parameters clearly demonstrates the importance of the superexchange chain geometry.

Liquidlike Spatial Distribution of Magnetic Droplets Revealed by Neutron Scattering inLa1−xCaxMnO3

Elastic neutron scattering experiments performed in semi-conducting La(1-x)Ca(x)MnO3 single crystals (x=0.05, 0.08), reveal new features in the problem of electronic phase separation and metal insulator transition. Below TN, the observation of a broad magnetic modulation in the q-dependent scattering intensity, centered at nearly identical qm whatever the q direction, can be explained by a liquid-like spatial distribution of similar magnetic droplets. A semi-quantitative description of their magnetic state, diameter, and average distance, can be done using a two-phase model. Such a picture can explain the anomalous characteristics of the spin wave branches and may result from unmixing forces between charge carriers predicted from the s-d model.

Explanation of the Cu spin-wave excitation gap in Nd CuO

By use of a mean-field approach the spin-wave dispersion of the Cu degrees of freedom in the undoped high-T C material Nd2CuO4is investigated. The experimentally observed sharp decrease of the Cu spin-wave gap with increasing temperature in the range \({T^*} < T < {T_N}\) is explained by a paramagnetic-like susceptibility of the Nd spins which couple to the Cu subsystem. The degeneracy of the “in-plane” and “out-of-plane” polarized Cu spin-wave branches is shown to be lifted by the uniaxial anisotropy of the Cu-Cu nearest-neighbor interaction.

Antiferromagnetic resonance in Bi2CuO4

Magnetic resonance of the low-frequency spin-wave branch in the Bi2CuO4 antiferromagnet with an easy-plane anisotropy has been studied. Angular, frequency, and temperature dependences of the position and width of the antiferromagnetic resonance (AFMR) line have been measured. Our measurements combined with earlier data [H. Ochta, K. Yoshida, T. Matsuya, T. Nanba, M. Motokawa, K. Yamada, Y. Endon, and S. Hosoya, J. Phys. Soc. Jpn. 61, 2921 (1992); E. W. Ong, G. H. Kwei, R. A. Robinson, B. L. Ramakrishna, and R. B. von Dreele, Phys. Rev. B 42, 4255 (1990) [ have allowed us to determine anisotropy constants of this material and to account for the unusual character of its static susceptibility anisotropy. The AFMR line shifts to the high-field side and broadens in a temperature range of 10–15 K, and the cause of this has remained unclear. In the low-temperature range the line shows a hysteresis corresponding to a static field magnitude several times as large as the spin-flop field. The position and width of the AFMR line depend sensitively on the sample preparation technique.

New spin-wave branch induced by the hole doping in La 1− xCa xMnO 3

Measurements of spin waves in La 1− x Ca x MnO 3, x = 0.08, T = 14 K, are reported and compared to x = 0.05. Two spin-wave branches are observed, one showing the characteristics expected for Mn spins coupled by exchange (strongly anisotropic), the other for Mn spins coupled through hole hopping (isotropic, with a quadratic law ω = Dq 2 + Δ at small q). Both spin-wave branches evolve with x. These two types of excitations are separated by an energy band which does not vary with x.

New spin dynamics in GMR Ca doped manganites

Spin dynamics have been studied on single crystals of weakly doped La 1− x Ca x MnO 3. At 17 K a spin-wave branch is found due to superexchange coupling (ferromagnetic in the a, b plane, increasing with x, antiferromagnetic along c decreasing with x). An additional isotropic and weakly dispersed spin-wave branch is observed around ferromagnetic zone centers at lower energy. This branch shows a gap decreasing with x and a stiffness constant increasing with x. The evolution of this branch with temperature is presented. In the paramagnetic phase the excitations become diffusive and the q-dependence of the width of the quasi-elastic spectra: Γ( q) = Γ 0 + Dq 2 reflects the dispersion law found in the ordered phase. These further excitations are attributed to the coupling induced by the mobile charge carriers with the localized spin lattice.

Influence of Ce-doping on the magnetic excitations of Nd in Nd 2− xCe xCuO 4

We present a study of the spin-wave excitations of Nd in Nd 2− x Ce x CuO 4 ( x = 0, 0.09, 0.15, 0.18) by inelastic neutron scattering. Upon Ce-doping one observes a softening of the spin-wave branches. This is mainly explained by the decrease of the CuNd exchange field at the Nd site, while the NdNd exchange interactions remain almost unchanged. Consequently a high density of states of spin-wave excitations at low energies is created. The resulting calculated specific-heat shows a large γ = C/ T-value at low temperatures, which indicates that the novel heavy-fermion behavior in these compounds is mainly due to magnetic correlations of the Nd ions.

Acoustic excitation of nuclear spin waves in the easy-plane antiferromagnetic material KMnF3

Ultrasound damping at T=4.2 K in single crystal easy-plane antiferromagnetic KMnF3 is studied experimentally as a function of the magnitude and direction of a constant magnetic field H at frequencies of 640–670 MHz, corresponding to the frequencies of nuclear spin waves. Two experimental situations are examined: in the first, the vector H lies in the easy magnetization plane (001), and in the second, H forms an angle with (001). For longitudinal ultrasound waves propagating along the hard magnetization axis [001], it is found that the damping depends resonantly on the magnitude of the field H. In the first case a single damping maximum is observed, and in the second, two damping peaks that are well resolved with respect to the field. The angular dependence of the resonance damping signals on the direction of the constant magnetic field is found to have a 90° periodicity in all cases. The observed effects are explained by resonant ultrasonic excitation of nuclear spin waves. On the basis of an analysis of the magnetoacoustic interaction energy, it is shown that in the first case, nonzero oscillations of the antiferromagnetism vector L occur only in the basal plane, while in the second, oscillations of L occur both in the basal and a vertical plane, which are associated, respectively, with two branches of the nuclear spin waves. It is also shown that the 90° periodicity in the angular dependence of the damping signals is associated with a fourth order [001] axis.

From mixed valence to pseudo-one-dimensional spin chains in Yb 4As 3

According to a recently developed model, the Yb 3+-ions in the heavy-fermion system Yb 4As 3 are positioned on parallel, quasi-one-dimensional chains at low temperatures. A small interchain coupling J′ leads to a strong magnetic-field dependence of the linear specific-heat coefficient γ. Within linear spin-wave approximation, it is shown that the six spin-wave excitation branches ω κ( q,H) determine the low-temperature thermodynamics of Yb 4As 3. Most importantly, field-induced gaps scaling as Δ ⋍ √JJ′ appear at q = γ,A . For J′ = 10 −4 J, a good agreement with the experimental variation of γ( H) achieved.

New dynamical features in La 0.95Ca 0.05MnO 3

Inelastic neutron scattering data of La 0.95Ca 0.05MnO 3 are reported and compared to those of LaMnO 3. At this low doping rate, the energy spectra are drastically changed. Around all ferromagnetic centers, a new magnetic mode appears which is very weakly dispersed. It coexists with a dispersed spin-wave branch, similar to that found in the pure system, showing a strong decrease of interplane coupling. The new mode, which reveals a magnetic coupling of distinct origin, could be a precursor effect of the dynamics in the metallic state

Parallel pump spin wave instability threshold in thin ferromagnetic films

A rigorous theory of parallel pump instability threshold of spin waves in ferromagnetic films is developed. Dipole and exchange interactions, the dipole-dipole hybridization of spin wave branches, and the magnetocrystalline and induced anisotropies are taken into account. Expressions for the parallel pump instability threshold are obtained in general form which can be used to calculate the threshold of a single ferromagnetic layer or a multilayered structure containing an arbitrary number of ferromagnetic films. The influences of the dipole-dipole hybridization of parametrically generated spin waves and of their nonreciprocal propagation properties on the threshold are discussed.

Metal-insulator transition in an one-dimensional two-band Hubbard-like model

We consider a one-dimensional two-band model of electrons on a lattice with equal nearest-neighbour hopping, an interband splitting Delta and a Hubbard-like repulsion U. The model is defined via the SU(4) generalization of Lieb and Wu's Bethe ansatz solution of the one-dimensional Hubbard model. At T=0 the model has a Mott metal-insulator transition at a critical value Uc( Delta ) for a band filling of exactly one electron per site. Uc decreases with Delta , being zero if the excited-electron band is empty, and Uc=2.981 when the bands are degenerate. We discuss the ground-state properties, the spectrum of elemental excitations, the specific heat and the magnetic susceptibility, in both the metallic and insulating phases, as a function of the crystal-field splitting for exactly one electron per site. There are four branches of elemental excitations: (i) charge excitations, (ii) crystalline-field excitations and (iii) two branches of spin waves. The Fermi velocity is finite in the metallic phase, diverges as the metal-insulator transition is approached from the metallic side and vanishes for the insulator. Each band contributes to the susceptibility with a term that is inversely proportional to the spin-wave velocity for that band. The low-temperature specific heat is proportional to T and to the sum of the inverses of the velocities of the four branches.

Surface spin waves in a semi-infinite ferromagnet with perpendicular surface anisotropy

The effect of perpendicular surface anisotropy on surface spin-wave excitations in a semi-infinite ferromagnet is studied by solving the coupled equations of motion of spin-wave annihilation and creation operators. The ground-state spin arrangement in our model is either as in-plane ordering or a canted one due to a competition between the perpendicular anisotropy in the surface and the bulk anisotropy parallel to the surface, and surface spin-wave frequencies are calculated in both phases. It is shown that at the transition point between these two distinct phases a low-lying surface spin-wave branch becomes soft, and consequently the ellipticity and the amplitude of spin precessions in the surface layer grow large in the vicinity of the transition point.

Four-spin exchange in Bi2CuO4 (abstract)

Magnetic properties of Bi2CuO4 single crystals are of interest owing to their relationship to the high-Tc superconductor problem. Earlier we carried out the magnetic and neutron scattering investigations on high quality large single crystals. The Bi2CuO4 crystal belongs to the space group P4/ncc and the magnetic group is the collinear plane antiferromagnet with a Neel temperature of 45.8 K. The three-dimensional character of the antiferromagnetic ordering is reflected by the value of the critical magnetization exponent 0.352. The sublattice magnetization depends very little on temperature up to about 15 K. The spin-wave spectrum contains two branches with energy gaps of 2.1 and 3.4 meV. The theoretical interpretation of the experimental data shows the necessity of introducing the tensor four-site exchange in the S=1/2 spin Hamiltonian with the tetragonal anisotropic bilinear exchange. This provides the gap characteristics of the acoustic branch of spin waves and the correct temperature dependence of the sublattice magnetization. This result shows the importance of the four-site exchange for the formation of Bi2CuO4 magnetic properties. From the similarity of magnetic subsystems, Bi2CuO4, and high-Tc superconductors it is suggested that the four-site exchange is responsible for magnetic ordering of the square spin S=1/2 lattice. We also suggest a thermodynamic theory of the temperature dependence of magnetic properties of Bi2CuO4.