Session V23: Spin Waves

Spin-wave interactions in quantum antiferromagnets

We study spin-wave interactions in quantum antiferromagnets in terms of Hermitian field operators representing staggered and ferromagnetic transverse spin fluctuations. In this parameterization, the two-body interaction vertex between staggered spin fluctuations vanishes at long wavelengths. We derive a new effective action for the staggered fluctuations and show that the quantum critical point separating the renormalized classical from the quantum disordered regime in $D > 1$ dimensions is characterized by an anomalous dimension of the field operator $\eta =D-1$. We further use this technique to derive the effective long-wavelength Euclidean action for the antiferromagnetic spin-waves of ordered antiferromagnets subject to a magnetic field. We point out, that the magnetic field dependence of the spin-wave dispersion predicted by the usual $O(3)$ nonlinear sigma model disagrees with spin-wave theory. We argue that the nonlinear sigma-model does not take into account all relevant spin-wave interactions and derive a modified effective action for the spin-waves which contains an additional quartic interaction. At zero temperature the corresponding vertex is relevant in the renormalization group sense below three dimensions.   

Spin dynamics of a Half-Metallic Ferromagnet

We determine the dispersion relations, and therefore, the collective modes of a model for a half-metallic ferromagnet by using two approaches: the particle-hole propagator method and the kinetic equation method. We formulate the latter by using the theory of spin polarized Fermi liquids. In both cases we express the results in terms of Landau interaction parameters and make the corresponding comparisons. We also calculate the velocities of propagation of the modes and study the spin stiffness. We discuss the results in the context of the currently available experimental data.   

Evolution of Spin-Wave Excitations in Ferromagnetic Metallic Manganites

The deviation of spin dynamics from the Double-Exchange mechanism in the ferromagnetic metallic manganites is an important yet unsettled issue in the understanding of the close coupling behavior between charge, lattice, orbital, and spin degrees of freedom in these doped ``colossal'' magnetoresistive materials. We have used neutron scattering to systematically study the spin-wave excitations of three ferromagnetic metallic $A_{1-x}A'_{x}$MnO$_{3}$ manganites (where $A$ and $A'$ are rare- and alkaline-earth ions respectively). By combining with previous work, we elucidate the systematics of the spin-spin interaction and interactions between spin and other degrees of freedom as a function of carrier concentration $x$, on-site disorder, and strength of the lattice distortion. The long wavelength spin dynamics show only a very weak dependence across the series, while the Curie temperatures vary substantially. The ratio of the fourth to the first neighbor exchange ($J_{4 }= J_{1})$ that controls the zone boundary magnon softening changes systematically with $x$, but does not depend on the other parameters. None of the prevailing models can account for these behaviors. The work was supported by NSF-DMR0453804, NSF-DMR0346826, DE-FG02-05ER46202 and DOE DE-FG02-04ER46125. ORNL is managed by UT-Battelle, LLC, for the U.S. DOE under contract DE-AC05-00OR22725.   

Coherent spin waves in epitaxial Fe films on GaAs (001)

Recently, coherent spin waves have been investigated in ferromagnetic thin film using optical pump-probe technique by exploiting the temperature dependence of the magnetic anisotropy. Here, we report on low-order spin wave modes in the dipole-exchange regime in Fe films epitaxially grown on GaAs (001). Three precession modes with zero in-plane wave-vector are observed by applying an external magnetic field along the in-plane hard axis. The lowest frequency mode can be well described by the uniform magnetization precession. The cubic magnetocrystalline anisotropy constant K$_{1}$/M$_{s}$ and effective demagnetization field 4$\pi $M$_{s}$ are determined to be 270 Oe and 17.5 KOe, respectively. The modes with higher frequencies correspond to the first- and second-order spin waves. An effective exchange stiffness constant of 0.8*10$^{-6}$ erg/cm is obtained from the calculation assuming the single sine- or cosine- type standing spin waves with free spins at both interfaces. The exchange constant is smaller than the values obtained from high order spin wave modes in ferromagnetic resonance experiments and neutron scattering. The difference may result from pinning effects, which modifies the location of the surface antinodes of the standing spin waves thereby changing their effective wavelength.   

Evolution of Cr's charge and spin density waves under GPa pressures

We trace the development of Cr's charge density wave (intensity 10$^{-4}$ $\times $ Bragg peak) and spin density wave (intensity 10$^{-8} \quad \times $ Bragg peak!) at GPa pressures at the Advanced Photon Source. Using a diamond anvil cell combined with a newly developed chemical filtering technique, we find a previously unknown incommensurate to commensurate transition at 2 GPa. We analyze the incommensurate Q-vector and intensity changes at the approach to the transition. At much higher pressures ($>$ 7 GPa), Cr's antiferromagnetism is completely suppressed and the evolution of the line shape at the approach to the quantum critical point directly reveals the role played by quantum fluctuations.   

Spin wave in quasi-equilibrium spin systems

Using the Landau Fermi liquid theory we can study a quasi- equilibrium spin system at T=0K to determine the dispersion relations of possible collective modes. By quasi-equilibrium spin system we refer to a spin system which has finite magnetization in the absence of an external magnetic field. Such a system has been realized in liquid helium and in spintronic materials using different methods; one of those is the optical pumping. We have predicted the existence of gapless spin wave modes in such a system. We compare the dispersion relations of different modes of the quasi- equilibrium system with those of a paramagnetic system, in the presence of a magnetic field, and a ferromagnetic system. Using the relaxation time approximation for the collision integral we study the effect of temperature on the dispersion relation of the modes. We will also present the behavior of the dynamical structure function as a function of frequency and discuss the contribution of different modes to the sum rules.   

Valence values of the cations in selenospinel Cu(Cr,Ti)$_2$Se$_4$

A long-standing issue about the Cu valency in selenospinel CuCr$_2$Se$_4$ was investigated by soft x-ray absorption spectroscopy (XAS) and magnetic circular dichroism (XMCD). Using the sensitivity of XAS and XMCD to the valence value of transition metal ion and its local symmetry, we checked the valence value of each cation in selenospinel CuCr$_x$Ti$_{2-x}$Se$_4$ ($x$ = 1.0, 1.1, 1.5, and 2.0) and obtained spectroscopic evidence that a small amount of the Cu cation changes the valency from Cu(I) to Cu(II) as the Cr concentration increases from 1.0 to 2.0. Dependence of the Cu(II) concentration and the mean field magnetic exchange energy on the Cr concentration suggests the Cu $d$-hole plays a crucial role in the intriguing magnetic/electrical properties of CuCr$_2$Se$_4$.   

The optical and acoustic magnetic excitations in optimum-doped superconductor $YBa_{2}Cu_{3}O_{6.95}$ ($T_{c} = 93 K$)

We use high-resolution inelastic neutron scattering to map out the full spin excitations spectra in $YBa_{2}Cu_{3}O_{6.95}$ ($T_{c} = 93 K$), probably the most studied high-transition temperature superconductor. In the bi-layer materials where there are two magnetic ions per unit cell, magnetic excitations have two distinct symmetries: odd (or acoustic) and even (or optical) channels. We show that magnetic excitations in the acoustic channel of $YBa_{2}Cu_{3}O_{6.95}$ are remarkably similar to that of $YBa_{2}Cu_{3}O_{6.6}$ [1] and $La_{1.88}Ba_ {0.12}CuO_{4}$ [2]. That is, they have the universal hour-glass shape with incommensurate spin fluctuations below the commensurate resonance. In the optical channel, we discovered incommensurate magnetic excitations in superconducting state. We establish the both the acoustic and optical magnetic excitations spectra of $YBa_{2}Cu_{3}O_{6.95}$ in absolute units and compare the results with underdoped $YBa_ {2}Cu_{3}O_{6.6}$ and single layer families of cuprate superconductors. [1] S. M. Hayden et al. Nature 429, 531 (2004);[2] J. M. Tranquada et al. Nature 429, 534 (2004).   

Spin excitations and phonons in La$_{1.5}$Sr$_{0.5}$CoO$_{4}$

Inelastic neutron scattering measurements were performed in La$_{1.5}$Sr$_{0.5}$CoO$_{4}$. At room temperature, the excitation spectrum is dominated by phonons. At low temperatures we see a mix of lattice vibrations and spin modes. By subtracting phonon contributions we are able to identify purely magnetic modes. A resonance mode was found at energies around 25-27meV, the nature of which is under investigation.   

Ferromagnetic Relaxation by Magnon Induced Currents

A theory for calculating spin wave relaxation times based on the magnon-electron interaction is developed. In a conducting ferromagnet the interaction between the conduction electrons and the magnons is important. The magnetic field generated by the spin wave is time dependent and therefore it creates an electric field in the system. In a metallic system, the fields drive the conduction electrons. These magnon induced currents help dissipate the energy of the system by Joule heating. Our study incorporates a thin film geometry and explores relaxation time for a wide range of magnon wave vectors spanning both the magnetostatic and exchange regimes. The relaxation time is calculated directly from the ratio of the energy density of the system and the power loss by magnon induced currents. We propose a wave vector dependent damping constant which approaches values as high as 0.2 for high conductivity metals such as permalloy, showing the large magnitude of the effect. The theory compares reasonably well with spin wave resonance experiments for lower modes. We propose the following picture of ferromagnetic relaxation in switching experiments. The initial rapid approach of magnetization direction to equilibrium is enabled by magnon-magnon scattering that converts the energy into the higher spin wave modes. These modes then decay at a slower pace via the magnon-electron interaction investigated in this work or by the trditionally invoked mechanisms in less pure, lower conductivity films.   

On the possibility of electrical detection of spin echoes in a ferromagnetic thin film

We show, via a straightforward calculation, that it is feasible to detect nuclear spin echoes in a ferromagnetic thin film by electrical means. The detection scheme is based on the deflection of the electronic magnetization when the nuclear magnetization reforms during an echo. This deflection is observed as a time-dependent magnetoresistive signal.   

Quantum vs. Thermal Annealing of Magnetic Domain Walls in Elemental Chromium

Dynamics of magnetic domain walls separating regions with different orientations of the spin (charge) density wave has been studied with x-ray photon correlated spectroscopy (XPCS) in bulk Cr samples. Measurements were carried out at temperatures ranging from 298K to 4K. Upon cooling down to 4K thermal exploration of energetic landscape is replaced by quantum tunneling between discrete low-energy configurations of domain walls. Direct comparison of coherent speckle pattern fluctuations for Bragg reflection and charge density wave satellite peak provides information about magnetic superlattice dynamics, deconvolved from the motion of underlying atomic lattice or beam components.