Spin wave relaxation and magnetic properties in [M/Cu] super-lattices; M=Fe, Co and Ni

In this work, we study the elementary excitations and magnetic properties of the [M/Cu] super-lattices with: M=Fe, Co and Ni, represented by a Heisenberg ferromagnetic system with N atomic planes. The nearest neighbour (NN), next nearest neighbour (NNN) exchange, dipolar interactions and surface anisotropy effects are taken into account and the Hamiltonian is studied in the framework of the linear spin wave theory. In the presence of the exchange alone, the excitation spectrum E(k) and the magnetization 〈S^z〉/S analytical expressions are obtained using the Green's function formalism. The obtained relaxation time of the magnon populations is nearly the same in the Fe and Co-based super-lattices, while these magnetic excitations would last much longer in the Ni-based super lattice. A numerical study of the surface anisotropy and long-ranged dipolar interaction combined effects are also reported. The exchange integral values deduced from a comparison with experience for the three super-lattices are coherent.     

Magnetic excitations in invar alloys, Fe65Ni35 and Fe3Pt

Magnetic excitations in Fe₆₅Ni₃₅ and ordered Fe₃Pt have been studied by neutron scattering. It has been found that both alloys show common anomalous dynamical properties. At 5 K, well defined magnon groups could be detected up to 80 meV, but the temperature variations of the magnetization M(T) cannot be explained only by the magnon excitations. The integrated intensity measurements of the magnon spectra suggest that no other excitations take part in M(T). The magnons dampen significantly with the linewidth Γ(q, T) obeying a relation Γ(q, T) = (Γ₀+aT)q² with 1. The results suggest that the magnetic excitations with volume change occur in a slow relaxation process.     

An effective-medium method: The phonon mechanism of anomaly formation in the magnon spectrum of a finite magnetic superlattice

An effective-medium method is developed and applied to calculate the effect of dynamic magnetoelastic interaction on the spin dynamics of a finite magnetic thin-layer superlattice in the long-wavelength approximation. It is shown that if the intra-and interlayer spin-spin interactions are predominantly due to indirect coupling via the long-range field of quasi-static magnetoelastic strains, then the magnon spectrum of such a superlattice exhibits anomalies which are absent in the usually considered case where collective spin-wave excitations are formed through magnetodipole interaction.     

Weyl and Nodal Ring Magnons in Three-Dimensional Honeycomb Lattices

We study the topological properties of magnon excitations in a wide class of three-dimensional(3 D) honeycomb lattices with ferromagnetic ground states. It is found that they host nodal ring magnon excitations. These rings locate on the same plane in the momentum space. The nodal ring degeneracy can be lifted by the DzyaloshinskiiMoriya interactions to form two Weyl points with opposite charges. We explicitly discuss these physics in the simplest 3 D honeycomb lattice and the hyerhoneycomb lattice, and show drumhead and arc surface states in the nodal ring and Weyl phases, respectively, due to the bulk-boundary correspondence.     

Effect of photons on the magnon splitting

The self energy of a uniform magnon due to splitting into two magnons of oppositely directed momenta is calculated in the presence of a magnon-photon interaction, and it is found that the magnons and the photons form excitations of mixed character. Thus there is a net correction to the magnon splitting due to an interference.     

Spin waves in Hubbard model

Gutzwiller's variational method has been used to study the spin waves in the ferromagnetic state of a narrow band. The spin wave energies are investigated in both the nondegenerate and the doubly degenerate bands. The electron correlation restricts the spin excitations and so improves the RPA solutions of the magnon energies. It is found that the bare intra-atomic interaction energies in the RPA solutions are replaced by smaller effective ones. In the case of a degenerate band model, contrary to the constant value as predicted by RPA, the Stoner gap parameter is reduced by the correlation effect.     

Coupled Band Gap Solitons in One-Dimensional Alternating Heisenberg Ferromagnetic Chains

The dynamics of coupled band gap solitons in one-dimensional Heisenberg ferromagnetic chains with bond alternation is considered analytically. Using the method of multiple scales the nonlinear coupled-mode equations (i.e. Manakov equations) for the upper cutoff mode of acoustic band and the lower cutoff mode of optical band are derived under the quasi-discreteness approximation. Due to the cross-phase modulation the type of soliton excitations may be changed and the vibrating frequencies of these soliton excitations may locate within or outside the gap of magnon frequency bands.     

Light scattering from two-magnon modes in K2NiF4

Measurements of Raman scattering from two magnon excitations in the two dimensional antiferromagnet K₂NiF₄ are extended to 275°K. The zeroth frequency moment of the magnon signal has a temperature dependence essentially the same as that found in the three dimensional antiferromagnet, NiF₂. No indications of critical behavior are apparent in the first frequency moment for K₂NiF₄ in contrast to the case of three dimensional antiferromagnets.     

Phase transitions driven by quasiparticle interactions

After a survey of the solid–liquid transition, driven by phonon–phonon interactions, attention is next focussed on two phase transitions caused by electron–phonon interactions. These are (i) the Barden–Cooper–Schrieffer pure metal superconducting transition and (ii) the original Peierls instability. These have closely similar forms for the respective transition temperatures, both being related to energy gaps. Spin–phonon interactions are then discussed in relation to spin-Peierls materials. Finally, magnon–magnon interactions are treated in the context of the ferromagnetic–paramagnetic transition in the itinerant electron systems Fe, Co and Ni. Heuristic and phenomenological arguments, plus of course experiment, provide the basis for the conclusions drawn here.     

Lifetimes of excited electrons in Fe and Ni: first-principles GW and the T-matrix theory

We present the results of an ab initio calculation of excited electron lifetimes in ferromagnetic materials which incorporates non-spin-flip and spin-flip processes within GW and T-matrix approaches. The method we develop is applied to low-energy electron excitations in Fe and Ni. It is found that the spin-wave generation in Fe essentially reduces the lifetimes of the spin-minority d states whereas the free-electron-like spin-minority states and all the spin-majority states are affected much less. The influence of spin-flip scattering on the lifetimes in Ni appears to be weak. The T-matrix non-spin-flip processes are important for the lifetimes of excited spin-minority states.     

Lifetimes of excited electrons in Fe and Ni: first-principles GW and the T-matrix theory

We present the results of an ab initio calculation of excited electron lifetimes in ferromagnetic materials which incorporates non-spin-flip and spin-flip processes within GW and T-matrix approaches. The method we develop is applied to low-energy electron excitations in Fe and Ni. It is found that the spin-wave generation in Fe essentially reduces the lifetimes of the spin-minority d states whereas the free-electron-like spin-minority states and all the spin-majority states are affected much less. The influence of spin-flip scattering on the lifetimes in Ni appears to be weak. The T-matrix non-spin-flip processes are important for the lifetimes of excited spin-minority states.     

Origin of hidden magnetic excitations in an invar alloy Fe65Ni35

Hidden magnetic excitations responsible for the Invar effect have been studied by polarized neutron scattering with polarization analysis. No low energy magnetic excitations could be found at the zone boundary, which excludes a simple weak ferromagnetism model for the Invar effect of the alloy. The result, however, suggests that there exist additional “dispersive” magnetic excitations which seem to develop along the magnon dispersions.     

Excitation spectra of one-dimensional spin-1/2 Fermi gas with an attraction

Using an exact Bethe ansatz solution, we rigorously study excitation spectra of the spin-1/2 Fermi gas (called Yang–Gaudin model) with an attractive interaction. Elementary excitations of this model involve particle-hole excitation, hole excitation and adding particles in the Fermi seas of pairs and unpaired fermions. The gapped magnon excitations in the spin sector show a ferromagnetic coupling to the Fermi sea of the single fermions. By numerically and analytically solving the Bethe ansatz equations and the thermodynamic Bethe ansatz equations of this model, we obtain excitation energies for various polarizations in the phase of the Fulde–Ferrell–Larkin–Ovchinnikov-like state. For a small momentum (long-wavelength limit) and in the strong interaction regime, we analytically obtained their linear dispersions with curvature corrections, effective masses as well as velocities in particle-hole excitations of pairs and unpaired fermions. Such a type of particle-hole excitations display a novel separation of collective motions of bosonic modes within paired and unpaired fermions. Finally, we also discuss magnon excitations in the spin sector and the application of Bragg spectroscopy for testing such separated charge excitation modes of pairs and single fermions.     

Magnetic ordering in digital alloys of group-IV semiconductors with 3<Emphasis Type="Italic">d</Emphasis>-transition metals

The ab initio investigation of the magnetic ordering in digital alloys consisting of monolayers of 3d-transition metals Ti, V, Cr, Mn, Fe, Co, and Ni introduced into the Si, Ge, and Si_0.5Ge_0.5 semiconductor hosts is reported. The calculations of the parameters of the exchange interactions and total-energy calculations indicate that the ferromagnetic order appears only in the manganese monolayers, whereas the antiferromagnetic order is more probable in V, Cr, and Fe monolayers, and Ti, Co, and Ni monolayers are nonmagnetic. The stability of the ferromagnetic phase in digital alloys containing manganese monolayers has been analyzed using the calculations of magnon spectra.     

The <Superscript>55</Superscript>Mn Spin Echo Test of Magnon BEC State in MnCO<Subscript>3</Subscript>

The coherent quantum state of magnons—Bose–Einstein condensate (BEC) has been observed in several types of antiferromagnets. According to the Bose statistics of magnons, BEC appears when the magnon density exceeds the critical density N _BEC and the magnon gas condenses to a quantum liquid. The BEC state is characterized by a coherent precession of the magnetization. In this paper, the first experiments showing the suppression of the spin echo signal by the magnon BEC is presented. These experiments confirm the coherence of magnetic excitations in the BEC state.     

Magnon mechanism of defect reactions in solids

A phenomenological theory is developed that describes the effect of a magnetic field on the defect reactions in a solid. The theory is based on a concept regarding the lattice magnetism according to which a defect induces a magnetoactive (magnon) branch in the spectrum of elementary excitations of a crystal lacking a magnetic structure in the absence of defects. The probability of a defect complex disintegrating in a magnetic field is calculated in terms of the magnon mechanism of the reaction.     

Spin excitations in an anisotropic bond-alternating quantum s = 1 chain in a magnetic field: contrast to haldane spin chains

Inelastic neutron scattering experiments on the S = 1 quasi-one-dimensional bond-alternating antiferromagnet Ni(C9D24N4)(NO2)ClO4 have been performed under magnetic fields below and above a critical field Hc at which the energy gap closes. Normal field dependence of Zeeman splitting of the excited triplet modes below Hc has been observed, but the highest mode is unusually small and smears out with increasing field. This can be explained by an interaction with a low-lying two magnon continuum at q(parallel) = pi that is present in dimerized chains but absent in uniform ones. Above Hc, we find only one excited mode, in stark contrast with three massive excitations previously observed in the structurally similar Haldane-gap material NDMAP [A. Zheludev, Phys. Rev. B 68, 134438 (2003)].     

Functional Magnon Network Blocks Based on Structures with Translational Symmetry Violation

Technical Physics - We have studied the properties of spin-wave excitations in a structure that is a junction of two regular magnon waveguides. The proposed structure enables the transmission of...     

On the energetics of transversal and longitudinal fluctuations of atomic magnetic moments

By constrained spin-density functional calculations we estimate the relative role of the longitudinal and transversal fluctuations of the magnetic moments in the series of 3d metals (bcc Fe, hcp and fcc Co, and fcc Ni) for weak excitations from the ferromagnetic ground state. It is shown that the importance of longitudinal fluctuations strongly varies from relatively small in bcc Fe to large in fcc Ni. This means that a consistent adiabatic treatment of the low-energy spin fluctuations should include independent longitudinal fluctuations.