Magnon Spectrum Research Articles

Structural and electronic response of U3Fe4Ge4 to high pressure

Structural, magnetic, and electrical properties have been studied on a U3Fe4Ge4 single crystal under hydrostatic pressure. The orthorhombic crystal structure is found to be stable up to 30 GPa, the highest applied pressure, but the compressibility is strongly anisotropic. Contrary to typical uranium intermetallics for which the softest lattice direction is along the shortest inter-uranium links, in U3Fe4Ge4 the lattice is compressed most in a perpendicular direction for the high pressure range. The elastic properties are modified considerably in the vicinity of 1 GPa when the b axis is transformed from least compressible to most compressible. The bulk modulus is found to be about 150 GPa. The anomalies in the elastic properties are reflected in the electronic properties that consistently indicate a change of the magnetic ground state from ferromagnetic to antiferromagnetic. Both types of order exhibit a gap in the magnon spectrum; however, it is twice as high for the ferromagnetic state. The magnetoresistance reveals field-induced transitions of different origins in the antiferromagnetic state along the easy and hard magnetization directions.

Magnetic excitation spectrum ofLuFe2O4measured with inelastic neutron scattering

We report neutron inelastic scattering measurements and analysis of the spectrum of magnons propagating within the Fe2O4 bilayers of LuFe2O4. The observed spectrum is consistent with six magnetic modes and a single prominent gap, which is compatible with a single bilayer magnetic unit cell containing six spins. We model the magnon dispersion by linear spin-wave theory and find very good agreement with the domain-averaged spectrum of a spin-charge bilayer superstructure comprising one Fe3+ -rich monolayer and one Fe2+ -rich monolayer. These findings indicate the existence of polar bilayers in LuFe2O4, contrary to recent studies that advocate a charge-segregated non-polar bilayer model. Weak scattering observed below the magnon gap suggests that a fraction of the bilayers contain other combinations of charged monolayers not included in the model. Refined values for the dominant exchange interactions are reported.

Non-Collinear Magnetic Configurations at Finite Temperature in Thin Films

The finite-temperature magnetism of a monolayer on a bcc (110) surface was examined using a model Hamiltonian containing ferromagnetic or antiferromagnetic exchange interactions, Dzyaloshinsky-Moriya (DM) interactions, and easy-axis on-site anisotropy. We examined the competition between the collinear ground state parallel to the easy axis and the spin spiral state in the plane perpendicular to this axis preferred by the DM interaction. Using approximative methods to calculate the magnon spectrum at finite temperatures, it was found that even if the ground state is collinear, increasing the DM interaction strongly decreases the critical temperature where this collinear order disappears. Using atomistic spin dynamics simulations, it was found that at this critical temperature the system transforms into the non-collinear state. Including external magnetic field helps stabilizing the ferromagnetic state. An effect due to the finite size of the magnetic monolayer was included in the model considering a different value for the anisotropy at the edges of the monolayer. This effect was shown to stabilize the spin spiral state by fixing the phase at the ends of the stripe.

Unified approach to electronic, thermodynamical, and transport properties ofFe3SiandFe3Alalloys

The electronic, thermodynamical, and transport properties of ordered ${\mathrm{Fe}}_{3}X$ ($X=\text{Al},\mathrm{Si}$) alloys are studied from first principles. We present here a unified approach to the phase stability, the estimate of the Curie temperature, the temperature dependence of sublattice magnetizations, magnon spectra, the spin-stiffnesses, and residual resistivities. An important feature of the present study is that all calculated physical properties are determined in the framework of the same first-principles electronic structure model combined with the effective Ising and Heisenberg Hamiltonians used for study of the thermodynamical properties of alloys. Curie temperatures, spin-stiffnesses, and magnon spectra are determined using the same calculated exchange integrals. Finally, the transport properties are calculated using the linear-response theory. Our theoretical estimates compare well with available experimental data. In particular, calculations predict (in agreement with experiment) the ordered $D{0}_{3}$ phase as the ground-state alloy structure, demonstrate that a correct relation of Curie temperatures of ${\mathrm{Fe}}_{3}\mathrm{Al}/{\mathrm{Fe}}_{3}\mathrm{Si}$ alloys can be obtained only by going beyond a simple mean-field approximation, provide reasonable estimates of spin-stiffnesses, and give resistivities compatible with structural disorder observed in the experiment. Although the calculated temperature dependences of the Fe magnetization on different sublattices are similar, they nevertheless deviate more than in the experiment, and we discuss a possible origin.

Magnon-skyrmion scattering in chiral magnets

Understanding interactions between skyrmions and magnons is an active area of research pursued by several groups. The authors of this manuscript present a comprehensive analytical theory for the effects of skyrmions on the magnon spectrum that can lead to a topological magnon Hall effect. Conversely, magnons affect the dynamics of skyrmions via an effective reactive force resulting in a large skyrmion Hall effect.

Magnetic excitations in a spin-orbit-coupledd4Mott insulator on the square lattice

We study the magnetic order and excitations in strong spin-orbit coupled, Van Vleck-type, $d^4$ Mott insulators on a square lattice. Extending the previous work, we include the tetragonal crystal field splitting and explore its effects on magnetic phase diagram and magnon spectra. Two different ordered phases, with in-plane and out-of-plane orientation of the staggered moments, are found for the higher and lower values of the crystal field splitting, respectively. The magnetic excitation spectra for paramagnetic and magnetically ordered phases are calculated and discussed in the context of a candidate spin-orbit $d^4$ Mott insulator Ca$_2$RuO$_4$.

S-duality and the giant magnon dispersion relation

We use S-duality and planarity to propose an argument for the non-renormalization of the dispersion relation of giant magnon solutions in type IIB string theory on AdS_5 x S^5. We compute the spectrum of giant magnons for (p,q)-strings from field theory at strong coupling by using the central charge properties of electrically and magnetically charged supersymmetric states in the Coulomb branch of N=4 super Yang-Mills. We argue that the coupling dependence of the giant magnon dispersion relation conjectured in the literature using integrability assumptions is in fact the only functional dependence compatible with S-duality.

Magnetoelastic modes and lifetime of magnons in thin yttrium iron garnet films

We calculate the effects of the spin-lattice coupling on the magnon spectrum of thin ferromagnetic films consisting of the magnetic insulator yttrium-iron garnet. The magnon-phonon hybridisation generates a characteristic minimum in the spin dynamic structure factor which quantitatively agrees with recent Brillouin light scattering experiments. We also show that at room temperature the phonon contribution to the magnon damping exhibits a rather complicated momentum dependence: In the exchange regime the magnon damping is dominated by Cherenkov type scattering processes, while in the long-wavelength dipolar regime these processes are subdominant and the magnon damping is two orders of magnitude smaller. We supplement our calculations by actual measurements of the magnon relaxation in the dipolar regime. Our theory provides a simple explanation of a recent experiment probing the different temperatures of the magnon and phonon gases in yttrium-iron garnet.

Stoner Instability Revisited: Emergence of Local Quantum Criticality?

We revisit Stoner instability, an old problem but in a modern point of view. An idea is to extract out dynamics of directional fluctuations of spins explicitly, resorting to the CP$^{1}$ representation and integrating over their amplitude fluctuations. As a result, we derive an effective field theory for ferromagnetic quantum phase transitions in terms of bosonic spinons and fermionic holons. We show that this effective field theory reproduces overdamped spin dynamics in a paramagnetic Fermi liquid and magnon spectrum in a ferromagnetic Fermi liquid. An interesting observation is that the velocity of spinons becomes zero, approaching the ferromagnetic quantum critical point, which implies emergence of local quantum criticality. Based on this scenario, we predict the $\omega/T$ scaling behavior near ferromagnetic quantum criticality beyond the conventional scenario of the weak-coupling approach.

Magnon spectra and strong spin-lattice coupling in magnetically frustratedMnB2O4(B=Mn,V): Inelastic light-scattering studies

The ferrimagnetic spinels MnB2O4 (B = Mn,V) exhibit a similar series of closely spaced magnetic and structural phase transitions at low temperatures, reflecting both magnetic frustration and a strong coupling between the spin and lattice degrees of freedom. Careful studies of excitations in MnB2O4 (B = Mn,V), and the evolution of these excitations with temperature, are important for obtaining a microscopic description of the role that magnetic excitations and spin-lattice coupling play in the low temperature phase transitions of these materials. We report an inelastic light (Raman) scattering study of the temperature and magnetic field dependences of one- and two-magnon excitations in MnV2O4 and Mn3O4. We observe a pair of q=0 one-magnon modes at 74 cm^{-1} and 81 cm^{-1} in MnV2O4, which is in contrast with the single 80 cm^{-1} q=0 magnon that has been reported for MnV2O4 based on previous neutron scattering measurements and spin wave calculations. Additionally, we find that the two-magnon energy of MnV2O4 decreases ("softens") with decreasing temperature below T_{N}, which we attribute to strong coupling between magnetic and vibrational excitations near the zone boundary.

Magneto- to electroactive transmutation of spin waves in ErMnO3.

The low-energy dynamical properties of the multiferroic hexagonal perovskite ErMnO3 have been studied by inelastic neutron scattering as well as terahertz and far infrared spectroscopies on a synchrotron source. From these complementary techniques, we have determined the magnon and crystal field spectra and identified a zone center magnon excitable only by the electric field of an electromagnetic wave. Using a comparison with the isostructural YMnO3 compound and crystal field calculations, we propose that this dynamical magnetoelectric process is due to the hybridization of a magnon with an electroactive crystal field transition.

Effects of high pressure on the structural, magnetic, and transport properties of the itinerant 5fferromagnetU2Fe3Ge

Crystal structure, magnetic, and transport properties of the ${\text{U}}_{2}$${\text{Fe}}_{3}$Ge compound have been studied under hydrostatic pressure. ${\text{U}}_{2}$${\text{Fe}}_{3}$Ge crystallizes in the hexagonal crystal structure of the ${\text{Mg}}_{2}$${\text{Cu}}_{3}$Si type, an ordered variant of the ${\text{MgZn}}_{2}$ Laves structure (C14). The Laves phase has proven highly stable; the crystal structure is maintained up to 27 GPa, the highest applied pressure. X-ray diffraction data revealed that the $c$ direction, where the shortest U-U bonds lie, is softer than the $a$ direction. The bulk modulus ${B}_{0}$ is \ensuremath{\sim}100 GPa. ${\text{U}}_{2}$${\text{Fe}}_{3}$Ge is an itinerant ferromagnet with the Curie temperature ${T}_{\mathrm{C}}$ = 55 K and the spontaneous magnetic moment ${M}_{\mathrm{s}}=1{\ensuremath{\mu}}_{\mathrm{B}}$ per formula unit at ambient pressure. The two parameters decrease rapidly under pressure with the rates $\mathrm{d}(\mathrm{ln}\phantom{\rule{0.16em}{0ex}}{M}_{s})/\mathrm{d}p=\ensuremath{-}0.33$ GPa${}^{\ensuremath{-}1}$ and $\mathrm{d}(\mathrm{ln}\phantom{\rule{0.16em}{0ex}}{T}_{\mathrm{C}})/\mathrm{d}p=\ensuremath{-}0.27$ GPa${}^{\ensuremath{-}1}$. The extrapolation of ${T}_{\mathrm{C}}$ and ${M}_{\mathrm{s}}$ indicates that the magnetic order will be suppressed between 3 and 4 GPa. Electrical resistivity was found to decrease gradually with increasing pressure. The data in the ferromagnetic state points to the opening of a gap in the magnon spectrum at higher pressures. ${\text{U}}_{2}$${\text{Fe}}_{3}$Ge is relatively more sensitive to pressure than isostructural ${\text{UNi}}_{2}$.

Spin wave spectra and spatially modulated structures in BiFeO3

The spin wave spectra of antiferromagnetic BiFeO3-type multiferronics are analyzed theoretically. The presence of a spatially modulated cycloidal antiferromagnetic structure leads to a countable number of frequency branches of two oscillatory modes (Goldstone and activation) for spin waves propagating along a cycloid. When there is no magnetic field and anisotropy, the magnon spectrum is characterized by the absence of frequency gaps. The spectral features of the spin oscillations with changing anisotropy and application of a magnetic field are identified and the limits on the existence of an antiferromagnetic cycloid are established up to its transformation into a conical structure. In the transverse direction the spin oscillations have a mixed character which indicates that the cycloid is stable with respect to bending throughout its domain of existence.

Relativistic and thermal effects on the magnon spectrum of a ferromagnetic monolayer

A spin model including magnetic anisotropy terms and Dzyaloshinsky–Moriya interactions is studied for the case of a ferromagnetic monolayer with C2v symmetry like Fe/W(110). Using the quasiclassical stochastic Landau–Lifshitz–Gilbert equations, the magnon spectrum of the system is derived using linear response theory. The Dzyaloshinsky–Moriya interaction leads to asymmetry in the spectrum, while the anisotropy terms induce a gap. It is shown that, in the presence of lattice defects, both the Dzyaloshinsky–Moriya interactions and the two-site anisotropy lead to a softening of the magnon energies. Two methods are developed to investigate the magnon spectrum at finite temperatures. The theoretical results are compared to atomistic spin dynamics simulations and good agreement is found between them.

Spin Conductivity in Two-Dimensional Non-Collinear Antiferromagnets

We propose a method to derive the spin current operator for non-collinear Heisenberg antiferromagnets. We show that the spin conductivity calculated by the spectral representation with the spin current satisfies the f-sum rule. We also study the spin conductivity at T=0 within spin wave theory. We show how the spin conductivity depends on the external magnetic field with changing magnon spectrum. We also find that the spin Drude weight vanishes for any external magnetic field at T=0.

Theory of asymmetric and negative differential magnon tunneling under temperature bias: Towards a spin Seebeck diode and transistor

We study the nonequilibrium transport for the asymmetric and negative differential magnon tunneling driven by temperature bias. We demonstrate that the many-body magnon interaction that makes the magnonic spectrum temperature-dependent is the crucial factor for the emergence of rectification and negative differential spin Seebeck effects in magnon tunneling junctions. When magnonic junctions have temperature-dependent density of states, reversing the temperature bias is able to give asymmetric spin currents and increasing temperature bias could give an anomalously decreasing magnonic spin current. We show that these properties are relevant for building spin Seebeck diodes and transistors, which could play important roles in controlling information and energy in magnonics and spin caloritronics.

Interatomic Exchange Interactions for Finite-Temperature Magnetism and Nonequilibrium Spin Dynamics

We derive ab inito exchange parameters for general noncollinear magnetic configurations, in terms of a multiple scattering formalism. We show that the general exchange formula has an anisotropiclike term even in the absence of spin-orbit coupling, and that this term is large, for instance, for collinear configuration in bcc Fe, whereas for fcc Ni it is quite small. We demonstrate that keeping this term leads to what one should consider a biquadratic effective spin Hamiltonian even in the case of collinear arrangement. In noncollinear systems this term results in new tensor elements that are important for exchange interactions at finite temperatures, but they have less importance at low temperature. To illustrate our results in practice, we calculate for bcc Fe magnon spectra obtained from configuration-dependent exchange parameters, where the configurations are determined by finite-temperature effects. Our theory results in the same quantitative results as the finite-temperature neutron scattering experiments.

Role of boundaries in micromagnetic calculations of magnonic spectra of arrays of magnetic nanoelements

We have used micromagnetic simulations performed with open and periodic boundary conditions to study the influence of the presence of array boundaries on the spectra and spatial profiles of collective spin-wave excitations in arrays of magnetic nanoelements. The spectra and spatial profiles of collective spin waves excited in isolated arrays of nanoelements and those forming a part of quasi-infinite arrays are qualitatively different even if the same excitation field is used in the simulations. In particular, the use of periodic boundary conditions suppresses the excitation of nonuniform collective modes by uniform excitation fields. However, the use of nonuniform excitation fields in combination with periodic boundary conditions is shown to enable investigation of the structure of magnonic dispersion curves for quasi-infinite arrays (magnonic crystals) in different directions in the reciprocal space and for different magnonic bands. The results obtained in the latter case show a perfect agreement with those obtained with the dynamical matrix method for infinite arrays of nanoelements of the same geometry and magnetic properties.

Spin-state crossover model for the magnetism of iron pnictides.

We propose a minimal model describing magnetic behavior of Fe-based superconductors. The key ingredient of the model is a dynamical mixing of quasidegenerate spin states of Fe(2+) ion by intersite electron hoppings, resulting in an effective local spin S(eff). The moments S(eff) tend to form singlet pairs and may condense into a spin nematic phase due to the emergent biquadratic exchange couplings. The long-range ordered part m of S(eff) varies widely, 0 ≤ m ≤ S(eff), but magnon spectra are universal and scale with S(eff), resolving the puzzle of large but fluctuating Fe moments. Unusual temperature dependences of a local moment and spin susceptibility are also explained.

Magnetic, transport and electronic properties of SmNi4Si compound

The polycrystalline sample of SmNi4Si was investigated by means of powder X-ray diffraction, magnetic susceptibility, magnetization, electrical resistivity and specific heat measurements. This compound crystallizes in the hexagonal CaCu5-type structure (space group P6/mmm) and orders magnetically at TC = 14.9 K. The magnetization, electrical resistivity and specific heat data indicate that the electronic ground state of the SmNi4Si has a ferromagnetic-like character. The magnitude of energy gap in the magnon spectrum calculated from electrical resistivity data revealed the presence of high influence of magnetocrystalline anisotropy effect. Our experimental studies were supported by first-principles calculations employing DFT method together with LSDA + U approach. We explore implication of U parameter for magnetization of SmNi4Si. Obtained theoretical result shows that the electronic structure and magnetic properties of SmNi4Si for U = 6 eV is closer to our experimental results.