Magnon Phonon Research Articles

Microscopic origin of subthermal magnons and the spin Seebeck effect

Recent experimental evidence points to low-energy magnons as the primary contributors to the spin Seebeck effect. This spectral dependence is puzzling since it is not observed on other thermocurrents in the same material. Here, we argue that the physical origin of this behavior is the magnon–magnon scattering mediated by phonons, in a process which conserves the number of magnons. To assess the importance and features of this kind of scattering, we derive the effective magnon–phonon interaction from a microscopic model, including band energy, a screened electron–electron interaction and the electron–phonon interaction. Unlike higher order magnon-only scattering, we find that the coupling with phonons induce a scattering which is very small for low-energy (or subthermal) magnons but increases sharply above a certain energy—rendering magnons above this energy poor spin-current transporters.

Prediction of electromagnons in antiferromagnetic ferroelectrics from first-principles: The case of BiFeO3

abstractA first-principles-based computational approach is developed to study finite-temperature complex dynamics in multiferroics. Application of the technique to one of the most well-known multiferroic, BiFeO3, predicts the existence of electromagnons that have so far remained elusive for first-principles computations. It is demonstrated that the most general form of the magnetoelectric coupling that is allowed in any multiferroic, may produce an electromagnon in antiferromagnetic ferroelectric. This finding could be explained from the basic theory of antiferromagnetic resonance. The temperature evolution of the electromagnon, magnon and soft phonon modes is studied to trace the intrinsic features of these excitations. A connection with experiment is also made.

Model calculation of thermal conductivity in antiferromagnets

A theoretical study is given of thermal conductivity in antiferromagnetic materials. The study has the advantage that the three-phonon interactions as well as the magnon phonon interactions have been represented by model operators that preserve the important properties of the exact collision operators. A new expression for thermal conductivity has been derived that involves the same terms obtained in our previous work in addition to two new terms. These two terms represent the conservation and quasi-conservation of wavevector that occur in the three-phonon Normal and Umklapp processes respectively. They gave appreciable contributions to the thermal conductivity and have led to an excellent quantitative agreement with the experimental measurements of the antiferromagnet FeCl2.

Phonon thermal transport and phonon–magnon coupling in polycrystalline BiFeO3 systems

Temperature-dependent thermal conductivity of polycrystalline BiFeO3, Bi0.9Ba0.05Ca0.05FeO2.95 and Bi0.9Ca0.1FeO2.95 materials was measured using a direct heat pulse technique. Thermal conductivity of the BiFeO3-based materials is analyzed using a phonon model to probe the thermal transport mechanisms in these ferrites. It is found that the calculated thermal conductivity of the BiFeO3-based compounds is in good agreement with experimental data. The suppression of the low-temperature phonon peak in the thermal conductivity of the doped BiFeO3 materials is mainly attributed to the phonon-point-defect scattering. In addition, the contribution of optical phonon–magnon resonance scattering to optical phonon thermal transport reveals the presence of phonon–magnon coupling in these BiFeO3 materials. Finally, magneto-thermal conductivity measurements show the magnon thermal transport in pure and doped BiFeO3 systems.

Theory of Ultrafast Demagnetization after Femtosecond Laser Pulses

Experimentally it has been found that a ferromagnetic metallic lm is strongly demagnetized within few hundred fs after exposure to a fs laser pulse. The theories of this ultrafast demagnetization are reviewed, especially the in uence of scatterings of the excited electrons at phonons and at magnons. The calculations for phonons and magnons are performed by the ab initio spin-density-functional electron theory for Ni and Fe, and they are based on Fermi's golden rule for transition rates. The application of this rule on the fs time scale is critically discussed in view of results from quantum-kinetic density-matrix calculations. It is shown that the experimentally observed demagnetization rates cannot be explained by spinip scatterings of electrons exclusively at phonons or exclusively at magnons. A combination of individual spinip electron phonon and spinip electron magnon processes is shown to be a potential candidate for the explanation of ultrafast demagnetization.

Andreev-spectroscopy study of unconventional superconductivity in MgB2:(La,Sr)MnO3 nanocomposite

Unconventional high-temperature superconductivity in MgB2:La0:65Sr0:35MnO3 (MgB:LSMO) nanocomposite has been found recently [V. N. Krivoruchko and V. Yu. Tarenkov, Phys. Rev. B 86, 10502 (2012)]. In this report, the symmetry of the nanocomposite superconducting order parameter and plausible pairing mechanisms have been studied by the point-contact Andreev-reflection (PCAR) spectroscopy. To clarify the experimental results obtained, we consider a model of a ferromagnetic superconductor, which assumes a coexistence of itinerant ferromagnetism and mixed-parity superconductivity. The Balian–Werthamer state, with quasiparticle gap topology of the same form as that of the ordinary s- wave state, fits the experimental data reasonably well. Utilizing the extended Eliashberg formalizm, we calculated the contribution of MgB2 in the total composite's conductivity and estimated the magnitude of the electron–phonon effects originated from MgB2 in I–V characteristics of the composite at above-gap energies. It was found that distinctive features observed in the PC spectra of the MgB:LSMO samples and conventionally attributed to the electron–phonon interaction cannot be related to the MgB2 phonons. It is argued that the detected singularities may be a manifestation of the electron-spectrum renormalizations due to strong magnetoelastic (magnon–phonon) interaction in LSMO.

Magneto-thermal conduction and magneto-caloric effect in poly and nano crystalline forms of multiferroic GdCrO3

Gadolinium chromite, GdCrO3, belongs to the family of rare earth chromites, exhibiting multiferroism with coupling between electric polarization and magnetic ordering. It is understood that the interaction between Gd3+ and Cr3+ ions is responsible for switchable polarization in this system. Below Néel temperature the spins of Cr3+ ions interact in anti-parallel through super exchange mechanism, giving rise to antiferromagnetic ordering at around 169 K in poly and nanocrystalline phases of this material. In order to understand the nature of spin–lattice coupling and magnon–phonon interaction in the intermediate temperature range (150–250 K), the magneto-thermal conduction and magneto-caloric effect in poly and nanocrystalline forms of this material are reported. These properties show anomalies around 169 K, which is described as due to spin–phonon coupling. When particle sizes are reduced to nanometer scales, thermal conductivity decreases significantly while specific heat capacity increases. The former is explained as due to reduction in phonon mean free path and phonon scattering from nanoparticle interfaces, while the latter is ascribed to contributions from Einstein oscillators at weakly bound atoms at the interfaces of nanocrystals.

Lattice and spin excitations in multiferroich-YMnO3

We used Raman and terahertz spectroscopies to investigate lattice and magnetic excitations and their cross-coupling in the hexagonal YMnO3 multiferroic. Two phonon modes are strongly affected by the magnetic order. Magnon excitations have been identified thanks to comparison with neutron measurements and spin wave calculations but no electromagnon has been observed. In addition, we evidenced two additional Raman active peaks. We have compared this observation with the anti-crossing between magnon and acoustic phonon branches measured by neutron. These optical measurements underly the unusual strong spin-phonon coupling.

Thermopower studies of rare earth doped lanthanum barium manganites

Influence of rare earth doping on electrical, magnetic and thermopower studies of La0.34Re0.33Ba0.33MnO3 compound was investigated. Ferro to paramagnetic transition and metal to insulator transition temperatures decrease with decreasing ionic radius of the dopant ion. Electrical resistivity in the entire temperature range is explained by phase separation model. The magnitude of Seebeck coefficient increases with increasing dopant ionic radius. A cross over from negative to positive sign has also been observed in thermopower data with decreasing A site ionic radius (〈rA〉). The low temperature thermopower data has been explained using a qualitative model containing diffusion; magnon drag and phonon drag effects while the paramagnetic insulating part has been analyzed using small polaron hopping mechanism.

Magneto-thermal conduction and phonon anomalies across magnetic transitions in multiferroic (poly and nanocrystalline) bismuth ferrite

Bismuth ferric oxide (BFO) or bismuth ferrite is a multiferroic material with perovskite structure in which ferroelectric and antiferromagnetic orderings coexist. The magneto-electric coupling in this material makes it interesting from fundamental physics and applications points of view. As a result of complex magneto-elastic coupling and spin-glass behavior at low temperatures, the material exhibits a number of phase transitions driven by magnetic ordering. Earlier reports indicate that the primary order parameter in these transitions is not polarization but are related to magnon mode softening. In order to throw more light on the magneto-elastic and phonon related properties of this material, we measured the thermal transport properties, thermal conductivity and specific heat capacity, in the presence of an external magnetic field and compared the results with the zero field case. Results are reported for polycrystalline as well as nanocrystalline samples of BFO between 140K and 250K. A photopyroelectric thermal wave technique has been employed for the measurements. Anomalies in thermal properties observed at 140K, 200K and 240K in polycrystalline samples as well as their changes with applied field are explained in terms of magneto-elastic and spin–phonon couplings. It is found that the transitions get less well defined and one of the transition temperatures get shifted upwards considerably as the particle sizes are reduced to nanometer scales. Particle size dependences of phonon and magnon–phonon scattering are invoked to explain these results.

Phonon–magnon coupling in CoF2 investigated by time-of-flight neutron spectroscopy

We report the results of inelastic neutron scattering investigation on the model antiferromagnet CoF2 by time-of-flight neutron spectroscopy. We measured the details of the scattering function S(Q,ω) as a function of temperature with two different incident neutron wavelengths. The temperature and Q dependence of the measured scattering function suggests the presence of magnon–phonon coupling in almost all branches. The present results are in agreement with the strong magnetoelastic effects observed previously.

Magnetic Phases of ZnCr2O4Revealed by Magneto-Optical Studies under Ultra-High Magnetic Fields of up to 600 T

Faraday rotation and magneto-optical absorption spectral measurements were conducted on a spinel oxide, ZnCr 2 O 4 , a prototype of three-dimensional geometrically frustrated magnet. The measurements were carried out at temperatures down to 4.6 K under ultra-high magnetic fields of up to 600 T. The ultra-high magnetic fields were generated by the electro-magnetic flux compression method. We obtained a precise magnetization curve up to a fully polarized phase, where the phase transition takes place above 400 T. The experimental magnetization curves were compared with those obtained by Monte Carlo calculations with an effective spin model including spin–lattice coupling up to fully saturated magnetization. The absorption spectral peaks of the intra- d -band transitions in Cr 3+ ions as well as the exciton–magnon–phonon transition were used for monitoring the crystal and magnetic structures subjected to a strong external magnetic field. A novel magnetic phase was found prior to the fully polarized phase, whi...

Exact and model operators for magnon–phonon interactions in antiferromagnets

A theoretical study is given of magnon–phonon interactions in antiferromagnetic materials. The roles of magnons and phonons as heat carriers and as sources of thermal resistance have been taken into consideration. The exact collision operator which represents the magnon–phonon interactions involved in the transport Boltzmann equations has been replaced by a model operator which possesses the same important properties. The effect of other scattering processes that either phonons or magnons are involved has also been investigated. A new expression for the thermal conductivity has been derived. It includes terms which represent both Normal and Umklapp magnon–phonon processes. The results obtained by using the new expression agree quantitatively with the experimental measurements on Fe Cl 2

Effect of optical phonon–magnon interaction on the magnon damping at finite temperature

A magnon–phonon interaction model is set up in a two-dimensional ferromagnetic compound square-lattice system. By using Matsubara Green function theory we have calculated the magnon damping on the main symmetric lines in the Brillouin zone, compared the influences of magnetic ion optical-mode phonon and non-magnetic ion optical-mode phonon on the magnon damping of the system, and discussed the influences of the parameter variations and temperature on the magnon damping. The results show that the coupling of the optical-mode phonon and the magnon plays an important role for the magnon damping, especially the longitudinal optical-mode phonon contributes the most for the magnon damping. It is also found that the effect of the non-magnetic ion’s optical-mode phonon on magnon damping is more significant than that of the magnetic ion. By the expression − Im Σ ∗ ( 1 ) ( k ) = ħ / ( 2 τ ) , the magnon life-time and the magnon line-width are discussed either.

One-magnon light scattering and spin-reorientation transition in epitaxial BiFeO 3 thin films

Raman scattering from one-magnon excitation has been observed for the first time in epitaxial BiFeO 3 thin films grown on (1 1 1) SrTiO 3 substrates. The intensities and the frequency of the magnon mode at 18.9 cm −1 (M 1) showed a discrepancy at the characteristic temperatures of ∼140 and 200 K and the magnon mode at 27.9 cm −1 (M 2) disappeared at ∼200 K suggesting spin-reorientation (SR) transition in the epitaxial BFO film. The dc susceptibility measurement showed a large discrepancy near these two temperatures evidently elucidating the spin-reorientation transition mechanism. The partial spectral weight of the magnon modes is believed to be transferred to the lowest phonon mode appearing at 72.8 cm −1 and higher magnon mode M 2 disappearing near 200 K reveal magnon–phonon coupling near to SR transition.

Ultrafast dynamics at lanthanide surfaces: microscopic interaction of the charge, lattice and spin subsystems

In solids the charge, lattice and spin degrees of freedom are coupled and the respective coupling strengths result in characteristic timescales on which excitations of one particular subsystem interact and equilibrate with the remaining subsystems. In ferromagnetic metals the respective elementary interaction processes occur on the pico- and femtosecond time scales. To elucidate these interaction mechanisms and the timescales we investigate the ultrafast dynamics of electrons, phonons and spins excited by an intense infrared optical pulse at Gd(0 0 0 1) and Tb(0 0 0 1) surfaces, employing complementary time-resolved methods of optical second harmonics generation and photoelectron spectroscopy. These surfaces exhibit rich dynamics including a collective response of the crystal lattice and the magnetization, manifested by a coupled coherent optical phonon–magnon mode at 3 THz. After a review of the earlier studies we present temperature- and fluence-dependent results of pump–probe experiments. The temperature dependence of the coherent phonon (CP) amplitude shows that a spin- and a charge-driven excitation mechanism can be separated. The spin-driven excitation dominates below the Curie temperature in agreement with ab initio model calculations that establish a displacement of the surface plane upon a change of the surface spin polarization. Analysis of the temperature-dependent CP damping shows an anomaly in the vicinity of the Curie temperature that is for Gd well described by phonon–magnon scattering. In the case of Tb an additional damping channel is required that could be related to local spin-flip excitations.

Magneto-transport and thermal properties of Pr2/3Ba1/3(Mn1−xSbx)O3 system

Magneto-transport and thermal studies on Sb-doped Pr2/3Ba1/3(Mn1−xSbx)O3 (0 ⩽ x ⩽ 0.03) manganites are reported here. Two transitions (at TP1 and TP2) observed in the electrical resistivity behaviour of pristine sample Pr2/3Ba1/3MnO3 shift to lower temperatures with an overall increase in the electrical resistivity. Electrical resistivity upturn observed at low temperatures gets enhanced with increasing Sb content and the minimum resistivity temperature shifts to higher temperatures. Intrinsic magneto-resistance (MR) at TP1 gets suppressed whereas extrinsic MR gets enhanced with Sb-doping. With higher Sb content two MR peaks are observed and MR at the low temperature peak is higher than at the high temperature peak. Thermoelectric power (TEP) of the pristine sample shows a transition from an insulating behaviour at high temperatures to a metallic behaviour at low temperatures. With Sb-doping TEP shows anomalous behaviour as a peak is developed near the phase transition (TP1) due to the weakening of the double-exchange mechanism and increase in magnetic inhomogeneity of the samples. Conduction at high temperatures is due to the hopping of small polarons and the activation energy increases due to the localization of carriers with Sb-doping. Electron–magnon interaction in the metallic region also gets suppressed with Sb-doping. Specific heat measurements exhibit a peak below TP1 and the area under the peak decreases, indicating the increase in magnetic inhomogeneity with Sb-doping. Thermal conductivity κ above TP1 is a result of the local anharmonic distortions associated with small polarons. The anomalous peak in κ around TP1 is attributed to a competition of the reduction in the Jahn–Teller distortion and magnon–phonon scattering enhancement.

Optical coupling to spin waves in the cycloidal multiferroicBiFeO3

The magnon and optical phonon spectrum of an incommensurate multiferroic such as $\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_{3}$ is considered in the framework of a phenomenological Landau theory. The resulting spin wave spectrum is quite distinct from commensurate substances due to soft mode anisotropy and magnon zone folding. The former allows electrical control of spin wave propagation via reorientation of the spontaneous ferroelectric moment. The latter gives rise to multiple magnetodielectric resonances due to the coupling of optical phonons at zero wave vector to magnons at integer multiples of the cycloid wave vector. These results show that the optical response of a multiferroic reveals much more about its magnetic excitations than previously anticipated on the basis of simpler models.

Dynamics of the quantum integer spinS one-dimensional Heisenberg antiferromagnet coupled to phonons

We present the low-temperature dynamic properties of the quantumone-dimensional Heisenberg antiferromagnet with integer spinS coupled to phonons. The calculation of the relaxation function is performedusing the combination of the memory function formalism with the modifiedspin-wave theory. The procedure includes up to two-magnon and magnon–phononprocesses. We show that the phonon–magnon coupling smooths the double-peakstructure obtained for the model in the absence of phonons, when the wavevectorq movesfrom q = π.

Magnon–phonon coupling in two-dimensional Heisenberg ferromagnetic system

A magnon–phonon interaction model is set up for a two-dimensional insulating ferromagnetic system. By using Matsubara function theory, we have studied the magnon spectrum and calculated the magnon dispersion curve on the main symmetric point and line in the Brillouin zone for various parameters of the system. The results show that at the boundary of Brillouin zone, there is a strong magnon softening. The contributions of longitudinal and transverse phonons on the magnon softening are compared and the influences of various parameters are also discussed.