Strong Magnon-phonon Coupling Research Articles

Direct observation of topological magnon polarons in a multiferroic material

Magnon polarons are novel elementary excitations possessing hybrid magnonic and phononic signatures, and are responsible for many exotic spintronic and magnonic phenomena. Despite long-term sustained experimental efforts in chasing for magnon polarons, direct spectroscopic evidence of their existence is hardly observed. Here, we report the direct observation of magnon polarons using neutron spectroscopy on a multiferroic Fe2Mo3O8 possessing strong magnon-phonon coupling. Specifically, below the magnetic ordering temperature, a gap opens at the nominal intersection of the original magnon and phonon bands, leading to two separated magnon-polaron bands. Each of the bands undergoes mixing, interconverting and reversing between its magnonic and phononic components. We attribute the formation of magnon polarons to the strong magnon-phonon coupling induced by Dzyaloshinskii-Moriya interaction. Intriguingly, we find that the band-inverted magnon polarons are topologically nontrivial. These results uncover exotic elementary excitations arising from the magnon-phonon coupling, and offer a new route to topological states by considering hybridizations between different types of fundamental excitations.

Thermal Hall effect in a van der Waals triangular magnet FeCl2

Thermal transport is a pivotal probe for studying low-energy, charge-neutral quasiparticles in insulating magnets. In this Letter, we report an observation of large magnetothermal conductivity and thermal Hall effect (THE) in a van der Waals antiferromagnet ${\mathrm{FeCl}}_{2}$. The magnetothermal conductivity reaches over $\ensuremath{\sim}700%$, indicating strong magnon-phonon coupling. Furthermore, we find an appreciable thermal Hall signal which changes sign concurrently with the spin-flip transition from the antiferromagnetic state to the polarized ferromagnetic state. Our theoretical calculations suggest that, in addition to the Berry curvature induced at the anticrossing points of the hybridized magnon and acoustic phonon modes of ${\mathrm{FeCl}}_{2}$, other mechanisms are needed to account for the magnitude of the observed THE.

Nuclear and magnetic spin structure of the antiferromagnetic triangular lattice compound LiCrTe2 investigated by mu ^+SR, neutron and X-ray diffraction

Two-dimensional (2D) triangular lattice antiferromagnets (2D-TLA) often manifest intriguing physical and technological properties, due to the strong interplay between lattice geometry and electronic properties. The recently synthesized 2-dimensional transition metal dichalcogenide LiCrTe_2, being a 2D-TLA, enriched the range of materials which can present such properties. In this work, muon spin rotation (mu ^+SR) and neutron powder diffraction (NPD) have been utilized to reveal the true magnetic nature and ground state of LiCrTe_2. From high-resolution NPD the magnetic spin order at base-temperature is not, as previously suggested, helical, but rather collinear antiferromagnetic (AFM) with ferromagnetic (FM) spin coupling within the ab-plane and AFM coupling along the c-axis. The value if the ordered magnetic Cr moment is established as mu _{textrm{Cr}}= 2.36~mu _{textrm{B}}. From detailed mu ^+SR measurements we observe an AFM ordering temperature T_{textrm{N}}approx 125 K. This value is remarkably higher than the one previously reported by magnetic bulk measurements. From mu ^+SR we are able to extract the magnetic order parameter, whose critical exponent allows us to categorize LiCrTe_2 in the 3D Heisenberg AFM universality class. Finally, by combining our magnetic studies with high-resolution synchrotron X-ray diffraction (XRD), we find a clear coupling between the nuclear and magnetic spin lattices. This suggests the possibility for a strong magnon–phonon coupling, similar to what has been previously observed in the closely related compound LiCrO_2.

Entanglement enhancement in cavity magnomechanics by an optical parametric amplifier

We propose a method to enhance bipartite and tripartite entanglement in cavity magnomechanics using an optical parametric amplifier (OPA). We analyze this system and identified parametric regimes where different types of entanglement are enhanced. We show that a proper choice of phase of the parametric amplifier leads to the enhancement of the bipartite entanglements. Moreover, the tripartite entanglement is also significantly enhanced in the presence of the OPA. The OPA not only enhances the strength of entanglement but also increases the domain of entanglement over a wider space of detunings as compared to the system when no OPA is present. Similarly, the robustness of entanglement against temperature is also enhanced. Another important consequence of the OPA is the fact that it relaxes the requirement of strong magnon-phonon coupling to generate cavity-magnon entanglement which is necessary for the case when the OPA is not present. We believe that the presented scheme is a step forward to realize robust quantum entanglement using current technology.

Magnetic control in the terahertz.

Strong magnon-phonon coupling may help develop superfast optical drives.

High-frequency magnetoacoustic resonance through strain-spin coupling in perpendicular magnetic multilayers.

It is desirable to experimentally demonstrate an extremely high resonant frequency, assisted by strain-spin coupling, in technologically important perpendicular magnetic materials for device applications. Here, we directly observe the coupling of magnons and phonons in both time and frequency domains upon femtosecond laser excitation. This strain-spin coupling leads to a magnetoacoustic resonance in perpendicular magnetic [Co/Pd] n multilayers, reaching frequencies in the extremely high frequency (EHF) band, e.g., 60 GHz. We propose a theoretical model to explain the physical mechanism underlying the strain-spin interaction. Our model explains the amplitude increase of the magnetoacoustic resonance state with time and quantitatively predicts the composition of the combined strain-spin state near the resonance. We also detail its precise dependence on the magnetostriction. The results of this work offer a potential pathway to manipulating both the magnitude and timing of EHF and strongly coupled magnon-phonon excitations.

The fifty years it has taken to understand the dynamics of UO2 in its ordered state

In 1966 Roger Cowley (together with Gerald Dolling) reported the first neutron inelastic scattering from the magnetic excitations from UO2 below its antiferromagnetic ordering temperature of 30 K. They showed the strong magnon–phonon coupling in this material and that the excitations appeared to contain an additional mode that was not anticipated. Cowley never returned to UO2, but showed a keen interest in the developments. Forty years after this pioneering work, unambiguous evidence was found (using resonance x-ray techniques) for the ordering below TN of the electric quadrupoles involving the anisotropy of the 5f charge distribution around the uranium nuclei. A further 10 years later, now armed with a full theory for the excitation spectrum expected for phonons, magnons, and quadrupoles, we can identify the latter as the source of the ‘extra’ mode reported first in 1966. The story is a long winding one, with the expected serendipity and dead ends, but is now (almost) completed.

Electromagnon dispersion probed by inelastic X-ray scattering in LiCrO2.

Inelastic X-ray scattering with meV energy resolution (IXS) is an ideal tool to measure collective excitations in solids and liquids. In non-resonant scattering condition, the cross-section is strongly dominated by lattice vibrations (phonons). However, it is possible to probe additional degrees of freedom such as magnetic fluctuations that are strongly coupled to the phonons. The IXS spectrum of the coupled system contains not only the phonon dispersion but also the so far undetected magnetic correlation function. Here we report the observation of strong magnon–phonon coupling in LiCrO2 that enables the measurement of magnetic correlations throughout the Brillouin zone via IXS. We find electromagnon excitations and electric dipole active two-magnon excitations in the magnetically ordered phase and heavily damped electromagnons in the paramagnetic phase of LiCrO2. We predict that several (frustrated) magnets with dominant direct exchange and non-collinear magnetism show surprisingly large IXS cross-section for magnons and multi-magnon processes.

Spontaneous decays of magneto-elastic excitations in non-collinear antiferromagnet (Y,Lu)MnO3

Magnons and phonons are fundamental quasiparticles in a solid and can be coupled together to form a hybrid quasi-particle. However, detailed experimental studies on the underlying Hamiltonian of this particle are rare for actual materials. Moreover, the anharmonicity of such magnetoelastic excitations remains largely unexplored, although it is essential for a proper understanding of their diverse thermodynamic behaviour and intrinsic zero-temperature decay. Here we show that in non-collinear antiferromagnets, a strong magnon–phonon coupling can significantly enhance the anharmonicity, resulting in the creation of magnetoelastic excitations and their spontaneous decay. By measuring the spin waves over the full Brillouin zone and carrying out anharmonic spin wave calculations using a Hamiltonian with an explicit magnon–phonon coupling, we have identified a hybrid magnetoelastic mode in (Y,Lu)MnO3 and quantified its decay rate and the exchange-striction coupling term required to produce it.

Phase transformations in multiferroic Bi1−xLaxFe1−yTiyO3 ceramics probed by temperature dependent Raman scattering

Optical phonons and phase transitions of Bi1−xLaxFe1−yTiyO3 (BLFTO, 0.02 ≤ x ≤ 0.12, 0.01 ≤ y ≤ 0.08) ceramics have been investigated by Raman scattering in the temperature range from 80 to 680 K. Four phase transitions around 140, 205, 570, and 640 K can be observed. The Raman modes are sensitive to the spin reorientation around 140 and 205 K, owing to the strong magnon-phonon coupling. The transformation around 570 K is a structural transition from rhombohedral to orthorhombic phase due to an external pressure induced by the chemical substitution. The anomalies of the phonon frequencies near Néel temperature TN have been discussed in the light of the multiferroicity. Moreover, it was found that the structural transition temperature and TN of BLFTO ceramics decrease towards room temperature with increasing doping composition as a result of size mismatch between substitution and host cations.

A large iron isotope effect in SmFeAsO1 - xF x and Ba1 - xK x Fe2As2

The recent discovery of superconductivity in oxypnictides with a critical transition temperature (T(C)) higher than the McMillan limit of 39 K (the theoretical maximum predicted by Bardeen-Cooper-Schrieffer theory) has generated great excitement. Theoretical calculations indicate that the electron-phonon interaction is not strong enough to give rise to such high transition temperatures, but strong ferromagnetic/antiferromagnetic fluctuations have been proposed to be responsible. Superconductivity and magnetism in pnictide superconductors, however, show a strong sensitivity to the crystal lattice, suggesting the possibility of unconventional electron-phonon coupling. Here we report the effect of oxygen and iron isotope substitution on T(C) and the spin-density wave (SDW) transition temperature (T(SDW)) in the SmFeAsO(1 - x)F(x) and Ba(1 - x)K(x)Fe(2)As(2) systems. The oxygen isotope effect on T(C) and T(SDW) is very small, while the iron isotope exponent alpha(C) = -dlnT(C)/dlnM is about 0.35 (0.5 corresponds to the full isotope effect). Surprisingly, the iron isotope exchange shows the same effect on T(SDW) as T(C). This indicates that electron-phonon interaction plays some role in the superconducting mechanism, but a simple electron-phonon coupling mechanism seems unlikely because a strong magnon-phonon coupling is included.

First-order phase transition inUO2:235Uand17ONMR study

A nuclear magnetic resonance (NMR) study is reported for a $5f$-cooperative Jahn-Teller (JT) system with antiferromagnetic ordering, ${\mathrm{UO}}_{2},$ where we have performed magnetic actinide ${(}^{235}\mathrm{U})$ as well as nonmagnetic ligand ${(}^{17}\mathrm{O})$ NMR studies in a $5f$-electron system. The observed ${}^{235}\mathrm{U}$ hyperfine interaction is consistent with an axially symmetric $5f$-wave-function character for ${\mathrm{U}}^{4+}$ below a first-order transition temperature ${T}_{\mathrm{N}}=30.8\mathrm{K}.$ Thus, the orbital degeneracy in the cubic crystalline field is lifted by a magnetic ordering combined with a JT distortion. On the other hand, modulation phenomena in the ${}^{17}\mathrm{O}$ spin-echo decay provide evidence of a lattice distortion just below ${T}_{\mathrm{N}},$ which gives rise to an axially symmetric electric-field gradient at the oxygen sites. These results indicate that, in a cooperative JT transition which occurs as a result of coupling between $5f$ quadrupoles and lattice distortion, the behavior of the $5f$ quadrupoles can be investigated using uranium NMR, while the lattice distortion can be studied with the nonmagnetic ligand NMR. Among magnetic structures and JT distortions, proposed on the basis of neutron scattering experiments and theoretical work, the noncollinear $3\mathit{k}$-type structure is the most likely, because either the $1\mathit{k}$ or $2\mathit{k}$ would cause an orthorhombic distortion. With respect to low-energy spin-wave excitations, nuclear spin-lattice relaxation rates ${T}_{1}^{\ensuremath{-}1}$ at both sites show a ${T}^{7}$ behavior below ${T}_{\mathrm{N}},$ which suggests the presence of gapless excitations due to strong magnon-phonon coupling.

Magnon broadening effects in double-layered manganite La 1.2Sr 1.8Mn 2O 7

The magnon line width of La 1.2Sr 1.8Mn 2O 7 near the Brillouin zone boundary is investigated from both theoretical and experimental points of view. Abrupt magnon broadening is ascribed to a strong magnon–phonon coupling. The magnon broadening observed in cubic perovskite manganites is also discussed.

Magnetic excitations in DyAl2

The spin-wave excitation spectrum of ferromagnetic DyAl2 has been measured at 5K by neutron inelastic scattering methods. The scattering at selected momentum transfers has also been measured at elevated temperatures. The results are used to obtain crystal-field terms and exchange interactions. Strong magnon-phonon coupling effects occur. At elevated temperatures the spin waves exhibit considerable lifetime effects well below Tc and an excited-state spin wave is observed.

'Pseudo-acoustic' magnon dispersion in yttrium iron garnet

Neutron spectroscopy measurements have confirmed an earlier intimation of an 'anomalous' anisotropy in the dispersion of 'acoustic magnon like' excitations in yttrium iron garnet Y3Fe5O12. The present experiments have shown this anisotropy to be large and intricate, with energies at a fixed excitation wavelength 25 Å, varying from 5 to 20 meV. A contention that such anisotropy might arise from strong magnon-phonon coupling effects is not supported by experimental investigation of (i) a spin-reorientation transition in Tb0.3Y2.7Fe5O12or (ii) applying variously directed magnetic fields to YIG itself. The experimental dispersion curves are replicated instead by linear spin-wave theory, with fitted values for all exchange interactions J1ad, J2ad, J3ad, J1aa, Δ1aa, J2aa, J3aa, J1dd, J2dd, J3dd, J4dd, J5ddcloser than a/2∼6.2 Å. In this new model, the experimentally observed 'acoustic magnon' anisotropy is attributed to a complex coexistence of acoustic and low-energy optic magnon branches: ensuing computations suggest that possible 'soft modes' of such 'pseudo-acoustic' branches may relate to previously reported 'cantings' of the ferrimagnetic structure in various substituted garnets. The new model also helps in understanding some reported anomalies in existing magnetisation, specific heat and microwave measurements.

Raman scattering study of spin dynamics in the quasi-1D Ising antiferromagnets CsCoCl3and CsCoBr3

Raman measurements on the quasi-1D Ising antiferromagnets CsCoCl3 and CsCoBr3 are presented. The magnons in both systems may be described by a spin-1/2 model and exhibit most interesting quantum features. These features are discussed in the 3D ordered and in the 1D short-range order regimes. Due to the hexagonal structure, 1D phenomena may be studied even in the presence of short-range order. Most of the effects may be described very satisfactorily by a recently developed theory, apart from strong magnon-phonon coupling in CsCoCl3. A set of parameters describing the magnons in these quantum systems is extracted from the data. It is suggested that 3D cooperative phenomena are still present in the 1D regime.

Changes in vibrational modes near the magnetic phase transition in magnetostrictive materials

Mössbauer studies of 57Fe in RFe 2 (R = Y, Tb, Dy, Ho) at temperatures 300 to 800 K and in R(Co 0.98Fe 0.02) 2 (R = Tb, Dy) at temperatures 85 to 300 K have been performed. The spectra yield the temperature dependence of the easy axis of magnetization, the magnetic hyperfine fields, electric field gradients and isomer shift. From the total Mössbauer absorption area the recoil free fraction was determined. The relative intensities of the various Mössbauer absorption lines yield the anisotropy in the recoil free fraction. In all compounds, except in YFe 2, all measured quantities show large variations near the magnetic phase transition temperature ( T c). The variation in the recoil free fraction and in its anisotropy and the variation in the electric field gradient are all consistent, quantitatively, with anisotropic softening in the vibration modes of the Fe nucleus near T c In RFe 2 the modes in which the iron vibrates along the local C 3h symmetry axes are softened less than the perpendicular modes. In RCo 2 it is the other way around. It seems, that these critical softening phenomena are due to the strong magnon-phonon coupling present in these materials and absent in YFe 2. This conclusion is confirmed by a theoretical analysis given in an adjacent paper. In it we use the Green function method to calculate the average of the square of vibrational amplitudes in a Debye solid with a strong magnon-phonon coupling at temperatures close to the magnetic phase transition. We show that these amplitudes may increase or decrease near T c depending on the details of the coupled system.

On the coupling of phonons to magnetic states

The magnetic field dependence of the thermal conductivity of MnCl 2-4H 2O reveals strong magnon-phonon coupling at temperatures well below the Néel temperature, but very weak spin-phonon coupling above this temperature. It appears that modulation of the exchange interaction is responsible for the strong coupling and this mechanism is absent above the transition temperature. Thus coupling of phonons to magnetic states of ordered and disordered magnetic systems differ qualitatively.