Magnon Exchange Research Articles

Spin swapping for an exchange magnon spin current

Spin swapping for an exchange magnon spin current

Interaction between magnon and skyrmion: Toward quantum magnonics

In recent years, magnon and spin texture are attracting great interest in condensed matter physics and magnetism. Magnonics is aiming to use magnon as information carriers to realize functions for storage, transmission, and processing. Magnetic skyrmion is representative spin texture due to its topologically nontrivial properties. Since skyrmions are topologically protected, their transformation to other spin configurations requires overcoming additional topological energy barriers. Therefore, skyrmions are more stable than other trivial spin textures. In addition, the characters of nanoscale size, quasiparticle properties, and various excitation modes make them a potential candidate for spintronic application. Magnon and skyrmion, as two fundamental excitations, can coexist in magnetic systems and interplay with each other through direct exchange interactions. In this review, we provide an overview of recent theoretical and experimental studies on magnon–skyrmion interactions. We mainly focus on three kinds of magnon–skyrmion interactions: (i) magnon scattering by skyrmion, (ii) skyrmion motion driven by magnon, and (iii) coupling between magnon and skyrmion modes. The first two kinds of interactions could be clearly explained by the wave-particle interaction model on the classical level. Alternatively, the last kind of interaction could be understood by the coupled harmonic oscillator model on the quantum level, which indicates fast energy exchange and hybrid magnon states. The exploration focused on quantum phenomena of magnon has led to the emerging field of quantum magnonics and promoted applications of magnon in quantum information storage and processing. In the end, we give a perspective on the exploration of magnon–skyrmion interaction in quantum magnonics.

Visualizing band selective enhancement of quasiparticle lifetime in a metallic ferromagnet

Electrons navigate more easily in a background of ordered magnetic moments than around randomly oriented ones. This fundamental quantum mechanical principle is due to their Bloch wave nature and also underlies ballistic electronic motion in a perfect crystal. As a result, a paramagnetic metal that develops ferromagnetic order often experiences a sharp drop in the resistivity. Despite the universality of this phenomenon, a direct observation of the impact of ferromagnetic order on the electronic quasiparticles in a magnetic metal is still lacking. Here we demonstrate that quasiparticles experience a significant enhancement of their lifetime in the ferromagnetic state of the low-density magnetic semimetal EuCd2As2, but this occurs only in selected bands and specific energy ranges. This is a direct consequence of the magnetically induced band splitting and the multi-orbital nature of the material. Our detailed study allows to disentangle different electronic scattering mechanisms due to non-magnetic disorder and magnon exchange. Such high momentum and energy dependence quasiparticle lifetime enhancement can lead to spin selective transport and potential spintronic applications.

Anatomy of inertial magnons in ferromagnetic nanostructures

We analyze dispersion relations of magnons in ferromagnetic nanostructures with uniaxial anisotropy taking into account inertial terms, i.e., magnetic nutation. Inertial effects are parametrized by the damping-independent parameter $\ensuremath{\beta}$, which allows for an unambiguous discrimination of inertial effects from Gilbert damping parameter $\ensuremath{\alpha}$. The analysis of magnon dispersion relation shows its two branches are modified by the inertial effect, albeit in different ways. The upper nutation branch starts at $\ensuremath{\omega}=1/\ensuremath{\beta}$, the lower branch coincides with ferromagnetic resonance (FMR) in the long-wavelength limit and deviates from the zero-inertia parabolic dependence $\ensuremath{\simeq}{\ensuremath{\omega}}_{\text{FMR}}+D{k}^{2}$ of the exchange magnon. Taking a realistic experimental geometry of magnetic thin films, nanowires, and nanodiscs, magnon eigenfrequencies, eigenvectors, and $Q$-factors are found to depend on the shape anisotropy. The possibility of phase-matched magnetoelastic excitation of nutation magnons is discussed and the condition was found to depend on $\ensuremath{\beta}$, exchange stiffness $D$, and the acoustic velocity.

Thermal Gating of Magnon Exchange in Magnetic Multilayers with Antiferromagnetic Spacers.

We observe a strong thermally controlled magnon-mediated interlayer coupling of two ferromagnetic layers via an antiferromagnetic spacer in spin-valve type trilayers. The effect manifests itself as a coherent switching as well as collective resonant precession of the two ferromagnets, which can be controlled by varying temperature and the spacer thickness. We explain the observed behavior as due to a strong hybridization of the ferro- and antiferromagnetic magnon modes in the trilayer at temperatures just below the Néel temperature of the antiferromagnetic spacer.

AC Josephson effect between two superfluid time crystals.

Quantum time crystals are systems characterized by spontaneously emerging periodic order in the time domain1. While originally a phase of broken time translation symmetry was a mere speculation2, a wide range of time crystals has been reported3-5. However, the dynamics and interactions between such systems have not been investigated experimentally. Here we study two adjacent quantum time crystals realized by two magnon condensates in superfluid 3He-B. We observe an exchange of magnons between the time crystals leading to opposite-phase oscillations in their populations-a signature of the AC Josephson effect6-while the defining periodic motion remains phase coherent throughout the experiment. Our results demonstrate that time crystals obey the general dynamics of quantum mechanics and offer a basis to further investigate the fundamental properties of these phases, opening pathways for possible applications in developing fields, such as quantum information processing.

Effects of molecular adsorption on the spin-wave spectrum and magnon relaxation in two-dimensional Cr2Ge2Te6.

In this work, we performed detailed first-principles calculation and theoretical analysis to investigate the effect of molecular adsorption on the spin-wave spectrum and magnon relaxation in a Cr2Ge2Te6 (CGT) monolayer. It is found that NH3, NO, and NO2 adsorption can enhance the exchange constant of CGT, which can result in a blue-shift in the spin-wave spectrum. At 30 K, by means of a thorough investigation of many possible lattice configurations excited by thermal fluctuation, we identify the magnon scattering rate from the intrinsic lattice vibrational modes, and find that the relaxation of optical and acoustic magnons exhibits a completely different wave vector dependence. Moreover, although the adsorption of NO2 and NH3 molecules has a negligible influence on the magnon-phonon interaction, the adsorption of NO molecules results in a significant increase in magnon scattering strength. In the long-wavelength limit, the interlayer vibrational modes induced by NO adsorption increase the magnon-phonon scattering strength by ∼12.7%. The remarkable interlayer magnon-phonon interaction is ascribed to the strong CGT-NO coupling and large molecular vibration amplitude. Considering the importance of magnon relaxation time in the application of spin devices, we suggest that both the impacts on the exchange interaction and scattering rate must be considered when manipulating two-dimensional magnets by surface functionalization.

Magnetic susceptibility and simulated neutron signal in the two-dimensional Hubbard model

We compute dynamic spin susceptibilities in the two-dimensional Hubbard model using the method of Dual Fermions and provide comparison to lattice Monte Carlo and cluster dynamical mean field theory. We examine the energy dispersion identified by peaks in ${\rm Im}\chi(\omega,q)$ which define spin modes and compare the exchange scale and magnon dispersion to neutron experiments on the parent La$_2$CuO$_4$ cuprate. We present the evolution of the spin excitations as a function of Hubbard interaction strengths and doping and explore the particle-hole asymmetry of the spin excitations. We also study the correlation lengths and the spin excitation dispersion peak structure and find a `Y'-shaped dispersion similar to neutron results on doped HgBa$_2$CuO$_{4+\delta}$.

Triplet superconductivity in ferromagnets due to magnon exchange

We consider the superconducting pairing induced by spin waves exchange in a ferromagnet with both conduction and localized electrons, the latter being described as spins. We use the microscopic Eliashberg theory to describe the pairing of conducting electrons and the RPA approach to treat the localized spins assuming an exchange coupling between the conducting electrons and spins. In the framework of non relativistic Hamiltonian twe found that he spin wave exchange results in equal spin electron pairing described by the two components of the order parameter, $\Delta^{\uparrow}$ (both spins up) and $\Delta^{\downarrow}$ (both spins down). Due to the conservation of total spin projection on the axis of the spontaneous ferromagnetic moment, the spin wave exchange at low temperatures includes an emission of magnons and an absorption of thermal magnons by the conduction electrons. The absorption and emission processes depend differently on the temperature, with the absorption being progressively suppressed as the temperature drops. As a result, the superconducting pairing exists only if the electron-spin wave exchange parameter $g$ exceeds some critical value $g_c$. At $g>g_c$ pairing vanishes if the temperature drops below the lowest point $T_{cl}$ or increases above the upper critical point $T_{ch} \approx T_m$ (the Curie temperature) where the spin waves cease to exist. This behavior inherent to the spin carrying glue is in an obvious disagreement with the results of conventional BCS approach which assumes that the effective electron-electron attraction is simply proportional to the static magnetic susceptibility.

Stoner versus Heisenberg: Ultrafast exchange reduction and magnon generation during laser-induced demagnetization

The excitation of a ferromagnetic film by a femtosecond laser pulse causes an unexpectedly fast quenching of the film's magnetization on subpicosecond time scales. The microscopic physical mechanisms responsible for this remain a scientific puzzle. The authors employ femtosecond extreme ultraviolet pulses produced by high harmonic generation to follow how the magnetization of a thin cobalt film evolves after the excitation by a 40-fs laser pulse. By measuring the time-, energy-, and angle-resolved magneto-optical response of the Co films across the ${M}_{2,3}$ absorption edge, they obtain a set of time-lapsed magnetic asymmetry spectra, which contain a wealth of information about the different mechanisms at work. When combined with advanced ab initio magneto-optical calculations, they identify two dominant contributions: first, a transient reduction of exchange splitting, and second, magnon excitation. This work thus distinguishes between two fundamental models of magnetism, the Stoner and Heisenberg models, which ascribe magnetization dynamics to an exchange splitting reduction and spin wave excitations, respectively.

Stoner vs. Heisenberg: Ultrafast exchange reduction and magnon generation during laser-induced demagnetization

Stoner vs. Heisenberg: Ultrafast exchange reduction and magnon generation during laser-induced demagnetization

Exchange magnon induced resistance asymmetry in permalloy spin-Hall oscillators

We investigate magnetization dynamics in a spin-Hall oscillator using a direct current measurement as well as conventional microwave spectrum analysis. When the current applies an anti-damping spin-transfer torque, we observe a change in resistance which we ascribe mainly to the excitation of incoherent exchange magnons. A simple model is developed based on the reduction of the effective saturation magnetization, quantitatively explaining the data. The observed phenomena highlight the importance of exchange magnons on the operation of spin-Hall oscillators.

Holes localized on a Skyrmion in a doped antiferromagnet on the honeycomb lattice: Symmetry analysis

Using the low-energy effective field theory for hole-doped antiferromagnets on the honeycomb lattice, we study the localization of holes on Skyrmions, as a potential mechanism for the preformation of Cooper pairs. In contrast to the square lattice case, for the standard radial profile of the Skyrmion on the honeycomb lattice, only holes residing in one of the two hole pockets can get localized. This differs qualitatively from hole pairs bound by magnon exchange, which is most attractive between holes residing in different momentum space pockets. On the honeycomb lattice, magnon exchange unambiguously leads to f-wave pairing, which is also observed experimentally. Using the collective-mode quantization of the Skyrmion, we determine the quantum numbers of the localized hole pairs. Again, f-wave symmetry is possible, but other competing pairing symmetries cannot be ruled out.

On magnon mediated Cooper pair formation in ferromagnetic superconductors

Identification of pairing mechanism leading to ferromagnetic superconductivity is one of the most challenging issues in condensed matter physics. Although different models have been proposed to explain this phenomenon, a quantitative understanding about this pairing is yet to be achieved. Using the localized-itinerant model, we find that in ferromagnetic superconducting materials both triplet pairing and singlet pairing of electrons are possible through magnon exchange depending upon whether the Debye cut off frequency of magnons is greater or lesser than the Hund's coupling (J) multiplied by average spin (S) per site. Taking into account the repulsive interaction due to the existence of paramagnons, we also find an expression for effective interaction potential between a pair of electrons with opposite spins. We apply the developed formalism in case of UGe2 and URhGe. The condition of singlet pairing is found to be fulfilled in these cases, as was previously envisaged by Suhl [Suhl, Phys. Rev. Lett. 87, 167007 (2001)]. We compute the critical temperatures of URhGe at ambient pressure and of UGe2 under different pressures for the first time through BCS equation. Thus, this work outlines a very simple way to evaluate critical temperature in case of a superconducting system. A close match with the available experimental results strongly supports our theoretical treatment.

Magnetic properties and magnon excitations in Fe(001) films grown on Ir(001)

We present a comprehensive study of magnetic properties and high wave-vector magnon excitations in epitaxial Fe(001) films grown on Ir(001) substrate. The magnetic properties are investigated by magneto-optical Kerr effect for various thicknesses of Fe film from 4 up to 27 monolayers. The magnon dispersion relation is obtained by means of spin-polarized electron energy loss spectroscopy. By comparing the experimental and theoretical results, we comment on the effect of the strain induced tetragonal distortion in the Fe film and the electronic hybridizations between the film and the substrate on the exchange interaction and magnon dispersion relation. The thickness-dependent measurements show that the magnon dispersion relation does not change significantly while changing the film thickness in the regime, where the film is uniformly strained. In addition, the measurements are also performed on thicker Fe films, where the films are relaxed into a bulklike structure. The magnon energies close to the zone boundary differ from the ones measured on thick Fe(110) films on W(110). However, the magnon dispersion relation measured for both systems can be well described within the Heisenberg model by using the same effective exchange parameters.

Systematic low-energy effective field theory for magnons and holes in an antiferromagnet on the honeycomb lattice

Based on a symmetry analysis of the microscopic Hubbard and t-J models, a systematic low-energy effective field theory is constructed for hole-doped antiferromagnets on the honeycomb lattice. In the antiferromagnetic phase, doped holes are massive due to the spontaneous breakdown of the $SU(2)_s$ symmetry, just as nucleons in QCD pick up their mass from spontaneous chiral symmetry breaking. In the broken phase the effective action contains a single-derivative term, similar to the Shraiman-Siggia term in the square lattice case. Interestingly, an accidental continuous spatial rotation symmetry arises at leading order. As an application of the effective field theory we consider one-magnon exchange between two holes and the formation of two-hole bound states. As an unambiguous prediction of the effective theory, the wave function for the ground state of two holes bound by magnon exchange exhibits $f$-wave symmetry.

Ferromagnetic and Spin-Wave Resonance in Fe<sub>x</sub>Ni<sub>1-X</sub> (0<x<1) Films

The spectrum of standing exchange magnon wave in FeNi alloy films obtained by thermal evaporation method is studied. The modification of dispersion law conditioned by magnetization fluctuations is detected. The films of invar structure having metastable bcc structure are most dispersing relative to magnetic state. The size of magnetic inhomogeneity was less than 5 nm.

New properties of magnon density in uniaxial anisotropic ferromagnet on the background of spin wave

The N-soliton solutions of magnetization in uniaxial anisotropic ferromagnet on the background of spin wave are presented by using the effective Darboux transformation method. With the analytical solutions new properties of magnon density is studied in detail. On the ground state background the magnon density is constant for the spin wave solution and the magnetic soliton, respectively. However, on the spin wave background the magnon density possesses of temporal or spatial periodic oscillation. Moreover, the soliton solution possess the breather character in its propagation along the ferromagnet. These results show that during soliton propagation a periodic magnon exchange occurs between the magnetic soliton and the spin wave background.

Magnetic field induced one-magnon Raman scattering in the magnon Bose-Einstein condensation phase ofTlCuCl3

We report the observation of the ${A}_{g}$-symmetric one-magnon Raman peak in the magnon Bose-Einstein condensation phase of ${\text{TlCuCl}}_{3}$. Its Raman shift traces the one-magnon energy at the magnetic $\ensuremath{\Gamma}$ point, and its intensity is proportional to the squared transverse magnetization. The appearance of the one-magnon Raman scattering originates from the exchange magnon Raman process and reflects the change of the magnetic-state symmetry. By using the bond-operator representation, we theoretically clarify the Raman selection rules, which are consistent with the experimental results.

Magnon exchange mechanism of ferromagnetic superconductivity

The magnon exchange mechanism of ferromagnetic superconductivity (FM superconductivity) was developed to explain in a natural way the fact that the superconductivity in ${\mathrm{UGe}}_{2},$ ${\mathrm{ZrZn}}_{2},$ and URhGe is confined to the ferromagnetic phase. The order parameter is a spin antiparallel component of a spin-1 triplet with zero spin projection. The transverse spin fluctuations are pair forming and the longitudinal ones are pair breaking. In the present paper, a superconducting solution, based on the magnon exchange mechanism, is obtained which closely matches the experiments with ${\mathrm{ZrZn}}_{2}$ and URhGe. The onset of superconductivity leads to the appearance of complicated Fermi surfaces in the spin-up and spin-down momentum distribution functions. Each of them consist of two pieces, but they are simple connected and can be made very small by varying the microscopic parameters. As a result, it is obtained that the specific heat depends on the temperature linearly, at low temperature, and the coefficient $\ensuremath{\gamma}=C/T$ is smaller in the superconducting phase than in the ferromagnetic one. The absence of a quantum transition from ferromagnetism to ferromagnetic superconductivity in a weak ferromagnets ${\mathrm{ZrZn}}_{2}$ and URhGe is explained accounting for the contribution of magnon self-interaction to the spin fluctuations' parameters. It is shown that in the presence of an external magnetic field the system undergoes a first-order quantum phase transition.