Handedness manipulation of propagating antiferromagnetic magnons

Antiferromagnetic magnons possess a distinctive feature absent in their ferromagnetic counterparts: the presence of two distinct handedness modes, the right-handed (RH) and left-handed (LH) precession modes. The magnon handedness determines the sign of spin polarization carried by the propagating magnon, which is indispensable for harnessing the diverse functionalities in magnonic devices, such as data encoding, magnon polarization-based logic systems, and quantum applications involving magnons. However, the control of coherently propagating magnon handedness in antiferromagnets has remained elusive. Here we demonstrate the manipulation and electrical readout of propagating magnon handedness in perpendicularly magnetized synthetic antiferromagnets (SAF). We find that the antiferromagnetic magnon handedness can be directly identified by measuring the inverse spin Hall effect (ISHE) voltage, which arises from the spin pumping effect caused by the propagating antiferromagnetic magnons in the SAF structure. The RH and LH modes of the magnon can be distinguishable when the SAF structure is sandwiched by heavy metals with the same sign of spin Hall angle. This work unveils promising avenues for harnessing the unique properties of antiferromagnetic magnons.

Damping of dipole-exchange spin waves in ferromagnetic thin films at elevated temperatures

A non-bosonic technique, based on the drone-fermion perturbation method and a high-density expansion, is employed to study the spin-wave (SW) scattering processes in a ferromagnetic thin film with exchange and dipole-dipole interactions. Specifically, the diagrammatic contributions to the spin-spin Green's functions are evaluated within a 1/zperturbation expansion, wherezis the number of spins interacting with any given spin. The results are used to calculate the SW damping at temperatures below the Curie temperatureT_C. It is found that, apart from the usual contributions due to three-magnon and four-magnon processes in the film, which are dominant at relatively low temperatures (consistent with boson expansion methods), there is an additional mechanism that becomes important for temperatures above about12TCThis is spin disorder damping, previously studied in bulk magnetic materials; it occurs when a spin wave is scattered by the instantaneous disorder produced when a longitudinal spin component undergoes a large thermal fluctuation. Numerical estimates are presented for thin films of Permalloy and EuO.

Tunable Magnon-Magnon Coupling Mediated by Dynamic Dipolar Interaction in Synthetic Antiferromagnets

We report an experimental observation of magnon-magnon coupling in interlayer exchange coupled synthetic antiferromagnets of FeCoB/Ru/FeCoB layers. An anticrossing gap of spin-wave resonance between acoustic and optic modes appears when the external magnetic field points to the direction tilted from the spin-wave propagation. The magnitude of the gap (i.e., coupling strength) can be controlled by changing the direction of the in-plane magnetic field and also enhanced by increasing the wave number of excited spin waves. We find that the coupling strength under the specified conditions is larger than the dissipation rates of both the resonance modes, indicating that a strong coupling regime is satisfied. A theoretical analysis based on the Landau-Lifshitz equation shows quantitative agreement with the experiments and indicates that the anticrossing gap appears when the exchange symmetry of two magnetizations is broken by the spin-wave excitation.

The effect of lattice structure on dipole-exchange spin waves in ultrathin ferromagnetic films

Calculations are reported relating to the dipole-exchange spin waves in ultrathin ferromagnetic films with different lattice structures such as simple cubic, body-centred cubic and face-centred cubic, using a microscopic, or Hamiltonian-based, theory. Both the short-range exchange and the long-range dipolar interactions are included in the Hamiltonian, together with an external magnetic field applied either parallel or perpendicular to the film surfaces. The use of phenomenological dipole-exchange boundary conditions, as in macroscopic theories, is avoided. Comparisons between our results and macroscopic theories generally give good agreement in a small wavevector limit. At larger wavevectors for the microscopic theory, it is found that the frequencies and the localization properties of the discrete spin waves show a sensitive dependence on the lattice structure.

Electromagnetic response of a dipole-coupled ellipsoidal bilayer

We derive an expression for the polarizability of an ellipsoidally shaped cell-like structure at field frequencies where membrane molecular resonances (vibrational and electronic) are important. We first present analytical results for the dielectric function of a flat, dipole coupled, bilayer consisting of molecules with one prominent resonance frequency. Due to the nature of the dipole coupling the dielectric function is different for the field being parallel or perpendicular to the bilayer normal with two new resonance frequencies omega=omega;(0 parallel) and omega=omega;(0 perpendicular ). We then combine this anisotropic bilayer dielectric function with the analytical solution of Gauss equation for an ellipsoid with an anisotropic coating (the coating dielectric function being different parallel and perpendicular to the coating normal) in order to find the polarizability of an ellipsoidal bilayer membrane. In particular, we find that for a thin-walled (compared to the size of the cell) membrane the resonance frequencies of the polarizability are the same as for a flat bilayer (independent of the cell shape). However, our analytic result for the geometric weights for the oscillator strengths is sensitive to the shape; the geometric weight for the omega=omega;(0 perpendicular ) (omega=omega;(0 parallel)) peak is largest when the external field is along the largest (smallest) axis. The geometric weights are shown to be constrained by three sum rules.