Mechanical Spin Waves in a Nonlocal Discrete Magnetoelastic Lattice with Negative Group Velocity

Structured materials can be engineered to support polarized waves generated by the rotation of the wave's displacement field, leading to spin angular momentum (SAM). This property can be leveraged to control the propagation of sound or vibrations through momentum locking or to induce nonreciprocal propagation via phonon-magnon coupling. However, the physical realization of elastic metamaterials that support SAM is a challenge. Discrete lattices can be used to realize mechanical systems that support SAM associated with the rotational motion of particles as the waves propagate. Here, a discrete elastic system is demonstrated, consisting of repulsive magnets mounted on cantilevers, which exhibit elastic spin waves arising from the coupling between the longitudinal and transverse motion of the magnets. It is shown that the direct observation of spin variation can be employed to detect lattice defects. The lattice also enables unidirectional wave propagation and can transmit waves with a negative group velocity due to nonlocal interactions.