On the Spin Superfluid Quantum Interference Device (Spin SQUID)

In easy-plane ferromagnets, the spin superfluid phase facilitates coherent spin transport. So far, experimental evidence remains elusive. In this Letter, we propose an indirect way to sense this phenomenon via a spin superfluid quantum interference device (Spin SQUID) — the analogy of its superconducting counterpart (SQUID). We theoretically investigate the static and dynamic properties of a quasi-one-dimensional easy-plane ferromagnetic insulator arranged in a ring configuration. This system includes a Josephson weak link and exhibits strong Dzyaloshinskii-Moriya interaction (DMI), which can be tuned by an in-plane electric field. The DMI can be interpreted as a gauge field that couples to the spin supercurrent through the Aharonov-Casher (AC) effect. We show that the properties of the spin superfluid are periodic with respect to the accumulated Aharanov-Casher phase and are, therefore, sensitive to the electric flux in units of an electric flux quantum. For readout, we propose to apply spectroscopic analysis to detect the frequency of the harmonic modes.