Nonlinear magnetization dynamics as a route to nonreciprocal phases, spin superfluidity, and analogue gravity

We identify a conventional ferromagnetic multilayer as a low-overhead platform where gain, loss, and nonlinearity are independently tunable, and show that balancing Gilbert damping with a dc drive stabilizes a spacetime-translation-symmetry-breaking chiral spin-superfluid limit cycle. This driven superflow supports emergent nonreciprocity: long-wavelength magnons of opposite chirality acquire intrinsically asymmetric dispersions and propagate direction-selectively, realizing a spin-superfluid diode. The asymmetry is flow-borne: it follows from broken Galilean invariance and does not require any built-in structural asymmetry or fine-tuned gain–loss balance. The linearized dynamics in the co-moving superfluid frame are intrinsically pseudo-Hermitian and, in the long-wavelength sector, can be mapped to a (1+1)D wave equation on curved spacetime. Modulation of applied currents engender sonic horizon that parametrically squeezes magnons and produces Hawking-like emission via particle–hole pair generation. These results establish a tabletop route from nonlinear magnetization dynamics to nonreciprocal transport, nonequilibrium phase transitions, and analogue-gravity kinematics.