Parity-Time Symmetric Spintronics and Spin Cavitronics

Non-Hermitian Hamiltonian obeying parity-time (PT) symmetry can exhibit real spectra and a spontaneous symmetry breaking accompanied by a real-to-complex spectral phase transition at the ‘exceptional point (EP)’. Here, we predict that a ferromagnet with gain (loss) is equivalent to an antiferromagnet with an equal amount of loss (gain), and show a ferromagnet to antiferromagnet phase transition in PT-symmetric magnetic bilayers when the balanced gain-loss parameter exceeds a critical value. We then generate the idea to a spin cavitronic device that allows the strong coupling between photons and magnons. We demonstrate a “Z”-shape transmission spectrum in the exact PT phase. The spectrum evolves to a step function when the polariton touches the third-order exceptional point. The estimated magnetic sensitivity can approach 10^-15 T Hz^-1/2 around the third-order EP. Our work paves the way for investigating the parity-time symmetry in spintronics and spin cavitronics, and for designing ultrasensitive magnetometers.