  We report a global effect induced by the local complex field, associated with
the spin-exchange interaction. High-order exceptional point up to ($N+1$)-level
coalescence is created at the critical local complex field applied to the
$N$-size quantum spin chain. The ($N+1$)-order coalescent level is a saturated
ferromagnetic ground state in the isotropic spin system. Remarkably, the final
state always approaches the ground state for an arbitrary initial state with
any number of spin flips; even if the initial state is orthogonal to the ground
state. Furthermore, the switch of macroscopic magnetization is solely driven by
the time and forms a hysteresis loop in the time domain. The retentivity and
coercivity of the hysteresis loop mainly rely on the non-Hermiticity. Our
findings highlight the cooperation of non-Hermiticity and the interaction in
quantum spin system, suggest a dynamical framework to realize magnetization,
and thus pave the way for the non-Hermitian quantum spin system.
