Chiral-lattice magnets can exhibit a variety of physical phenomena when time-reversal symmetry is broken by their magnetism. For example, nonreciprocal responses of (quasi)particles have been widely observed in chiral-lattice magnets with macroscopic magnetization. Meanwhile, time-reversal symmetry can also be broken in antiferromagnets without magnetization. Here we report an unconventional chirality-magnetism coupling in a chiral-lattice antiferromagnet Pb(TiO)Cu4(PO4)4 whose time-reversal symmetry is broken by an ordering of magnetic quadrupoles. Our experiments demonstrate that a sign of magnetic quadrupoles is controllable by a magnetic field only, which is generally impossible in consideration of the symmetry of magnetic quadrupoles. Furthermore, we find that the sign of magnetic quadrupoles stabilized by applying a magnetic field is reversed by a switching of the chirality. Our theoretical calculations and phenomenological approach reveal that this unusual coupling between the chirality and magnetic quadrupoles is mediated by the previously-unrecognized magnetic octupoles that emerge due to the chirality.
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