Commensurate to Incommensurate Transition in the Chiral Helimagnet CuB2O4

We have studied a commensurate to incommensurate transition in the novel helimagnet CuB 2 O 4 by means of high-frequency ESR. In the commensurate weak ferromagnetic (CWF) state, two ESR modes have been found at low temperatures, which are explained in terms of an electron paramagnetic resonance of the Cu 2+ spins at 8 d -sites and an antiferromagnetic resonance of the magnetically ordered Cu 2+ spins at 4 b -sites with the Dzyaloshinskii–Moriya (DM) interaction. The DM interaction among the Cu 2+ spins at 4 b -sites that carries the CWF state is evaluated quantitatively. The incommensurate helimagnetic state is discussed in terms of the magnetic frustration.