Magnetic-field dependence of the magnetic excitations in ordered

The effect of an external magnetic field H on the magnetic excitations in ordered $\chem{HoNi_{2}B_{2}C}$ is investigated by inelastic neutron scattering on single crystals for the first time. A Zeeman-like renormalization of the magnetic excitations is observed at fixed $T=2$$\un{K}$ and $H\parallel[1,1,0]$ up to $H=5$$\un{T}$. Above $H\sim 1.1$$\un{T}$ the renormalization occurs linearly, following the expectation for the $\chem{Ho^{3+}}$ crystal electric-field levels at finite fields. However, in the range $0<H<1.1$$\un{T}$ clear discontinuities mark the successive metamagnetic transitions reported for the ground state of the title compound. The comparison of the magnetic excitations between the antiferromagnetic ($H<0.4$$\un{T}$), helicoidal ($0.4\le H