Neutron diffraction study of anomalous high-field magnetic phases in TmNi₂B₂C

We present a (B,T)-phase diagram of the magnetic superconductor TmNi₂B₂C obtained by neutron scattering. The measurements were performed in magnetic fields up to 6 T applied along the crystalline a axis. The observed phases are characterized by three ordering vectors, Q_F=(0.094,0.094,0),Q_AI=(0.483,0,0), and Q_AII=(0.496,0,0), all with the magnetic moment along the c axis. In zero and low fields the Tm 4f-moments order in a long wavelength transverse spin density wave with Q=Q_F. The magnetic Q_AI structure is stabilized by an applied field of 1 T and a transition to Q_AII is observed at 4 T. For both transitions there is a broad temperature and field range of overlap between the different states. Surprisingly, we observe that the Q_A phases persist to increasingly higher temperatures when the field is increased. Doping with Yb has been introduced to partly suppress superconductivity. In (Tm_0.90Yb_0.10)Ni₂B₂C the Q_F→Q_AI phase transition is also observed but at a larger transition field compared to the undoped compound. In (Tm_0.85Yb_0.15)Ni₂B₂C the Q_F phase persists up to at least 1.8 T. The magnetic correlation length of the Q_AI phase in TmNi₂B₂C measured parallel and perpendicular to the magnetic field, is constant within 10% at all fields and temperatures.