Neutron diffraction study of anomalous high-field magnetic phases in ${\mathrm{TmNi}}_2{\mathrm{B}}_2\mathrm{C}$

We present a $\mathrm{}(B,T)\mathrm{}$-phase diagram of the magnetic superconductor ${\mathrm{TmNi}}_2{\mathrm{B}}_2\mathrm{C}$ 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, ${\mathbf{Q}}_{\mathrm{F}}=(0.094,0.094,0),{\mathbf{Q}}_{\mathrm{AI}}=(0.483,0,0),$ and ${\mathbf{Q}}_{\mathrm{AII}}=(0.496,0,0),$ 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 $\mathbf{Q}={\mathbf{Q}}_{\mathrm{F}}.$ The magnetic ${\mathbf{Q}}_{\mathrm{AI}}$ structure is stabilized by an applied field of 1 T and a transition to ${\mathbf{Q}}_{\mathrm{AII}}$ 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 ${\mathbf{Q}}_{\mathrm{A}}$ phases persist to increasingly higher temperatures when the field is increased. Doping with Yb has been introduced to partly suppress superconductivity. In $\left({\mathrm{Tm}}_{0.90}{\mathrm{Yb}}_{0.10}\right){\mathrm{Ni}}_2{\mathrm{B}}_2\mathrm{C}$ the ${\mathbf{Q}}_{\mathrm{F}}\to {\mathbf{Q}}_{\mathrm{AI}}$ phase transition is also observed but at a larger transition field compared to the undoped compound. In $\left({\mathrm{Tm}}_{0.85}{\mathrm{Yb}}_{0.15}\right){\mathrm{Ni}}_2{\mathrm{B}}_2\mathrm{C}$ the ${\mathbf{Q}}_{\mathrm{F}}$ phase persists up to at least 1.8 T. The magnetic correlation length of the ${\mathbf{Q}}_{\mathrm{AI}}$ phase in ${\mathrm{TmNi}}_2{\mathrm{B}}_2\mathrm{C}$ measured parallel and perpendicular to the magnetic field, is constant within 10% at all fields and temperatures.