Multiphase competition in the quantum XY pyrochlore antiferromagnet CdYb 2 Se 4 : Zero and applied magnetic field study

We study magnetic behavior of the ${\mathrm{Yb}}^{3+}$ ions on a frustrated pyrochlore lattice in the spinel ${\mathrm{CdYb}}_{2}{\mathrm{Se}}_{4}$. The crystal-electric-field parameters deduced from high-energy inelastic neutron scattering reveal a well-isolated ytterbium ground-state doublet with a weakly Ising character. Magnetic order studied by powder neutron diffraction evolves from the XY-type antiferromagnetic ${\mathrm{\ensuremath{\Gamma}}}_{5}$ state to a splayed icelike ferromagnet (both with $\mathbf{k}=0$) in applied magnetic field with ${B}_{c}=3\phantom{\rule{0.28em}{0ex}}\mathrm{T}$. Low-energy inelastic neutron scattering identifies weakly dispersive magnetic bands around 0.72 meV starting at $|\mathbf{Q}|=1.1\phantom{\rule{4pt}{0ex}}{\AA{}}^{\ensuremath{-}1}$ at zero field, which diminish with field and vanish above 3 T. We explain the observed magnetic behavior in framework of the nearest-neighbor anisotropic exchange model for effective $S=\frac{1}{2}$ Kramers doublets on the pyrochlore lattice. The estimated exchanges position the ${\mathrm{CdYb}}_{2}{\mathrm{Se}}_{4}$ spinel close to the phase boundary between the ${\mathrm{\ensuremath{\Gamma}}}_{5}$ and splayed ferromagnet states, similar to the Yb pyrochlores, suggesting an important role of the competition between these phases.