Structural and dynamic properties of the magnetic order in the 90-K superconductor Ho${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$

Zero-field ${\mu }^{+}$ spin-rotation measurements in a ceramic sample of Ho${\mathrm{Ba}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ reveal three-dimensional antiferromagnetic order below ≈5 K and some incoherent, perhaps two-dimensional, order up to 50 K in a diminishing fraction of the sample volume. With increasing temperature, at about 100 mK, the Ho moments change their orientation from parallel to the $c$ axis to parallel to the $a$ axis. The effective Ho moment is of the order of 2.6${\mathrm{\mu }}_{\mathit{B}}$. Fluctuating field components perpendicular to the static field components at the ${\mu }^{+}$ positions cause dynamical relaxation coexisting with static dipolar line broadening. Thermally activated slow "paramagnetic" fluctuations are observed up to 50 K.