  The magnetic-field and temperature dependencies of ultrasound propagation and
magnetization of single-crystalline CoCr2O4 have been studied in static and
pulsed magnetic fields up to 14 T and 62 T, respectively. Distinct anomalies
with significant changes in the sound velocity and attenuation are found in
this spinel compound at the onset of long-range incommensurate spiral-spin
order at T_s = 27 K and at the transition from the incommensurate to the
commensurate state at T_l = 14 K, evidencing strong spin-lattice coupling.
While the magnetization evolves gradually with field, steplike increments in
the ultrasound clearly signal a transition into a new magneto-structural state
between 6.2 and 16.5 K and at high magnetic fields. We argue that this is a
high-symmetry phase with only the longitudinal component of the magnetization
being ordered, while the transverse helical component remains disordered. This
phase is metastable in an extended H-T phase space.
