Unconventional Magnetostructural Transition in ${\mathrm{CoCr}}_2{\mathrm{O}}_4$ at High Magnetic Fields

The magnetic-field and temperature dependencies of the ultrasound propagation and magnetization of single-crystalline ${\mathrm{CoCr}}_2{\mathrm{O}}_4$ have been studied in static and pulsed magnetic fields up to 14 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 $T_s=27\mathrm{K}$ and at the transition from the incommensurate to the commensurate states at $T_l=14\mathrm{K}$, evidencing strong spin-lattice coupling. While the magnetization evolves gradually with the field, steplike increments in the ultrasound clearly signal a transition into a new magnetostructural 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.

Figure 1

Temperature dependence of the ZFC (full circles) and FC susceptibilities (open squares) for ${\mathrm{CoCr}}_2{\mathrm{O}}_4$ measured in a field of 10 mT applied along the $⟨001⟩$ axis. Inset: Temperature dependence of the inverse susceptibility measured in a field of 1 T. The solid line marks a Curie-Weiss fit at high temperatures.

Figure 2

Temperature dependencies of the sound velocity $v$ (right scale for the zero field) and its relative change $\Delta v/v_0$ (left scale) for ${\mathrm{CoCr}}_2{\mathrm{O}}_4$ measured in different static magnetic fields for ultrasound waves propagating along the $⟨111⟩$ axis. The inset shows $\Delta v/v_0$ for waves propagating along the $⟨001⟩$ axis at low temperatures for cooling and warming cycles. The arrows mark the magnetic phase transitions $T_C$, $T_s$, and $T_l$ observed at the zero magnetic field.

Figure 3

(a) Sound velocity $v$, (b) sound attenuation $\Delta \alpha$, and (c) magnetization $M$ in ${\mathrm{CoCr}}_2{\mathrm{O}}_4$ at different temperatures vs the magnetic field applied along the $⟨111⟩$ direction. The data for the up and down field sweeps are shown. The vertical arrow marks the transition field $H_t$ at 6.25 K. The tilted arrows indicate the field sweep directions. The curves in (b) and (c), except that for 1.5 K, are shifted along the vertical axis for clarity.

Figure 4

Schematic low-temperature phase diagram of ${\mathrm{CoCr}}_2{\mathrm{O}}_4$. The phase boundaries between the metastable phase and the helical C phase detected in the pulsed experiments are shown by dashed lines. The arrows connect the boundaries observed in increasing fields (open symbols) and decreasing fields (solid circles). The open squares and circles refer to the ultrasound data measured in the pulsed fields applied along the $⟨001⟩$ and $⟨111⟩$ directions, respectively.