  MnCr2O4 that exhibits spin frustration and complex spiral spin order is of
great interest from both fundamental as well as application-oriented
perspectives. Unlike CoCr2O4 whose ground state presents the coexistence of
commensurate spiral spin order (CSSO) and ferroelectric order, MnCr2O4 shows no
multiferroicity. One reason is that the spiral spin order is highly sensitive
to the oxygen concentration in MnCr2O4. Here, we have successfully grown
high-quality single-crystalline MnCr2O4 by the chemical vapor transport method.
We observe a new first-order magnetic transition from the incommensurate spiral
spin order (ICSSO) at 19.4 K to the CSSO at 17.4 K. This magnetic transition is
verified by magnetization, specific heat, and magnetoelectric measurements,
which also confirm that the ground state exhibits the coexistence of the CSSO
and magnetoelectricity below 17.4 K. Interestingly, the temperature evolution
of Raman spectra between 5.4 and 300 K suggests that the structure remains the
same. We also find that the phase-transition temperature of the CSSO decreases
as applied magnetic field increases up to 45 kOe.
