A novel magnetic phase prior to a saturation moment in frustrated spinel oxides in ultra-high magnetic fields

We have investigated the magnetic-field induced phases of a typical three-dimensional frustrated magnet, CdCr$_2$O$_4$, in magnetic fields of up to 120 T that is generated by the single-turn coil techniques. We focused on magnetic phase transitions in proximity of a saturated magnetization moment. We utilized both the electromagnetic induction method using magnetic pick-up coils and magneto-optical spectroscopies of the $d$-$d$ transitions and the exciton-magnon-phonon transitions to study the magnetic properties subjected to ultra-high magnetic fields. Anomalies were observed in magneto-optical absorption intensity as well as differential magnetization prior to a fully polarized magnetic phase (a vacuum state in the magnon picture), revealing a novel magnetic phase associated with changes in both crystal and magnetic structures accompanied by the first order phase transition. Magnetic superfluid state such as an umbrella-like magnetic structure or a spin nematic state, is proposed as a candidate for the novel magnetic phase, which is found universal in the series of chromium spinel oxides, $A$Cr$_2$O$_4$ ($A$ = Zn, Cd, Hg).