  High-field phase diagram of a chiral-lattice antiferromagnet Sr(TiO)Cu4(PO4)4
is studied by means of the ultrasound, dielectric, and magnetocaloric-effect
measurements. These experimental techniques reveal two new phase transitions at
high fields, which have not been resolved by the previous magnetization
experiments. Specifically, the c66 acoustic mode shows drastic changes with
hysteresis with applied fields along the c axis, indicating a strong
magneto-elastic coupling. Combined with the cluster mean-field theory, we
discuss the origins of these phase transitions. By considering the chiral-twist
effect of Cu4O12 cupola units, which is inherent to the chiral crystal
structure, the phase diagram is reasonably reproduced. The agreement between
the experiment and theory suggests that this material is a unique quasi
two-dimensional spin system with competing exchange interactions and chirality,
leading to the rich phase diagram.
