DescriptionThis dissertation seeks to study some polar magnets, especially hybrid improper ferroelectricity (HIF), one promising candidate to realize room{temperature multiferroics. In order to realize the designed properties, high-quality single crystals are required. First, we introduce the concept of hybrid improper ferroelectricity and discuss some intriguing domain features in the non-magnetic HIF, Sr-doped Ca3Ti2O7. Second, we present the detailed study of Ca3Mn2O7, the prototypic magnetic HIF. The discovery of interrelation between the domain structures and polarization switching is unprecedented. Finally, we pick up Fe2Mo3O8 from the list of polar magnets and unveil the hidden ferrimagnetism and giant magnetoelectric effect with a record high magnetoelectric coefficient.