This book presents the theoretical and experimental investigations on antiferromagnetically coupled ferrimagnets and reveals new aspects of ferrimagnetic dynamics in terms of the role of angular momentum. The purpose of this book is to show readers that antiferromagnets/ferrimagnets are useful in spintronic devices in that (1) The non­adiabatic spin­transfer torque in antiferromagnets acts as a staggered magnetic field, which can drive the magnetic domain walls, and (2) The Gilbert damping parameter α, the energy ­dissipation rate associated with the magnetic dynamics of ferrimagnets, is insensitive to temperature in contrast to the conventional understanding that the effective α of ferrimagnets diverges at the angular momentum compensation temperature. This book provides readers with a scientific platform of ferrimagnetic dynamics, which serves as a useful basis for realizing the next ­generation of spintronic devices.