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Spin wave spintronics (also known as magnonics) processes information by propagating spin waves with no charge displaced. Because dissipation is thus minimized this is rapidly becoming an important subject of research within the larger area of spintronics. The logic states in magnonic circuitry can be defined either by the phase or by the amplitude of the spin wave. In both cases, a π-phase shifter plays a crucial role in performing logical operations. The first spin wave logic gate was experimentally demonstrated by Kostylev et al. They utilized an inhomogeneous magnetic field to control the phase difference between spin waves propagating in different arms of a Mach-Zehnder interferometer -and thus the amplitude of the output spin wave. Later, Schneider et al and Lee et al developed a complete set of logic gates such as NOR, XOR and AND, based on spin wave interferometry. However, all of theses gates are controlled by a current-induced magnetic field. As the devices shrink down, π-phase shift requires a larger electric current to induce stronger magnetic field, which inevitably increases the power-loss. Therefore, voltage-controlled spin wave electronics becomes an attractive alternative avenue towards nano-scale magnonics, where exchange spin waves are of primary interest.