Experimental Study of New Magnetic Circuit Elements Built from Nanomagnets for Magnetic Quantum-Dot Cellular Automata Logic Applications

Magnetic quantum-dot cellular automata (MQCA) systems are networks of closely-spaced, dipole-coupled, single-domain nanomagnets designed for digital computation. MQCA offers very low power dissipation with high integration density of functional devices, as QCA implementations do in general. In addition, MQCA can operate over a wide temperature range from sub-Kelvin to the Curie temperature. Information propagation and inversion have previously been demonstrated in MQCA. In this thesis, we perform a shape study of asymmetric magnets for MQCA gate design, and we demonstrate for the first time room temperature operation of a programmable MQCA majority-logic gate, i.e. the basic majority gate, with different length of the driver magnet. The samples were fabricated on silicon wafers by using high-resolution electron-beam lithography for patterning of electron beam evaporated ferromagnetic metals. The nanomagnet circuits were imaged by magnetic force microscopy (MFM), with which individual magnetization states were distinguished and mapped. Magnetic switching behavior was investigated in arrays of magnets with different aspect ratios. Switching fields were determined experimentally by MFM images taken after several independent demagnetizations. By increasing a magnet's length along its easy axis, its coercivity increases in that direction. This basic phenomenon was used for the design and fabrication of a programmable majority gate. The majority gate was demonstrated by employing NiFe polycrystalline nanomagnets with 60 nm x 90 nm lateral sizes. Drivers were provided by additional nanomagnets fabricated together with the gate, and the operation was tested by MFM. The work presented here is an experimental proof of the MQCA concept.