Nanomagnets that exhibit only two stable states of magnetization can represent digital bits. Magnetic random access memories (MRAM) store binary information in such nanomagnets, and fabrication of dense arrays of nanomagnets is also under development for application in hard disk drives (HDD). The latter faces the challenge of avoiding magnetic dipole interactions between the individual elements in the arrays, which limits data storage density. On the contrary, these interactions are utilized in the magnetic quantum-dot cellular automata (MQCA) system, which is a network of closely spaced, dipole-coupled, single-domain nanomagnets designed for digital computation. MQCA offers very low power dissipation together with high integration density of functional devices, as QCA implementations do in general. In addition, MQCA can operate over a wide range of temperatures from zero Kelvin to the Curie temperature. Information propagation and negation have previously been demonstrated in MQCA. In this dissertation room temperature operation of the basic MQCA logic gate, i.e. the three-input majority gate, is demonstrated for the first time.
The samples were fabricated on silicon wafers by using electron-beam lithography for patterning thermally evaporated ferromagnetic metals. The networks of nanomagnets were imaged by magnetic force microscopy (MFM), with which magnetization states of the individual nanomagnets were distinguished and mapped. Magnetic dipole-ordering in the networks was investigated in different samples. Average ordering lengths were calculated by statistical analysis of the MFM images that were taken after several independent demagnetization processes. The average ordering length was found to be dependent on shape and size of the nanomagnets, and it was limited by defects introduced during fabrication. The majority gate was demonstrated by employing 30 nm thick NiFe polycrystalline nanomagnets with 70 nm x 120 nm lateral sizes. Inputs 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 concept for the MQCA. The theoretical calculations can be found in the dissertation of Gyrgy Csaba.#