Magnetism and interactions within the single domain limit: reversal, logic and information storage
Magnetism and magnetic material research on mesoscopic length scale, especially on the size-dependent scaling laws have drawn much attention. From a scientific point of view, magnetic phenomena would vary dramatically as a function of length, corresponding to the multi-single magnetic domain transition. From an industrial point of view, the application of magnetic interaction within the single domain limit has been proposed and studied in the context of magnetic recording and information processing. In this thesis, we discuss the characteristic lengths in a magnetic system resulting from the competition between different energy terms. Further, we studied the two basic interactions within the single domain limit: dipole interaction and exchange interaction using two examples with practical applications: magnetic quantum-dot cellular automata (MQCA) logic and bit patterned media (BPM) magnetic recording. Dipole interactions within the single domain limit for shape-tuned nanomagnet array was studied in this thesis. We proposed a 45° clocking mechanism which would intrinsically eliminate clocking misalignments. This clocking field was demonstrated in both nanomagnet arrays for signal propagation and majority gates for logic operation. Correct output and robust performance were detected for both configurations. Further, more complicated design, two digital binary full adder, was carried out. This novel design provides high stability for logic operation of MQCA and potentially paves way for its application. In addition, the magnetization reversal in perpendicular magnetic anisotropy (PMA) system and its dependence on scaling law were studied. High quality epitaxial L10 FePdPt and FePt thin films on MgO (001) substrate was fabricated. For FePd binary alloy thin films, we observed that Pt addition and low temperature post annealing would facilitate L10 chemical ordering. For FePt thin film samples, reversal process and magnetic behavior were studied for both continuous film and patterned dots array. Further, patterned dots arrays within different length scale were developed to reveal the multi/single domain transition, which is observed to be around 200 nm for our L10 FePt thin film. Finally, sub-100 nm dots array in a large area was carried out for both L10 FePt monolayer and FePt/Co bilayer heterostructure. The high patterning quality was demonstrated in the uniform dot size distribution, and the magnetic properties of the dots array indicates a good candidate for the application of bit-patterned media (BPM) exchange coupled composite (ECC) magnetic recording.