Spintronic systems, where the spin degree of freedom is exploited for storing and processing information, have gained tremendous interest in the recent years because of their potential in revolutionizing the current electronics industry. Low-dimensional magnetic structures, such as epitaxial magnetic films on insulators or semiconductors, are particularly interesting because they form the key components in many important spintronic applications including gigabit nonvolatile memory, magnetic tunnel junction and spin injector. With advanced ultra-high-vacuum growth techniques such as molecular beam epitaxy, it is possible to create low-dimensional magnetic structures and magnet/semiconductor hybrid structures with atomic precision. Here in this thesis, I will show several interesting magnetic nanostructures grown using this method. These structures range from thin films of magnetic alloys, to ultrathin transition metal based magnetic layers on GaN and W(110) surfaces, and antiferromagnetic Mn3N2 nanopyramids. To study the magnetic properties of these structures down to nanoscale, and to correlate magnetism with structure, scanning tunneling microscopy (STM) and spin-polarized scanning tunneling microscopy (SP-STM) are employed.
As one example, in the Mn on GaN(0001) project, we have discovered a class of novel well-ordered striped superstructures with local sqrt3xsqrt3-R30 ordering. Combining STM and first-principles theory, we find that Mn atoms react with excessive surface Ga atoms and form a high-density two-dimensional MnxGa1-x structure. The surface electronic states are found to be dominated by the highly spin-polarized Mn d electrons. As a consequence, Mn atomic sites can be directly identified in STM images. For the narrowest stripes, calculations show a row-wise antiferromagnetic ground state, which is observed in real space at room temperature as an asymmetry in the density of states.
As the second example, I have applied SP-STM to study the nanoscale magnetization of antiferromagnetic Mn3N2 films. Results have shown that the surface exhibits a spin pyramid structure, where the magnetism is strongly correlated with the surface topography. Using spin-polarized dI/dV mapping, different layers can be clearly distinguished due to their different chemical and spin properties. We will show that it is possible to separate the contributions from both the electronic and the magnetic structure by applying a small magnetic field. The field rotates the tip magnetization axis causing concomitant change in the magnetic sensitivity while keeping the electronic structure unchanged. The rotation of the tip magnetization also revealed a perpendicular spin-reorientation on one subset of the surface terraces.
As shown through these projects, STM and SP-STM provide an unprecedented power to resolve magnetism and spin ordering with atomic precision. They also provide the ideal method for correlating magnetism with structure and electronic properties. By studying the interplay between structural defects such as anti-phase domain boundaries and magnetism, these projects not only unravel intriguing magnetic phenomena at reduced dimensions, but also provide methods for tailoring the material toward real-world applications.