This work focused on two different material systems, the L10-MnGa on η sub>⊥-Mn3N2, and α-Cr on MgO with further experimental developments on spin-polarized scanning tunneling microscopy tips. Our primary goal is the understanding of the spin configurations on magnetic materials, as well as morphological and crystallographic features on the surface that impact the magnetic structures.
The two aspects of a scanning tunneling microscopy tip, the macroscopic profile and the nanoscale apex, can be tailored by controlling the tension during electrochemical etching and the solution-electrode contact area via acetone vapor. The apex diameter is shown to be proportional to the square root of the tension, and is demonstrated over apex diameters of 150–500 nm. The apex was found to be created in four distinct shapes where a secondary etching can reshape the tip into a single geometry. Improvement in tip height and stability of the profile are demonstrated versus a non-acetone fabrication control. An optimal range of apex diameters 100–250 nm for the round tip geometry was found to be the best recipient for ultra-thin Fe coating. In this apex diameter range a good balance between tip sharpness and roundness was found.
Antiferromagnetic Cr films 100010 nm thick were grown by molecular beam epitaxy on MgO(001) substrates achieving an atomically smooth staircase morphology. The c(2×2) surface structure of the Cr(001) surface was found via atomically resolved scanning tunneling microscopy images and dI/dV spectroscopy. A practical method of interpreting spin-polarized scanning tunneling microscopy dI/dV images was developed and applied to the c(2×2) Cr(001) surface structure. An in-plane layerwise antiferromagnetic spin structure was observed with 180° spin reversal between atomic layers. The quantization axes of the c(2×2) spins was found to depend on the staircase with possible spin alignments along the (100), (010), and (110) planes.
Ferromagnetic L10-MnGa was grown by molecular beam epitaxy under ultra-high vacuum conditions to a 73 ± 5 nm thickness atop of 50 ± 5 nm thick molecular beam epitaxy grown antiferromagnetic η⊥--n3N2 on a MgO(001) substrate. The MnGa grew along the c-axis with an out-of-plane spacing of c = 3.707 ± 0.005 Å and a relaxed in-plane spacing of a = b = 4.0 ± 0.05 Å measured with X-ray diffraction and reflected high-energy electron diffraction respectively. Scanning tunneling microscopy, Auger electron spectroscopy, and reflection high energy electron diffraction, are combined with first-principles density functional theory calculations to determine the reconstructions of theL10-MnGa(001) surface. We find two lowest energy reconstructions of the MnGa(001) face: a (1×1) Ga-terminated structure and a (1×2) structure with a Mn replacing a Ga in the (1×1) Ga-terminated surface. The (1×2) reconstruction forms a row structure along [100]. The manganese:gallium stoichiometry within the surface based on theoretical modeling is in good agreement with experiment. Magnetic moment calculations for the two lowest energy structures reveal important surface and bulk effects leading to oscillatory total magnetization for ultra-thin MnGa(001) films.
Williamson-Hall analysis reveals 67 ± 17 nm tall columnar grains with a residual stress of 2.4 ± 0.26(x10-3). A radial distribution plot of screw dislocations observed in scanning tunneling microscopy images found an in-plane coherence length of 15 ± 5 nm. Reflection high-energy electron diffraction analysis of the in-plane lattice spacing during growth reveals a critical thickness of 0.7 ± 0.25 nm for the MnGa, by which the MnGa film relaxes by incorporating dislocations of both edge and screw type. Vibrating sample magnetometry was employed to produce hysteresis loops of the bilayer system. It is found that the dislocation density plays a chief role in understanding the measured moments per unit cell; where a large dislocation density lowers the moment per unit cell significantly due to chemical layering incoherence. Exchange bias was measured in this unconventional bilayer system with 66 ± 31 Oe shift to the left in the in-plane hysteresis loop with an exchange energy of 0.2 ± 0.1 erg/cc. Additionally, perpendicular magnetic anisotropy was observed with an anisotropy constant of 4.96 ± 0.01 Merg/cc.