UC Santa Cruz

Materials with long range magnetic order and strong spin-orbit coupling canexhibit unique physical phenomena when the materials are structured in novel configurations. Thin film growth via molecular beam epitaxy enables precise engineering of these materials into novel configurations by elemental doping and construction of bilayer structures. The effect of random competing single-ion anisotropies in antiferromagnets was studied using epitaxial MnxNi1−xF2 antiferromagnetic thin film alloys. Both MnF2 and NiF2 have the tetragonal rutile crystal structure, but MnF2 has an easy axis magnetic anisotropy along the c-axis of the unit cell while NiF2 has an easy plane magnetic anisotropy perpendicular to the c-axis. Crystal- lographic and magnetization measurements demonstrated that the thin film alloys exhibit epitaxial strain from the MgF2 (110) substrates, and that pure MnF2 thin films exhibit piezomagnetic effects due to the epitaxial strain. Mean field theory is used to calculate the exchange energies of the alloy system and predict the existence of an oblique antiferromagnetic phase. Magnetization measurements show evidence of this oblique antiferromagnetic phase in addition to an emergent magnetic phase that is believed to be either a magnetic glassy phase or a helical phase. Thin films of the topological insulator Bi2Te3 doped with Mn ions exhibit a spontaneous ferromagnetic moment below T ≈ 16 K. These Mn doped Bi2Te3 thin films are grown on several different substrates, hexagonal Al2O3 (0003), tetragonal MgF2 (110), and the tetragonal antiferromagnet NiF2 (110), with crystallographic characterization indicating single phase growth of the Mn doped Bi2Te3 film regardless of substrate. Electronic transport and magnetic moment measurements show that the ferromagnetic moment of the Mn doped Bi2Te3 thin films is enhanced as the Fermi level moves from the bulk conduc- tion band and towards the bulk band gap, suggesting that electronic surface states play an important role in mediating the ferromagnetic order. Mn doped Bi2Te3 grown on antiferromagnetic NiF2 show evidence that the ferromagnetic moment of the Mn doped Bi2Te3 film is suppressed, suggesting the existence of an interface effect between the two magnetic layers. The Fermi level of the co-doped topological insulator (BiSb)2Te3 can be tuned to lie in the bulk band gap by careful control of the (BiSb) stoichiometric ratio. Thin films of (BiSb)2Te3 are grown on both Al2O3 and antiferromag- netic MnF2. Perpendicular and parallel magnetoconductance measurements are performed and fit to several models of the magnetoconductance, including comparisons of the quasi-2D Hikami-Larkin-Nagaoka model to a model derived for 2D Dirac states. The fits of experimental data to theory suggest at im- proved conduction through the 2D topological surface states due to the tuned Fermi level. (BiSb)2Te3-MnF2 bilayers show evidence of enhanced magnetic scattering, suggesting the presence of magnetoelectric coupling effects at the interface.