MAGNETIC INTERACTION WITH SPIN-POLARIZED SURFACE STATES: Ferromagnetic insulators and Rashba semiconductors

Abstract

Spin-polarized surface states are quickly becoming a popular subject of study due to the interesting physical environment they present and the potential application of their properties in novel spintronic devices. The interaction of magnetism with these surface states is currently poorly understood but the realization of this effect is expected to produce exotic phenomena that are intriguing for both basic and applied purposes. Two routes were taken to better understand this magnetic interaction: finding ferromagnetic insulators that could be grown onto a topological insulator to investigate the proximity magnetic interaction and doping of magnetic elements into Rashba semiconductors to observe the effect of intrinsic magnetism on the spin-split state. CrSbSe\(_{3}\), Cr\(_{2}\)Sn\(_{3}\)Se\(_{7}\), and Cr\(_{2}\)Ge\(_{2}\)Te\(_{6}\) were synthesized as potential ferromagnetic insulators. Only crystals of Cr\(_{2}\)Ge\(_{2}\)Te\(_{6}\) were obtained. Single crystal X-ray diffraction experiments reveal that Cr\(_{2}\)Ge\(_{2}\)Te\(_{6}\) has a rhombohedral symmetry that is compatible with topological insulators like Bi\(_{2}\)Se\(_{3}\). Property measurements show that Cr\(_{2}\)Ge\(_{2}\)Te\(_{6}\) has a ferromagnetic ordering with a transition temperature of 65 K along with a resistivity higher than 240 \(\Omega\) cm. Magnetic data indicate this ferromagnet lacks any hysteresis at temperatures as low as 10 K. Magneto-optics studies done in collaboration revealing the absence of magnetic domain walls support this finding. These observations suggest complex magnetic behavior that is the subject of ongoing investigation. My work establishes this compound as a ferromagnetic insulator with a lattice symmetry compatible with the topological insulators. Cr\(_{2}\)Ge\(_{2}\)Te\(_{6}\) is thus an excellent candidate to try growing onto one of these materials to study the effects of magnetism on the topological surface state. The Rashba semiconductor BiTeI was doped with the magnetic transition metals Mn and V at several concentrations. While crystals of BiTeI were obtained, the samples showed no signs of magnetization implying that they were not successfully doped. BiTeBr was grown as it is predicted to have large spin-polarization similar to BiTeI. Electronic measurements show that the crystals are metallic with a resistivity on the order of 0.10 m\(\Omega\) cm. This high conductivity will hinder the compound's identification as a Rashba material due to the masking of any surface contribution. SbTeI was synthesized due to chemical analogy with BiTeI, but single crystals of the compound were not obtained. A surface electronic structure calculation of SbTeI reveals a semiconducting band gap of 0.1 eV with no apparent Rashba signature. This work shows that doping of BiTeI may not be the best approach to determine the magnetic effect on Rashba semiconductors. Moreover, my work did identify two potential Rashba compounds and yield single crystals of one of these. Both compounds are favorable candidates for magnetic doping if the Rashba effect is confirmed.