Spin injection from epitaxial Heusler alloy thin films into InGaAs/GaAs quantum wells

Abstract

A study has been carried out on the suitability of using the Heusler alloy, Co2MnGa as spin injector into GaAs. A range of appropriate theoretical and experimental tools has been employed. Our calculations predict Co2MnGa to have a spin polarization P = 63 %. Calculations have been made on off-stoichiometric crystals and crystals with site swapping defects. Significant decrease in the spin polarization has been predicted for disorder defects involving especially Co on Mn or Ga sites. From an estimate based on the calculated defect formation energies it is found that Mn on Co-sites are likely to exist with a concentration of 2 %, while Ga on Mn-sites are likely to exist with a concentration of 21 %. Epitaxial magnetic Co2MnGa thin films have been grown by molecular beam epitaxy. The reproducibility is approximately within 2 % of the correct stoichiometry. Furthermore epitaxial growth of hetero-structures of Co2MnGa and Al and AlxOy layers has been performed succesfully. In the growth optimization process, inductively coupled plasma optical emission spectroscopy has been found to be a reliable and efficient way of determining the stoichiometry of thin films. The thin films have been characterized structurally, magnetically, and electrically. The crystal structure has been studied by X-ray diffraction techniques. The expected Heusler structure L21 has been observed. Anisotropy of in-plane magnetization in near stoichiometric thin films grown on a 2 × 4 Ga-rich reconstructed GaAs surface has been observed while no anisotropy is seen for near stoichiometry thin films on an ordinary GaAs surface. Typically thin films grown on GaAs show lower saturation magnetization than expected from bulk properties. The electrical characterizations have revealed resistivities around ρ = 350μΩcm at 300 K. Generally, the near stoichiometric films have a very weak temperature dependence, with resistivity dropping slightly when cooled. A small room temperature anisotropic magneto resistance has been measured to 0.062 %. From transmission line measurements at low current, interface resistance in δ-doped GaAs-ferromagnet is found to typically 0.02-0.1 Ωmm2 for Fe and Co contacts but two orders of magnitude higher for the Co2MnGa contacts. Point contact Andreev reflection measurements on an off-stoichiometric thin film (Co2.4Mn1.6Ga) show a spin polarization of P ≈ 50 %. Furthermore spin injection into a InGaAs/GaAs quantum well have been characterized optically in the oblique Hanle geometry. The highest steady state spin polarization in the quantum well has been found to P ≈ 6 % at T = 5 K. The spin polarization decreases with increasing temperature and is unmeasurably small above 20 K, contrary to Fe reference samples exhibiting a room temperature steady state spin polarization of P = 2.3 %