Effect of magnetic ordering on the spin Hall angle of epitaxial Ho(0001) thin films.*

The operation of numerous spintronic devices are enabled through spin currents and thus, it is important to study how the generation, polarization and transmission of spin currents can be controlled by the local magnetic/electronic environment. Holmium is a rare earth metal which undergoes a paramagnetic to anti-ferromagnetic (AFM) transition at T_N=131K and an AFM to ferromagnetic (FM) transition at T_c=20K. The AFM phase is characterized by an incommensurate spin helix in which the average moments are ferromagnetically aligned within the basal planes but rotate from plane to plane along the c axis with an average turn angle varying from 50⁰/layer at T_N to 30⁰/layer at T_C . Ho is further expected to have a good charge to spin conversion efficiency owing to its high atomic number and initial results of poly-crystalline Ho films at room temperature show a spin Hall Angle (SHA) of 0.14^[1]. Here, we explore the SHA of epitaxial Ho thin films in the paramagnetic, AFM and FM phases to observe the effect of magnetic ordering on the generation of spin current. We deposited epitaxial Ho (0001) using Al₂O₃(0001) as a substrate with a 3-nm W(110) as a seed layer and confirm the different magnetic phases via magnetometry and transport measurements. We create the following thin-film stack: Al₂O₃(0001)/W(3)/Ho(10)/W(1)/Py(7)/W(1.5) (where the thicknesses are in nm) and study the SHA of Ho via spin torque ferromagnetic resonance experiments. We systematically vary the temperature down to 10K to study the charge to spin conversion efficiency. By relating various magnetic parameters such as effective magnetization, gilbert damping and SHA at various temperatures, we explore how the magnetic ordering affects the spin dynamics in this system.

References:

[1] N. Reynolds et al. Phys. Rev. B 95, 064412 (2017)