Tuning magnetic anisotropy in thin films helps create magnon valves

Magnetic insulators are promising for the study of spin currents and spin wave dynamics. Thulium iron garnet (TIG) is an attractive magnetic insulator candidate because of its perpendicular magnetic anisotropy (PMA) properties. By varying growth and annealing parameters, Vilela et al. investigated how to tune the PMA in TIG.

The group found the annealing temperature, the substrate’s lattice parameters and the thickness of the TIG film affect its in-plain strain, leading to a large impact on the film’s anisotropy. The duration of the annealing process and the surrounding gas played a smaller role.

Using these results to control PMA, the researchers demonstrated the fabrication of TIG/Au/TIG epitaxial trilayers. This geometry behaves as an insulating magnon valve, allowing only pure spin currents propagate through while blocking charge currents.

“This shows that it is possible to fabricate more complex heterostructures with magnetic insulators with PMA by adjusting the parameters of growth and thermal annealing,” said author Gilvânia Vilela.

To understand the parameters that play a role in PMA, the group grew TIG on crystalline substrates and performed post-growth thermal annealing at different temperatures between 600 and 900 degrees Celsius, ensuring this was the only factor being varied. Because a lattice mismatch in between TIG and the substrate is expected to cause mechanical stress and vary magnetic behavior, they estimated the in-plane strain in the film as the variation between the measured lattice constant and the expected, unstrained lattice constant.

Additional studies of the TIG/Au/TIG trilayer’s spin transport and spin dynamics properties are necessary in order to take advantage of its behavior as a magnon valve. Furthermore, TIG can be combined with topological insulators to observe the breaking of time-reversal symmetry.

Source: “Strain-tuned magnetic anisotropy in sputtered thulium iron garnet ultrathin films and TIG/Au/TIG valve structures,” by Gilvânia Vilela, Hang Chi, Gregory Stephen, Charles Settens, Preston Zhou, Yunbo Ou, Dhavala Suri, Don Heiman, and Jagadeesh S. Moodera, Journal of Applied Physics (2020). The article can be accessed at https://doi.org/10.1063/1.5135012 .