Widely used Platinum-based detection of pure spin currents under scrutiny

October 7, 2012 - In the recent Physical Review Letters article, Prof. Xiao's group with collaborators from the Johns Hopkins University and the Arizona State University have performed detailed magnetotransport measurements, both electrical and thermal, to conclusively show strong ferromagnetic characteristics in thin Platinum (Pt) films on the ferromagnetic insulator YIG due to the magnetic proximity effects.

The magnetism induced in Pt in proximity to ferromagnetic metals or insulators is quite analogous to the well-known proximity effects in superconductivity. Because of the large density of states of the d-electrons, Pt almost satisfies the Stoner criterion for the onset of ferromagnetic ordering which facilitates penetration of magnetic ordering from the adjacent ferromagnetic metals or insulators.

For centuries, Pt metal has been highly valued for its luster and rarity. With the technological revolution, Pt has found many applications owing to its high chemical inertness and catalytic properties. Recently, Pt metal has been employed most often as a detector of pure spin currents, which are not accompanied by any net charge flux and which pave the way for ultra-low dissipation devices.

For example, Pt thin films on ferromagnets have been indispensable in establishing virtually all the newly discovered pure spin current phenomena, including the spin Hall effect in metals, spin-dependent Seebeck effect in ferromagnetic metals, spin-Seebeck effect in ferromagnetic insulators, pure spin current driven spin-transfer torque, detection of spin pumping by precessing magnetization etc.

Given the prominent role of Pt in spin-based phenomena, it is imperative to ascertain the transport and magnetic characteristics of thin Pt films in contact with a ferromagnet. The study of Prof. Xiao and collaborators suggest that the suitability and the unique role of Pt on ferromagnets for detecting the pure spin current have been compromised. As a result, the pure spin current detected by a thin Pt is tainted with a spin polarized current. The pure spin current phenomena, such as the inverse spin Hall effect and the spin Seebeck effect have been contaminated with the anomalous Hall effect and the anomalously Nernst effect, respectively. The shortcoming of Pt notwithstanding, it is essential to either quantitatively determine the separate contributions or identify other metals that can unequivocally detect the pure spin current.

The research team at the University of Delaware was supported by DOE through the Center for Spintronics and Biodetection.