Theoretical approach and experimental investigation of spin current tunnelling in Ni80Fe20/Si3N4/Fe40Co37Zr11N12 magnetically collinear trilayer films with defined in-plane uniaxial anisotropy

Abstract

Ferromagnetic trilayer film systems with nonferromagnetic Si3N4 interface layers were fabricated by using magnetron sputtering. A post-annealing process was carried out at 400 °C for 2 h in a static magnetic field, in order to induce an in-plane uniaxial anisotropy. The present study introduces the exchange interaction between ferromagnetic layers which influences the static magnetisation properties as well as the natural resonance frequency behaviour and its damping features, i.e., damping and anti-damping effects. This is expressed by the total damping parameter αitot = αGi ± αspi, which is enhanced or reduced by a spin transfer torque damping parameter ± αspi whereupon Gilbert bulk damping αGi is considered to be independent. The static polarisation loops are slightly but noticeably different, caused by an interface with thickness up to 100 nm, which reflects a decreasing exchange interaction. With varying the interface thickness, the films also show a distinct variation in their frequency spectra. This leads to the conclusion that spin currents caused by spin pumping arise, and tunnelling within a Si3N4 insulating barrier generates spin transfer torque. This can be assumed because the observed ferromagnetic resonance dual lines show remarkable changes in their Full Width at Half Maxima (FWHM) at frequencies of approximately 600 MHz caused by the Ni80Fe20 layers and 2.32 GHz caused by the Fe40Co37Zr11N12 layers. Regarding the FWHM of the resonance frequency spectra in association with the damping constant αeff on the dependence of the interface layer thickness, one can observe an exponential curve shape for both ferromagnetic layers, which is obviously a sign for the interaction between the layers through a spin current tunnelling mechanism.