Magnetostatics and Dynamics of Ion Irradiated NiFe/Ta Multilayer Films Studied by Vector Network Analyzer Ferromagnetic Resonance

In the present work, the implications of ion irradiation on the magnetostatic and dynamic properties of soft magnetic Py/Ta (Py = Permalloy: Ni80Fe20) single and multilayer lms have been investigated with the main objective of nding a way to determine their saturation magnetization. Both polar magneto-optical Kerr eect (MOKE) and vector network analyzer ferromagnetic resonance (VNA-FMR) measurements have proven to be suitable methods to determine 0MS, circumventing the problem of the unknown eective magnetic volume that causes conventional techniques such as SQUID or VSM to fail. Provided there is no perpendicular anisotropy contribution in the samples, the saturation magnetization can be determined even in the case of strong interfacial mixing due to an inherently high number of Py/Ta interfaces and/or ion irradiation with high uences.
Another integral part of this work has been to construct a VNA-FMR spectrometer capable of performing both azimuthal and polar angle-dependent measurements using a magnet strong enough to saturate samples containing iron. Starting from scratch, this comprised numerous steps such as developing a suitable coplanar waveguide design, and writing the control, evaluation, and tting software.
With both increasing ion uence and number of Py/Ta interfaces, a decrease of saturation magnetization has been observed. In the case of the 10Py samples, an immediate decrease of 0MS already sets in at small ion uences. However, for the 1Py and 5Py samples, the saturation magnetization remains constant up to a certain ion uence, but then starts to rapidly decrease. Ne ion irradiation causes a mixing and broadening of the interfaces. Thus, the Py/Ta stacks undergo a transition from being polycrystalline to amorphous at a critical uence depending on the number of interfaces. The saturation magnetization is found to vanish at a Ta concentration of about 10{15 at.% in the Py layers. The samples possess a small uniaxial anisotropy, which remains virtually unaected by the ion uence, but slightly reduces with an increasing number of Py/Ta interfaces. In addition to magnetostatics, the dynamic properties of the samples have been investigated as well. The Gilbert damping parameter increases with both increasing number of Py/Ta interfaces and higher ion uences, with the former having a stronger in uence. The inhomogeneous linewidth broadening B0 increases as well with increasing number of Py/Ta interfaces, but slightly decreases for higher ion uences.