Magnetic Anisotropies and Dynamic Magnetic Properties in Multilayered Thin Films

Theoretical and experimental research on magnetic materials and magnetic devices intend to investigate novel materials and structures which can be used for the next generation spintronic devices. Moreover, it is essential to conduct fundamental research on new phenomena aiming for new generation of devices that can be faster, smaller, cheaper, and more reliable. Magnetic anisotropies have been widely used in spintronic devices. From the unidirectional exchange bias anisotropy that is used in magnetic read heads and giant magnetic resonance (GMR) sensors, to the interfacial perpendicular magnetic anisotropy (PMA) that is essential for magnetic tunnel junctions (MTJs). In the first chapter of this dissertation a short introduction to magnetization dynamics including experimental techniques is given. In the second chapter, the exchange bias anisotropy and the interfacial origin of relaxation in Ru/IrMn/CoFe/Ru exchange biased systems is discussed and investigated. The interfacial perpendicular anisotropy is observed in these systems and can be quantified using FMR technique. Such anisotropy can exist in a thin ferromagnetic film (such as NiFe) that is in proximity to a metallic (e.g.Ru) or insulating (e.g. SiO2) non-magnetic layer, which is the topic of the third chapter of this dissertation. In addition, experimental results confirm that spatial fluctuations of the uniaxial perpendicular anisotropy can push the easy axis of the magnetization in an orientation that is neither perpendicular to the film nor normal to it. The effect of the lateral fluctuation of the uniaxial anisotropy on the magnetic energy landscape and the magnetization dynamics of thin magnetic layers is reported in chapter four using micromagnetic simulations

Unidirectional and uniaxial anisotropies in the MnN/CoFeB exchange bias system

Rai A, Dunz M, Sapkota A, et al. Unidirectional and uniaxial anisotropies in the MnN/CoFeB exchange bias system. Journal of Magnetism and Magnetic Materials. 2019;485:374-380.We report on broadband and in-plane angle dependent ferromagnetic resonance measurements of MnN (30 nm)/CoFeB (t(CoFeB)) exchange bias bilayer systems with CoFeB thicknesses ranging from 5 to 20 nm. We find that the strong unidirectional anisotropy in this system is accompanied by a strong uniaxial in-plane anisotropy and a small interfacial perpendicular anisotropy (K-perpendicular to,K- i = 0.043 +/- 0.001 erg/cm(2)). The strength of the interfacial exchange bias coupling in this system Delta sigma(eb) = 0.193 +/- 0.005 erg/cm(2) is only 2.5 times larger than the interfacial uniaxial coupling strength Delta sigma(u) = 0.076 +/- 0.003 erg/cm(2). The relaxation in this system also shows a strong unidirectional and uniaxial in-plane anisotropy. While the observed thickness dependence of the uniaxial anisotropy of the relaxation is consistent with an interfacial two-magnon scattering contribution, we find that this is not the case for the unidirectional relaxation