Quantitative imaging of the magnetic fields of a nano-modulated permalloy thin film by electron holography

Dipolar magnetic stray fields are the origin of most coupling and interaction effects among patterned ferromagnetic micro- and nanostructures. For example, they are the source of Neel coupling in trilayer films with orange-peel-type or modulated interfaces [1], they are responsible for two-magnon scattering in ultrathin films [2] or uniaxial magnetic anisotropy, and many other effects. Although intuitively clear and easy to grasp the detailed magnetic configuration of nanostructured films is not easy to detect. Micromagnetic simulations are a possible method to calculate the stray fields and magnetic configuration. However, they results relies on the exact modeling of the magnetic structure and knowledge of sample parameters.
Off-axis electron holography (EH) using a transmission electron microscope (TEM) is the method of choice to image the magnetization and stray field of a ferromagnetic sample directly. Moreover it allows a quantitative analysis compared to conventional imaging of magnetic domains. Modern aberration corrected TEMs provide enough resolution to image the stray fields even of nanostructures.
In our work we have investigated those stray fields as well as the magnetic state of a thin permalloy film deposited on a rippled Si template quantitatively.
The rippled Si template was prepared by Xe+ ion beam erosion of a Si substrate under an oblique angle of 65°. The ion beam energy determines the period of the ripples, which was selected such to yield ripples with a periodicity of about 220 nm. A 30 nm thick Permalloy (Ni81Fe19) film with a 3 nm Cr cap layer was subsequently evaporated onto this template [4]. The film follows the morphology of the template as cross-checked by atomic force microscopy before and after deposition. From this sample a cross-section lamella was cut out and subsequently thinned to below 100 nm using a focused ion beam system for the transmission electron holography imaging. The geometry for the micromagnetic simulations was taken from the conventional TEM images.
The cross-sectional electron holographic measurements yield the electric and magnetic phase images [3]. The magnetic phase image allows to determine the local orientation of the magnetization inside the permalloy film (see Fig. 1). The magnetization follows nearly perfectly the surface modulation of the film with a saturation magnetization of roughly 1000 mT. Regarding the magnetic stray field outside the film the micromagnetic simulation shows the periodic change due to rising and falling ripple slopes in good agreement with the holographic images.
The resulting dipolar stray fields are around 10-20 mT [5].