Direct holographic depth‐ and lateral‐ imaging of nanoscale magnets generated by ion impact

Abstract

The alloy Fe 60 Al 40 is described by a paramagnetic B2 structure in its ordered phase, which transforms into a ferromagnetic A2 structure by chemical disordering that can be induced locally by ion irradiation [1,2]. This mechanism allows writing arbitrary magnetic nanostructures on paramagnetic thin films e.g. by means of a focused ion beam available in novel scanning ion microscopes. However, reproducible fabrication of nanoscale magnets requires knowledge about the depth and lateral distribution of the induced magnetization in dependence on irradiation parameters. Off‐Axis Electron Holography provides suitable insights by revealing the local distribution of the projected magnetic flux density with nanometer resolution [3]. By means of the coherent superposition of an electron wave passing through the object with one passing through vacuum, interference fringes can be formed at the detector plane encoding the amplitude and phase of the electron wave. The phase of an electron wave shifted by electric and magnetic fields of the object permits direct field mapping at the nanometer scale. In cross‐sectional samples of irradiated thin films, we studied the effect of the kinetic ion energy ranging from 5‐30 keV on the depth distribution of the induced magnetization [4]. In agreement with irradiation damage simulations [2], we found a magnetized film adjacent to the ion entrance surface growing in depth with increasing kinetic ion energy. We conclude that a homogeneous magnetization depth distribution in a 40 nm thick film requires a kinetic Ne + ion energy of at least 20 keV. The resolution of the ion beam nano‐pattering is mainly limited by the effect of lateral ion scattering blurring the magnetization distribution at the pattern edges. To study this effect, we fabricated 500 nm wide magnetized stripes separated by non‐ferromagnetic (i.e. non‐irradiated) spacers (Fig. 1) using a focused Ne + ion beam (2 nm probe size) at 25 keV in a helium ion microscope [5]. The flux distribution at the stripe facets is an indicator for the effect of lateral scattering but is difficult to directly interpret in terms of magnetization because of the superposition with stray fields. Therefore, we applied a magneto‐static model for the field distribution around the nanoscale magnet as a function of the magnetization blurring, which returns a width of lateral scattering of about 20 nm fitting best to experimental results [4].