Fabrication of magnetic metamaterials by focused-ion-beam direct writing

Nanoscale magnetic patterning has a great potential in the development of novel nanomagnetic devices, systems and materials. Nanostructured magnetic materials can show functionalities that cannot be achieved in their bulk constituents. Because of their emergent properties, these materials are often called metamaterials. Direct writing of magnetic patterns by focused ion-beam irradiation is a single step process, which is advantageous over traditional multi-step lithography techniques. Different approaches to ion-beam-induced magnetic patterning have been studied, for example modification of magnetic anisotropies, coercivity, exchange bias or the magnetization of the material. We have recently shown the way how to grow metastable fcc Fe thin films which can undergo magnetic (paramagnetic -> ferromagnetic) and structural (fcc->bcc) phase transformation upon ion beam irradiation. These films present a favorable system, where ferromagnetic elements can be selectively written by focused ion beam into a paramagnetic layer. The goal of the proposed project is to exploit the potential of the Ni-stabilized fcc Fe thin films for ion- beam-induced magnetic patterning and, consequently, for the fabrication of nanostructured magnetic metamaterials. Currently, we are able to prepare Ni-stabilized fcc Fe thin films on Cu(100) single crystal substrates. The necessity of the Cu single crystal substrate as the seed for the growth of metastable fcc Fe films seriously limits their use for fundamental as well as for applied (economical) reasons. We plan to transfer the deposition process of these films onto different types of substrates including semiconductor, dielectric and also novel types of materials such as perovskite oxides. For the growth of fcc Fe films, the substrate must exhibit a close structural match with similar lattice parameters as the grown film. To promote the fcc growth a thin Cu buffer layer may be placed between the substrate and the metastable Fe film. The possible candidates for a substrate are Si(100) (with a Cu buffer layer), diamond C(100) and perovskite oxides SrTiO3 and LaAlO3. The prepared films will be consequently patterned by focused ion beams and the (magnetic) resolution of the method and its dependence on the ion mass, energy and type of substrate will be explored. Also the magnetic properties of areas transformed by different ion species, energies and ion fluences will be studied. In the end, the capability of the method to prepare a nanostructured material with the properties unattainable in continuous magnetic thin films (e.g. magnonic crystal) will be demonstrated.

Nanoscale magnetic patterning has a great potential in the development of novel nanomagnetic devices, systems and materials. Nanostructured magnetic materials can show functionalities that cannot be achieved in their bulk constituents. The project "Fabrication of magnetic metamaterials by focused-ion-beam direct writing" was dedicated to a new method of creating such nanostructures. The method is based on a metastable, non-magnetic iron-nickel alloy, which can be converted into a ferromagnetic material by irradiation with a focused ion beam. Thereby, magnetic structures can be created with a resolution of about 100 nanometers. Apart from optimizing this method and creating various magnetic nanostructures, the project has led to the important discovery that the magnetic anisotropy, i.e., the possible directions of the magnetization (and, thereby, the magnetic field) can be controlled by this method. Currently, this cannot be achieved by any other method of creating magnetic nanostructures. Together with the favorable magnetic properties of the material used, these results will lead the way to the fabrication of novel devices based on magnetic spin waves.