Search

Writing nanoscale magnets with neon using a gas field ion source microscope

The use of focussed beams of ions to induce strong saturation magnetization (Ms) can lead to unprecedented flexibility in rapidly producing modulated magnetic materials of desired geometry. True flexibility is achieved if the process is non-destructive i.e., Ms is activated at the point of ion impact. Furthermore, the ion is ideally chemically inert and of low mass, such that it escapes the lattice after undergoing the collision process with host atoms and coming to rest. Here we describe the possibility of using a nano-focussed beam of Ne+ ions to generate ferromagnetic phases within initially non-ferromagnetic B2 materials such as Fe60Al40 and Fe50Rh50. The transition occurs as the penetrating ions displace atoms, and the subsequent vacancy creation and recombination leading to the formation of the chemically disordered A2 structure, which is ferromagnetic. The beam has a diameter of ≈ 2 nm and the transformed region is confined to the interaction volume of the ions with the host atoms, with a diameter < 50 nm. Such an ion-beam is readily available in a He+-ion microscope (where Ne+ can also be loaded). The locally formed ferromagnetic nanostructures are embedded within the electrically conducting B2 precursor material. In the case of Fe60Al40 ferromagnetic regions of Ms = 780 kAm-1 [1] are embedded within a non-ferromagnetic matrix. For Fe50Rh50, ferromagnetic regions of Ms = 1000 kAm-1 [2] can be embedded within an antiferromagnetic matrix. The ion-induced B2 → A2 phase transition is the most viable route to achieving well-defined ferromagnetic nanostructures of desired geometry within non-ferromagnetic metallic matrices. We describe our recent direct nanoscale imaging of ion-induced magnetic regions [3] and our attempts to demonstrate possible applications in spin-transport and magnonic devices. References: [1] R. Bali, S. Wintz, F. Meutzner, R. Hübner, R Boucher, A. A. Ünal, S Valencia, A. Neudert, K. Potzger, J. Bauch, F. Kronast, S. Facsko, J. Lindner, and J. Fassbender, Nano Letters 14, 2, 435 (2014). [2] Alireza Heidarian, Rantej Bali, Jörg Grenzer, Richard A. Wilhelm, Rene Heller, Oguz Yildirim, Jürgen Lindner and Kay Potzger, Nucl. Instrum. Methods Phys. Res. B 358, 251-254 (2015). [3] Falk Röder, Gregor Hlawacek, Sebastian Wintz, René Hübner, Lothar Bischoff, Hannes Lichte, Kay Potzger, Jürgen Lindner, Jürgen Fassbender, and Rantej Bali, under review, Scientific Reports (2015).

Besides imaging, gas field ion source (GFIS) based microscopes [1] are used for materials modification. This usually is based on the use of high fluence to either mill the sample material or implant Nobel gas ions into the target material [2]. Here, we present a novel route utilizing a Helium Ion Microscope (HIM) to form nano–sized magnets of arbitrary shape using very low fluences (6 × 1014 cm−2) of 20 keV–25 keV Neon ions. The fine Neon beam available in the HIM is used to locally switch 40nm thin Fe60Al40 films from the well ordered paramagnetic B2 structure into the ferromagnetic A2 structure [3, 4]. Planar structures potentially useful for applications such as spin valves or other spin–transport devices have been formed this way. Kerr Microscopy and off–axis TEM holography has been used to analyze the resulting magnetic nano–structures. Results on the energy depended depth of magnetization as well as on the lateral definition of the magnetic structures due to scattering are presented.

Regarding magnetic nanostructures, dipolar stray fields start to evolve with ongoing miniaturization, that grossly alter the magnetic properties. Besides, these fields play an important role in spinwave dynamics of periodic nanostructures like magnonic crystals. Hence, a detailed knowledge of the magnetic configuration under geometric confinements on the nanoscale is mandatory. We present a study of the magnetic fields arising from thin Permalloy films deposited on periodically modulated surfaces (ripples). These ripples were prepared using a self-organized ion beam erosion process generating modulations on the nanoscale. Cross-sectional transmission electron holography nowadays offers the necessary sensitivity and nanoscale lateral resolution to probe the magnetic configuration of such rippled films quantitatively. The imaging allows for an absolute analysis of the local magnetization inside the film as well as the accompanying stray fields outside. In addition, the evolving stray fields above the sample's surface were investigated by magnetic force microscopy.

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].

Domain boundaries in ferroic materials deviate from the bulk in both the structural and electronic properties. Their presence in the material influences the total energy of the system, the band structure and the magnetic and electric polarization. We report on a Density Functional Theory (DFT) approach within the Local Density Approximation on domain boundaries in multiferroic BiFeO3 (space group: R3c). Our model systems consist of the experimentally observed 71deg, 109deg and 180deg domain walls. The calculations were performed within the DFT software VASP, incorporating standard pseudopotentials and a plane wave basis set. A complete electronic and ionic relaxation of the model structures has been performed to yield details of the charge and structure modulation at the boundary including the deformation of the Fe-centered oxygen octahedron, the formation of electric dipole layers leading to a jump in the electrostatic potential, band gap narrowing and a domain wall dependent modification of the small ferromagnetic effect present in BiFeO3.