Magnetic patterning by electron beam assisted carbon lithography

The exploitation of the unique physical properties of thin films and heterostructures are opening intriguing opportunities for magnetic storage technology. These artificial materials will in fact enable novel architectures for a multitude of magnetic devices and sensors, promoting a significant improvement in storage density, functionality and efficiency. Their usage will also contribute to diminish the consumption of materials that are rare and difficult to extract, being often detrimental to the environment. With these objectives in mind, researchers are now looking with great attention at the combination of thin ferromagnetic layers with 2-dimensional crystals like graphene and transition metal dichalcogenides. Due to their layered structure, these systems exhibit very favorable magnetic properties, which can be tuned through thickness and interfacial interactions. For instance, graphene-cobalt stacks display an enhanced perpendicular magnetic anisotropy, a feature that is especially important for non-volatile memories. 
The fabrication of layered materials, however, is still a very challenging process. Not only it requires atomic precision in the deposition of the various layers but also the ability to create nano or microstructures of arbitrary shape. Conventional lithography in conjunction with chemical etching permits nowadays to sculpture the matter with great accuracy, at lateral resolution close to the nanometer. Yet, this approach poses an important limitation, that is, the material can only be shaped by erosion. The ability to vary the chemical composition, by adding atoms for example, is instead very desirable for many applications. To date, this can be done by stimulating the fragmentation of suitable carrier molecules using photons or electrons. So far, various methods based on focused beam induced processing methods have been devised, which can be readily employed to deposit carbonaceous layers and metallic nanostructures. These methods, however, cannot be applied when ultra-clean, ultra-high vacuum (UHV) conditions are needed, as happens for the case of semiconductor industry.
Here we present a novel lithographic method capable of laterally modifying the magnetic anisotropy of ultrathin cobalt films by depositing atomic carbon under UHV conditions. The method uses carbon monoxide (CO) as carbon carrier, and micro-focused, low-energy electron beams to locally stimulate molecular fragmentation. The underlying principle is rather simple: by stimulating CO dissociation, electrons promote the accumulation of carbon on the surface, which in turn induces an in-plane to out-of-plane spin reorientation in cobalt. Magnetic micro-patterns can be readily printed in the cobalt film following the two-step process illustrated in Figure 1a,b. Initially, the clean surface is exposed at a CO partial pressure of 10^-8mbar and irradiated using a very intense, focused electron beam. Subsequently, the specimen is heated in UHV slightly above 440 K. Such a mild thermal treatment causes CO to desorb from the non-irradiated surface regions, so that the clean surface is recovered. Conversely, the carbidic and graphitic species present in the e-beam irradiated regions remain strongly bonded to cobalt, changing its magnetic properties.
Photoelectron spectro-microscopy at the Nanospectroscopy beamline of the Elettra synchrotron permitted us to experimentally to validate our magnetic patterning method. The experiments were carried out on hcp Co/Re(0001), keeping the film thickness in the range from 4 to 6 atomic layers. This system exhibits in-plane magnetic anisotropy at room temperature. Figure 1c shows a micrometre-sized disk that was printed by irradiating the surface with 50 eV electrons while delivering a CO dose of about 10 L. As shown by low energy electron microscopy (LEEM), the disk morphology exhibits sharp edges. A cross-sectional cut across the image (see Figure 1d) reveals an edge width of less than 30 nm, resulting from both the lateral resolution of the microscope and the sample drift during irradiation. The image in Figure 1e illustrates the magnetic state of the cobalt film. The striped magnetic domains inside the irradiated area (with a period of about 120 nm) reveal that an in-plane to out-of-plane spin reorientation transition has occurred in the film. The amount of carbon needed to change the magnetic anisotropy of the film is small, just about 1/10 of a graphene monolayer, as we could quantify using laterally resolved x-ray photoelectron spectroscopy (XPS). 
 

Figure 1.  (left) Scheme of the protocol for printing chemo-magnetic patterns in ultrathin Co on Re(0001). (a) The film is exposed to CO at room temperature. The irradiation with a focused electron beam (yellow) stimulates the dissociation of the molecule, which results in the accumulation of atomic carbon on the surface. (b) Subsequently, the sample is annealed above 170 °C to desorb molecularly adsorbed CO from the non-irradiated surface regions. (c) LEEM image of an e-beam irradiated disk. Disk diameter: 1 μm; Co thickness: 4 atomic layers; irradiation energy: 50 eV; CO dose: 9.75 L; (d) Intensity profile across the orange line in the LEEM image in (c) and fit using a step function convoluted with a Gaussian of full width at half-maximum of 30 nm. The dashed blue lines indicate the 15–85% distance between minimum and maximum intensity. (e) XMCD-PEEM image of the same region at the Co L3 edge. (f) Intensity profiles across the blue and orange dashed lines in the XMCD-PEEM image in (e). The magnetic stripes indicate out-of-plane magnetic anisotropy. The stripe period is 120 nm. Adapted with permission from [1]. Copyright (2018) American Chemical Society.

We found out that the exposure to CO doses in excess of 1000 L permits to deposit a single layer of carbon on the surface [1]. A short thermal treatment at 380 °C in UHV was shown to greatly improve the ordering within this overlayer, which transforms into graphene. Correspondingly, a stripe magnetic pattern appears in the irradiated area, confirming the enhancement of out-of-plane magnetic anisotropy (see Figure 2a). Domains with opposite magnetization are separated by chiral Neél walls (Figure 2b). This is due to strong and asymmetric exchange interactions occurring at the two interfaces of the film, also known as the Dzyaloshinskii–Moriya interaction. This observation suggests that magnetic skyrmions can be stabilized in the Co film under appropriate conditions. Magnetic microscopy allowed us to image the nucleation of skyrmion bubbles in a similar, graphene-covered Co film upon reversing the magnetization with an external field. 
 

Figure 1.  (a) Co L3 edge XMCD-PEEM image illustrating the magnetic state of ultra-thin cobalt on Re(0001), onto which a disk-shaped graphitic overlayer was printed by e-beam irradiation in CO ambient (the specimen was exposed to a total CO dose of 2700 L) and subsequent thermal treatment at 380 °C. The e-beam irradiated area in the centre of the image exhibits out-of-plane magnetic anisotropy. Its surroundings are magnetized in-plane. (b) Magnified image of the region highlighted by the white dashed square in (a). The out-of-plane domains are separated by chiral Neél walls, revealed by the enhancement in image contrast at the domain boundaries. The red arrow represents the direction of the photon beam; the other symbols indicate the orientation of the magnetization vector. Adapted with permission from [1]. Copyright (2018) American Chemical Society.

In conclusion, our lithographic protocol enables to laterally control an adsorbate-induced spin reorientation transition, permitting to write the magnetic anisotropy within arbitrary regions of mesoscopic size. This capability may be used to induce confinement effects without destroying the lateral continuity of the Co film. Most importantly, one can create magnetic patterns in situ, without the need of further processing or transfer the sample to a dedicated lithographic set-up. We envisage applications in fundamental studies on magnetism. For instance, one can fabricate artificial heterostacks comprising different ferromagnetic layers, where the magnetic coupling is tuned by an interposed graphene spacer.

 

This research was conducted by the following research team:

Pietro Genoni¹, Francesca Genuzio², Tevfik Onur Menteş², Benito Santos², Alessandro Sala^2,3, Cristina Lenardi¹, and Andrea Locatelli²
 

¹ CIMAINA, Department of Physics, Università degli Studi di Milano, via Celoria 16, I-20133 Milan, Italy

² Elettra-Sincrotrone Trieste, S.S. 14 km 163.5 in AREA Science Park, Basovizza, I-34149 Trieste, Italy

³ Department of Physics, Università degli Studi di Trieste, Via Valerio 2, I-34127 Trieste, Italy

Contact persons:

Andrea Locatelli, email: