Ultrathin metal films

Although growth of ultrathin metal films is generally considered a mature subfield of surface science, surprises are still possible. That was the case about a decade ago when we discovered that ferromagnetic iron films thought to have face-centered cubic structure are actually body-centered cubic (See the Crystallography of Iron Films page of our STM Gallery, and this finding spurred a lot of new activities! Also the decades-old technique of pulsed laser deposition turned out to be highly interesting.

In the age of nanotechnology, one can easily imagine that demand will arise for various magnetic structures in the sub-micrometer range. Creating these structures by conventional lithography is a complicated and thus costly process. Irradiation by a focused ion beam, or an ion beam patterning system (developed at IMS Nanofabrication AG) provides a one-step alternative.

Iron films grown on Cu(100) are in a metastable nonmagnetic fcc state between about 1 – 2 nm thickness, or more, if suitable additives are used. It was one of the many clever ideas of Albert Biedermann, then postdoc in our group to initiate the transformation into the stable ferromagnetic bcc state by ion irradiation. We have verified this by magneto-optic Kerr-effect (MOKE) measurements. Using an ion beam patterner at IMS, we could exploit this phenomenon to write magnetic structures with a resolution of ≈100 nm, probably limited by the resolution of the magnetic force microscope used to image them as well as by the non-perfect focus reached on our comparably rough sample.

A more recent collaboration with our friends at CEITEC in Brno (CZ) was the key for an even more important finding: Using a properly chosen way of irradiating the sample with a focused ion-beam (FIB), it is possible to control the magnetic anisotropy of these structures! This paves the way to applications of the structures as waveguides for spin waves (magnons).

1. W. Rupp, A. Biedermann, B. Kamenik, R. Ritter, Ch. Klein, E. Platzgummer, M. Schmid, P. Varga
   Ion-beam induced fcc-bcc transition in ultrathin Fe films for ferromagnetic patterning
   Appl. Phys. Lett. 93 (2008) 063102 ⋅ full text
2. S. Shah Zaman, P. Dvorak, R. Ritter, A. Buchsbaum, D. Stickler, H.P. Oepen, M. Schmid, P. Varga
   In-situ magnetic nano-patterning of Fe films grown on Cu(100)
   J. Appl. Phys. 110 (2011) 024309 ⋅ full text
3. S. Shah Zaman, H. Oßmer, J. Jonner, Z. Novotný, A. Buchsbaum, M. Schmid, P. Varga
   Ion beam induced magnetic transformation of CO-stabilized fcc Fe films on Cu(100)
   Phys. Rev. B 82 (2010) 235401 ⋅ full text
4. J. Gloss, S. S. Zaman, J. Jonner, Z. Novotny, M. Schmid, P. Varga, and M. Urbánek
   Ion-beam-induced magnetic and structural phase transformation of Ni-stabilized face-centered-cubic Fe films on Cu(100)
   Appl. Phys. Lett. 103, 262405 (2013) ⋅ full text.
5. M. Urbánek, L. Flajšman, V. Křižáková, J. Gloss, M. Horký, M. Schmid, and P. Varga
   Focused ion beam direct writing of magnetic patterns with controlled structural and magnetic properties
   APL Mater. 6, 060701 (2018) ⋅ full text.
6. L. Flajšman, K. Wagner, M. Vaňatka, J. Gloss, V. Křižáková, M. Schmid, H. Schultheiss, M. Urbánek
   Zero-field propagation of spin waves in waveguides prepared by focused ion beam direct writing
   Phys. Rev. B 101, 014436 (2020) ⋅ full text.

Pulsed laser deposition (PLD) is a simple, yet powerful method to grow thin films of a large variety of materials, and it was widely studied in the applied physics community after the success of growing high-temperature superconductors PLD in the 1980ies. It is known since more than two decades that the particles ablated by by short laser pulses have high kinetic energy (dozens to hundreds of electron volts); when they impinge on the substrate this leads to differences in the morphology and structure of the films as compared to thermal evaporation. Nevertheless, the basic physics of the ablation process by short laser pulses and the processes on the substrate were insufficiently understood, and we could shed much light on this subject by our high-resolution STM studies.

We have combined a time-of-flight spectrometer for determination of the particles' energy with STM to study the surface structure at the substrate. It turned out that moderate particle energies are sufficient for implantation of ablated particles into the substrate, while higher kinetic energies are required for the creation of additional nuclei that modify the island density. As long as these additional nuclei are not formed, our results nicely fit calculations by classic nucleation theory, taking the time structure of the pulsed deposition into account.

1. A. Buchsbaum, G. Rauchbauer, P. Varga, M. Schmid
   Time-of-flight spectroscopy of the energy distribution of laser-ablated atoms and ions
   Rev. Sci. Instrum. 79 (2008) 043301 ⋅ full text
2. M. Schmid, C. Lenauer, A. Buchsbaum, F. Wimmer, G. Rauchbauer, P. Scheiber, G. Betz, P. Varga
   High island densities in pulsed laser deposition: Causes and implications
   Phys. Rev. Lett. 103 (2009) 076101 ⋅ full text