To create metrologically relevant magnetic reference materials on a nanoscale we use a variety of magnetic artefacts, such as conventional ferromagnets (iron, cobalt, permalloy), diluted magnetic semiconductors, functional oxides and molecular magnets, with the size ranging from 1 μm down to 5 nm.

Arrays of lithographically fabricated
ferromagnetic particles

Nanomagnets can be prepared by two main approaches namely ‘top-down’ and ‘bottom-up’ techniques. In the first approach we start from a continuous magnetic thin film deposited on a nonmagnetic substrate. The film is further patterned using a nanolithography technique in order to remove all ‘excess’ of material and leave individual magnetic dots. This step is followed by lift-off process chemical or dry etching and ion milling. The advantage of the ‘top-down’ approach is the possibility to preserve to some extent optimal magnetic properties typical for thin films, i.e. their epitaxiality, which leads to a predictable and controllable magneto-crystalline anisotropy of the nanomagnets [see References 1-4]. This research has been done in collaboration with CTH (Sweden).

Array of Ge_1-xMn_x nanowires in the template. A free-standing
Ge_1-xMn_x nanowires. An individual Co/Fe₃O₄ coaxial nanocable 

Self-assembled nanostructures is a rapidly developing branch of the bottom-up approach, which has recently demonstrated the highest industrial impact. Growth in nanotemplates is the most developed method for producing self-assembled 1D nanostructures. Porous aluminum oxide membranes consisting of packed columnar arrays of hexagonal cells, each with a cylindrical nanopore in the centre, are among the most flexible, robust and cheap. In our research we used porous aluminium membranes as growth matrices for arrays of metal, metal oxide, diluted magnetic semiconductor nanowires, hollow Co nanotubes and complex coaxial nanocables [see References 5-19]. All these structures have been grown using a Supercritical Fluid Deposition technique in collaboration with UCC, Ireland.

Fe cube with size of 17 nm

Alternatively, 0D magnetic nanoparticles can be reproducibly synthesised in different shapes (cubes, spheres) and sizes (from 3 to 200 nm) by wet-chemical and gas-phase synthesised methods. Traditionally, large arrays of these particles were considered as very attractive for the manufacturing of magnetic data storage media or biomedical applications, however we use individual magnetic beads because they are ideal metrological artifacts: their size and magnetic moment can be precisely tuned and measured by various magnetic sensors. This work is carried out in collaboration with Duisburg University, Germany.

Magnetic properties of large ensembles of nanoparticles are measured using a commercial SQUID magnetometer (Quantum Design). The system is open for commercial exploitation.

Selected References

1. On the realisation of artificial spin-chains.
   M. van Kampen, et al.
   J. of Phys.: Cond. Matter. 17, L27 (2005).
2. Room Temperature Ferromagnetism in Ge1-xMnx nanowires.
   O. Kazakova, et al.
   Phys. Rev. B. 72, 0944415 (2005).
3. Synthesis and Characterisation of Highly Ordered Cobalt-Magnetite Nanocable Arrays.
   B. Daly, et al.
   Small 2, 1299 (2006).
4. Tunable Magnetic Properties of Metal - Metal Oxide Coaxial Nanocables.
   O. Kazakova, et al.
   Phys. Rev. B. 74, 184413 (2006).
5. Influence of thermal coupling on spin avalanches in Mn12-acetate.
   C. H. Webster, et al.
   Phys. Rev. B 76, 12403 (2007).
6. Dilute Magnetic Semiconductor Nanowires
   J. S. Kulkarni, O. Kazakova and J. D. Holmes.
   Appl. Phys. A. Editorial Review. 85, 277 (2006).
7. Influence of thermal coupling on spin avalanches in Mn12-acetate
   C. H. Webster, O. Kazakova, A. Ya. Tzalenchuk, J. C. Gallop, P. W. Josephs-Franks, A. Hernandez-M?nguez, and J. Tejada.
   Phys. Rev. B 76, 012403 (2007).
8. Spin solitons and spin waves in chiral and racemic molecular based ferrimagnets
   R. Morgunov, M. V. Kirman, K. Inoue, Y. Tanimoto, J. Kishine, A. S. Ovchinnikov, and O. Kazakova,
   Phys. Rev. B 77, 184419 (2008).
9. Effect of dimensionality on the spin dynamics of GeMn systems: Electron spin resonance measurements
   O. Kazakova, R. Morgunov, J. Kulkarni, J. Holmes, and L. Ottaviano.
   Phys. Rev. B, 77, 235317 (2008).
10. Electron Spin Resonance and Microwave Magnetoresistance in Ge:Mn Thin Films
    R. Morgunov, M. Farle, M. Passacantando, L. Ottaviano, and O. Kazakova.
    Phys. Rev. B 78, 045206 (2008).
11. Single Crystalline Ge1-xMnx Nanowires as Building Blocks for Nanoelectronics
    M. I. van der Meulen, N. Petkov, M. A. Morris, O. Kazakova, X. Han, K. L. Wang, A. P. Jacobs, and J. D. Holmes,
    Nano Lett., 9, 50 (2009).
12. Percolation ferromagnetism and spin waves in Ge:Mn thin films
    R. B. Morgunov, A. I. Dmitriev and O. L. Kazakova,
    Phys. Rev. B 80, 085205 (2009).
13. Route to single magnetic particle detection: carbon nanotube decorated with a finite number of nanocubes
    I. Rod, O. Kazakova, D. C. Cox, M. Spasova, and M. Farle,
    Nanotechnology 20, 335301 (2009).
14. Unusual magnetism in templated NiS nanoparticles.
    L. Barry, J. D. Holmes, D. J. Otway, M. P. Copley, O. Kazakova, and M. A. Morris,
    J. Phys.: Condens. Matter. 22, 076001 (2010).
15. Nonlinear spin-wave phenomena in [Mn{(R/S)-pn}2]2[Mn{(R/S)-pn}2H2O][Cr(CN)6] molecular ferrimagnet
    R. B. Morgunov, F. B. Mushenok and O. Kazakova,
    Phys. Rev. B. 82, 134439 (2010).
16. Single phase room temperature ferromagnetism in Mn implanted amorphous Ge
    L. Ottaviano, A. Continenza, G. Profeta, G. Impellizzeri, A. Irrera, R. Gunnella, O. Kazakova,
    Phys. Rev. B. 83, 134426 (2011).
17. Synthesis and Magnetic Characterization of Co-axial Ge1-xMnx/a-Si Heterostructures
    S. Barth, O. Kazakova, S. Estrade, R. G. Hobbs, F. Peiro, M. A. Morris, and J. D. Holmes,
    Crystal Growth & Design 11, 5253 (2011).