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PGI Kolloquium:

Prof. Dr. Harald Brune,
École Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerland

PGI Lecture Hall, Building 04.8, 2nd Floor, Room 365

20.04.2018 11:00 Uhr

The Smallest Permanent Magnets

Until two years ago, the smallest permanent magnets were single molecular magnets. They are promising candidates for magnetic information storage, molecular spintronics, and quantum bits [1, 2]. Surface Science has opened up an alternative approach to this field in studying the magnetic properties of single atoms adsorbed onto surfaces. Spectacular properties such as very large magnetic anisotropies were reported, but until two years ago, all atoms were paramagnetic.

BruneCopyright: Prof. Dr. Brune

Very recently a major breakthrough was achieved by identifying systems where a single surface adsorbed atom can indeed be a stable magnet. Ho atoms on two monolayer thick MgO(100) films grown on Ag(100) where found to exhibit magnetic remanence up to 30 K and relaxation times of at least one hour at 2 K [3]. This result was obtained from ensemble measurements using X-ray magnetic circular dichroism (XMCD).

Spin-polarized scanning tunneling microscopy (STM) measurements demonstrated reading and writing of individual Ho atoms and confirmed their long magnetic lifetimes [4]. Very recent STM experiments show stable magnetization over two hours in external fields of 8 T that are applied opposite to the magnetization of the atoms and at temperatures of 30 K; the first spontaneous switching is observed at 45 K [5].

We have identified two more systems that exhibit permanent magnetism in single adatoms, namely Dy atoms graphene on Ir(111) [6] and Tb/MgO/Ag(100) [7]. These single atom magnets are all candidates for magnetic information storage; the combined coercitive field and thermal stability of Ho/MgO(100) outperforms the best single molecular magnets. Recently, electron paramagnetic resonance (ESR) has been realized on individual Fe atoms with the STM [8]. This gives access to the coherence time which is a benchmark testing the possibility of single atom qubits.

[1] S. Thiele, F. Balestro, R. Ballou, S. Klyatskaya, M. Ruben, and W. Wernsdorfer, Science 344, 1135 (2014).
[2] R. J. Blagg et al., Nat. Chem. 5, 673 (2013).
[3] F. Donati et al., Science 352, 318 (2016).
[4] F. D. Natterer et al. Nature 543, 226 (2017).
[5] F. D. Natterer, F. Donati, F. Patthey and H. Brune, arXiv:1712.07871, PRL submitted (2018).
[6] R. Baltic et al. Nanolett. 16, 7610 (2016).
[7] L. Persichetti et al., to be published.
[8] S. Baumann, W. Paul, T. Choi, C. P. Lutz, A. Ardavan, and A. J. Heinrich, Science 350, 417 (2015).


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