"Realizing the Smallest Surface Adsorbed Quantum Magnets"

Who: Harald Brune (Institute of Physics, EPFL, Switzerland)

Place: Donostia International Physics Center

Date: Thursday, 8 September 2016, 12:00

Realizing the Smallest Surface Adsorbed Quantum Magnets

The magnetic states of nanostructures are of high interest for information storage and
quantum information processing. This has triggered the search for the smallest possible
magnets. In devices, electrodes have to be attached to them for readout and manipulation of
the magnetic quantum states. This corresponds to their adsorption onto conducting surfaces.

Possible candidates for the smallest surface adsorbed magnets are molecules and small
metal clusters down to the ultimate size limit of single metal adatoms. The benchmarks for
quantum magnets are their magnetic relaxation and coherence times, T1 and T2. The first
determines how long information can be stored in a magnetic quantum state, and the second
defines the time one has to take out a quantum computation step.

Many molecular magnets that exhibit promising properties in bulk samples loose them
entirely when they are surface adsorbed, and all single metal adatoms reported so far are
perfect paramagnets, despite their high magnetic anisotropies [1]. This is predominantly due
to scattering with metal conduction electrons. In the case of molecules, conformational
changes induced by surface adsorption may add to it. Finally, for higher temperature, phonon-induced magnetization reversal starts to override electron scattering.

We show that these problems can largely be overcome by using oxide, graphene, and
hexagonal boron-nitride spacer layers that significantly enhance the magnetic lifetimes of the
surface adsorbed species as compared to direct adsorption onto a metal substrate. Pc2Tb
double-decker molecules adsorbed on MgO(100) thin films grown on Ag(100) have much
longer spin-relaxation time and wider hysteresis than in bulk samples [2]. Ho atoms on the
same surface are the first single atom magnets [3]. They exhibit spin-relaxation times of an
hour at 2 K and display hysteresis up to 30 K, thus outperforming best molecular magnets.

[1] I. G. Rau et al., Science 344, 988 (2014).
[2] C. Wäckerlin et al., Adv. Mater. 28 5195 (2016).
[3] F. Donati et al., Science 352 318 (2016).

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