Physikalisches Institut (PHI)

Single-Molecule Magnetism

        The basic building blocks in our spintronic devices are Single-Molecule Magnets (SMMs). As a purely molecular property, SMMs display slow relaxation of the magnetization in the absence of an external magnetic field and have the potential use in information storage and quantum computing. SMMs consist of an inner magnetic core (with one or several metal ions) and a surrounding shell of ligands that can be tailored to influence the magnetic properties, bind SMMs to surfaces or onto junctions. SMMs display a range of quantum effects that are observable up to higher and higher temperatures due to the steady progress in molecular design. These effects include quantum tunnelling of the magnetization, Berry phase interference and quantum coherence.

Fig. 1: Four famous SMMs (from left to right): Fe8, Mn12, TbPc2 and Tb2Pc3 (last one with magnetic easy axes).

 

Studies of SMMs with micro-SQUID measurements:

        For the purpose of studying SMMs and individual magnetic particles, we developed and are continuously improving a micro-SQUID setup, operational in the temperature range from 20 mK to 9 K and with a field up to 7 Tesla. Furthermore, the field sweeping rates can be as high as 30 T/s with a field stability better than a microtesla and a time resolution of 1 ms or less, allowing for short-time measurements. The magnetic field can be applied in any direction with a precision better than 0.1 degrees. The magnetometer consists of an array of micro-SQUIDs which are suitable for studying single crystals as small as 10 micrometers, which are placed directly on the array. Further improvements have also led us to the development of the nano-SQUID device. 

Fig. 2: A "sionludi" cryostat used for the micro-SQUID measurements (left), and a schematic representation of the magnetometer (right). 

         On our setup, we measure hysteresis profiles, quantum tunnelling of the magnetisation and relaxation curves of the SMMs. This enables us a quick identification of the most promising compounds for in-depth studies. Furthermore, we use special methods to study decoherence effects, such as the quantum hole digging method.

Schematic representation of the energy landscape of a SMM and hysteresis loops of single crystals

Fig. 3: (a) Schematic representation of the energy diagram of a Mn12-SMM with a spin ground state S = 10. (b) Hysteresis loops of single crystals of a Mn12-SMM at different temperatures, exhibiting steps due to resonant quantum tunneling between energy levels.

 

Recent objectives and developments:

  • We are working towards an experimental setup for higher temperatures with micro-Hall probes and towards improving the time resolution of our measurements.
  • Because magnetic fields are not easily localised, we are interested in using electrical fields and understanding their effects on the properties of electronic and nuclear spins. 
  • Concerning data storage and quantum computing, we are working on special DC- and AC- sequences which we can implement on molecular magnets.
  • We are studying SMMs with multiple exchange-coupled lanthanide ions, some of which possess toroidal magnetic moments, whilst others are potential nuclear qubits with very large Hilbert spaces, adjustable both with the choice of the lanthanide isotope and with the molecular design. 

 

Further reading:

1 Leuenberger, M. N.; Loss, D. Nature 2001, 410 (6830), 789-793.
2 Wernsdorfer, W.; Sessoli, R. Science 1999, 284 (5411), 133-135.
3 Wernsdorfer, W., et al. Nature 2002, 416 (6879), 406-409.
4 Ishikawa, N., et al. Angew. Chem. Int. Ed. 2005, 44 (19), 2931-5.
5 Gatteschi, D., et al. Molecular Nanomagnets. Oxford University Press: New York, 2006.
6 Wernsdorfer, W. Nat. Mater. 2007, 6 (3), 174-176.
7 Wernsdorfer, W. Adv. Chem. Phys.; John Wiley & Sons, Inc.: Hoboken, NJ, USA, 2007; 99-190.
8 Bogani, L.; Wernsdorfer, W. Nat. Mater. 2008, 7 (3), 179-186.
9 Wernsdorfer, W. ‎Supercond. Sci. Technol. 2009, 22 (6), 064013.
10 Liu, J., et al. J. Am. Chem. Soc. 2016, 138 (16), 5441-50.
11 Moreno-Pineda, E., et al. Angew. Chem. Int. Ed. 2017, 56 (33), 9915-9919.
12 Vignesh, K. R., et al. Nat. Commun. 2017, 8 (1), 1023.
13 Moreno-Pineda, E., et al. Inorg. Chem. 2018, 57 (16), 9873-9879.
14 Morita, T., et al. J. Am. Chem. Soc. 2018, 140 (8), 2995-3007.