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Condensed matter

Condensed matter

Superconductor transition switches single-molecule magnet

04 Mar 2020 Isabelle Dumé
single molecule magnet lab
Roberta Sessoli and Giulia Serrano in their lab. (Credit: University of Florence)

A superconductor can switch the magnetic moment of a single-molecule magnet placed on top of it. This novel phenomenon, discovered by researchers in Italy, occurs because of quantum tunnelling of magnetic spins, and might be exploited in future quantum information technologies.

Single-molecule magnets are paramagnetic materials that can switch their magnetization between two states – “spin up” and “spin down”, for example. At low temperatures, these molecular complexes retain their magnetic state even in the absence of a magnetic field because reversing the magnetization would require them to overcome an energy barrier. This magnetic “memory” effect could be exploited in spintronics and quantum computing applications since the spins can act as stable quantum bits, or qubits.

According to study lead author Giulia Serrano, the combination of molecular magnets and superconductors is currently a hot research topic. Among other findings, researchers have discovered that monolayers of paramagnetic molecules can influence the temperature at which an adjacent layer of material becomes superconducting (that is, conducting electricity with no resistance). This change in the superconducting transition temperature Tc occurs because the paramagnetic monolayers create local states in the bandgap of the superconductor.

Influence of the superconducting transition

Serrano and colleagues in Roberta Sessoli’s group at the University of Florence have now found that this interaction also works in the opposite sense: a material undergoing a superconducting transition can influence the spin dynamics of nearby single-molecule magnets. In their experiments, the researchers studied clusters of four iron atoms (Fe4) incorporated into the structure of a complex molecule containing ligands derived from a trialcohol. The geometry of this molecule keeps the iron atoms in a propeller-like arrangement that protects the high spin of the Fe4 magnetic core at low temperatures.

The team did their experiments in a ultrahigh vacuum chamber, where they used a thermal sublimation technique to deposit the Fe4 clusters onto the surface of lead (111). This material, a type-I superconductor, changes from a metal to a superconductor at a Tc of 7.2 K., but Serrano explains that superconductivity is only established if the applied magnetic field is lower than the critical field Hc. For lead, Hc is around 800 oersteds.

The researchers then analysed the magnetism of the Fe4 using synchrotron light and a technique called X-ray magnetic circular dichroism (XMCD). They found that at Hc, the lead superconductor switches the magnetization state of the Fe4 by “activating” the resonant quantum tunnelling of its magnetic spins. Quantum tunnelling is the process by which quantum particles can penetrate energy barriers that would be insurmountable to classical objects.

A new magnetization switching mechanism

Serrano and colleagues say this phenomenon is a new magnetization switching mechanism – a hypothesis they backed up by observing magnetic hysteresis loops, which show how the magnetic flux density, B, of a material changes as a function of an applied magnetic field, H.

As lead undergoes its superconducting transition, an increasing fraction of it enters a so-called Meissner state, which occurs when a material placed in a magnetic field expels magnetic flux from its interior as it becomes a superconductor. This state has the effect of locally cancelling the external magnetic field of the molecular magnet and “unblocking” its magnetization state.

“Single-molecule magnets in contact with these superconducting lead regions thus switch their ‘blocked’ magnetisation state to a resonant quantum tunnelling regime by the activation of the quantum tunnelling process,” Serrano tells Physics World.

As the lead transitions to the superconducting state, the number of switching events increases as more regions of the material become superconducting. This can be seen as a gradual decrease of the magnetization value in the hysteresis loop of the single-molecule magnet, she says.

The beginning of a novel research field

Serrano says that the team’s observations open new perspectives for using such hybrid systems in quantum information technologies. As well as being exploited as qubits with a magnetization that can be switched quickly, single-molecule magnets could also be used as local sensors for probing the superconducting state, she adds.

According to the researchers, who report their work in Nature Materials, the new result heralds the beginning of a novel research field aimed at better understanding how single-molecule magnets – and magnetic molecules in general – interact with various kinds of superconductors. They suggest that superconductors with complex domain structures, such as vortex states, would be particularly interesting to study.

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