Molecular magnets

Historically most materials in magnetic applications are based on inorganic materials. Recently, however, organic and molecular materials have begun to show increasing promise. We perform research on molecular magnets using a variety of techniques, including muon-spin rotation and electron spin resonance.

Recent research

Entangling muon and fluorine moments in molecular magnets

We have exploited quantum entanglement to probe the surroundings of muons in matter. The muon, a type of subatomic particle, is often implanted into materials to probe the internal magnetic fields. In the past, such studies have been criticised since the exact environment of the muon is often mysterious. Our work, which exploits the quantum mechanical interaction of the muon and the nuclei of fluorine atoms, allows us to identify the atomic surroundings of the muon. We also suggest ways to trap muons in known configurations allowing muons studies access to new information.

Muon-Fluorine Entangled States in Molecular Magnets

T. Lancaster, S. J. Blundell, P. J. Baker, M. L. Brooks, W. Hayes, F. L. Pratt, J. L. Manson, M. M. Conner, and J. A. Schlueter,
Phys. Rev. Lett. 99, 267601 (2007) Link

Nanoscale magnets show new promise as quantum information processors

Together with collaborators at the Universities of Manchester and Princeton, we have moved a step closer to making a new kind of information processing device called a quantum computer by using tiny magnets, each one made out of a single molecule. In a quantum computer, the information-carrying elements are permitted to exist in strange quantum states, known as qubits. Our results show that the time for which the qubits can be stable can significantly exceed the time it takes to perform operations on them. This is the crucial prerequisite for the deployment of these systems in quantum information applications. More information.

Will spin-relaxation times in molecular magnets permit quantum information processing?

Arzhang Ardavan, Olivier Rival, John J.L. Morton, Stephen J. Blundell, Alexei M. Tyryshkin, Grigore A. Timco, and Richard E.P. Winnpenny,
Phys. Rev. Lett. 98, 057201 (2007) Link

A highly ideal Heisenberg antiferromagnetic spin-chain

The organic radical-ion salt DEOCC-TCNQF4 contains linear chains of stacked molecules with significant Heisenberg antiferromagnet interactions along the chain and extremely weak interactions between the chains. Zero-field muon-spin rotation has confirmed the absence of long-range magnetic order down to 20 mK and field-depdendent muon-spin rotation is found to be consistent with diffusive motion of the spin excitations. The anisotropic spin dynamics and the upper boundary for magnetic ordering temperature both indicate interchain magnetic coupling J' below 7 mK. As the intrachain coupling J is 110 K, |J'/J| is significantly less than 0.0001. This system provides one of the most ideal examples of the one-dimensional S=1/2 Heisenberg antiferromagnet yet discovered.

Low-Temperature Spin Diffusion in a Highly Ideal S=1/2 Heisenberg Antiferromagnetic Chain Studied by Muon Spin Relaxation

F. L. Pratt, S. J. Blundell, T. Lancaster, C. Baines and S. Takagi,
Phys. Rev. Lett. 96, 247203 (2006) Link

Novel magnets made from the strongest known hydrogen bond

A bifluoride building block has been used to make a three-dimensional coordination polymer for the first time. The structure is very thermally stable (up to 200 degrees celsius) due to the exceptional strength of the hydrogen bonds within it; bifluoride contains the strongest known hydrogen bond. The structure contains copper ions bound to pyrazine molecules in a planar square. Bifluoride ions (HF2-) sit above and below the copper ions. Each pyrazine molecule can bond to one copper ion at each end, to give a potentially infinite copper-pyrazine plane. Bifluoride ions therefore act as bridges between the planes. The magnetic properties of the material were measured using muons, showing that antiferromagnetic order occurs below 1.45 Kelvin. More information may be found in the press release.

[Cu(HF₂)(pyz)₂]BF₄ (pyz = pyrazine): long-range magnetic ordering in a pseudo-cubic coordination polymer comprised of bridging HF₂^- and pyrzine ligands

J L Manson, M M Conner, J A Schlueter, T Lancaster, S J Blundell, M L Brooks, F L Pratt, T Papageorgiou, A D Bianchi, J Wosnitza and M-H Whangbo,
Chem. Commun., 2006 Link

Magnetic order in a quasi-one dimensional magnetic chain

Copper pyrazine dinitrate consists of chains of Cu2+ ions which each have spin-1/2. The chains are reasonably well isolated from one another and this means that long range magnetic order had not been observed in it because long range order is not possible in one-dimension. However, our measurements using muons have recently picked up the signature of magnetic order below 107 mK in this material. At these very low temperatures the interchain coupling, though weak, starts to have an effect and the system "realises" that it is really three-dimensional. Muons are incredibly useful for detecting this effect since conventional bulk probes are rather ineffective in very anisotropic systems.

Magnetic order in the quasi-one-dimensional spin-1/2 molecular chain compound copper pyrazine dinitrate

T. Lancaster, S. J. Blundell, M. L. Brooks, P. J. Baker, F. L. Pratt, J. L. Manson, C. P. Landee and C. Baines,
Phys. Rev. B 73, R020410 (2006) Link

Review article

Purely organic ferromagnets, based upon nitronyl nitroxide radicals, show long range magnetic order at very low temperatures in the region of 1 K, while sulfur based radicals show weak ferromagnetism at temperatures up to 36 K. It is also possible to prepare molecule based magnets in which transition metal ions are used to provide the magnetic moment, but organic groups mediate the interactions. This strategy has produced magnetic materials with a large variety of structures, including chains, layered systems and three-dimensional networks, some of which show ordering at room temperature and some of which have very high coercivity. Even if long range magnetic order is not achieved, the spin crossover effect may be observed, which has important applications. Further magnetic materials may be obtained by constructing charge transfer salts, which can produce metallic molecular magnets. Another development is single-molecule magnets, formed by preparing small magnetic clusters. These materials can show macroscopic quantum tunnelling of the magnetization and may have uses as memory devices or in quantum computation applications.

Organic and molecular magnets

S. J. Blundell and F. L. Pratt,
J. Phys.: Condens. Matter 16, R771 (2004) Link