Disorderly magnets – a new ‘spin liquid’ shows its potential

A rare-earth oxide with liquid-like magnetic behaviour could be the key to a future generation of quantum electronic devices.

Studies of materials with unusual magnetic characteristics have intrigued physicists and electronic engineers for decades. They not only provide valuable theoretical insights into the often complex quantum behaviour of electrons in solids and liquids, they also offer huge potential as the basis for the next generation of devices for information processing and storage.

Magnetism arises because electrons behave like little magnetic needles due to their property called spin. Its direction pointing up or down produces a tiny magnetic field. In the most common magnetic materials like iron, the atoms are arranged in a regular crystalline array and the spins of their outermost electrons interact so that they all align in the same direction, thus producing a bulk ferromagnetic order; the electronic spins in some materials may also interact such that neighbouring spins point in alternating directions to produce antiferromagnetism resulting in a zero net magnetic moment.

In more exotic materials, such as mixed oxides of rare-earth metals, magnetic behaviour can be more interesting. Rare-earth elements are at the heavy end of the Periodic Table, and their atoms have more outer electrons that generate more complex magnetic behaviour in compounds containing them. A particular effect that has fascinated physicists for 50 years is magnetic ‘frustration’. This is when the spins of magnetic ions experience competing interactions with those at neighbouring sites so as to pull their alignment in opposite directions. The classic model is that of a triangular crystal lattice of magnetic atoms that want to align antiferromagnetically but can satisfy the alternating alignment for only two of three atoms in the triangle at any one time. The result is that the spin directions remain thoroughly disordered and may perpetually fluctuate due to quantum effects.

This strange state is known as a spin liquid – by analogy with a liquid such as water in which the molecules move around randomly yet interact with near neighbours in a way that affects water’s overall properties. In a spin liquid, the magnetic spins continually alter their direction randomly but also produce locally correlated patterns.

Quantum Entanglement

Why is a spin liquid so interesting? Well, the magnetic disorder operates right at the quantum level. The theory predicts that the different quantum states of the individual spins can become highly ‘entangled’, forming a so-called superposition of states. This extreme correlation is the basis behind quantum computing, which is predicted to drive the next revolution in information technology. Quantum superposition, which characterises the basic binary unit of quantum computing, tends to be rather fragile. However, a spin liquid could provide a form of entanglement robust to external fluctuations. 

So far, only a few spin-liquid candidate systems have been identified. However, none has possessed spin correlations that were highly anisotropic, that is different in different directions. Recently, however, a research group in Slovenia, together with an international team studied a new potential spin liquid – a mixed rare-earth oxide called neodymium heptatantalate (NdTa₇O₁₉). Its basic crystal unit structure consists of a triangular lattice of magnetic neodymium ions arranged in layers that are fairly well separated by inter-layers of tantalum oxide. This leads to frustrated and highly anisotropic antiferromagnetic interactions within the neodymium layers but much weaker interactions across from layer to layer. Theory indicates that such a setting could stabilise a spin-liquid ground state.

To investigate whether this was the case, the team employed a suite of magnetic experimental techniques at different facilities. Neutron scattering, in particular, is ideal for studying magnetic structure and correlated behaviour because neutrons also have spin, which couples to spins in crystalline magnetic materials. The resulting neutron measurements then provide information about the nature of the magnetism. A complementary set of magnetic experiments was carried out – going down as close to absolute zero as possible where thermal dynamic effects could no longer swamp the more subtle quantum-spin-liquid behaviour.

Bulk magnetic measurements indicated that even at very low temperatures, there was no magnetic ordering or freezing of spin direction, thus suggesting a disordered spin state – as did neutron diffraction measurements which did not discover any magnetic structure. Inelastic neutron scattering measurements provided further evidence for a spin liquid, and measured the relative strengths of correlations within the neodymium triangle planes. Experiments carried out at the ILL using polarised neutrons (with all spins aligned) revealed increased diffuse magnetic scattering at 50 millikelvins due to short-range antiferromagnetic spin correlations, indicating that that the nearest-neighbour interactions within the layers were much stronger than the longer-range ones across the layers.

The entangled nearest-neighbour in-plane spins are particularly significant in investigations of quantum materials where triangular frustration exerts these unusual magnetic effects. This rare-earth tantalate thus points the way to a new family of quantum spin liquids that could provide a platform for developing quantum information technologies of the future.