Resistance is fragmented: how magnets defy disorder

Some magnetic materials feature peculiar states the fundamental understanding of which may pave the way for future applications. Neutron experiments have just revealed that they can be unexpectedly stable with respect to microscopic disorder.

From our fridge door magnet to MRI machines, magnets are used in a plethora of everyday applications. Research and development continuously work on optimising and designing novel magnetic materials. In order to tap into future areas of application, a detailed understanding of the fundamental microscopic properties of magnets is required. This is the focus of a recently published study.

All materials, magnetic or not, consist of atoms. Their electrons spin around the atomic nucleus, generating a magnetic field similar to a tiny magnet. Magnetic ions feature partially filled electronic shells that results in (atomic) magnetic moments whereas in non-magnetic ions or atoms filled electronic shells cancel out the magnetism.

An important property used to describe magnets are their atomic magnetic moments. Interestingly, some magnetic materials can assume exotic states with respect to the arrangement of their magnetic moments. Theoreticians have predicted that this should occur in the magnetic pyrochlore Nd₂Zr₂O₇(Nd = neodymium, Zr = zirconium, O = oxygen). At low temperature, this material features a particular, unconventional ordered state known as the “all-in-all-out configuration” (AIAO). This exotic order is accompanied by specific motions of the moments (excitations), which depends on the interactions coupling these moments.

Neutrons behave like nanomagnets and are therefore the ideal probe for investigating such intriguing magnetic states, more specifically through the dynamics of the moments which is the most prominent signature of the exotic state. This inspired an international team of researchers to undertake a series of neutron experiments on Nd₂Zr₂O₇to obtain a better understanding of its AIAO states.

"A fundamental question is: do the excitations, characteristic of this state, fade away or resist to microscopic disorder?", explains Elsa Lhotel, the corresponding author of the publication. To introduce disorder into the material, the team used atomic substitution. For example, in some samples, magnetic correlations were "diluted" by replacing neodymium by non-magnetic lanthanum (La) ions. In others, zirconium was replaced by titanium (Ti), disturbing the local magnetic symmetry. "Additionally, we produced a version of the magnet in which zirconium ions were replaced by a mixture of several non-magnetic ions", says Mélanie Léger, the first author of the study.

The different samples were measured on various instruments, notably including ILL’s time-of-flight (TOF) instruments PANTHER and IN5. "We observed magnetic excitations in a series of samples with an increasing degree of substitution of zirconium by titanium", says Elsa Lhotel. "Interestingly, we did not observe a large difference between low and high substitution degrees."

"ILL’s time-of-flight instruments are ideally suited for these experiments", adds Jacques Ollivier, the main responsible of IN5. "Their setup allows us to record a large range of energy transfers and length scales at excellent resolutions." The inelastic results were confirmed by neutron diffraction, in which the different substitution degrees were also observed to have little influence on the material structure. The team concluded that the neodymium-based compounds they produced are highly resistant towards disorder. In fact, the disorder even appeared to stabilise the AIAO ground state. The impressive stability of the pyrochlore studied here is a promising asset for its further optimisation in view of applications.

This study illustrates the importance of neutrons for detailed studies of the molecular properties of materials, notably including magnets. Such explorations can ultimately pave the way towards the development of novel, tunable materials for a large variety of applications such as, for example, quantum computing.

Images: Example spectra measured on PANTHER, SHARP and IN5, respectively.

Reference: Léger, M., Vayer, F., Hatnean, M. C., Damay, F., Decorse, C., Berardan, D., Fåk, B., Zanotti, J.-M., Berrod, Q., Ollivier, J., Embs, J. P., Fennell, T., Sheptyakov, D., Petit, S., & Lhotel, E. (2024). Impact of disorder in Nd-based pyrochlore magnets. Physical Review B, 109 (22), 224416. https://doi.org/10.1103/PhysRevB.109.224416

ILL instruments: PANTHER, IN5, SHARP

ILL contacts: Jacques Ollivier