Do quantum fluctuations create magnetism at oxide surfaces?

15 May 2019

Over the past 50 years scientists have come to understand the magnetism of transition-metal oxides, and how to make use of them in applications ranging from permanent magnets and data storage, to microwave components and ferrofluids for biomedical applications.

Today, our understanding of oxide magnetism takes a new turn. Prof. Michael Coey, AMBER and School of Physics, Trinity College, argues in a Commentary published in Nature Materials, that quantum fluctuations of the vacuum may be causing weak magnetism at the surface of oxides even in materials where magnetic transition-metal atoms are absent: magnetic oxides with no magnetic ions.

The problem of magnetism in metal oxides that contain only a few percent of magnetic metal atoms dates back to 2000, when surprising claims were made that thin films of these materials were ferromagnetic above room temperature. It was hoped then that the oxides were dilute magnetic semiconductors that would open a new era of ‘spin‘ electronics based of the magnetism of spinning electrons. But that was not to be! Numerous studies were published on oxide films apparently exhibiting ferromagnetism when doped with various magnetic or even non-magnetic metal atoms, but the data were difficult to reproduce, and they failed to pinpoint the source of the magnetism as coming from any of the atoms present in the films. Contamination in the experimental processing of the oxides came to light. Sources of contamination included ferromagnetic material external to the thin films such as iron from dyes, substrate heaters and tweezers or magnetite particles from ambient air. This caused materials scientists to dismiss the magnetism as some sort of dirt effect or measurement artefact.

But Prof. Coey suggests otherwise: “We find that these oxides really do become very weakly magnetic at room temperature: it is a surface effect that we are able to turn on and off by various simple physical or chemical treatments. One way to destroy the magnetism is simply to place the sample in a solution of asprin. The key question is this: how can we explain magnetism without any magnetic material? It is difficult puzzle”.

The hypothesis Prof. Coey considers in his commentary as the most likely to explain the new magnetic effect is also the most controversial; namely that quantum fluctuations of the vacuum may be responsible for the observation. This argument relies on a prediction from quantum physics that everything is bathed in a universal sea of zero-point energy that sets the lowest state of every point in empty space. The energy is not directly observable and cannot be harvested, but it is manifest in the weak van der Waals and Casimir forces.

As Prof. Coey outlines: “We think that we can explain the very weak magnetism in oxides in terms of these fluctuations, which cause the surface electrons to enter a collective orbiting state where they can respond together to a magnetic field. It is an idea that opens up fascinating new questions for the scientific community”.

As the paper concludes: “If the hypothesis could be definitively established, the prospect of an influence of zero-point fluctuations on the physical properties of condensed matter is intriguing. All life has evolved in this environment. Who knows what could have been the consequences?”

The full paper is available at the Nature Materials website: https://doi.org/10.1038/s41563-019-0365-9

Coey, J. M. D. 2019. Magnetism in d0 oxides. Nature Materials (1476-4660)