A new understanding of antiferromagnetism

Ferromagnetism is the form of magnetism with which we are all familiar in everyday life. It is the phenomenon by which materials such as iron, when placed in an external magnetic field, become magnetised and remain so for a period after the material is extracted from the field. The term ‘ferromagnet’ can be used for any material that exhibits a net magnetic moment in the absence of an external field.

Strictly speaking, however, this definition applies only to materials in which all the magnetic ions add a net positive contribution to the magnetisation. If some of the ions are partially anti-aligned, the material is said to be ferrimagnetic. And if the spins of the ions are aligned in a regular pattern, with neighbouring spins pointing in the opposite direction, the material is an antiferromagnet. Such magnetic alignments occur only below a certain critical temperature dependent on the material. This is called the Curie Point in the case of ferromagnets, and the Néel Temperature with antiferromagnets.

In antiferromagnets, the magnetism is undetectable at a macroscopic level, and is instead confined to tiny regions where the atoms behave as individual magnets, with the bulk material being magnetically neutral. Chromium is an example of a metal that displays antiferromagnetic behaviour.

X-ray speckle pattern of an antiferromagnetic domain wall (© Center for Nanoscale Materials, Argonne National Laboratory)

Antiferromagnetism remains a relatively little understood phenomenon, but physicists at University College London in the UK, and the University of Chicago and the Argonne National Laboratory (ANL) in the US, recently used x-ray holography to study the internal workings of antiferromagnets.

As well as producing the first holograms of antiferromagnets, the research showed that their magnetic domains shift with time over micrometre distances, even at temperatures well below the corresponding Néel Temperature of the material. The researchers say that the most likely explanation for this is to be found in quantum mechanics, and the experiments open the door to the future exploitation of antiferromagnets in emerging technologies such as quantum computing.

“The key finding of our research provides information on the stability of domain walls in antiferromagnets,” says Oleg Shpyrko, lead author of the study published recently in the journal Nature. “Understanding this is the first step towards engineering antiferromagnets into useful nanoscale devices that exploit it.”

Antiferromagnetic films are currently used as pinning layers in computer hard drives, and since they are relatively insensitive to stray magnetic fields, antiferromagnets preserve a reference direction for hard drive read/write heads. The materials are also used in spintronic devices, where information is carried by electron spin rather than charge. The stability of magnetic polarisation in the thin films used today is often taken for granted, but the new discoveries show that in thicker films and bulk materials, magnetic domain walls may not be so stable.

The latest research used x-ray photon correlation spectroscopy to produce speckle patterns, or holograms which provide a unique fingerprint of a particular magnetic domain configuration. This is possible as the internal order of antiferromagnets is roughly the same as the wavelength of x-rays (<10 nm). But it has only become possible in the last few years with the availability of coherent x-ray sources such as the Advanced Photon Source, located at the Argonne National Laboratory.

“The next step in our research is to explore how doping pure chromium with vanadium – chromium’s neighbour in the periodic table – affects antiferromagnetism in general, and domain stability in particular,” says Shpyrko. “Vanadium is known to suppress chromium’s antiferromagnetism, resulting in a so-called quantum criticality. On the other hand, randomly distributed dopant vanadium atoms may also act as pinning centres. We are also exploring the application of coherent x-ray speckle to the study of other materials that show similar domain texture, such as colossal magnetoresistive compounds.”

Figure: By observing changes in coherent x-ray speckle patterns such as this, researchers can investigate the nanoscale dynamics of antiferromagnetic domain walls, and observe a crossover from classical to quantum behaviour (© Center for Nanoscale Materials, Argonne National Laboratory).

Further reading: Direct measurement of antiferromagnetic domain fluctuations, Shpyrko et al., Nature 447, 68 (2007).

Article first published in Nanomaterials News.