Frustrated Magnets

By: David Bradley

Scientists sometimes take magnetism for granted. But some materials behave badly and scientists funded by the UK's Engineering and Physical Sciences Research Council (EPSRC) are trying to find out why. They are looking at the conventional wisdom of modern theories and finding it does not always stick to exotic magnets.

Scientists from archaeologists to zoologists are attracted to magnetic measurements. Archaeologists use magnetic artefacts to date sites, while a zoologist might be investigating the effects of magnetism on bird brains. Magnetism is fundamental to science, but despite their ubiquity scientists still cannot fully explain magnetic materials.
  Professor of Solid State Chemistry at Edinburgh University, Andrew Harrison hopes with the help of EPSRC funding to get inside some exotic magnets, which could provide insight into more common magnetic materials and ultimately help in the design of computer memory and electrical devices. "The study of the fundamental properties of magnets gives us valuable insights into the principles that govern the structure of solids," he explains, "This has implications that stretch beyond magnetism and into superconductivity."

  The conventional picture of a magnet says each atom in a material acts like a tiny magnet. The magnetic moment, or direction of the North-South divide on the atoms line up parallel (in permanent ferromagnets like iron) or anti-parallel in an up-down arrangement in antiferromagnetic materials, such as manganese oxide. "The reason why such materials behave this way," says Professor Harrison, "is that below a certain critical temperature the magnetic moments 'freeze', or become locked in position." For an iron magnet this critical temperature is well above room temperature but for other materials they have to be cooled near to absolute zero before they become magnetic. The picture was fairly simple until high-temperature cuprate superconductors were discovered and started throwing up strange results. For instance, some of these materials, at first sight seemed not to have frozen magnetic moments. Such findings inspired researchers to seek new understanding of magnetic materials.

  Because the magnetic moments in some superconducting magnets were very small, quantum fluctuations in their orientation overpower the usual forces that normally lock the magnetic moments in place. In the case of lanthanum cuprate, this is not quite enough to upset the conventional picture but a new ingredient - frustration - changes completely the conventional picture. Frustration is found in materials where the magnetic moments are on a triangular rather than a square lattice. "You cannot physically align all the moments antiparallel with their neighbours!" says Harrison. In such a 'frustrated' lattice, the conventional forces between magnetic moments are much reduced and the quantum fluctuations are more influential. This kind of magnet may never freeze and the material fluctuates between different states with the moments twitching between the sides of the triangles. The result is that the material exists as a "spin fluid" and such materials could help explain magnetism and possibly superconductivity.

  According to Harrison, however, materials that have a small magnetic moment in a frustrated lattice are very rare and have proved difficult to synthesize. "Such a material is one of the Holy Grails in this area of science" he muses There are many materials with triangular lattices, such as vanadium(II) chloride, but they have conventional magnetism. "The challenge is to swap the elements such as vanadium, which have relatively large atomic moments, for magnetic copper ions, which have small atomic moments, while retaining a triangular lattice. So far the wrong lattice forms," adds Harrison, "It's as if nature doesn't want to produce such a material with this kind of unstable ground state, what happens is that the magnet distorts to some other form as it cools down."
  Harrison and collaborators have tried an alternative material type over the last few years. A material of chemical formula ABO2 (A and B are two different metals, O2 is oxygen) can crystallize with the rock salt, sodium chloride NaCl, structure but instead of Na-Cl-Na-Cl