Most everyday magnets are ferromagnets like iron, whose internal “magnetic domains” (tiny bar magnet-like regions within the larger magnet) can be made to line up preferentially in one direction to create a magnetic field outside the material. In an antiferromagnet, however, adjacent domains line-up antiparallel to each other, thereby cancelling each other’s magnetic field overall. However, unlike normal non-magnetic materials with randomized domain directions, the antiferromagnet’s antiparallel ordering can exhibit interesting behaviours at small scales, especially near material interfaces. These behaviours have been exploited, for example, by the multibillion dollar magnetic hard-drive industry.
A team of researchers led by David Cortie, Andrew MacFarlane and Robert Kiefl working at the βNMR facility at TRIUMF have made the first application of the facility’s depth-resolved ion-beam technique to detect the NMR signal from the near-surface region of an antiferromagnet. Their initial work was on the prototypical antiferromagnet α-Fe2O3 (110), which was identified as an ideal test case. In the βNMR technique, 8Li+ ions are implanted precisely underneath material surfaces at varying depths with roughly 10 nanometre precision (see Figure 2). Detecting the resulting lithium decay electron serves to enhance the NMR signal by a factor of a billion. The combination provides extreme sensitivity for characterizing local magnetic fields at nanometre scales, essential for probing such fields near material interfaces.
The group discovered new behaviour in the spin reorientation transition near the α-Fe2O3 (110) surface that did not fit within the established hierarchy of near-surface phase transitions (see Figure 3). Their results showed conclusively that βNMR can measure phase transitions, structure and dynamics near antiferromagnetic interfaces, thereby providing a new tool for optimizing functionality in novel nanostructures. βNMR looks forward to greatly increased uptime once TRIUMF’s upcoming ARIEL facility comes online, and so we can all look forward to many more breakthroughs coming from βNMR in the future.
For more information, the research paper can be found here.