Applied Radioisotopes

Towards more data-dense hard drives

bNMR Investigation of the Depth-Dependent Magnetic Properties of an Antiferromagnetic Surface Hard drives use disks made of magnetic material to store information, and an electromagnet in the read/write head writes information to the disk by magnetizing small sections of the disk. Increasing the information on a hard drive requires shrinking the size of the magnetic sections and this means the near-surface regions are increasingly important. The prototypical antiferromagnet α-Fe2O3 has a first-order transition known as the Morin transition at 260 K, where the orientation of antiferromagnetic order with respect to the crystal lattice undergoes an abrupt change. In this work the static spin orientation and dynamic spin correlations within nanometers from the surface of a single crystal was studied via the nuclear spin polarization of implanted 8Li ions and detected via bNMR spectroscopy. As reported in Physical Review Letters (2016), the experiment found that the Morin transition temperature was independent of depth from 1 to 100 nm from the free (110) surface but the fluctuations of the electronic spins are faster near the crystal surface and decay into the bulk over a characteristic length of 11 nm.  The results suggest the magnetic order parameter undergoes a continuous gradient rather than a phase separation of bulk vs. surface magnetism. Whereas previous studies made use of nanoparticles to achieve sufficient near-surface volume fraction to extract a signal, bNMR spectroscopy allowed a depth-resolved characterisation of the magnetic order parameter into a macroscopic single crystal of α-Fe2O3, differentiating free-surface and finite-size effects on magnetic order.