betaNMR reveals nanoscale surface details in topological insulators

bNMR reveals nanoscale surface details in topological insulators: Wolfgang Pauli said, “God made the bulk; surfaces were invented by the devil”, a recognition of the fact that theories and experimental measurements of near-surface properties are very difficult. Surfaces may be difficult to study, but it is where much interesting physics arise. Topological insulators (TI) are materials where the bulk is an insulator but whose surface contains conducting states, which means that electrons can only move along the surface of the material. Topologically protected states could act as a source of spin-polarized electrons with properties relevant to spintronics applications including quantum computing. As published in the Proceedings of the National Academy of Sciences (2015) researchers at CMMS used bNMR spectroscopy as a nano-scale depth-resolved probe of magnetism and conductivity within about 10 nm of the free surface of (Bi,Sb)2Te3. This depth-dependent study of electronic and magnetic properties of TI epitaxial layers using implanted, spbin-polarized ⁸Li⁺ ions reveals differences in the band structure between the near-surface and deeper into the bulk material.    

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.