Bigger capacity for big data

31 Aug 2015 NUS scientists at the Department of Physics and NUSNNI-Nanocore have discovered an unusual magnetic effect in nanolayers of an oxide of lanthanum and manganese.

Magnetism in nanoscale layers only a few tens of atoms thick is one of the foundations of the big data revolution – for example, all the information downloaded from the internet is stored magnetically on hard disks in server farms dotted across the World. A team led by Profs Ariando and Venky VENKATESAN from the Department of Physics in NUS, with colleagues from Netherlands, the United States and Ireland has uncovered a new twist to the story of atomically thin magnetic layers.

Prof Ariando explained the new discovery is that the magnetism of an oxide of lanthanum and manganese (LaMnO₃) layer is switched on abruptly when the number of manganese atomic layers changes from 5 to 6. This is achieved by growing thin layers of stacks of individual LaMnO₃ cells, each only four tenths of a nanometer thick on a perfectly flat crystal of nonmagnetic SrTiO₃, using a technique called pulsed laser deposition (see Figure). By adding a sixth atomic layer, the magnetism switches from antiferromagnetic (antiferromagnets produce no magnetic field) to ferromagnetic. Such an abrupt transition has never been seen before. Using a Scanning SQUID Microscope, an instrument that uses superconducting electronics to measure magnetic fields with exquisite sensitivity (one hundred thousand times smaller than the Earth’s field), a direct image of the change in magnetic properties was obtained. This magnetic imaging was conducted by Dr Xiao Renshaw WANG, who is a PhD graduate from Physics and NUSNNI (supervised by Profs Ariando and Venkatesan), working with Prof Hans HILGENKAMP at the MESA+ Institute of the University of Twente in The Netherlands. The abrupt switch from anti-ferromagnetism to ferromagnetism just by adding one extra atomic layer can be explained due to an avalanche of electronic charge inside the LaMnO₃ from the top surface of the film to the bottom.

The next plans are to use local electric fields to controllably turn on/off the magnetism of its 5-layer films, and explore potential applications in microwave devices and magnetic recording. With magnetic memory elements approaching nano dimensions, this technique promises new approaches in magnetic recording and computing. The National Research Foundation (NRF) and Ministry of Education (MOE) of Singapore supported this research.

Figure showing the magnetic layer being grown by depositing atomic-layer by atomic-layer of LaMnO3 on a substrate crystal. After the growth, the magnetic field is recorded by scanning a tiny superconducting coil over the surface of the grown LaMnO3 film. [Image credit: Renshaw Wang]

Reference

X Renshaw Wang et al. “Imaging and control of ferromagnetism in LaMnO3/SrTiO3 heterostructures” Science 349 (2015) 716.