The coupled magnetic and electronic properties of rare-earth doped GaN semiconductors have attracted a lot of interest by researchers from industry and academia in recent years. Researchers from University of Strathclyde, Osaka University (Japan) and Diamond Light Source have investigated Eu-doped GaN at Diamond beamlines B18 and I06. Their results have been published in Nature’s Scientific Reports.
Unlike other trivalent rare-earth ions, Eu3+ ions have no magnetic moment in the ground state. In this research the presence of an induced magnetic moment was demonstrated using Optically Detected X-ray Magnetic Circular Dichroism (XMCD). Using the detection of X-ray Excited Optical Luminescence (XEOL) from Eu3+ at 622 nm scientists uniquely probed the magnetic moment of the 7F2 excited state involved in optical emission. Interestingly, the same emission line is already used in red light-emitting devices so every time these devices are switched on magnetism is induced.
The 400nm thick GaN samples with just 0.11 at% of Eu were grown on sapphire substrates at 1000°C, using a complex method called ‘organometallic vapour phase epitaxy’. Samples were then studied on beamline I06 using XMCD, which compares the X-ray absorption spectra of left- and right-circularly polarised X-rays. XMCD was measured using surface-sensitive total electron yield (TEY) and bulk-sensitive XEOL. The shift of Eu M5 XMCD peak from 1128.3 eV in TEY to 1129.8 eV in XEOL was explained by the presence of magnetic Eu2+ ions on the surface and Eu3+ ions being in excited 7F2 state in the bulk. Theoretical calculations reproduced the energy shift observed in the experiment. X-ray Absorption Near Edge Structure (XANES) was measured on B18 beamline to confirm that bulk of the sample did not contain Eu2+ ions.
“This is the first time that an induced magnetic moment of Eu3+ ions in this excited state has been detected directly using XEOL detection of XMCD. As Eu3+ ions do not possess magnetic moment in the ground state this opens up the possibility that the magnetism can be switched on and off by shining a light or switching on and off voltage. This is a very interesting prospect that could lead to development of novel magneto-optoelectronic devices in the future. The next stage is to investigate the reported room-temperature ferromagnetic behaviour of Eu-doped GaN and whether this relates to the induced magnetic moment observed here.”Slava Kachkanov, principal investigator, Diamond Light Source
Induced magnetic moment of Eu3+ ions in GaN, Slava Kachkanov, M. J. Wallace, Gerrit van der Laan, Sarnjeet Dhesi, Stuart Cavill, Y. Fujiwara, K. P. O'Donnell, Scientific Reports 2 DOI: 10.1038/srep00969
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