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
Diamond Light Source is the UK's national synchrotron science facility, located at the Harwell Science and Innovation Campus in Oxfordshire.
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