Spin polarized conduction and valence band states in GdN
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Spin polarized conduction and valence band states in GdN Muhammad Azeem1,2 1
Department of Applied Physics and Astronomy, University of Sharjah, University City Road,
P.O. Box 27272, Sharjah, United Arab Emirates. 2
MacDiarmid Institute for Advanced Materials and Nanotechnology, School of Chemical and
Physical Sciences, Victoria University of Wellington, Wellington 6140, New Zealand. Abstract Optical transmission spectra are collected in the ferromagnetic state of GdN at various temperatures. The temperature dependent extinction coefficient extracted from the transmission measurements show that the conduction and valence band states are spin polarized. The results confirm the LSDA+U estimates of the energy gap associated with the majority/minority-spins and also the magnitude of the spin splitting. Introduction Gd metal has half-filled 4f orbitals with 7 electrons making it an ideal metal for the spintronic applications. It has been shown that the nitride compound of Gd metal has semiconducting ground state [1] however there are also reports for GdN single crystals to be metallic [2]. On the magnetic ordering, Duan et al [3] proposed a next-nearest-neighbour (NNN) antiferromagnetic super exchange mechanism and RKKY-type nearest neighbour exchange co-exist in GdN compounds. A carrier-mediated ferromagnetism in GdN is proposed by Sharma et al [4]. Further, Senapati et al [5] speculates an antiferromagnetic phase present in GdN, introduced due to the local strain produced by nitrogen vacancies. Electronic structure calculations [3, 6, 7] provide a varying interpretations to the structure of electronic bands in GdN. Initial theoretical studies proposed a half-metallic [8,9] and then the semiconducting nature of GdN [10,11]. A theoretical study by Larson et al [10] showed that GdN, like EuO, is a ferromagnetic semiconductor with a Curie temperature of 68 K. The first systematic investigation of the direct energy gap of GdN was done by Trodahl et al on the GaN protected samples [12]. The LSDA+U band structure calculations were, then, tuned to the experimentally obtained energy gap value, Eg=1.3 eV [12], which returned a semiconducting solution. The maximum of the valence band in GdN lies at the Brillouin zone center and the conduction band minimum at the X point. It is thus an indirect-gap semiconductor with a minimum direct gap situated at X. The conduction band is predominantly of Gd 5d character, and the ferromagnetic f-d exchange interaction ensures that the majority conduction band states lie lower in energy. In contrast the N 2p electrons that dominate the valence band, experience an antiferromagnetic exchange with the Gd 5d electrons, so that the valence band maximum is of majority spin. The average of majority and minority spin gaps corresponds closely to the 1.3 eV,
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