First-principles Study of Nitrogen Vacancies in GdN

Punya, Atchara; Cheiwchanchamnangij, Tawinan; Thiess, Alexander; Lambrecht, Walter R. L.

doi:10.1557/opl.2011.383

Cited by 37 publications

(42 citation statements)

References 17 publications

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“…(16) and (17) are converged to about 1 meV, which translates to an uncertainty of 3 K. (ii) Based on Monte Carlo calculations with two (as given in Table VI 17 Khazen et al, 80 Leuenberger et al, 11 and Yoshitomi et al, 81 which vary in value also depending on film thickness, strain, grain size, stoichiometry, and N vacancies. 82,83 We conclude that our results are in very good agreement with the experimental situation. Comparing our results to other theoretical values exhibited in Table VI we note that our coupling constants J 1 and J 2 obtained with HSE are significantly higher, which gives rise to a higher Curie temperature, in agreement with experiment.…”

Section: Magnetic Order and Critical Temperaturesupporting

confidence: 88%

HSE hybrid functional within the FLAPW method and its application to GdN

Schlipf,

Betzinger,

Friedrich

et al. 2011

Phys. Rev. B

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We present an implementation of the Heyd-Scuseria-Ernzerhof (HSE) hybrid functional within the fullpotential linearized augmented-plane-wave (FLAPW) method. Pivotal to the HSE functional is the screened electron-electron interaction, which we separate into the bare Coulomb interaction and the remainder. Both terms give rise to exchange potentials, which sum up to the screened nonlocal exchange potential of HSE. We evaluate the former with the help of an auxiliary basis, defined in such a way that the bare Coulomb matrix becomes sparse. The latter, which is a slowly varying function in real space, is computed efficiently in reciprocal space. This approach is general and can be applied to a whole class of screened hybrid functionals. We obtain excellent agreement of band gaps and lattice constants for prototypical semiconductors and insulators with electronic-structure calculations using plane-wave or Gaussian basis sets. We apply the HSE hybrid functional to examine the ground-state properties of rocksalt GdN, which have been controversially discussed in literature. Our results indicate that there is a half-metal to insulator transition occurring between the theoretically optimized lattice constant at 0 K and the experimental lattice constant at room temperature. Overall, we attain good agreement with experimental data for band transitions, magnetic moments, and the Curie temperature.

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Section: Magnetic Order and Critical Temperaturesupporting

confidence: 88%

HSE hybrid functional within the FLAPW method and its application to GdN

Schlipf,

Betzinger,

Friedrich

et al. 2011

Phys. Rev. B

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“…In view of the dominant doping role played by nitrogen vacancies (V N ) it is here that we seek an enhanced exchange interaction that may nucleate ferromagnetism at 70 K. As a guide we use the V N energy levels calculated by Punya et al [41]; each vacancy is expected to bind two electrons in a singlet state, with a third electron bound only weakly below the conduction band in the dilute limit. As the temperature lowers the carriers begin to freeze out of the conduction band to occupy the third V N level.…”

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confidence: 99%

Role of magnetic polarons in ferromagnetic GdN

et al. 2013

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We report an interplay between magnetism and charge transport in the ferromagnetic semiconductor GdN, pointing to the formation of magnetic polarons centred on nitrogen vacancies. The scenario goes some way to resolving a long-standing disagreement between the measured and predicted Curie temperature in GdN. It further constitutes an extension of concepts that relate closely to the behaviour of ferromagnetic semiconductors generally, and EuO in particular.PACS numbers: 75.50. Pp, Intrinsic ferromagnetic semiconductors, which offer the freedom to dope into specific conducting channels without destroying their magnetic behaviour, are of obvious technological interest. From a fundamental point of view they are again interesting; it is a substantial challenge to understand the interplay between their magnetic and transport properties. Among the earliest discovered, and even now the most studied, is EuO, which shows a magnetoresistance as large as thirteen orders of magnitude across the metal-insulator transition at the Curie temperature (T C ) [1][2][3], the largest in any compound. The strong transport-magnetism interplay is further emphasised by the enhanced T C seen in electron-doped samples [4,5]. Debate continues about whether the ferromagnetic transition is homogeneous [2,6,7] or whether it involves magnetic polarons nucleating around magnetic impurities [3,[8][9][10][11][12].GdN, the prototypical rare-earth nitride compound, provides a rich set of comparisons with EuO. Divalent Eu and trivalent Gd share the same half-filled 4f shell, with S = 7/2, L = 0, and a net moment of 7 µ B . They share also the same rock salt structure, their reported T C are both near 70 K [13][14][15][16], and GdN also shows a strong magnetoresistance at T C [13,17]. Theoretical treatments of reproduce the measured electronic band features well, showing agreement as regards its semiconductor nature [13,14], the direct band gap [22][23][24], and features in the conduction-band density of states [13,25]. Even more important for device development is that GdN, unlike EuO, has a dispersive valence band of delocalised nitrogen states, which raises the possibility that it can support both n− and p−type conduction. Its potential in realistic spintronics has already been demonstrated by its use in a spin filter [26].In contrast the ferromagnetic exchange mechanism is still poorly understood. The atomic-like nature of the 4f electrons necessitates indirect exchange, but those levels lie too far below the Fermi level to call on the third-order perturbation theory applied to EuO [4,27,28]. The validity of a proposed exchange channel involving the excited Gd 4f8 level is uncertain [29]; in particular note these levels are similarly far from Fermi level [25]. Small energy differences are found among various spin orderings determined within the LDSA+U treatment that has successfully reproduced the band structure, but they lead to an estimated T C below 25 K [30], far below the experimental values. There is even a 1980 report [31], raised again re...

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“…For GdN, the apparently non-overlapping Gd f-and N p-states lead to weak coupling, yet the T c is comparable to that of EuO and consistently observed to be higher than predicted. 26 There exists much controversy surrounding the role of the O p-states in driving the magnetic ordering in EuO, with recent calculations suggesting that, despite evidence for strong p-f hybridization, and that the superexchange parameter, J 2, depends on the position of the 2p band, the 4f-2p-4f exchange remains negligible compared with the 5d-4f and 6s-4f indirect exchange. 27 Recent computational work has suggested that the N-vacancies in GdN may play a more active role in the magnetic ordering than previously believed; however their inclusion in the calculation cannot reconcile the large discrepancy between the theoretical and experimental values of T c .…”

mentioning

confidence: 99%

“…27 Recent computational work has suggested that the N-vacancies in GdN may play a more active role in the magnetic ordering than previously believed; however their inclusion in the calculation cannot reconcile the large discrepancy between the theoretical and experimental values of T c . 26 Furthermore, Granville et al have previously shown the T c for GdN to be carrierindependent, suggesting the dominant exchange mechanism to be intrinsic in nature. 11 For HoN, we expect the p-f hybridization to yield stronger coupling between the Ho f-and N p-states, as is observed in EuO-how this will affect the calculated T c remains to be investigated.…”

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confidence: 99%

p-f hybridization in the ferromagnetic semiconductor HoN

Brown

Downes

McMahon

et al. 2012

Applied Physics Letters

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The electronic structure of thin film HoN has been studied using soft x-ray spectroscopy. The combination of soft x-ray emission, x-ray absorption, and photoemission techniques yields direct evidence for hybridization between the N 2p and the Ho 4f states, previously unseen in this or related rare earth nitride systems. The N 2p states extend up to 10 eV below the Fermi level to nearly twice the binding energy as previously believed. Optical spectroscopy yields a minimum direct gap of 1.48 eV. In light of these results, we identify HoN as a prime candidate for spin-diffusion and spintronics applications.

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First-principles Study of Nitrogen Vacancies in GdN

Cited by 37 publications

References 17 publications

HSE hybrid functional within the FLAPW method and its application to GdN

HSE hybrid functional within the FLAPW method and its application to GdN

Role of magnetic polarons in ferromagnetic GdN

p-f hybridization in the ferromagnetic semiconductor HoN

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