Role of magnetic polarons in ferromagnetic GdN

Abstract

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...

Luminescence properties of magnetic polarons in EuTe: Theoretical description and experiments in magnetic fields up to 28 T

“…If the Xf interaction overpowers the ff one, then a magnetic polaron may be formed, which is when the photoexcited electron breaks the natural antiferromagnetic alignment in its vicinity, generating a local nonzero magnetic moment. Magnetic polarons may also arise due to electrons bound to impurities and defects, and it has been suggested that they may be exploited to raise the critical temperature in rare-earth ferromagnets [13,14].…”

“…However, technological interest waned when it was discovered that EuTe is an antiferromagnet with a very low Néel temperature (T N = 9.6 K), which is impractical for device applications. Nevertheless, EuTe and other Eu chalcogenides remain ideal magnetic systems that can be used to test the principles of operation of spin-related devices [8], and to investigate many-body physical phenomena, e.g., spin waves [9] and magnetic polarons [2,4,[10][11][12][13][14].…”

Electric field and photo-excited control of the carrier concentration in GdN

“…A Curie temperature (T C ) near 70 K establishes its RE-RE exchange as the strongest within the series. 4,5,15 It has been suggested that a homogeneous ferromagnetic (FM) phase exists only below 50 K, and that the commonly quoted 70 K T C signals a FM phase nucleated by magnetic polarons near nitrogen vacancies, 16 but even that 50 K is higher than the T C for any other member of the REN series. The promise of GdN has been demonstrated recently in a superconductor-GdN-superconductor tunnel junction that provides excellent spin selectivity.…”

Large negative magnetoresistance in reactive sputtered polycrystalline GdNx films

“…The film fabricated at f N2 ¼ 20 sccm shows an upturn of magnetization below 20 K, which is similar to the shoulder observed in the intermediate-doped epitaxial GdN film. 26 The shoulder is ascribed to the formation of magnetic polarons centered on N vacancies. 26 Similarly, at f N2 ¼ 20 sccm, the upturn of magnetization below 20 K can be attributed to the ferromagnetism brought about by N vacancies when carriers freeze out of conduction band to occupy V N level and provide a large local electron density that can mediate exchange between neighboring Gd ions, forming a magnetic polaron.…”

“…26 The shoulder is ascribed to the formation of magnetic polarons centered on N vacancies. 26 Similarly, at f N2 ¼ 20 sccm, the upturn of magnetization below 20 K can be attributed to the ferromagnetism brought about by N vacancies when carriers freeze out of conduction band to occupy V N level and provide a large local electron density that can mediate exchange between neighboring Gd ions, forming a magnetic polaron. Figure 1(d) shows the temperature-dependent normalized resistivity q(T)/q(305 K) of polycrystalline GdN x films.…”