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...

Europium nitride is semiconducting and contains nonmagnetic Eu3+, but substoichiometric EuN has Eu in a mix of 2+ and 3+ charge states. We show that at Eu2+ concentrations near 15%-20% EuN is ferromagnetic with a Curie temperature as high as 120 K. The Eu3+ polarization follows that of the Eu2+, confirming that the ferromagnetism is intrinsic to the EuN which is, thus, a novel diluted magnetic semiconductor. Transport measurements shed light on the likely exchange mechanisms.