Europium nitride: a novel diluted magnetic semiconductor

Ferromagnetism, where a material spontaneously forms a net magnetisation, is one of the oldest and most striking examples of correlated electron phenomena. Most ferromagnets are metals, but under the right conditions some semiconductors can also become ferromagnetic if they are doped with a small concentration of magnetic ions. Such so-called diluted magnetic semiconductors (DMSs) offer exciting possibilities in the field of spintronics, where the functionality of electronic devices is enhanced by exploiting the intrinsic magnetic moment of electrons in addition to their charge. The most well studied diluted magnetic semiconductors are formed by adding manganese to III-V semiconductors such as GaAs. Mobile charges in the semiconductor host mediate magnetic interactions between the manganese ions, allowing them to form an ordered magnetic state. Understanding the details of the interactions remains a challenge even after more than two decades of study [1], and the field remains an active research area. We have recently added a novel material, europium nitride, to the list of known DMSs, and we have shown that it exhibits some surprising properties that distinguish it from other materials in the class [2].

Europium nitride (EuN) is a member of the rare-earth nitride series, many of which are ferromagnets at low temperature. However, EuN is not expected to be magnetic because of the particular configuration of the electrons in the Eu³⁺ ions. The team at Victoria University in Wellington had already discovered that in EuN thin films prepared with a slight deficit of nitrogen some of the europium ions convert to the 2+ charge state. Eu²⁺ does carry a magnetic moment, but the moments of the individual 2+ ions had not previously been observed to order into an overall magnetic state [3]. The new research showed that, once about 20% of the europium becomes Eu²⁺, magnetic ordering sets in below a critical temperature of 125 K (see Figure 15), which is one of the highest temperatures of confirmed DMS systems.

Fig. 15: Ferromagnetic response of a nitrogen deficient europium nitride film. The solid lines represent SQUID magnetometry measurements, while the red symbols represent element specific magnetisation measured by XMCD. The XMCD results prove that the ferromagnetism originates in the EuN rather than in an impurity phase.

Fig. 16: X-ray absorption and XMCD results from a ferromagnetic EuN film. Magnetic polarisation of europium ions in both 2+ and 3+ charge states is evident.

Proof that the magnetism is intrinsic to EuN came from X-ray magnetic circular dichroism (XMCD) results obtained at beamline ID12. This powerful technique independently measured the magnetic contribution from the Eu²⁺ and Eu³⁺ (see Figure 16). The magnetic signal from the Eu²⁺ component of the EuN closely follows the magnetisation of the whole sample (solid circles in Figure 15), confirming the central role of Eu²⁺ in the magnetic state. More surprisingly, a strong magnetic signal was also found on the Eu³⁺. This proves that the Eu²⁺ remains coupled in the EuN matrix rather than forming a separate magnetic impurity phase. Furthermore, it indicates that the Eu³⁺ that forms the bulk of the material is playing far more than a passive role in the magnetism. Instead, the Eu³⁺ is acting as a magnetically polarisable background that enhances the magnetic interactions between the Eu²⁺ ions beyond that which can be provided by the small density of conduction electrons in the material. This is in striking contrast to conventional DMS systems where the host semiconductor is magnetically inert, and thus represents a new paradigm in the DMS field.

Principal publication and authors
Do Le Binh (a), B.J. Ruck (a),  F. Natali (a), H. Warring (a),  H.J. Trodahl (a), E.-M. Anton (a),  C. Meyer (b), L. Ranno (b), F. Wilhelm (c), and A. Rogalev (c), Phys. Rev. Lett. 111, 167206 (2013).
(a) The MacDiarmid Institute for Advanced Materials and Nanotechnology, Victoria University of Wellington, Wellington (New Zealand)
(b) Institut Néel, Centre National de la Recherche Scientifique and Université Joseph Fourier, Grenoble (France)
(c) ESRF

References
[1] T. Dietl and H. Ohno, Rev. Mod. Phys. 86, 187 (2014); T. Jungwirth,  J. Wunderlich, V. Novák, K. Olejník,  B.L. Gallagher, R.P. Campion,  K.W. Edmonds, A.W. Rushforth,  A.J. Ferguson and P. Nemec, Rev. Mod. Phys. 86, 855 (2014).
[2] Do Le Binh, B.J. Ruck, F. Natali,  H. Warring, H.J. Trodahl, E.-M. Anton,  C. Meyer, L. Ranno, F. Wilhelm and  A. Rogalev, Phys. Rev. Lett. 111, 167206 (2013).
[3] B.J. Ruck, H.J. Trodahl, J. Richter,  J. Criginski-Cezar, F. Wilhelm,  A. Rogalev, V. Antonov, Do Le Binh,  F. Natali and C. Meyer, Phys. Rev. B 83, 174404 (2011).