Europium nitride: a novel diluted magnetic semiconductor

15-05-2014

A research team from Victoria University of Wellington in New Zealand, working with scientists from the ESRF and an expert in magnetism from CNRS Grenoble, have recently discovered an entirely new diluted magnetic semiconductor. The discovery was made in a material called europium nitride and unambiguously confirmed using X-ray magnetic circular dichroism (XMCD).

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Diluted magnetic semiconductors (DMSs) are a class of materials that are typically formed by incorporating magnetic ions such as manganese into conventional semiconductors like gallium arsenide. They are of interest in the field of spintronics that seeks to exploit the intrinsic magnetism of electrons in electronic devices. DMSs offer particular advantages over typical magnetic metals like iron when it comes to injecting spin polarised electric currents into semiconductor devices, a key step in realising spintronics technologies. Attempts to study DMS systems are especially difficult because the magnetic ions often segregate into nanosized impurity phases that give a spurious magnetic signal.

Europium nitride (EuN) is a member of the rare-earth nitride series, many of which are both magnetic and semiconducting. However, EuN is not expected to be magnetic because of the particular configuration of the electrons in the Eu3+ ion. We had already discovered that EuN thin films prepared with a slight deficit of nitrogen contain some of the europium ions in the 2+ charge state. These ions do carry a magnetic moment, but the moments of the individual 2+ ions had not previously been observed to interact to give an overall ferromagnetic state [1]. The new research showed that if about 20% of the europium is in the Eu2+ state, magnetic ordering sets in below a critical temperature of 125 K (see Figure 1), which is one of the highest temperatures of confirmed DMS systems.

Ferromagnetic response of a nitrogen deficient europium nitride film.

Figure 1. 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. The inset shows a hysteresis loop measured at 5 K, confirming the ferromagnetism.

Proof that the magnetism is intrinsic to the EuN was provided by XMCD studies at beamline ID12. In these studies, the magnetic contribution from the Eu2+ and Eu3+ ions in the films were probed independently (see Figure 2). The magnetic signal from the Eu2+ component of the EuN was found to closely follow the overall magnetisation of the whole sample (solid circles in Figure 1) confirming that the magnetism comes from the film. More significantly, a strong magnetic signal was also found on the Eu3+, which confirms that the Eu2+ remains part of the EuN rather than forming a separate magnetic impurity phase.

X-ray absorption and XMCD results from a ferromagnetic EuN film.

Figure 2. 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.

We have gone on to demonstrate that the magnetism is closely linked to the electrical conductivity of the films. This implies that the so-called exchange interaction that aligns the magnetic moments of the Eu2+ ions might be mediated by conduction electrons, as in conventional diluted magnetic semiconductors. However, the magnetic polarisation of the Eu3+ revealed by the XMCD measurements suggests that the mechanism is more complicated, with the Eu3+ essentially acting as a magnetically polarisable background that enhances the magnetic interactions. This contrasts with conventional DMS systems where the host semiconductor is magnetically inert.

 

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), Europium nitride: A novel diluted magnetic semiconductor, Phys. Rev. Lett. 111, 167206 (2013).
(a) The MacDiarmid Institute for Advanced Materials and Nanotechnology, Victoria University of Wellington (New Zealand)
(b) Institut Néel, Centre National de la Recherche Scientifique and Université Joseph Fourier, Grenoble (France)
(c) ESRF

 

References
[1] B.J. Ruck, H.J. Trodahl, J. Richter, J. Criginski-Cezar, F. Wilhelm, A. Rogalev, V. Antonov, Do Le Binh, F. Natali, C. Meyer, Phys. Rev. B 83, 174404 (2011).

 

Top image: The XMCD end station at beamline ID12 used in this study of europium nitride features a 17 Tesla magnet.