Room temperature magnetic semiconductor for spintronics

18-06-2013

Materials for spintronic applications need to have both magnetic and semiconducting properties. But an intrinsic semiconductor that is magnetic at room temperature has yet to be found. The europium chalcogenides (EuO, EuS, EuSe, EuTe) are promising wide band-gap semiconductors, but they have very low Curie temperatures. Here, for the first time, we demonstrate that EuS can be spin-polarised at room temperature in proximity to a ferromagnet, such as Co, using XMCD at the Eu-L2,3 edges. This result suggests that manipulation of the electron spin and charge in these materials should be feasible in the near future.

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Europium sulphide (EuS) is a natural ferromagnetic semiconductor. When compared to layered systems of the type ferromagnet/semiconductor, it has the advantage that spin-polarised electrons are created within the semiconductor itself. The disadvantage of EuS is a low Curie temperature (TC) of 16.6 K. Previous work with EuS nanospheres in a Co matrix or layered Co/EuS systems with relatively large spheres or layers (larger than about 3.5 nm) showed also some enhancement of TC of EuS due to the proximity to Co but no convincing evidence for ferromagnetism at room temperature (R.T.) of EuS was provided [1].

Most work in the literature has focused on diluted magnetic semiconductors. However, recent measurements carried out at the ESRF’s spectroscopy beamline ID12 proved that there is no sign of intrinsic ferromagnetic interaction between Co substitution (dopant) ions in ZnO in diluted semiconductors. The Co ions showed purely paramagnetic or superparamagnetic response [2].

In this work, Co/EuS multilayers were prepared by e-beam evaporation in ultra-high vacuum. Both, X-ray reflectivity and successive high-resolution cross-section transmission electron microscopy showed a good quality of the layering [see right inset of Figure 1 for a Co(4 nm)/EuS(3.5 nm) multilayer]. However, defects resulted in a partial loss of stoichiometry and the appearance of both divalent and trivalent Eu (as described in Ref. [3], a few Eu3S4 crystallites also appear, green circle in the inset of Figure 1). Only divalent Eu contributes to ferromagnetism because trivalent europium possesses J = 0.

 

R.T. spin polarisation of Eu in EuS layers in proximity to Co layers.

Figure 1. R.T. spin polarisation of Eu in EuS layers in proximity to Co layers. In the inset, element-specific hysteresis loops are traced for both, Co and EuS, at R.T. The cross-section transmission electron microscopy images support the successful formation of the Co/EuS multilayers. (We thank Dr. A. Delimitis for the microscopy images).

Element-specific XMCD measurements were performed at the L2,3 edges of Eu at ID12 at temperatures between R.T. and 2.5 K and under magnetic fields up to 17 T. Figure 1 shows the XMCD signal at the L2 edge of Eu recorded on a Co(7 nm)/EuS(2 nm) multilayer at R.T. Comparison with the low temperature (T = 2.5 K) XMCD signal leads to the conclusion that divalent Eu shows a relatively large magnetic response at R.T., almost equal to that of bulk Ni. Element-specific hysteresis loops at R.T. (inset) showed that there is no need for an external field to spin-polarise EuS, so this is an effect of the direct exchange coupling Jint at the interface with Co. We also observed that the Co and Eu moments are always antiparallel. No significant change in the magnitude of the magnetisation of each element occurs up to 0.65 T.

We suggest that two factors contribute to the strong magnetic response of EuS at R.T. The first is the high susceptibility of EuS layers to effective fields due to the J = 7/2 character of the system. In Figure 2, we show that the same effective field, in a mean-field theory (MFT) approach, polarises a J=7/2 system by a factor of almost three more than a J=1/2 system at the same reduced temperature above TC. The second was the very thin layers used in this work that approach the two dimensional-limit (2 nm of EuS correspond to about 6 atomic (100) planes). Two-dimensional ferromagnets are not stable against thermal fluctuations at finite temperatures. However, even moderate fields are able to suppress the spin fluctuations and result in a significant enhancement of the TC of ultrathin ferromagnetic films [4]. Similarly, in exchange coupled layers such as Co and EuS, the exchange coupling at the interface plays the role of an effective field. This was shown [3] to be of the order of 1 T. It is interesting to note that an MFT approach that does not take into account the presence of two-dimensional magnetic fluctuations in ultrathin films would substantially underestimate (even by one order of magnitude) the enhancement of TC in these layers [4].

MFT curves show that a J=7/2 system is more susceptible than a J=1/2 system for moderate effective fields at the same reduced temperature

Figure 2. MFT curves show that a J=7/2 system is more susceptible than a J=1/2 system for moderate effective fields at the same reduced temperature t = T/TC (here TC is supposed to be 16.6 K in both calculations, therefore, t scales as T). However, the strong spin polarisation of the 2 nm EuS by Co cannot be understood by MFT. The reduced dimensionality of EuS layers may provide a more plausible explanation.

Our work shows promising indications for the employment of Co/EuS or similar layered structures in spintronics. The ferromagnetism at R.T. in combination with tuneable optical properties of very thin EuS layers due to quantum confinement effects [5] may render EuS-based layered structures useful for novel optoelectronics-spintronics applications.

 

Principal publication and authors
Direct evidence for significant spin-polarization of EuS in Co/EuS multilayers at room temperature, S.D. Pappas (a), P. Poulopoulos (a,b,c), B. Lewitz (b), A. Straub (b), A. Goschew (b), V. Kapaklis (d), F. Wilhelm (e), A. Rogalev (e), and P. Fumagalli (b), Scientific Reports 3, 1333 (2013).
(a) Laboratory of High-Tech Materials, School of Engineering, University of Patras (Greece)
(b) Institut für Experimentalphysik, Freie Universität Berlin (Germany)
(c) Materials Science Department, University of Patras (Greece)
(d) Department of Physics and Astronomy, Uppsala University (Sweden)
(e) ESRF

 

References
[1] P. Fumagalli et al., Phys. Rev. B 57, 14294 (1998).
[2] A. Ney et al., Phys. Rev. Lett. 100, 157201 (2008).
[3] B. Lewitz et al., SPIN 2, 1250016 (2012).
[4] P.J. Jensen et al., Phys. Rev. B 60, R14994 (1999).
[5] P. Poulopoulos et al., Appl. Phys. Lett. 100, 211910 (2012).

 

 

Top image: Using polarised X-rays to study spintronic materials.