Effect of annealing on the structure and magnetic properties of Co2FeAl0.5Si0.5 thin films on Ge(111)
Introduction
Efficient spin-injection from half-metallic ferromagnets into semiconductors is crucial for the development of hybrid spintronic devices such as spin transistors [[1], [2], [3], [4]]. Half-metallic ferromagnets such as Co-based full Heusler alloys that are 100% spin-polarized at the Fermi-level are ideal candidates for such devices [[5], [6], [7], [8]]. In particular, Co2FeSi0.5Al0.5 (CFAS) shows great promise due to its high Curie temperature, high magnetic moment, mid-gap Fermi level, and low Gilbert damping constant [[9], [10], [11], [12], [13], [14]]. In addition to its thermal stability, CFAS has excellent lattice match with Ge (only ∼0.4% lattice mismatch). Furthermore, in comparison to the CFAS/Si interface previously investigated by our group [5], recent studies show that the CFAS/Ge interface preserves both the high spin-polarisation of the CFAS film as well as its magnetic moment in the interface vicinity, making this system an excellent platform for spin-based device applications [5,6].
Despite the predicted ideal properties of fully ordered L21 single crystal half-metallic Heusler alloys, structural defects and chemical disorder can drastically change the spin polarisation [7,15,16] in Heusler thin films. In order to improve spin-electronic properties, thermal treatment is essential not only to remove the point- and extended structural defects but also to achieve the chemically-ordered L21 phase throughout the film [17,18]. Annealing at elevated temperatures in the range of 500–700 °C is a standard approach undertaken for various heterostructures such as CPP-SVs and MTJs based on half-metallic Heusler electrodes [11,15,19,20]. In the case of hybrid heterostructures, annealing at such relatively high temperatures presents a large barrier for improving the film properties due to an extensive intermixing between the semiconducting substrate and Heusler film which usually occurs even at lower temperatures. In contrast to CFAS/Si where extensive intermixing has been observed even for room temperature deposition [5], recent studies of CFAS films on Ge(111) have demonstrated atomically and chemically sharp interfaces for room temperature deposition [6]. The structural phase of the room temperature deposited CFAS is B2 and defects such as antiphase boundaries, which locally destroy the spin polarisation, have been observed [7]. Therefore, it is important to determine how the annealing affects overall film structure as well as the atomic and chemical structure of the CFAS/Ge interface. Keeping the structural and chemical integrity of the half-metal/semiconductor interface is crucial since it determines the heterostructure band alignment, Schottky barrier height and ultimately the spin injection efficiency.
In this letter, we present a systematic study of the effect of annealing temperature on the CFAS/Ge heterostructure with a goal of establishing a correlation between structure, magnetic properties and interface integrity in the temperature range from 350 to 550 °C. Vibrating sample magnetometer (VSM) measurements show stable overall magnetisation and film coercivity up to 450 °C. The decrease of the Gilbert damping constant, measured by ferromagnetic resonance (FMR), within the same temperature range indicates improved structural/chemical ordering of the CFAS film. A further increase in the annealing temperature results in an increase of resonance linewidth suggesting secondary magnetic phase formations, which is also reflected in a decrease of magnetisation and an increase of coercivity of the film. Aberration-corrected scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS) measurements reveal that the changes in magnetic properties of the CFAS film are correlated to structural changes, mainly as a result of an interdiffusion between the Ge substrate and the CFAS film. Based on the structural data obtained by electron microscopy, we also perform first-principles calculations which correlate the drop of observed magnetisation with the structure of the annealed specimens. This work clearly shows that forming the L21 phase on Ge substrates is hindered by the strong interdiffusion of the Ge and the Heusler film. Hence the standard annealing procedures that require annealing above 500 °C are not suitable for hybrid spintronic structures.
Section snippets
Experimental
The samples were prepared by co-deposition of Co, Fe, Si and Al using low-temperature molecular beam epitaxy [21,22]. A 18 nm-thick CFAS film was deposited on a pre-cleaned 10 × 10 mm2 Ge (111) substrate at room temperature. Prior to loading Ge(111) substrates into the chamber, their surfaces were chemically cleaned with an aqueous 1% HF solution to remove any native oxide and contamination. Annealing experiments were carried out inside a UHV chamber with a base pressure of 8 × 10−11 mbar. Each
Results and discussion
Fig. 1(a) is an overview of the as-grown film, showing a well-defined interface with the Ge substrate. The SAED pattern (not shown) taken from a region including both the Ge(111) substrate and CFAS film demonstrates that the as-grown film is a single crystal structure; it also reveals the epitaxial crystallographic relationship between the film and substrate: CFAS(111)||Ge(111) and CFAS(10)||Ge(10). Fig. 1(b) is a HAADF STEM image, showing a detailed view of the interface region, where the
Conclusion
We showed that Co2FeAl0.5Si0.5 films deposited at room temperature on Ge(111) substrates have B2 ordering with magnetic properties strongly dependent on further annealing at a range of temperatures. The decrease of Gilbert damping for the films annealed up to 450 °C indicates improved chemical ordering of Co2FeAl0.5Si0.5. Consequently due to the very similar magnetic moment per unit cell we did not observe any change in saturation magnetisation up to 450 °C. Annealing at higher temperatures
Acknowledgments
This work was funded by the Engineering and Physical Sciences Research Council (EPSRC) through grants EP/K03278X/1 and EP/K032852/1. The SuperSTEM Laboratory is the U.K. National Facility for Aberration-Corrected STEM, supported by the EPSRC.
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