Iron silicides have an excellent potential as so-called "spintronics" devices since the system has various structures and properties. For instance, D03 Fe3Si is ferromagnetic metal and orthorhombic β-FeSi2 is semiconductor which possibly emits near infra-red light for the fiber communication. So far, many authors have focused on the synthesis of light-emitting β-FeSi2, however, not intensively on the silicide magnetism, which is another important part of the spintronics.
Recently, solid phase epitaxial (SPE) growth of iron silicides on Si(111) substrates has been systematically studied [1], as functions of deposited Fe thickness Θ on Si(111)7x7 and subsequently annealing temperature T where Si(111)1x1-Fe surface appears at Θ ~ 1-6 ML and T < 600 K, and Si(111)2x2-Fe at Θ ~ 1-13 ML and T ~ 650-800 K. The structure models based on the B2 (CsCl) c-FeSi(111) ultra-thin film have been proposed for these surfaces [2]: B-type stacking of c-FeSi(111) on Si(111). Ref. 2 suggested B8-type interface (B-type stacking and eight-fold Fe at the interface) suitable among A5, B5, A7, B7, A8 and B8 models.
To study magnetism of the 1x1- and 2x2-Fe surfaces, we prepared high quality samples in ultra-high vacuum and carried out in-situ LEED, RHEED, STM, SMOKE and ex-situ SQUID measurements. The SMOKE results showed ferromagnetic minor loops at 40K for both surfaces. The SQUID showed quit high magnetization for both and the hysteresis loop at 5 K for the 2x2-Fe, after the air exposure. These implies that the Si(111)1x1- and 2x2-Fe surfaces are super-paramagnetic and ferromagnetic, respectively, and that the ferromagnetic moment arises from the c-FeSi(111) film or the interface.
Our DFT calculation showed that c-FeSi(111) bulk has negligible magnetic moment and that the A4, B4, A5 and B5-type (not A7-B8) interfaces have non-zero magnetic moment. The ex-situ XRD measurement strongly suggested the B5-type interface. Therefore, we consider that the ferromagnetic moment of the 1x1- and 2x2-Fe surfaces arise from the B5-type interface.
References:
[1] K. Kataoka et al., Phys. Rev. B 74, 155406 (2006).
[2] S. Walter et al., J. Phys.: Condens. Matter 15, 5207 (2003). |