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Tuning the Magnetic and Electronic Properties of FexSi1-x Thin Films for Spintronics

  • Author(s): Karel, Julie Elizabeth
  • Advisor(s): Hellman, Frances
  • et al.
Abstract

Advances in traditional CMOS devices, in pursuit of Moore's Law, have lead to the detrimental side effects of increased energy consumption and heat generation. Spintronic (spin-electronic) devices are a potential alternative to standard charge-based devices where the electron spin carries the information instead. Many proposed spintronic devices require a spin-injector, a material that can produce a highly spin-polarized current, and consequently significant work has gone into identifying these types of materials. GayMn1-yAs, the canonical dilute magnetic semiconductor, has been touted as a promising material in this capacity since it is theoretically predicted to be 100% spin polarized and offers the possibility to electrically tune the ferromagnetism. However, the Curie temperature remains low (~150 K), making the material unsuitable for room-temperature spintronic applications.

This dissertation investigated the magnetic and electronic properties of a potentially better alternative: off-stoichimetry, bcc-like FexSi1-x thin films (0.43800 K) and theoretically predicted high spin polarization (100%). However, little work has been done on off-stoichiometry FexSi1-x thin films (0.43xSi1-x system is unique in that thin film growth techniques allow access to varying degrees of both chemical and structural order over a wide composition range. In the crystalline system, three different bcc-like structures (D03, B2, A2), each with a different degree of chemical order, are possible. The A2 structure is a chemically disordered random bcc solid solution, and the B2 structure is a partially ordered CsCl structure with Fe on the cube corner sites and Fe/Si randomly arranged on the body center sites. Finally, the D03 structure is chemically ordered with Fe on the cube corners and Fe and Si alternating in the body centers. Amorphous FexSi1-x thin films can also be fabricated, allowing for a comprehensive and direct comparison of the magnetic properties. This work probed the effects of chemical and structural disorder on the magnetic and electronic properties of FexSi1-x thin films.

The local chemical order in epitaxial FexSi1-x thin films was characterized using conversion electron Mössbauer spectrometry (CEMS); X-ray absorption fine structure (XAFS) and density functional theory (DFT) were used to characterize the local environments in the amorphous films. CEMS showed films have B2 chemical order for x≤0.65 and D03 for x>0.65. Even very far from the equilibrium composition, x=0.75, the films still tended towards chemical order; the A2 structure was not successfully fabricated. Both theoretical DFT calculations and X-ray absorption fine structure for the amorphous materials indicate a local atomic structure that is well-ordered for Fe-Si pairs and less ordered for Fe-Fe; calculated and experimental interatomic distances are similar to a bcc structure, however with a decreased coordination number. Experimental and theoretical number densities in the amorphous structures are less than in the crystalline phase.

The magnetism was found to strongly depend on the chemical order for both the crystalline and amorphous structures. The chemically disordered A2 structure has more Fe-Fe pairs than the chemically ordered B2 or D03 structures, leading to a larger predicted moment. The magnetic moments for the B2 and D03 structures are not significantly different. They should, in fact, be essentially the same since the first nearest neighbor environments are the same; on average there are the same number of Fe-Fe first nearest neighbor pairs in both structures. Only the second nearest neighbor environments, which have a weaker effect on the magnetic moment, are different. An enhanced magnetic moment due to enhanced spin and orbital moments was observed in all amorphous films versus crystalline films of the same composition. The amorphous local environments (based on the fraction of Fe-Fe nearest neighbors, N1Fe-Fe/CN1) are approximately intermediate between the chemically disordered A2 structure and the chemically ordered D03 or B2 structures; the amorphous materials, while structurally disordered, are only partially chemically disordered. The amorphous materials have a different structure; there are however more Fe-Fe pairs than the D03 or B2 structures (although less than A2), explaining the observed enhanced moment.

Not surprisingly, the electronic properties were also found to depend strongly on chemical and structural order, based on hard X-ray photoemission spectroscopy and DFT calculations. The core-level peaks in the amorphous structure (x=0.67) show little broadening despite a significant energy shift, suggesting that the local environment around the Si atoms is different than in the crystalline materials but far more uniform than expected, consistent with XAFS results, which showed that Si is well-ordered. A well-resolved Si 2p spin-orbit splitting for two epitaxial alloys, x=0.72 (D03) and 0.67 (B2) suggests that nearest-neighbor interactions are the dominant effect on binding energy for the Si atoms in the sample. The Si 2p peak in the amorphous sample also shows spin-orbit splitting, another indication that the local structure around each Si atom is relatively well defined. The valence bands show a broadening of the features when chemical and structural disorder is increased, consistent with theoretical band structure calculations for D03, B2, A2 and amorphous structures.

The electronic structure calculations reveal that the spin-polarization, |P|, is relatively insensitive to x in the amorphous structures and is negative and comparable in magnitude to the B2 structure. It is larger, by more than a factor of 3, than |P| in the hypothetical A2 structure. The D03 structure has the largest |P|. Remarkably, Andreev reflection measurements reveal that the spin polarization in the amorphous film (x=0.65) is significantly larger than the epitaxial (B2) film (x=0.65). In fact, the spin polarization of the amorphous film is larger than spin polarization measurements by Andreev reflection reported on an x=0.75 (D03) epitaxial film.

Lastly, the anomalous Hall effect, observed in all films, was very large in the amorphous films versus epitaxial films with the same composition. To investigate the origins of the AHE, σxy/Mz was plotted versus σxx, allowing for comparison to recent theoretical calculations. In the epitaxial films, σxy/Mz is constant, meaning the AHE is dominated by the intrinsic mechanism, as predicted theoretically in this moderate longitudinal conductivity regime. The AHE in the low conductivity regime (amorphous films) shows a scaling with conductivity similar to that seen in low conductivity GaMnAs films despite much larger disorder and carrier concentration in the amorphous films. The AHE scaling in these material systems was compared to other materials in the low conductivity regime, and all were found to be approximately linearly dependent on the longitudinal conductivity, suggesting dependence on the number of carriers.

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