Localization effects in the disordered Ta interlayer of multilayer Ta–FeNi films: Evidence from dc transport and spectroscopic ellipsometry study

Abstract

Using dc transport and wide-band spectroscopic ellipsometry techniques, we study localization effects in the disordered metallic Ta interlayer of different thicknesses in the multilayer films (MLFs) (Ta–FeNi)N grown by rf sputtering deposition. In the grown MLFs, the FeNi layer was 0.52 nm thick, while the Ta layer thickness varied between 1.2 and 4.6 nm. The Ta layer dielectric function was extracted from the Drude-Lorentz simulation. The dc transport study of the MLFs implies non-metallic (dρ/dT<0) behavior, with negative temperature coefficient of resistivity (TCR). The TCR absolute value increases upon increasing the Ta interlayer thickness, indicating enhanced electron localization. With that, the free charge carrier Drude response decreases. Moreover, the pronounced changes occur in the extended spectral range, involving the higher-energy Lorentz bands. The Drude dc conductivity drops below the weak localization limit for the thick Ta layer. The global band structure reconstruction may indicate the formation of a nearly localized many-body electron state.

Control of Mooij correlations at the nanoscale in the disordered metallic Ta–nanoisland FeNi multilayers

“…We would like to note that the estimated dc conductivity values σ 0 =1/ρ 0 (see Table 1) are in good agreement with the optical dc limit σ 1 (ω→0) of the Ta layer Drude response in the MLF samples N1, N2, and N3 of ≃ 2050 (4.6 nm), ≃ 2550 (2.3 nm), and ≃ 4650 (1.3 nm) −1 • cm −1 (depicted in Fig. 5), resulting from our recent spectroscopic ellipsometry study 50 .…”

“…8a-d and in our previous papers (see Fig. 4d 30,50 ) one can see the following systematic changes (i) the Drude dc conductivity decreases below the MIR limit and the low-energy band shifts to a higher energy of 2.7 eV in the sample N2, (ii) the Drude dc conductivity increases above the MIR limit and the low-energy band shifts to the lower energy about 1 eV in the samples N5 and N6, and (iii) the intensity of the low-energy band decreases on the conductivity enhancement in the sample N6. According to our simulations, the best metallicity properties among the whole MLF sample series N5-N8 are demonstrated by the Ta layer in the sample N8 (see Table 2 and Fig.…”

“…In the MLF sample N1, incorporating www.nature.com/scientificreports/ www.nature.com/scientificreports/ such FeNi nanoisland layers, the Drude dc conductivity drops well below the MIR limit (see Table 1) and the low-energy band at 2 eV becomes essentially suppressed, while the pronounced increase in the higher-energy band around 6-8 eV is observed (see Fig. 4e from our previous article 50 ), which can be associated with new localised electronic states. According to the obtained results, these new localisation phenomena introduced by an additional strong magnetic disorder and long-range many-body interactions between giant magnetic moments of FeNi nanoislands via the remaining free itinerant charge carriers of a disordered metal by means of RKKY-type indirect exchange, lead to the additional Ta layer resistivity increase (normalized to the MIR limit value ρ * ∼ 300 µ�•cm) up to 60%.…”

“…The quality of the fit was verified by the coincidence within the specified accuracy of less than 5% with the ellipsometric angles �(ω) and �(ω) measured at two angles of incidence of 65 • and 70 • . It was verified that the simulation in the framework of the multilayer model (Ta-FeNi) N -Ta/Sitall, where the Ta-FeNi interface roughness is explicitly included, does not improve the fit (for more details, see our recently published study 50 ). In particular, we expect that the Ta-FeNi interface roughness is essentially incorporated in the effective dielectric function of the nanoisland FeNi layer.…”

“…The results of the dc transport study ( vs ) for (i) the MLF samples N1–N3, N5, and N6 are represented by square symbols (see also Table 1 ), and for (ii) the 50- and 33-nm thick single-layer Ta films are shown by asterisks marked as 1 and 2, respectively 31 . The results of the optical study are represented by ( vs 1/ ) and include the optical Drude dc limit resistivities (1/ ) for the Ta layer in the MLFs N1–N3 (orange circles) 50 and N5 (black circle) and N6 (green circle) (see also Fig. 8 a,b and Table 2 ).…”