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

Abstract

Localisation phenomena in highly disordered metals close to the extreme conditions determined by the Mott-Ioffe-Regel (MIR) limit when the electron mean free path is approximately equal to the interatomic distance is a challenging problem. Here, to shed light on these localisation phenomena, we studied the dc transport and optical conductivity properties of nanoscaled multilayered films composed of disordered metallic Ta and magnetic FeNi nanoisland layers, where ferromagnetic FeNi nanoislands have giant magnetic moments of 10$$^3$$ 3 –10$$^5$$ 5 Bohr magnetons ($$\mu _{\mathrm{B}}$$ μ B ). In these multilayered structures, FeNi nanoisland giant magnetic moments are interacting due to the indirect exchange forces acting via the Ta electron subsystem. We discovered that the localisation phenomena in the disordered Ta layer lead to a decrease in the Drude contribution of free charge carriers and the appearance of the low-energy electronic excitations in the 1–2 eV spectral range characteristic of electronic correlations, which may accompany the formation of electronic inhomogeneities. From the consistent results of the dc transport and optical studies we found that with an increase in the FeNi layer thickness across the percolation threshold evolution from the superferromagnetic to ferromagnetic behaviour within the FeNi layer leads to the delocalisation of Ta electrons from the associated localised electronic states. On the contrary, we discovered that when the FeNi layer is discontinuous and represented by randomly distributed superparamagnetic FeNi nanoislands, the Ta layer normalized dc conductivity falls down below the MIR limit by about 60%. The discovered effect leading to the dc conductivity fall below the MIR limit can be associated with non-ergodicity and purely quantum (many-body) localisation phenomena, which need to be challenged further.

Bi layer properties in the Bi-FeNi GMR-type structures probed by spectroscopic ellipsometry

“…[13]). Indeed, strong magnetic and spatial disorder induced by magnetic FeNi nanoislands, as well as long-range many-body interactions between magnetic moments of permalloy nanoislands [17], may lead to specific localization of free charge carriers [28]. However, the surface conductivity (or interface) states for the 1.4 nm layer in the Bi-FeNi(1.8 nm) multilayers may be topologically nontrivial and, in this case, the electrons cannot be backscattered by impurities.…”

Bi Layer Properties in the Bi–FeNi GMR-Type Structures Probed by Spectroscopic Ellipsometry

“…[13], Figure 4d). Indeed, a strong magnetic and spatial disorder induced by magnetic FeNi nanoislands, as well as long-range many-body interactions between the magnetic moments of permalloy nanoislands [17], may lead to a specific localization of free charge carriers [28]. However, the surface conductivity (or interface) states for the 1.4 nm layer in the Bi-FeNi(1.8 nm) multilayers may be topologically nontrivial and, in this case, the electrons cannot be backscattered by impurities.…”

Bismuth layer properties in the ultrathin Bi-FeNi multilayer films probed by spectroscopic ellipsometry

“…Here, with decreasing the FeNi strong SPM-type fluctuations of giant magnetic moments of the FeNi nanoislands become important [9,11]. In our recent study [8], we reported that this leads to localization phenomena in the GMR-type MLFs, introduced by an additional strong magnetic disorder and long-range many-body interactions magnetic of FeNi nanoislands. Therefore, the lack of evidence on the surface metallic states for the Bi layer in the [Bi(2.5, 2.0, 1.4 nm)-FeNi(0.8, nm)] 16 MLFs can be referred to strong SPMtype fluctuations of giant magnetic moments of FM FeNi nanoislands, leading to strong scattering and localization of free charge carriers in the surface layer.…”

“…Recently, we have demonstrated that the electronic properties of the free and localized Ta charge carriers in (Ta-FeNi) N multilayer films (MLFs) can be studied by spectroscopic ellipsometry (SE) [7,8]. Here, we explore the elaborated SE approach to gain insights into the electron band structure and surface electronic properties of ultrathin Bi layers in real GMR-type (Bi-FeNi) N MLF structures, incorporating nanoisland FeNi layers (see the scheme of the (Bi-FeNi) N MLFs investigated in the present study by SE in Fig.…”

“…In the prepared (Bi-FeNi) N MLF samples, the FeNi layer nominal thickness was 0.8 and 1.2 nm, the thickness of the Bi layer was 0.6, 1.4, 2.0, and 2.5 nm, and the number of Bi/FeNi bilayers was N = 16. In our recent study, (Ta-FeNi) MLFs grown by rf sputtering deposition onto Sitall-glass substrates, including ultrathin 0.52 nm-thick FeNi layers, were characterized by scanning/transmission electron microscopy (STEM) (for details see [8]). Here, the grown Bi-FeNi MLF samples were characterized by the atomic-force microscopy (AFM), Xray diffraction, and X-ray reflectivity (see supplementary material to this article).…”