Influence of f–d Interaction on Tunnel Magnetoresistance and Magnetoimpedance in Island Fe/Gd₂O₃ Nanostructures

The potential for amplifying tunnel magnetoresistance and magnetoimpedance in island nanofilms without energy consumption or the use of amplifying devices was studied. Such amplification was observed for the films deposited on a Gd₂O₃ layer instead of a glass substrate. The enhancement is due to the f–d exchange interaction established between atoms with unfilled f- and d-electron shells, present in both Fe and Gd₂O₃. The f–d exchange interaction enhances the ordering of the magnetic structure within the Fe ferromagnetic layer, increases its magnetization, and subsequently improves the properties that depend on this magnetization. Iron and Gd₂O₃ were selected because the magnetic moments of Fe in the iron group and Gd in the lanthanide series are among the highest effective ones: μ_Fe = 7.13 μ_B and μ_Gd = 7.95 μ_B. This, according to theory, determines the high energy of the f–d exchange interaction. The island morphology of Fe films deposited on a Gd₂O₃ layer enabled electron tunneling under the influence of the f–d exchange interaction. This allowed the study of tunnel magnetoresistance under direct current and magnetoimpedance under alternating current influenced by the f–d exchange interaction. The frequency dependence of the active component Z', the reactive component Z'', and the total impedance Z of Fe films on glass and Gd₂O₃ substrates, without and under a constant magnetic field of 7500 Oe, was analyzed. These characteristics are used to determine the frequency dependence of tunnel magnetoimpedance and estimate tunnel magnetoresistance for Fe island films on glass and Gd₂O₃ substrates. These characteristics were found to be 55 % higher, on average, for Fe deposited on Gd₂O₃ than for Fe on glass, both at a low frequency of 0.1 Hz for tunnel magnetoresistance and at higher frequencies of 1–10⁵ Hz for tunnel magnetoimpedance. This results from the influence of f–d exchange interaction on electron tunneling between the iron islands.