Collective magnetic response of inhomogeneous nanoisland FeNi films around the percolation transition

Abstract

By using superconducting quantum interference device (SQUID) magnetometry we investigated anisotropic high-field (H 7 T) low-temperature (10 K) magnetization response of inhomogeneous nanoisland FeNi films grown by rf sputtering deposition on Sitall (TiO2) glass substrates. In the grown FeNi films, the FeNi layer nominal thickness varied from 0.6 to 2.5 nm, across the percolation transition at the dc ≃ 1.8 nm. We discovered that, beyond conventional spin-magnetism of Fe21Ni79 permalloy, the extracted out-of-plane magnetization response of the nanoisland FeNi films is not saturated in the range of investigated magnetic fields and exhibits paramagnetic-like behavior. We found that the anomalous out-of-plane magnetization response exhibits an escalating slope with increase in the nominal film thickness from 0.6 to 1.1 nm, however, it decreases with further increase in the film thickness, and then practically vanishes on approaching the FeNi film percolation threshold. At the same time, the in-plane response demonstrates saturation behavior above 1.5-2 T, competing with anomalously large diamagnetic-like response, which becomes pronounced at high magnetic fields. It is possible that the supported-metal interaction leads to the creation of a thin charge-transfer (CT) layer and a Schottky barrier at the FeNi film/Sitall (TiO2) interface. Then, in the system with nanoscale circular domains, the observed anomalous paramagnetic-like magnetization response can be associated with a large orbital moment of the localized electrons. In addition, the inhomogeneous nanoisland FeNi films can possess spontaneous ordering of toroidal moments, which can be either of orbital or spin origin. The system with toroidal inhomogeneity can lead to anomalously strong diamagnetic-like response. The observed magnetization response is determined by the interplay between the paramagnetic-and diamagnetic-like contributions.

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

“…Another paradigm is represented by the MLF sample N1, where the 0.52-nm thick FeNi layer is far from the percolation transition and the FeNi layers are well separated from each other (by 4.6 nm). Earlier, we established that ultrathin monolayers of FeNi nanoislands exhibit superparamagnetic-like (SPM) behaviour at room temperature, associated with their giant magnetic moments of 10 3 -10 5 µ B 36,40 . 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.…”

“…Near the percolation transition within the FeNi layers strong magnetic dipole-dipole and exchange interactions between the FM nanoislands become relevant. It was shown that in quasi-2D systems of FM nanoparticles collective superferromagnetic (SFM) states can exist in their self-assembled local arrangements (clusters) at comparatively high temperatures 36,39,40 . We suppose that in here GMR-like case is realized, where the interlayer coupling between two neighboring FeNi layers is driven by the indirect exchange interactions via the itinerant charge carriers of the Ta spacer layer.…”

“…5 ). We discovered that the dc transport of the Ta layer strongly depends on the structural properties of the FeNi layers around the metal-insulator percolation transition, which, in turn, determine their magnetic properties 40 and, thereby, the stregth of magnetic coupling between neighboring FeNi layers mediated by the RKKY-type interaction. The entanglement of the RKKY-type interaction with the itinerant electrons of the Ta spacer lies in the root of the observed Mooij correlations driven by the interplay between the strength of the magnetic interaction and localisation of electons in the disordered Ta–nanoisland FeNi MLF structures.…”

“…1). Here, with increasing the FeNi layer thickness across the percolation threshold, the evolution from superparamagnetic (SPM) through superferromagnetic (SFM) to ferromagnetic (FM) behaviour within the FeNi layer [39][40][41] will modify the strength of the indirect exchange interaction between neighbouring FeNi layers. This, in turn, will strongly influence the itinerant charge carriers of the Ta layer promoting conditions for their enhanced localisation or delocalisation.…”

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

“…1). Here with increasing the FeNi layer thickness across the percolation threshold the evolution from superparamagnetic (SPM) through superferromagnetic (SFM) to ferromagnetic (FM) behaviour within the FeNi layer [29][30][31] will modify the strength of the indirect exchange interaction in ensembles of magnetic FeNi nanoislands. This, in turn, will strongly influence the itinerant charge carriers of the Ta layer, promoting conditions for their enhanced localisation or delocalisation, leading to the appearance or disappearance of electronic "puddles".…”