Discontinuous Metal-insulator Multilayers Research Articles

Probing the morphology of epitaxial Fe/MgO discontinuous multilayers by magnetometric technique

Abstract Discontinuous metal-insulator multilayers (DMIMs) are a special type of nanostructures with a layered arrangement of metallic particles sandwiched between continuous insulating layers. DMIMs exhibit moderate tunneling magnetoresistance (TMR) ratio but enhanced low-field sensitivity, which makes them promising candidates for magnetic field sensors. Recently we have grown epitaxial Fe/MgO DMIMs on MgO (0 0 1) single crystal substrates at different deposition conditions. Here, based on the analysis of magnetic isotherms in a broad temperature range, the effect of deposition temperature (TS) on microstructure of DMIMs is being studied and compared with the results of Transmission Electron Microscopy. It is shown that metallic layers consist of flat nanoparticles whose average size decreases, and their crystallinity improves with the increase of TS.

Magnetic Properties of Epitaxial Discontinuous Fe/MgO Multilayers

We report magnetic, dynamic and transport properties of discontinuous metal-insulator multilayers Fe/MgO grown on amorphous Corning glass and single-crystalline MgO (001) substrates. The films of structure Substrate/MgO (3 nm)/[Fe (0.6 nm)/MgO (3.0 nm)] x 10 were prepared in ultra-high vacuum conditions using Pulsed Laser Deposition. It was shown that conditions of epitaxial growth are favorable for MgO substrates. As a result a substantial increase of tunneling magnetoresistance caused by spin-filtering effect was observed and reasonably theoretically explained. The value of TMR - 9.2% at room temperature in 18 kOe magnetic field is three times higher comparing to that for the samples grown on Corning glass substrates.

Single-particle blocking and collective magnetic states in discontinuous CoFe/Al2O3 multilayers

Discontinuous metal–insulator multilayers (DMIMs) of [CoFe(tn)/Al2O3]m containing soft ferromagnetic (FM) Co80Fe20 nanoparticles embedded discontinuously in a diamagnetic insulating Al2O3 matrix are ideal systems to study interparticle interaction effects. Here the CoFe nanoparticles are treated as superspins with random size, position and anisotropy. At low particle density, namely nominal layer thickness tn = 0.5 nm, single-particle blocking phenomena are observed due to the absence of large enough interparticle interactions. However at 0.5 nm < tn < 1.1 nm, the particles encounter strong interactions which give rise to a superspin glass (SSG) phase. The SSG phase has been characterized by memory effect, ageing, dynamic scaling, etc. With further increase in particle concentration (1.1 nm < tn < 1.4 nm) and, hence, smaller interparticle distances, strong interactions lead to a FM-like state which is called superferromagnetic (SFM). The SFM state has been characterized by several techniques, e.g. dynamic hysteresis, Cole–Cole plots extracted from ac susceptibility, polarized neutron reflectometry, etc. Moreover, the SFM domains could be imaged by x-ray photoemission electron microscopy and magneto-optic Kerr effect microscopy. At tn > 1.4 nm physical percolation occurs between the particles and the samples are no longer discontinuous and then termed as metal insulating multilayers. Competition between long- and short-ranged dipolar interactions leads to an oscillating magnetization depth profile from CoFe layer to CoFe layer with an incommensurate periodicity.

Determination of the percolation threshold in Fe/MgO magnetic granular multilayers

The evolution of the morphology, magnetic and transport properties of Fe(t nm)/MgO(3.0 nm) multilayers with respect to the nominal metallic layer thickness wasinvestigated. A comparison with existing experimental data on discontinuousmetal–insulator multilayers, ultrathin epitaxial Fe films on MgO substrates and granularcermet films is made. It is confirmed that the deposition conditions and the materialcomposition play a crucial role in the percolation process. Nominal thicknessesof Fe layers at which an infinite metallic cluster is formed and the conditionsfor continuous Fe coverage were determined. Different methods of percolationthreshold detection were analysed. We show that investigation of the temperaturedependence of resistance in nanostructures could lead to an overestimation of thepercolation threshold value, while magnetic measurements alone could lead to itsunderestimation.

X-ray absorption spectroscopy study of granular Fe / Si3N4 systems

[Fe (tFe) / Si3N4 (3 nm)] multilayers were prepared by sequential magnetron sputtering. The Fe layer thickness has been varied for every sample. Magnetic properties show a gradual evolution from a ferromagnetic state for samples with larger metal thickness to a granular behaviour for the samples with the smallest Fe layer thickness. Microstructural features such as average thickness and granularity, as well as the local order around Fe atoms, were studied by x-ray reflectometry and x-ray absorption spectroscopy, respectively. X-ray reflectometry suggests that the formation of discontinuous metal-insulator multilayers is produced at tFe ≤ 1.3 nm. Extended x-ray absorption fine structure (EXAFS) analysis shows a reduction, of the Fe-Fe coordination shell as the metal layer thickness decreases. Moreover, a new phase emerges, and it is visible at the samples with tFe ≤ 1.3 nm. This coordination shell is attributed to the formation of Fe-N bonds likely placed at the interface regions. X-ray absorption near edge spectroscopy (XANES) at the Fe K-edge shows as well the evolution from the metallic spectrum to a combined contribution of 2 phases as the metal layer thickness decreases. XANES calculations performed within the real-space multiple-scattering formalism of two nanometric phases for metal bcc Fe and tetrahedral FeN in zinc-blende structure provide a successful explanation of the XANES spectral evolution. The appearance of a new phase linked to the interface regions obtained by X-ray absorption analysis suggests the granular morphology of samples with tFe ≤ 1.3 nm.

Magnetic states of discontinuous Co 80Fe 20–Al 2O 3 multilayers

Discontinuous metal–insulator multilayers [Co 80Fe 20( t)/Al 2O 3(3 nm)] n were studied by SQUID magnetometry and AC susceptometry. CoFe forms ferromagnetic particles in the Al 2O 3 matrix. In this paper, we focus on the field dependence of the AC susceptibility of samples with t=1.3 nm. We find strong evidence for transitions from a superpara- to a superferromagnetic and, finally, to a reentrant superspin glass state.

AC susceptibility studies of discontinuous Co 80Fe 20/Al 2O 3 multilayers

Discontinuous metal–insulator multilayers [Co 80Fe 20( t)/Al 2O 3(3 nm)] 10 are prepared by ion-beam sputtering and investigated by SQUID magnetometry and AC susceptometry. For 1.0⩽ t⩽1.6 nm, two coexisting subsystems of dipolar coupled nanoparticles are encountered. While a spin-glass-like freezing particulate fraction dominates at t=1.0 nm, a percolating dipolar ferromagnetic fraction becomes increasingly important at higher nominal thickness of t.

Interacting ferromagnetic nanoparticles in discontinuousCo80Fe20/Al2O3multilayers: From superspin glass to reentrant superferromagnetism

Dipolar superferromagnetism with reentrant low-temperature superspin glass behavior is observed on a randomly distributed ferromagnetic nanoparticle systems in discontinuous metal-insulator multilayers $[{\mathrm{Co}}_{80}{\mathrm{Fe}}_{20}(t)/{\mathrm{Al}}_{2}{\mathrm{O}}_{3}(3\mathrm{}\mathrm{n}\mathrm{m}){]}_{10}$ with nominal thickness $1.1l~tl~1.3\mathrm{nm}$ by use of ac susceptometry and dc magnetometry. At $t=1.0\mathrm{nm},$ superspin glass-like freezing is evidenced by the criticality of dynamic and nonlinear susceptibilities.